carbon-lang/toolchain/check/cpp_thunk.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/check/cpp_thunk.h"

#include "clang/AST/GlobalDecl.h"
#include "clang/AST/Mangle.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/Sema.h"
#include "toolchain/check/call.h"
#include "toolchain/check/context.h"
#include "toolchain/check/control_flow.h"
#include "toolchain/check/convert.h"
#include "toolchain/check/literal.h"
#include "toolchain/check/type.h"
#include "toolchain/check/type_completion.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

// Returns the C++ thunk mangled name given the callee function.
static auto GenerateThunkMangledName(
    clang::MangleContext& mangle_context,
    const clang::FunctionDecl& callee_function_decl) -> std::string {
  RawStringOstream mangled_name_stream;
  mangle_context.mangleName(clang::GlobalDecl(&callee_function_decl),
                            mangled_name_stream);
  mangled_name_stream << ".carbon_thunk";

  return mangled_name_stream.TakeStr();
}

// Returns true if a C++ thunk is required for the given type. A C++ thunk is
// required for any type except for void, pointer types and signed 32-bit and
// 64-bit integers.
static auto IsThunkRequiredForType(Context& context, SemIR::TypeId type_id)
    -> bool {
  if (!type_id.has_value() || type_id == SemIR::ErrorInst::TypeId) {
    return false;
  }

  type_id = context.types().GetUnqualifiedType(type_id);

  switch (context.types().GetAsInst(type_id).kind()) {
    case SemIR::PointerType::Kind: {
      return false;
    }

    case SemIR::ClassType::Kind: {
      if (!context.types().IsComplete(type_id)) {
        // Signed integers of 32 or 64 bits should be completed when imported.
        return true;
      }

      auto int_info = context.types().TryGetIntTypeInfo(type_id);
      if (!int_info || !int_info->bit_width.has_value()) {
        return true;
      }

      llvm::APInt bit_width = context.ints().Get(int_info->bit_width);
      return bit_width != 32 && bit_width != 64;
    }

    default:
      return true;
  }
}

auto IsCppThunkRequired(Context& context, const SemIR::Function& function)
    -> bool {
  if (!function.clang_decl_id.has_value()) {
    return false;
  }

  SemIR::TypeId return_type_id =
      function.GetDeclaredReturnType(context.sem_ir());
  if (return_type_id.has_value()) {
    // TODO: Support non-void return values.
    return false;
  }

  bool thunk_required_for_param = false;
  for (auto param_id :
       context.inst_blocks().GetOrEmpty(function.call_params_id)) {
    if (param_id == SemIR::ErrorInst::InstId) {
      return false;
    }
    if (!thunk_required_for_param &&
        IsThunkRequiredForType(
            context,
            context.insts().GetAs<SemIR::AnyParam>(param_id).type_id)) {
      thunk_required_for_param = true;
    }
  }

  return thunk_required_for_param;
}

// Returns whether the type is a pointer or a signed int of 32 or 64 bits.
static auto IsSimpleAbiType(clang::ASTContext& ast_context,
                            clang::QualType type) -> bool {
  if (type->isPointerType()) {
    return true;
  }

  if (const auto* builtin_type = type->getAs<clang::BuiltinType>()) {
    if (builtin_type->isSignedInteger()) {
      uint64_t type_size = ast_context.getIntWidth(type);
      return type_size == 32 || type_size == 64;
    }
  }

  return false;
}

namespace {
// Information about the callee of a thunk.
struct CalleeFunctionInfo {
  explicit CalleeFunctionInfo(clang::FunctionDecl* decl) : decl(decl) {
    const auto* method_decl = dyn_cast<clang::CXXMethodDecl>(decl);
    has_object_parameter = method_decl && !method_decl->isStatic() &&
                           !isa<clang::CXXConstructorDecl>(method_decl);
    if (has_object_parameter && method_decl->isImplicitObjectMemberFunction()) {
      implicit_this_type = method_decl->getThisType();
    }
  }

  // Returns whether this callee has an implicit `this` parameter.
  auto has_implicit_object_parameter() const -> bool {
    return !implicit_this_type.isNull();
  }

  // Returns whether this callee has an explicit `this` parameter.
  auto has_explicit_object_parameter() const -> bool {
    return has_object_parameter && !has_implicit_object_parameter();
  }

  // Returns the number of parameters the thunk should have.
  auto num_thunk_params() const -> unsigned {
    return has_implicit_object_parameter() + decl->getNumParams();
  }

  auto GetThunkParamIndex(unsigned callee_param_index) const -> unsigned {
    return has_implicit_object_parameter() + callee_param_index;
  }

  // The callee function.
  clang::FunctionDecl* decl;

  // Whether the callee has an object parameter, which might be explicit or
  // implicit.
  bool has_object_parameter;

  // If the callee has an implicit object parameter, the corresponding `this`
  // type. Otherwise a null type.
  clang::QualType implicit_this_type;
};
}  // namespace

// Creates the thunk parameter types given the callee function.
static auto BuildThunkParameterTypes(clang::ASTContext& ast_context,
                                     CalleeFunctionInfo callee_info)
    -> llvm::SmallVector<clang::QualType> {
  llvm::SmallVector<clang::QualType> thunk_param_types;
  thunk_param_types.reserve(callee_info.num_thunk_params());
  if (callee_info.has_implicit_object_parameter()) {
    thunk_param_types.push_back(ast_context.getAttributedType(
        clang::NullabilityKind::NonNull, callee_info.implicit_this_type,
        callee_info.implicit_this_type));
  }

  for (const clang::ParmVarDecl* callee_param :
       callee_info.decl->parameters()) {
    clang::QualType param_type = callee_param->getType();
    bool is_simple_abi_type = IsSimpleAbiType(ast_context, param_type);
    if (!is_simple_abi_type) {
      clang::QualType pointer_type = ast_context.getPointerType(param_type);
      param_type = ast_context.getAttributedType(
          clang::NullabilityKind::NonNull, pointer_type, pointer_type);
    }
    thunk_param_types.push_back(param_type);
  }

  CARBON_CHECK(thunk_param_types.size() == callee_info.num_thunk_params());
  return thunk_param_types;
}

// Returns the thunk parameters using the callee function parameter identifiers.
static auto BuildThunkParameters(clang::ASTContext& ast_context,
                                 CalleeFunctionInfo callee_info,
                                 clang::FunctionDecl* thunk_function_decl)
    -> llvm::SmallVector<clang::ParmVarDecl*> {
  clang::SourceLocation clang_loc = callee_info.decl->getLocation();

  const auto* thunk_function_proto_type =
      thunk_function_decl->getFunctionType()->getAs<clang::FunctionProtoType>();

  llvm::SmallVector<clang::ParmVarDecl*> thunk_params;
  unsigned num_thunk_params = thunk_function_decl->getNumParams();
  thunk_params.reserve(num_thunk_params);

  if (callee_info.has_implicit_object_parameter()) {
    clang::ParmVarDecl* thunk_param =
        clang::ParmVarDecl::Create(ast_context, thunk_function_decl, clang_loc,
                                   clang_loc, &ast_context.Idents.get("this"),
                                   thunk_function_proto_type->getParamType(0),
                                   nullptr, clang::SC_None, nullptr);
    thunk_params.push_back(thunk_param);
  }

  for (unsigned i : llvm::seq(callee_info.decl->getNumParams())) {
    clang::ParmVarDecl* thunk_param = clang::ParmVarDecl::Create(
        ast_context, thunk_function_decl, clang_loc, clang_loc,
        callee_info.decl->getParamDecl(i)->getIdentifier(),
        thunk_function_proto_type->getParamType(
            callee_info.GetThunkParamIndex(i)),
        nullptr, clang::SC_None, nullptr);
    thunk_params.push_back(thunk_param);
  }

  CARBON_CHECK(thunk_params.size() == num_thunk_params);
  return thunk_params;
}

// Returns the thunk function declaration given the callee function and the
// thunk parameter types.
static auto CreateThunkFunctionDecl(
    Context& context, CalleeFunctionInfo callee_info,
    llvm::ArrayRef<clang::QualType> thunk_param_types) -> clang::FunctionDecl* {
  clang::ASTContext& ast_context = context.ast_context();
  clang::SourceLocation clang_loc = callee_info.decl->getLocation();

  clang::IdentifierInfo& identifier_info = ast_context.Idents.get(
      callee_info.decl->getNameAsString() + "__carbon_thunk");

  auto ext_proto_info = clang::FunctionProtoType::ExtProtoInfo();
  clang::QualType thunk_function_type = ast_context.getFunctionType(
      callee_info.decl->getReturnType(), thunk_param_types, ext_proto_info);

  clang::DeclContext* decl_context = ast_context.getTranslationUnitDecl();
  // TODO: Thunks should not have external linkage, consider using `SC_Static`.
  clang::FunctionDecl* thunk_function_decl = clang::FunctionDecl::Create(
      ast_context, decl_context, clang_loc, clang_loc,
      clang::DeclarationName(&identifier_info), thunk_function_type,
      /*TInfo=*/nullptr, clang::SC_Extern);
  decl_context->addDecl(thunk_function_decl);

  thunk_function_decl->setParams(
      BuildThunkParameters(ast_context, callee_info, thunk_function_decl));

  // Set always_inline.
  thunk_function_decl->addAttr(
      clang::AlwaysInlineAttr::CreateImplicit(ast_context));

  // Set asm("<callee function mangled name>.carbon_thunk").
  thunk_function_decl->addAttr(clang::AsmLabelAttr::CreateImplicit(
      ast_context,
      GenerateThunkMangledName(*context.sem_ir().clang_mangle_context(),
                               *callee_info.decl),
      clang_loc));

  return thunk_function_decl;
}

// Builds a reference to the given parameter thunk, which is carrying a value of
// the given type.
static auto BuildThunkParamRef(clang::Sema& sema,
                               clang::FunctionDecl* thunk_function_decl,
                               unsigned thunk_index, clang::QualType type)
    -> clang::Expr* {
  clang::ParmVarDecl* thunk_param =
      thunk_function_decl->getParamDecl(thunk_index);
  clang::SourceLocation clang_loc = thunk_param->getLocation();

  clang::Expr* call_arg = sema.BuildDeclRefExpr(
      thunk_param, thunk_param->getType().getNonReferenceType(),
      clang::VK_LValue, clang_loc);
  if (thunk_param->getType() != type) {
    // TODO: Consider inserting a cast to an rvalue. Note that we currently
    // pass pointers to non-temporary objects as the argument when calling a
    // thunk, so we'll need to either change that or generate different thunks
    // depending on whether we're moving from each parameter.
    clang::ExprResult deref_result =
        sema.BuildUnaryOp(nullptr, clang_loc, clang::UO_Deref, call_arg);
    CARBON_CHECK(deref_result.isUsable());
    call_arg = deref_result.get();
  }
  return call_arg;
}

// Builds a reference to the parameter thunk parameter corresponding to the
// given callee parameter index.
static auto BuildParamRefForCalleeArg(clang::Sema& sema,
                                      clang::FunctionDecl* thunk_function_decl,
                                      CalleeFunctionInfo callee_info,
                                      unsigned callee_index) -> clang::Expr* {
  unsigned thunk_index = callee_info.GetThunkParamIndex(callee_index);
  return BuildThunkParamRef(
      sema, thunk_function_decl, thunk_index,
      callee_info.decl->getParamDecl(callee_index)->getType());
}

// Builds an argument list for the callee function by creating suitable uses of
// the corresponding thunk parameters.
static auto BuildCalleeArgs(clang::Sema& sema,
                            clang::FunctionDecl* thunk_function_decl,
                            CalleeFunctionInfo callee_info)
    -> llvm::SmallVector<clang::Expr*> {
  llvm::SmallVector<clang::Expr*> call_args;
  // The object parameter is always passed as `self`, not in the callee argument
  // list, so the first argument corresponds to the second parameter if there is
  // an explicit object parameter and the first parameter otherwise.
  unsigned first_param = callee_info.has_explicit_object_parameter();
  unsigned num_params = callee_info.decl->getNumParams();
  call_args.reserve(num_params - first_param);
  for (unsigned callee_index : llvm::seq(first_param, num_params)) {
    call_args.push_back(BuildParamRefForCalleeArg(sema, thunk_function_decl,
                                                  callee_info, callee_index));
  }
  return call_args;
}

// Builds the thunk function body which calls the callee function using the call
// args and returns the callee function return value. Returns nullptr on
// failure.
static auto BuildThunkBody(clang::Sema& sema,
                           clang::FunctionDecl* thunk_function_decl,
                           CalleeFunctionInfo callee_info)
    -> clang::StmtResult {
  clang::SourceLocation clang_loc = callee_info.decl->getLocation();

  // If the callee has an object parameter, build a member access expression as
  // the callee. Otherwise, build a regular reference to the function.
  clang::ExprResult callee;
  if (callee_info.has_object_parameter) {
    auto* object_param_ref =
        BuildThunkParamRef(sema, thunk_function_decl, 0,
                           callee_info.has_implicit_object_parameter()
                               ? thunk_function_decl->getParamDecl(0)->getType()
                               : callee_info.decl->getParamDecl(0)->getType());
    bool is_arrow = callee_info.has_implicit_object_parameter();
    auto object =
        sema.PerformMemberExprBaseConversion(object_param_ref, is_arrow);
    if (object.isInvalid()) {
      return clang::StmtError();
    }
    callee = sema.BuildMemberExpr(
        object.get(), is_arrow, clang_loc, clang::NestedNameSpecifierLoc(),
        clang::SourceLocation(), callee_info.decl,
        clang::DeclAccessPair::make(callee_info.decl, clang::AS_public),
        /*HadMultipleCandidates=*/false, clang::DeclarationNameInfo(),
        sema.getASTContext().BoundMemberTy, clang::VK_PRValue,
        clang::OK_Ordinary);
  } else {
    callee =
        sema.BuildDeclRefExpr(callee_info.decl, callee_info.decl->getType(),
                              clang::VK_PRValue, clang_loc);
  }

  if (callee.isInvalid()) {
    return clang::StmtError();
  }

  // Build the argument list.
  llvm::SmallVector<clang::Expr*> call_args =
      BuildCalleeArgs(sema, thunk_function_decl, callee_info);

  clang::ExprResult call = sema.BuildCallExpr(nullptr, callee.get(), clang_loc,
                                              call_args, clang_loc);
  if (!call.isUsable()) {
    return clang::StmtError();
  }

  // TODO: Consider building a CompoundStmt holding this to make our result more
  // closely resemble a real C++ function.
  return sema.BuildReturnStmt(clang_loc, call.get());
}

auto BuildCppThunk(Context& context, const SemIR::Function& callee_function)
    -> clang::FunctionDecl* {
  clang::FunctionDecl* callee_function_decl =
      context.sem_ir()
          .clang_decls()
          .Get(callee_function.clang_decl_id)
          .decl->getAsFunction();
  CARBON_CHECK(callee_function_decl);

  CalleeFunctionInfo callee_info(callee_function_decl);

  // Build the thunk function declaration.
  auto thunk_param_types =
      BuildThunkParameterTypes(context.ast_context(), callee_info);
  clang::FunctionDecl* thunk_function_decl =
      CreateThunkFunctionDecl(context, callee_info, thunk_param_types);

  // Build the thunk function body.
  clang::Sema& sema = context.sem_ir().clang_ast_unit()->getSema();
  clang::Sema::ContextRAII context_raii(sema, thunk_function_decl);
  sema.ActOnStartOfFunctionDef(nullptr, thunk_function_decl);
  clang::StmtResult body =
      BuildThunkBody(sema, thunk_function_decl, callee_info);
  sema.ActOnFinishFunctionBody(thunk_function_decl, body.get());
  if (body.isInvalid()) {
    return nullptr;
  }

  return thunk_function_decl;
}

auto PerformCppThunkCall(Context& context, SemIR::LocId loc_id,
                         SemIR::FunctionId callee_function_id,
                         llvm::ArrayRef<SemIR::InstId> callee_arg_ids,
                         SemIR::InstId thunk_callee_id) -> SemIR::InstId {
  llvm::ArrayRef<SemIR::InstId> callee_function_params =
      context.inst_blocks().GetOrEmpty(
          context.functions().Get(callee_function_id).call_params_id);

  llvm::ArrayRef<SemIR::InstId> thunk_function_params =
      context.inst_blocks().GetOrEmpty(
          context.functions()
              .Get(GetCalleeFunction(context.sem_ir(), thunk_callee_id)
                       .function_id)
              .call_params_id);

  size_t num_params = callee_function_params.size();
  CARBON_CHECK(thunk_function_params.size() == num_params);
  CARBON_CHECK(callee_arg_ids.size() == num_params);
  llvm::SmallVector<SemIR::InstId> thunk_arg_ids;
  thunk_arg_ids.reserve(num_params);
  for (auto [callee_param_inst_id, thunk_param_inst_id, callee_arg_id] :
       llvm::zip(callee_function_params, thunk_function_params,
                 callee_arg_ids)) {
    SemIR::TypeId callee_param_type_id =
        context.insts().GetAs<SemIR::AnyParam>(callee_param_inst_id).type_id;
    SemIR::TypeId thunk_param_type_id =
        context.insts().GetAs<SemIR::AnyParam>(thunk_param_inst_id).type_id;

    SemIR::InstId arg_id = callee_arg_id;
    if (callee_param_type_id != thunk_param_type_id) {
      arg_id = Convert(context, loc_id, arg_id,
                       {.kind = ConversionTarget::CppThunkRef,
                        .type_id = callee_param_type_id});
      arg_id = AddInst<SemIR::AddrOf>(
          context, loc_id,
          {.type_id = GetPointerType(
               context, context.types().GetInstId(callee_param_type_id)),
           .lvalue_id = arg_id});
      arg_id =
          ConvertToValueOfType(context, loc_id, arg_id, thunk_param_type_id);
    }
    thunk_arg_ids.push_back(arg_id);
  }

  return PerformCall(context, loc_id, thunk_callee_id, thunk_arg_ids);
}

}  // namespace Carbon::Check