// Copyright 2020 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "src/tint/reader/spirv/function.h"

#include <algorithm>
#include <array>

#include "src/tint/ast/assignment_statement.h"
#include "src/tint/ast/bitcast_expression.h"
#include "src/tint/ast/break_statement.h"
#include "src/tint/ast/builtin_attribute.h"
#include "src/tint/ast/builtin_value.h"
#include "src/tint/ast/call_statement.h"
#include "src/tint/ast/continue_statement.h"
#include "src/tint/ast/discard_statement.h"
#include "src/tint/ast/if_statement.h"
#include "src/tint/ast/loop_statement.h"
#include "src/tint/ast/return_statement.h"
#include "src/tint/ast/stage_attribute.h"
#include "src/tint/ast/switch_statement.h"
#include "src/tint/ast/unary_op_expression.h"
#include "src/tint/ast/variable_decl_statement.h"
#include "src/tint/sem/builtin_type.h"
#include "src/tint/transform/spirv_atomic.h"
#include "src/tint/type/depth_texture.h"
#include "src/tint/type/sampled_texture.h"
#include "src/tint/type/texture_dimension.h"
#include "src/tint/utils/hashmap.h"
#include "src/tint/utils/hashset.h"

// Terms:
//    CFG: the control flow graph of the function, where basic blocks are the
//    nodes, and branches form the directed arcs.  The function entry block is
//    the root of the CFG.
//
//    Suppose H is a header block (i.e. has an OpSelectionMerge or OpLoopMerge).
//    Then:
//    - Let M(H) be the merge block named by the merge instruction in H.
//    - If H is a loop header, i.e. has an OpLoopMerge instruction, then let
//      CT(H) be the continue target block named by the OpLoopMerge
//      instruction.
//    - If H is a selection construct whose header ends in
//      OpBranchConditional with true target %then and false target %else,
//      then  TT(H) = %then and FT(H) = %else
//
// Determining output block order:
//    The "structured post-order traversal" of the CFG is a post-order traversal
//    of the basic blocks in the CFG, where:
//      We visit the entry node of the function first.
//      When visiting a header block:
//        We next visit its merge block
//        Then if it's a loop header, we next visit the continue target,
//      Then we visit the block's successors (whether it's a header or not)
//        If the block ends in an OpBranchConditional, we visit the false target
//        before the true target.
//
//    The "reverse structured post-order traversal" of the CFG is the reverse
//    of the structured post-order traversal.
//    This is the order of basic blocks as they should be emitted to the WGSL
//    function. It is the order computed by ComputeBlockOrder, and stored in
//    the |FunctionEmiter::block_order_|.
//    Blocks not in this ordering are ignored by the rest of the algorithm.
//
//    Note:
//     - A block D in the function might not appear in this order because
//       no block in the order branches to D.
//     - An unreachable block D might still be in the order because some header
//       block in the order names D as its continue target, or merge block,
//       or D is reachable from one of those otherwise-unreachable continue
//       targets or merge blocks.
//
// Terms:
//    Let Pos(B) be the index position of a block B in the computed block order.
//
// CFG intervals and valid nesting:
//
//    A correctly structured CFG satisfies nesting rules that we can check by
//    comparing positions of related blocks.
//
//    If header block H is in the block order, then the following holds:
//
//      Pos(H) < Pos(M(H))
//
//      If CT(H) exists, then:
//
//         Pos(H) <= Pos(CT(H))
//         Pos(CT(H)) < Pos(M)
//
//    This gives us the fundamental ordering of blocks in relation to a
//    structured construct:
//      The blocks before H in the block order, are not in the construct
//      The blocks at M(H) or later in the block order, are not in the construct
//      The blocks in a selection headed at H are in positions [ Pos(H),
//      Pos(M(H)) ) The blocks in a loop construct headed at H are in positions
//      [ Pos(H), Pos(CT(H)) ) The blocks in the continue construct for loop
//      headed at H are in
//        positions [ Pos(CT(H)), Pos(M(H)) )
//
//      Schematically, for a selection construct headed by H, the blocks are in
//      order from left to right:
//
//                 ...a-b-c H d-e-f M(H) n-o-p...
//
//           where ...a-b-c: blocks before the selection construct
//           where H and d-e-f: blocks in the selection construct
//           where M(H) and n-o-p...: blocks after the selection construct
//
//      Schematically, for a loop construct headed by H that is its own
//      continue construct, the blocks in order from left to right:
//
//                 ...a-b-c H=CT(H) d-e-f M(H) n-o-p...
//
//           where ...a-b-c: blocks before the loop
//           where H is the continue construct; CT(H)=H, and the loop construct
//           is *empty*
//           where d-e-f... are other blocks in the continue construct
//           where M(H) and n-o-p...: blocks after the continue construct
//
//      Schematically, for a multi-block loop construct headed by H, there are
//      blocks in order from left to right:
//
//                 ...a-b-c H d-e-f CT(H) j-k-l M(H) n-o-p...
//
//           where ...a-b-c: blocks before the loop
//           where H and d-e-f: blocks in the loop construct
//           where CT(H) and j-k-l: blocks in the continue construct
//           where M(H) and n-o-p...: blocks after the loop and continue
//           constructs
//

using namespace tint::number_suffixes;  // NOLINT

namespace tint::reader::spirv {

namespace {

constexpr uint32_t kMaxVectorLen = 4;

/// @param inst a SPIR-V instruction
/// @returns Returns the opcode for an instruciton
inline spv::Op opcode(const spvtools::opt::Instruction& inst) {
    return inst.opcode();
}
/// @param inst a SPIR-V instruction pointer
/// @returns Returns the opcode for an instruciton
inline spv::Op opcode(const spvtools::opt::Instruction* inst) {
    return inst->opcode();
}

// Gets the AST unary opcode for the given SPIR-V opcode, if any
// @param opcode SPIR-V opcode
// @param ast_unary_op return parameter
// @returns true if it was a unary operation
bool GetUnaryOp(spv::Op opcode, ast::UnaryOp* ast_unary_op) {
    switch (opcode) {
        case spv::Op::OpSNegate:
        case spv::Op::OpFNegate:
            *ast_unary_op = ast::UnaryOp::kNegation;
            return true;
        case spv::Op::OpLogicalNot:
            *ast_unary_op = ast::UnaryOp::kNot;
            return true;
        case spv::Op::OpNot:
            *ast_unary_op = ast::UnaryOp::kComplement;
            return true;
        default:
            break;
    }
    return false;
}

/// Converts a SPIR-V opcode for a WGSL builtin function, if there is a
/// direct translation. Returns nullptr otherwise.
/// @returns the WGSL builtin function name for the given opcode, or nullptr.
const char* GetUnaryBuiltInFunctionName(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpAny:
            return "any";
        case spv::Op::OpAll:
            return "all";
        case spv::Op::OpIsNan:
            return "isNan";
        case spv::Op::OpIsInf:
            return "isInf";
        case spv::Op::OpTranspose:
            return "transpose";
        default:
            break;
    }
    return nullptr;
}

// Converts a SPIR-V opcode to its corresponding AST binary opcode, if any
// @param opcode SPIR-V opcode
// @returns the AST binary op for the given opcode, or kNone
ast::BinaryOp ConvertBinaryOp(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpIAdd:
        case spv::Op::OpFAdd:
            return ast::BinaryOp::kAdd;
        case spv::Op::OpISub:
        case spv::Op::OpFSub:
            return ast::BinaryOp::kSubtract;
        case spv::Op::OpIMul:
        case spv::Op::OpFMul:
        case spv::Op::OpVectorTimesScalar:
        case spv::Op::OpMatrixTimesScalar:
        case spv::Op::OpVectorTimesMatrix:
        case spv::Op::OpMatrixTimesVector:
        case spv::Op::OpMatrixTimesMatrix:
            return ast::BinaryOp::kMultiply;
        case spv::Op::OpUDiv:
        case spv::Op::OpSDiv:
        case spv::Op::OpFDiv:
            return ast::BinaryOp::kDivide;
        case spv::Op::OpUMod:
        case spv::Op::OpSMod:
        case spv::Op::OpFRem:
            return ast::BinaryOp::kModulo;
        case spv::Op::OpLogicalEqual:
        case spv::Op::OpIEqual:
        case spv::Op::OpFOrdEqual:
            return ast::BinaryOp::kEqual;
        case spv::Op::OpLogicalNotEqual:
        case spv::Op::OpINotEqual:
        case spv::Op::OpFOrdNotEqual:
            return ast::BinaryOp::kNotEqual;
        case spv::Op::OpBitwiseAnd:
            return ast::BinaryOp::kAnd;
        case spv::Op::OpBitwiseOr:
            return ast::BinaryOp::kOr;
        case spv::Op::OpBitwiseXor:
            return ast::BinaryOp::kXor;
        case spv::Op::OpLogicalAnd:
            return ast::BinaryOp::kAnd;
        case spv::Op::OpLogicalOr:
            return ast::BinaryOp::kOr;
        case spv::Op::OpUGreaterThan:
        case spv::Op::OpSGreaterThan:
        case spv::Op::OpFOrdGreaterThan:
            return ast::BinaryOp::kGreaterThan;
        case spv::Op::OpUGreaterThanEqual:
        case spv::Op::OpSGreaterThanEqual:
        case spv::Op::OpFOrdGreaterThanEqual:
            return ast::BinaryOp::kGreaterThanEqual;
        case spv::Op::OpULessThan:
        case spv::Op::OpSLessThan:
        case spv::Op::OpFOrdLessThan:
            return ast::BinaryOp::kLessThan;
        case spv::Op::OpULessThanEqual:
        case spv::Op::OpSLessThanEqual:
        case spv::Op::OpFOrdLessThanEqual:
            return ast::BinaryOp::kLessThanEqual;
        default:
            break;
    }
    // It's not clear what OpSMod should map to.
    // https://bugs.chromium.org/p/tint/issues/detail?id=52
    return ast::BinaryOp::kNone;
}

// If the given SPIR-V opcode is a floating point unordered comparison,
// then returns the binary float comparison for which it is the negation.
// Othewrise returns BinaryOp::kNone.
// @param opcode SPIR-V opcode
// @returns operation corresponding to negated version of the SPIR-V opcode
ast::BinaryOp NegatedFloatCompare(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpFUnordEqual:
            return ast::BinaryOp::kNotEqual;
        case spv::Op::OpFUnordNotEqual:
            return ast::BinaryOp::kEqual;
        case spv::Op::OpFUnordLessThan:
            return ast::BinaryOp::kGreaterThanEqual;
        case spv::Op::OpFUnordLessThanEqual:
            return ast::BinaryOp::kGreaterThan;
        case spv::Op::OpFUnordGreaterThan:
            return ast::BinaryOp::kLessThanEqual;
        case spv::Op::OpFUnordGreaterThanEqual:
            return ast::BinaryOp::kLessThan;
        default:
            break;
    }
    return ast::BinaryOp::kNone;
}

// Returns the WGSL standard library function for the given
// GLSL.std.450 extended instruction operation code.  Unknown
// and invalid opcodes map to the empty string.
// @returns the WGSL standard function name, or an empty string.
std::string GetGlslStd450FuncName(uint32_t ext_opcode) {
    switch (ext_opcode) {
        case GLSLstd450FAbs:
        case GLSLstd450SAbs:
            return "abs";
        case GLSLstd450Acos:
            return "acos";
        case GLSLstd450Asin:
            return "asin";
        case GLSLstd450Atan:
            return "atan";
        case GLSLstd450Atan2:
            return "atan2";
        case GLSLstd450Ceil:
            return "ceil";
        case GLSLstd450UClamp:
        case GLSLstd450SClamp:
        case GLSLstd450NClamp:
        case GLSLstd450FClamp:  // FClamp is less prescriptive about NaN operands
            return "clamp";
        case GLSLstd450Cos:
            return "cos";
        case GLSLstd450Cosh:
            return "cosh";
        case GLSLstd450Cross:
            return "cross";
        case GLSLstd450Degrees:
            return "degrees";
        case GLSLstd450Determinant:
            return "determinant";
        case GLSLstd450Distance:
            return "distance";
        case GLSLstd450Exp:
            return "exp";
        case GLSLstd450Exp2:
            return "exp2";
        case GLSLstd450FaceForward:
            return "faceForward";
        case GLSLstd450FindILsb:
            return "firstTrailingBit";
        case GLSLstd450FindSMsb:
            return "firstLeadingBit";
        case GLSLstd450FindUMsb:
            return "firstLeadingBit";
        case GLSLstd450Floor:
            return "floor";
        case GLSLstd450Fma:
            return "fma";
        case GLSLstd450Fract:
            return "fract";
        case GLSLstd450InverseSqrt:
            return "inverseSqrt";
        case GLSLstd450Ldexp:
            return "ldexp";
        case GLSLstd450Length:
            return "length";
        case GLSLstd450Log:
            return "log";
        case GLSLstd450Log2:
            return "log2";
        case GLSLstd450NMax:
        case GLSLstd450FMax:  // FMax is less prescriptive about NaN operands
        case GLSLstd450UMax:
        case GLSLstd450SMax:
            return "max";
        case GLSLstd450NMin:
        case GLSLstd450FMin:  // FMin is less prescriptive about NaN operands
        case GLSLstd450UMin:
        case GLSLstd450SMin:
            return "min";
        case GLSLstd450FMix:
            return "mix";
        case GLSLstd450Normalize:
            return "normalize";
        case GLSLstd450PackSnorm4x8:
            return "pack4x8snorm";
        case GLSLstd450PackUnorm4x8:
            return "pack4x8unorm";
        case GLSLstd450PackSnorm2x16:
            return "pack2x16snorm";
        case GLSLstd450PackUnorm2x16:
            return "pack2x16unorm";
        case GLSLstd450PackHalf2x16:
            return "pack2x16float";
        case GLSLstd450Pow:
            return "pow";
        case GLSLstd450FSign:
        case GLSLstd450SSign:
            return "sign";
        case GLSLstd450Radians:
            return "radians";
        case GLSLstd450Reflect:
            return "reflect";
        case GLSLstd450Refract:
            return "refract";
        case GLSLstd450Round:
        case GLSLstd450RoundEven:
            return "round";
        case GLSLstd450Sin:
            return "sin";
        case GLSLstd450Sinh:
            return "sinh";
        case GLSLstd450SmoothStep:
            return "smoothstep";
        case GLSLstd450Sqrt:
            return "sqrt";
        case GLSLstd450Step:
            return "step";
        case GLSLstd450Tan:
            return "tan";
        case GLSLstd450Tanh:
            return "tanh";
        case GLSLstd450Trunc:
            return "trunc";
        case GLSLstd450UnpackSnorm4x8:
            return "unpack4x8snorm";
        case GLSLstd450UnpackUnorm4x8:
            return "unpack4x8unorm";
        case GLSLstd450UnpackSnorm2x16:
            return "unpack2x16snorm";
        case GLSLstd450UnpackUnorm2x16:
            return "unpack2x16unorm";
        case GLSLstd450UnpackHalf2x16:
            return "unpack2x16float";

        default:
            // TODO(dneto) - The following are not implemented.
            // They are grouped semantically, as in GLSL.std.450.h.

        case GLSLstd450Asinh:
        case GLSLstd450Acosh:
        case GLSLstd450Atanh:

        case GLSLstd450Modf:
        case GLSLstd450ModfStruct:
        case GLSLstd450IMix:

        case GLSLstd450Frexp:
        case GLSLstd450FrexpStruct:

        case GLSLstd450PackDouble2x32:
        case GLSLstd450UnpackDouble2x32:

        case GLSLstd450InterpolateAtCentroid:
        case GLSLstd450InterpolateAtSample:
        case GLSLstd450InterpolateAtOffset:
            break;
    }
    return "";
}

// Returns the WGSL standard library function builtin for the
// given instruction, or sem::BuiltinType::kNone
sem::BuiltinType GetBuiltin(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpBitCount:
            return sem::BuiltinType::kCountOneBits;
        case spv::Op::OpBitFieldInsert:
            return sem::BuiltinType::kInsertBits;
        case spv::Op::OpBitFieldSExtract:
        case spv::Op::OpBitFieldUExtract:
            return sem::BuiltinType::kExtractBits;
        case spv::Op::OpBitReverse:
            return sem::BuiltinType::kReverseBits;
        case spv::Op::OpDot:
            return sem::BuiltinType::kDot;
        case spv::Op::OpDPdx:
            return sem::BuiltinType::kDpdx;
        case spv::Op::OpDPdy:
            return sem::BuiltinType::kDpdy;
        case spv::Op::OpFwidth:
            return sem::BuiltinType::kFwidth;
        case spv::Op::OpDPdxFine:
            return sem::BuiltinType::kDpdxFine;
        case spv::Op::OpDPdyFine:
            return sem::BuiltinType::kDpdyFine;
        case spv::Op::OpFwidthFine:
            return sem::BuiltinType::kFwidthFine;
        case spv::Op::OpDPdxCoarse:
            return sem::BuiltinType::kDpdxCoarse;
        case spv::Op::OpDPdyCoarse:
            return sem::BuiltinType::kDpdyCoarse;
        case spv::Op::OpFwidthCoarse:
            return sem::BuiltinType::kFwidthCoarse;
        default:
            break;
    }
    return sem::BuiltinType::kNone;
}

// @param opcode a SPIR-V opcode
// @returns true if the given instruction is an image access instruction
// whose first input operand is an OpSampledImage value.
bool IsSampledImageAccess(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpImageSampleImplicitLod:
        case spv::Op::OpImageSampleExplicitLod:
        case spv::Op::OpImageSampleDrefImplicitLod:
        case spv::Op::OpImageSampleDrefExplicitLod:
        // WGSL doesn't have *Proj* texturing; spirv reader emulates it.
        case spv::Op::OpImageSampleProjImplicitLod:
        case spv::Op::OpImageSampleProjExplicitLod:
        case spv::Op::OpImageSampleProjDrefImplicitLod:
        case spv::Op::OpImageSampleProjDrefExplicitLod:
        case spv::Op::OpImageGather:
        case spv::Op::OpImageDrefGather:
        case spv::Op::OpImageQueryLod:
            return true;
        default:
            break;
    }
    return false;
}

// @param opcode a SPIR-V opcode
// @returns true if the given instruction is an atomic operation.
bool IsAtomicOp(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpAtomicLoad:
        case spv::Op::OpAtomicStore:
        case spv::Op::OpAtomicExchange:
        case spv::Op::OpAtomicCompareExchange:
        case spv::Op::OpAtomicCompareExchangeWeak:
        case spv::Op::OpAtomicIIncrement:
        case spv::Op::OpAtomicIDecrement:
        case spv::Op::OpAtomicIAdd:
        case spv::Op::OpAtomicISub:
        case spv::Op::OpAtomicSMin:
        case spv::Op::OpAtomicUMin:
        case spv::Op::OpAtomicSMax:
        case spv::Op::OpAtomicUMax:
        case spv::Op::OpAtomicAnd:
        case spv::Op::OpAtomicOr:
        case spv::Op::OpAtomicXor:
        case spv::Op::OpAtomicFlagTestAndSet:
        case spv::Op::OpAtomicFlagClear:
        case spv::Op::OpAtomicFMinEXT:
        case spv::Op::OpAtomicFMaxEXT:
        case spv::Op::OpAtomicFAddEXT:
            return true;
        default:
            break;
    }
    return false;
}

// @param opcode a SPIR-V opcode
// @returns true if the given instruction is an image sampling, gather,
// or gather-compare operation.
bool IsImageSamplingOrGatherOrDrefGather(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpImageSampleImplicitLod:
        case spv::Op::OpImageSampleExplicitLod:
        case spv::Op::OpImageSampleDrefImplicitLod:
        case spv::Op::OpImageSampleDrefExplicitLod:
            // WGSL doesn't have *Proj* texturing; spirv reader emulates it.
        case spv::Op::OpImageSampleProjImplicitLod:
        case spv::Op::OpImageSampleProjExplicitLod:
        case spv::Op::OpImageSampleProjDrefImplicitLod:
        case spv::Op::OpImageSampleProjDrefExplicitLod:
        case spv::Op::OpImageGather:
        case spv::Op::OpImageDrefGather:
            return true;
        default:
            break;
    }
    return false;
}

// @param opcode a SPIR-V opcode
// @returns true if the given instruction is an image access instruction
// whose first input operand is an OpImage value.
bool IsRawImageAccess(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpImageRead:
        case spv::Op::OpImageWrite:
        case spv::Op::OpImageFetch:
            return true;
        default:
            break;
    }
    return false;
}

// @param opcode a SPIR-V opcode
// @returns true if the given instruction is an image query instruction
bool IsImageQuery(spv::Op opcode) {
    switch (opcode) {
        case spv::Op::OpImageQuerySize:
        case spv::Op::OpImageQuerySizeLod:
        case spv::Op::OpImageQueryLevels:
        case spv::Op::OpImageQuerySamples:
        case spv::Op::OpImageQueryLod:
            return true;
        default:
            break;
    }
    return false;
}

// @returns the merge block ID for the given basic block, or 0 if there is none.
uint32_t MergeFor(const spvtools::opt::BasicBlock& bb) {
    // Get the OpSelectionMerge or OpLoopMerge instruction, if any.
    auto* inst = bb.GetMergeInst();
    return inst == nullptr ? 0 : inst->GetSingleWordInOperand(0);
}

// @returns the continue target ID for the given basic block, or 0 if there
// is none.
uint32_t ContinueTargetFor(const spvtools::opt::BasicBlock& bb) {
    // Get the OpLoopMerge instruction, if any.
    auto* inst = bb.GetLoopMergeInst();
    return inst == nullptr ? 0 : inst->GetSingleWordInOperand(1);
}

// A structured traverser produces the reverse structured post-order of the
// CFG of a function.  The blocks traversed are the transitive closure (minimum
// fixed point) of:
//  - the entry block
//  - a block reached by a branch from another block in the set
//  - a block mentioned as a merge block or continue target for a block in the
//  set
class StructuredTraverser {
  public:
    explicit StructuredTraverser(const spvtools::opt::Function& function) : function_(function) {
        for (auto& block : function_) {
            id_to_block_[block.id()] = &block;
        }
    }

    // Returns the reverse postorder traversal of the CFG, where:
    //  - a merge block always follows its associated constructs
    //  - a continue target always follows the associated loop construct, if any
    // @returns the IDs of blocks in reverse structured post order
    std::vector<uint32_t> ReverseStructuredPostOrder() {
        visit_order_.Clear();
        visited_.clear();
        VisitBackward(function_.entry()->id());

        std::vector<uint32_t> order(visit_order_.rbegin(), visit_order_.rend());
        return order;
    }

  private:
    // Executes a depth first search of the CFG, where right after we visit a
    // header, we will visit its merge block, then its continue target (if any).
    // Also records the post order ordering.
    void VisitBackward(uint32_t id) {
        if (id == 0) {
            return;
        }
        if (visited_.count(id)) {
            return;
        }
        visited_.insert(id);

        const spvtools::opt::BasicBlock* bb = id_to_block_[id];  // non-null for valid modules
        VisitBackward(MergeFor(*bb));
        VisitBackward(ContinueTargetFor(*bb));

        // Visit successors. We will naturally skip the continue target and merge
        // blocks.
        auto* terminator = bb->terminator();
        const auto opcode = terminator->opcode();
        if (opcode == spv::Op::OpBranchConditional) {
            // Visit the false branch, then the true branch, to make them come
            // out in the natural order for an "if".
            VisitBackward(terminator->GetSingleWordInOperand(2));
            VisitBackward(terminator->GetSingleWordInOperand(1));
        } else if (opcode == spv::Op::OpBranch) {
            VisitBackward(terminator->GetSingleWordInOperand(0));
        } else if (opcode == spv::Op::OpSwitch) {
            // TODO(dneto): Consider visiting the labels in literal-value order.
            utils::Vector<uint32_t, 32> successors;
            bb->ForEachSuccessorLabel(
                [&successors](const uint32_t succ_id) { successors.Push(succ_id); });
            for (auto succ_id : successors) {
                VisitBackward(succ_id);
            }
        }

        visit_order_.Push(id);
    }

    const spvtools::opt::Function& function_;
    std::unordered_map<uint32_t, const spvtools::opt::BasicBlock*> id_to_block_;
    utils::Vector<uint32_t, 32> visit_order_;
    std::unordered_set<uint32_t> visited_;
};

/// A StatementBuilder for ast::SwitchStatement
/// @see StatementBuilder
struct SwitchStatementBuilder final : public Castable<SwitchStatementBuilder, StatementBuilder> {
    /// Constructor
    /// @param cond the switch statement condition
    explicit SwitchStatementBuilder(const ast::Expression* cond) : condition(cond) {}

    /// @param builder the program builder
    /// @returns the built ast::SwitchStatement
    const ast::SwitchStatement* Build(ProgramBuilder* builder) const override {
        // We've listed cases in reverse order in the switch statement.
        // Reorder them to match the presentation order in WGSL.
        auto reversed_cases = cases;
        std::reverse(reversed_cases.begin(), reversed_cases.end());

        return builder->create<ast::SwitchStatement>(Source{}, condition,
                                                     std::move(reversed_cases));
    }

    /// Switch statement condition
    const ast::Expression* const condition;
    /// Switch statement cases
    utils::Vector<ast::CaseStatement*, 4> cases;
};

/// A StatementBuilder for ast::IfStatement
/// @see StatementBuilder
struct IfStatementBuilder final : public Castable<IfStatementBuilder, StatementBuilder> {
    /// Constructor
    /// @param c the if-statement condition
    explicit IfStatementBuilder(const ast::Expression* c) : cond(c) {}

    /// @param builder the program builder
    /// @returns the built ast::IfStatement
    const ast::IfStatement* Build(ProgramBuilder* builder) const override {
        return builder->create<ast::IfStatement>(Source{}, cond, body, else_stmt);
    }

    /// If-statement condition
    const ast::Expression* const cond;
    /// If-statement block body
    const ast::BlockStatement* body = nullptr;
    /// Optional if-statement else statement
    const ast::Statement* else_stmt = nullptr;
};

/// A StatementBuilder for ast::LoopStatement
/// @see StatementBuilder
struct LoopStatementBuilder final : public Castable<LoopStatementBuilder, StatementBuilder> {
    /// @param builder the program builder
    /// @returns the built ast::LoopStatement
    ast::LoopStatement* Build(ProgramBuilder* builder) const override {
        return builder->create<ast::LoopStatement>(Source{}, body, continuing);
    }

    /// Loop-statement block body
    const ast::BlockStatement* body = nullptr;
    /// Loop-statement continuing body
    /// @note the mutable keyword here is required as all non-StatementBuilders
    /// `ast::Node`s are immutable and are referenced with `const` pointers.
    /// StatementBuilders however exist to provide mutable state while the
    /// FunctionEmitter is building the function. All StatementBuilders are
    /// replaced with immutable AST nodes when Finalize() is called.
    mutable const ast::BlockStatement* continuing = nullptr;
};

/// @param decos a list of parsed decorations
/// @returns true if the decorations include a SampleMask builtin
bool HasBuiltinSampleMask(utils::VectorRef<const ast::Attribute*> decos) {
    if (auto* builtin = ast::GetAttribute<ast::BuiltinAttribute>(decos)) {
        return builtin->builtin == ast::BuiltinValue::kSampleMask;
    }
    return false;
}

}  // namespace

BlockInfo::BlockInfo(const spvtools::opt::BasicBlock& bb) : basic_block(&bb), id(bb.id()) {}

BlockInfo::~BlockInfo() = default;

DefInfo::DefInfo(size_t the_index,
                 const spvtools::opt::Instruction& def_inst,
                 uint32_t the_block_pos)
    : index(the_index), inst(def_inst), local(DefInfo::Local(the_block_pos)) {}

DefInfo::DefInfo(size_t the_index, const spvtools::opt::Instruction& def_inst)
    : index(the_index), inst(def_inst) {}

DefInfo::~DefInfo() = default;

DefInfo::Local::Local(uint32_t the_block_pos) : block_pos(the_block_pos) {}

DefInfo::Local::Local(const Local& other) = default;

DefInfo::Local::~Local() = default;

ast::Node* StatementBuilder::Clone(CloneContext*) const {
    return nullptr;
}

FunctionEmitter::FunctionEmitter(ParserImpl* pi,
                                 const spvtools::opt::Function& function,
                                 const EntryPointInfo* ep_info)
    : parser_impl_(*pi),
      ty_(pi->type_manager()),
      builder_(pi->builder()),
      ir_context_(*(pi->ir_context())),
      def_use_mgr_(ir_context_.get_def_use_mgr()),
      constant_mgr_(ir_context_.get_constant_mgr()),
      type_mgr_(ir_context_.get_type_mgr()),
      fail_stream_(pi->fail_stream()),
      namer_(pi->namer()),
      function_(function),
      sample_mask_in_id(0u),
      sample_mask_out_id(0u),
      ep_info_(ep_info) {
    PushNewStatementBlock(nullptr, 0, nullptr);
}

FunctionEmitter::FunctionEmitter(ParserImpl* pi, const spvtools::opt::Function& function)
    : FunctionEmitter(pi, function, nullptr) {}

FunctionEmitter::FunctionEmitter(FunctionEmitter&& other)
    : parser_impl_(other.parser_impl_),
      ty_(other.ty_),
      builder_(other.builder_),
      ir_context_(other.ir_context_),
      def_use_mgr_(ir_context_.get_def_use_mgr()),
      constant_mgr_(ir_context_.get_constant_mgr()),
      type_mgr_(ir_context_.get_type_mgr()),
      fail_stream_(other.fail_stream_),
      namer_(other.namer_),
      function_(other.function_),
      sample_mask_in_id(other.sample_mask_out_id),
      sample_mask_out_id(other.sample_mask_in_id),
      ep_info_(other.ep_info_) {
    other.statements_stack_.Clear();
    PushNewStatementBlock(nullptr, 0, nullptr);
}

FunctionEmitter::~FunctionEmitter() = default;

FunctionEmitter::StatementBlock::StatementBlock(const Construct* construct,
                                                uint32_t end_id,
                                                FunctionEmitter::CompletionAction completion_action)
    : construct_(construct), end_id_(end_id), completion_action_(completion_action) {}

FunctionEmitter::StatementBlock::StatementBlock(StatementBlock&& other) = default;

FunctionEmitter::StatementBlock::~StatementBlock() = default;

void FunctionEmitter::StatementBlock::Finalize(ProgramBuilder* pb) {
    TINT_ASSERT(Reader, !finalized_ /* Finalize() must only be called once */);

    for (size_t i = 0; i < statements_.Length(); i++) {
        if (auto* sb = statements_[i]->As<StatementBuilder>()) {
            statements_[i] = sb->Build(pb);
        }
    }

    if (completion_action_ != nullptr) {
        completion_action_(statements_);
    }

    finalized_ = true;
}

void FunctionEmitter::StatementBlock::Add(const ast::Statement* statement) {
    TINT_ASSERT(Reader, !finalized_ /* Add() must not be called after Finalize() */);
    statements_.Push(statement);
}

void FunctionEmitter::PushNewStatementBlock(const Construct* construct,
                                            uint32_t end_id,
                                            CompletionAction action) {
    statements_stack_.Push(StatementBlock{construct, end_id, action});
}

void FunctionEmitter::PushGuard(const std::string& guard_name, uint32_t end_id) {
    TINT_ASSERT(Reader, !statements_stack_.IsEmpty());
    TINT_ASSERT(Reader, !guard_name.empty());
    // Guard control flow by the guard variable.  Introduce a new
    // if-selection with a then-clause ending at the same block
    // as the statement block at the top of the stack.
    const auto& top = statements_stack_.Back();

    auto* cond = builder_.Expr(Source{}, guard_name);
    auto* builder = AddStatementBuilder<IfStatementBuilder>(cond);

    PushNewStatementBlock(top.GetConstruct(), end_id, [=](const StatementList& stmts) {
        builder->body = create<ast::BlockStatement>(Source{}, stmts, utils::Empty);
    });
}

void FunctionEmitter::PushTrueGuard(uint32_t end_id) {
    TINT_ASSERT(Reader, !statements_stack_.IsEmpty());
    const auto& top = statements_stack_.Back();

    auto* cond = MakeTrue(Source{});
    auto* builder = AddStatementBuilder<IfStatementBuilder>(cond);

    PushNewStatementBlock(top.GetConstruct(), end_id, [=](const StatementList& stmts) {
        builder->body = create<ast::BlockStatement>(Source{}, stmts, utils::Empty);
    });
}

FunctionEmitter::StatementList FunctionEmitter::ast_body() {
    TINT_ASSERT(Reader, !statements_stack_.IsEmpty());
    auto& entry = statements_stack_[0];
    entry.Finalize(&builder_);
    return entry.GetStatements();
}

const ast::Statement* FunctionEmitter::AddStatement(const ast::Statement* statement) {
    TINT_ASSERT(Reader, !statements_stack_.IsEmpty());
    if (statement != nullptr) {
        statements_stack_.Back().Add(statement);
    }
    return statement;
}

const ast::Statement* FunctionEmitter::LastStatement() {
    TINT_ASSERT(Reader, !statements_stack_.IsEmpty());
    auto& statement_list = statements_stack_.Back().GetStatements();
    TINT_ASSERT(Reader, !statement_list.IsEmpty());
    return statement_list.Back();
}

bool FunctionEmitter::Emit() {
    if (failed()) {
        return false;
    }
    // We only care about functions with bodies.
    if (function_.cbegin() == function_.cend()) {
        return true;
    }

    // The function declaration, corresponding to how it's written in SPIR-V,
    // and without regard to whether it's an entry point.
    FunctionDeclaration decl;
    if (!ParseFunctionDeclaration(&decl)) {
        return false;
    }

    bool make_body_function = true;
    if (ep_info_) {
        TINT_ASSERT(Reader, !ep_info_->inner_name.empty());
        if (ep_info_->owns_inner_implementation) {
            // This is an entry point, and we want to emit it as a wrapper around
            // an implementation function.
            decl.name = ep_info_->inner_name;
        } else {
            // This is a second entry point that shares an inner implementation
            // function.
            make_body_function = false;
        }
    }

    if (make_body_function) {
        auto* body = MakeFunctionBody();
        if (!body) {
            return false;
        }

        builder_.AST().AddFunction(create<ast::Function>(
            decl.source, builder_.Symbols().Register(decl.name), std::move(decl.params),
            decl.return_type->Build(builder_), body, std::move(decl.attributes), utils::Empty));
    }

    if (ep_info_ && !ep_info_->inner_name.empty()) {
        return EmitEntryPointAsWrapper();
    }

    return success();
}

const ast::BlockStatement* FunctionEmitter::MakeFunctionBody() {
    TINT_ASSERT(Reader, statements_stack_.Length() == 1);

    if (!EmitBody()) {
        return nullptr;
    }

    // Set the body of the AST function node.
    if (statements_stack_.Length() != 1) {
        Fail() << "internal error: statement-list stack should have 1 "
                  "element but has "
               << statements_stack_.Length();
        return nullptr;
    }

    statements_stack_[0].Finalize(&builder_);
    auto& statements = statements_stack_[0].GetStatements();
    auto* body = create<ast::BlockStatement>(Source{}, statements, utils::Empty);

    // Maintain the invariant by repopulating the one and only element.
    statements_stack_.Clear();
    PushNewStatementBlock(constructs_[0].get(), 0, nullptr);

    return body;
}

bool FunctionEmitter::EmitPipelineInput(std::string var_name,
                                        const Type* var_type,
                                        AttributeList* attrs,
                                        utils::Vector<int, 8> index_prefix,
                                        const Type* tip_type,
                                        const Type* forced_param_type,
                                        ParameterList* params,
                                        StatementList* statements) {
    // TODO(dneto): Handle structs where the locations are annotated on members.
    tip_type = tip_type->UnwrapAlias();
    if (auto* ref_type = tip_type->As<Reference>()) {
        tip_type = ref_type->type;
    }

    // Recursively flatten matrices, arrays, and structures.
    return Switch(
        tip_type,
        [&](const Matrix* matrix_type) -> bool {
            index_prefix.Push(0);
            const auto num_columns = static_cast<int>(matrix_type->columns);
            const Type* vec_ty = ty_.Vector(matrix_type->type, matrix_type->rows);
            for (int col = 0; col < num_columns; col++) {
                index_prefix.Back() = col;
                if (!EmitPipelineInput(var_name, var_type, attrs, index_prefix, vec_ty,
                                       forced_param_type, params, statements)) {
                    return false;
                }
            }
            return success();
        },
        [&](const Array* array_type) -> bool {
            if (array_type->size == 0) {
                return Fail() << "runtime-size array not allowed on pipeline IO";
            }
            index_prefix.Push(0);
            const Type* elem_ty = array_type->type;
            for (int i = 0; i < static_cast<int>(array_type->size); i++) {
                index_prefix.Back() = i;
                if (!EmitPipelineInput(var_name, var_type, attrs, index_prefix, elem_ty,
                                       forced_param_type, params, statements)) {
                    return false;
                }
            }
            return success();
        },
        [&](const Struct* struct_type) -> bool {
            const auto& members = struct_type->members;
            index_prefix.Push(0);
            for (size_t i = 0; i < members.size(); ++i) {
                index_prefix.Back() = static_cast<int>(i);
                AttributeList member_attrs(*attrs);
                if (!parser_impl_.ConvertPipelineDecorations(
                        struct_type,
                        parser_impl_.GetMemberPipelineDecorations(*struct_type,
                                                                  static_cast<int>(i)),
                        &member_attrs)) {
                    return false;
                }
                if (!EmitPipelineInput(var_name, var_type, &member_attrs, index_prefix, members[i],
                                       forced_param_type, params, statements)) {
                    return false;
                }
                // Copy the location as updated by nested expansion of the member.
                parser_impl_.SetLocation(attrs,
                                         ast::GetAttribute<ast::LocationAttribute>(member_attrs));
            }
            return success();
        },
        [&](Default) {
            const bool is_builtin = ast::HasAttribute<ast::BuiltinAttribute>(*attrs);

            const Type* param_type = is_builtin ? forced_param_type : tip_type;

            const auto param_name = namer_.MakeDerivedName(var_name + "_param");
            // Create the parameter.
            // TODO(dneto): Note: If the parameter has non-location decorations,
            // then those decoration AST nodes will be reused between multiple
            // elements of a matrix, array, or structure.  Normally that's
            // disallowed but currently the SPIR-V reader will make duplicates when
            // the entire AST is cloned at the top level of the SPIR-V reader flow.
            // Consider rewriting this to avoid this node-sharing.
            params->Push(builder_.Param(param_name, param_type->Build(builder_), *attrs));

            // Add a body statement to copy the parameter to the corresponding
            // private variable.
            const ast::Expression* param_value = builder_.Expr(param_name);
            const ast::Expression* store_dest = builder_.Expr(var_name);

            // Index into the LHS as needed.
            auto* current_type = var_type->UnwrapAlias()->UnwrapRef()->UnwrapAlias();
            for (auto index : index_prefix) {
                Switch(
                    current_type,
                    [&](const Matrix* matrix_type) {
                        store_dest = builder_.IndexAccessor(store_dest, builder_.Expr(i32(index)));
                        current_type = ty_.Vector(matrix_type->type, matrix_type->rows);
                    },
                    [&](const Array* array_type) {
                        store_dest = builder_.IndexAccessor(store_dest, builder_.Expr(i32(index)));
                        current_type = array_type->type->UnwrapAlias();
                    },
                    [&](const Struct* struct_type) {
                        store_dest = builder_.MemberAccessor(
                            store_dest, parser_impl_.GetMemberName(*struct_type, index));
                        current_type = struct_type->members[static_cast<size_t>(index)];
                    });
            }

            if (is_builtin && (tip_type != forced_param_type)) {
                // The parameter will have the WGSL type, but we need bitcast to
                // the variable store type.
                param_value = builder_.Bitcast(tip_type->Build(builder_), param_value);
            }

            statements->Push(builder_.Assign(store_dest, param_value));

            // Increment the location attribute, in case more parameters will
            // follow.
            IncrementLocation(attrs);

            return success();
        });
}

void FunctionEmitter::IncrementLocation(AttributeList* attributes) {
    for (auto*& attr : *attributes) {
        if (auto* loc_attr = attr->As<ast::LocationAttribute>()) {
            // Replace this location attribute with a new one with one higher index.
            // The old one doesn't leak because it's kept in the builder's AST node
            // list.
            attr = builder_.Location(
                loc_attr->source, AInt(loc_attr->expr->As<ast::IntLiteralExpression>()->value + 1));
        }
    }
}

bool FunctionEmitter::EmitPipelineOutput(std::string var_name,
                                         const Type* var_type,
                                         AttributeList* decos,
                                         utils::Vector<int, 8> index_prefix,
                                         const Type* tip_type,
                                         const Type* forced_member_type,
                                         StructMemberList* return_members,
                                         ExpressionList* return_exprs) {
    tip_type = tip_type->UnwrapAlias();
    if (auto* ref_type = tip_type->As<Reference>()) {
        tip_type = ref_type->type;
    }

    // Recursively flatten matrices, arrays, and structures.
    return Switch(
        tip_type,
        [&](const Matrix* matrix_type) {
            index_prefix.Push(0);
            const auto num_columns = static_cast<int>(matrix_type->columns);
            const Type* vec_ty = ty_.Vector(matrix_type->type, matrix_type->rows);
            for (int col = 0; col < num_columns; col++) {
                index_prefix.Back() = col;
                if (!EmitPipelineOutput(var_name, var_type, std::move(decos), index_prefix, vec_ty,
                                        forced_member_type, return_members, return_exprs)) {
                    return false;
                }
            }
            return success();
        },
        [&](const Array* array_type) -> bool {
            if (array_type->size == 0) {
                return Fail() << "runtime-size array not allowed on pipeline IO";
            }
            index_prefix.Push(0);
            const Type* elem_ty = array_type->type;
            for (int i = 0; i < static_cast<int>(array_type->size); i++) {
                index_prefix.Back() = i;
                if (!EmitPipelineOutput(var_name, var_type, std::move(decos), index_prefix, elem_ty,
                                        forced_member_type, return_members, return_exprs)) {
                    return false;
                }
            }
            return success();
        },
        [&](const Struct* struct_type) -> bool {
            const auto& members = struct_type->members;
            index_prefix.Push(0);
            for (int i = 0; i < static_cast<int>(members.size()); ++i) {
                index_prefix.Back() = i;
                AttributeList member_attrs(*decos);
                if (!parser_impl_.ConvertPipelineDecorations(
                        struct_type, parser_impl_.GetMemberPipelineDecorations(*struct_type, i),
                        &member_attrs)) {
                    return false;
                }
                if (!EmitPipelineOutput(var_name, var_type, &member_attrs, index_prefix,
                                        members[static_cast<size_t>(i)], forced_member_type,
                                        return_members, return_exprs)) {
                    return false;
                }
                // Copy the location as updated by nested expansion of the member.
                parser_impl_.SetLocation(decos,
                                         ast::GetAttribute<ast::LocationAttribute>(member_attrs));
            }
            return success();
        },
        [&](Default) {
            const bool is_builtin = ast::HasAttribute<ast::BuiltinAttribute>(*decos);

            const Type* member_type = is_builtin ? forced_member_type : tip_type;
            // Derive the member name directly from the variable name.  They can't
            // collide.
            const auto member_name = namer_.MakeDerivedName(var_name);
            // Create the member.
            // TODO(dneto): Note: If the parameter has non-location decorations,
            // then those decoration AST nodes  will be reused between multiple
            // elements of a matrix, array, or structure.  Normally that's
            // disallowed but currently the SPIR-V reader will make duplicates when
            // the entire AST is cloned at the top level of the SPIR-V reader flow.
            // Consider rewriting this to avoid this node-sharing.
            return_members->Push(
                builder_.Member(member_name, member_type->Build(builder_), *decos));

            // Create an expression to evaluate the part of the variable indexed by
            // the index_prefix.
            const ast::Expression* load_source = builder_.Expr(var_name);

            // Index into the variable as needed to pick out the flattened member.
            auto* current_type = var_type->UnwrapAlias()->UnwrapRef()->UnwrapAlias();
            for (auto index : index_prefix) {
                Switch(
                    current_type,
                    [&](const Matrix* matrix_type) {
                        load_source = builder_.IndexAccessor(load_source, i32(index));
                        current_type = ty_.Vector(matrix_type->type, matrix_type->rows);
                    },
                    [&](const Array* array_type) {
                        load_source = builder_.IndexAccessor(load_source, i32(index));
                        current_type = array_type->type->UnwrapAlias();
                    },
                    [&](const Struct* struct_type) {
                        load_source = builder_.MemberAccessor(
                            load_source, parser_impl_.GetMemberName(*struct_type, index));
                        current_type = struct_type->members[static_cast<size_t>(index)];
                    });
            }

            if (is_builtin && (tip_type != forced_member_type)) {
                // The member will have the WGSL type, but we need bitcast to
                // the variable store type.
                load_source = builder_.Bitcast(forced_member_type->Build(builder_), load_source);
            }
            return_exprs->Push(load_source);

            // Increment the location attribute, in case more parameters will
            // follow.
            IncrementLocation(decos);

            return success();
        });
}

bool FunctionEmitter::EmitEntryPointAsWrapper() {
    Source source;

    // The statements in the body.
    utils::Vector<const ast::Statement*, 8> stmts;

    FunctionDeclaration decl;
    decl.source = source;
    decl.name = ep_info_->name;
    const ast::Type* return_type = nullptr;  // Populated below.

    // Pipeline inputs become parameters to the wrapper function, and
    // their values are saved into the corresponding private variables that
    // have already been created.
    for (uint32_t var_id : ep_info_->inputs) {
        const auto* var = def_use_mgr_->GetDef(var_id);
        TINT_ASSERT(Reader, var != nullptr);
        TINT_ASSERT(Reader, opcode(var) == spv::Op::OpVariable);
        auto* store_type = GetVariableStoreType(*var);
        auto* forced_param_type = store_type;
        AttributeList param_decos;
        if (!parser_impl_.ConvertDecorationsForVariable(var_id, &forced_param_type, &param_decos,
                                                        true)) {
            // This occurs, and is not an error, for the PointSize builtin.
            if (!success()) {
                // But exit early if an error was logged.
                return false;
            }
            continue;
        }

        // We don't have to handle initializers because in Vulkan SPIR-V, Input
        // variables must not have them.

        const auto var_name = namer_.GetName(var_id);

        bool ok = true;
        if (HasBuiltinSampleMask(param_decos)) {
            // In Vulkan SPIR-V, the sample mask is an array. In WGSL it's a scalar.
            // Use the first element only.
            auto* sample_mask_array_type = store_type->UnwrapRef()->UnwrapAlias()->As<Array>();
            TINT_ASSERT(Reader, sample_mask_array_type);
            ok = EmitPipelineInput(var_name, store_type, &param_decos, {0},
                                   sample_mask_array_type->type, forced_param_type, &decl.params,
                                   &stmts);
        } else {
            // The normal path.
            ok = EmitPipelineInput(var_name, store_type, &param_decos, {}, store_type,
                                   forced_param_type, &decl.params, &stmts);
        }
        if (!ok) {
            return false;
        }
    }

    // Call the inner function.  It has no parameters.
    stmts.Push(builder_.CallStmt(source, builder_.Call(source, ep_info_->inner_name)));

    // Pipeline outputs are mapped to the return value.
    if (ep_info_->outputs.IsEmpty()) {
        // There is nothing to return.
        return_type = ty_.Void()->Build(builder_);
    } else {
        // Pipeline outputs are converted to a structure that is written
        // to just before returning.

        const auto return_struct_name = namer_.MakeDerivedName(ep_info_->name + "_out");
        const auto return_struct_sym = builder_.Symbols().Register(return_struct_name);

        // Define the structure.
        StructMemberList return_members;
        ExpressionList return_exprs;

        const auto& builtin_position_info = parser_impl_.GetBuiltInPositionInfo();

        for (uint32_t var_id : ep_info_->outputs) {
            if (var_id == builtin_position_info.per_vertex_var_id) {
                // The SPIR-V gl_PerVertex variable has already been remapped to
                // a gl_Position variable.  Substitute the type.
                const Type* param_type = ty_.Vector(ty_.F32(), 4);
                AttributeList out_decos{
                    create<ast::BuiltinAttribute>(source, ast::BuiltinValue::kPosition)};

                const auto var_name = namer_.GetName(var_id);
                return_members.Push(
                    builder_.Member(var_name, param_type->Build(builder_), out_decos));
                return_exprs.Push(builder_.Expr(var_name));

            } else {
                const auto* var = def_use_mgr_->GetDef(var_id);
                TINT_ASSERT(Reader, var != nullptr);
                TINT_ASSERT(Reader, opcode(var) == spv::Op::OpVariable);
                const Type* store_type = GetVariableStoreType(*var);
                const Type* forced_member_type = store_type;
                AttributeList out_decos;
                if (!parser_impl_.ConvertDecorationsForVariable(var_id, &forced_member_type,
                                                                &out_decos, true)) {
                    // This occurs, and is not an error, for the PointSize builtin.
                    if (!success()) {
                        // But exit early if an error was logged.
                        return false;
                    }
                    continue;
                }

                const auto var_name = namer_.GetName(var_id);
                bool ok = true;
                if (HasBuiltinSampleMask(out_decos)) {
                    // In Vulkan SPIR-V, the sample mask is an array. In WGSL it's a
                    // scalar. Use the first element only.
                    auto* sample_mask_array_type =
                        store_type->UnwrapRef()->UnwrapAlias()->As<Array>();
                    TINT_ASSERT(Reader, sample_mask_array_type);
                    ok = EmitPipelineOutput(var_name, store_type, &out_decos, {0},
                                            sample_mask_array_type->type, forced_member_type,
                                            &return_members, &return_exprs);
                } else {
                    // The normal path.
                    ok = EmitPipelineOutput(var_name, store_type, &out_decos, {}, store_type,
                                            forced_member_type, &return_members, &return_exprs);
                }
                if (!ok) {
                    return false;
                }
            }
        }

        if (return_members.IsEmpty()) {
            // This can occur if only the PointSize member is accessed, because we
            // never emit it.
            return_type = ty_.Void()->Build(builder_);
        } else {
            // Create and register the result type.
            auto* str =
                create<ast::Struct>(Source{}, return_struct_sym, return_members, AttributeList{});
            parser_impl_.AddTypeDecl(return_struct_sym, str);
            return_type = builder_.ty.Of(str);

            // Add the return-value statement.
            stmts.Push(builder_.Return(
                source, builder_.Call(source, return_type, std::move(return_exprs))));
        }
    }

    auto* body = create<ast::BlockStatement>(source, stmts, utils::Empty);
    AttributeList fn_attrs;
    fn_attrs.Push(create<ast::StageAttribute>(source, ep_info_->stage));

    if (ep_info_->stage == ast::PipelineStage::kCompute) {
        auto& size = ep_info_->workgroup_size;
        if (size.x != 0 && size.y != 0 && size.z != 0) {
            const ast::Expression* x = builder_.Expr(i32(size.x));
            const ast::Expression* y = size.y ? builder_.Expr(i32(size.y)) : nullptr;
            const ast::Expression* z = size.z ? builder_.Expr(i32(size.z)) : nullptr;
            fn_attrs.Push(create<ast::WorkgroupAttribute>(Source{}, x, y, z));
        }
    }

    builder_.AST().AddFunction(create<ast::Function>(
        source, builder_.Symbols().Register(ep_info_->name), std::move(decl.params), return_type,
        body, std::move(fn_attrs), AttributeList{}));

    return true;
}

bool FunctionEmitter::ParseFunctionDeclaration(FunctionDeclaration* decl) {
    if (failed()) {
        return false;
    }

    const std::string name = namer_.Name(function_.result_id());

    // Surprisingly, the "type id" on an OpFunction is the result type of the
    // function, not the type of the function.  This is the one exceptional case
    // in SPIR-V where the type ID is not the type of the result ID.
    auto* ret_ty = parser_impl_.ConvertType(function_.type_id());
    if (failed()) {
        return false;
    }
    if (ret_ty == nullptr) {
        return Fail() << "internal error: unregistered return type for function with ID "
                      << function_.result_id();
    }

    ParameterList ast_params;
    function_.ForEachParam([this, &ast_params](const spvtools::opt::Instruction* param) {
        // Valid SPIR-V requires function call parameters to be non-null
        // instructions.
        TINT_ASSERT(Reader, param != nullptr);
        const Type* const type = IsHandleObj(*param)
                                     ? parser_impl_.GetHandleTypeForSpirvHandle(*param)
                                     : parser_impl_.ConvertType(param->type_id());

        if (type != nullptr) {
            auto* ast_param = parser_impl_.MakeParameter(param->result_id(), type, AttributeList{});
            // Parameters are treated as const declarations.
            ast_params.Push(ast_param);
            // The value is accessible by name.
            identifier_types_.emplace(param->result_id(), type);
        } else {
            // We've already logged an error and emitted a diagnostic. Do nothing
            // here.
        }
    });
    if (failed()) {
        return false;
    }
    decl->name = name;
    decl->params = std::move(ast_params);
    decl->return_type = ret_ty;
    decl->attributes.Clear();

    return success();
}

bool FunctionEmitter::IsHandleObj(const spvtools::opt::Instruction& obj) {
    TINT_ASSERT(Reader, obj.type_id() != 0u);
    auto* spirv_type = type_mgr_->GetType(obj.type_id());
    TINT_ASSERT(Reader, spirv_type);
    return spirv_type->AsImage() || spirv_type->AsSampler() ||
           (spirv_type->AsPointer() &&
            (static_cast<spv::StorageClass>(spirv_type->AsPointer()->storage_class()) ==
             spv::StorageClass::UniformConstant));
}

bool FunctionEmitter::IsHandleObj(const spvtools::opt::Instruction* obj) {
    return (obj != nullptr) && IsHandleObj(*obj);
}

const Type* FunctionEmitter::GetVariableStoreType(const spvtools::opt::Instruction& var_decl_inst) {
    const auto type_id = var_decl_inst.type_id();
    // Normally we use the SPIRV-Tools optimizer to manage types.
    // But when two struct types have the same member types and decorations,
    // but differ only in member names, the two struct types will be
    // represented by a single common internal struct type.
    // So avoid the optimizer's representation and instead follow the
    // SPIR-V instructions themselves.
    const auto* ptr_ty = def_use_mgr_->GetDef(type_id);
    const auto store_ty_id = ptr_ty->GetSingleWordInOperand(1);
    const auto* result = parser_impl_.ConvertType(store_ty_id);
    return result;
}

bool FunctionEmitter::EmitBody() {
    RegisterBasicBlocks();

    if (!TerminatorsAreValid()) {
        return false;
    }
    if (!RegisterMerges()) {
        return false;
    }

    ComputeBlockOrderAndPositions();
    if (!VerifyHeaderContinueMergeOrder()) {
        return false;
    }
    if (!LabelControlFlowConstructs()) {
        return false;
    }
    if (!FindSwitchCaseHeaders()) {
        return false;
    }
    if (!ClassifyCFGEdges()) {
        return false;
    }
    if (!FindIfSelectionInternalHeaders()) {
        return false;
    }

    if (!RegisterSpecialBuiltInVariables()) {
        return false;
    }
    if (!RegisterLocallyDefinedValues()) {
        return false;
    }
    FindValuesNeedingNamedOrHoistedDefinition();

    if (!EmitFunctionVariables()) {
        return false;
    }
    if (!EmitFunctionBodyStatements()) {
        return false;
    }
    return success();
}

void FunctionEmitter::RegisterBasicBlocks() {
    for (auto& block : function_) {
        block_info_[block.id()] = std::make_unique<BlockInfo>(block);
    }
}

bool FunctionEmitter::TerminatorsAreValid() {
    if (failed()) {
        return false;
    }

    const auto entry_id = function_.begin()->id();
    for (const auto& block : function_) {
        if (!block.terminator()) {
            return Fail() << "Block " << block.id() << " has no terminator";
        }
    }
    for (const auto& block : function_) {
        block.WhileEachSuccessorLabel([this, &block, entry_id](const uint32_t succ_id) -> bool {
            if (succ_id == entry_id) {
                return Fail() << "Block " << block.id() << " branches to function entry block "
                              << entry_id;
            }
            if (!GetBlockInfo(succ_id)) {
                return Fail() << "Block " << block.id() << " in function "
                              << function_.DefInst().result_id() << " branches to " << succ_id
                              << " which is not a block in the function";
            }
            return true;
        });
    }
    return success();
}

bool FunctionEmitter::RegisterMerges() {
    if (failed()) {
        return false;
    }

    const auto entry_id = function_.begin()->id();
    for (const auto& block : function_) {
        const auto block_id = block.id();
        auto* block_info = GetBlockInfo(block_id);
        if (!block_info) {
            return Fail() << "internal error: block " << block_id
                          << " missing; blocks should already "
                             "have been registered";
        }

        if (const auto* inst = block.GetMergeInst()) {
            auto terminator_opcode = opcode(block.terminator());
            switch (opcode(inst)) {
                case spv::Op::OpSelectionMerge:
                    if ((terminator_opcode != spv::Op::OpBranchConditional) &&
                        (terminator_opcode != spv::Op::OpSwitch)) {
                        return Fail() << "Selection header " << block_id
                                      << " does not end in an OpBranchConditional or "
                                         "OpSwitch instruction";
                    }
                    break;
                case spv::Op::OpLoopMerge:
                    if ((terminator_opcode != spv::Op::OpBranchConditional) &&
                        (terminator_opcode != spv::Op::OpBranch)) {
                        return Fail() << "Loop header " << block_id
                                      << " does not end in an OpBranch or "
                                         "OpBranchConditional instruction";
                    }
                    break;
                default:
                    break;
            }

            const uint32_t header = block.id();
            auto* header_info = block_info;
            const uint32_t merge = inst->GetSingleWordInOperand(0);
            auto* merge_info = GetBlockInfo(merge);
            if (!merge_info) {
                return Fail() << "Structured header block " << header
                              << " declares invalid merge block " << merge;
            }
            if (merge == header) {
                return Fail() << "Structured header block " << header
                              << " cannot be its own merge block";
            }
            if (merge_info->header_for_merge) {
                return Fail() << "Block " << merge
                              << " declared as merge block for more than one header: "
                              << merge_info->header_for_merge << ", " << header;
            }
            merge_info->header_for_merge = header;
            header_info->merge_for_header = merge;

            if (opcode(inst) == spv::Op::OpLoopMerge) {
                if (header == entry_id) {
                    return Fail() << "Function entry block " << entry_id
                                  << " cannot be a loop header";
                }
                const uint32_t ct = inst->GetSingleWordInOperand(1);
                auto* ct_info = GetBlockInfo(ct);
                if (!ct_info) {
                    return Fail() << "Structured header " << header
                                  << " declares invalid continue target " << ct;
                }
                if (ct == merge) {
                    return Fail() << "Invalid structured header block " << header
                                  << ": declares block " << ct
                                  << " as both its merge block and continue target";
                }
                if (ct_info->header_for_continue) {
                    return Fail() << "Block " << ct
                                  << " declared as continue target for more than one header: "
                                  << ct_info->header_for_continue << ", " << header;
                }
                ct_info->header_for_continue = header;
                header_info->continue_for_header = ct;
            }
        }

        // Check single-block loop cases.
        bool is_single_block_loop = false;
        block_info->basic_block->ForEachSuccessorLabel(
            [&is_single_block_loop, block_id](const uint32_t succ) {
                if (block_id == succ) {
                    is_single_block_loop = true;
                }
            });
        const auto ct = block_info->continue_for_header;
        block_info->is_continue_entire_loop = ct == block_id;
        if (is_single_block_loop && !block_info->is_continue_entire_loop) {
            return Fail() << "Block " << block_id
                          << " branches to itself but is not its own continue target";
        }
        // It's valid for a the header of a multi-block loop header to declare
        // itself as its own continue target.
    }
    return success();
}

void FunctionEmitter::ComputeBlockOrderAndPositions() {
    block_order_ = StructuredTraverser(function_).ReverseStructuredPostOrder();

    for (uint32_t i = 0; i < block_order_.size(); ++i) {
        GetBlockInfo(block_order_[i])->pos = i;
    }
    // The invalid block position is not the position of any block that is in the
    // order.
    assert(block_order_.size() <= kInvalidBlockPos);
}

bool FunctionEmitter::VerifyHeaderContinueMergeOrder() {
    // Verify interval rules for a structured header block:
    //
    //    If the CFG satisfies structured control flow rules, then:
    //    If header H is reachable, then the following "interval rules" hold,
    //    where M(H) is H's merge block, and CT(H) is H's continue target:
    //
    //      Pos(H) < Pos(M(H))
    //
    //      If CT(H) exists, then:
    //         Pos(H) <= Pos(CT(H))
    //         Pos(CT(H)) < Pos(M)
    //
    for (auto block_id : block_order_) {
        const auto* block_info = GetBlockInfo(block_id);
        const auto merge = block_info->merge_for_header;
        if (merge == 0) {
            continue;
        }
        // This is a header.
        const auto header = block_id;
        const auto* header_info = block_info;
        const auto header_pos = header_info->pos;
        const auto merge_pos = GetBlockInfo(merge)->pos;

        // Pos(H) < Pos(M(H))
        // Note: When recording merges we made sure H != M(H)
        if (merge_pos <= header_pos) {
            return Fail() << "Header " << header << " does not strictly dominate its merge block "
                          << merge;
            // TODO(dneto): Report a path from the entry block to the merge block
            // without going through the header block.
        }

        const auto ct = block_info->continue_for_header;
        if (ct == 0) {
            continue;
        }
        // Furthermore, this is a loop header.
        const auto* ct_info = GetBlockInfo(ct);
        const auto ct_pos = ct_info->pos;
        // Pos(H) <= Pos(CT(H))
        if (ct_pos < header_pos) {
            Fail() << "Loop header " << header << " does not dominate its continue target " << ct;
        }
        // Pos(CT(H)) < Pos(M(H))
        // Note: When recording merges we made sure CT(H) != M(H)
        if (merge_pos <= ct_pos) {
            return Fail() << "Merge block " << merge << " for loop headed at block " << header
                          << " appears at or before the loop's continue "
                             "construct headed by "
                             "block "
                          << ct;
        }
    }
    return success();
}

bool FunctionEmitter::LabelControlFlowConstructs() {
    // Label each block in the block order with its nearest enclosing structured
    // control flow construct. Populates the |construct| member of BlockInfo.

    //  Keep a stack of enclosing structured control flow constructs.  Start
    //  with the synthetic construct representing the entire function.
    //
    //  Scan from left to right in the block order, and check conditions
    //  on each block in the following order:
    //
    //        a. When you reach a merge block, the top of the stack should
    //           be the associated header. Pop it off.
    //        b. When you reach a header, push it on the stack.
    //        c. When you reach a continue target, push it on the stack.
    //           (A block can be both a header and a continue target.)
    //        c. When you reach a block with an edge branching backward (in the
    //           structured order) to block T:
    //            T should be a loop header, and the top of the stack should be a
    //            continue target associated with T.
    //            This is the end of the continue construct. Pop the continue
    //            target off the stack.
    //
    //       Note: A loop header can declare itself as its own continue target.
    //
    //       Note: For a single-block loop, that block is a header, its own
    //       continue target, and its own backedge block.
    //
    //       Note: We pop the merge off first because a merge block that marks
    //       the end of one construct can be a single-block loop.  So that block
    //       is a merge, a header, a continue target, and a backedge block.
    //       But we want to finish processing of the merge before dealing with
    //       the loop.
    //
    //      In the same scan, mark each basic block with the nearest enclosing
    //      header: the most recent header for which we haven't reached its merge
    //      block. Also mark the the most recent continue target for which we
    //      haven't reached the backedge block.

    TINT_ASSERT(Reader, block_order_.size() > 0);
    constructs_.Clear();
    const auto entry_id = block_order_[0];

    // The stack of enclosing constructs.
    utils::Vector<Construct*, 4> enclosing;

    // Creates a control flow construct and pushes it onto the stack.
    // Its parent is the top of the stack, or nullptr if the stack is empty.
    // Returns the newly created construct.
    auto push_construct = [this, &enclosing](size_t depth, Construct::Kind k, uint32_t begin_id,
                                             uint32_t end_id) -> Construct* {
        const auto begin_pos = GetBlockInfo(begin_id)->pos;
        const auto end_pos =
            end_id == 0 ? uint32_t(block_order_.size()) : GetBlockInfo(end_id)->pos;
        const auto* parent = enclosing.IsEmpty() ? nullptr : enclosing.Back();
        auto scope_end_pos = end_pos;
        // A loop construct is added right after its associated continue construct.
        // In that case, adjust the parent up.
        if (k == Construct::kLoop) {
            TINT_ASSERT(Reader, parent);
            TINT_ASSERT(Reader, parent->kind == Construct::kContinue);
            scope_end_pos = parent->end_pos;
            parent = parent->parent;
        }
        constructs_.Push(std::make_unique<Construct>(parent, static_cast<int>(depth), k, begin_id,
                                                     end_id, begin_pos, end_pos, scope_end_pos));
        Construct* result = constructs_.Back().get();
        enclosing.Push(result);
        return result;
    };

    // Make a synthetic kFunction construct to enclose all blocks in the function.
    push_construct(0, Construct::kFunction, entry_id, 0);
    // The entry block can be a selection construct, so be sure to process
    // it anyway.

    for (uint32_t i = 0; i < block_order_.size(); ++i) {
        const auto block_id = block_order_[i];
        TINT_ASSERT(Reader, block_id > 0);
        auto* block_info = GetBlockInfo(block_id);
        TINT_ASSERT(Reader, block_info);

        if (enclosing.IsEmpty()) {
            return Fail() << "internal error: too many merge blocks before block " << block_id;
        }
        const Construct* top = enclosing.Back();

        while (block_id == top->end_id) {
            // We've reached a predeclared end of the construct.  Pop it off the
            // stack.
            enclosing.Pop();
            if (enclosing.IsEmpty()) {
                return Fail() << "internal error: too many merge blocks before block " << block_id;
            }
            top = enclosing.Back();
        }

        const auto merge = block_info->merge_for_header;
        if (merge != 0) {
            // The current block is a header.
            const auto header = block_id;
            const auto* header_info = block_info;
            const auto depth = static_cast<size_t>(1 + top->depth);
            const auto ct = header_info->continue_for_header;
            if (ct != 0) {
                // The current block is a loop header.
                // We should see the continue construct after the loop construct, so
                // push the loop construct last.

                // From the interval rule, the continue construct consists of blocks
                // in the block order, starting at the continue target, until just
                // before the merge block.
                top = push_construct(depth, Construct::kContinue, ct, merge);
                // A loop header that is its own continue target will have an
                // empty loop construct. Only create a loop construct when
                // the continue target is *not* the same as the loop header.
                if (header != ct) {
                    // From the interval rule, the loop construct consists of blocks
                    // in the block order, starting at the header, until just
                    // before the continue target.
                    top = push_construct(depth, Construct::kLoop, header, ct);

                    // If the loop header branches to two different blocks inside the loop
                    // construct, then the loop body should be modeled as an if-selection
                    // construct
                    utils::Vector<uint32_t, 4> targets;
                    header_info->basic_block->ForEachSuccessorLabel(
                        [&targets](const uint32_t target) { targets.Push(target); });
                    if ((targets.Length() == 2u) && targets[0] != targets[1]) {
                        const auto target0_pos = GetBlockInfo(targets[0])->pos;
                        const auto target1_pos = GetBlockInfo(targets[1])->pos;
                        if (top->ContainsPos(target0_pos) && top->ContainsPos(target1_pos)) {
                            // Insert a synthetic if-selection
                            top = push_construct(depth + 1, Construct::kIfSelection, header, ct);
                        }
                    }
                }
            } else {
                // From the interval rule, the selection construct consists of blocks
                // in the block order, starting at the header, until just before the
                // merge block.
                const auto branch_opcode = opcode(header_info->basic_block->terminator());
                const auto kind = (branch_opcode == spv::Op::OpBranchConditional)
                                      ? Construct::kIfSelection
                                      : Construct::kSwitchSelection;
                top = push_construct(depth, kind, header, merge);
            }
        }

        TINT_ASSERT(Reader, top);
        block_info->construct = top;
    }

    // At the end of the block list, we should only have the kFunction construct
    // left.
    if (enclosing.Length() != 1) {
        return Fail() << "internal error: unbalanced structured constructs when "
                         "labeling structured constructs: ended with "
                      << enclosing.Length() - 1 << " unterminated constructs";
    }
    const auto* top = enclosing[0];
    if (top->kind != Construct::kFunction || top->depth != 0) {
        return Fail() << "internal error: outermost construct is not a function?!";
    }

    return success();
}

bool FunctionEmitter::FindSwitchCaseHeaders() {
    if (failed()) {
        return false;
    }
    for (auto& construct : constructs_) {
        if (construct->kind != Construct::kSwitchSelection) {
            continue;
        }
        const auto* branch = GetBlockInfo(construct->begin_id)->basic_block->terminator();

        // Mark the default block
        const auto default_id = branch->GetSingleWordInOperand(1);
        auto* default_block = GetBlockInfo(default_id);
        // A default target can't be a backedge.
        if (construct->begin_pos >= default_block->pos) {
            // An OpSwitch must dominate its cases.  Also, it can't be a self-loop
            // as that would be a backedge, and backedges can only target a loop,
            // and loops use an OpLoopMerge instruction, which can't precede an
            // OpSwitch.
            return Fail() << "Switch branch from block " << construct->begin_id
                          << " to default target block " << default_id << " can't be a back-edge";
        }
        // A default target can be the merge block, but can't go past it.
        if (construct->end_pos < default_block->pos) {
            return Fail() << "Switch branch from block " << construct->begin_id
                          << " to default block " << default_id
                          << " escapes the selection construct";
        }
        if (default_block->default_head_for) {
            // An OpSwitch must dominate its cases, including the default target.
            return Fail() << "Block " << default_id
                          << " is declared as the default target for two OpSwitch "
                             "instructions, at blocks "
                          << default_block->default_head_for->begin_id << " and "
                          << construct->begin_id;
        }
        if ((default_block->header_for_merge != 0) &&
            (default_block->header_for_merge != construct->begin_id)) {
            // The switch instruction for this default block is an alternate path to
            // the merge block, and hence the merge block is not dominated by its own
            // (different) header.
            return Fail() << "Block " << default_block->id
                          << " is the default block for switch-selection header "
                          << construct->begin_id << " and also the merge block for "
                          << default_block->header_for_merge << " (violates dominance rule)";
        }

        default_block->default_head_for = construct.get();
        default_block->default_is_merge = default_block->pos == construct->end_pos;

        // Map a case target to the list of values selecting that case.
        std::unordered_map<uint32_t, utils::Vector<uint64_t, 4>> block_to_values;
        utils::Vector<uint32_t, 4> case_targets;
        std::unordered_set<uint64_t> case_values;

        // Process case targets.
        for (uint32_t iarg = 2; iarg + 1 < branch->NumInOperands(); iarg += 2) {
            const auto value = branch->GetInOperand(iarg).AsLiteralUint64();
            const auto case_target_id = branch->GetSingleWordInOperand(iarg + 1);

            if (case_values.count(value)) {
                return Fail() << "Duplicate case value " << value << " in OpSwitch in block "
                              << construct->begin_id;
            }
            case_values.insert(value);
            if (block_to_values.count(case_target_id) == 0) {
                case_targets.Push(case_target_id);
            }
            block_to_values[case_target_id].Push(value);
        }

        for (uint32_t case_target_id : case_targets) {
            auto* case_block = GetBlockInfo(case_target_id);

            case_block->case_values = std::move(block_to_values[case_target_id]);

            // A case target can't be a back-edge.
            if (construct->begin_pos >= case_block->pos) {
                // An OpSwitch must dominate its cases.  Also, it can't be a self-loop
                // as that would be a backedge, and backedges can only target a loop,
                // and loops use an OpLoopMerge instruction, which can't preceded an
                // OpSwitch.
                return Fail() << "Switch branch from block " << construct->begin_id
                              << " to case target block " << case_target_id
                              << " can't be a back-edge";
            }
            // A case target can be the merge block, but can't go past it.
            if (construct->end_pos < case_block->pos) {
                return Fail() << "Switch branch from block " << construct->begin_id
                              << " to case target block " << case_target_id
                              << " escapes the selection construct";
            }
            if (case_block->header_for_merge != 0 &&
                case_block->header_for_merge != construct->begin_id) {
                // The switch instruction for this case block is an alternate path to
                // the merge block, and hence the merge block is not dominated by its
                // own (different) header.
                return Fail() << "Block " << case_block->id
                              << " is a case block for switch-selection header "
                              << construct->begin_id << " and also the merge block for "
                              << case_block->header_for_merge << " (violates dominance rule)";
            }

            // Mark the target as a case target.
            if (case_block->case_head_for) {
                // An OpSwitch must dominate its cases.
                return Fail() << "Block " << case_target_id
                              << " is declared as the switch case target for two OpSwitch "
                                 "instructions, at blocks "
                              << case_block->case_head_for->begin_id << " and "
                              << construct->begin_id;
            }
            case_block->case_head_for = construct.get();
        }
    }
    return success();
}

BlockInfo* FunctionEmitter::HeaderIfBreakable(const Construct* c) {
    if (c == nullptr) {
        return nullptr;
    }
    switch (c->kind) {
        case Construct::kLoop:
        case Construct::kSwitchSelection:
            return GetBlockInfo(c->begin_id);
        case Construct::kContinue: {
            const auto* continue_target = GetBlockInfo(c->begin_id);
            return GetBlockInfo(continue_target->header_for_continue);
        }
        default:
            break;
    }
    return nullptr;
}

const Construct* FunctionEmitter::SiblingLoopConstruct(const Construct* c) const {
    if (c == nullptr || c->kind != Construct::kContinue) {
        return nullptr;
    }
    const uint32_t continue_target_id = c->begin_id;
    const auto* continue_target = GetBlockInfo(continue_target_id);
    const uint32_t header_id = continue_target->header_for_continue;
    if (continue_target_id == header_id) {
        // The continue target is the whole loop.
        return nullptr;
    }
    const auto* candidate = GetBlockInfo(header_id)->construct;
    // Walk up the construct tree until we hit the loop.  In future
    // we might handle the corner case where the same block is both a
    // loop header and a selection header. For example, where the
    // loop header block has a conditional branch going to distinct
    // targets inside the loop body.
    while (candidate && candidate->kind != Construct::kLoop) {
        candidate = candidate->parent;
    }
    return candidate;
}

bool FunctionEmitter::ClassifyCFGEdges() {
    if (failed()) {
        return false;
    }

    // Checks validity of CFG edges leaving each basic block.  This implicitly
    // checks dominance rules for headers and continue constructs.
    //
    // For each branch encountered, classify each edge (S,T) as:
    //    - a back-edge
    //    - a structured exit (specific ways of branching to enclosing construct)
    //    - a normal (forward) edge, either natural control flow or a case fallthrough
    //
    // If more than one block is targeted by a normal edge, then S must be a
    // structured header.
    //
    // Term: NEC(B) is the nearest enclosing construct for B.
    //
    // If edge (S,T) is a normal edge, and NEC(S) != NEC(T), then
    //    T is the header block of its NEC(T), and
    //    NEC(S) is the parent of NEC(T).

    for (const auto src : block_order_) {
        TINT_ASSERT(Reader, src > 0);
        auto* src_info = GetBlockInfo(src);
        TINT_ASSERT(Reader, src_info);
        const auto src_pos = src_info->pos;
        const auto& src_construct = *(src_info->construct);

        // Compute the ordered list of unique successors.
        utils::Vector<uint32_t, 4> successors;
        {
            std::unordered_set<uint32_t> visited;
            src_info->basic_block->ForEachSuccessorLabel(
                [&successors, &visited](const uint32_t succ) {
                    if (visited.count(succ) == 0) {
                        successors.Push(succ);
                        visited.insert(succ);
                    }
                });
        }

        // There should only be one backedge per backedge block.
        uint32_t num_backedges = 0;

        // Track destinations for normal forward edges, either kForward or kCaseFallThrough.
        // These count toward the need to have a merge instruction.  We also track kIfBreak edges
        // because when used with normal forward edges, we'll need to generate a flow guard
        // variable.
        utils::Vector<uint32_t, 4> normal_forward_edges;
        utils::Vector<uint32_t, 4> if_break_edges;

        if (successors.IsEmpty() && src_construct.enclosing_continue) {
            // Kill and return are not allowed in a continue construct.
            return Fail() << "Invalid function exit at block " << src
                          << " from continue construct starting at "
                          << src_construct.enclosing_continue->begin_id;
        }

        for (const auto dest : successors) {
            const auto* dest_info = GetBlockInfo(dest);
            // We've already checked terminators are valid.
            TINT_ASSERT(Reader, dest_info);
            const auto dest_pos = dest_info->pos;

            // Insert the edge kind entry and keep a handle to update
            // its classification.
            EdgeKind& edge_kind = src_info->succ_edge[dest];

            if (src_pos >= dest_pos) {
                // This is a backedge.
                edge_kind = EdgeKind::kBack;
                num_backedges++;
                const auto* continue_construct = src_construct.enclosing_continue;
                if (!continue_construct) {
                    return Fail() << "Invalid backedge (" << src << "->" << dest << "): " << src
                                  << " is not in a continue construct";
                }
                if (src_pos != continue_construct->end_pos - 1) {
                    return Fail() << "Invalid exit (" << src << "->" << dest
                                  << ") from continue construct: " << src
                                  << " is not the last block in the continue construct "
                                     "starting at "
                                  << src_construct.begin_id << " (violates post-dominance rule)";
                }
                const auto* ct_info = GetBlockInfo(continue_construct->begin_id);
                TINT_ASSERT(Reader, ct_info);
                if (ct_info->header_for_continue != dest) {
                    return Fail() << "Invalid backedge (" << src << "->" << dest
                                  << "): does not branch to the corresponding loop header, "
                                     "expected "
                                  << ct_info->header_for_continue;
                }
            } else {
                // This is a forward edge.
                // For now, classify it that way, but we might update it.
                edge_kind = EdgeKind::kForward;

                // Exit from a continue construct can only be from the last block.
                const auto* continue_construct = src_construct.enclosing_continue;
                if (continue_construct != nullptr) {
                    if (continue_construct->ContainsPos(src_pos) &&
                        !continue_construct->ContainsPos(dest_pos) &&
                        (src_pos != continue_construct->end_pos - 1)) {
                        return Fail()
                               << "Invalid exit (" << src << "->" << dest
                               << ") from continue construct: " << src
                               << " is not the last block in the continue construct "
                                  "starting at "
                               << continue_construct->begin_id << " (violates post-dominance rule)";
                    }
                }

                // Check valid structured exit cases.

                if (edge_kind == EdgeKind::kForward) {
                    // Check for a 'break' from a loop or from a switch.
                    const auto* breakable_header =
                        HeaderIfBreakable(src_construct.enclosing_loop_or_continue_or_switch);
                    if (breakable_header != nullptr) {
                        if (dest == breakable_header->merge_for_header) {
                            // It's a break.
                            edge_kind =
                                (breakable_header->construct->kind == Construct::kSwitchSelection)
                                    ? EdgeKind::kSwitchBreak
                                    : EdgeKind::kLoopBreak;
                        }
                    }
                }

                if (edge_kind == EdgeKind::kForward) {
                    // Check for a 'continue' from within a loop.
                    const auto* loop_header = HeaderIfBreakable(src_construct.enclosing_loop);
                    if (loop_header != nullptr) {
                        if (dest == loop_header->continue_for_header) {
                            // It's a continue.
                            edge_kind = EdgeKind::kLoopContinue;
                        }
                    }
                }

                if (edge_kind == EdgeKind::kForward) {
                    const auto& header_info = *GetBlockInfo(src_construct.begin_id);
                    if (dest == header_info.merge_for_header) {
                        // Branch to construct's merge block.  The loop break and
                        // switch break cases have already been covered.
                        edge_kind = EdgeKind::kIfBreak;
                    }
                }

                // A forward edge into a case construct that comes from something
                // other than the OpSwitch is actually a fallthrough.
                if (edge_kind == EdgeKind::kForward) {
                    const auto* switch_construct =
                        (dest_info->case_head_for ? dest_info->case_head_for
                                                  : dest_info->default_head_for);
                    if (switch_construct != nullptr) {
                        if (src != switch_construct->begin_id) {
                            edge_kind = EdgeKind::kCaseFallThrough;
                        }
                    }
                }

                // The edge-kind has been finalized.

                if ((edge_kind == EdgeKind::kForward) ||
                    (edge_kind == EdgeKind::kCaseFallThrough)) {
                    normal_forward_edges.Push(dest);
                }
                if (edge_kind == EdgeKind::kIfBreak) {
                    if_break_edges.Push(dest);
                }

                if ((edge_kind == EdgeKind::kForward) ||
                    (edge_kind == EdgeKind::kCaseFallThrough)) {
                    // Check for an invalid forward exit out of this construct.
                    if (dest_info->pos > src_construct.end_pos) {
                        // In most cases we're bypassing the merge block for the source
                        // construct.
                        auto end_block = src_construct.end_id;
                        const char* end_block_desc = "merge block";
                        if (src_construct.kind == Construct::kLoop) {
                            // For a loop construct, we have two valid places to go: the
                            // continue target or the merge for the loop header, which is
                            // further down.
                            const auto loop_merge =
                                GetBlockInfo(src_construct.begin_id)->merge_for_header;
                            if (dest_info->pos >= GetBlockInfo(loop_merge)->pos) {
                                // We're bypassing the loop's merge block.
                                end_block = loop_merge;
                            } else {
                                // We're bypassing the loop's continue target, and going into
                                // the middle of the continue construct.
                                end_block_desc = "continue target";
                            }
                        }
                        return Fail() << "Branch from block " << src << " to block " << dest
                                      << " is an invalid exit from construct starting at block "
                                      << src_construct.begin_id << "; branch bypasses "
                                      << end_block_desc << " " << end_block;
                    }

                    // Check dominance.

                    //      Look for edges that violate the dominance condition: a branch
                    //      from X to Y where:
                    //        If Y is in a nearest enclosing continue construct headed by
                    //        CT:
                    //          Y is not CT, and
                    //          In the structured order, X appears before CT order or
                    //          after CT's backedge block.
                    //        Otherwise, if Y is in a nearest enclosing construct
                    //        headed by H:
                    //          Y is not H, and
                    //          In the structured order, X appears before H or after H's
                    //          merge block.

                    const auto& dest_construct = *(dest_info->construct);
                    if (dest != dest_construct.begin_id && !dest_construct.ContainsPos(src_pos)) {
                        return Fail()
                               << "Branch from " << src << " to " << dest << " bypasses "
                               << (dest_construct.kind == Construct::kContinue ? "continue target "
                                                                               : "header ")
                               << dest_construct.begin_id << " (dominance rule violated)";
                    }
                }

                // Error on the fallthrough at the end in order to allow the better error messages
                // from the above checks to happen.
                if (edge_kind == EdgeKind::kCaseFallThrough) {
                    return Fail() << "Fallthrough not permitted in WGSL";
                }
            }  // end forward edge
        }      // end successor

        if (num_backedges > 1) {
            return Fail() << "Block " << src << " has too many backedges: " << num_backedges;
        }
        if ((normal_forward_edges.Length() > 1) && (src_info->merge_for_header == 0)) {
            return Fail() << "Control flow diverges at block " << src << " (to "
                          << normal_forward_edges[0] << ", " << normal_forward_edges[1]
                          << ") but it is not a structured header (it has no merge "
                             "instruction)";
        }
        if ((normal_forward_edges.Length() + if_break_edges.Length() > 1) &&
            (src_info->merge_for_header == 0)) {
            // There is a branch to the merge of an if-selection combined
            // with an other normal forward branch.  Control within the
            // if-selection needs to be gated by a flow predicate.
            for (auto if_break_dest : if_break_edges) {
                auto* head_info = GetBlockInfo(GetBlockInfo(if_break_dest)->header_for_merge);
                // Generate a guard name, but only once.
                if (head_info->flow_guard_name.empty()) {
                    const std::string guard = "guard" + std::to_string(head_info->id);
                    head_info->flow_guard_name = namer_.MakeDerivedName(guard);
                }
            }
        }
    }

    return success();
}

bool FunctionEmitter::FindIfSelectionInternalHeaders() {
    if (failed()) {
        return false;
    }
    for (auto& construct : constructs_) {
        if (construct->kind != Construct::kIfSelection) {
            continue;
        }
        auto* if_header_info = GetBlockInfo(construct->begin_id);
        const auto* branch = if_header_info->basic_block->terminator();
        const auto true_head = branch->GetSingleWordInOperand(1);
        const auto false_head = branch->GetSingleWordInOperand(2);

        auto* true_head_info = GetBlockInfo(true_head);
        auto* false_head_info = GetBlockInfo(false_head);
        const auto true_head_pos = true_head_info->pos;
        const auto false_head_pos = false_head_info->pos;

        const bool contains_true = construct->ContainsPos(true_head_pos);
        const bool contains_false = construct->ContainsPos(false_head_pos);

        // The cases for each edge are:
        //  - kBack: invalid because it's an invalid exit from the selection
        //  - kSwitchBreak ; record this for later special processing
        //  - kLoopBreak ; record this for later special processing
        //  - kLoopContinue ; record this for later special processing
        //  - kIfBreak; normal case, may require a guard variable.
        //  - kFallThrough; invalid exit from the selection
        //  - kForward; normal case

        if_header_info->true_kind = if_header_info->succ_edge[true_head];
        if_header_info->false_kind = if_header_info->succ_edge[false_head];
        if (contains_true) {
            if_header_info->true_head = true_head;
        }
        if (contains_false) {
            if_header_info->false_head = false_head;
        }

        if (contains_true && (true_head_info->header_for_merge != 0) &&
            (true_head_info->header_for_merge != construct->begin_id)) {
            // The OpBranchConditional instruction for the true head block is an
            // alternate path to the merge block of a construct nested inside the
            // selection, and hence the merge block is not dominated by its own
            // (different) header.
            return Fail() << "Block " << true_head << " is the true branch for if-selection header "
                          << construct->begin_id << " and also the merge block for header block "
                          << true_head_info->header_for_merge << " (violates dominance rule)";
        }
        if (contains_false && (false_head_info->header_for_merge != 0) &&
            (false_head_info->header_for_merge != construct->begin_id)) {
            // The OpBranchConditional instruction for the false head block is an
            // alternate path to the merge block of a construct nested inside the
            // selection, and hence the merge block is not dominated by its own
            // (different) header.
            return Fail() << "Block " << false_head
                          << " is the false branch for if-selection header " << construct->begin_id
                          << " and also the merge block for header block "
                          << false_head_info->header_for_merge << " (violates dominance rule)";
        }

        if (contains_true && contains_false && (true_head_pos != false_head_pos)) {
            // This construct has both a "then" clause and an "else" clause.
            //
            // We have this structure:
            //
            //   Option 1:
            //
            //     * condbranch
            //        * true-head (start of then-clause)
            //        ...
            //        * end-then-clause
            //        * false-head (start of else-clause)
            //        ...
            //        * end-false-clause
            //        * premerge-head
            //        ...
            //     * selection merge
            //
            //   Option 2:
            //
            //     * condbranch
            //        * true-head (start of then-clause)
            //        ...
            //        * end-then-clause
            //        * false-head (start of else-clause) and also premerge-head
            //        ...
            //        * end-false-clause
            //     * selection merge
            //
            //   Option 3:
            //
            //     * condbranch
            //        * false-head (start of else-clause)
            //        ...
            //        * end-else-clause
            //        * true-head (start of then-clause) and also premerge-head
            //        ...
            //        * end-then-clause
            //     * selection merge
            //
            // The premerge-head exists if there is a kForward branch from the end
            // of the first clause to a block within the surrounding selection.
            // The first clause might be a then-clause or an else-clause.
            const auto second_head = std::max(true_head_pos, false_head_pos);
            const auto end_first_clause_pos = second_head - 1;
            TINT_ASSERT(Reader, end_first_clause_pos < block_order_.size());
            const auto end_first_clause = block_order_[end_first_clause_pos];
            uint32_t premerge_id = 0;
            uint32_t if_break_id = 0;
            for (auto& then_succ_iter : GetBlockInfo(end_first_clause)->succ_edge) {
                const uint32_t dest_id = then_succ_iter.first;
                const auto edge_kind = then_succ_iter.second;
                switch (edge_kind) {
                    case EdgeKind::kIfBreak:
                        if_break_id = dest_id;
                        break;
                    case EdgeKind::kForward: {
                        if (construct->ContainsPos(GetBlockInfo(dest_id)->pos)) {
                            // It's a premerge.
                            if (premerge_id != 0) {
                                // TODO(dneto): I think this is impossible to trigger at this
                                // point in the flow. It would require a merge instruction to
                                // get past the check of "at-most-one-forward-edge".
                                return Fail()
                                       << "invalid structure: then-clause headed by block "
                                       << true_head << " ending at block " << end_first_clause
                                       << " has two forward edges to within selection"
                                       << " going to " << premerge_id << " and " << dest_id;
                            }
                            premerge_id = dest_id;
                            auto* dest_block_info = GetBlockInfo(dest_id);
                            if_header_info->premerge_head = dest_id;
                            if (dest_block_info->header_for_merge != 0) {
                                // Premerge has two edges coming into it, from the then-clause
                                // and the else-clause. It's also, by construction, not the
                                // merge block of the if-selection.  So it must not be a merge
                                // block itself. The OpBranchConditional instruction for the
                                // false head block is an alternate path to the merge block, and
                                // hence the merge block is not dominated by its own (different)
                                // header.
                                return Fail()
                                       << "Block " << premerge_id << " is the merge block for "
                                       << dest_block_info->header_for_merge
                                       << " but has alternate paths reaching it, starting from"
                                       << " blocks " << true_head << " and " << false_head
                                       << " which are the true and false branches for the"
                                       << " if-selection header block " << construct->begin_id
                                       << " (violates dominance rule)";
                            }
                        }
                        break;
                    }
                    default:
                        break;
                }
            }
            if (if_break_id != 0 && premerge_id != 0) {
                return Fail() << "Block " << end_first_clause << " in if-selection headed at block "
                              << construct->begin_id << " branches to both the merge block "
                              << if_break_id << " and also to block " << premerge_id
                              << " later in the selection";
            }
        }
    }
    return success();
}

bool FunctionEmitter::EmitFunctionVariables() {
    if (failed()) {
        return false;
    }
    for (auto& inst : *function_.entry()) {
        if (opcode(inst) != spv::Op::OpVariable) {
            continue;
        }
        auto* var_store_type = GetVariableStoreType(inst);
        if (failed()) {
            return false;
        }
        const ast::Expression* initializer = nullptr;
        if (inst.NumInOperands() > 1) {
            // SPIR-V initializers are always constants.
            // (OpenCL also allows the ID of an OpVariable, but we don't handle that
            // here.)
            initializer = parser_impl_.MakeConstantExpression(inst.GetSingleWordInOperand(1)).expr;
            if (!initializer) {
                return false;
            }
        }
        auto* var = parser_impl_.MakeVar(inst.result_id(), type::AddressSpace::kNone,
                                         var_store_type, initializer, AttributeList{});
        auto* var_decl_stmt = create<ast::VariableDeclStatement>(Source{}, var);
        AddStatement(var_decl_stmt);
        auto* var_type = ty_.Reference(var_store_type, type::AddressSpace::kNone);
        identifier_types_.emplace(inst.result_id(), var_type);
    }
    return success();
}

TypedExpression FunctionEmitter::AddressOfIfNeeded(TypedExpression expr,
                                                   const spvtools::opt::Instruction* inst) {
    if (inst && expr) {
        if (auto* spirv_type = type_mgr_->GetType(inst->type_id())) {
            if (expr.type->Is<Reference>() && spirv_type->AsPointer()) {
                return AddressOf(expr);
            }
        }
    }
    return expr;
}

TypedExpression FunctionEmitter::MakeExpression(uint32_t id) {
    if (failed()) {
        return {};
    }
    switch (GetSkipReason(id)) {
        case SkipReason::kDontSkip:
            break;
        case SkipReason::kOpaqueObject:
            Fail() << "internal error: unhandled use of opaque object with ID: " << id;
            return {};
        case SkipReason::kSinkPointerIntoUse: {
            // Replace the pointer with its source reference expression.
            auto source_expr = GetDefInfo(id)->sink_pointer_source_expr;
            TINT_ASSERT(Reader, source_expr.type->Is<Reference>());
            return source_expr;
        }
        case SkipReason::kPointSizeBuiltinValue: {
            return {ty_.F32(), create<ast::FloatLiteralExpression>(
                                   Source{}, 1.0, ast::FloatLiteralExpression::Suffix::kF)};
        }
        case SkipReason::kPointSizeBuiltinPointer:
            Fail() << "unhandled use of a pointer to the PointSize builtin, with ID: " << id;
            return {};
        case SkipReason::kSampleMaskInBuiltinPointer:
            Fail() << "unhandled use of a pointer to the SampleMask builtin, with ID: " << id;
            return {};
        case SkipReason::kSampleMaskOutBuiltinPointer: {
            // The result type is always u32.
            auto name = namer_.Name(sample_mask_out_id);
            return TypedExpression{ty_.U32(), builder_.Expr(Source{}, name)};
        }
    }
    auto type_it = identifier_types_.find(id);
    if (type_it != identifier_types_.end()) {
        // We have a local named definition: function parameter, let, or var
        // declaration.
        auto name = namer_.Name(id);
        auto* type = type_it->second;
        return TypedExpression{type, builder_.Expr(Source{}, name)};
    }
    if (parser_impl_.IsScalarSpecConstant(id)) {
        auto name = namer_.Name(id);
        return TypedExpression{parser_impl_.ConvertType(def_use_mgr_->GetDef(id)->type_id()),
                               builder_.Expr(Source{}, name)};
    }
    if (singly_used_values_.count(id)) {
        auto expr = std::move(singly_used_values_[id]);
        singly_used_values_.erase(id);
        return expr;
    }
    const auto* spirv_constant = constant_mgr_->FindDeclaredConstant(id);
    if (spirv_constant) {
        return parser_impl_.MakeConstantExpression(id);
    }
    const auto* inst = def_use_mgr_->GetDef(id);
    if (inst == nullptr) {
        Fail() << "ID " << id << " does not have a defining SPIR-V instruction";
        return {};
    }
    switch (opcode(inst)) {
        case spv::Op::OpVariable: {
            // This occurs for module-scope variables.
            auto name = namer_.Name(id);
            // Construct the reference type, mapping storage class correctly.
            const auto* type =
                RemapPointerProperties(parser_impl_.ConvertType(inst->type_id(), PtrAs::Ref), id);
            return TypedExpression{type, builder_.Expr(Source{}, name)};
        }
        case spv::Op::OpUndef:
            // Substitute a null value for undef.
            // This case occurs when OpUndef appears at module scope, as if it were
            // a constant.
            return parser_impl_.MakeNullExpression(parser_impl_.ConvertType(inst->type_id()));

        default:
            break;
    }
    if (const spvtools::opt::BasicBlock* const bb = ir_context_.get_instr_block(id)) {
        if (auto* block = GetBlockInfo(bb->id())) {
            if (block->pos == kInvalidBlockPos) {
                // The value came from a block not in the block order.
                // Substitute a null value.
                return parser_impl_.MakeNullExpression(parser_impl_.ConvertType(inst->type_id()));
            }
        }
    }
    Fail() << "unhandled expression for ID " << id << "\n" << inst->PrettyPrint();
    return {};
}

bool FunctionEmitter::EmitFunctionBodyStatements() {
    // Dump the basic blocks in order, grouped by construct.

    // We maintain a stack of StatementBlock objects, where new statements
    // are always written to the topmost entry of the stack. By this point in
    // processing, we have already recorded the interesting control flow
    // boundaries in the BlockInfo and associated Construct objects. As we
    // enter a new statement grouping, we push onto the stack, and also schedule
    // the statement block's completion and removal at a future block's ID.

    // Upon entry, the statement stack has one entry representing the whole
    // function.
    TINT_ASSERT(Reader, !constructs_.IsEmpty());
    Construct* function_construct = constructs_[0].get();
    TINT_ASSERT(Reader, function_construct != nullptr);
    TINT_ASSERT(Reader, function_construct->kind == Construct::kFunction);
    // Make the first entry valid by filling in the construct field, which
    // had not been computed at the time the entry was first created.
    // TODO(dneto): refactor how the first construct is created vs.
    // this statements stack entry is populated.
    TINT_ASSERT(Reader, statements_stack_.Length() == 1);
    statements_stack_[0].SetConstruct(function_construct);

    for (auto block_id : block_order()) {
        if (!EmitBasicBlock(*GetBlockInfo(block_id))) {
            return false;
        }
    }
    return success();
}

bool FunctionEmitter::EmitBasicBlock(const BlockInfo& block_info) {
    // Close off previous constructs.
    while (!statements_stack_.IsEmpty() && (statements_stack_.Back().GetEndId() == block_info.id)) {
        statements_stack_.Back().Finalize(&builder_);
        statements_stack_.Pop();
    }
    if (statements_stack_.IsEmpty()) {
        return Fail() << "internal error: statements stack empty at block " << block_info.id;
    }

    // Enter new constructs.

    utils::Vector<const Construct*, 4> entering_constructs;  // inner most comes first
    {
        auto* here = block_info.construct;
        auto* const top_construct = statements_stack_.Back().GetConstruct();
        while (here != top_construct) {
            // Only enter a construct at its header block.
            if (here->begin_id == block_info.id) {
                entering_constructs.Push(here);
            }
            here = here->parent;
        }
    }
    // What constructs can we have entered?
    // - It can't be kFunction, because there is only one of those, and it was
    //   already on the stack at the outermost level.
    // - We have at most one of kSwitchSelection, or kLoop because each of those
    //   is headed by a block with a merge instruction (OpLoopMerge for kLoop,
    //   and OpSelectionMerge for kSwitchSelection).
    // - When there is a kIfSelection, it can't contain another construct,
    //   because both would have to have their own distinct merge instructions
    //   and distinct terminators.
    // - A kContinue can contain a kContinue
    //   This is possible in Vulkan SPIR-V, but Tint disallows this by the rule
    //   that a block can be continue target for at most one header block. See
    //   test BlockIsContinueForMoreThanOneHeader. If we generalize this,
    //   then by a dominance argument, the inner loop continue target can only be
    //   a single-block loop.
    // TODO(dneto): Handle this case.
    // - If a kLoop is on the outside, its terminator is either:
    //   - an OpBranch, in which case there is no other construct.
    //   - an OpBranchConditional, in which case there is either an kIfSelection
    //     (when both branch targets are different and are inside the loop),
    //     or no other construct (because the branch targets are the same,
    //     or one of them is a break or continue).
    // - All that's left is a kContinue on the outside, and one of
    //   kIfSelection, kSwitchSelection, kLoop on the inside.
    //
    //   The kContinue can be the parent of the other.  For example, a selection
    //   starting at the first block of a continue construct.
    //
    //   The kContinue can't be the child of the other because either:
    //     - The other can't be kLoop because:
    //        - If the kLoop is for a different loop then the kContinue, then
    //          the kContinue must be its own loop header, and so the same
    //          block is two different loops. That's a contradiction.
    //        - If the kLoop is for a the same loop, then this is a contradiction
    //          because a kContinue and its kLoop have disjoint block sets.
    //     - The other construct can't be a selection because:
    //       - The kContinue construct is the entire loop, i.e. the continue
    //         target is its own loop header block.  But then the continue target
    //         has an OpLoopMerge instruction, which contradicts this block being
    //         a selection header.
    //       - The kContinue is in a multi-block loop that is has a non-empty
    //         kLoop; and the selection contains the kContinue block but not the
    //         loop block. That breaks dominance rules. That is, the continue
    //         target is dominated by that loop header, and so gets found by the
    //         block traversal on the outside before the selection is found. The
    //         selection is inside the outer loop.
    //
    // So we fall into one of the following cases:
    //  - We are entering 0 or 1 constructs, or
    //  - We are entering 2 constructs, with the outer one being a kContinue or
    //    kLoop, the inner one is not a continue.
    if (entering_constructs.Length() > 2) {
        return Fail() << "internal error: bad construct nesting found";
    }
    if (entering_constructs.Length() == 2) {
        auto inner_kind = entering_constructs[0]->kind;
        auto outer_kind = entering_constructs[1]->kind;
        if (outer_kind != Construct::kContinue && outer_kind != Construct::kLoop) {
            return Fail() << "internal error: bad construct nesting. Only a Continue "
                             "or a Loop construct can be outer construct on same block.  "
                             "Got outer kind "
                          << int(outer_kind) << " inner kind " << int(inner_kind);
        }
        if (inner_kind == Construct::kContinue) {
            return Fail() << "internal error: unsupported construct nesting: "
                             "Continue around Continue";
        }
        if (inner_kind != Construct::kIfSelection && inner_kind != Construct::kSwitchSelection &&
            inner_kind != Construct::kLoop) {
            return Fail() << "internal error: bad construct nesting. Continue around "
                             "something other than if, switch, or loop";
        }
    }

    // Enter constructs from outermost to innermost.
    // kLoop and kContinue push a new statement-block onto the stack before
    // emitting statements in the block.
    // kIfSelection and kSwitchSelection emit statements in the block and then
    // emit push a new statement-block. Only emit the statements in the block
    // once.

    // Have we emitted the statements for this block?
    bool emitted = false;

    // When entering an if-selection or switch-selection, we will emit the WGSL
    // construct to cause the divergent branching.  But otherwise, we will
    // emit a "normal" block terminator, which occurs at the end of this method.
    bool has_normal_terminator = true;

    for (auto iter = entering_constructs.rbegin(); iter != entering_constructs.rend(); ++iter) {
        const Construct* construct = *iter;

        switch (construct->kind) {
            case Construct::kFunction:
                return Fail() << "internal error: nested function construct";

            case Construct::kLoop:
                if (!EmitLoopStart(construct)) {
                    return false;
                }
                if (!EmitStatementsInBasicBlock(block_info, &emitted)) {
                    return false;
                }
                break;

            case Construct::kContinue:
                if (block_info.is_continue_entire_loop) {
                    if (!EmitLoopStart(construct)) {
                        return false;
                    }
                    if (!EmitStatementsInBasicBlock(block_info, &emitted)) {
                        return false;
                    }
                } else {
                    if (!EmitContinuingStart(construct)) {
                        return false;
                    }
                }
                break;

            case Construct::kIfSelection:
                if (!EmitStatementsInBasicBlock(block_info, &emitted)) {
                    return false;
                }
                if (!EmitIfStart(block_info)) {
                    return false;
                }
                has_normal_terminator = false;
                break;

            case Construct::kSwitchSelection:
                if (!EmitStatementsInBasicBlock(block_info, &emitted)) {
                    return false;
                }
                if (!EmitSwitchStart(block_info)) {
                    return false;
                }
                has_normal_terminator = false;
                break;
        }
    }

    // If we aren't starting or transitioning, then emit the normal
    // statements now.
    if (!EmitStatementsInBasicBlock(block_info, &emitted)) {
        return false;
    }

    if (has_normal_terminator) {
        if (!EmitNormalTerminator(block_info)) {
            return false;
        }
    }
    return success();
}

bool FunctionEmitter::EmitIfStart(const BlockInfo& block_info) {
    // The block is the if-header block.  So its construct is the if construct.
    auto* construct = block_info.construct;
    TINT_ASSERT(Reader, construct->kind == Construct::kIfSelection);
    TINT_ASSERT(Reader, construct->begin_id == block_info.id);

    const uint32_t true_head = block_info.true_head;
    const uint32_t false_head = block_info.false_head;
    const uint32_t premerge_head = block_info.premerge_head;

    const std::string guard_name = block_info.flow_guard_name;
    if (!guard_name.empty()) {
        // Declare the guard variable just before the "if", initialized to true.
        auto* guard_var = builder_.Var(guard_name, builder_.ty.bool_(), MakeTrue(Source{}));
        auto* guard_decl = create<ast::VariableDeclStatement>(Source{}, guard_var);
        AddStatement(guard_decl);
    }

    const auto condition_id = block_info.basic_block->terminator()->GetSingleWordInOperand(0);
    auto* cond = MakeExpression(condition_id).expr;
    if (!cond) {
        return false;
    }
    // Generate the code for the condition.
    auto* builder = AddStatementBuilder<IfStatementBuilder>(cond);

    // Compute the block IDs that should end the then-clause and the else-clause.

    // We need to know where the *emitted* selection should end, i.e. the intended
    // merge block id.  That should be the current premerge block, if it exists,
    // or otherwise the declared merge block.
    //
    // This is another way to think about it:
    //   If there is a premerge, then there are three cases:
    //    - premerge_head is different from the true_head and false_head:
    //      - Premerge comes last. In effect, move the selection merge up
    //        to where the premerge begins.
    //    - premerge_head is the same as the false_head
    //      - This is really an if-then without an else clause.
    //        Move the merge up to where the premerge is.
    //    - premerge_head is the same as the true_head
    //      - This is really an if-else without an then clause.
    //        Emit it as:   if (cond) {} else {....}
    //        Move the merge up to where the premerge is.
    const uint32_t intended_merge = premerge_head ? premerge_head : construct->end_id;

    // then-clause:
    //   If true_head exists:
    //     spans from true head to the earlier of the false head (if it exists)
    //     or the selection merge.
    //   Otherwise:
    //     ends at from the false head (if it exists), otherwise the selection
    //     end.
    const uint32_t then_end = false_head ? false_head : intended_merge;

    // else-clause:
    //   ends at the premerge head (if it exists) or at the selection end.
    const uint32_t else_end = premerge_head ? premerge_head : intended_merge;

    const bool true_is_break = (block_info.true_kind == EdgeKind::kSwitchBreak) ||
                               (block_info.true_kind == EdgeKind::kLoopBreak);
    const bool false_is_break = (block_info.false_kind == EdgeKind::kSwitchBreak) ||
                                (block_info.false_kind == EdgeKind::kLoopBreak);
    const bool true_is_continue = block_info.true_kind == EdgeKind::kLoopContinue;
    const bool false_is_continue = block_info.false_kind == EdgeKind::kLoopContinue;

    // Push statement blocks for the then-clause and the else-clause.
    // But make sure we do it in the right order.
    auto push_else = [this, builder, else_end, construct, false_is_break, false_is_continue]() {
        // Push the else clause onto the stack first.
        PushNewStatementBlock(construct, else_end, [=](const StatementList& stmts) {
            // Only set the else-clause if there are statements to fill it.
            if (!stmts.IsEmpty()) {
                // The "else" consists of the statement list from the top of
                // statements stack, without an "else if" condition.
                builder->else_stmt = create<ast::BlockStatement>(Source{}, stmts, utils::Empty);
            }
        });
        if (false_is_break) {
            AddStatement(create<ast::BreakStatement>(Source{}));
        }
        if (false_is_continue) {
            AddStatement(create<ast::ContinueStatement>(Source{}));
        }
    };

    if (!true_is_break && !true_is_continue &&
        (GetBlockInfo(else_end)->pos < GetBlockInfo(then_end)->pos)) {
        // Process the else-clause first.  The then-clause will be empty so avoid
        // pushing onto the stack at all.
        push_else();
    } else {
        // Blocks for the then-clause appear before blocks for the else-clause.
        // So push the else-clause handling onto the stack first. The else-clause
        // might be empty, but this works anyway.

        // Handle the premerge, if it exists.
        if (premerge_head) {
            // The top of the stack is the statement block that is the parent of the
            // if-statement. Adding statements now will place them after that 'if'.
            if (guard_name.empty()) {
                // We won't have a flow guard for the premerge.
                // Insert a trivial if(true) { ... } around the blocks from the
                // premerge head until the end of the if-selection.  This is needed
                // to ensure uniform reconvergence occurs at the end of the if-selection
                // just like in the original SPIR-V.
                PushTrueGuard(construct->end_id);
            } else {
                // Add a flow guard around the blocks in the premerge area.
                PushGuard(guard_name, construct->end_id);
            }
        }

        push_else();
        if (true_head && false_head && !guard_name.empty()) {
            // There are non-trivial then and else clauses.
            // We have to guard the start of the else.
            PushGuard(guard_name, else_end);
        }

        // Push the then clause onto the stack.
        PushNewStatementBlock(construct, then_end, [=](const StatementList& stmts) {
            builder->body = create<ast::BlockStatement>(Source{}, stmts, utils::Empty);
        });
        if (true_is_break) {
            AddStatement(create<ast::BreakStatement>(Source{}));
        }
        if (true_is_continue) {
            AddStatement(create<ast::ContinueStatement>(Source{}));
        }
    }

    return success();
}

bool FunctionEmitter::EmitSwitchStart(const BlockInfo& block_info) {
    // The block is the if-header block.  So its construct is the if construct.
    auto* construct = block_info.construct;
    TINT_ASSERT(Reader, construct->kind == Construct::kSwitchSelection);
    TINT_ASSERT(Reader, construct->begin_id == block_info.id);
    const auto* branch = block_info.basic_block->terminator();

    const auto selector_id = branch->GetSingleWordInOperand(0);
    // Generate the code for the selector.
    auto selector = MakeExpression(selector_id);
    if (!selector) {
        return false;
    }
    // First, push the statement block for the entire switch.
    auto* swch = AddStatementBuilder<SwitchStatementBuilder>(selector.expr);

    // Grab a pointer to the case list.  It will get buried in the statement block
    // stack.
    PushNewStatementBlock(construct, construct->end_id, nullptr);

    // We will push statement-blocks onto the stack to gather the statements in
    // the default clause and cases clauses. Determine the list of blocks
    // that start each clause.
    utils::Vector<const BlockInfo*, 4> clause_heads;

    // Collect the case clauses, even if they are just the merge block.
    // First the default clause.
    const auto default_id = branch->GetSingleWordInOperand(1);
    const auto* default_info = GetBlockInfo(default_id);
    clause_heads.Push(default_info);
    // Now the case clauses.
    for (uint32_t iarg = 2; iarg + 1 < branch->NumInOperands(); iarg += 2) {
        const auto case_target_id = branch->GetSingleWordInOperand(iarg + 1);
        clause_heads.Push(GetBlockInfo(case_target_id));
    }

    std::stable_sort(
        clause_heads.begin(), clause_heads.end(),
        [](const BlockInfo* lhs, const BlockInfo* rhs) { return lhs->pos < rhs->pos; });
    // Remove duplicates
    {
        // Use read index r, and write index w.
        // Invariant: w <= r;
        size_t w = 0;
        for (size_t r = 0; r < clause_heads.Length(); ++r) {
            if (clause_heads[r] != clause_heads[w]) {
                ++w;  // Advance the write cursor.
            }
            clause_heads[w] = clause_heads[r];
        }
        // We know it's not empty because it always has at least a default clause.
        TINT_ASSERT(Reader, !clause_heads.IsEmpty());
        clause_heads.Resize(w + 1);
    }

    // Push them on in reverse order.
    const auto last_clause_index = clause_heads.Length() - 1;
    for (size_t i = last_clause_index;; --i) {
        // Create a list of integer literals for the selector values leading to
        // this case clause.
        utils::Vector<const ast::CaseSelector*, 4> selectors;
        const bool has_selectors = clause_heads[i]->case_values.has_value();
        if (has_selectors) {
            auto values = clause_heads[i]->case_values.value();
            std::stable_sort(values.begin(), values.end());
            for (auto value : values) {
                // The rest of this module can handle up to 64 bit switch values.
                // The Tint AST handles 32-bit values.
                const uint32_t value32 = uint32_t(value & 0xFFFFFFFF);
                if (selector.type->IsUnsignedScalarOrVector()) {
                    selectors.Push(create<ast::CaseSelector>(
                        Source{}, create<ast::IntLiteralExpression>(
                                      Source{}, value32, ast::IntLiteralExpression::Suffix::kU)));
                } else {
                    selectors.Push(create<ast::CaseSelector>(
                        Source{},
                        create<ast::IntLiteralExpression>(Source{}, static_cast<int32_t>(value32),
                                                          ast::IntLiteralExpression::Suffix::kI)));
                }
            }

            if ((default_info == clause_heads[i]) && construct->ContainsPos(default_info->pos)) {
                // Generate a default selector
                selectors.Push(create<ast::CaseSelector>(Source{}));
            }
        } else {
            // Generate a default selector
            selectors.Push(create<ast::CaseSelector>(Source{}));
        }
        TINT_ASSERT(Reader, !selectors.IsEmpty());

        // Where does this clause end?
        const auto end_id =
            (i + 1 < clause_heads.Length()) ? clause_heads[i + 1]->id : construct->end_id;

        // Reserve the case clause slot in swch->cases, push the new statement block
        // for the case, and fill the case clause once the block is generated.
        auto case_idx = swch->cases.Length();
        swch->cases.Push(nullptr);
        PushNewStatementBlock(construct, end_id, [=](const StatementList& stmts) {
            auto* body = create<ast::BlockStatement>(Source{}, stmts, utils::Empty);
            swch->cases[case_idx] = create<ast::CaseStatement>(Source{}, selectors, body);
        });

        if (i == 0) {
            break;
        }
    }

    return success();
}

bool FunctionEmitter::EmitLoopStart(const Construct* construct) {
    auto* builder = AddStatementBuilder<LoopStatementBuilder>();
    PushNewStatementBlock(construct, construct->end_id, [=](const StatementList& stmts) {
        builder->body = create<ast::BlockStatement>(Source{}, stmts, utils::Empty);
    });
    return success();
}

bool FunctionEmitter::EmitContinuingStart(const Construct* construct) {
    // A continue construct has the same depth as its associated loop
    // construct. Start a continue construct.
    auto* loop_candidate = LastStatement();
    auto* loop = loop_candidate->As<LoopStatementBuilder>();
    if (loop == nullptr) {
        return Fail() << "internal error: starting continue construct, "
                         "expected loop on top of stack";
    }
    PushNewStatementBlock(construct, construct->end_id, [=](const StatementList& stmts) {
        loop->continuing = create<ast::BlockStatement>(Source{}, stmts, utils::Empty);
    });

    return success();
}

bool FunctionEmitter::EmitNormalTerminator(const BlockInfo& block_info) {
    const auto& terminator = *(block_info.basic_block->terminator());
    switch (opcode(terminator)) {
        case spv::Op::OpReturn:
            AddStatement(builder_.Return(Source{}));
            return true;
        case spv::Op::OpReturnValue: {
            auto value = MakeExpression(terminator.GetSingleWordInOperand(0));
            if (!value) {
                return false;
            }
            AddStatement(builder_.Return(Source{}, value.expr));
            return true;
        }
        case spv::Op::OpKill:
            // For now, assume SPIR-V OpKill has same semantics as WGSL discard.
            // TODO(dneto): https://github.com/gpuweb/gpuweb/issues/676
            AddStatement(builder_.Discard(Source{}));
            return true;
        case spv::Op::OpUnreachable:
            // Translate as if it's a return. This avoids the problem where WGSL
            // requires a return statement at the end of the function body.
            {
                const auto* result_type = type_mgr_->GetType(function_.type_id());
                if (result_type->AsVoid() != nullptr) {
                    AddStatement(builder_.Return(Source{}));
                } else {
                    auto* ast_type = parser_impl_.ConvertType(function_.type_id());
                    AddStatement(builder_.Return(Source{}, parser_impl_.MakeNullValue(ast_type)));
                }
            }
            return true;
        case spv::Op::OpBranch: {
            const auto dest_id = terminator.GetSingleWordInOperand(0);
            AddStatement(MakeBranch(block_info, *GetBlockInfo(dest_id)));
            return true;
        }
        case spv::Op::OpBranchConditional: {
            // If both destinations are the same, then do the same as we would
            // for an unconditional branch (OpBranch).
            const auto true_dest = terminator.GetSingleWordInOperand(1);
            const auto false_dest = terminator.GetSingleWordInOperand(2);
            if (true_dest == false_dest) {
                // This is like an unconditional branch.
                AddStatement(MakeBranch(block_info, *GetBlockInfo(true_dest)));
                return true;
            }

            const EdgeKind true_kind = block_info.succ_edge.find(true_dest)->second;
            const EdgeKind false_kind = block_info.succ_edge.find(false_dest)->second;
            auto* const true_info = GetBlockInfo(true_dest);
            auto* const false_info = GetBlockInfo(false_dest);
            auto* cond = MakeExpression(terminator.GetSingleWordInOperand(0)).expr;
            if (!cond) {
                return false;
            }

            // We have two distinct destinations. But we only get here if this
            // is a normal terminator; in particular the source block is *not* the
            // start of an if-selection or a switch-selection.  So at most one branch
            // is a kForward, kCaseFallThrough, or kIfBreak.

            if (true_kind == EdgeKind::kCaseFallThrough ||
                false_kind == EdgeKind::kCaseFallThrough) {
                return Fail() << "Fallthrough not supported in WGSL";
            }

            // In the case of a continuing block a `break-if` needs to be emitted for either an
            // if-break or an if-else-break statement. This only happens inside the continue block.
            // It's possible for a continue block to also be the loop block, so checks are needed
            // that this is a continue construct and the header construct will cause a continuing
            // construct to be emitted. (i.e. the header is not `continue is entire loop`.
            bool needs_break_if = false;
            if ((true_kind == EdgeKind::kLoopBreak || false_kind == EdgeKind::kLoopBreak) &&
                block_info.construct && block_info.construct->kind == Construct::Kind::kContinue) {
                auto* header = GetBlockInfo(block_info.construct->begin_id);

                TINT_ASSERT(Reader, header->construct &&
                                        header->construct->kind == Construct::Kind::kContinue);
                if (!header->is_continue_entire_loop) {
                    needs_break_if = true;
                }
            }

            // At this point, at most one edge is kForward or kIfBreak.

            // If this is a continuing block and a `break` is to be emitted, then this needs to be
            // converted to a `break-if`. This may involve inverting the condition if this was a
            // `break-unless`.
            if (needs_break_if) {
                if (true_kind == EdgeKind::kLoopBreak && false_kind == EdgeKind::kLoopBreak) {
                    // Both branches break ... ?
                    return Fail() << "Both branches of if inside continuing break.";
                }

                if (true_kind == EdgeKind::kLoopBreak) {
                    AddStatement(create<ast::BreakIfStatement>(Source{}, cond));
                } else {
                    AddStatement(create<ast::BreakIfStatement>(
                        Source{},
                        create<ast::UnaryOpExpression>(Source{}, ast::UnaryOp::kNot, cond)));
                }
                return true;

            } else {
                // Emit an 'if' statement to express the *other* branch as a conditional
                // break or continue.  Either or both of these could be nullptr.
                // (A nullptr is generated for kIfBreak, kForward, or kBack.)
                // Also if one of the branches is an if-break out of an if-selection
                // requiring a flow guard, then get that flow guard name too.  It will
                // come from at most one of these two branches.
                std::string flow_guard;
                auto* true_branch = MakeBranchDetailed(block_info, *true_info, &flow_guard);
                auto* false_branch = MakeBranchDetailed(block_info, *false_info, &flow_guard);

                AddStatement(MakeSimpleIf(cond, true_branch, false_branch));
                if (!flow_guard.empty()) {
                    PushGuard(flow_guard, statements_stack_.Back().GetEndId());
                }
            }
            return true;
        }
        case spv::Op::OpSwitch:
            // An OpSelectionMerge must precede an OpSwitch.  That is clarified
            // in the resolution to Khronos-internal SPIR-V issue 115.
            // A new enough version of the SPIR-V validator checks this case.
            // But issue an error in this case, as a defensive measure.
            return Fail() << "invalid structured control flow: found an OpSwitch "
                             "that is not preceded by an "
                             "OpSelectionMerge: "
                          << terminator.PrettyPrint();
        default:
            break;
    }
    return success();
}

const ast::Statement* FunctionEmitter::MakeBranchDetailed(const BlockInfo& src_info,
                                                          const BlockInfo& dest_info,
                                                          std::string* flow_guard_name_ptr) {
    auto kind = src_info.succ_edge.find(dest_info.id)->second;
    switch (kind) {
        case EdgeKind::kBack:
            // Nothing to do. The loop backedge is implicit.
            break;
        case EdgeKind::kSwitchBreak: {
            // Don't bother with a break at the end of a case/default clause.
            const auto header = dest_info.header_for_merge;
            TINT_ASSERT(Reader, header != 0);
            const auto* exiting_construct = GetBlockInfo(header)->construct;
            TINT_ASSERT(Reader, exiting_construct->kind == Construct::kSwitchSelection);
            const auto candidate_next_case_pos = src_info.pos + 1;
            // Leaving the last block from the last case?
            if (candidate_next_case_pos == dest_info.pos) {
                // No break needed.
                return nullptr;
            }
            // Leaving the last block from not-the-last-case?
            if (exiting_construct->ContainsPos(candidate_next_case_pos)) {
                const auto* candidate_next_case =
                    GetBlockInfo(block_order_[candidate_next_case_pos]);
                if (candidate_next_case->case_head_for == exiting_construct ||
                    candidate_next_case->default_head_for == exiting_construct) {
                    // No break needed.
                    return nullptr;
                }
            }
            // We need a break.
            return create<ast::BreakStatement>(Source{});
        }
        case EdgeKind::kLoopBreak:
            return create<ast::BreakStatement>(Source{});
        case EdgeKind::kLoopContinue:
            // An unconditional continue to the next block is redundant and ugly.
            // Skip it in that case.
            if (dest_info.pos == 1 + src_info.pos) {
                break;
            }
            // Otherwise, emit a regular continue statement.
            return create<ast::ContinueStatement>(Source{});
        case EdgeKind::kIfBreak: {
            const auto& flow_guard = GetBlockInfo(dest_info.header_for_merge)->flow_guard_name;
            if (!flow_guard.empty()) {
                if (flow_guard_name_ptr != nullptr) {
                    *flow_guard_name_ptr = flow_guard;
                }
                // Signal an exit from the branch.
                return create<ast::AssignmentStatement>(Source{}, builder_.Expr(flow_guard),
                                                        MakeFalse(Source{}));
            }

            // For an unconditional branch, the break out to an if-selection
            // merge block is implicit.
            break;
        }
        case EdgeKind::kCaseFallThrough: {
            Fail() << "Fallthrough not supported in WGSL";
            return nullptr;
        }
        case EdgeKind::kForward:
            // Unconditional forward branch is implicit.
            break;
    }
    return nullptr;
}

const ast::Statement* FunctionEmitter::MakeSimpleIf(const ast::Expression* condition,
                                                    const ast::Statement* then_stmt,
                                                    const ast::Statement* else_stmt) const {
    if ((then_stmt == nullptr) && (else_stmt == nullptr)) {
        return nullptr;
    }
    StatementList if_stmts;
    if (then_stmt != nullptr) {
        if_stmts.Push(then_stmt);
    }
    auto* if_block = create<ast::BlockStatement>(Source{}, if_stmts, utils::Empty);

    const ast::Statement* else_block = nullptr;
    if (else_stmt) {
        else_block = create<ast::BlockStatement>(StatementList{else_stmt}, utils::Empty);
    }

    auto* if_stmt = create<ast::IfStatement>(Source{}, condition, if_block, else_block);

    return if_stmt;
}

bool FunctionEmitter::EmitStatementsInBasicBlock(const BlockInfo& block_info,
                                                 bool* already_emitted) {
    if (*already_emitted) {
        // Only emit this part of the basic block once.
        return true;
    }
    // Returns the given list of local definition IDs, sorted by their index.
    auto sorted_by_index = [this](auto& ids) {
        auto sorted = ids;
        std::stable_sort(sorted.begin(), sorted.end(),
                         [this](const uint32_t lhs, const uint32_t rhs) {
                             return GetDefInfo(lhs)->index < GetDefInfo(rhs)->index;
                         });
        return sorted;
    };

    // Emit declarations of hoisted variables, in index order.
    for (auto id : sorted_by_index(block_info.hoisted_ids)) {
        const auto* def_inst = def_use_mgr_->GetDef(id);
        TINT_ASSERT(Reader, def_inst);
        // Compute the store type.  Pointers are not storable, so there is
        // no need to remap pointer properties.
        auto* store_type = parser_impl_.ConvertType(def_inst->type_id());
        AddStatement(create<ast::VariableDeclStatement>(
            Source{}, parser_impl_.MakeVar(id, type::AddressSpace::kNone, store_type, nullptr,
                                           AttributeList{})));
        auto* type = ty_.Reference(store_type, type::AddressSpace::kNone);
        identifier_types_.emplace(id, type);
    }

    // Emit regular statements.
    const spvtools::opt::BasicBlock& bb = *(block_info.basic_block);
    const auto* terminator = bb.terminator();
    const auto* merge = bb.GetMergeInst();  // Might be nullptr
    for (auto& inst : bb) {
        if (&inst == terminator || &inst == merge || opcode(inst) == spv::Op::OpLabel ||
            opcode(inst) == spv::Op::OpVariable) {
            continue;
        }
        if (!EmitStatement(inst)) {
            return false;
        }
    }

    // Emit assignments to carry values to phi nodes in potential destinations.
    // Do it in index order.
    if (!block_info.phi_assignments.IsEmpty()) {
        // Keep only the phis that are used.
        utils::Vector<BlockInfo::PhiAssignment, 4> worklist;
        worklist.Reserve(block_info.phi_assignments.Length());
        for (const auto assignment : block_info.phi_assignments) {
            if (GetDefInfo(assignment.phi_id)->local->num_uses > 0) {
                worklist.Push(assignment);
            }
        }
        // Sort them.
        std::stable_sort(
            worklist.begin(), worklist.end(),
            [this](const BlockInfo::PhiAssignment& lhs, const BlockInfo::PhiAssignment& rhs) {
                return GetDefInfo(lhs.phi_id)->index < GetDefInfo(rhs.phi_id)->index;
            });

        // Generate assignments to the phi variables being fed by this
        // block.  It must act as a parallel assignment. So first capture the
        // current value of any value that will be overwritten, then generate
        // the assignments.

        // The set of IDs that are read  by the assignments.
        utils::Hashset<uint32_t, 8> read_set;
        for (const auto assignment : worklist) {
            read_set.Add(assignment.value_id);
        }
        // Generate a let-declaration to capture the current value of each phi
        // that will be both read and written.
        utils::Hashmap<uint32_t, Symbol, 8> copied_phis;
        for (const auto assignment : worklist) {
            const auto phi_id = assignment.phi_id;
            if (read_set.Contains(phi_id)) {
                auto copy_name = namer_.MakeDerivedName(namer_.Name(phi_id) + "_c" +
                                                        std::to_string(block_info.id));
                auto copy_sym = builder_.Symbols().Register(copy_name);
                copied_phis.GetOrCreate(phi_id, [copy_sym]() { return copy_sym; });
                AddStatement(builder_.WrapInStatement(
                    builder_.Let(copy_sym, builder_.Expr(namer_.Name(phi_id)))));
            }
        }

        // Generate assignments to the phi vars.
        for (const auto assignment : worklist) {
            const auto phi_id = assignment.phi_id;
            auto* const lhs_expr = builder_.Expr(namer_.Name(phi_id));
            // If RHS value is actually a phi we just cpatured, then use it.
            auto copy_sym = copied_phis.Find(assignment.value_id);
            auto* const rhs_expr =
                copy_sym ? builder_.Expr(*copy_sym) : MakeExpression(assignment.value_id).expr;
            AddStatement(builder_.Assign(lhs_expr, rhs_expr));
        }
    }

    *already_emitted = true;
    return true;
}

bool FunctionEmitter::EmitConstDefinition(const spvtools::opt::Instruction& inst,
                                          TypedExpression expr) {
    if (!expr) {
        return false;
    }

    // Do not generate pointers that we want to sink.
    if (GetDefInfo(inst.result_id())->skip == SkipReason::kSinkPointerIntoUse) {
        return true;
    }

    expr = AddressOfIfNeeded(expr, &inst);
    expr.type = RemapPointerProperties(expr.type, inst.result_id());
    auto* let = parser_impl_.MakeLet(inst.result_id(), expr.type, expr.expr);
    if (!let) {
        return false;
    }
    AddStatement(create<ast::VariableDeclStatement>(Source{}, let));
    identifier_types_.emplace(inst.result_id(), expr.type);
    return success();
}

bool FunctionEmitter::EmitConstDefOrWriteToHoistedVar(const spvtools::opt::Instruction& inst,
                                                      TypedExpression expr) {
    return WriteIfHoistedVar(inst, expr) || EmitConstDefinition(inst, expr);
}

bool FunctionEmitter::WriteIfHoistedVar(const spvtools::opt::Instruction& inst,
                                        TypedExpression expr) {
    const auto result_id = inst.result_id();
    const auto* def_info = GetDefInfo(result_id);
    if (def_info && def_info->requires_hoisted_var_def) {
        auto name = namer_.Name(result_id);
        // Emit an assignment of the expression to the hoisted variable.
        AddStatement(create<ast::AssignmentStatement>(Source{}, builder_.Expr(name), expr.expr));
        return true;
    }
    return false;
}

bool FunctionEmitter::EmitStatement(const spvtools::opt::Instruction& inst) {
    if (failed()) {
        return false;
    }
    const auto result_id = inst.result_id();
    const auto type_id = inst.type_id();

    if (type_id != 0) {
        const auto& builtin_position_info = parser_impl_.GetBuiltInPositionInfo();
        if (type_id == builtin_position_info.struct_type_id) {
            return Fail() << "operations producing a per-vertex structure are not "
                             "supported: "
                          << inst.PrettyPrint();
        }
        if (type_id == builtin_position_info.pointer_type_id) {
            return Fail() << "operations producing a pointer to a per-vertex "
                             "structure are not "
                             "supported: "
                          << inst.PrettyPrint();
        }
    }

    // Handle combinatorial instructions.
    const auto* def_info = GetDefInfo(result_id);
    if (def_info) {
        TypedExpression combinatorial_expr;
        if (def_info->skip == SkipReason::kDontSkip) {
            combinatorial_expr = MaybeEmitCombinatorialValue(inst);
            if (!success()) {
                return false;
            }
        }
        // An access chain or OpCopyObject can generate a skip.
        if (def_info->skip != SkipReason::kDontSkip) {
            return true;
        }

        if (combinatorial_expr.expr != nullptr) {
            // If the expression is combinatorial, then it's not a direct access
            // of a builtin variable.
            TINT_ASSERT(Reader, def_info->local.has_value());
            if (def_info->requires_hoisted_var_def || def_info->requires_named_let_def ||
                def_info->local->num_uses != 1) {
                // Generate a const definition or an assignment to a hoisted definition
                // now and later use the const or variable name at the uses of this
                // value.
                return EmitConstDefOrWriteToHoistedVar(inst, combinatorial_expr);
            }
            // It is harmless to defer emitting the expression until it's used.
            // Any supporting statements have already been emitted.
            singly_used_values_.insert(std::make_pair(result_id, combinatorial_expr));
            return success();
        }
    }
    if (failed()) {
        return false;
    }

    if (IsImageQuery(opcode(inst))) {
        return EmitImageQuery(inst);
    }

    if (IsSampledImageAccess(opcode(inst)) || IsRawImageAccess(opcode(inst))) {
        return EmitImageAccess(inst);
    }

    if (IsAtomicOp(opcode(inst))) {
        return EmitAtomicOp(inst);
    }

    switch (opcode(inst)) {
        case spv::Op::OpNop:
            return true;

        case spv::Op::OpStore: {
            auto ptr_id = inst.GetSingleWordInOperand(0);
            const auto value_id = inst.GetSingleWordInOperand(1);

            const auto ptr_type_id = def_use_mgr_->GetDef(ptr_id)->type_id();
            const auto& builtin_position_info = parser_impl_.GetBuiltInPositionInfo();
            if (ptr_type_id == builtin_position_info.pointer_type_id) {
                return Fail() << "storing to the whole per-vertex structure is not supported: "
                              << inst.PrettyPrint();
            }

            TypedExpression rhs = MakeExpression(value_id);
            if (!rhs) {
                return false;
            }

            TypedExpression lhs;

            // Handle exceptional cases
            switch (GetSkipReason(ptr_id)) {
                case SkipReason::kPointSizeBuiltinPointer:
                    if (IsFloatOne(value_id)) {
                        // Don't store to PointSize
                        return true;
                    }
                    return Fail() << "cannot store a value other than constant 1.0 to "
                                     "PointSize builtin: "
                                  << inst.PrettyPrint();

                case SkipReason::kSampleMaskOutBuiltinPointer:
                    lhs = MakeExpression(sample_mask_out_id);
                    if (lhs.type->Is<Pointer>()) {
                        // LHS of an assignment must be a reference type.
                        // Convert the LHS to a reference by dereferencing it.
                        lhs = Dereference(lhs);
                    }
                    // The private variable is an array whose element type is already of
                    // the same type as the value being stored into it.  Form the
                    // reference into the first element.
                    lhs.expr = create<ast::IndexAccessorExpression>(
                        Source{}, lhs.expr, parser_impl_.MakeNullValue(ty_.I32()));
                    if (auto* ref = lhs.type->As<Reference>()) {
                        lhs.type = ref->type;
                    }
                    if (auto* arr = lhs.type->As<Array>()) {
                        lhs.type = arr->type;
                    }
                    TINT_ASSERT(Reader, lhs.type);
                    break;
                default:
                    break;
            }

            // Handle an ordinary store as an assignment.
            if (!lhs) {
                lhs = MakeExpression(ptr_id);
            }
            if (!lhs) {
                return false;
            }

            if (lhs.type->Is<Pointer>()) {
                // LHS of an assignment must be a reference type.
                // Convert the LHS to a reference by dereferencing it.
                lhs = Dereference(lhs);
            }

            AddStatement(create<ast::AssignmentStatement>(Source{}, lhs.expr, rhs.expr));
            return success();
        }

        case spv::Op::OpLoad: {
            // Memory accesses must be issued in SPIR-V program order.
            // So represent a load by a new const definition.
            const auto ptr_id = inst.GetSingleWordInOperand(0);
            const auto skip_reason = GetSkipReason(ptr_id);

            switch (skip_reason) {
                case SkipReason::kPointSizeBuiltinPointer:
                    GetDefInfo(inst.result_id())->skip = SkipReason::kPointSizeBuiltinValue;
                    return true;
                case SkipReason::kSampleMaskInBuiltinPointer: {
                    auto name = namer_.Name(sample_mask_in_id);
                    const ast::Expression* id_expr = builder_.Expr(Source{}, name);
                    // SampleMask is an array in Vulkan SPIR-V. Always access the first
                    // element.
                    id_expr = create<ast::IndexAccessorExpression>(
                        Source{}, id_expr, parser_impl_.MakeNullValue(ty_.I32()));

                    auto* loaded_type = parser_impl_.ConvertType(inst.type_id());

                    if (!loaded_type->IsIntegerScalar()) {
                        return Fail() << "loading the whole SampleMask input array is not "
                                         "supported: "
                                      << inst.PrettyPrint();
                    }

                    auto expr = TypedExpression{loaded_type, id_expr};
                    return EmitConstDefinition(inst, expr);
                }
                default:
                    break;
            }
            auto expr = MakeExpression(ptr_id);
            if (!expr) {
                return false;
            }

            // The load result type is the storage type of its operand.
            if (expr.type->Is<Pointer>()) {
                expr = Dereference(expr);
            } else if (auto* ref = expr.type->As<Reference>()) {
                expr.type = ref->type;
            } else {
                Fail() << "OpLoad expression is not a pointer or reference";
                return false;
            }

            return EmitConstDefOrWriteToHoistedVar(inst, expr);
        }

        case spv::Op::OpCopyMemory: {
            // Generate an assignment.
            auto lhs = MakeOperand(inst, 0);
            auto rhs = MakeOperand(inst, 1);
            // Ignore any potential memory operands. Currently they are all for
            // concepts not in WGSL:
            //   Volatile
            //   Aligned
            //   Nontemporal
            //   MakePointerAvailable ; Vulkan memory model
            //   MakePointerVisible   ; Vulkan memory model
            //   NonPrivatePointer    ; Vulkan memory model

            if (!success()) {
                return false;
            }

            // LHS and RHS pointers must be reference types in WGSL.
            if (lhs.type->Is<Pointer>()) {
                lhs = Dereference(lhs);
            }
            if (rhs.type->Is<Pointer>()) {
                rhs = Dereference(rhs);
            }

            AddStatement(create<ast::AssignmentStatement>(Source{}, lhs.expr, rhs.expr));
            return success();
        }

        case spv::Op::OpCopyObject: {
            // Arguably, OpCopyObject is purely combinatorial. On the other hand,
            // it exists to make a new name for something. So we choose to make
            // a new named constant definition.
            auto value_id = inst.GetSingleWordInOperand(0);
            const auto skip = GetSkipReason(value_id);
            if (skip != SkipReason::kDontSkip) {
                GetDefInfo(inst.result_id())->skip = skip;
                GetDefInfo(inst.result_id())->sink_pointer_source_expr =
                    GetDefInfo(value_id)->sink_pointer_source_expr;
                return true;
            }
            auto expr = AddressOfIfNeeded(MakeExpression(value_id), &inst);
            if (!expr) {
                return false;
            }
            return EmitConstDefOrWriteToHoistedVar(inst, expr);
        }

        case spv::Op::OpPhi: {
            // The value will be in scope, available for reading from the phi ID.
            return true;
        }

        case spv::Op::OpOuterProduct:
            // Synthesize an outer product expression in its own statement.
            return EmitConstDefOrWriteToHoistedVar(inst, MakeOuterProduct(inst));

        case spv::Op::OpVectorInsertDynamic:
            // Synthesize a vector insertion in its own statements.
            return MakeVectorInsertDynamic(inst);

        case spv::Op::OpCompositeInsert:
            // Synthesize a composite insertion in its own statements.
            return MakeCompositeInsert(inst);

        case spv::Op::OpFunctionCall:
            return EmitFunctionCall(inst);

        case spv::Op::OpControlBarrier:
            return EmitControlBarrier(inst);

        case spv::Op::OpExtInst:
            if (parser_impl_.IsIgnoredExtendedInstruction(inst)) {
                return true;
            }
            break;

        case spv::Op::OpIAddCarry:
        case spv::Op::OpISubBorrow:
        case spv::Op::OpUMulExtended:
        case spv::Op::OpSMulExtended:
            return Fail() << "extended arithmetic is not finalized for WGSL: "
                             "https://github.com/gpuweb/gpuweb/issues/1565: "
                          << inst.PrettyPrint();

        default:
            break;
    }
    return Fail() << "unhandled instruction with opcode " << uint32_t(opcode(inst)) << ": "
                  << inst.PrettyPrint();
}

TypedExpression FunctionEmitter::MakeOperand(const spvtools::opt::Instruction& inst,
                                             uint32_t operand_index) {
    auto expr = MakeExpression(inst.GetSingleWordInOperand(operand_index));
    if (!expr) {
        return {};
    }
    return parser_impl_.RectifyOperandSignedness(inst, std::move(expr));
}

TypedExpression FunctionEmitter::MaybeEmitCombinatorialValue(
    const spvtools::opt::Instruction& inst) {
    if (inst.result_id() == 0) {
        return {};
    }

    const auto op = opcode(inst);

    const Type* ast_type = nullptr;
    if (inst.type_id()) {
        ast_type = parser_impl_.ConvertType(inst.type_id());
        if (!ast_type) {
            Fail() << "couldn't convert result type for: " << inst.PrettyPrint();
            return {};
        }
    }

    auto binary_op = ConvertBinaryOp(op);
    if (binary_op != ast::BinaryOp::kNone) {
        auto arg0 = MakeOperand(inst, 0);
        auto arg1 =
            parser_impl_.RectifySecondOperandSignedness(inst, arg0.type, MakeOperand(inst, 1));
        if (!arg0 || !arg1) {
            return {};
        }
        auto* binary_expr =
            create<ast::BinaryExpression>(Source{}, binary_op, arg0.expr, arg1.expr);
        TypedExpression result{ast_type, binary_expr};
        return parser_impl_.RectifyForcedResultType(result, inst, arg0.type);
    }

    auto unary_op = ast::UnaryOp::kNegation;
    if (GetUnaryOp(op, &unary_op)) {
        auto arg0 = MakeOperand(inst, 0);
        auto* unary_expr = create<ast::UnaryOpExpression>(Source{}, unary_op, arg0.expr);
        TypedExpression result{ast_type, unary_expr};
        return parser_impl_.RectifyForcedResultType(result, inst, arg0.type);
    }

    const char* unary_builtin_name = GetUnaryBuiltInFunctionName(op);
    if (unary_builtin_name != nullptr) {
        ExpressionList params;
        params.Push(MakeOperand(inst, 0).expr);
        return {ast_type, builder_.Call(Source{}, unary_builtin_name, std::move(params))};
    }

    const auto builtin = GetBuiltin(op);
    if (builtin != sem::BuiltinType::kNone) {
        return MakeBuiltinCall(inst);
    }

    if (op == spv::Op::OpFMod) {
        return MakeFMod(inst);
    }

    if (op == spv::Op::OpAccessChain || op == spv::Op::OpInBoundsAccessChain) {
        return MakeAccessChain(inst);
    }

    if (op == spv::Op::OpBitcast) {
        return {ast_type,
                builder_.Bitcast(Source{}, ast_type->Build(builder_), MakeOperand(inst, 0).expr)};
    }

    if (op == spv::Op::OpShiftLeftLogical || op == spv::Op::OpShiftRightLogical ||
        op == spv::Op::OpShiftRightArithmetic) {
        auto arg0 = MakeOperand(inst, 0);
        // The second operand must be unsigned. It's ok to wrap the shift amount
        // since the shift is modulo the bit width of the first operand.
        auto arg1 = parser_impl_.AsUnsigned(MakeOperand(inst, 1));

        switch (op) {
            case spv::Op::OpShiftLeftLogical:
                binary_op = ast::BinaryOp::kShiftLeft;
                break;
            case spv::Op::OpShiftRightLogical:
                arg0 = parser_impl_.AsUnsigned(arg0);
                binary_op = ast::BinaryOp::kShiftRight;
                break;
            case spv::Op::OpShiftRightArithmetic:
                arg0 = parser_impl_.AsSigned(arg0);
                binary_op = ast::BinaryOp::kShiftRight;
                break;
            default:
                break;
        }
        TypedExpression result{
            ast_type, create<ast::BinaryExpression>(Source{}, binary_op, arg0.expr, arg1.expr)};
        return parser_impl_.RectifyForcedResultType(result, inst, arg0.type);
    }

    auto negated_op = NegatedFloatCompare(op);
    if (negated_op != ast::BinaryOp::kNone) {
        auto arg0 = MakeOperand(inst, 0);
        auto arg1 = MakeOperand(inst, 1);
        auto* binary_expr =
            create<ast::BinaryExpression>(Source{}, negated_op, arg0.expr, arg1.expr);
        auto* negated_expr =
            create<ast::UnaryOpExpression>(Source{}, ast::UnaryOp::kNot, binary_expr);
        return {ast_type, negated_expr};
    }

    if (op == spv::Op::OpExtInst) {
        if (parser_impl_.IsIgnoredExtendedInstruction(inst)) {
            // Ignore it but don't error out.
            return {};
        }
        if (!parser_impl_.IsGlslExtendedInstruction(inst)) {
            Fail() << "unhandled extended instruction import with ID "
                   << inst.GetSingleWordInOperand(0);
            return {};
        }
        return EmitGlslStd450ExtInst(inst);
    }

    if (op == spv::Op::OpCompositeConstruct) {
        ExpressionList operands;
        for (uint32_t iarg = 0; iarg < inst.NumInOperands(); ++iarg) {
            auto operand = MakeOperand(inst, iarg);
            if (!operand) {
                return {};
            }
            operands.Push(operand.expr);
        }
        return {ast_type, builder_.Call(Source{}, ast_type->Build(builder_), std::move(operands))};
    }

    if (op == spv::Op::OpCompositeExtract) {
        return MakeCompositeExtract(inst);
    }

    if (op == spv::Op::OpVectorShuffle) {
        return MakeVectorShuffle(inst);
    }

    if (op == spv::Op::OpVectorExtractDynamic) {
        return {ast_type, create<ast::IndexAccessorExpression>(Source{}, MakeOperand(inst, 0).expr,
                                                               MakeOperand(inst, 1).expr)};
    }

    if (op == spv::Op::OpConvertSToF || op == spv::Op::OpConvertUToF ||
        op == spv::Op::OpConvertFToS || op == spv::Op::OpConvertFToU) {
        return MakeNumericConversion(inst);
    }

    if (op == spv::Op::OpUndef) {
        // Replace undef with the null value.
        return parser_impl_.MakeNullExpression(ast_type);
    }

    if (op == spv::Op::OpSelect) {
        return MakeSimpleSelect(inst);
    }

    if (op == spv::Op::OpArrayLength) {
        return MakeArrayLength(inst);
    }

    // builtin readonly function
    // glsl.std.450 readonly function

    // Instructions:
    //    OpSatConvertSToU // Only in Kernel (OpenCL), not in WebGPU
    //    OpSatConvertUToS // Only in Kernel (OpenCL), not in WebGPU
    //    OpUConvert // Only needed when multiple widths supported
    //    OpSConvert // Only needed when multiple widths supported
    //    OpFConvert // Only needed when multiple widths supported
    //    OpConvertPtrToU // Not in WebGPU
    //    OpConvertUToPtr // Not in WebGPU
    //    OpPtrCastToGeneric // Not in Vulkan
    //    OpGenericCastToPtr // Not in Vulkan
    //    OpGenericCastToPtrExplicit // Not in Vulkan

    return {};
}

TypedExpression FunctionEmitter::EmitGlslStd450ExtInst(const spvtools::opt::Instruction& inst) {
    const auto ext_opcode = inst.GetSingleWordInOperand(1);

    if (ext_opcode == GLSLstd450Ldexp) {
        // WGSL requires the second argument to be signed.
        // Use a type initializer to convert it, which is the same as a bitcast.
        // If the value would go from very large positive to negative, then the
        // original result would have been infinity.  And since WGSL
        // implementations may assume that infinities are not present, then we
        // don't have to worry about that case.
        auto e1 = MakeOperand(inst, 2);
        auto e2 = ToSignedIfUnsigned(MakeOperand(inst, 3));

        return {e1.type, builder_.Call(Source{}, "ldexp", utils::Vector{e1.expr, e2.expr})};
    }

    auto* result_type = parser_impl_.ConvertType(inst.type_id());

    if (result_type->IsScalar()) {
        // Some GLSLstd450 builtins have scalar forms not supported by WGSL.
        // Emulate them.
        switch (ext_opcode) {
            case GLSLstd450Determinant: {
                auto m = MakeOperand(inst, 2);
                TINT_ASSERT(Reader, m.type->Is<Matrix>());
                return {ty_.F32(), builder_.Call(Source{}, "determinant", m.expr)};
            }

            case GLSLstd450Normalize:
                // WGSL does not have scalar form of the normalize builtin.
                // The answer would be 1 anyway, so return that directly.
                return {ty_.F32(), builder_.Expr(1_f)};

            case GLSLstd450FaceForward: {
                // If dot(Nref, Incident) < 0, the result is Normal, otherwise -Normal.
                // Also: select(-normal,normal, Incident*Nref < 0)
                // (The dot product of scalars is their product.)
                // Use a multiply instead of comparing floating point signs. It should
                // be among the fastest operations on a GPU.
                auto normal = MakeOperand(inst, 2);
                auto incident = MakeOperand(inst, 3);
                auto nref = MakeOperand(inst, 4);
                TINT_ASSERT(Reader, normal.type->Is<F32>());
                TINT_ASSERT(Reader, incident.type->Is<F32>());
                TINT_ASSERT(Reader, nref.type->Is<F32>());
                return {
                    ty_.F32(),
                    builder_.Call(
                        Source{}, "select",
                        utils::Vector{
                            create<ast::UnaryOpExpression>(Source{}, ast::UnaryOp::kNegation,
                                                           normal.expr),
                            normal.expr,
                            create<ast::BinaryExpression>(
                                Source{}, ast::BinaryOp::kLessThan,
                                builder_.Mul({}, incident.expr, nref.expr), builder_.Expr(0_f)),
                        }),
                };
            }

            case GLSLstd450Reflect: {
                // Compute  Incident - 2 * Normal * Normal * Incident
                auto incident = MakeOperand(inst, 2);
                auto normal = MakeOperand(inst, 3);
                TINT_ASSERT(Reader, incident.type->Is<F32>());
                TINT_ASSERT(Reader, normal.type->Is<F32>());
                return {
                    ty_.F32(),
                    builder_.Sub(
                        incident.expr,
                        builder_.Mul(2_f, builder_.Mul(normal.expr,
                                                       builder_.Mul(normal.expr, incident.expr))))};
            }

            case GLSLstd450Refract: {
                // It's a complicated expression. Compute it in two dimensions, but
                // with a 0-valued y component in both the incident and normal vectors,
                // then take the x component of that result.
                auto incident = MakeOperand(inst, 2);
                auto normal = MakeOperand(inst, 3);
                auto eta = MakeOperand(inst, 4);
                TINT_ASSERT(Reader, incident.type->Is<F32>());
                TINT_ASSERT(Reader, normal.type->Is<F32>());
                TINT_ASSERT(Reader, eta.type->Is<F32>());
                if (!success()) {
                    return {};
                }
                const Type* f32 = eta.type;
                return {
                    f32,
                    builder_.MemberAccessor(
                        builder_.Call(Source{}, "refract",
                                      utils::Vector{
                                          builder_.vec2<tint::f32>(incident.expr, 0_f),
                                          builder_.vec2<tint::f32>(normal.expr, 0_f),
                                          eta.expr,
                                      }),
                        "x"),
                };
            }
            default:
                break;
        }
    } else {
        switch (ext_opcode) {
            case GLSLstd450MatrixInverse:
                return EmitGlslStd450MatrixInverse(inst);
        }
    }

    const auto name = GetGlslStd450FuncName(ext_opcode);
    if (name.empty()) {
        Fail() << "unhandled GLSL.std.450 instruction " << ext_opcode;
        return {};
    }

    ExpressionList operands;
    const Type* first_operand_type = nullptr;
    // All parameters to GLSL.std.450 extended instructions are IDs.
    for (uint32_t iarg = 2; iarg < inst.NumInOperands(); ++iarg) {
        TypedExpression operand = MakeOperand(inst, iarg);
        if (first_operand_type == nullptr) {
            first_operand_type = operand.type;
        }
        operands.Push(operand.expr);
    }
    auto* call = builder_.Call(Source{}, name, std::move(operands));
    TypedExpression call_expr{result_type, call};
    return parser_impl_.RectifyForcedResultType(call_expr, inst, first_operand_type);
}

TypedExpression FunctionEmitter::EmitGlslStd450MatrixInverse(
    const spvtools::opt::Instruction& inst) {
    auto mat = MakeOperand(inst, 2);
    auto* mat_ty = mat.type->As<Matrix>();
    TINT_ASSERT(Reader, mat_ty);
    TINT_ASSERT(Reader, mat_ty->columns == mat_ty->rows);
    auto& pb = builder_;

    auto idx = [&](size_t row, size_t col) {
        return pb.IndexAccessor(pb.IndexAccessor(mat.expr, u32(row)), u32(col));
    };

    // Compute and save determinant to a let
    auto* det = pb.Div(1.0_f, pb.Call(Source{}, "determinant", mat.expr));
    auto s = pb.Symbols().New("s");
    AddStatement(pb.Decl(pb.Let(s, det)));

    // Returns (a * b) - (c * d)
    auto sub_mul2 = [&](auto* a, auto* b, auto* c, auto* d) {
        return pb.Sub(pb.Mul(a, b), pb.Mul(c, d));
    };

    // Returns (a * b) - (c * d) + (e * f)
    auto sub_add_mul3 = [&](auto* a, auto* b, auto* c, auto* d, auto* e, auto* f) {
        return pb.Add(pb.Sub(pb.Mul(a, b), pb.Mul(c, d)), pb.Mul(e, f));
    };

    // Returns (a * b) + (c * d) - (e * f)
    auto add_sub_mul3 = [&](auto* a, auto* b, auto* c, auto* d, auto* e, auto* f) {
        return pb.Sub(pb.Add(pb.Mul(a, b), pb.Mul(c, d)), pb.Mul(e, f));
    };

    // Returns -a
    auto neg = [&](auto&& a) { return pb.Negation(a); };

    switch (mat_ty->columns) {
        case 2: {
            // a, b
            // c, d
            auto* a = idx(0, 0);
            auto* b = idx(0, 1);
            auto* c = idx(1, 0);
            auto* d = idx(1, 1);

            // s * d, -s * b, -s * c, s * a
            auto* r = pb.mat2x2<f32>(  //
                pb.vec2<f32>(pb.Mul(s, d), pb.Mul(neg(s), b)),
                pb.vec2<f32>(pb.Mul(neg(s), c), pb.Mul(s, a)));
            return {mat.type, r};
        }

        case 3: {
            // a, b, c,
            // d, e, f,
            // g, h, i
            auto* a = idx(0, 0);
            auto* b = idx(0, 1);
            auto* c = idx(0, 2);
            auto* d = idx(1, 0);
            auto* e = idx(1, 1);
            auto* f = idx(1, 2);
            auto* g = idx(2, 0);
            auto* h = idx(2, 1);
            auto* i = idx(2, 2);

            auto r = pb.Mul(s,               //
                            pb.mat3x3<f32>(  //
                                pb.vec3<f32>(
                                    // e * i - f * h
                                    sub_mul2(e, i, f, h),
                                    // c * h - b * i
                                    sub_mul2(c, h, b, i),
                                    // b * f - c * e
                                    sub_mul2(b, f, c, e)),
                                pb.vec3<f32>(
                                    // f * g - d * i
                                    sub_mul2(f, g, d, i),
                                    // a * i - c * g
                                    sub_mul2(a, i, c, g),
                                    // c * d - a * f
                                    sub_mul2(c, d, a, f)),
                                pb.vec3<f32>(
                                    // d * h - e * g
                                    sub_mul2(d, h, e, g),
                                    // b * g - a * h
                                    sub_mul2(b, g, a, h),
                                    // a * e - b * d
                                    sub_mul2(a, e, b, d))));
            return {mat.type, r};
        }

        case 4: {
            // a, b, c, d,
            // e, f, g, h,
            // i, j, k, l,
            // m, n, o, p
            auto* a = idx(0, 0);
            auto* b = idx(0, 1);
            auto* c = idx(0, 2);
            auto* d = idx(0, 3);
            auto* e = idx(1, 0);
            auto* f = idx(1, 1);
            auto* g = idx(1, 2);
            auto* h = idx(1, 3);
            auto* i = idx(2, 0);
            auto* j = idx(2, 1);
            auto* k = idx(2, 2);
            auto* l = idx(2, 3);
            auto* m = idx(3, 0);
            auto* n = idx(3, 1);
            auto* o = idx(3, 2);
            auto* p = idx(3, 3);

            // kplo = k * p - l * o, jpln = j * p - l * n, jokn = j * o - k * n;
            auto* kplo = sub_mul2(k, p, l, o);
            auto* jpln = sub_mul2(j, p, l, n);
            auto* jokn = sub_mul2(j, o, k, n);

            // gpho = g * p - h * o, fphn = f * p - h * n, fogn = f * o - g * n;
            auto* gpho = sub_mul2(g, p, h, o);
            auto* fphn = sub_mul2(f, p, h, n);
            auto* fogn = sub_mul2(f, o, g, n);

            // glhk = g * l - h * k, flhj = f * l - h * j, fkgj = f * k - g * j;
            auto* glhk = sub_mul2(g, l, h, k);
            auto* flhj = sub_mul2(f, l, h, j);
            auto* fkgj = sub_mul2(f, k, g, j);

            // iplm = i * p - l * m, iokm = i * o - k * m, ephm = e * p - h * m;
            auto* iplm = sub_mul2(i, p, l, m);
            auto* iokm = sub_mul2(i, o, k, m);
            auto* ephm = sub_mul2(e, p, h, m);

            // eogm = e * o - g * m, elhi = e * l - h * i, ekgi = e * k - g * i;
            auto* eogm = sub_mul2(e, o, g, m);
            auto* elhi = sub_mul2(e, l, h, i);
            auto* ekgi = sub_mul2(e, k, g, i);

            // injm = i * n - j * m, enfm = e * n - f * m, ejfi = e * j - f * i;
            auto* injm = sub_mul2(i, n, j, m);
            auto* enfm = sub_mul2(e, n, f, m);
            auto* ejfi = sub_mul2(e, j, f, i);

            auto r = pb.Mul(s,               //
                            pb.mat4x4<f32>(  //
                                pb.vec4<f32>(
                                    // f * kplo - g * jpln + h * jokn
                                    sub_add_mul3(f, kplo, g, jpln, h, jokn),
                                    // -b * kplo + c * jpln - d * jokn
                                    add_sub_mul3(neg(b), kplo, c, jpln, d, jokn),
                                    // b * gpho - c * fphn + d * fogn
                                    sub_add_mul3(b, gpho, c, fphn, d, fogn),
                                    // -b * glhk + c * flhj - d * fkgj
                                    add_sub_mul3(neg(b), glhk, c, flhj, d, fkgj)),
                                pb.vec4<f32>(
                                    // -e * kplo + g * iplm - h * iokm
                                    add_sub_mul3(neg(e), kplo, g, iplm, h, iokm),
                                    // a * kplo - c * iplm + d * iokm
                                    sub_add_mul3(a, kplo, c, iplm, d, iokm),
                                    // -a * gpho + c * ephm - d * eogm
                                    add_sub_mul3(neg(a), gpho, c, ephm, d, eogm),
                                    // a * glhk - c * elhi + d * ekgi
                                    sub_add_mul3(a, glhk, c, elhi, d, ekgi)),
                                pb.vec4<f32>(
                                    // e * jpln - f * iplm + h * injm
                                    sub_add_mul3(e, jpln, f, iplm, h, injm),
                                    // -a * jpln + b * iplm - d * injm
                                    add_sub_mul3(neg(a), jpln, b, iplm, d, injm),
                                    // a * fphn - b * ephm + d * enfm
                                    sub_add_mul3(a, fphn, b, ephm, d, enfm),
                                    // -a * flhj + b * elhi - d * ejfi
                                    add_sub_mul3(neg(a), flhj, b, elhi, d, ejfi)),
                                pb.vec4<f32>(
                                    // -e * jokn + f * iokm - g * injm
                                    add_sub_mul3(neg(e), jokn, f, iokm, g, injm),
                                    // a * jokn - b * iokm + c * injm
                                    sub_add_mul3(a, jokn, b, iokm, c, injm),
                                    // -a * fogn + b * eogm - c * enfm
                                    add_sub_mul3(neg(a), fogn, b, eogm, c, enfm),
                                    // a * fkgj - b * ekgi + c * ejfi
                                    sub_add_mul3(a, fkgj, b, ekgi, c, ejfi))));
            return {mat.type, r};
        }
    }

    const auto ext_opcode = inst.GetSingleWordInOperand(1);
    Fail() << "invalid matrix size for " << GetGlslStd450FuncName(ext_opcode);
    return {};
}

const ast::Identifier* FunctionEmitter::Swizzle(uint32_t i) {
    if (i >= kMaxVectorLen) {
        Fail() << "vector component index is larger than " << kMaxVectorLen - 1 << ": " << i;
        return nullptr;
    }
    const char* names[] = {"x", "y", "z", "w"};
    return builder_.Ident(names[i & 3]);
}

const ast::Identifier* FunctionEmitter::PrefixSwizzle(uint32_t n) {
    switch (n) {
        case 1:
            return builder_.Ident("x");
        case 2:
            return builder_.Ident("xy");
        case 3:
            return builder_.Ident("xyz");
        default:
            break;
    }
    Fail() << "invalid swizzle prefix count: " << n;
    return nullptr;
}

TypedExpression FunctionEmitter::MakeFMod(const spvtools::opt::Instruction& inst) {
    auto x = MakeOperand(inst, 0);
    auto y = MakeOperand(inst, 1);
    if (!x || !y) {
        return {};
    }
    // Emulated with: x - y * floor(x / y)
    auto* div = builder_.Div(x.expr, y.expr);
    auto* floor = builder_.Call("floor", div);
    auto* y_floor = builder_.Mul(y.expr, floor);
    auto* res = builder_.Sub(x.expr, y_floor);
    return {x.type, res};
}

TypedExpression FunctionEmitter::MakeAccessChain(const spvtools::opt::Instruction& inst) {
    if (inst.NumInOperands() < 1) {
        // Binary parsing will fail on this anyway.
        Fail() << "invalid access chain: has no input operands";
        return {};
    }

    const auto base_id = inst.GetSingleWordInOperand(0);
    const auto base_skip = GetSkipReason(base_id);
    if (base_skip != SkipReason::kDontSkip) {
        // This can occur for AccessChain with no indices.
        GetDefInfo(inst.result_id())->skip = base_skip;
        GetDefInfo(inst.result_id())->sink_pointer_source_expr =
            GetDefInfo(base_id)->sink_pointer_source_expr;
        return {};
    }

    auto ptr_ty_id = def_use_mgr_->GetDef(base_id)->type_id();
    uint32_t first_index = 1;
    const auto num_in_operands = inst.NumInOperands();

    bool sink_pointer = false;
    // The current WGSL expression for the pointer, starting with the base
    // pointer and updated as each index is incorported.  The important part
    // is the pointee (or "store type").  The address space and access mode will
    // be patched as needed at the very end, via RemapPointerProperties.
    TypedExpression current_expr;

    // If the variable was originally gl_PerVertex, then in the AST we
    // have instead emitted a gl_Position variable.
    // If computing the pointer to the Position builtin, then emit the
    // pointer to the generated gl_Position variable.
    // If computing the pointer to the PointSize builtin, then mark the
    // result as skippable due to being the point-size pointer.
    // If computing the pointer to the ClipDistance or CullDistance builtins,
    // then error out.
    {
        const auto& builtin_position_info = parser_impl_.GetBuiltInPositionInfo();
        if (base_id == builtin_position_info.per_vertex_var_id) {
            // We only support the Position member.
            const auto* member_index_inst =
                def_use_mgr_->GetDef(inst.GetSingleWordInOperand(first_index));
            if (member_index_inst == nullptr) {
                Fail() << "first index of access chain does not reference an instruction: "
                       << inst.PrettyPrint();
                return {};
            }
            const auto* member_index_const = constant_mgr_->GetConstantFromInst(member_index_inst);
            if (member_index_const == nullptr) {
                Fail() << "first index of access chain into per-vertex structure is "
                          "not a constant: "
                       << inst.PrettyPrint();
                return {};
            }
            const auto* member_index_const_int = member_index_const->AsIntConstant();
            if (member_index_const_int == nullptr) {
                Fail() << "first index of access chain into per-vertex structure is "
                          "not a constant integer: "
                       << inst.PrettyPrint();
                return {};
            }
            const auto member_index_value = member_index_const_int->GetZeroExtendedValue();
            if (member_index_value != builtin_position_info.position_member_index) {
                if (member_index_value == builtin_position_info.pointsize_member_index) {
                    if (auto* def_info = GetDefInfo(inst.result_id())) {
                        def_info->skip = SkipReason::kPointSizeBuiltinPointer;
                        return {};
                    }
                } else {
                    // TODO(dneto): Handle ClipDistance and CullDistance
                    Fail() << "accessing per-vertex member " << member_index_value
                           << " is not supported. Only Position is supported, and "
                              "PointSize is ignored";
                    return {};
                }
            }

            // Skip past the member index that gets us to Position.
            first_index = first_index + 1;
            // Replace the gl_PerVertex reference with the gl_Position reference
            ptr_ty_id = builtin_position_info.position_member_pointer_type_id;

            auto name = namer_.Name(base_id);
            current_expr.expr = builder_.Expr(name);
            current_expr.type = parser_impl_.ConvertType(ptr_ty_id, PtrAs::Ref);
        }
    }

    // A SPIR-V access chain is a single instruction with multiple indices
    // walking down into composites.  The Tint AST represents this as
    // ever-deeper nested indexing expressions. Start off with an expression
    // for the base, and then bury that inside nested indexing expressions.
    if (!current_expr) {
        current_expr = MakeOperand(inst, 0);
        if (current_expr.type->Is<Pointer>()) {
            current_expr = Dereference(current_expr);
        }
    }
    const auto constants = constant_mgr_->GetOperandConstants(&inst);

    const auto* ptr_type_inst = def_use_mgr_->GetDef(ptr_ty_id);
    if (!ptr_type_inst || (opcode(ptr_type_inst) != spv::Op::OpTypePointer)) {
        Fail() << "Access chain %" << inst.result_id() << " base pointer is not of pointer type";
        return {};
    }
    spv::StorageClass address_space =
        static_cast<spv::StorageClass>(ptr_type_inst->GetSingleWordInOperand(0));
    uint32_t pointee_type_id = ptr_type_inst->GetSingleWordInOperand(1);

    // Build up a nested expression for the access chain by walking down the type
    // hierarchy, maintaining |pointee_type_id| as the SPIR-V ID of the type of
    // the object pointed to after processing the previous indices.
    for (uint32_t index = first_index; index < num_in_operands; ++index) {
        const auto* index_const = constants[index] ? constants[index]->AsIntConstant() : nullptr;
        const int64_t index_const_val = index_const ? index_const->GetSignExtendedValue() : 0;
        const ast::Expression* next_expr = nullptr;

        const auto* pointee_type_inst = def_use_mgr_->GetDef(pointee_type_id);
        if (!pointee_type_inst) {
            Fail() << "pointee type %" << pointee_type_id << " is invalid after following "
                   << (index - first_index) << " indices: " << inst.PrettyPrint();
            return {};
        }
        switch (opcode(pointee_type_inst)) {
            case spv::Op::OpTypeVector:
                if (index_const) {
                    // Try generating a MemberAccessor expression
                    const auto num_elems = pointee_type_inst->GetSingleWordInOperand(1);
                    if (index_const_val < 0 || num_elems <= index_const_val) {
                        Fail() << "Access chain %" << inst.result_id() << " index %"
                               << inst.GetSingleWordInOperand(index) << " value " << index_const_val
                               << " is out of bounds for vector of " << num_elems << " elements";
                        return {};
                    }
                    if (uint64_t(index_const_val) >= kMaxVectorLen) {
                        Fail() << "internal error: swizzle index " << index_const_val
                               << " is too big. Max handled index is " << kMaxVectorLen - 1;
                    }
                    next_expr = create<ast::MemberAccessorExpression>(
                        Source{}, current_expr.expr, Swizzle(uint32_t(index_const_val)));
                } else {
                    // Non-constant index. Use array syntax
                    next_expr = create<ast::IndexAccessorExpression>(Source{}, current_expr.expr,
                                                                     MakeOperand(inst, index).expr);
                }
                // All vector components are the same type.
                pointee_type_id = pointee_type_inst->GetSingleWordInOperand(0);
                // Sink pointers to vector components.
                sink_pointer = true;
                break;
            case spv::Op::OpTypeMatrix:
                // Use array syntax.
                next_expr = create<ast::IndexAccessorExpression>(Source{}, current_expr.expr,
                                                                 MakeOperand(inst, index).expr);
                // All matrix components are the same type.
                pointee_type_id = pointee_type_inst->GetSingleWordInOperand(0);
                break;
            case spv::Op::OpTypeArray:
                next_expr = create<ast::IndexAccessorExpression>(Source{}, current_expr.expr,
                                                                 MakeOperand(inst, index).expr);
                pointee_type_id = pointee_type_inst->GetSingleWordInOperand(0);
                break;
            case spv::Op::OpTypeRuntimeArray:
                next_expr = create<ast::IndexAccessorExpression>(Source{}, current_expr.expr,
                                                                 MakeOperand(inst, index).expr);
                pointee_type_id = pointee_type_inst->GetSingleWordInOperand(0);
                break;
            case spv::Op::OpTypeStruct: {
                if (!index_const) {
                    Fail() << "Access chain %" << inst.result_id() << " index %"
                           << inst.GetSingleWordInOperand(index)
                           << " is a non-constant index into a structure %" << pointee_type_id;
                    return {};
                }
                const auto num_members = pointee_type_inst->NumInOperands();
                if ((index_const_val < 0) || num_members <= uint64_t(index_const_val)) {
                    Fail() << "Access chain %" << inst.result_id() << " index value "
                           << index_const_val << " is out of bounds for structure %"
                           << pointee_type_id << " having " << num_members << " members";
                    return {};
                }
                auto name = namer_.GetMemberName(pointee_type_id, uint32_t(index_const_val));

                next_expr = builder_.MemberAccessor(Source{}, current_expr.expr, name);
                pointee_type_id = pointee_type_inst->GetSingleWordInOperand(
                    static_cast<uint32_t>(index_const_val));
                break;
            }
            default:
                Fail() << "Access chain with unknown or invalid pointee type %" << pointee_type_id
                       << ": " << pointee_type_inst->PrettyPrint();
                return {};
        }
        const auto pointer_type_id = type_mgr_->FindPointerToType(pointee_type_id, address_space);
        auto* type = parser_impl_.ConvertType(pointer_type_id, PtrAs::Ref);
        TINT_ASSERT(Reader, type && type->Is<Reference>());
        current_expr = TypedExpression{type, next_expr};
    }

    if (sink_pointer) {
        // Capture the reference so that we can sink it into the point of use.
        GetDefInfo(inst.result_id())->skip = SkipReason::kSinkPointerIntoUse;
        GetDefInfo(inst.result_id())->sink_pointer_source_expr = current_expr;
    }

    current_expr.type = RemapPointerProperties(current_expr.type, inst.result_id());
    return current_expr;
}

TypedExpression FunctionEmitter::MakeCompositeExtract(const spvtools::opt::Instruction& inst) {
    // This is structurally similar to creating an access chain, but
    // the SPIR-V instruction has literal indices instead of IDs for indices.

    auto composite_index = 0u;
    auto first_index_position = 1;
    TypedExpression current_expr(MakeOperand(inst, composite_index));
    if (!current_expr) {
        return {};
    }

    const auto composite_id = inst.GetSingleWordInOperand(composite_index);
    auto current_type_id = def_use_mgr_->GetDef(composite_id)->type_id();

    return MakeCompositeValueDecomposition(inst, current_expr, current_type_id,
                                           first_index_position);
}

TypedExpression FunctionEmitter::MakeCompositeValueDecomposition(
    const spvtools::opt::Instruction& inst,
    TypedExpression composite,
    uint32_t composite_type_id,
    int index_start) {
    // This is structurally similar to creating an access chain, but
    // the SPIR-V instruction has literal indices instead of IDs for indices.

    // A SPIR-V composite extract is a single instruction with multiple
    // literal indices walking down into composites.
    // A SPIR-V composite insert is similar but also tells you what component
    // to inject. This function is responsible for the the walking-into part
    // of composite-insert.
    //
    // The Tint AST represents this as ever-deeper nested indexing expressions.
    // Start off with an expression for the composite, and then bury that inside
    // nested indexing expressions.

    auto current_expr = composite;
    auto current_type_id = composite_type_id;

    auto make_index = [this](uint32_t literal) {
        return create<ast::IntLiteralExpression>(Source{}, literal,
                                                 ast::IntLiteralExpression::Suffix::kU);
    };

    // Build up a nested expression for the decomposition by walking down the type
    // hierarchy, maintaining |current_type_id| as the SPIR-V ID of the type of
    // the object pointed to after processing the previous indices.
    const auto num_in_operands = inst.NumInOperands();
    for (uint32_t index = static_cast<uint32_t>(index_start); index < num_in_operands; ++index) {
        const uint32_t index_val = inst.GetSingleWordInOperand(index);

        const auto* current_type_inst = def_use_mgr_->GetDef(current_type_id);
        if (!current_type_inst) {
            Fail() << "composite type %" << current_type_id << " is invalid after following "
                   << (index - static_cast<uint32_t>(index_start))
                   << " indices: " << inst.PrettyPrint();
            return {};
        }
        const char* operation_name = nullptr;
        switch (opcode(inst)) {
            case spv::Op::OpCompositeExtract:
                operation_name = "OpCompositeExtract";
                break;
            case spv::Op::OpCompositeInsert:
                operation_name = "OpCompositeInsert";
                break;
            default:
                Fail() << "internal error: unhandled " << inst.PrettyPrint();
                return {};
        }
        const ast::Expression* next_expr = nullptr;
        switch (opcode(current_type_inst)) {
            case spv::Op::OpTypeVector: {
                // Try generating a MemberAccessor expression. That result in something
                // like  "foo.z", which is more idiomatic than "foo[2]".
                const auto num_elems = current_type_inst->GetSingleWordInOperand(1);
                if (num_elems <= index_val) {
                    Fail() << operation_name << " %" << inst.result_id() << " index value "
                           << index_val << " is out of bounds for vector of " << num_elems
                           << " elements";
                    return {};
                }
                if (index_val >= kMaxVectorLen) {
                    Fail() << "internal error: swizzle index " << index_val
                           << " is too big. Max handled index is " << kMaxVectorLen - 1;
                    return {};
                }
                next_expr = create<ast::MemberAccessorExpression>(Source{}, current_expr.expr,
                                                                  Swizzle(index_val));
                // All vector components are the same type.
                current_type_id = current_type_inst->GetSingleWordInOperand(0);
                break;
            }
            case spv::Op::OpTypeMatrix: {
                // Check bounds
                const auto num_elems = current_type_inst->GetSingleWordInOperand(1);
                if (num_elems <= index_val) {
                    Fail() << operation_name << " %" << inst.result_id() << " index value "
                           << index_val << " is out of bounds for matrix of " << num_elems
                           << " elements";
                    return {};
                }
                if (index_val >= kMaxVectorLen) {
                    Fail() << "internal error: swizzle index " << index_val
                           << " is too big. Max handled index is " << kMaxVectorLen - 1;
                }
                // Use array syntax.
                next_expr = create<ast::IndexAccessorExpression>(Source{}, current_expr.expr,
                                                                 make_index(index_val));
                // All matrix components are the same type.
                current_type_id = current_type_inst->GetSingleWordInOperand(0);
                break;
            }
            case spv::Op::OpTypeArray:
                // The array size could be a spec constant, and so it's not always
                // statically checkable.  Instead, rely on a runtime index clamp
                // or runtime check to keep this safe.
                next_expr = create<ast::IndexAccessorExpression>(Source{}, current_expr.expr,
                                                                 make_index(index_val));
                current_type_id = current_type_inst->GetSingleWordInOperand(0);
                break;
            case spv::Op::OpTypeRuntimeArray:
                Fail() << "can't do " << operation_name
                       << " on a runtime array: " << inst.PrettyPrint();
                return {};
            case spv::Op::OpTypeStruct: {
                const auto num_members = current_type_inst->NumInOperands();
                if (num_members <= index_val) {
                    Fail() << operation_name << " %" << inst.result_id() << " index value "
                           << index_val << " is out of bounds for structure %" << current_type_id
                           << " having " << num_members << " members";
                    return {};
                }
                auto name = namer_.GetMemberName(current_type_id, uint32_t(index_val));

                next_expr = builder_.MemberAccessor(Source{}, current_expr.expr, name);
                current_type_id = current_type_inst->GetSingleWordInOperand(index_val);
                break;
            }
            default:
                Fail() << operation_name << " with bad type %" << current_type_id << ": "
                       << current_type_inst->PrettyPrint();
                return {};
        }
        current_expr = TypedExpression{parser_impl_.ConvertType(current_type_id), next_expr};
    }
    return current_expr;
}

const ast::Expression* FunctionEmitter::MakeTrue(const Source& source) const {
    return create<ast::BoolLiteralExpression>(source, true);
}

const ast::Expression* FunctionEmitter::MakeFalse(const Source& source) const {
    return create<ast::BoolLiteralExpression>(source, false);
}

TypedExpression FunctionEmitter::MakeVectorShuffle(const spvtools::opt::Instruction& inst) {
    const auto vec0_id = inst.GetSingleWordInOperand(0);
    const auto vec1_id = inst.GetSingleWordInOperand(1);
    const spvtools::opt::Instruction& vec0 = *(def_use_mgr_->GetDef(vec0_id));
    const spvtools::opt::Instruction& vec1 = *(def_use_mgr_->GetDef(vec1_id));
    const auto vec0_len = type_mgr_->GetType(vec0.type_id())->AsVector()->element_count();
    const auto vec1_len = type_mgr_->GetType(vec1.type_id())->AsVector()->element_count();

    // Idiomatic vector accessors.

    // Generate an ast::TypeInitializer expression.
    // Assume the literal indices are valid, and there is a valid number of them.
    auto source = GetSourceForInst(inst);
    const Vector* result_type = As<Vector>(parser_impl_.ConvertType(inst.type_id()));
    ExpressionList values;
    for (uint32_t i = 2; i < inst.NumInOperands(); ++i) {
        const auto index = inst.GetSingleWordInOperand(i);
        if (index < vec0_len) {
            auto expr = MakeExpression(vec0_id);
            if (!expr) {
                return {};
            }
            values.Push(create<ast::MemberAccessorExpression>(source, expr.expr, Swizzle(index)));
        } else if (index < vec0_len + vec1_len) {
            const auto sub_index = index - vec0_len;
            TINT_ASSERT(Reader, sub_index < kMaxVectorLen);
            auto expr = MakeExpression(vec1_id);
            if (!expr) {
                return {};
            }
            values.Push(
                create<ast::MemberAccessorExpression>(source, expr.expr, Swizzle(sub_index)));
        } else if (index == 0xFFFFFFFF) {
            // By rule, this maps to OpUndef.  Instead, make it zero.
            values.Push(parser_impl_.MakeNullValue(result_type->type));
        } else {
            Fail() << "invalid vectorshuffle ID %" << inst.result_id()
                   << ": index too large: " << index;
            return {};
        }
    }
    return {result_type, builder_.Call(source, result_type->Build(builder_), std::move(values))};
}

bool FunctionEmitter::RegisterSpecialBuiltInVariables() {
    size_t index = def_info_.size();
    for (auto& special_var : parser_impl_.special_builtins()) {
        const auto id = special_var.first;
        const auto builtin = special_var.second;
        const auto* var = def_use_mgr_->GetDef(id);
        def_info_[id] = std::make_unique<DefInfo>(index, *var);
        ++index;
        auto& def = def_info_[id];
        // Builtins are always defined outside the function.
        switch (builtin) {
            case spv::BuiltIn::PointSize:
                def->skip = SkipReason::kPointSizeBuiltinPointer;
                break;
            case spv::BuiltIn::SampleMask: {
                // Distinguish between input and output variable.
                const auto storage_class =
                    static_cast<spv::StorageClass>(var->GetSingleWordInOperand(0));
                if (storage_class == spv::StorageClass::Input) {
                    sample_mask_in_id = id;
                    def->skip = SkipReason::kSampleMaskInBuiltinPointer;
                } else {
                    sample_mask_out_id = id;
                    def->skip = SkipReason::kSampleMaskOutBuiltinPointer;
                }
                break;
            }
            case spv::BuiltIn::SampleId:
            case spv::BuiltIn::InstanceIndex:
            case spv::BuiltIn::VertexIndex:
            case spv::BuiltIn::LocalInvocationIndex:
            case spv::BuiltIn::LocalInvocationId:
            case spv::BuiltIn::GlobalInvocationId:
            case spv::BuiltIn::WorkgroupId:
            case spv::BuiltIn::NumWorkgroups:
                break;
            default:
                return Fail() << "unrecognized special builtin: " << int(builtin);
        }
    }
    return true;
}

bool FunctionEmitter::RegisterLocallyDefinedValues() {
    // Create a DefInfo for each value definition in this function.
    size_t index = def_info_.size();
    for (auto block_id : block_order_) {
        const auto* block_info = GetBlockInfo(block_id);
        const auto block_pos = block_info->pos;
        for (const auto& inst : *(block_info->basic_block)) {
            const auto result_id = inst.result_id();
            if ((result_id == 0) || opcode(inst) == spv::Op::OpLabel) {
                continue;
            }
            def_info_[result_id] = std::make_unique<DefInfo>(index, inst, block_pos);
            ++index;
            auto& info = def_info_[result_id];

            const auto* type = type_mgr_->GetType(inst.type_id());
            if (type) {
                // Determine address space and access mode for pointer values. Do this in
                // order because we might rely on the storage class for a previously-visited
                // definition.
                // Logical pointers can't be transmitted through OpPhi, so remaining
                // pointer definitions are SSA values, and their definitions must be
                // visited before their uses.
                if (type->AsPointer()) {
                    switch (opcode(inst)) {
                        case spv::Op::OpUndef:
                            return Fail() << "undef pointer is not valid: " << inst.PrettyPrint();
                        case spv::Op::OpVariable:
                            info->pointer = GetPointerInfo(result_id);
                            break;
                        case spv::Op::OpAccessChain:
                        case spv::Op::OpInBoundsAccessChain:
                        case spv::Op::OpCopyObject:
                            // Inherit from the first operand. We need this so we can pick up
                            // a remapped storage buffer.
                            info->pointer = GetPointerInfo(inst.GetSingleWordInOperand(0));
                            break;
                        default:
                            return Fail() << "pointer defined in function from unknown opcode: "
                                          << inst.PrettyPrint();
                    }
                }
                auto* unwrapped = type;
                while (auto* ptr = unwrapped->AsPointer()) {
                    unwrapped = ptr->pointee_type();
                }
                if (unwrapped->AsSampler() || unwrapped->AsImage() || unwrapped->AsSampledImage()) {
                    // Defer code generation until the instruction that actually acts on
                    // the image.
                    info->skip = SkipReason::kOpaqueObject;
                }
            }
        }
    }
    return true;
}

DefInfo::Pointer FunctionEmitter::GetPointerInfo(uint32_t id) {
    // Compute the result from first principles, for a variable.
    auto get_from_root_identifier =
        [&](const spvtools::opt::Instruction& inst) -> DefInfo::Pointer {
        // WGSL root identifiers (or SPIR-V "memory object declarations") are
        // either variables or function parameters.
        switch (opcode(inst)) {
            case spv::Op::OpVariable: {
                if (const auto* module_var = parser_impl_.GetModuleVariable(id)) {
                    return DefInfo::Pointer{module_var->declared_address_space,
                                            module_var->declared_access};
                }
                // Local variables are always Function storage class, with default
                // access mode.
                return DefInfo::Pointer{type::AddressSpace::kFunction, type::Access::kUndefined};
            }
            case spv::Op::OpFunctionParameter: {
                const auto* type = As<Pointer>(parser_impl_.ConvertType(inst.type_id()));
                // For access mode, kUndefined is ok for now, since the
                // only non-default access mode on a pointer would be for a storage
                // buffer, and baseline SPIR-V doesn't allow passing pointers to
                // buffers as function parameters.
                // If/when the SPIR-V path supports variable pointers, then we
                // can pointers to read-only storage buffers passed as
                // parameters.  In that case we need to do a global analysis to
                // determine what the formal argument parameter type should be,
                // whether it has read_only or read_write access mode.
                return DefInfo::Pointer{type->address_space, type::Access::kUndefined};
            }
            default:
                break;
        }
        TINT_ASSERT(Reader, false && "expected a memory object declaration");
        return {};
    };

    auto where = def_info_.find(id);
    if (where != def_info_.end()) {
        const auto& info = where->second;
        if (opcode(info->inst) == spv::Op::OpVariable) {
            // Ignore the cache in this case and compute it from scratch.
            // That's because for a function-scope OpVariable is a
            // locally-defined value.  So its cache entry has been created
            // with a default PointerInfo object, which has invalid data.
            //
            // Instead, you might think that we could forget this weirdness
            // and instead have more standard cache-like behaviour. But then
            // for non-function-scope variables we look up information
            // from a saved ast::Var. But some builtins don't correspond
            // to a declared ast::Var. This is simpler and more reliable.
            return get_from_root_identifier(info->inst);
        }
        // Use the cached value.
        return info->pointer;
    }
    const auto* inst = def_use_mgr_->GetDef(id);
    TINT_ASSERT(Reader, inst);
    return get_from_root_identifier(*inst);
}

const Type* FunctionEmitter::RemapPointerProperties(const Type* type, uint32_t result_id) {
    if (auto* ast_ptr_type = As<Pointer>(type)) {
        const auto pi = GetPointerInfo(result_id);
        return ty_.Pointer(ast_ptr_type->type, pi.address_space, pi.access);
    }
    if (auto* ast_ptr_type = As<Reference>(type)) {
        const auto pi = GetPointerInfo(result_id);
        return ty_.Reference(ast_ptr_type->type, pi.address_space, pi.access);
    }
    return type;
}

void FunctionEmitter::FindValuesNeedingNamedOrHoistedDefinition() {
    // Mark vector operands of OpVectorShuffle as needing a named definition,
    // but only if they are defined in this function as well.
    auto require_named_const_def = [&](const spvtools::opt::Instruction& inst,
                                       int in_operand_index) {
        const auto id = inst.GetSingleWordInOperand(static_cast<uint32_t>(in_operand_index));
        auto* const operand_def = GetDefInfo(id);
        if (operand_def) {
            operand_def->requires_named_let_def = true;
        }
    };
    for (auto& id_def_info_pair : def_info_) {
        const auto& inst = id_def_info_pair.second->inst;
        const auto op = opcode(inst);
        if ((op == spv::Op::OpVectorShuffle) || (op == spv::Op::OpOuterProduct)) {
            // We might access the vector operands multiple times. Make sure they
            // are evaluated only once.
            require_named_const_def(inst, 0);
            require_named_const_def(inst, 1);
        }
        if (parser_impl_.IsGlslExtendedInstruction(inst)) {
            // Some emulations of GLSLstd450 instructions evaluate certain operands
            // multiple times. Ensure their expressions are evaluated only once.
            switch (inst.GetSingleWordInOperand(1)) {
                case GLSLstd450FaceForward:
                    // The "normal" operand expression is used twice in code generation.
                    require_named_const_def(inst, 2);
                    break;
                case GLSLstd450Reflect:
                    require_named_const_def(inst, 2);  // Incident
                    require_named_const_def(inst, 3);  // Normal
                    break;
                default:
                    break;
            }
        }
    }

    // Scan uses of locally defined IDs, finding their first and last uses, in
    // block order.

    // Updates the span of block positions that this value is used in.
    // Ignores values defined outside this function.
    auto record_value_use = [this](uint32_t id, const BlockInfo* block_info) {
        if (auto* def_info = GetDefInfo(id)) {
            if (def_info->local.has_value()) {
                auto& local_def = def_info->local.value();
                // Update usage count.
                local_def.num_uses++;
                // Update usage span.
                local_def.first_use_pos = std::min(local_def.first_use_pos, block_info->pos);
                local_def.last_use_pos = std::max(local_def.last_use_pos, block_info->pos);

                // Determine whether this ID is defined in a different construct
                // from this use.
                const auto defining_block = block_order_[local_def.block_pos];
                const auto* def_in_construct = GetBlockInfo(defining_block)->construct;
                if (def_in_construct != block_info->construct) {
                    local_def.used_in_another_construct = true;
                }
            }
        }
    };
    for (auto block_id : block_order_) {
        const auto* block_info = GetBlockInfo(block_id);
        for (const auto& inst : *(block_info->basic_block)) {
            // Update bookkeeping for locally-defined IDs used by this instruction.
            if (opcode(inst) == spv::Op::OpPhi) {
                // For an OpPhi defining value P, an incoming value V from parent block B is
                // counted as being "used" at block B, not at the block containing the Phi.
                // That's because we will create a variable PHI_P to hold the phi value, and
                // in the code generated for block B, create assignment `PHI_P = V`.
                // To make the WGSL scopes work, both P and V are counted as being "used"
                // in the parent block B.

                const auto phi_id = inst.result_id();
                auto& phi_local_def = GetDefInfo(phi_id)->local.value();
                phi_local_def.is_phi = true;

                // Track all the places where we need to mention the variable,
                // so we can place its declaration.  First, record the location of
                // the read from the variable.
                // Record the assignments that will propagate values from predecessor
                // blocks.
                for (uint32_t i = 0; i + 1 < inst.NumInOperands(); i += 2) {
                    const uint32_t incoming_value_id = inst.GetSingleWordInOperand(i);
                    const uint32_t pred_block_id = inst.GetSingleWordInOperand(i + 1);
                    auto* pred_block_info = GetBlockInfo(pred_block_id);
                    // The predecessor might not be in the block order at all, so we
                    // need this guard.
                    if (IsInBlockOrder(pred_block_info)) {
                        // Track where the incoming value needs to be in scope.
                        record_value_use(incoming_value_id, block_info);

                        // Track where P needs to be in scope.  It's not an ordinary use, so don't
                        // count it as one.
                        const auto pred_pos = pred_block_info->pos;
                        phi_local_def.first_use_pos =
                            std::min(phi_local_def.first_use_pos, pred_pos);
                        phi_local_def.last_use_pos = std::max(phi_local_def.last_use_pos, pred_pos);

                        // Record the assignment that needs to occur at the end
                        // of the predecessor block.
                        pred_block_info->phi_assignments.Push({phi_id, incoming_value_id});
                    }
                }

                // Schedule the declaration of the state variable.
                const auto* enclosing_construct =
                    GetEnclosingScope(phi_local_def.first_use_pos, phi_local_def.last_use_pos);
                GetBlockInfo(enclosing_construct->begin_id)->phis_needing_state_vars.Push(phi_id);
            } else {
                inst.ForEachInId([block_info, &record_value_use](const uint32_t* id_ptr) {
                    record_value_use(*id_ptr, block_info);
                });
            }
        }
    }

    // For an ID defined in this function, determine if its evaluation and
    // potential declaration needs special handling:
    // - Compensate for the fact that dominance does not map directly to scope.
    //   A definition could dominate its use, but a named definition in WGSL
    //   at the location of the definition could go out of scope by the time
    //   you reach the use.  In that case, we hoist the definition to a basic
    //   block at the smallest scope enclosing both the definition and all
    //   its uses.
    // - If value is used in a different construct than its definition, then it
    //   needs a named constant definition.  Otherwise we might sink an
    //   expensive computation into control flow, and hence change performance.
    for (auto& id_def_info_pair : def_info_) {
        const auto def_id = id_def_info_pair.first;
        auto* def_info = id_def_info_pair.second.get();
        if (!def_info->local.has_value()) {
            // Never hoist a variable declared at module scope.
            // This occurs for builtin variables, which are mapped to module-scope
            // private variables.
            continue;
        }
        auto& local_def = def_info->local.value();

        if (local_def.num_uses == 0) {
            // There is no need to adjust the location of the declaration.
            continue;
        }

        const auto* def_in_construct = GetBlockInfo(block_order_[local_def.block_pos])->construct;
        // A definition in the first block of an kIfSelection or kSwitchSelection
        // occurs before the branch, and so that definition should count as
        // having been defined at the scope of the parent construct.
        if (local_def.block_pos == def_in_construct->begin_pos) {
            if ((def_in_construct->kind == Construct::kIfSelection) ||
                (def_in_construct->kind == Construct::kSwitchSelection)) {
                def_in_construct = def_in_construct->parent;
            }
        }

        // We care about the earliest between the place of definition, and the first
        // use of the value.
        const auto first_pos = std::min(local_def.block_pos, local_def.first_use_pos);
        const auto last_use_pos = local_def.last_use_pos;

        bool should_hoist_to_let = false;
        bool should_hoist_to_var = false;
        if (local_def.is_phi) {
            // We need to generate a variable, and assignments to that variable in
            // all the phi parent blocks.
            should_hoist_to_var = true;
        } else if (!def_in_construct->ContainsPos(first_pos) ||
                   !def_in_construct->ContainsPos(last_use_pos)) {
            // To satisfy scoping, we have to hoist the definition out to an enclosing
            // construct.
            should_hoist_to_var = true;
        } else {
            // Avoid moving combinatorial values across constructs.  This is a
            // simple heuristic to avoid changing the cost of an operation
            // by moving it into or out of a loop, for example.
            if ((def_info->pointer.address_space == type::AddressSpace::kUndefined) &&
                local_def.used_in_another_construct) {
                should_hoist_to_let = true;
            }
        }

        if (should_hoist_to_var || should_hoist_to_let) {
            const auto* enclosing_construct = GetEnclosingScope(first_pos, last_use_pos);
            if (should_hoist_to_let && (enclosing_construct == def_in_construct)) {
                // We can use a plain 'let' declaration.
                def_info->requires_named_let_def = true;
            } else {
                // We need to make a hoisted variable definition.
                // TODO(dneto): Handle non-storable types, particularly pointers.
                def_info->requires_hoisted_var_def = true;
                auto* hoist_to_block = GetBlockInfo(enclosing_construct->begin_id);
                hoist_to_block->hoisted_ids.Push(def_id);
            }
        }
    }
}

const Construct* FunctionEmitter::GetEnclosingScope(uint32_t first_pos, uint32_t last_pos) const {
    const auto* enclosing_construct = GetBlockInfo(block_order_[first_pos])->construct;
    TINT_ASSERT(Reader, enclosing_construct != nullptr);
    // Constructs are strictly nesting, so follow parent pointers
    while (enclosing_construct && !enclosing_construct->ScopeContainsPos(last_pos)) {
        // The scope of a continue construct is enclosed in its associated loop
        // construct, but they are siblings in our construct tree.
        const auto* sibling_loop = SiblingLoopConstruct(enclosing_construct);
        // Go to the sibling loop if it exists, otherwise walk up to the parent.
        enclosing_construct = sibling_loop ? sibling_loop : enclosing_construct->parent;
    }
    // At worst, we go all the way out to the function construct.
    TINT_ASSERT(Reader, enclosing_construct != nullptr);
    return enclosing_construct;
}

TypedExpression FunctionEmitter::MakeNumericConversion(const spvtools::opt::Instruction& inst) {
    const auto op = opcode(inst);
    auto* requested_type = parser_impl_.ConvertType(inst.type_id());
    auto arg_expr = MakeOperand(inst, 0);
    if (!arg_expr) {
        return {};
    }
    arg_expr.type = arg_expr.type->UnwrapRef();

    const Type* expr_type = nullptr;
    if ((op == spv::Op::OpConvertSToF) || (op == spv::Op::OpConvertUToF)) {
        if (arg_expr.type->IsIntegerScalarOrVector()) {
            expr_type = requested_type;
        } else {
            Fail() << "operand for conversion to floating point must be integral "
                      "scalar or vector: "
                   << inst.PrettyPrint();
        }
    } else if (op == spv::Op::OpConvertFToU) {
        if (arg_expr.type->IsFloatScalarOrVector()) {
            expr_type = parser_impl_.GetUnsignedIntMatchingShape(arg_expr.type);
        } else {
            Fail() << "operand for conversion to unsigned integer must be floating "
                      "point scalar or vector: "
                   << inst.PrettyPrint();
        }
    } else if (op == spv::Op::OpConvertFToS) {
        if (arg_expr.type->IsFloatScalarOrVector()) {
            expr_type = parser_impl_.GetSignedIntMatchingShape(arg_expr.type);
        } else {
            Fail() << "operand for conversion to signed integer must be floating "
                      "point scalar or vector: "
                   << inst.PrettyPrint();
        }
    }
    if (expr_type == nullptr) {
        // The diagnostic has already been emitted.
        return {};
    }

    ExpressionList params;
    params.Push(arg_expr.expr);
    TypedExpression result{expr_type, builder_.Call(GetSourceForInst(inst),
                                                    expr_type->Build(builder_), std::move(params))};

    if (requested_type == expr_type) {
        return result;
    }
    return {requested_type,
            builder_.Bitcast(GetSourceForInst(inst), requested_type->Build(builder_), result.expr)};
}

bool FunctionEmitter::EmitFunctionCall(const spvtools::opt::Instruction& inst) {
    // We ignore function attributes such as Inline, DontInline, Pure, Const.
    auto name = namer_.Name(inst.GetSingleWordInOperand(0));
    auto* function = create<ast::Identifier>(Source{}, builder_.Symbols().Register(name));

    ExpressionList args;
    for (uint32_t iarg = 1; iarg < inst.NumInOperands(); ++iarg) {
        uint32_t arg_id = inst.GetSingleWordInOperand(iarg);
        TypedExpression expr;

        if (IsHandleObj(def_use_mgr_->GetDef(arg_id))) {
            // For textures and samplers, use the memory object declaration
            // instead.
            const auto usage = parser_impl_.GetHandleUsage(arg_id);
            const auto* mem_obj_decl =
                parser_impl_.GetMemoryObjectDeclarationForHandle(arg_id, usage.IsTexture());
            expr = MakeExpression(mem_obj_decl->result_id());
            // Pass the handle through instead of a pointer to the handle.
            expr.type = parser_impl_.GetHandleTypeForSpirvHandle(*mem_obj_decl);
        } else {
            expr = MakeOperand(inst, iarg);
        }
        if (!expr) {
            return false;
        }
        // Functions cannot use references as parameters, so we need to pass by
        // pointer if the operand is of pointer type.
        expr = AddressOfIfNeeded(expr, def_use_mgr_->GetDef(inst.GetSingleWordInOperand(iarg)));
        args.Push(expr.expr);
    }
    if (failed()) {
        return false;
    }
    auto* call_expr = create<ast::CallExpression>(Source{}, function, std::move(args));
    auto* result_type = parser_impl_.ConvertType(inst.type_id());
    if (!result_type) {
        return Fail() << "internal error: no mapped type result of call: " << inst.PrettyPrint();
    }

    if (result_type->Is<Void>()) {
        return nullptr != AddStatement(builder_.CallStmt(Source{}, call_expr));
    }

    return EmitConstDefOrWriteToHoistedVar(inst, {result_type, call_expr});
}

bool FunctionEmitter::EmitControlBarrier(const spvtools::opt::Instruction& inst) {
    uint32_t operands[3];
    for (uint32_t i = 0; i < 3; i++) {
        auto id = inst.GetSingleWordInOperand(i);
        if (auto* constant = constant_mgr_->FindDeclaredConstant(id)) {
            operands[i] = constant->GetU32();
        } else {
            return Fail() << "invalid or missing operands for control barrier";
        }
    }

    uint32_t execution = operands[0];
    uint32_t memory = operands[1];
    uint32_t semantics = operands[2];

    if (execution != uint32_t(spv::Scope::Workgroup)) {
        return Fail() << "unsupported control barrier execution scope: "
                      << "expected Workgroup (2), got: " << execution;
    }
    if (semantics & uint32_t(spv::MemorySemanticsMask::AcquireRelease)) {
        semantics &= ~static_cast<uint32_t>(spv::MemorySemanticsMask::AcquireRelease);
    } else {
        return Fail() << "control barrier semantics requires acquire and release";
    }
    if (semantics & uint32_t(spv::MemorySemanticsMask::WorkgroupMemory)) {
        if (memory != uint32_t(spv::Scope::Workgroup)) {
            return Fail() << "workgroupBarrier requires workgroup memory scope";
        }
        AddStatement(builder_.CallStmt(builder_.Call("workgroupBarrier")));
        semantics &= ~static_cast<uint32_t>(spv::MemorySemanticsMask::WorkgroupMemory);
    }
    if (semantics & uint32_t(spv::MemorySemanticsMask::UniformMemory)) {
        if (memory != uint32_t(spv::Scope::Device)) {
            return Fail() << "storageBarrier requires device memory scope";
        }
        AddStatement(builder_.CallStmt(builder_.Call("storageBarrier")));
        semantics &= ~static_cast<uint32_t>(spv::MemorySemanticsMask::UniformMemory);
    }
    if (semantics) {
        return Fail() << "unsupported control barrier semantics: " << semantics;
    }
    return true;
}

TypedExpression FunctionEmitter::MakeBuiltinCall(const spvtools::opt::Instruction& inst) {
    const auto builtin = GetBuiltin(opcode(inst));
    auto* name = sem::str(builtin);
    auto* ident = create<ast::Identifier>(Source{}, builder_.Symbols().Register(name));

    ExpressionList params;
    const Type* first_operand_type = nullptr;
    for (uint32_t iarg = 0; iarg < inst.NumInOperands(); ++iarg) {
        TypedExpression operand = MakeOperand(inst, iarg);
        if (first_operand_type == nullptr) {
            first_operand_type = operand.type;
        }
        params.Push(operand.expr);
    }
    auto* call_expr = create<ast::CallExpression>(Source{}, ident, std::move(params));
    auto* result_type = parser_impl_.ConvertType(inst.type_id());
    if (!result_type) {
        Fail() << "internal error: no mapped type result of call: " << inst.PrettyPrint();
        return {};
    }
    TypedExpression call{result_type, call_expr};
    return parser_impl_.RectifyForcedResultType(call, inst, first_operand_type);
}

TypedExpression FunctionEmitter::MakeSimpleSelect(const spvtools::opt::Instruction& inst) {
    auto condition = MakeOperand(inst, 0);
    auto true_value = MakeOperand(inst, 1);
    auto false_value = MakeOperand(inst, 2);

    // SPIR-V validation requires:
    // - the condition to be bool or bool vector, so we don't check it here.
    // - true_value false_value, and result type to match.
    // - you can't select over pointers or pointer vectors, unless you also have
    //   a VariablePointers* capability, which is not allowed in by WebGPU.
    auto* op_ty = true_value.type->UnwrapRef();
    if (op_ty->Is<Vector>() || op_ty->IsFloatScalar() || op_ty->IsIntegerScalar() ||
        op_ty->Is<Bool>()) {
        ExpressionList params;
        params.Push(false_value.expr);
        params.Push(true_value.expr);
        // The condition goes last.
        params.Push(condition.expr);
        return {op_ty, create<ast::CallExpression>(
                           Source{},
                           create<ast::Identifier>(Source{}, builder_.Symbols().Register("select")),
                           std::move(params))};
    }
    return {};
}

Source FunctionEmitter::GetSourceForInst(const spvtools::opt::Instruction& inst) const {
    return parser_impl_.GetSourceForInst(&inst);
}

const spvtools::opt::Instruction* FunctionEmitter::GetImage(
    const spvtools::opt::Instruction& inst) {
    if (inst.NumInOperands() == 0) {
        Fail() << "not an image access instruction: " << inst.PrettyPrint();
        return nullptr;
    }
    // The image or sampled image operand is always the first operand.
    const auto image_or_sampled_image_operand_id = inst.GetSingleWordInOperand(0);
    const auto* image =
        parser_impl_.GetMemoryObjectDeclarationForHandle(image_or_sampled_image_operand_id, true);
    if (!image) {
        Fail() << "internal error: couldn't find image for " << inst.PrettyPrint();
        return nullptr;
    }
    return image;
}

const Texture* FunctionEmitter::GetImageType(const spvtools::opt::Instruction& image) {
    const Type* type = parser_impl_.GetHandleTypeForSpirvHandle(image);
    if (!parser_impl_.success()) {
        Fail();
        return {};
    }
    TINT_ASSERT(Reader, type != nullptr);
    if (auto* result = type->UnwrapAll()->As<Texture>()) {
        return result;
    }
    Fail() << "invalid texture type for " << image.PrettyPrint();
    return {};
}

const ast::Expression* FunctionEmitter::GetImageExpression(const spvtools::opt::Instruction& inst) {
    auto* image = GetImage(inst);
    if (!image) {
        return nullptr;
    }
    auto name = namer_.Name(image->result_id());
    return builder_.Expr(GetSourceForInst(inst), name);
}

const ast::Expression* FunctionEmitter::GetSamplerExpression(
    const spvtools::opt::Instruction& inst) {
    // The sampled image operand is always the first operand.
    const auto image_or_sampled_image_operand_id = inst.GetSingleWordInOperand(0);
    const auto* image =
        parser_impl_.GetMemoryObjectDeclarationForHandle(image_or_sampled_image_operand_id, false);
    if (!image) {
        Fail() << "internal error: couldn't find sampler for " << inst.PrettyPrint();
        return nullptr;
    }
    auto name = namer_.Name(image->result_id());
    return builder_.Expr(GetSourceForInst(inst), name);
}

bool FunctionEmitter::EmitImageAccess(const spvtools::opt::Instruction& inst) {
    ExpressionList args;
    const auto op = opcode(inst);

    // Form the texture operand.
    const spvtools::opt::Instruction* image = GetImage(inst);
    if (!image) {
        return false;
    }
    args.Push(GetImageExpression(inst));

    // Form the sampler operand, if needed.
    if (IsSampledImageAccess(op)) {
        // Form the sampler operand.
        if (auto* sampler = GetSamplerExpression(inst)) {
            args.Push(sampler);
        } else {
            return false;
        }
    }

    // Find the texture type.
    const auto* texture_type = parser_impl_.GetHandleTypeForSpirvHandle(*image)->As<Texture>();
    if (!texture_type) {
        return Fail();
    }

    // This is the SPIR-V operand index.  We're done with the first operand.
    uint32_t arg_index = 1;

    // Push the coordinates operands.
    auto coords = MakeCoordinateOperandsForImageAccess(inst);
    if (coords.IsEmpty()) {
        return false;
    }
    for (auto* coord : coords) {
        args.Push(coord);
    }
    // Skip the coordinates operand.
    arg_index++;

    const auto num_args = inst.NumInOperands();

    // Consumes the depth-reference argument, pushing it onto the end of
    // the parameter list. Issues a diagnostic and returns false on error.
    auto consume_dref = [&]() -> bool {
        if (arg_index < num_args) {
            args.Push(MakeOperand(inst, arg_index).expr);
            arg_index++;
        } else {
            return Fail() << "image depth-compare instruction is missing a Dref operand: "
                          << inst.PrettyPrint();
        }
        return true;
    };

    std::string builtin_name;
    bool use_level_of_detail_suffix = true;
    bool is_dref_sample = false;
    bool is_gather_or_dref_gather = false;
    bool is_non_dref_sample = false;
    switch (op) {
        case spv::Op::OpImageSampleImplicitLod:
        case spv::Op::OpImageSampleExplicitLod:
        case spv::Op::OpImageSampleProjImplicitLod:
        case spv::Op::OpImageSampleProjExplicitLod:
            is_non_dref_sample = true;
            builtin_name = "textureSample";
            break;
        case spv::Op::OpImageSampleDrefImplicitLod:
        case spv::Op::OpImageSampleDrefExplicitLod:
        case spv::Op::OpImageSampleProjDrefImplicitLod:
        case spv::Op::OpImageSampleProjDrefExplicitLod:
            is_dref_sample = true;
            builtin_name = "textureSampleCompare";
            if (!consume_dref()) {
                return false;
            }
            break;
        case spv::Op::OpImageGather:
            is_gather_or_dref_gather = true;
            builtin_name = "textureGather";
            if (!texture_type->Is<DepthTexture>()) {
                // The explicit component is the *first* argument in WGSL.
                ExpressionList gather_args;
                gather_args.Push(ToI32(MakeOperand(inst, arg_index)).expr);
                for (auto* arg : args) {
                    gather_args.Push(arg);
                }
                args = std::move(gather_args);
            }
            // Skip over the component operand, even for depth textures.
            arg_index++;
            break;
        case spv::Op::OpImageDrefGather:
            is_gather_or_dref_gather = true;
            builtin_name = "textureGatherCompare";
            if (!consume_dref()) {
                return false;
            }
            break;
        case spv::Op::OpImageFetch:
        case spv::Op::OpImageRead:
            // Read a single texel from a sampled or storage image.
            builtin_name = "textureLoad";
            use_level_of_detail_suffix = false;
            break;
        case spv::Op::OpImageWrite:
            builtin_name = "textureStore";
            use_level_of_detail_suffix = false;
            if (arg_index < num_args) {
                auto texel = MakeOperand(inst, arg_index);
                auto* converted_texel = ConvertTexelForStorage(inst, texel, texture_type);
                if (!converted_texel) {
                    return false;
                }

                args.Push(converted_texel);
                arg_index++;
            } else {
                return Fail() << "image write is missing a Texel operand: " << inst.PrettyPrint();
            }
            break;
        default:
            return Fail() << "internal error: unrecognized image access: " << inst.PrettyPrint();
    }

    // Loop over the image operands, looking for extra operands to the builtin.
    // Except we uroll the loop.
    uint32_t image_operands_mask = 0;
    if (arg_index < num_args) {
        image_operands_mask = inst.GetSingleWordInOperand(arg_index);
        arg_index++;
    }
    if (arg_index < num_args && (image_operands_mask & uint32_t(spv::ImageOperandsMask::Bias))) {
        if (is_dref_sample) {
            return Fail() << "WGSL does not support depth-reference sampling with "
                             "level-of-detail bias: "
                          << inst.PrettyPrint();
        }
        if (is_gather_or_dref_gather) {
            return Fail() << "WGSL does not support image gather with "
                             "level-of-detail bias: "
                          << inst.PrettyPrint();
        }
        builtin_name += "Bias";
        args.Push(MakeOperand(inst, arg_index).expr);
        image_operands_mask ^= uint32_t(spv::ImageOperandsMask::Bias);
        arg_index++;
    }
    if (arg_index < num_args && (image_operands_mask & uint32_t(spv::ImageOperandsMask::Lod))) {
        if (use_level_of_detail_suffix) {
            builtin_name += "Level";
        }
        if (is_dref_sample || is_gather_or_dref_gather) {
            // Metal only supports Lod = 0 for comparison sampling without
            // derivatives.
            // Vulkan SPIR-V does not allow Lod with OpImageGather or
            // OpImageDrefGather.
            if (!IsFloatZero(inst.GetSingleWordInOperand(arg_index))) {
                return Fail() << "WGSL comparison sampling without derivatives "
                                 "requires level-of-detail 0.0"
                              << inst.PrettyPrint();
            }
            // Don't generate the Lod argument.
        } else {
            // Generate the Lod argument.
            TypedExpression lod = MakeOperand(inst, arg_index);
            // When sampling from a depth texture, the Lod operand must be an I32.
            if (texture_type->Is<DepthTexture>()) {
                // Convert it to a signed integer type.
                lod = ToI32(lod);
            }
            args.Push(lod.expr);
        }

        image_operands_mask ^= uint32_t(spv::ImageOperandsMask::Lod);
        arg_index++;
    } else if ((op == spv::Op::OpImageFetch || op == spv::Op::OpImageRead) &&
               !texture_type->IsAnyOf<DepthMultisampledTexture, MultisampledTexture>()) {
        // textureLoad requires an explicit level-of-detail parameter for
        // non-multisampled texture types.
        args.Push(parser_impl_.MakeNullValue(ty_.I32()));
    }
    if (arg_index + 1 < num_args &&
        (image_operands_mask & uint32_t(spv::ImageOperandsMask::Grad))) {
        if (is_dref_sample) {
            return Fail() << "WGSL does not support depth-reference sampling with "
                             "explicit gradient: "
                          << inst.PrettyPrint();
        }
        if (is_gather_or_dref_gather) {
            return Fail() << "WGSL does not support image gather with "
                             "explicit gradient: "
                          << inst.PrettyPrint();
        }
        builtin_name += "Grad";
        args.Push(MakeOperand(inst, arg_index).expr);
        args.Push(MakeOperand(inst, arg_index + 1).expr);
        image_operands_mask ^= uint32_t(spv::ImageOperandsMask::Grad);
        arg_index += 2;
    }
    if (arg_index < num_args &&
        (image_operands_mask & uint32_t(spv::ImageOperandsMask::ConstOffset))) {
        if (!IsImageSamplingOrGatherOrDrefGather(op)) {
            return Fail() << "ConstOffset is only permitted for sampling, gather, or "
                             "depth-reference gather operations: "
                          << inst.PrettyPrint();
        }
        switch (texture_type->dims) {
            case type::TextureDimension::k2d:
            case type::TextureDimension::k2dArray:
            case type::TextureDimension::k3d:
                break;
            default:
                return Fail() << "ConstOffset is only permitted for 2D, 2D Arrayed, "
                                 "and 3D textures: "
                              << inst.PrettyPrint();
        }

        args.Push(ToSignedIfUnsigned(MakeOperand(inst, arg_index)).expr);
        image_operands_mask ^= uint32_t(spv::ImageOperandsMask::ConstOffset);
        arg_index++;
    }
    if (arg_index < num_args && (image_operands_mask & uint32_t(spv::ImageOperandsMask::Sample))) {
        // TODO(dneto): only permitted with ImageFetch
        args.Push(ToI32(MakeOperand(inst, arg_index)).expr);
        image_operands_mask ^= uint32_t(spv::ImageOperandsMask::Sample);
        arg_index++;
    }
    if (image_operands_mask) {
        return Fail() << "unsupported image operands (" << image_operands_mask
                      << "): " << inst.PrettyPrint();
    }

    // If any of the arguments are nullptr, then we've failed.
    if (std::any_of(args.begin(), args.end(), [](auto* expr) { return expr == nullptr; })) {
        return false;
    }

    auto* call_expr = builder_.Call(Source{}, builtin_name, std::move(args));

    if (inst.type_id() != 0) {
        // It returns a value.
        const ast::Expression* value = call_expr;

        // The result type, derived from the SPIR-V instruction.
        auto* result_type = parser_impl_.ConvertType(inst.type_id());
        auto* result_component_type = result_type;
        if (auto* result_vector_type = As<Vector>(result_type)) {
            result_component_type = result_vector_type->type;
        }

        // For depth textures, the arity might mot match WGSL:
        //  Operation           SPIR-V                     WGSL
        //   normal sampling     vec4  ImplicitLod          f32
        //   normal sampling     vec4  ExplicitLod          f32
        //   compare sample      f32   DrefImplicitLod      f32
        //   compare sample      f32   DrefExplicitLod      f32
        //   texel load          vec4  ImageFetch           f32
        //   normal gather       vec4  ImageGather          vec4
        //   dref gather         vec4  ImageDrefGather      vec4
        // Construct a 4-element vector with the result from the builtin in the
        // first component.
        if (texture_type->IsAnyOf<DepthTexture, DepthMultisampledTexture>()) {
            if (is_non_dref_sample || (op == spv::Op::OpImageFetch)) {
                value = builder_.Call(Source{},
                                      result_type->Build(builder_),  // a vec4
                                      utils::Vector{
                                          value,
                                          parser_impl_.MakeNullValue(result_component_type),
                                          parser_impl_.MakeNullValue(result_component_type),
                                          parser_impl_.MakeNullValue(result_component_type),
                                      });
            }
        }

        // If necessary, convert the result to the signedness of the instruction
        // result type. Compare the SPIR-V image's sampled component type with the
        // component of the result type of the SPIR-V instruction.
        auto* spirv_image_type =
            parser_impl_.GetSpirvTypeForHandleOrHandleMemoryObjectDeclaration(*image);
        if (!spirv_image_type || (opcode(spirv_image_type) != spv::Op::OpTypeImage)) {
            return Fail() << "invalid image type for image memory object declaration "
                          << image->PrettyPrint();
        }
        auto* expected_component_type =
            parser_impl_.ConvertType(spirv_image_type->GetSingleWordInOperand(0));
        if (expected_component_type != result_component_type) {
            // This occurs if one is signed integer and the other is unsigned integer,
            // or vice versa. Perform a bitcast.
            value = builder_.Bitcast(Source{}, result_type->Build(builder_), call_expr);
        }
        if (!expected_component_type->Is<F32>() && IsSampledImageAccess(op)) {
            // WGSL permits sampled image access only on float textures.
            // Reject this case in the SPIR-V reader, at least until SPIR-V validation
            // catches up with this rule and can reject it earlier in the workflow.
            return Fail() << "sampled image must have float component type";
        }

        EmitConstDefOrWriteToHoistedVar(inst, {result_type, value});
    } else {
        // It's an image write. No value is returned, so make a statement out
        // of the call.
        AddStatement(builder_.CallStmt(Source{}, call_expr));
    }
    return success();
}

bool FunctionEmitter::EmitImageQuery(const spvtools::opt::Instruction& inst) {
    // TODO(dneto): Reject cases that are valid in Vulkan but invalid in WGSL.
    const spvtools::opt::Instruction* image = GetImage(inst);
    if (!image) {
        return false;
    }
    auto* texture_type = GetImageType(*image);
    if (!texture_type) {
        return false;
    }

    const auto op = opcode(inst);
    switch (op) {
        case spv::Op::OpImageQuerySize:
        case spv::Op::OpImageQuerySizeLod: {
            ExpressionList exprs;
            // Invoke textureDimensions.
            // If the texture is arrayed, combine with the result from
            // textureNumLayers.
            ExpressionList dims_args{GetImageExpression(inst)};
            if (op == spv::Op::OpImageQuerySizeLod) {
                dims_args.Push(MakeOperand(inst, 1).expr);
            }
            const ast::Expression* dims_call =
                builder_.Call(Source{}, "textureDimensions", std::move(dims_args));
            auto dims = texture_type->dims;
            if ((dims == type::TextureDimension::kCube) ||
                (dims == type::TextureDimension::kCubeArray)) {
                // textureDimension returns a 3-element vector but SPIR-V expects 2.
                dims_call =
                    create<ast::MemberAccessorExpression>(Source{}, dims_call, PrefixSwizzle(2));
            }
            exprs.Push(dims_call);
            if (ast::IsTextureArray(dims)) {
                auto num_layers =
                    builder_.Call(Source{}, "textureNumLayers", GetImageExpression(inst));
                exprs.Push(num_layers);
            }
            auto* result_type = parser_impl_.ConvertType(inst.type_id());
            auto* unsigned_type = ty_.AsUnsigned(result_type);
            TypedExpression expr = {
                unsigned_type,
                // If `exprs` holds multiple expressions, then these are the calls to
                // textureDimensions() and textureNumLayers(), and these need to be placed into a
                // vector initializer - otherwise, just emit the single expression to omit an
                // unnecessary cast.
                (exprs.Length() > 1)
                    ? builder_.Call(Source{}, unsigned_type->Build(builder_), std::move(exprs))
                    : exprs[0],
            };

            expr = ToSignedIfUnsigned(expr);

            return EmitConstDefOrWriteToHoistedVar(inst, expr);
        }
        case spv::Op::OpImageQueryLod:
            return Fail() << "WGSL does not support querying the level of detail of an image: "
                          << inst.PrettyPrint();
        case spv::Op::OpImageQueryLevels:
        case spv::Op::OpImageQuerySamples: {
            const auto* name =
                (op == spv::Op::OpImageQueryLevels) ? "textureNumLevels" : "textureNumSamples";
            const ast::Expression* ast_expr =
                builder_.Call(Source{}, name, GetImageExpression(inst));
            auto* result_type = parser_impl_.ConvertType(inst.type_id());
            // The SPIR-V result type must be integer scalar.
            // The WGSL bulitin returns u32.
            // If they aren't the same then convert the result.
            if (!result_type->Is<U32>()) {
                ast_expr =
                    builder_.Call(Source{}, result_type->Build(builder_), utils::Vector{ast_expr});
            }
            TypedExpression expr{result_type, ast_expr};
            return EmitConstDefOrWriteToHoistedVar(inst, expr);
        }
        default:
            break;
    }
    return Fail() << "unhandled image query: " << inst.PrettyPrint();
}

bool FunctionEmitter::EmitAtomicOp(const spvtools::opt::Instruction& inst) {
    auto emit_atomic = [&](sem::BuiltinType builtin, std::initializer_list<TypedExpression> args) {
        // Split args into params and expressions
        ParameterList params;
        params.Reserve(args.size());
        ExpressionList exprs;
        exprs.Reserve(args.size());
        size_t i = 0;
        for (auto& a : args) {
            params.Push(builder_.Param("p" + std::to_string(i++), a.type->Build(builder_)));
            exprs.Push(a.expr);
        }

        // Function return type
        const ast::Type* ret_type = nullptr;
        if (inst.type_id() != 0) {
            ret_type = parser_impl_.ConvertType(inst.type_id())->Build(builder_);
        } else {
            ret_type = builder_.ty.void_();
        }

        // Emit stub, will be removed by transform::SpirvAtomic
        auto sym = builder_.Symbols().New(std::string("stub_") + sem::str(builtin));
        auto* stub_deco = builder_.ASTNodes().Create<transform::SpirvAtomic::Stub>(
            builder_.ID(), builder_.AllocateNodeID(), builtin);
        auto* stub =
            create<ast::Function>(Source{}, sym, std::move(params), ret_type,
                                  /* body */ nullptr,
                                  AttributeList{
                                      stub_deco,
                                      builder_.Disable(ast::DisabledValidation::kFunctionHasNoBody),
                                  },
                                  AttributeList{});
        builder_.AST().AddFunction(stub);

        // Emit call to stub, will be replaced with call to atomic builtin by transform::SpirvAtomic
        auto* call = builder_.Call(Source{}, sym, std::move(exprs));
        if (inst.type_id() != 0) {
            auto* result_type = parser_impl_.ConvertType(inst.type_id());
            TypedExpression expr{result_type, call};
            return EmitConstDefOrWriteToHoistedVar(inst, expr);
        }
        AddStatement(builder_.CallStmt(call));

        return true;
    };

    auto oper = [&](uint32_t index) -> TypedExpression {  //
        return MakeOperand(inst, index);
    };

    auto lit = [&](int v) -> TypedExpression {
        auto* result_type = parser_impl_.ConvertType(inst.type_id());
        if (result_type->Is<I32>()) {
            return TypedExpression(result_type, builder_.Expr(i32(v)));
        } else if (result_type->Is<U32>()) {
            return TypedExpression(result_type, builder_.Expr(u32(v)));
        }
        return {};
    };

    switch (opcode(inst)) {
        case spv::Op::OpAtomicLoad:
            return emit_atomic(sem::BuiltinType::kAtomicLoad, {oper(/*ptr*/ 0)});
        case spv::Op::OpAtomicStore:
            return emit_atomic(sem::BuiltinType::kAtomicStore,
                               {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicExchange:
            return emit_atomic(sem::BuiltinType::kAtomicExchange,
                               {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicCompareExchange:
        case spv::Op::OpAtomicCompareExchangeWeak:
            return emit_atomic(sem::BuiltinType::kAtomicCompareExchangeWeak,
                               {oper(/*ptr*/ 0), /*value*/ oper(5), /*comparator*/ oper(4)});
        case spv::Op::OpAtomicIIncrement:
            return emit_atomic(sem::BuiltinType::kAtomicAdd, {oper(/*ptr*/ 0), lit(1)});
        case spv::Op::OpAtomicIDecrement:
            return emit_atomic(sem::BuiltinType::kAtomicSub, {oper(/*ptr*/ 0), lit(1)});
        case spv::Op::OpAtomicIAdd:
            return emit_atomic(sem::BuiltinType::kAtomicAdd, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicISub:
            return emit_atomic(sem::BuiltinType::kAtomicSub, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicSMin:
            return emit_atomic(sem::BuiltinType::kAtomicMin, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicUMin:
            return emit_atomic(sem::BuiltinType::kAtomicMin, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicSMax:
            return emit_atomic(sem::BuiltinType::kAtomicMax, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicUMax:
            return emit_atomic(sem::BuiltinType::kAtomicMax, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicAnd:
            return emit_atomic(sem::BuiltinType::kAtomicAnd, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicOr:
            return emit_atomic(sem::BuiltinType::kAtomicOr, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicXor:
            return emit_atomic(sem::BuiltinType::kAtomicXor, {oper(/*ptr*/ 0), oper(/*value*/ 3)});
        case spv::Op::OpAtomicFlagTestAndSet:
        case spv::Op::OpAtomicFlagClear:
        case spv::Op::OpAtomicFMinEXT:
        case spv::Op::OpAtomicFMaxEXT:
        case spv::Op::OpAtomicFAddEXT:
            return Fail() << "unsupported atomic op: " << inst.PrettyPrint();

        default:
            break;
    }
    return Fail() << "unhandled atomic op: " << inst.PrettyPrint();
}

FunctionEmitter::ExpressionList FunctionEmitter::MakeCoordinateOperandsForImageAccess(
    const spvtools::opt::Instruction& inst) {
    if (!parser_impl_.success()) {
        Fail();
        return {};
    }
    const spvtools::opt::Instruction* image = GetImage(inst);
    if (!image) {
        return {};
    }
    if (inst.NumInOperands() < 1) {
        Fail() << "image access is missing a coordinate parameter: " << inst.PrettyPrint();
        return {};
    }

    // In SPIR-V for Shader, coordinates are:
    //  - floating point for sampling, dref sampling, gather, dref gather
    //  - integral for fetch, read, write
    // In WGSL:
    //  - floating point for sampling, dref sampling, gather, dref gather
    //  - signed integral for textureLoad, textureStore
    //
    // The only conversions we have to do for WGSL are:
    //  - When the coordinates are unsigned integral, convert them to signed.
    //  - Array index is always i32

    // The coordinates parameter is always in position 1.
    TypedExpression raw_coords(MakeOperand(inst, 1));
    if (!raw_coords) {
        return {};
    }
    const Texture* texture_type = GetImageType(*image);
    if (!texture_type) {
        return {};
    }
    type::TextureDimension dim = texture_type->dims;
    // Number of regular coordinates.
    uint32_t num_axes = static_cast<uint32_t>(ast::NumCoordinateAxes(dim));
    bool is_arrayed = ast::IsTextureArray(dim);
    if ((num_axes == 0) || (num_axes > 3)) {
        Fail() << "unsupported image dimensionality for " << texture_type->TypeInfo().name
               << " prompted by " << inst.PrettyPrint();
    }
    bool is_proj = false;
    switch (opcode(inst)) {
        case spv::Op::OpImageSampleProjImplicitLod:
        case spv::Op::OpImageSampleProjExplicitLod:
        case spv::Op::OpImageSampleProjDrefImplicitLod:
        case spv::Op::OpImageSampleProjDrefExplicitLod:
            is_proj = true;
            break;
        default:
            break;
    }

    const auto num_coords_required = num_axes + (is_arrayed ? 1 : 0) + (is_proj ? 1 : 0);
    uint32_t num_coords_supplied = 0;
    // Get the component type.  The raw_coords might have been hoisted into
    // a 'var' declaration, so unwrap the referenece if needed.
    auto* component_type = raw_coords.type->UnwrapRef();
    if (component_type->IsFloatScalar() || component_type->IsIntegerScalar()) {
        num_coords_supplied = 1;
    } else if (auto* vec_type = As<Vector>(component_type)) {
        component_type = vec_type->type;
        num_coords_supplied = vec_type->size;
    }
    if (num_coords_supplied == 0) {
        Fail() << "bad or unsupported coordinate type for image access: " << inst.PrettyPrint();
        return {};
    }
    if (num_coords_required > num_coords_supplied) {
        Fail() << "image access required " << num_coords_required
               << " coordinate components, but only " << num_coords_supplied
               << " provided, in: " << inst.PrettyPrint();
        return {};
    }

    ExpressionList result;

    // Generates the expression for the WGSL coordinates, when it is a prefix
    // swizzle with num_axes.  If the result would be unsigned, also converts
    // it to a signed value of the same shape (scalar or vector).
    // Use a lambda to make it easy to only generate the expressions when we
    // will actually use them.
    auto prefix_swizzle_expr = [this, num_axes, component_type, is_proj,
                                raw_coords]() -> const ast::Expression* {
        auto* swizzle_type =
            (num_axes == 1) ? component_type : ty_.Vector(component_type, num_axes);
        auto* swizzle = create<ast::MemberAccessorExpression>(Source{}, raw_coords.expr,
                                                              PrefixSwizzle(num_axes));
        if (is_proj) {
            auto* q =
                create<ast::MemberAccessorExpression>(Source{}, raw_coords.expr, Swizzle(num_axes));
            auto* proj_div = builder_.Div(swizzle, q);
            return ToSignedIfUnsigned({swizzle_type, proj_div}).expr;
        } else {
            return ToSignedIfUnsigned({swizzle_type, swizzle}).expr;
        }
    };

    if (is_arrayed) {
        // The source must be a vector. It has at least one coordinate component
        // and it must have an array component.  Use a vector swizzle to get the
        // first `num_axes` components.
        result.Push(prefix_swizzle_expr());

        // Now get the array index.
        const ast::Expression* array_index =
            builder_.MemberAccessor(raw_coords.expr, Swizzle(num_axes));
        if (component_type->IsFloatScalar()) {
            // When converting from a float array layer to integer, Vulkan requires
            // round-to-nearest, with preference for round-to-nearest-even.
            // But i32(f32) in WGSL has unspecified rounding mode, so we have to
            // explicitly specify the rounding.
            array_index = builder_.Call("round", array_index);
        }
        // Convert it to a signed integer type, if needed.
        result.Push(ToI32({component_type, array_index}).expr);
    } else {
        if (num_coords_supplied == num_coords_required && !is_proj) {
            // Pass the value through, with possible unsigned->signed conversion.
            result.Push(ToSignedIfUnsigned(raw_coords).expr);
        } else {
            // There are more coordinates supplied than needed. So the source type
            // is a vector. Use a vector swizzle to get the first `num_axes`
            // components.
            result.Push(prefix_swizzle_expr());
        }
    }
    return result;
}

const ast::Expression* FunctionEmitter::ConvertTexelForStorage(
    const spvtools::opt::Instruction& inst,
    TypedExpression texel,
    const Texture* texture_type) {
    auto* storage_texture_type = As<StorageTexture>(texture_type);
    auto* src_type = texel.type->UnwrapRef();
    if (!storage_texture_type) {
        Fail() << "writing to other than storage texture: " << inst.PrettyPrint();
        return nullptr;
    }
    const auto format = storage_texture_type->format;
    auto* dest_type = parser_impl_.GetTexelTypeForFormat(format);
    if (!dest_type) {
        Fail();
        return nullptr;
    }

    // The texel type is always a 4-element vector.
    const uint32_t dest_count = 4u;
    TINT_ASSERT(Reader, dest_type->Is<Vector>() && dest_type->As<Vector>()->size == dest_count);
    TINT_ASSERT(Reader, dest_type->IsFloatVector() || dest_type->IsUnsignedIntegerVector() ||
                            dest_type->IsSignedIntegerVector());

    if (src_type == dest_type) {
        return texel.expr;
    }

    // Component type must match floatness, or integral signedness.
    if ((src_type->IsFloatScalarOrVector() != dest_type->IsFloatVector()) ||
        (src_type->IsUnsignedScalarOrVector() != dest_type->IsUnsignedIntegerVector()) ||
        (src_type->IsSignedScalarOrVector() != dest_type->IsSignedIntegerVector())) {
        Fail() << "invalid texel type for storage texture write: component must be "
                  "float, signed integer, or unsigned integer "
                  "to match the texture channel type: "
               << inst.PrettyPrint();
        return nullptr;
    }

    const auto required_count = parser_impl_.GetChannelCountForFormat(format);
    TINT_ASSERT(Reader, 0 < required_count && required_count <= 4);

    const uint32_t src_count = src_type->IsScalar() ? 1 : src_type->As<Vector>()->size;
    if (src_count < required_count) {
        Fail() << "texel has too few components for storage texture: " << src_count
               << " provided but " << required_count << " required, in: " << inst.PrettyPrint();
        return nullptr;
    }

    // It's valid for required_count < src_count. The extra components will
    // be written out but the textureStore will ignore them.

    if (src_count < dest_count) {
        // Expand the texel to a 4 element vector.
        auto* component_type = src_type->IsScalar() ? src_type : src_type->As<Vector>()->type;
        src_type = ty_.Vector(component_type, dest_count);
        ExpressionList exprs;
        exprs.Push(texel.expr);
        for (auto i = src_count; i < dest_count; i++) {
            exprs.Push(parser_impl_.MakeNullExpression(component_type).expr);
        }
        texel.expr = builder_.Call(Source{}, src_type->Build(builder_), std::move(exprs));
    }

    return texel.expr;
}

TypedExpression FunctionEmitter::ToI32(TypedExpression value) {
    if (!value || value.type->Is<I32>()) {
        return value;
    }
    return {ty_.I32(), builder_.Call(Source{}, builder_.ty.i32(), utils::Vector{value.expr})};
}

TypedExpression FunctionEmitter::ToSignedIfUnsigned(TypedExpression value) {
    if (!value || !value.type->IsUnsignedScalarOrVector()) {
        return value;
    }
    if (auto* vec_type = value.type->As<Vector>()) {
        auto* new_type = ty_.Vector(ty_.I32(), vec_type->size);
        return {new_type, builder_.Call(new_type->Build(builder_), utils::Vector{value.expr})};
    }
    return ToI32(value);
}

TypedExpression FunctionEmitter::MakeArrayLength(const spvtools::opt::Instruction& inst) {
    if (inst.NumInOperands() != 2) {
        // Binary parsing will fail on this anyway.
        Fail() << "invalid array length: requires 2 operands: " << inst.PrettyPrint();
        return {};
    }
    const auto struct_ptr_id = inst.GetSingleWordInOperand(0);
    const auto field_index = inst.GetSingleWordInOperand(1);
    const auto struct_ptr_type_id = def_use_mgr_->GetDef(struct_ptr_id)->type_id();
    // Trace through the pointer type to get to the struct type.
    const auto struct_type_id = def_use_mgr_->GetDef(struct_ptr_type_id)->GetSingleWordInOperand(1);
    const auto field_name = namer_.GetMemberName(struct_type_id, field_index);
    if (field_name.empty()) {
        Fail() << "struct index out of bounds for array length: " << inst.PrettyPrint();
        return {};
    }

    auto member_expr = MakeExpression(struct_ptr_id);
    if (!member_expr) {
        return {};
    }
    if (member_expr.type->Is<Pointer>()) {
        member_expr = Dereference(member_expr);
    }
    auto* member_access = builder_.MemberAccessor(Source{}, member_expr.expr, field_name);

    // Generate the builtin function call.
    auto* call_expr = builder_.Call(Source{}, "arrayLength", builder_.AddressOf(member_access));

    return {parser_impl_.ConvertType(inst.type_id()), call_expr};
}

TypedExpression FunctionEmitter::MakeOuterProduct(const spvtools::opt::Instruction& inst) {
    // Synthesize the result.
    auto col = MakeOperand(inst, 0);
    auto row = MakeOperand(inst, 1);
    auto* col_ty = As<Vector>(col.type);
    auto* row_ty = As<Vector>(row.type);
    auto* result_ty = As<Matrix>(parser_impl_.ConvertType(inst.type_id()));
    if (!col_ty || !col_ty || !result_ty || result_ty->type != col_ty->type ||
        result_ty->type != row_ty->type || result_ty->columns != row_ty->size ||
        result_ty->rows != col_ty->size) {
        Fail() << "invalid outer product instruction: bad types " << inst.PrettyPrint();
        return {};
    }

    // Example:
    //    c : vec3 column vector
    //    r : vec2 row vector
    //    OuterProduct c r : mat2x3 (2 columns, 3 rows)
    //    Result:
    //      | c.x * r.x   c.x * r.y |
    //      | c.y * r.x   c.y * r.y |
    //      | c.z * r.x   c.z * r.y |

    ExpressionList result_columns;
    for (uint32_t icol = 0; icol < result_ty->columns; icol++) {
        ExpressionList result_row;
        auto* row_factor = create<ast::MemberAccessorExpression>(Source{}, row.expr, Swizzle(icol));
        for (uint32_t irow = 0; irow < result_ty->rows; irow++) {
            auto* column_factor =
                create<ast::MemberAccessorExpression>(Source{}, col.expr, Swizzle(irow));
            auto* elem = create<ast::BinaryExpression>(Source{}, ast::BinaryOp::kMultiply,
                                                       row_factor, column_factor);
            result_row.Push(elem);
        }
        result_columns.Push(
            builder_.Call(Source{}, col_ty->Build(builder_), std::move(result_row)));
    }
    return {result_ty,
            builder_.Call(Source{}, result_ty->Build(builder_), std::move(result_columns))};
}

bool FunctionEmitter::MakeVectorInsertDynamic(const spvtools::opt::Instruction& inst) {
    // For
    //    %result = OpVectorInsertDynamic %type %src_vector %component %index
    // there are two cases.
    //
    // Case 1:
    //   The %src_vector value has already been hoisted into a variable.
    //   In this case, assign %src_vector to that variable, then write the
    //   component into the right spot:
    //
    //    hoisted = src_vector;
    //    hoisted[index] = component;
    //
    // Case 2:
    //   The %src_vector value is not hoisted. In this case, make a temporary
    //   variable with the %src_vector contents, then write the component,
    //   and then make a let-declaration that reads the value out:
    //
    //    var temp : type = src_vector;
    //    temp[index] = component;
    //    let result : type = temp;
    //
    //   Then use result everywhere the original SPIR-V id is used.  Using a const
    //   like this avoids constantly reloading the value many times.

    auto* type = parser_impl_.ConvertType(inst.type_id());
    auto src_vector = MakeOperand(inst, 0);
    auto component = MakeOperand(inst, 1);
    auto index = MakeOperand(inst, 2);

    std::string var_name;
    auto original_value_name = namer_.Name(inst.result_id());
    const bool hoisted = WriteIfHoistedVar(inst, src_vector);
    if (hoisted) {
        // The variable was already declared in an earlier block.
        var_name = original_value_name;
        // Assign the source vector value to it.
        builder_.Assign({}, builder_.Expr(var_name), src_vector.expr);
    } else {
        // Synthesize the temporary variable.
        // It doesn't correspond to a SPIR-V ID, so we don't use the ordinary
        // API in parser_impl_.
        var_name = namer_.MakeDerivedName(original_value_name);

        auto* temp_var = builder_.Var(var_name, type->Build(builder_), type::AddressSpace::kNone,
                                      src_vector.expr);

        AddStatement(builder_.Decl({}, temp_var));
    }

    auto* lhs = create<ast::IndexAccessorExpression>(Source{}, builder_.Expr(var_name), index.expr);
    if (!lhs) {
        return false;
    }

    AddStatement(builder_.Assign(lhs, component.expr));

    if (hoisted) {
        // The hoisted variable itself stands for this result ID.
        return success();
    }
    // Create a new let-declaration that is initialized by the contents
    // of the temporary variable.
    return EmitConstDefinition(inst, {type, builder_.Expr(var_name)});
}

bool FunctionEmitter::MakeCompositeInsert(const spvtools::opt::Instruction& inst) {
    // For
    //    %result = OpCompositeInsert %type %object %composite 1 2 3 ...
    // there are two cases.
    //
    // Case 1:
    //   The %composite value has already been hoisted into a variable.
    //   In this case, assign %composite to that variable, then write the
    //   component into the right spot:
    //
    //    hoisted = composite;
    //    hoisted[index].x = object;
    //
    // Case 2:
    //   The %composite value is not hoisted. In this case, make a temporary
    //   variable with the %composite contents, then write the component,
    //   and then make a let-declaration that reads the value out:
    //
    //    var temp : type = composite;
    //    temp[index].x = object;
    //    let result : type = temp;
    //
    //   Then use result everywhere the original SPIR-V id is used.  Using a const
    //   like this avoids constantly reloading the value many times.
    //
    //   This technique is a combination of:
    //   - making a temporary variable and constant declaration, like what we do
    //     for VectorInsertDynamic, and
    //   - building up an access-chain like access like for CompositeExtract, but
    //     on the left-hand side of the assignment.

    auto* type = parser_impl_.ConvertType(inst.type_id());
    auto component = MakeOperand(inst, 0);
    auto src_composite = MakeOperand(inst, 1);

    std::string var_name;
    auto original_value_name = namer_.Name(inst.result_id());
    const bool hoisted = WriteIfHoistedVar(inst, src_composite);
    if (hoisted) {
        // The variable was already declared in an earlier block.
        var_name = original_value_name;
        // Assign the source composite value to it.
        builder_.Assign({}, builder_.Expr(var_name), src_composite.expr);
    } else {
        // Synthesize a temporary variable.
        // It doesn't correspond to a SPIR-V ID, so we don't use the ordinary
        // API in parser_impl_.
        var_name = namer_.MakeDerivedName(original_value_name);
        auto* temp_var = builder_.Var(var_name, type->Build(builder_), type::AddressSpace::kNone,
                                      src_composite.expr);
        AddStatement(builder_.Decl({}, temp_var));
    }

    TypedExpression seed_expr{type, builder_.Expr(var_name)};

    // The left-hand side of the assignment *looks* like a decomposition.
    TypedExpression lhs = MakeCompositeValueDecomposition(inst, seed_expr, inst.type_id(), 2);
    if (!lhs) {
        return false;
    }

    AddStatement(builder_.Assign(lhs.expr, component.expr));

    if (hoisted) {
        // The hoisted variable itself stands for this result ID.
        return success();
    }
    // Create a new let-declaration that is initialized by the contents
    // of the temporary variable.
    return EmitConstDefinition(inst, {type, builder_.Expr(var_name)});
}

TypedExpression FunctionEmitter::AddressOf(TypedExpression expr) {
    auto* ref = expr.type->As<Reference>();
    if (!ref) {
        Fail() << "AddressOf() called on non-reference type";
        return {};
    }
    return {
        ty_.Pointer(ref->type, ref->address_space),
        create<ast::UnaryOpExpression>(Source{}, ast::UnaryOp::kAddressOf, expr.expr),
    };
}

TypedExpression FunctionEmitter::Dereference(TypedExpression expr) {
    auto* ptr = expr.type->As<Pointer>();
    if (!ptr) {
        Fail() << "Dereference() called on non-pointer type";
        return {};
    }
    return {
        ptr->type,
        create<ast::UnaryOpExpression>(Source{}, ast::UnaryOp::kIndirection, expr.expr),
    };
}

bool FunctionEmitter::IsFloatZero(uint32_t value_id) {
    if (const auto* c = constant_mgr_->FindDeclaredConstant(value_id)) {
        if (const auto* float_const = c->AsFloatConstant()) {
            return 0.0f == float_const->GetFloatValue();
        }
        if (c->AsNullConstant()) {
            // Valid SPIR-V requires it to be a float value anyway.
            return true;
        }
    }
    return false;
}

bool FunctionEmitter::IsFloatOne(uint32_t value_id) {
    if (const auto* c = constant_mgr_->FindDeclaredConstant(value_id)) {
        if (const auto* float_const = c->AsFloatConstant()) {
            return 1.0f == float_const->GetFloatValue();
        }
    }
    return false;
}

FunctionEmitter::FunctionDeclaration::FunctionDeclaration() = default;
FunctionEmitter::FunctionDeclaration::~FunctionDeclaration() = default;

}  // namespace tint::reader::spirv

TINT_INSTANTIATE_TYPEINFO(tint::reader::spirv::StatementBuilder);
TINT_INSTANTIATE_TYPEINFO(tint::reader::spirv::SwitchStatementBuilder);
TINT_INSTANTIATE_TYPEINFO(tint::reader::spirv::IfStatementBuilder);
TINT_INSTANTIATE_TYPEINFO(tint::reader::spirv::LoopStatementBuilder);