// 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.

#ifndef SRC_READER_SPIRV_PARSER_IMPL_H_
#define SRC_READER_SPIRV_PARSER_IMPL_H_

#include <cstdint>
#include <memory>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#include "source/opt/constants.h"
#include "source/opt/decoration_manager.h"
#include "source/opt/ir_context.h"
#include "source/opt/module.h"
#include "source/opt/type_manager.h"
#include "source/opt/types.h"
#include "spirv-tools/libspirv.hpp"
#include "src/ast/expression.h"
#include "src/ast/struct_member_decoration.h"
#include "src/program_builder.h"
#include "src/reader/reader.h"
#include "src/reader/spirv/entry_point_info.h"
#include "src/reader/spirv/enum_converter.h"
#include "src/reader/spirv/fail_stream.h"
#include "src/reader/spirv/namer.h"
#include "src/reader/spirv/usage.h"
#include "src/source.h"
#include "src/type/alias_type.h"
#include "src/type/array_type.h"
#include "src/type/pointer_type.h"
#include "src/type/type.h"

/// This is the implementation of the SPIR-V parser for Tint.

/// Notes on terminology:
///
/// A WGSL "handle" is an opaque object used for accessing a resource via
/// special builtins.  In SPIR-V, a handle is stored a variable in the
/// UniformConstant storage class.  The handles supported by SPIR-V are:
///   - images, both sampled texture and storage image
///   - samplers
///   - combined image+sampler
///   - acceleration structures for raytracing.
///
/// WGSL only supports samplers and images, but calls images "textures".
/// When emitting errors, we aim to use terminology most likely to be
/// familiar to Vulkan SPIR-V developers.  We will tend to use "image"
/// and "sampler" instead of "handle".

namespace tint {
namespace reader {
namespace spirv {

/// The binary representation of a SPIR-V decoration enum followed by its
/// operands, if any.
/// Example:   { SpvDecorationBlock }
/// Example:   { SpvDecorationArrayStride, 16 }
using Decoration = std::vector<uint32_t>;
using DecorationList = std::vector<Decoration>;

/// An AST expression with its type.
struct TypedExpression {
  /// The type
  type::Type* type = nullptr;
  /// The expression
  ast::Expression* expr = nullptr;
};

/// Parser implementation for SPIR-V.
class ParserImpl : Reader {
 public:
  /// Creates a new parser
  /// @param input the input data to parse
  explicit ParserImpl(const std::vector<uint32_t>& input);
  /// Destructor
  ~ParserImpl() override;

  /// Run the parser
  /// @returns true if the parse was successful, false otherwise.
  bool Parse() override;

  /// @returns the program. The program builder in the parser will be reset
  /// after this.
  Program program() override;

  /// @returns a reference to the internal builder, without building the
  /// program. To be used only for testing.
  ProgramBuilder& builder() { return builder_; }

  /// Logs failure, ands return a failure stream to accumulate diagnostic
  /// messages. By convention, a failure should only be logged along with
  /// a non-empty string diagnostic.
  /// @returns the failure stream
  FailStream& Fail() {
    success_ = false;
    return fail_stream_;
  }

  /// @return true if failure has not yet occurred
  bool success() const { return success_; }

  /// @returns the accumulated error string
  const std::string error() { return errors_.str(); }

  /// Builds an internal representation of the SPIR-V binary,
  /// and parses it into a Tint AST module.  Diagnostics are emitted
  /// to the error stream.
  /// @returns true if it was successful.
  bool BuildAndParseInternalModule() {
    return BuildInternalModule() && ParseInternalModule();
  }
  /// Builds an internal representation of the SPIR-V binary,
  /// and parses the module, except functions, into a Tint AST module.
  /// Diagnostics are emitted to the error stream.
  /// @returns true if it was successful.
  bool BuildAndParseInternalModuleExceptFunctions() {
    return BuildInternalModule() && ParseInternalModuleExceptFunctions();
  }

  /// @returns the set of SPIR-V IDs for imports of the "GLSL.std.450"
  /// extended instruction set.
  const std::unordered_set<uint32_t>& glsl_std_450_imports() const {
    return glsl_std_450_imports_;
  }

  /// Converts a SPIR-V type to a Tint type, and saves it for fast lookup.
  /// If the type is only used for builtins, then register that specially,
  /// and return null.  If the type is a sampler, image, or sampled image, then
  /// return the Void type, because those opaque types are handled in a
  /// different way.
  /// On failure, logs an error and returns null.  This should only be called
  /// after the internal representation of the module has been built.
  /// @param type_id the SPIR-V ID of a type.
  /// @returns a Tint type, or nullptr
  type::Type* ConvertType(uint32_t type_id);

  /// Emits an alias type declaration for the given type, if necessary, and
  /// also updates the mapping of the SPIR-V type ID to the alias type.
  /// Do so for the types requiring user-specified names:
  /// - struct types
  /// - decorated arrays and runtime arrays
  /// TODO(dneto): I expect images and samplers to require names as well.
  /// This is a no-op if the parser has already failed.
  /// @param type_id the SPIR-V ID for the type
  /// @param type the type that might get an alias
  void MaybeGenerateAlias(uint32_t type_id,
                          const spvtools::opt::analysis::Type* type);

  /// @returns the fail stream object
  FailStream& fail_stream() { return fail_stream_; }
  /// @returns the namer object
  Namer& namer() { return namer_; }
  /// @returns a borrowed pointer to the internal representation of the module.
  /// This is null until BuildInternalModule has been called.
  spvtools::opt::IRContext* ir_context() { return ir_context_.get(); }

  /// Gets the list of decorations for a SPIR-V result ID.  Returns an empty
  /// vector if the ID is not a result ID, or if no decorations target that ID.
  /// The internal representation must have already been built.
  /// @param id SPIR-V ID
  /// @returns the list of decorations on the given ID
  DecorationList GetDecorationsFor(uint32_t id) const;
  /// Gets the list of decorations for the member of a struct.  Returns an empty
  /// list if the `id` is not the ID of a struct, or if the member index is out
  /// of range, or if the target member has no decorations.
  /// The internal representation must have already been built.
  /// @param id SPIR-V ID of a struct
  /// @param member_index the member within the struct
  /// @returns the list of decorations on the member
  DecorationList GetDecorationsForMember(uint32_t id,
                                         uint32_t member_index) const;

  /// Converts a SPIR-V struct member decoration. If the decoration is
  /// recognized but deliberately dropped, then returns nullptr without a
  /// diagnostic. On failure, emits a diagnostic and returns nullptr.
  /// @param struct_type_id the ID of the struct type
  /// @param member_index the index of the member
  /// @param decoration an encoded SPIR-V Decoration
  /// @returns the corresponding ast::StructuMemberDecoration
  ast::StructMemberDecoration* ConvertMemberDecoration(
      uint32_t struct_type_id,
      uint32_t member_index,
      const Decoration& decoration);

  /// Returns a string for the given type.  If the type ID is invalid,
  /// then the resulting string only names the type ID.
  /// @param type_id the SPIR-V ID for the type
  /// @returns a string description of the type.
  std::string ShowType(uint32_t type_id);

  /// Builds the internal representation of the SPIR-V module.
  /// Assumes the module is somewhat well-formed.  Normally you
  /// would want to validate the SPIR-V module before attempting
  /// to build this internal representation. Also computes a topological
  /// ordering of the functions.
  /// This is a no-op if the parser has already failed.
  /// @returns true if the parser is still successful.
  bool BuildInternalModule();

  /// Walks the internal representation of the module to populate
  /// the AST form of the module.
  /// This is a no-op if the parser has already failed.
  /// @returns true if the parser is still successful.
  bool ParseInternalModule();

  /// Records line numbers for each instruction.
  void RegisterLineNumbers();

  /// Walks the internal representation of the module, except for function
  /// definitions, to populate the AST form of the module.
  /// This is a no-op if the parser has already failed.
  /// @returns true if the parser is still successful.
  bool ParseInternalModuleExceptFunctions();

  /// Destroys the internal representation of the SPIR-V module.
  void ResetInternalModule();

  /// Registers extended instruction imports.  Only "GLSL.std.450" is supported.
  /// This is a no-op if the parser has already failed.
  /// @returns true if parser is still successful.
  bool RegisterExtendedInstructionImports();

  /// Returns true when the given instruction is an extended instruction
  /// for GLSL.std.450.
  /// @param inst a SPIR-V instruction
  /// @returns true if its an SpvOpExtInst for GLSL.std.450
  bool IsGlslExtendedInstruction(const spvtools::opt::Instruction& inst) const;

  /// Registers user names for SPIR-V objects, from OpName, and OpMemberName.
  /// Also synthesizes struct field names.  Ensures uniqueness for names for
  /// SPIR-V IDs, and uniqueness of names of fields within any single struct.
  /// This is a no-op if the parser has already failed.
  /// @returns true if parser is still successful.
  bool RegisterUserAndStructMemberNames();

  /// Register entry point information.
  /// This is a no-op if the parser has already failed.
  /// @returns true if parser is still successful.
  bool RegisterEntryPoints();

  /// Register Tint AST types for SPIR-V types, including type aliases as
  /// needed.  This is a no-op if the parser has already failed.
  /// @returns true if parser is still successful.
  bool RegisterTypes();

  /// Register sampler and texture usage for memory object declarations.
  /// This must be called after we've registered line numbers for all
  /// instructions. This is a no-op if the parser has already failed.
  /// @returns true if parser is still successful.
  bool RegisterHandleUsage();

  /// Emit const definitions for scalar specialization constants generated
  /// by one of OpConstantTrue, OpConstantFalse, or OpSpecConstant.
  /// This is a no-op if the parser has already failed.
  /// @returns true if parser is still successful.
  bool EmitScalarSpecConstants();

  /// Emits module-scope variables.
  /// This is a no-op if the parser has already failed.
  /// @returns true if parser is still successful.
  bool EmitModuleScopeVariables();

  /// Emits functions, with callees preceding their callers.
  /// This is a no-op if the parser has already failed.
  /// @returns true if parser is still successful.
  bool EmitFunctions();

  /// Emits a single function, if it has a body.
  /// This is a no-op if the parser has already failed.
  /// @param f the function to emit
  /// @returns true if parser is still successful.
  bool EmitFunction(const spvtools::opt::Function& f);

  /// Creates an AST Variable node for a SPIR-V ID, including any attached
  /// decorations, unless it's an ignorable builtin variable.
  /// @param id the SPIR-V result ID
  /// @param sc the storage class, which cannot be ast::StorageClass::kNone
  /// @param type the type
  /// @param is_const if true, the variable is const
  /// @param constructor the variable constructor
  /// @param decorations the variable decorations
  /// @returns a new Variable node, or null in the ignorable variable case and
  /// in the error case
  ast::Variable* MakeVariable(uint32_t id,
                              ast::StorageClass sc,
                              type::Type* type,
                              bool is_const,
                              ast::Expression* constructor,
                              ast::VariableDecorationList decorations);

  /// Creates an AST expression node for a SPIR-V constant.
  /// @param id the SPIR-V ID of the constant
  /// @returns a new expression
  TypedExpression MakeConstantExpression(uint32_t id);

  /// Creates an AST expression node for the null value for the given type.
  /// @param type the AST type
  /// @returns a new expression
  ast::Expression* MakeNullValue(type::Type* type);

  /// Converts a given expression to the signedness demanded for an operand
  /// of the given SPIR-V instruction, if required.  If the instruction assumes
  /// signed integer operands, and `expr` is unsigned, then return an
  /// as-cast expression converting it to signed. Otherwise, return
  /// `expr` itself.  Similarly, convert as required from unsigned
  /// to signed. Assumes all SPIR-V types have been mapped to AST types.
  /// @param inst the SPIR-V instruction
  /// @param expr an expression
  /// @returns expr, or a cast of expr
  TypedExpression RectifyOperandSignedness(
      const spvtools::opt::Instruction& inst,
      TypedExpression&& expr);

  /// Returns the "forced" result type for the given SPIR-V instruction.
  /// If the WGSL result type for an operation has a more strict rule than
  /// requried by SPIR-V, then we say the result type is "forced".  This occurs
  /// for signed integer division (OpSDiv), for example, where the result type
  /// in WGSL must match the operand types.
  /// @param inst the SPIR-V instruction
  /// @param first_operand_type the AST type for the first operand.
  /// @returns the forced AST result type, or nullptr if no forcing is required.
  type::Type* ForcedResultType(const spvtools::opt::Instruction& inst,
                               type::Type* first_operand_type);

  /// Returns a signed integer scalar or vector type matching the shape (scalar,
  /// vector, and component bit width) of another type, which itself is a
  /// numeric scalar or vector. Returns null if the other type does not meet the
  /// requirement.
  /// @param other the type whose shape must be matched
  /// @returns the signed scalar or vector type
  type::Type* GetSignedIntMatchingShape(type::Type* other);

  /// Returns a signed integer scalar or vector type matching the shape (scalar,
  /// vector, and component bit width) of another type, which itself is a
  /// numeric scalar or vector. Returns null if the other type does not meet the
  /// requirement.
  /// @param other the type whose shape must be matched
  /// @returns the unsigned scalar or vector type
  type::Type* GetUnsignedIntMatchingShape(type::Type* other);

  /// Wraps the given expression in an as-cast to the given expression's type,
  /// when the underlying operation produces a forced result type different
  /// from the expression's result type. Otherwise, returns the given expression
  /// unchanged.
  /// @param expr the expression to pass through or to wrap
  /// @param inst the SPIR-V instruction
  /// @param first_operand_type the AST type for the first operand.
  /// @returns the forced AST result type, or nullptr if no forcing is required.
  TypedExpression RectifyForcedResultType(
      TypedExpression expr,
      const spvtools::opt::Instruction& inst,
      type::Type* first_operand_type);

  /// @returns the registered boolean type.
  type::Type* Bool() const { return bool_type_; }

  /// Bookkeeping used for tracking the "position" builtin variable.
  struct BuiltInPositionInfo {
    /// The ID for the gl_PerVertex struct containing the Position builtin.
    uint32_t struct_type_id = 0;
    /// The member index for the Position builtin within the struct.
    uint32_t position_member_index = 0;
    /// The member index for the PointSize builtin within the struct.
    uint32_t pointsize_member_index = 0;
    /// The ID for the member type, which should map to vec4<f32>.
    uint32_t position_member_type_id = 0;
    /// The ID of the type of a pointer to the struct in the Output storage
    /// class class.
    uint32_t pointer_type_id = 0;
    /// The SPIR-V storage class.
    SpvStorageClass storage_class = SpvStorageClassOutput;
    /// The ID of the type of a pointer to the Position member.
    uint32_t position_member_pointer_type_id = 0;
    /// The ID of the gl_PerVertex variable, if it was declared.
    /// We'll use this for the gl_Position variable instead.
    uint32_t per_vertex_var_id = 0;
  };
  /// @returns info about the gl_Position builtin variable.
  const BuiltInPositionInfo& GetBuiltInPositionInfo() {
    return builtin_position_;
  }

  /// Returns the source record for the SPIR-V instruction with the given
  /// result ID.
  /// @param id the SPIR-V result id.
  /// @return the Source record, or a default one
  Source GetSourceForResultIdForTest(uint32_t id) const;
  /// 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;

  /// @param str a candidate identifier
  /// @returns true if the given string is a valid WGSL identifier.
  static bool IsValidIdentifier(const std::string& str);

  /// Returns true if the given SPIR-V ID is a declared specialization constant,
  /// generated by one of OpConstantTrue, OpConstantFalse, or OpSpecConstant
  /// @param id a SPIR-V result ID
  /// @returns true if the ID is a scalar spec constant.
  bool IsScalarSpecConstant(uint32_t id) {
    return scalar_spec_constants_.find(id) != scalar_spec_constants_.end();
  }

  /// For a SPIR-V ID that might define a sampler, image, or sampled image
  /// value, return the SPIR-V instruction that represents the memory object
  /// declaration for the object.  If we encounter an OpSampledImage along the
  /// way, follow the image operand when follow_image is true; otherwise follow
  /// the sampler operand. Returns nullptr if we can't trace back to a memory
  /// object declaration.  Emits an error and returns nullptr when the scan
  /// fails due to a malformed module. This method can be used any time after
  /// BuildInternalModule has been invoked.
  /// @param id the SPIR-V ID of the sampler, image, or sampled image
  /// @param follow_image indicates whether to follow the image operand of
  /// OpSampledImage
  /// @returns the memory object declaration for the handle, or nullptr
  const spvtools::opt::Instruction* GetMemoryObjectDeclarationForHandle(
      uint32_t id,
      bool follow_image);

  /// Returns the handle usage for a memory object declaration.
  /// @param id SPIR-V ID of a sampler or image OpVariable or
  /// OpFunctionParameter
  /// @returns the handle usage, or an empty usage object.
  Usage GetHandleUsage(uint32_t id) const;

  /// Returns the SPIR-V type for the sampler or image type for the given
  /// variable in UniformConstant storage class, or function parameter pointing
  /// into the UniformConstant storage class .  Returns null and emits an
  /// error on failure.
  /// @param var the OpVariable instruction or OpFunctionParameter
  /// @returns the Tint AST type for the sampler or texture, or null on error
  const spvtools::opt::Instruction*
  GetSpirvTypeForHandleMemoryObjectDeclaration(
      const spvtools::opt::Instruction& var);

  /// Returns the AST type for the pointer-to-sampler or pointer-to-texture type
  /// for the given variable in UniformConstant storage class.  Returns null and
  /// emits an error on failure.
  /// @param var the OpVariable instruction
  /// @returns the Tint AST type for the poiner-to-{sampler|texture} or null on
  /// error
  type::Pointer* GetTypeForHandleVar(const spvtools::opt::Instruction& var);

  /// Returns the channel component type corresponding to the given image
  /// format.
  /// @param format image texel format
  /// @returns the component type, one of f32, i32, u32
  type::Type* GetComponentTypeForFormat(type::ImageFormat format);

  /// Returns texel type corresponding to the given image format.
  /// @param format image texel format
  /// @returns the texel format
  type::Type* GetTexelTypeForFormat(type::ImageFormat format);

  /// Returns the SPIR-V instruction with the given ID, or nullptr.
  /// @param id the SPIR-V result ID
  /// @returns the instruction, or nullptr on error
  const spvtools::opt::Instruction* GetInstructionForTest(uint32_t id) const;

  /// A map of SPIR-V identifiers to builtins
  using BuiltInsMap = std::unordered_map<uint32_t, SpvBuiltIn>;

  /// @returns a map of builtins that should be handled specially by code
  /// generation. Either the builtin does not exist in WGSL, or a type
  /// conversion must be implemented on load and store.
  const BuiltInsMap& special_builtins() const { return special_builtins_; }

  /// @param builtin the SPIR-V builtin variable kind
  /// @returns the SPIR-V ID for the variable defining the given builtin, or 0
  uint32_t IdForSpecialBuiltIn(SpvBuiltIn builtin) const {
    // Do a linear search.
    for (const auto& entry : special_builtins_) {
      if (entry.second == builtin) {
        return entry.first;
      }
    }
    return 0;
  }

 private:
  /// Converts a specific SPIR-V type to a Tint type. Integer case
  type::Type* ConvertType(const spvtools::opt::analysis::Integer* int_ty);
  /// Converts a specific SPIR-V type to a Tint type. Float case
  type::Type* ConvertType(const spvtools::opt::analysis::Float* float_ty);
  /// Converts a specific SPIR-V type to a Tint type. Vector case
  type::Type* ConvertType(const spvtools::opt::analysis::Vector* vec_ty);
  /// Converts a specific SPIR-V type to a Tint type. Matrix case
  type::Type* ConvertType(const spvtools::opt::analysis::Matrix* mat_ty);
  /// Converts a specific SPIR-V type to a Tint type. RuntimeArray case
  /// @param rtarr_ty the Tint type
  type::Type* ConvertType(
      const spvtools::opt::analysis::RuntimeArray* rtarr_ty);
  /// Converts a specific SPIR-V type to a Tint type. Array case
  /// @param arr_ty the Tint type
  type::Type* ConvertType(const spvtools::opt::analysis::Array* arr_ty);
  /// Converts a specific SPIR-V type to a Tint type. Struct case.
  /// SPIR-V allows distinct struct type definitions for two OpTypeStruct
  /// that otherwise have the same set of members (and struct and member
  /// decorations).  However, the SPIRV-Tools always produces a unique
  /// `spvtools::opt::analysis::Struct` object in these cases. For this type
  /// conversion, we need to have the original SPIR-V ID because we can't always
  /// recover it from the optimizer's struct type object. This also lets us
  /// preserve member names, which are given by OpMemberName which is normally
  /// not significant to the optimizer's module representation.
  /// @param type_id the SPIR-V ID for the type.
  /// @param struct_ty the Tint type
  type::Type* ConvertType(uint32_t type_id,
                          const spvtools::opt::analysis::Struct* struct_ty);
  /// Converts a specific SPIR-V type to a Tint type. Pointer case
  /// The pointer to gl_PerVertex maps to nullptr, and instead is recorded
  /// in member #builtin_position_.
  /// @param type_id the SPIR-V ID for the type.
  /// @param ptr_ty the Tint type
  type::Type* ConvertType(uint32_t type_id,
                          const spvtools::opt::analysis::Pointer* ptr_ty);

  /// Parses the array or runtime-array decorations.
  /// @param spv_type the SPIR-V array or runtime-array type.
  /// @param decorations the populated decoration list
  /// @returns true on success.
  bool ParseArrayDecorations(const spvtools::opt::analysis::Type* spv_type,
                             ast::ArrayDecorationList* decorations);

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

  // The SPIR-V binary we're parsing
  std::vector<uint32_t> spv_binary_;

  // The program builder.
  ProgramBuilder builder_;

  // Is the parse successful?
  bool success_ = true;
  // Collector for diagnostic messages.
  std::stringstream errors_;
  FailStream fail_stream_;
  spvtools::MessageConsumer message_consumer_;

  // The registered boolean type.
  type::Type* bool_type_;

  // An object used to store and generate names for SPIR-V objects.
  Namer namer_;
  // An object used to convert SPIR-V enums to Tint enums
  EnumConverter enum_converter_;

  // The internal representation of the SPIR-V module and its context.
  spvtools::Context tools_context_;
  // All the state is owned by ir_context_.
  std::unique_ptr<spvtools::opt::IRContext> ir_context_;
  // The following are borrowed pointers to the internal state of ir_context_.
  spvtools::opt::Module* module_ = nullptr;
  spvtools::opt::analysis::DefUseManager* def_use_mgr_ = nullptr;
  spvtools::opt::analysis::ConstantManager* constant_mgr_ = nullptr;
  spvtools::opt::analysis::TypeManager* type_mgr_ = nullptr;
  spvtools::opt::analysis::DecorationManager* deco_mgr_ = nullptr;

  // The functions ordered so that callees precede their callers.
  std::vector<const spvtools::opt::Function*> topologically_ordered_functions_;

  // Maps an instruction to its source location. If no OpLine information
  // is in effect for the instruction, map the instruction to its position
  // in the SPIR-V module, counting by instructions, where the first
  // instruction is line 1.
  std::unordered_map<const spvtools::opt::Instruction*, Source::Location>
      inst_source_;

  // The set of IDs that are imports of the GLSL.std.450 extended instruction
  // sets.
  std::unordered_set<uint32_t> glsl_std_450_imports_;

  // Maps a SPIR-V type ID to the corresponding Tint type.
  std::unordered_map<uint32_t, type::Type*> id_to_type_;

  // Maps an unsigned type corresponding to the given signed type.
  std::unordered_map<type::Type*, type::Type*> signed_type_for_;
  // Maps an signed type corresponding to the given unsigned type.
  std::unordered_map<type::Type*, type::Type*> unsigned_type_for_;

  // Bookkeeping for the gl_Position builtin.
  // In Vulkan SPIR-V, it's the 0 member of the gl_PerVertex structure.
  // But in WGSL we make a module-scope variable:
  //    [[position]] var<in> gl_Position : vec4<f32>;
  // The builtin variable was detected if and only if the struct_id is non-zero.
  BuiltInPositionInfo builtin_position_;

  // SPIR-V type IDs that are either:
  // - a struct type decorated by BufferBlock
  // - an array, runtime array containing one of these
  // - a pointer type to one of these
  // These are the types "enclosing" a buffer block with the old style
  // representation: using Uniform storage class and BufferBlock decoration
  // on the struct.  The new style is to use the StorageBuffer storage class
  // and Block decoration.
  std::unordered_set<uint32_t> remap_buffer_block_type_;

  // The struct types with only read-only members.
  std::unordered_set<type::Type*> read_only_struct_types_;

  // The IDs of scalar spec constants
  std::unordered_set<uint32_t> scalar_spec_constants_;

  // Maps function_id to a list of entrypoint information
  std::unordered_map<uint32_t, std::vector<EntryPointInfo>>
      function_to_ep_info_;

  // Maps from a SPIR-V ID to its underlying memory object declaration,
  // following image paths. This a memoization table for
  // GetMemoryObjectDeclarationForHandle. (A SPIR-V memory object declaration is
  // an OpVariable or an OpFunctinParameter with pointer type).
  std::unordered_map<uint32_t, const spvtools::opt::Instruction*>
      mem_obj_decl_image_;
  // Maps from a SPIR-V ID to its underlying memory object declaration,
  // following sampler paths. This a memoization table for
  // GetMemoryObjectDeclarationForHandle.
  std::unordered_map<uint32_t, const spvtools::opt::Instruction*>
      mem_obj_decl_sampler_;

  // Maps a memory-object-declaration instruction to any sampler or texture
  // usages implied by usages of the memory-object-declaration.
  std::unordered_map<const spvtools::opt::Instruction*, Usage> handle_usage_;
  // The inferred pointer type for the given handle variable.
  std::unordered_map<const spvtools::opt::Instruction*, type::Pointer*>
      handle_type_;

  /// Maps the SPIR-V ID of a module-scope builtin variable that should be
  /// ignored or type-converted, to its builtin kind.
  /// See also BuiltInPositionInfo which is a separate mechanism for a more
  /// complex case of replacing an entire structure.
  BuiltInsMap special_builtins_;
};

}  // namespace spirv
}  // namespace reader
}  // namespace tint

#endif  // SRC_READER_SPIRV_PARSER_IMPL_H_