Create clang::MangleContext once per file instead of once per C++ thunk (#5924)

Part of #5514.

Author: bricknerb

toolchain/check/cpp_thunk.cpp

@@ -20,17 +20,11 @@ namespace Carbon::Check {
 
 // Returns the C++ thunk mangled name given the callee function.
 static auto GenerateThunkMangledName(
-    clang::ASTContext& ast_context,
+    clang::MangleContext& mangle_context,
     const clang::FunctionDecl& callee_function_decl) -> std::string {
   RawStringOstream mangled_name_stream;
-  {
-    // TODO: Create `MangleContext` once.
-    std::unique_ptr<clang::MangleContext> mangle_context(
-        ast_context.createMangleContext());
-    mangle_context->mangleName(clang::GlobalDecl(&callee_function_decl),
-                               mangled_name_stream);
-  }
-
+  mangle_context.mangleName(clang::GlobalDecl(&callee_function_decl),
+                            mangled_name_stream);
   mangled_name_stream << ".carbon_thunk";
 
   return mangled_name_stream.TakeStr();
@@ -185,9 +179,9 @@ static auto BuildThunkParameters(
 // Returns the thunk function declaration given the callee function and the
 // thunk parameter types.
 static auto CreateThunkFunctionDecl(
-    clang::ASTContext& ast_context,
-    const clang::FunctionDecl& callee_function_decl,
+    Context& context, const clang::FunctionDecl& callee_function_decl,
     llvm::ArrayRef<clang::QualType> thunk_param_types) -> clang::FunctionDecl* {
+  clang::ASTContext& ast_context = context.ast_context();
   clang::SourceLocation clang_loc = callee_function_decl.getLocation();
 
   clang::IdentifierInfo& identifier_info = ast_context.Idents.get(
@@ -216,7 +210,9 @@ static auto CreateThunkFunctionDecl(
 
   // Set asm("<callee function mangled name>.carbon_thunk").
   thunk_function_decl->addAttr(clang::AsmLabelAttr::CreateImplicit(
-      ast_context, GenerateThunkMangledName(ast_context, callee_function_decl),
+      ast_context,
+      GenerateThunkMangledName(*context.sem_ir().clang_mangle_context(),
+                               callee_function_decl),
       clang_loc));
 
   return thunk_function_decl;
@@ -278,8 +274,6 @@ static auto BuildThunkBody(clang::Sema& sema,
 
 auto BuildCppThunk(Context& context, const SemIR::Function& callee_function)
     -> clang::FunctionDecl* {
-  clang::ASTContext& ast_context = context.ast_context();
-
   clang::FunctionDecl* callee_function_decl =
       context.sem_ir()
           .clang_decls()
@@ -289,9 +283,9 @@ auto BuildCppThunk(Context& context, const SemIR::Function& callee_function)
 
   // Build the thunk function declaration.
   auto [thunk_param_types, param_type_changed] =
-      BuildThunkParameterTypes(ast_context, *callee_function_decl);
+      BuildThunkParameterTypes(context.ast_context(), *callee_function_decl);
   clang::FunctionDecl* thunk_function_decl = CreateThunkFunctionDecl(
-      ast_context, *callee_function_decl, thunk_param_types);
+      context, *callee_function_decl, thunk_param_types);
 
   // Build the thunk function body.
   clang::Sema& sema = context.sem_ir().cpp_ast()->getSema();

toolchain/sem_ir/file.cpp

@@ -8,6 +8,7 @@
 #include <string>
 #include <utility>
 
+#include "clang/AST/Mangle.h"
 #include "common/check.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
@@ -54,6 +55,8 @@ File::File(const Parse::Tree* parse_tree, CheckIRId check_ir_id,
   }
 }
 
+File::~File() = default;
+
 auto File::Verify() const -> ErrorOr<Success> {
   // Invariants don't necessarily hold for invalid IR.
   if (has_errors_) {
@@ -147,4 +150,9 @@ auto File::CollectMemUsage(MemUsage& mem_usage, llvm::StringRef label) const
   mem_usage.Collect(MemUsage::ConcatLabel(label, "types_"), types_);
 }
 
+auto File::set_cpp_ast(clang::ASTUnit* cpp_ast) -> void {
+  cpp_ast_ = cpp_ast;
+  clang_mangle_context_.reset(cpp_ast->getASTContext().createMangleContext());
+}
+
 }  // namespace Carbon::SemIR

toolchain/sem_ir/file.h

@@ -76,6 +76,7 @@ class File : public Printable<File> {
                 SharedValueStores& value_stores, std::string filename);
 
   File(const File&) = delete;
+  ~File();
   auto operator=(const File&) -> File& = delete;
 
   // Verifies that invariants of the semantics IR hold.
@@ -198,7 +199,10 @@ class File : public Printable<File> {
   // TODO: When the AST can be created before creating `File`, initialize the
   // pointer in the constructor and remove this function. This is part of
   // https://github.com/carbon-language/carbon-lang/issues/4666
-  auto set_cpp_ast(clang::ASTUnit* cpp_ast) -> void { cpp_ast_ = cpp_ast; }
+  auto set_cpp_ast(clang::ASTUnit* cpp_ast) -> void;
+  auto clang_mangle_context() -> clang::MangleContext* {
+    return clang_mangle_context_.get();
+  }
   auto clang_decls() -> ClangDeclStore& { return clang_decls_; }
   auto clang_decls() const -> const ClangDeclStore& { return clang_decls_; }
   auto names() const -> NameStoreWrapper {
@@ -333,6 +337,10 @@ class File : public Printable<File> {
   // `Cpp` imports.
   clang::ASTUnit* cpp_ast_ = nullptr;
 
+  // The Clang mangle context for the target in the ASTContext. Initialized
+  // together with `cpp_ast_`.
+  std::unique_ptr<clang::MangleContext> clang_mangle_context_;
+
   // Clang AST declarations pointing to the AST and their mapped Carbon
   // instructions. When calling `Lookup()`, `inst_id` is ignored. `Add()` will
   // not add multiple entries with the same `decl` and different `inst_id`.