carbon-lang/toolchain/check/cpp/export.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/export.h"

#include "toolchain/check/cpp/import.h"
#include "toolchain/check/cpp/location.h"
#include "toolchain/check/cpp/type_mapping.h"
#include "toolchain/check/function.h"
#include "toolchain/check/thunk.h"
#include "toolchain/check/type.h"
#include "toolchain/sem_ir/mangler.h"

namespace Carbon::Check {

// Create a `clang::FunctionDecl` for the given Carbon function. This
// can be used to call the Carbon function from C++. The Carbon
// function's ABI must be compatible with C++.
//
// The resulting decl is used to allow a generated C++ function to call
// a generated Carbon function.
static auto BuildCppFunctionDeclForCarbonFn(Context& context,
                                            SemIR::LocId loc_id,
                                            SemIR::FunctionId function_id)
    -> clang::FunctionDecl* {
  auto clang_loc = GetCppLocation(context, loc_id);

  const SemIR::Function& function = context.functions().Get(function_id);

  // Get parameters types.
  auto carbon_function_params =
      context.inst_blocks().Get(function.call_param_patterns_id);
  llvm::SmallVector<clang::QualType> cpp_param_types;
  for (auto param_inst_id : carbon_function_params) {
    auto scrutinee_type_id = ExtractScrutineeType(
        context.sem_ir(), context.insts().Get(param_inst_id).type_id());
    auto cpp_type = MapToCppType(context, scrutinee_type_id);
    if (cpp_type.isNull()) {
      context.TODO(loc_id, "failed to map C++ type to Carbon");
      return nullptr;
    }
    auto ref_type = context.ast_context().getLValueReferenceType(cpp_type);
    cpp_param_types.push_back(ref_type);
  }

  CARBON_CHECK(function.return_type_inst_id == SemIR::TypeInstId::None);
  auto cpp_return_type = context.ast_context().VoidTy;

  auto cpp_function_type = context.ast_context().getFunctionType(
      cpp_return_type, cpp_param_types,
      clang::FunctionProtoType::ExtProtoInfo());

  auto* identifier_info = GetClangIdentifierInfo(context, function.name_id);
  CARBON_CHECK(identifier_info, "function with non-identifier name {0}",
               function.name_id);

  auto* function_decl = clang::FunctionDecl::Create(
      context.ast_context(), context.ast_context().getTranslationUnitDecl(),
      /*StartLoc=*/clang_loc, /*NLoc=*/clang_loc, identifier_info,
      cpp_function_type, /*TInfo=*/nullptr, clang::SC_Extern);

  // Build parameter decls.
  llvm::SmallVector<clang::ParmVarDecl*> param_var_decls;
  for (auto [i, type] : llvm::enumerate(cpp_param_types)) {
    clang::ParmVarDecl* param = clang::ParmVarDecl::Create(
        context.ast_context(), function_decl, /*StartLoc=*/clang_loc,
        /*IdLoc=*/clang_loc, /*Id=*/nullptr, type, /*TInfo=*/nullptr,
        clang::SC_None, /*DefArg=*/nullptr);
    param_var_decls.push_back(param);
  }
  function_decl->setParams(param_var_decls);

  // Mangle the function name and attach it to the `FunctionDecl`.
  SemIR::Mangler m(context.sem_ir(), context.total_ir_count());
  std::string mangled_name = m.Mangle(function_id, SemIR::SpecificId::None);
  function_decl->addAttr(
      clang::AsmLabelAttr::Create(context.ast_context(), mangled_name));

  return function_decl;
}

// Create the declaration of the C++ thunk.
static auto BuildCppToCarbonThunkDecl(
    Context& context, SemIR::LocId loc_id, clang::DeclContext* decl_context,
    clang::DeclarationName thunk_name,
    llvm::ArrayRef<clang::QualType> thunk_param_types) -> clang::FunctionDecl* {
  clang::ASTContext& ast_context = context.ast_context();

  auto clang_loc = GetCppLocation(context, loc_id);

  clang::DeclarationNameInfo name_info(thunk_name, clang_loc);

  auto ext_proto_info = clang::FunctionProtoType::ExtProtoInfo();
  clang::QualType thunk_function_type = ast_context.getFunctionType(
      ast_context.VoidTy, thunk_param_types, ext_proto_info);

  auto* tinfo =
      ast_context.getTrivialTypeSourceInfo(thunk_function_type, clang_loc);

  const bool uses_fp_intrin = false;
  const bool inline_specified = true;
  const auto constexpr_kind = clang::ConstexprSpecKind::Unspecified;
  const auto trailing_requires_clause = clang::AssociatedConstraint();

  clang::FunctionDecl* thunk_function_decl = nullptr;
  if (auto* parent_class = dyn_cast<clang::CXXRecordDecl>(decl_context)) {
    // TODO: Support non-static methods.
    thunk_function_decl = clang::CXXMethodDecl::Create(
        ast_context, parent_class, clang_loc, name_info, thunk_function_type,
        tinfo, clang::SC_Static, uses_fp_intrin, inline_specified,
        constexpr_kind, clang_loc, trailing_requires_clause);
    // TODO: Map Carbon access to C++ access.
    thunk_function_decl->setAccess(clang::AS_public);
  } else {
    thunk_function_decl = clang::FunctionDecl::Create(
        ast_context, decl_context, clang_loc, name_info, thunk_function_type,
        tinfo, clang::SC_None, uses_fp_intrin, inline_specified,
        /*hasWrittenPrototype=*/true, constexpr_kind, trailing_requires_clause);
  }
  decl_context->addDecl(thunk_function_decl);

  llvm::SmallVector<clang::ParmVarDecl*> param_var_decls;
  for (auto [i, type] : llvm::enumerate(thunk_param_types)) {
    clang::ParmVarDecl* thunk_param = clang::ParmVarDecl::Create(
        ast_context, thunk_function_decl, /*StartLoc=*/clang_loc,
        /*IdLoc=*/clang_loc, /*Id=*/nullptr, type,
        /*TInfo=*/nullptr, clang::SC_None, /*DefArg=*/nullptr);
    param_var_decls.push_back(thunk_param);
  }
  thunk_function_decl->setParams(param_var_decls);

  // Force the thunk to be inlined and discarded.
  thunk_function_decl->addAttr(
      clang::AlwaysInlineAttr::CreateImplicit(ast_context));
  thunk_function_decl->addAttr(
      clang::InternalLinkageAttr::CreateImplicit(ast_context));

  return thunk_function_decl;
}

// Create the body of a C++ thunk that calls a Carbon thunk. The
// arguments are passed by reference to the callee.
static auto BuildCppToCarbonThunkBody(clang::Sema& sema,
                                      clang::FunctionDecl* function_decl,
                                      clang::FunctionDecl* callee_function_decl)
    -> clang::StmtResult {
  clang::SourceLocation clang_loc = function_decl->getLocation();

  clang::ExprResult callee = sema.BuildDeclRefExpr(
      callee_function_decl, callee_function_decl->getType(), clang::VK_PRValue,
      clang_loc);

  llvm::SmallVector<clang::Expr*> call_args;
  for (auto* param : function_decl->parameters()) {
    clang::Expr* call_arg =
        sema.BuildDeclRefExpr(param, param->getType().getNonReferenceType(),
                              clang::VK_LValue, clang_loc);
    call_args.push_back(call_arg);
  }
  clang::ExprResult call = sema.BuildCallExpr(nullptr, callee.get(), clang_loc,
                                              call_args, clang_loc);
  CARBON_CHECK(call.isUsable());

  return call.get();
}

// Create a C++ thunk that calls the Carbon thunk. The C++ thunk's
// parameter types are mapped from the parameters of the target function
// with `MapToCppType`. (Note that the target function here is the
// callee of the Carbon thunk.)
static auto BuildCppToCarbonThunk(
    Context& context, SemIR::LocId loc_id, clang::DeclContext* decl_context,
    llvm::StringRef base_name, clang::FunctionDecl* carbon_function_decl,
    llvm::ArrayRef<SemIR::TypeId> callee_param_type_ids)
    -> clang::FunctionDecl* {
  // Create the thunk's name.
  llvm::SmallString<64> thunk_name = base_name;
  thunk_name += "__cpp_thunk";
  auto& thunk_ident = context.ast_context().Idents.get(thunk_name);

  llvm::SmallVector<clang::QualType> param_types;
  for (auto type_id : callee_param_type_ids) {
    auto cpp_type = MapToCppType(context, type_id);
    if (cpp_type.isNull()) {
      context.TODO(loc_id, "failed to map C++ type to Carbon");
      return nullptr;
    }
    param_types.push_back(cpp_type);
  }

  auto* thunk_function_decl = BuildCppToCarbonThunkDecl(
      context, loc_id, decl_context, &thunk_ident, param_types);

  // Build the thunk function body.
  clang::Sema& sema = context.clang_sema();
  clang::Sema::ContextRAII context_raii(sema, thunk_function_decl);
  sema.ActOnStartOfFunctionDef(nullptr, thunk_function_decl);
  clang::StmtResult body = BuildCppToCarbonThunkBody(sema, thunk_function_decl,
                                                     carbon_function_decl);
  sema.ActOnFinishFunctionBody(thunk_function_decl, body.get());
  CARBON_CHECK(!body.isInvalid());

  context.clang_sema().getASTConsumer().HandleTopLevelDecl(
      clang::DeclGroupRef(thunk_function_decl));
  return thunk_function_decl;
}

// Create a Carbon thunk that calls `callee`. The thunk's parameters are
// all references to the callee parameter type.
static auto BuildCarbonToCarbonThunk(
    Context& context, SemIR::LocId loc_id, const SemIR::Function& callee,
    llvm::ArrayRef<SemIR::TypeId> callee_param_type_ids) -> SemIR::FunctionId {
  // Create the thunk's name.
  llvm::SmallString<64> thunk_name =
      context.names().GetFormatted(callee.name_id);
  thunk_name += "__carbon_thunk";
  auto& ident = context.ast_context().Idents.get(thunk_name);
  auto thunk_name_id =
      SemIR::NameId::ForIdentifier(context.identifiers().Add(ident.getName()));

  auto carbon_thunk_function_id =
      MakeGeneratedFunctionDecl(context, loc_id,
                                {.parent_scope_id = callee.parent_scope_id,
                                 .name_id = thunk_name_id,
                                 .param_type_ids = callee_param_type_ids,
                                 .params_are_refs = true})
          .second;
  BuildThunkDefinition(context, carbon_thunk_function_id,
                       carbon_thunk_function_id, callee.first_decl_id(),
                       callee.first_decl_id());

  return carbon_thunk_function_id;
}

auto GetReverseInteropFunctionDecl(Context& context, SemIR::LocId loc_id,
                                   clang::DeclContext& decl_context,
                                   SemIR::FunctionId callee_function_id)
    -> clang::FunctionDecl* {
  const SemIR::Function& callee = context.functions().Get(callee_function_id);

  if (callee.return_type_inst_id != SemIR::TypeInstId::None) {
    context.TODO(loc_id,
                 "unsupported: C++ calling a Carbon function with "
                 "return type other than `()`");
    return nullptr;
  }

  if (callee.generic_id.has_value()) {
    context.TODO(loc_id,
                 "unsupported: C++ calling a Carbon function with "
                 "generic parameters");
    return nullptr;
  }

  if (callee.call_param_ranges.implicit_size() != 0) {
    context.TODO(loc_id,
                 "unsupported: C++ calling a Carbon function with "
                 "an implicit parameter");
    return nullptr;
  }

  // Get the parameter types of the Carbon function being called.
  auto callee_function_params =
      context.inst_blocks().Get(callee.call_param_patterns_id);
  llvm::SmallVector<SemIR::TypeId> callee_param_type_ids;
  for (auto callee_param_inst_id : callee_function_params) {
    auto scrutinee_type_id = ExtractScrutineeType(
        context.sem_ir(), context.insts().Get(callee_param_inst_id).type_id());
    callee_param_type_ids.push_back(scrutinee_type_id);
  }

  // Create a Carbon thunk that calls the callee. The thunk's parameters
  // are all references so that the ABI is compatible with C++ callers.
  auto carbon_thunk_function_id =
      BuildCarbonToCarbonThunk(context, loc_id, callee, callee_param_type_ids);

  // Create a `clang::FunctionDecl` that can be used to call the Carbon thunk.
  auto* carbon_function_decl = BuildCppFunctionDeclForCarbonFn(
      context, loc_id, carbon_thunk_function_id);
  if (!carbon_function_decl) {
    return nullptr;
  }

  // Create a C++ thunk that calls the Carbon thunk.
  return BuildCppToCarbonThunk(context, loc_id, &decl_context,
                               context.names().GetFormatted(callee.name_id),
                               carbon_function_decl, callee_param_type_ids);
}

}  // namespace Carbon::Check