carbon-lang/toolchain/lower/file_context.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "toolchain/lower/file_context.h"

#include <memory>
#include <optional>
#include <string>
#include <utility>

#include "clang/CodeGen/ModuleBuilder.h"
#include "common/check.h"
#include "common/pretty_stack_trace_function.h"
#include "common/vlog.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/Linker/Linker.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
#include "toolchain/base/kind_switch.h"
#include "toolchain/lower/clang_global_decl.h"
#include "toolchain/lower/constant.h"
#include "toolchain/lower/function_context.h"
#include "toolchain/lower/mangler.h"
#include "toolchain/lower/options.h"
#include "toolchain/lower/specific_coalescer.h"
#include "toolchain/sem_ir/absolute_node_id.h"
#include "toolchain/sem_ir/diagnostic_loc_converter.h"
#include "toolchain/sem_ir/entry_point.h"
#include "toolchain/sem_ir/expr_info.h"
#include "toolchain/sem_ir/file.h"
#include "toolchain/sem_ir/function.h"
#include "toolchain/sem_ir/generic.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/inst.h"
#include "toolchain/sem_ir/inst_categories.h"
#include "toolchain/sem_ir/inst_kind.h"
#include "toolchain/sem_ir/pattern.h"
#include "toolchain/sem_ir/stringify.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Lower {

FileContext::FileContext(Context& context, const SemIR::File& sem_ir,
                         const SemIR::InstNamer* inst_namer,
                         llvm::raw_ostream* vlog_stream)
    : context_(&context),
      sem_ir_(&sem_ir),
      inst_namer_(inst_namer),
      vlog_stream_(vlog_stream),
      functions_(LoweredFunctionStore::MakeForOverwrite(sem_ir.functions())),
      specific_functions_(sem_ir.specifics(), nullptr),
      types_(LoweredTypeStore::MakeWithExplicitSize(
          sem_ir.insts().GetIdTag(), sem_ir.insts().size(), nullptr)),
      constants_(LoweredConstantStore::MakeWithExplicitSize(
          sem_ir.insts().GetIdTag(), sem_ir.insts().size(), nullptr)),
      lowered_specifics_(sem_ir.generics(),
                         llvm::SmallVector<SemIR::SpecificId>()),
      coalescer_(vlog_stream_, sem_ir.specifics()),
      vtables_(decltype(vtables_)::MakeForOverwrite(sem_ir.vtables())),
      specific_vtables_(sem_ir.specifics(), nullptr) {
  // Initialization that relies on invariants of the class.
  cpp_code_generator_ = CreateCppCodeGenerator();
  CARBON_CHECK(!sem_ir.has_errors(),
               "Generating LLVM IR from invalid SemIR::File is unsupported.");
}

// TODO: Move this to lower.cpp.
auto FileContext::PrepareToLower() -> void {
  if (cpp_code_generator_) {
    // Clang code generation should not actually modify the AST, but isn't
    // const-correct.
    cpp_code_generator_->Initialize(
        const_cast<clang::ASTContext&>(clang_ast_unit()->getASTContext()));

    // Work around `visitLocalTopLevelDecls` not being const. It doesn't modify
    // the AST unit other than triggering deserialization.
    auto* non_const_ast_unit = const_cast<clang::ASTUnit*>(clang_ast_unit());

    // Emit any top-level declarations now.
    // TODO: This may miss things that we need to emit which are handed to the
    // ASTConsumer in other ways. Instead of doing this, we should create the
    // CodeGenerator earlier and register it as an ASTConsumer before we parse
    // the C++ inputs.
    non_const_ast_unit->visitLocalTopLevelDecls(
        cpp_code_generator_.get(),
        [](void* codegen_ptr, const clang::Decl* decl) {
          auto* codegen = static_cast<clang::CodeGenerator*>(codegen_ptr);
          // CodeGenerator won't modify the declaration it's given, but we can
          // only call it via the ASTConsumer interface which doesn't know that.
          auto* non_const_decl = const_cast<clang::Decl*>(decl);
          codegen->HandleTopLevelDecl(clang::DeclGroupRef(non_const_decl));
          return true;
        });
  }

  // Lower all types that were required to be complete.
  for (auto type_id : sem_ir_->types().complete_types()) {
    if (type_id.index >= 0) {
      types_.Set(type_id, BuildType(sem_ir_->types().GetInstId(type_id)));
    }
  }

  // Lower function declarations.
  for (auto [id, _] : sem_ir_->functions().enumerate()) {
    functions_.Set(id, BuildFunctionDecl(id));
  }

  // TODO: Split vtable declaration creation from definition creation to avoid
  // redundant vtable definitions for imported vtables.
  for (const auto& [id, vtable] : sem_ir_->vtables().enumerate()) {
    const auto& class_info = sem_ir().classes().Get(vtable.class_id);
    // Vtables can't be generated for generics, only for their specifics - and
    // must be done lazily based on the use of those specifics.
    if (!class_info.generic_id.has_value()) {
      vtables_.Set(id, BuildVtable(vtable, SemIR::SpecificId::None));
    }
  }

  // Lower constants.
  LowerConstants(*this, constants_);
}

// TODO: Move this to lower.cpp.
auto FileContext::LowerDefinitions() -> void {
  // Lower global variable definitions.
  // TODO: Storing both a `constants_` array and a separate `global_variables_`
  // map is redundant.
  for (auto inst_id :
       sem_ir().inst_blocks().Get(sem_ir().top_inst_block_id())) {
    // Only `VarStorage` indicates a global variable declaration in the
    // top instruction block.
    if (auto var = sem_ir().insts().TryGetAs<SemIR::VarStorage>(inst_id)) {
      // Get the global variable declaration. We created this when lowering the
      // constant unless the variable is unnamed, in which case we need to
      // create it now.
      llvm::GlobalVariable* llvm_var = nullptr;
      if (auto const_id = sem_ir().constant_values().Get(inst_id);
          const_id.is_constant()) {
        llvm_var = cast<llvm::GlobalVariable>(GetConstant(const_id, inst_id));
      } else {
        llvm_var = BuildGlobalVariableDecl(*var);
      }

      // Convert the declaration of this variable into a definition by adding an
      // initializer.
      global_variables_.Insert(inst_id, llvm_var);
      llvm_var->setInitializer(
          llvm::Constant::getNullValue(llvm_var->getValueType()));
    }
  }

  // Lower function definitions.
  for (auto [id, fn_info] : sem_ir_->functions().enumerate()) {
    // If we created a declaration and the function definition is not imported,
    // build a definition.
    if (functions_.Get(id) && fn_info.definition_id.has_value() &&
        !sem_ir().insts().GetImportSource(fn_info.definition_id).has_value()) {
      BuildFunctionDefinition(id);
    }
  }

  // Append `__global_init` to `llvm::global_ctors` to initialize global
  // variables.
  if (auto global_ctor_id = sem_ir().global_ctor_id();
      global_ctor_id.has_value()) {
    const auto& global_ctor = sem_ir().functions().Get(global_ctor_id);
    BuildFunctionBody(global_ctor_id, SemIR::SpecificId::None, global_ctor,
                      *this, global_ctor);
    llvm::appendToGlobalCtors(llvm_module(),
                              GetFunction(sem_ir().global_ctor_id()),
                              /*Priority=*/0);
  }
}

auto FileContext::Finalize() -> void {
  if (cpp_code_generator_) {
    // Clang code generation should not actually modify the AST, but isn't
    // const-correct.
    cpp_code_generator_->HandleTranslationUnit(
        const_cast<clang::ASTContext&>(clang_ast_unit()->getASTContext()));
    bool link_error = llvm::Linker::linkModules(
        /*Dest=*/llvm_module(),
        /*Src=*/std::unique_ptr<llvm::Module>(
            cpp_code_generator_->ReleaseModule()));
    CARBON_CHECK(!link_error);
  }

  // Find equivalent specifics (from the same generic), replace all uses and
  // remove duplicately lowered function definitions.
  coalescer_.CoalesceEquivalentSpecifics(lowered_specifics_,
                                         specific_functions_);
}

auto FileContext::CreateCppCodeGenerator()
    -> std::unique_ptr<clang::CodeGenerator> {
  if (!clang_ast_unit()) {
    return nullptr;
  }

  RawStringOstream clang_module_name_stream;
  clang_module_name_stream << llvm_module().getName() << ".clang";

  // Do not emit Clang's name and version as the creator of the output file.
  cpp_code_gen_options_.EmitVersionIdentMetadata = false;

  return std::unique_ptr<clang::CodeGenerator>(clang::CreateLLVMCodeGen(
      clang_ast_unit()->getASTContext().getDiagnostics(),
      clang_module_name_stream.TakeStr(), context().file_system(),
      cpp_header_search_options_, cpp_preprocessor_options_,
      cpp_code_gen_options_, llvm_context()));
}

auto FileContext::GetConstant(SemIR::ConstantId const_id,
                              SemIR::InstId use_inst_id) -> llvm::Value* {
  auto const_inst_id = sem_ir().constant_values().GetInstId(const_id);
  auto* const_value = constants_.Get(const_inst_id);

  // For value expressions and initializing expressions, the value produced by
  // a constant instruction is a value representation of the constant. For
  // initializing expressions, `FinishInit` will perform a copy if needed.
  switch (auto cat = SemIR::GetExprCategory(sem_ir(), const_inst_id)) {
    case SemIR::ExprCategory::Value:
    case SemIR::ExprCategory::Initializing:
      break;

    case SemIR::ExprCategory::DurableRef:
    case SemIR::ExprCategory::EphemeralRef:
      // Constant reference expressions lower to an address.
      return const_value;

    case SemIR::ExprCategory::NotExpr:
    case SemIR::ExprCategory::Error:
    case SemIR::ExprCategory::Mixed:
      CARBON_FATAL("Unexpected category {0} for lowered constant {1}", cat,
                   sem_ir().insts().Get(const_inst_id));
  };

  auto value_rep = SemIR::ValueRepr::ForType(
      sem_ir(), sem_ir().insts().Get(const_inst_id).type_id());
  if (value_rep.kind != SemIR::ValueRepr::Pointer) {
    return const_value;
  }

  // The value representation is a pointer. Generate a variable to hold the
  // value, or find and reuse an existing one.
  if (auto result = global_variables().Lookup(const_inst_id)) {
    return result.value();
  }

  // Include both the name of the constant, if any, and the point of use in
  // the name of the variable.
  llvm::StringRef const_name;
  llvm::StringRef use_name;
  if (inst_namer_) {
    const_name = inst_namer_->GetUnscopedNameFor(const_inst_id);
    if (use_inst_id.has_value()) {
      use_name = inst_namer_->GetUnscopedNameFor(use_inst_id);
    }
  }

  // We always need to give the global a name even if the instruction namer
  // doesn't have one to use.
  if (const_name.empty()) {
    const_name = "const";
  }
  if (use_name.empty()) {
    use_name = "anon";
  }
  llvm::StringRef sep = (use_name[0] == '.') ? "" : ".";

  auto* global_variable = new llvm::GlobalVariable(
      llvm_module(), GetType(sem_ir().GetPointeeType(value_rep.type_id)),
      /*isConstant=*/true, llvm::GlobalVariable::InternalLinkage, const_value,
      const_name + sep + use_name);

  global_variables_.Insert(const_inst_id, global_variable);
  return global_variable;
}

auto FileContext::GetOrCreateFunction(SemIR::FunctionId function_id,
                                      SemIR::SpecificId specific_id)
    -> llvm::Function* {
  // If we have already lowered a declaration of this function, just return it.
  auto** result = GetFunctionAddr(function_id, specific_id);
  if (!*result) {
    *result = BuildFunctionDecl(function_id, specific_id);
  }
  return *result;
}

auto FileContext::BuildFunctionTypeInfo(const SemIR::Function& function,
                                        SemIR::SpecificId specific_id)
    -> FunctionTypeInfo {
  const auto return_info =
      SemIR::ReturnTypeInfo::ForFunction(sem_ir(), function, specific_id);

  if (!return_info.is_valid()) {
    // The return type has not been completed, create a trivial type instead.
    return {.type =
                llvm::FunctionType::get(llvm::Type::getVoidTy(llvm_context()),
                                        /*isVarArg=*/false)};
  }

  auto get_llvm_type = [&](SemIR::TypeId type_id) -> llvm::Type* {
    if (!type_id.has_value()) {
      return nullptr;
    }
    return GetType(type_id);
  };

  // TODO: expose the `Call` parameter patterns in `Function`, and use them here
  // instead of reconstructing them via the syntactic parameter lists.
  auto implicit_param_patterns =
      sem_ir().inst_blocks().GetOrEmpty(function.implicit_param_patterns_id);
  auto param_patterns =
      sem_ir().inst_blocks().GetOrEmpty(function.param_patterns_id);

  auto* return_type = get_llvm_type(return_info.type_id);

  llvm::SmallVector<llvm::Type*> param_types;
  // Compute the return type to use for the LLVM function. If the initializing
  // representation doesn't produce a value, set the return type to void.
  // TODO: For the `Run` entry point, remap return type to i32 if it doesn't
  // return a value.
  llvm::Type* function_return_type =
      (return_info.is_valid() &&
       return_info.init_repr.kind == SemIR::InitRepr::ByCopy)
          ? return_type
          : llvm::Type::getVoidTy(llvm_context());

  // TODO: Consider either storing `param_inst_ids` somewhere so that we can
  // reuse it from `BuildFunctionDefinition` and when building calls, or factor
  // out a mechanism to compute the mapping between parameters and arguments on
  // demand.
  llvm::SmallVector<SemIR::InstId> param_inst_ids;
  auto max_llvm_params = (return_info.has_return_slot() ? 1 : 0) +
                         implicit_param_patterns.size() + param_patterns.size();
  param_types.reserve(max_llvm_params);
  param_inst_ids.reserve(max_llvm_params);
  auto return_param_id = SemIR::InstId::None;
  if (return_info.has_return_slot()) {
    param_types.push_back(
        llvm::PointerType::get(llvm_context(), /*AddressSpace=*/0));
    return_param_id = function.return_slot_pattern_id;
    param_inst_ids.push_back(return_param_id);
  }
  for (auto param_pattern_id : llvm::concat<const SemIR::InstId>(
           implicit_param_patterns, param_patterns)) {
    // TODO: Handle a general pattern here, rather than assuming that each
    // parameter pattern contains at most one binding.
    auto param_pattern_info = SemIR::Function::GetParamPatternInfoFromPatternId(
        sem_ir(), param_pattern_id);
    if (!param_pattern_info) {
      continue;
    }
    // TODO: Use a more general mechanism to determine if the binding is a
    // reference binding.
    if (param_pattern_info->inst.kind == SemIR::RefParamPattern::Kind) {
      param_types.push_back(
          llvm::PointerType::get(llvm_context(), /*AddressSpace=*/0));
      param_inst_ids.push_back(param_pattern_id);
      continue;
    }
    auto param_type_id = ExtractScrutineeType(
        sem_ir(), SemIR::GetTypeOfInstInSpecific(sem_ir(), specific_id,
                                                 param_pattern_info->inst_id));
    CARBON_CHECK(
        !param_type_id.AsConstantId().is_symbolic(),
        "Found symbolic type id after resolution when lowering type {0}.",
        param_pattern_info->inst.type_id);
    switch (auto value_rep = SemIR::ValueRepr::ForType(sem_ir(), param_type_id);
            value_rep.kind) {
      case SemIR::ValueRepr::Unknown:
        // This parameter type is incomplete. Fallback to describing the
        // function type as `void()`.
        return {.type = llvm::FunctionType::get(
                    llvm::Type::getVoidTy(llvm_context()),
                    /*isVarArg=*/false)};
      case SemIR::ValueRepr::Dependent:
        CARBON_FATAL("Lowering function with dependent parameter type");
      case SemIR::ValueRepr::None:
        break;
      case SemIR::ValueRepr::Copy:
      case SemIR::ValueRepr::Custom:
      case SemIR::ValueRepr::Pointer:
        auto* param_types_to_add = get_llvm_type(value_rep.type_id);
        param_types.push_back(param_types_to_add);
        param_inst_ids.push_back(param_pattern_id);
        break;
    }
  }
  return {.type = llvm::FunctionType::get(function_return_type, param_types,
                                          /*isVarArg=*/false),
          .param_inst_ids = std::move(param_inst_ids),
          .return_type = return_type,
          .return_param_id = return_param_id};
}

auto FileContext::HandleReferencedCppFunction(clang::FunctionDecl* cpp_decl)
    -> void {
  // TODO: To support recursive inline functions, collect all calls to
  // `HandleTopLevelDecl()` in a custom `ASTConsumer` configured in the
  // `ASTUnit`, and replay them in lowering in the `CodeGenerator`. See
  // https://discord.com/channels/655572317891461132/768530752592805919/1370509111585935443
  clang::FunctionDecl* cpp_def = cpp_decl->getDefinition();
  if (!cpp_def) {
    return;
  }

  // Create the LLVM function (`CodeGenModule::GetOrCreateLLVMFunction()`)
  // so that code generation (`CodeGenModule::EmitGlobal()`) would see this
  // function name (`CodeGenModule::getMangledName()`), and will generate
  // its definition.
  llvm::Constant* function_address =
      cpp_code_generator_->GetAddrOfGlobal(CreateGlobalDecl(cpp_def),
                                           /*isForDefinition=*/false);
  CARBON_CHECK(function_address);
}

auto FileContext::HandleReferencedSpecificFunction(
    SemIR::FunctionId function_id, SemIR::SpecificId specific_id,
    llvm::Type* llvm_type) -> void {
  CARBON_CHECK(specific_id.has_value());

  // Add this specific function to a list of specific functions whose
  // definitions we need to emit.
  // TODO: Don't do this if we know this function is emitted as a
  // non-discardable symbol in the IR for some other file.
  context().AddPendingSpecificFunctionDefinition({.context = this,
                                                  .function_id = function_id,
                                                  .specific_id = specific_id});

  // Create a unique fingerprint for the function type.
  // For now, we compute the function type fingerprint only for specifics,
  // though we might need it for all functions in order to create a canonical
  // fingerprint across translation units.
  coalescer_.CreateTypeFingerprint(specific_id, llvm_type);
}

auto FileContext::BuildFunctionDecl(SemIR::FunctionId function_id,
                                    SemIR::SpecificId specific_id)
    -> llvm::Function* {
  const auto& function = sem_ir().functions().Get(function_id);

  // Don't lower generic functions. Note that associated functions in interfaces
  // have `Self` in scope, so are implicitly generic functions.
  if (function.generic_id.has_value() && !specific_id.has_value()) {
    return nullptr;
  }

  // Don't lower builtins.
  if (function.builtin_function_kind() != SemIR::BuiltinFunctionKind::None) {
    return nullptr;
  }

  // Don't lower C++ functions that use a thunk. We will never reference them
  // directly, and their signatures would not be expected to match the
  // corresponding C++ function anyway.
  if (function.special_function_kind ==
      SemIR::Function::SpecialFunctionKind::HasCppThunk) {
    return nullptr;
  }

  // TODO: Consider tracking whether the function has been used, and only
  // lowering it if it's needed.

  auto function_type_info = BuildFunctionTypeInfo(function, specific_id);

  // TODO: For an imported inline function, consider generating an
  // `available_externally` definition.
  auto linkage = specific_id.has_value() ? llvm::Function::LinkOnceODRLinkage
                                         : llvm::Function::ExternalLinkage;

  Mangler m(*this);
  std::string mangled_name = m.Mangle(function_id, specific_id);
  if (auto* existing = llvm_module().getFunction(mangled_name)) {
    // We might have already lowered this function while lowering a different
    // file. That's OK.
    // TODO: Check-fail or maybe diagnose if the two LLVM functions are not
    // produced by declarations of the same Carbon function. Name collisions
    // between non-private members of the same library should have been
    // diagnosed by check if detected, but it's not clear that check will always
    // be able to see this problem. In theory, name collisions could also occur
    // due to fingerprint collision.
    return existing;
  }

  // If this is a C++ function, tell Clang that we referenced it.
  if (auto clang_decl_id = sem_ir().functions().Get(function_id).clang_decl_id;
      clang_decl_id.has_value()) {
    CARBON_CHECK(!specific_id.has_value(),
                 "Specific functions cannot have C++ definitions");
    HandleReferencedCppFunction(
        sem_ir().clang_decls().Get(clang_decl_id).key.decl->getAsFunction());
    // TODO: Check that the signature and mangling generated by Clang and the
    // one we generated are the same.
  }

  // If this is a specific function, we may need to do additional work to emit
  // its definition.
  if (specific_id.has_value()) {
    HandleReferencedSpecificFunction(function_id, specific_id,
                                     function_type_info.type);
  }

  auto* llvm_function = llvm::Function::Create(function_type_info.type, linkage,
                                               mangled_name, llvm_module());

  CARBON_CHECK(llvm_function->getName() == mangled_name,
               "Mangled name collision: {0}", mangled_name);

  // Set up parameters and the return slot.
  for (auto [inst_id, arg] : llvm::zip_equal(function_type_info.param_inst_ids,
                                             llvm_function->args())) {
    auto name_id = SemIR::NameId::None;
    if (inst_id == function_type_info.return_param_id) {
      name_id = SemIR::NameId::ReturnSlot;
      arg.addAttr(llvm::Attribute::getWithStructRetType(
          llvm_context(), function_type_info.return_type));
    } else {
      name_id = SemIR::GetPrettyNameFromPatternId(sem_ir(), inst_id);
    }
    arg.setName(sem_ir().names().GetIRBaseName(name_id));
  }

  return llvm_function;
}

// Find the file and function ID describing the definition of a function.
static auto GetFunctionDefinition(const SemIR::File* decl_ir,
                                  SemIR::FunctionId function_id)
    -> std::pair<const SemIR::File*, SemIR::FunctionId> {
  // Find the file containing the definition.
  auto decl_id = decl_ir->functions().Get(function_id).definition_id;
  if (!decl_id.has_value()) {
    // Function is not defined.
    return {nullptr, SemIR::FunctionId::None};
  }

  // Find the function declaration this function was originally imported from.
  while (true) {
    auto import_inst_id = decl_ir->insts().GetImportSource(decl_id);
    if (!import_inst_id.has_value()) {
      break;
    }
    auto import_inst = decl_ir->import_ir_insts().Get(import_inst_id);
    decl_ir = decl_ir->import_irs().Get(import_inst.ir_id()).sem_ir;
    decl_id = import_inst.inst_id();
  }

  auto decl_ir_function_id =
      decl_ir->insts().GetAs<SemIR::FunctionDecl>(decl_id).function_id;
  return {decl_ir, decl_ir_function_id};
}

auto FileContext::BuildFunctionDefinition(SemIR::FunctionId function_id,
                                          SemIR::SpecificId specific_id)
    -> void {
  auto [definition_ir, definition_ir_function_id] =
      GetFunctionDefinition(&sem_ir(), function_id);
  if (!definition_ir) {
    // Function is probably defined in another file; not an error.
    return;
  }

  const auto& definition_function =
      definition_ir->functions().Get(definition_ir_function_id);
  BuildFunctionBody(
      function_id, specific_id, sem_ir().functions().Get(function_id),
      context().GetFileContext(definition_ir), definition_function);
}

auto FileContext::BuildFunctionBody(SemIR::FunctionId function_id,
                                    SemIR::SpecificId specific_id,
                                    const SemIR::Function& declaration_function,
                                    FileContext& definition_context,
                                    const SemIR::Function& definition_function)
    -> void {
  // On crash, report the function we were lowering.
  PrettyStackTraceFunction stack_trace_entry([&](llvm::raw_ostream& output) {
    SemIR::DiagnosticLocConverter converter(
        &context().tree_and_subtrees_getters(), &sem_ir());
    auto converted =
        converter.Convert(SemIR::LocId(declaration_function.definition_id),
                          /*token_only=*/false);
    converted.loc.FormatLocation(output);
    output << "Lowering function ";
    if (specific_id.has_value()) {
      output << SemIR::StringifySpecific(sem_ir(), specific_id);
    } else {
      output << SemIR::StringifyConstantInst(
          sem_ir(), declaration_function.definition_id);
    }
    output << "\n";
    // Crash output has a tab indent; try to indent slightly past that.
    converted.loc.FormatSnippet(output, /*indent=*/10);
  });

  // Note that `definition_function` is potentially from a different SemIR::File
  // than the one that this file context represents. Any lowering done for
  // values derived from `definition_function` should use `definition_context`
  // instead of our context.
  const auto& definition_ir = definition_context.sem_ir();

  auto* llvm_function = GetFunction(function_id, specific_id);
  CARBON_CHECK(llvm_function,
               "Attempting to define function that was not declared");

  const auto& body_block_ids = definition_function.body_block_ids;
  CARBON_DCHECK(!body_block_ids.empty(),
                "No function body blocks found during lowering.");

  // Store which specifics were already lowered (with definitions) for each
  // generic.
  if (declaration_function.generic_id.has_value() && specific_id.has_value()) {
    // TODO: We should track this in the definition context instead so that we
    // can deduplicate specifics from different files.
    AddLoweredSpecificForGeneric(declaration_function.generic_id, specific_id);
  }

  // Set attributes on the function definition.
  {
    llvm::AttrBuilder attr_builder(llvm_context());
    attr_builder.addAttribute(llvm::Attribute::NoUnwind);

    // TODO: We should take the opt level from the SemIR file; it might not be
    // the same for all files in a compilation.
    if (context().opt_level() == Lower::OptimizationLevel::None) {
      // --optimize=none disables all optimizations for this function.
      attr_builder.addAttribute(llvm::Attribute::OptimizeNone);
      attr_builder.addAttribute(llvm::Attribute::NoInline);
    } else {
      // Otherwise, always inline thunks.
      if (definition_function.special_function_kind ==
          SemIR::Function::SpecialFunctionKind::Thunk) {
        attr_builder.addAttribute(llvm::Attribute::AlwaysInline);
      }

      // Convert --optimize=size into optsize and minsize.
      if (context().opt_level() == Lower::OptimizationLevel::Size) {
        attr_builder.addAttribute(llvm::Attribute::OptimizeForSize);
        attr_builder.addAttribute(llvm::Attribute::MinSize);
      }

      // TODO: Should we generate an InlineHint for some functions? Perhaps for
      // those defined in the API file?
    }
    llvm_function->addFnAttrs(attr_builder);
  }

  FunctionContext function_lowering(
      definition_context, llvm_function, *this, specific_id,
      coalescer_.InitializeFingerprintForSpecific(specific_id),
      definition_context.BuildDISubprogram(definition_function, llvm_function),
      vlog_stream_);

  // Add parameters to locals.
  // TODO: This duplicates the mapping between sem_ir instructions and LLVM
  // function parameters that was already computed in BuildFunctionDecl.
  // We should only do that once.
  auto call_param_ids = definition_ir.inst_blocks().GetOrEmpty(
      definition_function.call_params_id);
  int param_index = 0;

  // TODO: Find a way to ensure this code and the function-call lowering use
  // the same parameter ordering.

  // Lowers the given parameter. Must be called in LLVM calling convention
  // parameter order.
  auto lower_param = [&](SemIR::InstId param_id) {
    // Get the value of the parameter from the function argument.
    llvm::Value* param_value;

    // The `type_id` of a parameter tracks the parameter's type.
    CARBON_CHECK(definition_ir.insts().Is<SemIR::AnyParam>(param_id));
    auto param_type = function_lowering.GetTypeIdOfInst(param_id);
    if (function_lowering.GetValueRepr(param_type).repr.kind !=
        SemIR::ValueRepr::None) {
      param_value = llvm_function->getArg(param_index);
      ++param_index;
    } else {
      param_value =
          llvm::PoisonValue::get(function_lowering.GetType(param_type));
    }
    // The value of the parameter is the value of the argument.
    function_lowering.SetLocal(param_id, param_value);
  };

  // Lower the return slot parameter.
  if (declaration_function.return_slot_pattern_id.has_value()) {
    auto call_param_id = call_param_ids.consume_back();
    // The LLVM calling convention has the return slot first rather than last.
    // Note that this queries whether there is a return slot at the LLVM level,
    // whereas `function.return_slot_pattern_id.has_value()` queries whether
    // there is a return slot at the SemIR level.
    if (SemIR::ReturnTypeInfo::ForFunction(sem_ir(), declaration_function,
                                           specific_id)
            .has_return_slot()) {
      lower_param(call_param_id);
    } else {
      // The return slot might still be mentioned as a destination location, but
      // shouldn't actually be used for anything, so we can use a poison value
      // for it.
      function_lowering.SetLocal(call_param_id,
                                 llvm::PoisonValue::get(llvm::PointerType::get(
                                     llvm_context(), /*AddressSpace=*/0)));
    }
  }

  // Lower the remaining call parameters.
  for (auto param_id : call_param_ids) {
    lower_param(param_id);
  }

  auto decl_block_id = SemIR::InstBlockId::None;
  if (function_id == sem_ir().global_ctor_id()) {
    decl_block_id = SemIR::InstBlockId::Empty;
  } else {
    decl_block_id =
        definition_ir.insts()
            .GetAs<SemIR::FunctionDecl>(definition_function.latest_decl_id())
            .decl_block_id;
  }

  // Lowers the contents of decl_block_id into the corresponding LLVM block,
  // creating it if it doesn't already exist.
  auto lower_block = [&](SemIR::InstBlockId block_id) {
    CARBON_VLOG("Lowering {0}\n", block_id);
    auto* llvm_block = function_lowering.GetBlock(block_id);
    // Keep the LLVM blocks in lexical order.
    llvm_block->moveBefore(llvm_function->end());
    function_lowering.builder().SetInsertPoint(llvm_block);
    function_lowering.LowerBlockContents(block_id);
  };

  lower_block(decl_block_id);

  // If the decl block is empty, reuse it as the first body block. We don't do
  // this when the decl block is non-empty so that any branches back to the
  // first body block don't also re-execute the decl.
  llvm::BasicBlock* block = function_lowering.builder().GetInsertBlock();
  if (block->empty() &&
      function_lowering.TryToReuseBlock(body_block_ids.front(), block)) {
    // Reuse this block as the first block of the function body.
  } else {
    function_lowering.builder().CreateBr(
        function_lowering.GetBlock(body_block_ids.front()));
  }

  // Lower all blocks.
  for (auto block_id : body_block_ids) {
    lower_block(block_id);
  }

  // LLVM requires that the entry block has no predecessors.
  auto* entry_block = &llvm_function->getEntryBlock();
  if (entry_block->hasNPredecessorsOrMore(1)) {
    auto* new_entry_block = llvm::BasicBlock::Create(
        llvm_context(), "entry", llvm_function, entry_block);
    llvm::BranchInst::Create(entry_block, new_entry_block);
  }

  // Emit fingerprint accumulated inside the function context.
  function_lowering.EmitFinalFingerprint();
}

auto FileContext::BuildDISubprogram(const SemIR::Function& function,
                                    const llvm::Function* llvm_function)
    -> llvm::DISubprogram* {
  if (!context().di_compile_unit()) {
    return nullptr;
  }
  auto name = sem_ir().names().GetAsStringIfIdentifier(function.name_id);
  CARBON_CHECK(name, "Unexpected special name for function: {0}",
               function.name_id);
  auto loc = GetLocForDI(function.definition_id);
  // TODO: Add more details here, including real subroutine type (once type
  // information is built), etc.
  return context().di_builder().createFunction(
      context().di_compile_unit(), *name, llvm_function->getName(),
      /*File=*/context().di_builder().createFile(loc.filename, ""),
      /*LineNo=*/loc.line_number,
      context().di_builder().createSubroutineType(
          context().di_builder().getOrCreateTypeArray({})),
      /*ScopeLine=*/0, llvm::DINode::FlagZero,
      llvm::DISubprogram::SPFlagDefinition);
}

// BuildTypeForInst is used to construct types for FileContext::BuildType below.
// Implementations return the LLVM type for the instruction. This first overload
// is the fallback handler for non-type instructions.
template <typename InstT>
  requires(InstT::Kind.is_type() == SemIR::InstIsType::Never)
static auto BuildTypeForInst(FileContext& /*context*/, InstT inst)
    -> llvm::Type* {
  CARBON_FATAL("Cannot use inst as type: {0}", inst);
}

template <typename InstT>
  requires(InstT::Kind.is_symbolic_when_type())
static auto BuildTypeForInst(FileContext& context, InstT /*inst*/)
    -> llvm::Type* {
  // Treat non-monomorphized symbolic types as opaque.
  return llvm::StructType::get(context.llvm_context());
}

static auto BuildTypeForInst(FileContext& context, SemIR::ArrayType inst)
    -> llvm::Type* {
  return llvm::ArrayType::get(
      context.GetType(context.sem_ir().types().GetTypeIdForTypeInstId(
          inst.element_type_inst_id)),
      *context.sem_ir().GetArrayBoundValue(inst.bound_id));
}

static auto BuildTypeForInst(FileContext& context, SemIR::BoolType /*inst*/)
    -> llvm::Type* {
  // TODO: We may want to have different representations for `bool` storage
  // (`i8`) versus for `bool` values (`i1`).
  return llvm::Type::getInt1Ty(context.llvm_context());
}

static auto BuildTypeForInst(FileContext& context, SemIR::ClassType inst)
    -> llvm::Type* {
  auto object_repr_id = context.sem_ir()
                            .classes()
                            .Get(inst.class_id)
                            .GetObjectRepr(context.sem_ir(), inst.specific_id);
  return context.GetType(object_repr_id);
}

template <typename InstT>
  requires(SemIR::Internal::HasInstCategory<SemIR::AnyQualifiedType, InstT>)
static auto BuildTypeForInst(FileContext& context, InstT inst) -> llvm::Type* {
  return context.GetType(
      context.sem_ir().types().GetTypeIdForTypeInstId(inst.inner_id));
}

static auto BuildTypeForInst(FileContext& context, SemIR::CustomLayoutType inst)
    -> llvm::Type* {
  auto layout = context.sem_ir().custom_layouts().Get(inst.layout_id);
  return llvm::ArrayType::get(llvm::Type::getInt8Ty(context.llvm_context()),
                              layout[SemIR::CustomLayoutId::SizeIndex]);
}

static auto BuildTypeForInst(FileContext& context,
                             SemIR::ImplWitnessAssociatedConstant inst)
    -> llvm::Type* {
  return context.GetType(inst.type_id);
}

static auto BuildTypeForInst(FileContext& /*context*/,
                             SemIR::ErrorInst /*inst*/) -> llvm::Type* {
  // This is a complete type but uses of it should never be lowered.
  return nullptr;
}

static auto BuildTypeForInst(FileContext& context, SemIR::FloatType inst)
    -> llvm::Type* {
  return llvm::Type::getFloatingPointTy(context.llvm_context(),
                                        inst.float_kind.Semantics());
}

static auto BuildTypeForInst(FileContext& context, SemIR::IntType inst)
    -> llvm::Type* {
  auto width =
      context.sem_ir().insts().TryGetAs<SemIR::IntValue>(inst.bit_width_id);
  CARBON_CHECK(width, "Can't lower int type with symbolic width");
  return llvm::IntegerType::get(
      context.llvm_context(),
      context.sem_ir().ints().Get(width->int_id).getZExtValue());
}

static auto BuildTypeForInst(FileContext& context, SemIR::PointerType /*inst*/)
    -> llvm::Type* {
  return llvm::PointerType::get(context.llvm_context(), /*AddressSpace=*/0);
}

static auto BuildTypeForInst(FileContext& /*context*/,
                             SemIR::PatternType /*inst*/) -> llvm::Type* {
  CARBON_FATAL("Unexpected pattern type in lowering");
}

static auto BuildTypeForInst(FileContext& context, SemIR::StructType inst)
    -> llvm::Type* {
  auto fields = context.sem_ir().struct_type_fields().Get(inst.fields_id);
  llvm::SmallVector<llvm::Type*> subtypes;
  subtypes.reserve(fields.size());
  for (auto field : fields) {
    subtypes.push_back(context.GetType(
        context.sem_ir().types().GetTypeIdForTypeInstId(field.type_inst_id)));
  }
  return llvm::StructType::get(context.llvm_context(), subtypes);
}

static auto BuildTypeForInst(FileContext& context, SemIR::TupleType inst)
    -> llvm::Type* {
  // TODO: Investigate special-casing handling of empty tuples so that they
  // can be collectively replaced with LLVM's void, particularly around
  // function returns. LLVM doesn't allow declaring variables with a void
  // type, so that may require significant special casing.
  auto elements = context.sem_ir().inst_blocks().Get(inst.type_elements_id);
  llvm::SmallVector<llvm::Type*> subtypes;
  subtypes.reserve(elements.size());
  for (auto type_id : context.sem_ir().types().GetBlockAsTypeIds(elements)) {
    subtypes.push_back(context.GetType(type_id));
  }
  return llvm::StructType::get(context.llvm_context(), subtypes);
}

static auto BuildTypeForInst(FileContext& context, SemIR::TypeType /*inst*/)
    -> llvm::Type* {
  return context.GetTypeType();
}

static auto BuildTypeForInst(FileContext& context, SemIR::VtableType /*inst*/)
    -> llvm::Type* {
  return llvm::Type::getVoidTy(context.llvm_context());
}

static auto BuildTypeForInst(FileContext& context,
                             SemIR::SpecificFunctionType /*inst*/)
    -> llvm::Type* {
  return llvm::PointerType::get(context.llvm_context(), 0);
}

template <typename InstT>
  requires(
      InstT::Kind.template IsAnyOf<
          SemIR::AssociatedEntityType, SemIR::AutoType, SemIR::BoundMethodType,
          SemIR::CharLiteralType, SemIR::CppOverloadSetType, SemIR::CppVoidType,
          SemIR::FacetType, SemIR::FloatLiteralType, SemIR::FunctionType,
          SemIR::FunctionTypeWithSelfType, SemIR::GenericClassType,
          SemIR::GenericInterfaceType, SemIR::GenericNamedConstraintType,
          SemIR::InstType, SemIR::IntLiteralType, SemIR::NamespaceType,
          SemIR::WhereExpr, SemIR::WitnessType, SemIR::UnboundElementType>())
static auto BuildTypeForInst(FileContext& context, InstT /*inst*/)
    -> llvm::Type* {
  // Return an empty struct as a placeholder.
  // TODO: Should we model an interface as a witness table, or an associated
  // entity as an index?
  return llvm::StructType::get(context.llvm_context());
}

auto FileContext::BuildType(SemIR::InstId inst_id) -> llvm::Type* {
  // Use overload resolution to select the implementation, producing compile
  // errors when BuildTypeForInst isn't defined for a given instruction.
  CARBON_KIND_SWITCH(sem_ir_->insts().Get(inst_id)) {
#define CARBON_SEM_IR_INST_KIND(Name)     \
  case CARBON_KIND(SemIR::Name inst): {   \
    return BuildTypeForInst(*this, inst); \
  }
#include "toolchain/sem_ir/inst_kind.def"
  }
}

auto FileContext::BuildGlobalVariableDecl(SemIR::VarStorage var_storage)
    -> llvm::GlobalVariable* {
  Mangler m(*this);
  auto mangled_name = m.MangleGlobalVariable(var_storage.pattern_id);
  auto linkage = llvm::GlobalVariable::ExternalLinkage;

  // If the variable doesn't have an externally-visible name, demote it to
  // internal linkage and invent a plausible name that shouldn't collide with
  // any of our real manglings.
  if (mangled_name.empty()) {
    linkage = llvm::GlobalVariable::InternalLinkage;
    if (inst_namer_) {
      mangled_name =
          ("var.anon" + inst_namer_->GetUnscopedNameFor(var_storage.pattern_id))
              .str();
    }
  }

  auto* type = GetType(var_storage.type_id);
  return new llvm::GlobalVariable(llvm_module(), type,
                                  /*isConstant=*/false, linkage,
                                  /*Initializer=*/nullptr, mangled_name);
}

auto FileContext::GetLocForDI(SemIR::InstId inst_id) -> Context::LocForDI {
  return context().GetLocForDI(
      GetAbsoluteNodeId(sem_ir_, SemIR::LocId(inst_id)).back());
}

auto FileContext::BuildVtable(const SemIR::Vtable& vtable,
                              SemIR::SpecificId specific_id)
    -> llvm::GlobalVariable* {
  const auto& class_info = sem_ir().classes().Get(vtable.class_id);

  Mangler m(*this);
  std::string mangled_name = m.MangleVTable(class_info, specific_id);

  if (sem_ir()
          .insts()
          .GetImportSource(class_info.first_owning_decl_id)
          .has_value()) {
    // Emit a declaration of an imported vtable using a(n opaque) pointer type.
    // This doesn't have to match the definition that appears elsewhere, it'll
    // still get merged correctly.
    auto* gv = new llvm::GlobalVariable(
        llvm_module(),
        llvm::PointerType::get(llvm_context(), /*AddressSpace=*/0),
        /*isConstant=*/true, llvm::GlobalValue::ExternalLinkage, nullptr,
        mangled_name);
    gv->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
    return gv;
  }

  auto vtable_inst_block =
      sem_ir().inst_blocks().Get(vtable.virtual_functions_id);

  auto* entry_type = llvm::IntegerType::getInt32Ty(llvm_context());
  auto* table_type = llvm::ArrayType::get(entry_type, vtable_inst_block.size());

  auto* llvm_vtable = new llvm::GlobalVariable(
      llvm_module(), table_type, /*isConstant=*/true,
      llvm::GlobalValue::ExternalLinkage, nullptr, mangled_name);

  auto* i32_type = llvm::IntegerType::getInt32Ty(llvm_context());
  auto* i64_type = llvm::IntegerType::getInt64Ty(llvm_context());
  auto* vtable_const_int =
      llvm::ConstantExpr::getPtrToInt(llvm_vtable, i64_type);

  llvm::SmallVector<llvm::Constant*> vfuncs;
  vfuncs.reserve(vtable_inst_block.size());

  for (auto fn_decl_id : vtable_inst_block) {
    auto [_1, _2, fn_id, fn_specific_id] =
        DecomposeVirtualFunction(sem_ir(), fn_decl_id, specific_id);

    vfuncs.push_back(llvm::ConstantExpr::getTrunc(
        llvm::ConstantExpr::getSub(
            llvm::ConstantExpr::getPtrToInt(
                GetOrCreateFunction(fn_id, fn_specific_id), i64_type),
            vtable_const_int),
        i32_type));
  }

  llvm_vtable->setInitializer(llvm::ConstantArray::get(table_type, vfuncs));
  llvm_vtable->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);

  return llvm_vtable;
}

}  // namespace Carbon::Lower