src/tint/lang/spirv/reader/ast_parser/function.h

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#ifndef SRC_TINT_LANG_SPIRV_READER_AST_PARSER_FUNCTION_H_
#define SRC_TINT_LANG_SPIRV_READER_AST_PARSER_FUNCTION_H_

#include <limits>
#include <memory>
#include <optional>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#include "src/tint/lang/spirv/reader/ast_parser/ast_parser.h"
#include "src/tint/lang/spirv/reader/ast_parser/attributes.h"
#include "src/tint/lang/spirv/reader/ast_parser/construct.h"
#include "src/tint/lang/wgsl/program/program_builder.h"
#include "src/tint/utils/text/string_stream.h"

namespace tint::spirv::reader::ast_parser {

/// Kinds of CFG edges.
//
// The edge kinds are used in many ways.
//
// For example, consider the edges leaving a basic block and going to distinct targets. If the
// total number of kForward + kIfBreak + kCaseFallThrough edges is more than 1, then the block must
// be a structured header, i.e. it needs a merge instruction to declare the control flow divergence
// and associated reconvergence point.  Those those edge kinds count toward divergence because
// SPIR-V is designed to easily map back to structured control flow in GLSL (and C).  In GLSL and C,
// those forward-flow edges don't have a special statement to express them.  The other forward
// edges: kSwitchBreak, kLoopBreak, and kLoopContinue directly map to 'break', 'break', and
// 'continue', respectively.
enum class EdgeKind {
    // A back-edge: An edge from a node to one of its ancestors in a depth-first
    // search from the entry block.
    kBack,
    // An edge from a node to the merge block of the nearest enclosing switch,
    // where there is no intervening loop.
    kSwitchBreak,
    // An edge from a node to the merge block of the nearest enclosing loop, where
    // there is no intervening switch.
    // The source block is a "break block" as defined by SPIR-V.
    kLoopBreak,
    // An edge from a node in a loop body to the associated continue target, where
    // there are no other intervening loops or switches.
    // The source block is a "continue block" as defined by SPIR-V.
    kLoopContinue,
    // An edge from a node to the merge block of the nearest enclosing structured
    // construct, but which is neither a kSwitchBreak or a kLoopBreak.
    // This can only occur for an "if" selection, i.e. where the selection
    // header ends in OpBranchConditional.
    kIfBreak,
    // An edge from one switch case to the next sibling switch case. Note, this is not valid in WGSL
    // at the moment and will trigger an ICE if encountered. It is here for completeness.
    kCaseFallThrough,
    // None of the above.
    kForward
};

/// The number used to represent an invalid block position
static constexpr uint32_t kInvalidBlockPos = ~0u;

/// Bookkeeping info for a basic block.
struct BlockInfo {
    /// Constructor
    /// @param bb internal representation of the basic block
    explicit BlockInfo(const spvtools::opt::BasicBlock& bb);
    ~BlockInfo();

    /// The internal representation of the basic block.
    const spvtools::opt::BasicBlock* basic_block;

    /// The ID of the OpLabel instruction that starts this block.
    uint32_t id = 0;

    /// The position of this block in the reverse structured post-order.
    /// If the block is not in that order, then this remains the invalid value.
    uint32_t pos = kInvalidBlockPos;

    /// If this block is a header, then this is the ID of the merge block.
    uint32_t merge_for_header = 0;
    /// If this block is a loop header, then this is the ID of the continue
    /// target.
    uint32_t continue_for_header = 0;
    /// If this block is a merge, then this is the ID of the header.
    uint32_t header_for_merge = 0;
    /// If this block is a continue target, then this is the ID of the loop
    /// header.
    uint32_t header_for_continue = 0;
    /// Is this block a continue target which is its own loop header block?
    /// In this case the continue construct is the entire loop.  The associated
    /// "loop construct" is empty, and not represented.
    bool is_continue_entire_loop = false;

    /// The immediately enclosing structured construct. If this block is not
    /// in the block order at all, then this is still nullptr.
    const Construct* construct = nullptr;

    /// Maps the ID of a successor block (in the CFG) to its edge classification.
    std::unordered_map<uint32_t, EdgeKind> succ_edge;

    /// The following fields record relationships among blocks in a selection
    /// construct for an OpSwitch instruction.

    /// If not null, then the pointed-at construct is a selection for an OpSwitch,
    /// and this block is a case target for it.  We say this block "heads" the
    /// case construct.
    const Construct* case_head_for = nullptr;
    /// If not null, then the pointed-at construct is a selection for an OpSwitch,
    /// and this block is the default target for it.  We say this block "heads"
    /// the default case construct.
    const Construct* default_head_for = nullptr;
    /// Is this a default target for a switch, and is it also the merge for its
    /// switch?
    bool default_is_merge = false;
    /// The list of switch values that cause a branch to this block.
    std::optional<tint::Vector<uint64_t, 4>> case_values;

    /// The following fields record relationships among blocks in a selection
    /// construct for an OpBranchConditional instruction.

    /// When this block is an if-selection header, this is the edge kind
    /// for the true branch.
    EdgeKind true_kind = EdgeKind::kForward;
    /// When this block is an if-selection header, this is the edge kind
    /// for the false branch.
    EdgeKind false_kind = EdgeKind::kForward;
    /// If not 0, then this block is an if-selection header, and `true_head` is
    /// the target id of the true branch on the OpBranchConditional, and that
    /// target is inside the if-selection.
    uint32_t true_head = 0;
    /// If not 0, then this block is an if-selection header, and `false_head`
    /// is the target id of the false branch on the OpBranchConditional, and
    /// that target is inside the if-selection.
    uint32_t false_head = 0;
    /// If not 0, then this block is an if-selection header, and when following
    /// the flow via the true and false branches, control first reconverges at
    /// the block with ID `premerge_head`, and `premerge_head` is still inside
    /// the if-selection.
    uint32_t premerge_head = 0;
    /// If non-empty, then this block is an if-selection header, and control flow
    /// in the body must be guarded by a boolean flow variable with this name.
    /// This occurs when a block in this selection has both an if-break edge, and
    /// also a different normal forward edge but without a merge instruction.
    std::string flow_guard_name = "";

    /// The result IDs that this block is responsible for declaring as a
    /// hoisted variable.
    /// @see DefInfo#requires_hoisted_var_def
    tint::Vector<uint32_t, 4> hoisted_ids;

    /// A PhiAssignment represents the assignment of a value to the state
    /// variable associated with an OpPhi in a successor block.
    struct PhiAssignment {
        /// The ID of an OpPhi receiving a value from this basic block.
        uint32_t phi_id;
        /// The ID of the value carried to the given OpPhi.
        uint32_t value_id;
    };
    /// If this basic block branches to a visited basic block containing phis,
    /// then this is the list of writes to the variables associated those phis.
    tint::Vector<PhiAssignment, 4> phi_assignments;
    /// The IDs of OpPhi instructions which require their associated state
    /// variable to be declared in this basic block.
    tint::Vector<uint32_t, 4> phis_needing_state_vars;
};

/// Writes the BlockInfo to the stream
/// @param o the stream
/// @param bi the BlockInfo
/// @returns the stream so calls can be chained
template <typename STREAM, typename = traits::EnableIfIsOStream<STREAM>>
auto& operator<<(STREAM& o, const BlockInfo& bi) {
    o << "BlockInfo{"
      << " id: " << bi.id << " pos: " << bi.pos << " merge_for_header: " << bi.merge_for_header
      << " continue_for_header: " << bi.continue_for_header
      << " header_for_merge: " << bi.header_for_merge
      << " is_continue_entire_loop: " << int(bi.is_continue_entire_loop) << "}";
    return o;
}

/// Reasons for avoiding generating an intermediate value.
enum class SkipReason {
    /// `kDontSkip`: The value should be generated. Used for most values.
    kDontSkip,

    /// For remaining cases, the value is not generated.

    /// `kOpaqueObject`: used for any intermediate value which is an sampler,
    /// image,
    /// or sampled image, or any pointer to such object. Code is generated
    /// for those objects only when emitting the image instructions that access
    /// the image (read, write, sample, gather, fetch, or query). For example,
    /// when encountering an OpImageSampleExplicitLod, a call to the
    /// textureSampleLevel builtin function will be emitted, and the call will
    /// directly reference the underlying texture and sampler (variable or
    /// function parameter).
    kOpaqueObject,

    /// `kSinkPointerIntoUse`: used to avoid emitting certain pointer expressions,
    /// by instead generating their reference expression directly at the point of
    /// use. For example, we apply this to OpAccessChain when indexing into a
    /// vector, to avoid generating address-of vector component expressions.
    kSinkPointerIntoUse,

    /// `kPointSizeBuiltinPointer`: the value is a pointer to the Position builtin
    /// variable.  Don't generate its address.  Avoid generating stores to this
    /// pointer.
    kPointSizeBuiltinPointer,
    /// `kPointSizeBuiltinValue`: the value is the value loaded from the
    /// PointSize builtin. Use 1.0f instead, because that's the only value
    /// supported by WebGPU.
    kPointSizeBuiltinValue,

    /// `kSampleMaskInBuiltinPointer`: the value is a pointer to the SampleMaskIn
    /// builtin input variable.  Don't generate its address.
    kSampleMaskInBuiltinPointer,

    /// `kSampleMaskOutBuiltinPointer`: the value is a pointer to the SampleMask
    /// builtin output variable.
    kSampleMaskOutBuiltinPointer,
};

/// Bookkeeping info for a SPIR-V ID defined in the function, or some
/// module-scope variables. This will be valid for result IDs that are:
/// - defined in the function and:
///    - instructions that are not OpLabel, and not OpFunctionParameter
///    - are defined in a basic block visited in the block-order for the
///    function.
/// - certain module-scope builtin variables.
struct DefInfo {
    /// Constructor for a locally defined value.
    /// @param index an ordering index for uniqueness.
    /// @param def_inst the SPIR-V instruction defining the ID
    /// @param block_pos the position of the first basic block dominated by the
    ///   definition
    DefInfo(size_t index, const spvtools::opt::Instruction& def_inst, uint32_t block_pos);
    /// Constructor for a value defined at module scope.
    /// @param index an ordering index for uniqueness.
    /// @param def_inst the SPIR-V instruction defining the ID
    DefInfo(size_t index, const spvtools::opt::Instruction& def_inst);

    /// Destructor.
    ~DefInfo();

    /// An index for uniquely and deterministically ordering all DefInfo records
    /// in a function.
    const size_t index = 0;

    /// The SPIR-V instruction that defines the ID.
    const spvtools::opt::Instruction& inst;

    /// Information about a definition created inside a function.
    struct Local {
        /// Constructor.
        /// @param block_pos the position of the basic block defining the value.
        explicit Local(uint32_t block_pos);
        /// Copy constructor.
        /// @param other the original object to copy from.
        Local(const Local& other);
        /// Destructor.
        ~Local();

        /// The position of the basic block defininig the value, in function
        /// block order.
        /// See method `FunctionEmitter::ComputeBlockOrderAndPositions` for block
        /// ordering.
        const uint32_t block_pos = 0;

        /// The number of uses of this ID.
        uint32_t num_uses = 0;

        /// The block position of the first use of this ID, or MAX_UINT if it is not
        /// used at all.  The "first" ordering is determined by the function block
        /// order.  The first use of an ID might be in an OpPhi that precedes the
        /// definition of the ID.
        /// The ID defined by an OpPhi is counted as being "used" in each of its
        /// parent blocks.
        uint32_t first_use_pos = std::numeric_limits<uint32_t>::max();
        /// The block position of the last use of this ID, or 0 if it is not used
        /// at all.  The "last" ordering is determined by the function block order.
        /// The ID defined by an OpPhi is counted as being "used" in each of its
        /// parent blocks.
        uint32_t last_use_pos = 0;

        /// Is this value used in a construct other than the one in which it was
        /// defined?
        bool used_in_another_construct = false;
        /// Is this ID an OpPhi?
        bool is_phi = false;
    };

    /// Information about a definition inside the function. Populated if and only
    /// if the definition actually is inside the function.
    std::optional<Local> local;

    /// True if this ID requires a WGSL 'let' definition, due to context. It
    /// might get one anyway (so this is *not* an if-and-only-if condition).
    bool requires_named_let_def = false;

    /// True if this ID must map to a WGSL variable declaration before the
    /// corresponding position of the ID definition in SPIR-V.  This compensates
    /// for the difference between dominance and scoping. An SSA definition can
    /// dominate all its uses, but the construct where it is defined does not
    /// enclose all the uses, and so if it were declared as a WGSL let-
    /// declaration at the point of its SPIR-V definition, then the WGSL name
    /// would go out of scope too early. Fix that by creating a variable at the
    /// top of the smallest construct that encloses both the definition and all
    /// its uses. Then the original SPIR-V definition maps to a WGSL assignment
    /// to that variable, and each SPIR-V use becomes a WGSL read from the
    /// variable.
    /// TODO(dneto): This works for constants of storable type, but not, for
    /// example, pointers. crbug.com/tint/98
    bool requires_hoisted_var_def = false;

    /// Information about a pointer value, used to construct its WGSL type.
    struct Pointer {
        /// The address space to use for this value, if it is of pointer type.
        /// This is required to carry an address space override from a storage
        /// buffer expressed in the old style (with Uniform address space)
        /// that needs to be remapped to StorageBuffer address space.
        /// This is kInvalid for non-pointers.
        core::AddressSpace address_space = core::AddressSpace::kUndefined;

        /// The declared access mode.
        core::Access access = core::Access::kUndefined;
    };

    /// The expression to use when sinking pointers into their use.
    /// When encountering a use of this instruction, we will emit this expression
    /// instead.
    TypedExpression sink_pointer_source_expr = {};

    /// Collected information about a pointer value.
    Pointer pointer;

    /// The reason, if any, that this value should be ignored.
    /// Normally no values are ignored.  This field can be updated while
    /// generating code because sometimes we only discover necessary facts
    /// in the middle of generating code.
    SkipReason skip = SkipReason::kDontSkip;
};

/// Writes the DefInfo to the stream
/// @param o the stream
/// @param di the DefInfo
/// @returns the stream so calls can be chained
template <typename STREAM, typename = traits::EnableIfIsOStream<STREAM>>
auto& operator<<(STREAM& o, const DefInfo& di) {
    o << "DefInfo{"
      << " inst.result_id: " << di.inst.result_id();
    if (di.local.has_value()) {
        const auto& dil = di.local.value();
        o << " block_pos: " << dil.block_pos << " num_uses: " << dil.num_uses
          << " first_use_pos: " << dil.first_use_pos << " last_use_pos: " << dil.last_use_pos
          << " used_in_another_construct: " << (dil.used_in_another_construct ? "true" : "false")
          << " is_phi: " << (dil.is_phi ? "true" : "false") << "";
    }
    o << " requires_named_let_def: " << (di.requires_named_let_def ? "true" : "false")
      << " requires_hoisted_var_def: " << (di.requires_hoisted_var_def ? "true" : "false");
    if (di.pointer.address_space != core::AddressSpace::kUndefined) {
        o << " sc:" << int(di.pointer.address_space);
    }
    switch (di.skip) {
        case SkipReason::kDontSkip:
            break;
        case SkipReason::kOpaqueObject:
            o << " skip:opaque";
            break;
        case SkipReason::kSinkPointerIntoUse:
            o << " skip:sink_pointer";
            break;
        case SkipReason::kPointSizeBuiltinPointer:
            o << " skip:pointsize_pointer";
            break;
        case SkipReason::kPointSizeBuiltinValue:
            o << " skip:pointsize_value";
            break;
        case SkipReason::kSampleMaskInBuiltinPointer:
            o << " skip:samplemaskin_pointer";
            break;
        case SkipReason::kSampleMaskOutBuiltinPointer:
            o << " skip:samplemaskout_pointer";
            break;
    }
    o << "}";
    return o;
}

/// A placeholder Statement that exists for the duration of building a
/// StatementBlock. Once the StatementBlock is built, Build() will be called to
/// construct the final AST node, which will be used in the place of this
/// StatementBuilder.
/// StatementBuilders are used to simplify construction of AST nodes that will
/// become immutable. The builders may hold mutable state while the
/// StatementBlock is being constructed, which becomes an immutable node on
/// StatementBlock::Finalize().
class StatementBuilder : public Castable<StatementBuilder, ast::Statement> {
  public:
    /// Constructor
    StatementBuilder() : Base(GenerationID(), ast::NodeID(), Source{}) {}

    /// @param builder the program builder
    /// @returns the build AST node
    virtual const ast::Statement* Build(ProgramBuilder* builder) const = 0;

  private:
    Node* Clone(ast::CloneContext&) const override;
};

/// A FunctionEmitter emits a SPIR-V function onto a Tint AST module.
class FunctionEmitter {
    using StructMemberList = tint::Vector<const ast::StructMember*, 8>;
    using ExpressionList = tint::Vector<const ast::Expression*, 8>;
    using ParameterList = tint::Vector<const ast::Parameter*, 8>;
    using StatementList = tint::Vector<const ast::Statement*, 8>;

  public:
    /// Creates a FunctionEmitter, and prepares to write to the AST module
    /// in `pi`
    /// @param pi a ASTParser which has already executed BuildInternalModule
    /// @param function the function to emit
    FunctionEmitter(ASTParser* pi, const spvtools::opt::Function& function);
    /// Creates a FunctionEmitter, and prepares to write to the AST module
    /// in `pi`
    /// @param pi a ASTParser which has already executed BuildInternalModule
    /// @param function the function to emit
    /// @param ep_info entry point information for this function, or nullptr
    FunctionEmitter(ASTParser* pi,
                    const spvtools::opt::Function& function,
                    const EntryPointInfo* ep_info);
    /// Move constructor. Only valid when the other object was newly created.
    /// @param other the emitter to clone
    FunctionEmitter(FunctionEmitter&& other);
    /// Destructor
    ~FunctionEmitter();

    /// Emits the function to AST module.
    /// @return whether emission succeeded
    bool Emit();

    /// @returns true if emission has not yet failed.
    bool success() const { return fail_stream_.status(); }
    /// @returns true if emission has failed.
    bool failed() const { return !success(); }

    /// Finalizes any StatementBuilders returns the body of the function.
    /// Must only be called once, and to be used only for testing.
    /// @returns the body of the function.
    StatementList ast_body();

    /// Records failure.
    /// @returns a FailStream on which to emit diagnostics.
    FailStream& Fail() { return fail_stream_.Fail(); }

    /// @returns the parser implementation
    ASTParser* parser() { return &parser_impl_; }

    /// Emits the entry point as a wrapper around its implementation function.
    /// Pipeline inputs become formal parameters, and pipeline outputs become
    /// return values.
    /// @returns false if emission failed.
    bool EmitEntryPointAsWrapper();

    /// Creates one or more entry point input parameters corresponding to a part of an input
    /// variable.  The part of the input variable is specfied by the `index_prefix`, which
    /// successively indexes into the variable. Also generates the assignment statements that copy
    /// the input parameter to the corresponding part of the variable.  Assumes the variable has
    /// already been created in the Private address space.
    /// @param var_name The name of the variable
    /// @param var_type The store type of the variable
    /// @param attributes The variable's attributes
    /// @param index_prefix Indices stepping into the variable, indicating what part of the variable
    /// to populate.
    /// @param tip_type The type of the component inside variable, after indexing with the indices
    /// in `index_prefix`.
    /// @param forced_param_type The type forced by WGSL, if the variable is a builtin, otherwise
    /// the same as var_type.
    /// @param params The parameter list where the new parameter is appended.
    /// @param statements The statement list where the assignment is appended.
    /// @returns false if emission failed
    bool EmitPipelineInput(std::string var_name,
                           const Type* var_type,
                           tint::Vector<int, 8> index_prefix,
                           const Type* tip_type,
                           const Type* forced_param_type,
                           Attributes& attributes,
                           ParameterList& params,
                           StatementList& statements);

    /// Creates one or more struct members from an output variable, and the
    /// expressions that compute the value they contribute to the entry point
    /// return value.  The part of the output variable is specfied
    /// by the `index_prefix`, which successively indexes into the variable.
    /// Assumes the variable has already been created in the Private address space
    /// @param var_name The name of the variable
    /// @param var_type The store type of the variable
    /// @param index_prefix Indices stepping into the variable, indicating what part of the variable
    /// to populate.
    /// @param tip_type The type of the component inside variable, after indexing with the indices
    /// in `index_prefix`.
    /// @param forced_member_type The type forced by WGSL, if the variable is a builtin, otherwise
    /// the same as var_type.
    /// @param attributes The variable's attributes
    /// @param return_members The struct member list where the new member is added.
    /// @param return_exprs The expression list where the return expression is added.
    /// @returns false if emission failed
    bool EmitPipelineOutput(std::string var_name,
                            const Type* var_type,
                            tint::Vector<int, 8> index_prefix,
                            const Type* tip_type,
                            const Type* forced_member_type,
                            Attributes& attributes,
                            StructMemberList& return_members,
                            ExpressionList& return_exprs);

    /// Updates the attribute list, replacing an existing Location attribute
    /// with another having one higher location value. Does nothing if no
    /// location attribute exists.
    /// Assumes the list contains at most one Location attribute.
    /// @param attributes the attribute list to modify
    void IncrementLocation(Attributes& attributes);

    /// Create an ast::BlockStatement representing the body of the function.
    /// This creates the statement stack, which is non-empty for the lifetime
    /// of the function.
    /// @returns the body of the function, or null on error
    const ast::BlockStatement* MakeFunctionBody();

    /// Emits the function body, populating the bottom entry of the statements
    /// stack.
    /// @returns false if emission failed.
    bool EmitBody();

    /// Records a mapping from block ID to a BlockInfo struct.
    /// Populates `block_info_`
    void RegisterBasicBlocks();

    /// Verifies that terminators only branch to labels in the current function.
    /// Assumes basic blocks have been registered.
    /// @returns true if terminators are valid
    bool TerminatorsAreValid();

    /// Populates merge-header cross-links and BlockInfo#is_continue_entire_loop.
    /// Also verifies that merge instructions go to blocks in the same function.
    /// Assumes basic blocks have been registered, and terminators are valid.
    /// @returns false if registration fails
    bool RegisterMerges();

    /// Determines the output order for the basic blocks in the function.
    /// Populates `block_order_` and BlockInfo#pos.
    /// Assumes basic blocks have been registered.
    void ComputeBlockOrderAndPositions();

    /// @returns the reverse structured post order of the basic blocks in
    /// the function.
    const std::vector<uint32_t>& block_order() const { return block_order_; }

    /// Verifies that the orderings among a structured header, continue target,
    /// and merge block are valid. Assumes block order has been computed, and
    /// merges are valid and recorded.
    /// @returns false if invalid nesting was detected
    bool VerifyHeaderContinueMergeOrder();

    /// Labels each basic block with its nearest enclosing structured construct.
    /// Populates BlockInfo#construct and the `constructs_` list.
    /// Assumes terminators are valid and merges have been registered, block
    /// order has been computed, and each block is labeled with its position.
    /// Checks nesting of structured control flow constructs.
    /// @returns false if bad nesting has been detected
    bool LabelControlFlowConstructs();

    /// @returns the structured constructs
    const ConstructList& constructs() const { return constructs_; }

    /// Marks blocks targets of a switch, either as the head of a case or
    /// as the default target.
    /// @returns false on failure
    bool FindSwitchCaseHeaders();

    /// Classifies the successor CFG edges for the ordered basic blocks.
    /// Also checks validity of each edge (populates BlockInfo#succ_edge).
    /// Implicitly checks dominance rules for headers and continue constructs.
    /// Assumes each block has been labeled with its control flow construct.
    /// @returns false on failure
    bool ClassifyCFGEdges();

    /// Marks the blocks within a selection construct that are the first blocks
    /// in the "then" clause, the "else" clause, and the "premerge" clause.
    /// The head of the premerge clause is the block, if it exists, at which
    /// control flow reconverges from the "then" and "else" clauses, but before
    /// before the merge block for that selection.   The existence of a premerge
    /// should be an exceptional case, but is allowed by the structured control
    /// flow rules.
    /// @returns false if bad nesting has been detected.
    bool FindIfSelectionInternalHeaders();

    /// Creates a DefInfo record for each module-scope builtin variable
    /// that should be handled specially.  Either it's ignored, or its store
    /// type is converted on load.
    /// Populates the `def_info_` mapping for such IDs.
    /// @returns false on failure
    bool RegisterSpecialBuiltInVariables();

    /// Creates a DefInfo record for each locally defined SPIR-V ID.
    /// Populates the `def_info_` mapping with basic results for such IDs.
    /// @returns false on failure
    bool RegisterLocallyDefinedValues();

    /// Returns the pointer information needed for the given SPIR-V ID.
    /// Assumes the given ID yields a value of pointer type.  For IDs
    /// corresponding to WGSL root identifiers (i.e. OpVariable or
    /// OpFunctionParameter), the info is computed from scratch.
    /// Otherwise, this looks up pointer info from a base pointer whose
    /// data is cached in def_info_.
    /// @param id a SPIR-V ID for a pointer value
    /// @returns the associated Pointer info
    DefInfo::Pointer GetPointerInfo(uint32_t id);

    /// Remaps the address space and access mode for the type of a
    /// locally-defined value, if necessary. If it's not a pointer or reference
    /// type, then the result is a copy of the `type` argument.
    /// @param type the AST type
    /// @param result_id the SPIR-V ID for the locally defined value
    /// @returns an possibly updated type
    const Type* RemapPointerProperties(const Type* type, uint32_t result_id);

    /// Marks locally defined values when they should get a 'let'
    /// definition in WGSL, or a 'var' definition at an outer scope.
    /// This occurs in several cases:
    ///  - When a SPIR-V instruction might use the dynamically computed value
    ///    only once, but the WGSL code might reference it multiple times.
    ///    For example, this occurs for the vector operands of OpVectorShuffle.
    ///    In this case the definition's DefInfo#requires_named_let_def property
    ///    is set to true.
    ///  - When a definition and at least one of its uses are not in the
    ///    same structured construct.
    ///    In this case the definition's DefInfo#requires_named_let_def property
    ///    is set to true.
    ///  - When a definition is in a construct that does not enclose all the
    ///    uses.  In this case the definition's DefInfo#requires_hoisted_var_def
    ///    property is set to true.
    /// Updates the `def_info_` mapping.
    void FindValuesNeedingNamedOrHoistedDefinition();

    /// Emits declarations of function variables.
    /// @returns false if emission failed.
    bool EmitFunctionVariables();

    /// Emits statements in the body.
    /// @returns false if emission failed.
    bool EmitFunctionBodyStatements();

    /// Emits a basic block.
    /// @param block_info the block to emit
    /// @returns false if emission failed.
    bool EmitBasicBlock(const BlockInfo& block_info);

    /// Emits an IfStatement, including its condition expression, and sets
    /// up the statement stack to accumulate subsequent basic blocks into
    /// the "then" and "else" clauses.
    /// @param block_info the if-selection header block
    /// @returns false if emission failed.
    bool EmitIfStart(const BlockInfo& block_info);

    /// Emits a SwitchStatement, including its condition expression, and sets
    /// up the statement stack to accumulate subsequent basic blocks into
    /// the default clause and case clauses.
    /// @param block_info the switch-selection header block
    /// @returns false if emission failed.
    bool EmitSwitchStart(const BlockInfo& block_info);

    /// Emits a LoopStatement, and pushes a new StatementBlock to accumulate
    /// the remaining instructions in the current block and subsequent blocks
    /// in the loop.
    /// @param construct the loop construct
    /// @returns false if emission failed.
    bool EmitLoopStart(const Construct* construct);

    /// Emits a ContinuingStatement, and pushes a new StatementBlock to accumulate
    /// the remaining instructions in the current block and subsequent blocks
    /// in the continue construct.
    /// @param construct the continue construct
    /// @returns false if emission failed.
    bool EmitContinuingStart(const Construct* construct);

    /// Emits the non-control-flow parts of a basic block, but only once.
    /// The `already_emitted` parameter indicates whether the code has already
    /// been emitted, and is used to signal that this invocation actually emitted
    /// it.
    /// @param block_info the block to emit
    /// @param already_emitted the block to emit
    /// @returns false if the code had not yet been emitted, but emission failed
    bool EmitStatementsInBasicBlock(const BlockInfo& block_info, bool* already_emitted);

    /// Emits code for terminators, but that aren't part of entering or
    /// resolving structured control flow. That is, if the basic block
    /// terminator calls for it, emit the fallthrough break, continue, return, or kill commands.
    /// @param block_info the block with the terminator to emit (if any)
    /// @returns false if emission failed
    bool EmitNormalTerminator(const BlockInfo& block_info);

    /// Returns a new statement to represent the given branch representing a
    /// "normal" terminator, as in the sense of EmitNormalTerminator.  If no
    /// WGSL statement is required, the statement will be nullptr. This method
    /// tries to avoid emitting a 'break' statement when that would be redundant
    /// in WGSL due to implicit breaking out of a switch.
    /// @param src_info the source block
    /// @param dest_info the destination block
    /// @returns the new statement, or a null statement
    const ast::Statement* MakeBranch(const BlockInfo& src_info, const BlockInfo& dest_info) {
        return MakeBranchDetailed(src_info, dest_info, nullptr);
    }

    /// Returns a new statement to represent the given branch representing a
    /// "normal" terminator, as in the sense of EmitNormalTerminator.  If no
    /// WGSL statement is required, the statement will be nullptr. This method tries to avoid
    /// emitting a 'break' statement when that would be redundant in WGSL due to implicit breaking
    /// out of a switch. If the control flow edge is an if-break for an if-selection with a
    /// control flow guard, then return that guard name via `flow_guard_name_ptr`
    /// when that parameter is not null.
    /// @param src_info the source block
    /// @param dest_info the destination block
    /// @param flow_guard_name_ptr return parameter for control flow guard name
    /// @returns the new statement, or a null statement
    const ast::Statement* MakeBranchDetailed(const BlockInfo& src_info,
                                             const BlockInfo& dest_info,
                                             std::string* flow_guard_name_ptr);

    /// Returns a new if statement with the given statements as the then-clause
    /// and the else-clause.  Either or both clauses might be nullptr. If both
    /// are nullptr, then don't make a new statement and instead return nullptr.
    /// @param condition the branching condition
    /// @param then_stmt the statement for the then clause of the if, or nullptr
    /// @param else_stmt the statement for the else clause of the if, or nullptr
    /// @returns the new statement, or nullptr
    const ast::Statement* MakeSimpleIf(const ast::Expression* condition,
                                       const ast::Statement* then_stmt,
                                       const ast::Statement* else_stmt) const;

    /// Emits the statements for an normal-terminator OpBranchConditional
    /// where one branch is a case fall through (the true branch if and only
    /// if `fall_through_is_true_branch` is true), and the other branch is
    /// goes to a different destination, named by `other_dest`.
    /// @param src_info the basic block from which we're branching
    /// @param cond the branching condition
    /// @param other_edge_kind the edge kind from the source block to the other
    /// destination
    /// @param other_dest the other branching destination
    /// @param fall_through_is_true_branch true when the fall-through is the true
    /// branch
    /// @returns the false if emission fails
    bool EmitConditionalCaseFallThrough(const BlockInfo& src_info,
                                        const ast::Expression* cond,
                                        EdgeKind other_edge_kind,
                                        const BlockInfo& other_dest,
                                        bool fall_through_is_true_branch);

    /// Emits a normal instruction: not a terminator, label, or variable
    /// declaration.
    /// @param inst the instruction
    /// @returns false if emission failed.
    bool EmitStatement(const spvtools::opt::Instruction& inst);

    /// Emits a const definition for the typed value in `ast_expr`, and
    /// records it as the translation for the result ID from `inst`.
    /// @param inst the SPIR-V instruction defining the value
    /// @param ast_expr the already-computed AST expression for the value
    /// @returns false if emission failed.
    bool EmitConstDefinition(const spvtools::opt::Instruction& inst, TypedExpression ast_expr);

    /// Emits a write of the typed value in `ast_expr` to a hoisted variable
    /// for the given SPIR-V ID, if that ID has a hoisted declaration. Otherwise,
    /// emits a const definition instead.
    /// @param inst the SPIR-V instruction defining the value
    /// @param ast_expr the already-computed AST expression for the value
    /// @returns false if emission failed.
    bool EmitConstDefOrWriteToHoistedVar(const spvtools::opt::Instruction& inst,
                                         TypedExpression ast_expr);

    /// If the result ID of the given instruction is hoisted, then emits
    /// a statement to write the expression to the hoisted variable, and
    /// returns true.  Otherwise return false.
    /// @param inst the SPIR-V instruction defining a value.
    /// @param ast_expr the expression to assign.
    /// @returns true if the instruction has an associated hoisted variable.
    bool WriteIfHoistedVar(const spvtools::opt::Instruction& inst, TypedExpression ast_expr);

    /// Makes an expression from a SPIR-V ID.
    /// if the SPIR-V result type is a pointer.
    /// @param id the SPIR-V ID of the value
    /// @returns an AST expression for the instruction, or an invalid
    /// TypedExpression on error.
    TypedExpression MakeExpression(uint32_t id);

    /// Creates an expression and supporting statements for a combinatorial
    /// instruction, or returns null.  A SPIR-V instruction is combinatorial
    /// if it has no side effects and its result depends only on its operands,
    /// and not on accessing external state like memory or the state of other
    /// invocations.  Statements are only created if required to provide values
    /// to the expression. Supporting statements are not required to be
    /// combinatorial.
    /// @param inst a SPIR-V instruction representing an exrpression
    /// @returns an AST expression for the instruction, or nullptr.
    TypedExpression MaybeEmitCombinatorialValue(const spvtools::opt::Instruction& inst);

    /// Creates an expression and supporting statements for the a GLSL.std.450
    /// extended instruction.
    /// @param inst a SPIR-V OpExtInst instruction from GLSL.std.450
    /// @returns an AST expression for the instruction, or nullptr.
    TypedExpression EmitGlslStd450ExtInst(const spvtools::opt::Instruction& inst);

    /// Creates an expression for the GLSL.std.450 matrix `inverse` extended instruction.
    /// @param inst a SPIR-V OpExtInst instruction from GLSL.std.450
    /// @returns an AST expression for the instruction, or nullptr.
    TypedExpression EmitGlslStd450MatrixInverse(const spvtools::opt::Instruction& inst);

    /// Creates an expression for OpCompositeExtract
    /// @param inst an OpCompositeExtract instruction.
    /// @returns an AST expression for the instruction, or nullptr.
    TypedExpression MakeCompositeExtract(const spvtools::opt::Instruction& inst);

    /// Creates an expression for indexing into a composite value.  The literal
    /// indices that step into the value start at instruction input operand
    /// `start_index` and run to the end of the instruction.
    /// @param inst the original instruction
    /// @param composite the typed expression for the composite
    /// @param composite_type_id the SPIR-V type ID for the composite
    /// @param index_start the index of the first operand in `inst` that is an
    /// index into the composite type
    /// @returns an AST expression for the decomposed composite, or {} on error
    TypedExpression MakeCompositeValueDecomposition(const spvtools::opt::Instruction& inst,
                                                    TypedExpression composite,
                                                    uint32_t composite_type_id,
                                                    int index_start);

    /// Creates an expression for OpVectorShuffle
    /// @param inst an OpVectorShuffle instruction.
    /// @returns an AST expression for the instruction, or nullptr.
    TypedExpression MakeVectorShuffle(const spvtools::opt::Instruction& inst);

    /// Creates an expression for a numeric conversion.
    /// @param inst a numeric conversion instruction
    /// @returns an AST expression for the instruction, or nullptr.
    TypedExpression MakeNumericConversion(const spvtools::opt::Instruction& inst);

    /// Gets the block info for a block ID, if any exists
    /// @param id the SPIR-V ID of the OpLabel instruction starting the block
    /// @returns the block info for the given ID, if it exists, or nullptr
    BlockInfo* GetBlockInfo(uint32_t id) const {
        auto where = block_info_.find(id);
        if (where == block_info_.end()) {
            return nullptr;
        }
        return where->second.get();
    }

    /// Is the block, represented by info, in the structured block order?
    /// @param info the block
    /// @returns true if the block is in the structured block order.
    bool IsInBlockOrder(const BlockInfo* info) const {
        return info && info->pos != kInvalidBlockPos;
    }

    /// Gets the definition info for a result ID.
    /// @param id the SPIR-V ID of local definition.
    /// @returns the definition info for the given ID, if it exists, or nullptr
    DefInfo* GetDefInfo(uint32_t id) const {
        auto where = def_info_.find(id);
        if (where == def_info_.end()) {
            return nullptr;
        }
        return where->second.get();
    }
    /// Returns the skip reason for a result ID.
    /// @param id SPIR-V result ID
    /// @returns the skip reason for the given ID, or SkipReason::kDontSkip
    SkipReason GetSkipReason(uint32_t id) const {
        if (auto* def_info = GetDefInfo(id)) {
            return def_info->skip;
        }
        return SkipReason::kDontSkip;
    }

    /// Returns the most deeply nested structured construct which encloses the
    /// WGSL scopes of names declared in both block positions. Each position must
    /// be a valid index into the function block order array.
    /// @param first_pos the first block position
    /// @param last_pos the last block position
    /// @returns the smallest construct containing both positions
    const Construct* GetEnclosingScope(uint32_t first_pos, uint32_t last_pos) const;

    /// Finds loop construct associated with a continue construct, if it exists.
    /// Returns nullptr if:
    ///  - the given construct is not a continue construct
    ///  - the continue construct does not have an associated loop construct
    ///    (the continue target is also the loop header block)
    /// @param c the continue construct
    /// @returns the associated loop construct, or nullptr
    const Construct* SiblingLoopConstruct(const Construct* c) const;

    /// Returns an identifier expression for the swizzle name of the given
    /// index into a vector.  Emits an error and returns nullptr if the
    /// index is out of range, i.e. 4 or higher.
    /// @param i index of the subcomponent
    /// @returns the identifier expression for the `i`'th component
    const ast::Identifier* Swizzle(uint32_t i);

    /// Returns an identifier expression for the swizzle name of the first
    /// `n` elements of a vector.  Emits an error and returns nullptr if `n`
    /// is out of range, i.e. 4 or higher.
    /// @param n the number of components in the swizzle
    /// @returns the swizzle identifier for the first n elements of a vector
    const ast::Identifier* PrefixSwizzle(uint32_t n);

    /// Converts SPIR-V image coordinates from an image access instruction
    /// (e.g. OpImageSampledImplicitLod) into an expression list consisting of
    /// the texture coordinates, and an integral array index if the texture is
    /// arrayed. The texture coordinate is a scalar for 1D textures, a vector of
    /// 2 elements for a 2D texture, and a vector of 3 elements for a 3D or
    /// Cube texture. Excess components are ignored, e.g. if the SPIR-V
    /// coordinate is a 4-element vector but the image is a 2D non-arrayed
    /// texture then the 3rd and 4th components are ignored.
    /// On failure, issues an error and returns an empty expression list.
    /// @param image_access the image access instruction
    /// @returns an ExpressionList of the coordinate and array index (if any)
    ExpressionList MakeCoordinateOperandsForImageAccess(
        const spvtools::opt::Instruction& image_access);

    /// Returns the given value as an i32. If it's already an i32 then simply returns @p value.
    /// Otherwise, wrap the value in a TypeInitializer expression.
    /// @param value the value to pass through or convert
    /// @returns the value as an i32 value.
    TypedExpression ToI32(TypedExpression value);

    /// Returns the given value as an u32. If it's already an u32 then simply returns @p value.
    /// Otherwise, wrap the value in a TypeInitializer expression.
    /// @param value the value to pass through or convert
    /// @returns the value as an u32 value.
    TypedExpression ToU32(TypedExpression value);

    /// Returns the given value as a signed integer type of the same shape if the value is unsigned
    /// scalar or vector, by wrapping the value with a TypeInitializer expression.  Returns the
    /// value itself if the value was already signed.
    /// @param value the value to pass through or convert
    /// @returns the value itself, or converted to signed integral
    TypedExpression ToSignedIfUnsigned(TypedExpression value);

    /// @param value_id the value identifier to check
    /// @returns true if the given SPIR-V id represents a constant float 0.
    bool IsFloatZero(uint32_t value_id);
    /// @param value_id the value identifier to check
    /// @returns true if the given SPIR-V id represents a constant float 1.
    bool IsFloatOne(uint32_t value_id);

  private:
    /// FunctionDeclaration contains the parsed information for a function header.
    struct FunctionDeclaration {
        /// Constructor
        FunctionDeclaration();
        /// Destructor
        ~FunctionDeclaration();

        /// Parsed header source
        Source source;
        /// Function name
        std::string name;
        /// Function parameters
        ParameterList params;
        /// Function return type
        const Type* return_type;
        /// Function attributes
        Attributes attributes;
    };

    /// Parse the function declaration, which comprises the name, parameters, and
    /// return type, populating `decl`.
    /// @param decl the FunctionDeclaration to populate
    /// @returns true if emission has not yet failed.
    bool ParseFunctionDeclaration(FunctionDeclaration* decl);

    /// @param obj a SPIR-V instruction with a result ID and a type ID
    /// @returns true if the object is an image, a sampler, or a pointer to
    /// an image or a sampler
    bool IsHandleObj(const spvtools::opt::Instruction& obj);

    /// @param obj a SPIR-V instruction with a result ID and a type ID
    /// @returns true if the object is an image, a sampler, or a pointer to
    /// an image or a sampler
    bool IsHandleObj(const spvtools::opt::Instruction* obj);

    /// @returns the store type for the OpVariable instruction, or
    /// null on failure.
    const Type* GetVariableStoreType(const spvtools::opt::Instruction& var_decl_inst);

    /// Returns an expression for an instruction operand. Signedness conversion is
    /// performed to match the result type of the SPIR-V instruction.
    /// @param inst the SPIR-V instruction
    /// @param operand_index the index of the operand, counting 0 as the first
    /// input operand
    /// @returns a new expression node
    TypedExpression MakeOperand(const spvtools::opt::Instruction& inst, uint32_t operand_index);

    /// Returns an expression for a SPIR-V OpFMod instruction.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    TypedExpression MakeFMod(const spvtools::opt::Instruction& inst);

    /// Returns an expression for a SPIR-V OpAccessChain or OpInBoundsAccessChain
    /// instruction.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    TypedExpression MakeAccessChain(const spvtools::opt::Instruction& inst);

    /// Emits a function call.  On failure, emits a diagnostic and returns false.
    /// @param inst the SPIR-V function call instruction
    /// @returns false if emission failed
    bool EmitFunctionCall(const spvtools::opt::Instruction& inst);

    /// Emits a control barrier builtin.  On failure, emits a diagnostic and
    /// returns false.
    /// @param inst the SPIR-V control barrier instruction
    /// @returns false if emission failed
    bool EmitControlBarrier(const spvtools::opt::Instruction& inst);

    /// Returns an expression for a SPIR-V instruction that maps to a WGSL
    /// builtin function call.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    TypedExpression MakeBuiltinCall(const spvtools::opt::Instruction& inst);

    /// Returns an expression for a SPIR-V instruction that maps to the extractBits WGSL
    /// builtin function call, with special handling to cast offset and count to u32, if needed.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    TypedExpression MakeExtractBitsCall(const spvtools::opt::Instruction& inst);

    /// Returns an expression for a SPIR-V instruction that maps to the insertBits WGSL
    /// builtin function call, with special handling to cast offset and count to u32, if needed.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    TypedExpression MakeInsertBitsCall(const spvtools::opt::Instruction& inst);

    /// Returns an expression for a SPIR-V OpArrayLength instruction.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    TypedExpression MakeArrayLength(const spvtools::opt::Instruction& inst);

    /// Generates an expression for a SPIR-V OpOuterProduct instruction.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    TypedExpression MakeOuterProduct(const spvtools::opt::Instruction& inst);

    /// Generates statements for a SPIR-V OpVectorInsertDynamic instruction.
    /// Registers a const declaration for the result.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    bool MakeVectorInsertDynamic(const spvtools::opt::Instruction& inst);

    /// Generates statements for a SPIR-V OpComposite instruction.
    /// Registers a const declaration for the result.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    bool MakeCompositeInsert(const spvtools::opt::Instruction& inst);

    /// Get the SPIR-V instruction for the image memory object declaration for
    /// the image operand to the given instruction.
    /// @param inst the SPIR-V instruction
    /// @returns a SPIR-V OpVariable or OpFunctionParameter instruction, or null
    /// on error
    const spvtools::opt::Instruction* GetImage(const spvtools::opt::Instruction& inst);

    /// Get the AST texture the SPIR-V image memory object declaration.
    /// @param inst the SPIR-V memory object declaration for the image.
    /// @returns a texture type, or null on error
    const Texture* GetImageType(const spvtools::opt::Instruction& inst);

    /// Get the expression for the image operand from the first operand to the
    /// given instruction.
    /// @param inst the SPIR-V instruction
    /// @returns an identifier expression, or null on error
    const ast::Expression* GetImageExpression(const spvtools::opt::Instruction& inst);

    /// Get the expression for the sampler operand from the first operand to the
    /// given instruction.
    /// @param inst the SPIR-V instruction
    /// @returns an identifier expression, or null on error
    const ast::Expression* GetSamplerExpression(const spvtools::opt::Instruction& inst);

    /// Emits a texture builtin function call for a SPIR-V instruction that
    /// accesses an image or sampled image.
    /// @param inst the SPIR-V instruction
    /// @returns true on success, false on error
    bool EmitImageAccess(const spvtools::opt::Instruction& inst);

    /// Emits statements to implement a SPIR-V image query.
    /// @param inst the SPIR-V instruction
    /// @returns an expression
    bool EmitImageQuery(const spvtools::opt::Instruction& inst);

    /// Emits statements to implement a SPIR-V atomic op.
    /// @param inst the SPIR-V instruction
    /// @returns true on success, false on error
    bool EmitAtomicOp(const spvtools::opt::Instruction& inst);

    /// Converts the given texel to match the type required for the storage
    /// texture with the given type. In WGSL the texel value is always provided
    /// as a 4-element vector, but the component type is determined by the
    /// texel channel type. See "Texel Formats for Storage Textures" in the WGSL
    /// spec. Returns an expression, or emits an error and returns nullptr.
    /// @param inst the image access instruction (used for diagnostics)
    /// @param texel the texel
    /// @param texture_type the type of the storage texture
    /// @returns the texel, after necessary conversion.
    const ast::Expression* ConvertTexelForStorage(const spvtools::opt::Instruction& inst,
                                                  TypedExpression texel,
                                                  const Texture* texture_type);

    /// Returns an expression for an OpSelect, if its operands are scalars
    /// or vectors. These translate directly to WGSL select.  Otherwise, return
    /// an expression with a null owned expression
    /// @param inst the SPIR-V OpSelect instruction
    /// @returns a typed expression, or one with a null owned expression
    TypedExpression MakeSimpleSelect(const spvtools::opt::Instruction& inst);

    /// Finds the header block for a structured construct that we can "break"
    /// out from, from deeply nested control flow, if such a block exists.
    /// If the construct is:
    ///  - a switch selection: return the selection header (ending in OpSwitch)
    ///  - a loop construct: return the loop header block
    ///  - a continue construct: return the loop header block
    /// Otherwise, return nullptr.
    /// @param c a structured construct, or nullptr
    /// @returns the block info for the structured header we can "break" from,
    /// or nullptr
    BlockInfo* HeaderIfBreakable(const Construct* c);

    /// Appends a new statement to the top of the statement stack.
    /// Does nothing if the statement is null.
    /// @param statement the new statement
    /// @returns a pointer to the statement.
    const ast::Statement* AddStatement(const ast::Statement* statement);

    /// AddStatementBuilder() constructs and adds the StatementBuilder of type
    /// `T` to the top of the statement stack.
    /// @param args the arguments forwarded to the T constructor
    /// @return the built StatementBuilder
    template <typename T, typename... ARGS>
    T* AddStatementBuilder(ARGS&&... args) {
        TINT_ASSERT(!statements_stack_.IsEmpty());
        return statements_stack_.Back().AddStatementBuilder<T>(std::forward<ARGS>(args)...);
    }

    /// Returns the source record for the given instruction.
    /// @param inst the SPIR-V instruction
    /// @return the Source record, or a default one
    Source GetSourceForInst(const spvtools::opt::Instruction& inst) const;

    /// @returns the last statement in the top of the statement stack.
    const ast::Statement* LastStatement();

    using CompletionAction = std::function<void(const StatementList&)>;

    // A StatementBlock represents a braced-list of statements while it is being
    // constructed.
    class StatementBlock {
      public:
        StatementBlock(const Construct* construct,
                       uint32_t end_id,
                       CompletionAction completion_action);
        StatementBlock(StatementBlock&&);
        ~StatementBlock();

        StatementBlock(const StatementBlock&) = delete;
        StatementBlock& operator=(const StatementBlock&) = delete;

        /// Replaces any StatementBuilders with the built result, and calls the
        /// completion callback (if set). Must only be called once, after all
        /// statements have been added with Add().
        /// @param builder the program builder
        void Finalize(ProgramBuilder* builder);

        /// Add() adds `statement` to the block.
        /// Add() must not be called after calling Finalize().
        void Add(const ast::Statement* statement);

        /// AddStatementBuilder() constructs and adds the StatementBuilder of type
        /// `T` to the block.
        /// Add() must not be called after calling Finalize().
        /// @param args the arguments forwarded to the T constructor
        /// @return the built StatementBuilder
        template <typename T, typename... ARGS>
        T* AddStatementBuilder(ARGS&&... args) {
            auto builder = std::make_unique<T>(std::forward<ARGS>(args)...);
            auto* ptr = builder.get();
            Add(ptr);
            builders_.emplace_back(std::move(builder));
            return ptr;
        }

        /// @param construct the construct which this construct constributes to
        void SetConstruct(const Construct* construct) { construct_ = construct; }

        /// @return the construct to which this construct constributes
        const Construct* GetConstruct() const { return construct_; }

        /// @return the ID of the block at which the completion action should be
        /// triggered and this statement block discarded. This is often the `end_id`
        /// of `construct` itself.
        uint32_t GetEndId() const { return end_id_; }

        /// @return the list of statements being built, if this construct is not a
        /// switch.
        const StatementList& GetStatements() const { return statements_; }

      private:
        /// The construct to which this construct constributes.
        const Construct* construct_;
        /// The ID of the block at which the completion action should be triggered
        /// and this statement block discarded. This is often the `end_id` of
        /// `construct` itself.
        const uint32_t end_id_;
        /// The completion action finishes processing this statement block.
        FunctionEmitter::CompletionAction const completion_action_;
        /// The list of statements being built, if this construct is not a switch.
        StatementList statements_;

        /// Owned statement builders
        std::vector<std::unique_ptr<StatementBuilder>> builders_;
        /// True if Finalize() has been called.
        bool finalized_ = false;
    };

    /// Pushes an empty statement block onto the statements stack.
    /// @param action the completion action for this block
    void PushNewStatementBlock(const Construct* construct,
                               uint32_t end_id,
                               CompletionAction action);

    /// Emits an if-statement whose condition is the given flow guard
    /// variable, and pushes onto the statement stack the corresponding
    /// statement block ending (and not including) the given block.
    /// @param flow_guard name of the flow guard variable
    /// @param end_id first block after the if construct.
    void PushGuard(const std::string& flow_guard, uint32_t end_id);

    /// Emits an if-statement with 'true' condition, and pushes onto the
    /// statement stack the corresponding statement block ending (and not
    /// including) the given block.
    /// @param end_id first block after the if construct.
    void PushTrueGuard(uint32_t end_id);

    /// @returns a boolean true expression.
    const ast::Expression* MakeTrue(const Source&) const;

    /// @returns a boolean false expression.
    const ast::Expression* MakeFalse(const Source&) const;

    /// @param expr the expression to take the address of
    /// @returns a TypedExpression that is the address-of `expr` (`&expr`)
    /// @note `expr` must be a reference type
    TypedExpression AddressOf(TypedExpression expr);

    /// Returns AddressOf(expr) if expr is has reference type and
    /// the instruction has a pointer result type.  Otherwise returns expr.
    /// @param expr the expression to take the address of
    /// @returns a TypedExpression that is the address-of `expr` (`&expr`)
    /// @note `expr` must be a reference type
    TypedExpression AddressOfIfNeeded(TypedExpression expr, const spvtools::opt::Instruction* inst);

    /// @param expr the expression to dereference
    /// @returns a TypedExpression that is the dereference-of `expr` (`*expr`)
    /// @note `expr` must be a pointer type
    TypedExpression Dereference(TypedExpression expr);

    /// Creates a new `ast::Node` owned by the ProgramBuilder.
    /// @param args the arguments to pass to the type initializer
    /// @returns the node pointer
    template <typename T, typename... ARGS>
    T* create(ARGS&&... args) const {
        return builder_.create<T>(std::forward<ARGS>(args)...);
    }

    using PtrAs = ASTParser::PtrAs;

    ASTParser& parser_impl_;
    TypeManager& ty_;
    ProgramBuilder& builder_;
    spvtools::opt::IRContext& ir_context_;
    spvtools::opt::analysis::DefUseManager* def_use_mgr_;
    spvtools::opt::analysis::ConstantManager* constant_mgr_;
    spvtools::opt::analysis::TypeManager* type_mgr_;
    FailStream& fail_stream_;
    Namer& namer_;
    const spvtools::opt::Function& function_;

    // The SPIR-V ID for the SampleMask input variable.
    uint32_t sample_mask_in_id;
    // The SPIR-V ID for the SampleMask output variable.
    uint32_t sample_mask_out_id;

    // A stack of statement lists. Each list is contained in a construct in
    // the next deeper element of stack. The 0th entry represents the statements
    // for the entire function.  This stack is never empty.
    // The `construct` member for the 0th element is only valid during the
    // lifetime of the EmitFunctionBodyStatements method.
    tint::Vector<StatementBlock, 8> statements_stack_;

    // The map of IDs that have already had an identifier name generated for it,
    // to their Type.
    std::unordered_map<uint32_t, const Type*> identifier_types_;
    // Mapping from SPIR-V ID that is used at most once, to its AST expression.
    std::unordered_map<uint32_t, TypedExpression> singly_used_values_;

    // The IDs of basic blocks, in reverse structured post-order (RSPO).
    // This is the output order for the basic blocks.
    std::vector<uint32_t> block_order_;

    // Mapping from block ID to its bookkeeping info.
    std::unordered_map<uint32_t, std::unique_ptr<BlockInfo>> block_info_;

    // Mapping from a result ID to its bookkeeping info.  This may be
    // either a result ID defined in the function body, or the ID of a
    // module-scope variable.
    std::unordered_map<uint32_t, std::unique_ptr<DefInfo>> def_info_;

    // Structured constructs, where enclosing constructs precede their children.
    ConstructList constructs_;

    // Information about entry point, if this function is referenced by one
    const EntryPointInfo* ep_info_ = nullptr;
};

}  // namespace tint::spirv::reader::ast_parser

#endif  // SRC_TINT_LANG_SPIRV_READER_AST_PARSER_FUNCTION_H_