Support accessing Carbon class fields from C++ (#7119)

When any field of a Carbon class is access from C++ for the first time,
all fields are exported as `clang::FieldDecl`s (this is necessary
because clang fields have an internal index that is initialized on first
use).

`ClangDeclStore` now provides bidirectional mapping. This allows looking
up a `ClangDeclId` by `InstId`, so when Carbon class fields are exported
they can be looked up that way.

toolchain/check/cpp/export.cpp

@@ -167,6 +167,166 @@ auto ExportClassToCpp(Context& context, SemIR::LocId loc_id,
   return record_decl;
 }
 
+// Get the `StructTypeField`s from a class's object repr.
+static auto GetStructTypeFields(Context& context,
+                                const SemIR::Class& class_info)
+    -> llvm::ArrayRef<SemIR::StructTypeField> {
+  if (class_info.adapt_id.has_value()) {
+    // The representation of an adapter won't necessarily be a
+    // struct. Return an empty array since adapters can't declare
+    // fields.
+    return {};
+  }
+
+  auto object_repr_type_id =
+      class_info.GetObjectRepr(context.sem_ir(), SemIR::SpecificId::None);
+  auto struct_type =
+      context.types().GetAs<SemIR::StructType>(object_repr_type_id);
+  return context.struct_type_fields().Get(struct_type.fields_id);
+}
+
+static auto LookupClassFieldByStructField(
+    const Context& context, const SemIR::NameScope& class_scope,
+    const SemIR::StructTypeField& struct_field)
+    -> std::optional<SemIR::InstStore::GetAsWithIdResult<SemIR::FieldDecl>> {
+  if (auto entry_id = class_scope.Lookup(struct_field.name_id)) {
+    auto field_inst_id =
+        class_scope.GetEntry(*entry_id).result.target_inst_id();
+    return context.insts().TryGetAsWithId<SemIR::FieldDecl>(field_inst_id);
+  }
+  return std::nullopt;
+}
+
+// Creates a `clang::FieldDecl` for a Carbon class field. Returns
+// nullptr if an error occurs.
+static auto CreateCppFieldDecl(Context& context,
+                               clang::CXXRecordDecl* record_decl,
+                               SemIR::InstId field_inst_id,
+                               const SemIR::FieldDecl& field_decl)
+    -> clang::FieldDecl* {
+  // Get the field's C++ type.
+  auto unbound_element_type =
+      context.types().GetAs<SemIR::UnboundElementType>(field_decl.type_id);
+  auto cpp_type =
+      MapToCppType(context, context.types().GetTypeIdForTypeInstId(
+                                unbound_element_type.element_type_inst_id));
+  if (cpp_type.isNull()) {
+    context.TODO(field_inst_id, "failed to map Carbon type to C++");
+    return nullptr;
+  }
+
+  // Get the field's C++ identifier.
+  auto* identifier_info = GetClangIdentifierInfo(context, field_decl.name_id);
+  CARBON_CHECK(identifier_info, "field with non-identifier name {0}",
+               field_decl.name_id);
+
+  // Create the `clang::FieldDecl`.
+  auto clang_loc = GetCppLocation(context, SemIR::LocId(field_inst_id));
+  auto* cpp_field_decl = clang::FieldDecl::Create(
+      context.ast_context(), record_decl, /*StartLoc=*/clang_loc,
+      /*IdLoc=*/clang_loc, identifier_info, cpp_type, /*TInfo=*/nullptr,
+      /*BW=*/nullptr,
+      /*Mutable=*/true, clang::ICIS_NoInit);
+  cpp_field_decl->setAccess(clang::AS_public);
+  record_decl->addHiddenDecl(cpp_field_decl);
+
+  return cpp_field_decl;
+}
+
+auto ExportAllFieldsToCpp(Context& context, SemIR::Class& class_info) -> void {
+  if (class_info.fields_exported) {
+    return;
+  }
+
+  const auto& class_scope = context.name_scopes().Get(class_info.scope_id);
+
+  for (const auto& struct_field : GetStructTypeFields(context, class_info)) {
+    auto class_field =
+        LookupClassFieldByStructField(context, class_scope, struct_field);
+    if (!class_field) {
+      continue;
+    }
+
+    // Map the parent scope into the C++ AST.
+    auto* decl_context = ExportNameScopeToCpp(
+        context, SemIR::LocId(class_field->inst_id), class_info.scope_id);
+    if (!decl_context) {
+      continue;
+    }
+
+    auto* cpp_field_decl =
+        CreateCppFieldDecl(context, cast<clang::CXXRecordDecl>(decl_context),
+                           class_field->inst_id, class_field->inst);
+    if (!cpp_field_decl) {
+      continue;
+    }
+
+    // Create and store the `ClangDeclId`.
+    auto key = SemIR::ClangDeclKey::ForNonFunctionDecl(cpp_field_decl);
+    context.clang_decls().Add({.key = key, .inst_id = class_field->inst_id});
+  }
+
+  class_info.fields_exported = true;
+}
+
+auto ExportFieldToCpp(Context& context, SemIR::InstId field_inst_id,
+                      SemIR::FieldDecl field_decl) -> clang::FieldDecl* {
+  // Get the `SemIR::Class` that contains the `field_decl`.
+  auto unbound_element_type =
+      context.types().GetAs<SemIR::UnboundElementType>(field_decl.type_id);
+  SemIR::TypeId class_type_id = context.types().GetTypeIdForTypeInstId(
+      unbound_element_type.class_type_inst_id);
+  auto class_type = context.types().GetAs<SemIR::ClassType>(class_type_id);
+  auto& class_info = context.classes().Get(class_type.class_id);
+
+  // If the class's fields haven't already been exported, do so now.
+  ExportAllFieldsToCpp(context, class_info);
+
+  // Get the exported `clang::FieldDecl`.
+  auto clang_decl_id = context.clang_decls().Lookup(field_inst_id);
+  if (clang_decl_id == SemIR::ClangDeclId::None) {
+    return nullptr;
+  }
+  return cast<clang::FieldDecl>(
+      context.clang_decls().Get(clang_decl_id).key.decl);
+}
+
+auto CalculateCppFieldOffsets(
+    Context& context, SemIR::ClassId class_id,
+    llvm::DenseMap<const clang::FieldDecl*, uint64_t>& field_offsets) -> bool {
+  auto class_info = context.classes().Get(class_id);
+  const auto& class_scope = context.name_scopes().Get(class_info.scope_id);
+
+  auto class_layout = SemIR::ObjectLayout::Empty();
+  for (const auto& struct_field : GetStructTypeFields(context, class_info)) {
+    auto field_type_id = context.sem_ir().types().GetTypeIdForTypeInstId(
+        struct_field.type_inst_id);
+    auto field_layout = context.sem_ir()
+                            .types()
+                            .GetCompleteTypeInfo(field_type_id)
+                            .object_layout;
+
+    // Use the field's name to look up the corresponding entry in the
+    // class. If it's a `FieldDecl`, write out the offset of the
+    // corresponding `clang::FieldDecl`.
+    auto class_field =
+        LookupClassFieldByStructField(context, class_scope, struct_field);
+    if (class_field) {
+      auto* cpp_field_decl =
+          ExportFieldToCpp(context, class_field->inst_id, class_field->inst);
+      if (!cpp_field_decl) {
+        return false;
+      }
+      field_offsets.insert(
+          {cpp_field_decl, class_layout.FieldOffset(field_layout).bits()});
+    }
+
+    class_layout.AppendField(field_layout);
+  }
+
+  return true;
+}
+
 namespace {
 struct FunctionInfo {
   struct Param {

toolchain/check/cpp/export.h

@@ -31,6 +31,29 @@ auto ExportClassToCpp(Context& context, SemIR::LocId loc_id,
                       SemIR::InstId class_inst_id, SemIR::ClassType class_type)
     -> clang::TagDecl*;
 
+// Export all `SemIR::FieldDecl`s in the class body as `clang::FieldDecl`s.
+auto ExportAllFieldsToCpp(Context& context, SemIR::Class& class_info) -> void;
+
+// Exports a Carbon class field into C++.
+//
+// If the field has already been exported, returns the existing C++
+// field.
+//
+// If the field has not already been exported, *all* fields of the class
+// are exported, and then the requested C++ field is returned.
+//
+// Returns nullptr if the class could not be exported and an error was
+// diagnosed.
+auto ExportFieldToCpp(Context& context, SemIR::InstId field_inst_id,
+                      SemIR::FieldDecl field_decl) -> clang::FieldDecl*;
+
+// Get the field offset for each field in a class.
+//
+// Returns true on success, false if any error occurs.
+auto CalculateCppFieldOffsets(
+    Context& context, SemIR::ClassId class_id,
+    llvm::DenseMap<const clang::FieldDecl*, uint64_t>& field_offsets) -> bool;
+
 // Get a `clang::FunctionDecl` that can be used to call a Carbon function.
 auto ExportFunctionToCpp(Context& context, SemIR::LocId loc_id,
                          SemIR::FunctionId function_id) -> clang::FunctionDecl*;

toolchain/check/cpp/generate_ast.cpp

@@ -436,6 +436,9 @@ auto CarbonExternalASTSource::MapInstIdToClangDeclOrType(LookupResult lookup)
       return ExportFunctionToCpp(*context_, SemIR::LocId(target_inst_id),
                                  callee_function->function_id);
     }
+    case CARBON_KIND(SemIR::FieldDecl field_decl): {
+      return ExportFieldToCpp(*context_, target_inst_id, field_decl);
+    }
     default:
       return nullptr;
   }
@@ -609,7 +612,7 @@ auto CarbonExternalASTSource::CompleteType(clang::TagDecl* tag_decl) -> void {
     return;
   }
 
-  const auto& class_info = context_->classes().Get(class_type.class_id);
+  auto& class_info = context_->classes().Get(class_type.class_id);
   class_decl->startDefinition();
   CARBON_CHECK(class_decl->hasDefinition());
 
@@ -636,8 +639,9 @@ auto CarbonExternalASTSource::CompleteType(clang::TagDecl* tag_decl) -> void {
     }
   }
 
-  // TODO: Import fields, plus any special member functions that affect class
-  // properties.
+  ExportAllFieldsToCpp(*context_, class_info);
+
+  // TODO: Import any special member functions that affect class properties.
   class_decl->completeDefinition();
 }
 
@@ -668,8 +672,8 @@ auto CarbonExternalASTSource::layoutRecordType(
   size = layout.size.bytes() * 8;
   alignment = layout.alignment.bits();
 
-  // TODO: Add field offsets once we import fields.
-  static_cast<void>(field_offsets);
+  // Fill in `field_offsets`.
+  CalculateCppFieldOffsets(*context_, class_type.class_id, field_offsets);
 
   // Add offset for base class, if any.
   if (const auto* class_decl = dyn_cast<clang::CXXRecordDecl>(record_decl);

toolchain/check/testdata/interop/cpp/class/export/field.carbon

@@ -0,0 +1,123 @@
+// 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-FILE: toolchain/testing/testdata/min_prelude/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/class/export/field.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/class/export/field.carbon
+
+// --- field.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class A {
+  var x: i32;
+  var y: i32;
+}
+
+inline Cpp '''
+void F() {
+  Carbon::A a;
+  a.x = 12;
+  a.y = 34;
+}
+''';
+
+// --- field_inheritence.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+base class A {
+  var x: i32;
+}
+
+class B {
+  extend base: A;
+  var y: i32;
+}
+
+inline Cpp '''
+void F() {
+  Carbon::B b;
+  b.x = 12;
+  b.y = 34;
+}
+''';
+
+// --- fail_nonexistent_field.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class A {}
+
+inline Cpp '''
+void F() {
+  Carbon::A a;
+  // CHECK:STDERR: fail_nonexistent_field.carbon:[[@LINE+4]]:5: error: no member named 'x' in 'Carbon::A' [CppInteropParseError]
+  // CHECK:STDERR:    13 |   a.x = 12;
+  // CHECK:STDERR:       |   ~ ^
+  // CHECK:STDERR:
+  a.x = 12;
+  // CHECK:STDERR: fail_nonexistent_field.carbon:[[@LINE+4]]:5: error: no member named 'y' in 'Carbon::A' [CppInteropParseError]
+  // CHECK:STDERR:    18 |   a.y = 34;
+  // CHECK:STDERR:       |   ~ ^
+  // CHECK:STDERR:
+  a.y = 34;
+}
+''';
+
+// --- fail_unsupported_field_type.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class A {
+  // CHECK:STDERR: fail_unsupported_field_type.carbon:[[@LINE+4]]:7: error: semantics TODO: `failed to map Carbon type to C++` [SemanticsTodo]
+  // CHECK:STDERR:   var x: ();
+  // CHECK:STDERR:       ^~~~~
+  // CHECK:STDERR:
+  var x: ();
+  // CHECK:STDERR: fail_unsupported_field_type.carbon:[[@LINE+4]]:7: error: semantics TODO: `failed to map Carbon type to C++` [SemanticsTodo]
+  // CHECK:STDERR:   var y: ();
+  // CHECK:STDERR:       ^~~~~
+  // CHECK:STDERR:
+  var y: ();
+}
+
+inline Cpp '''
+void F() {
+  Carbon::A a;
+  // CHECK:STDERR: fail_unsupported_field_type.carbon:[[@LINE+4]]:5: error: no member named 'x' in 'Carbon::A' [CppInteropParseError]
+  // CHECK:STDERR:    24 |   a.x;
+  // CHECK:STDERR:       |   ~ ^
+  // CHECK:STDERR:
+  a.x;
+  // CHECK:STDERR: fail_unsupported_field_type.carbon:[[@LINE+4]]:5: error: no member named 'y' in 'Carbon::A' [CppInteropParseError]
+  // CHECK:STDERR:    29 |   a.y;
+  // CHECK:STDERR:       |   ~ ^
+  // CHECK:STDERR:
+  a.y;
+}
+''';
+
+// --- fail_adapter.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class A {
+  adapt ();
+}
+
+inline Cpp '''
+void F() {
+  Carbon::A a;
+  // CHECK:STDERR: fail_adapter.carbon:[[@LINE+4]]:5: error: no member named 'x' in 'Carbon::A' [CppInteropParseError]
+  // CHECK:STDERR:    15 |   a.x = 12;
+  // CHECK:STDERR:       |   ~ ^
+  // CHECK:STDERR:
+  a.x = 12;
+}
+''';

toolchain/check/testdata/interop/cpp/class/export/layout.carbon

@@ -32,7 +32,7 @@ library "[[@TEST_NAME]]";
 import Cpp;
 
 class C {
-  var a: ()*;
+  var a: i32*;
   var b: i32;
 }
 

toolchain/lower/testdata/interop/cpp/reverse/class.carbon

@@ -34,7 +34,7 @@ library "[[@TEST_NAME]]";
 import Cpp;
 
 base class A {
-  var x: ()*;
+  var x: i32*;
   var y: i32;
 }
 
@@ -70,6 +70,29 @@ void h() {
 }
 ''';
 
+// --- field.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+class C {
+  var a: i32;
+  var b: i32;
+}
+
+inline Cpp '''
+void F(Carbon::C& c) {
+  c.a = 12;
+  c.b = 34;
+}
+''';
+
+fn G() {
+  var c: C = {.a = 0, .b = 0};
+  Cpp.F(ref c);
+}
+
 // CHECK:STDOUT: ; ModuleID = 'pointer.carbon'
 // CHECK:STDOUT: source_filename = "pointer.carbon"
 // CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
@@ -129,9 +152,9 @@ void h() {
 // CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
 // CHECK:STDOUT: target triple = "x86_64-unknown-linux-gnu"
 // CHECK:STDOUT:
-// CHECK:STDOUT: %"class.Carbon::A" = type { [12 x i8] }
-// CHECK:STDOUT: %"class.Carbon::B" = type { [16 x i8] }
-// CHECK:STDOUT: %"class.Carbon::C" = type { [16 x i8] }
+// CHECK:STDOUT: %"class.Carbon::A" = type <{ ptr, i32 }>
+// CHECK:STDOUT: %"class.Carbon::B" = type { %"class.Carbon::A", i32 }
+// CHECK:STDOUT: %"class.Carbon::C" = type { %"class.Carbon::A", i32 }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: mustprogress uwtable
 // CHECK:STDOUT: define dso_local void @_Z1fv() #0 {
@@ -269,3 +292,116 @@ void h() {
 // CHECK:STDOUT: !30 = !{!31}
 // CHECK:STDOUT: !31 = !DILocalVariable(arg: 1, scope: !29, type: !21)
 // CHECK:STDOUT: !32 = !DILocation(line: 23, column: 1, scope: !29)
+// CHECK:STDOUT: ; ModuleID = 'field.carbon'
+// CHECK:STDOUT: source_filename = "field.carbon"
+// CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+// CHECK:STDOUT: target triple = "x86_64-unknown-linux-gnu"
+// CHECK:STDOUT:
+// CHECK:STDOUT: %"class.Carbon::C" = type { i32, i32 }
+// CHECK:STDOUT:
+// CHECK:STDOUT: @C.val.loc19_3 = internal constant { i32, i32 } zeroinitializer
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: mustprogress nounwind uwtable
+// CHECK:STDOUT: define dso_local void @_Z1FRN6Carbon1CE(ptr noundef nonnull align 4 dereferenceable(8) %c) #0 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %c.addr = alloca ptr, align 8
+// CHECK:STDOUT:   store ptr %c, ptr %c.addr, align 8, !tbaa !11
+// CHECK:STDOUT:   %0 = load ptr, ptr %c.addr, align 8, !tbaa !11, !nonnull !14, !align !15
+// CHECK:STDOUT:   %a = getelementptr inbounds nuw %"class.Carbon::C", ptr %0, i32 0, i32 0
+// CHECK:STDOUT:   store i32 12, ptr %a, align 4, !tbaa !16
+// CHECK:STDOUT:   %1 = load ptr, ptr %c.addr, align 8, !tbaa !11, !nonnull !14, !align !15
+// CHECK:STDOUT:   %b = getelementptr inbounds nuw %"class.Carbon::C", ptr %1, i32 0, i32 1
+// CHECK:STDOUT:   store i32 34, ptr %b, align 4, !tbaa !18
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nounwind
+// CHECK:STDOUT: define void @_CG.Main() #1 !dbg !19 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %c.var = alloca { i32, i32 }, align 8, !dbg !22
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %c.var), !dbg !22
+// CHECK:STDOUT:   %.loc19_29.3.a = getelementptr inbounds nuw { i32, i32 }, ptr %c.var, i32 0, i32 0, !dbg !23
+// CHECK:STDOUT:   %.loc19_29.6.b = getelementptr inbounds nuw { i32, i32 }, ptr %c.var, i32 0, i32 1, !dbg !23
+// CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %c.var, ptr align 4 @C.val.loc19_3, i64 8, i1 false), !dbg !22
+// CHECK:STDOUT:   call void @_Z1FRN6Carbon1CE(ptr %c.var), !dbg !24
+// CHECK:STDOUT:   call void @"_COp.71e25083d63c4d6c:core.Destroy.Core"(ptr %c.var), !dbg !22
+// CHECK:STDOUT:   ret void, !dbg !25
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nounwind
+// CHECK:STDOUT: define weak_odr void @"_COp.7e389eab4a7e5487:core.Destroy.Core"(ptr %self) #1 !dbg !26 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   ret void, !dbg !32
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nounwind
+// CHECK:STDOUT: define weak_odr void @"_COp.fb32c4dfca65e378:core.Destroy.Core"(ptr %self) #1 !dbg !33 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   ret void, !dbg !39
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nounwind
+// CHECK:STDOUT: define weak_odr void @"_COp.71e25083d63c4d6c:core.Destroy.Core"(ptr %self) #1 !dbg !40 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   ret void, !dbg !43
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
+// CHECK:STDOUT: declare void @llvm.lifetime.start.p0(ptr captures(none)) #2
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nocallback nofree nounwind willreturn memory(argmem: readwrite)
+// CHECK:STDOUT: declare void @llvm.memcpy.p0.p0.i64(ptr noalias writeonly captures(none), ptr noalias readonly captures(none), i64, i1 immarg) #3
+// CHECK:STDOUT:
+// CHECK:STDOUT: attributes #0 = { mustprogress nounwind uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
+// CHECK:STDOUT: attributes #1 = { nounwind }
+// CHECK:STDOUT: attributes #2 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
+// CHECK:STDOUT: attributes #3 = { nocallback nofree nounwind willreturn memory(argmem: readwrite) }
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1, !2, !3, !4}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!5}
+// CHECK:STDOUT: !llvm.errno.tbaa = !{!7}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = !{i32 8, !"PIC Level", i32 2}
+// CHECK:STDOUT: !3 = !{i32 7, !"PIE Level", i32 2}
+// CHECK:STDOUT: !4 = !{i32 7, !"uwtable", i32 2}
+// CHECK:STDOUT: !5 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !6, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !6 = !DIFile(filename: "field.carbon", directory: "")
+// CHECK:STDOUT: !7 = !{!8, !8, i64 0}
+// CHECK:STDOUT: !8 = !{!"int", !9, i64 0}
+// CHECK:STDOUT: !9 = !{!"omnipotent char", !10, i64 0}
+// CHECK:STDOUT: !10 = !{!"Simple C++ TBAA"}
+// CHECK:STDOUT: !11 = !{!12, !12, i64 0}
+// CHECK:STDOUT: !12 = !{!"p1 _ZTSN6Carbon1CE", !13, i64 0}
+// CHECK:STDOUT: !13 = !{!"any pointer", !9, i64 0}
+// CHECK:STDOUT: !14 = !{}
+// CHECK:STDOUT: !15 = !{i64 4}
+// CHECK:STDOUT: !16 = !{!17, !8, i64 0}
+// CHECK:STDOUT: !17 = !{!"_ZTSN6Carbon1CE", !8, i64 0, !8, i64 4}
+// CHECK:STDOUT: !18 = !{!17, !8, i64 4}
+// CHECK:STDOUT: !19 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !6, line: 18, type: !20, spFlags: DISPFlagDefinition, unit: !5)
+// CHECK:STDOUT: !20 = !DISubroutineType(types: !21)
+// CHECK:STDOUT: !21 = !{null}
+// CHECK:STDOUT: !22 = !DILocation(line: 19, column: 3, scope: !19)
+// CHECK:STDOUT: !23 = !DILocation(line: 19, column: 14, scope: !19)
+// CHECK:STDOUT: !24 = !DILocation(line: 20, column: 3, scope: !19)
+// CHECK:STDOUT: !25 = !DILocation(line: 18, column: 1, scope: !19)
+// CHECK:STDOUT: !26 = distinct !DISubprogram(name: "Op", linkageName: "_COp.7e389eab4a7e5487:core.Destroy.Core", scope: null, file: !6, line: 19, type: !27, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !30)
+// CHECK:STDOUT: !27 = !DISubroutineType(types: !28)
+// CHECK:STDOUT: !28 = !{null, !29}
+// CHECK:STDOUT: !29 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+// CHECK:STDOUT: !30 = !{!31}
+// CHECK:STDOUT: !31 = !DILocalVariable(arg: 1, scope: !26, type: !29)
+// CHECK:STDOUT: !32 = !DILocation(line: 19, column: 3, scope: !26)
+// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "Op", linkageName: "_COp.fb32c4dfca65e378:core.Destroy.Core", scope: null, file: !6, line: 19, type: !34, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !37)
+// CHECK:STDOUT: !34 = !DISubroutineType(types: !35)
+// CHECK:STDOUT: !35 = !{null, !36}
+// CHECK:STDOUT: !36 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
+// CHECK:STDOUT: !37 = !{!38}
+// CHECK:STDOUT: !38 = !DILocalVariable(arg: 1, scope: !33, type: !36)
+// CHECK:STDOUT: !39 = !DILocation(line: 19, column: 3, scope: !33)
+// CHECK:STDOUT: !40 = distinct !DISubprogram(name: "Op", linkageName: "_COp.71e25083d63c4d6c:core.Destroy.Core", scope: null, file: !6, line: 19, type: !34, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !41)
+// CHECK:STDOUT: !41 = !{!42}
+// CHECK:STDOUT: !42 = !DILocalVariable(arg: 1, scope: !40, type: !36)
+// CHECK:STDOUT: !43 = !DILocation(line: 19, column: 3, scope: !40)

toolchain/sem_ir/clang_decl.cpp

@@ -41,4 +41,34 @@ auto ClangDecl::Print(llvm::raw_ostream& out) const -> void {
   out << "{key: " << key << ", inst_id: " << inst_id << "}";
 }
 
+ClangDeclStore::ClangDeclStore(CheckIRId check_ir_id) : values_(check_ir_id) {}
+
+auto ClangDeclStore::Add(ClangDecl value) -> ClangDeclId {
+  auto id = values_.Add(value);
+  inst_id_to_clang_decl_id_.Insert(value.inst_id, id);
+  return id;
+}
+
+auto ClangDeclStore::Lookup(ClangDeclKey key) const -> ClangDeclId {
+  return values_.Lookup(key);
+}
+
+auto ClangDeclStore::Lookup(InstId inst_id) const -> ClangDeclId {
+  if (auto result = inst_id_to_clang_decl_id_.Lookup(inst_id)) {
+    return result.value();
+  }
+  return ClangDeclId::None;
+}
+
+auto ClangDeclStore::OutputYaml() const -> Yaml::OutputMapping {
+  return values_.OutputYaml();
+}
+
+auto ClangDeclStore::CollectMemUsage(MemUsage& mem_usage,
+                                     llvm::StringRef label) const -> void {
+  values_.CollectMemUsage(mem_usage, label);
+  mem_usage.Collect(MemUsage::ConcatLabel(label, "inst_id_to_clang_decl_id_"),
+                    inst_id_to_clang_decl_id_);
+}
+
 }  // namespace Carbon::SemIR

toolchain/sem_ir/clang_decl.h

@@ -118,9 +118,38 @@ struct ClangDecl : public Printable<ClangDecl> {
   auto GetAsKey() const -> ClangDeclKey { return key; }
 };
 
-// Use the AST node pointer directly when doing `Lookup` to find an ID.
-using ClangDeclStore =
-    CanonicalValueStore<ClangDeclId, ClangDeclKey, Tag<CheckIRId>, ClangDecl>;
+// Canonical storage for `ClangDecl`s. Provides bidirectional mapping
+// between `ClangDeclId`s and `InstId`s.
+class ClangDeclStore {
+ public:
+  explicit ClangDeclStore(CheckIRId check_ir_id);
+
+  // Adds a `ClangDecl`, returning an ID to reference it.
+  auto Add(ClangDecl value) -> ClangDeclId;
+
+  // Looks up a `ClangDecl` by `ClangDeclId`.
+  auto Get(ClangDeclId id) const -> const ClangDecl& { return values_.Get(id); }
+
+  // Looks up a `ClangDeclId` by `ClangDeclKey`.
+  auto Lookup(ClangDeclKey key) const -> ClangDeclId;
+
+  // Looks up a `ClangDeclId` by `InstId`.
+  auto Lookup(InstId inst_id) const -> ClangDeclId;
+
+  auto OutputYaml() const -> Yaml::OutputMapping;
+
+  auto CollectMemUsage(MemUsage& mem_usage, llvm::StringRef label) const
+      -> void;
+
+ private:
+  // Canonical storage for `ClangDecl`s. Allows mapping from a
+  // `ClangDeclId` to an `InstId`.
+  CanonicalValueStore<ClangDeclId, ClangDeclKey, Tag<CheckIRId>, ClangDecl>
+      values_;
+
+  // Map from `InstId` to `ClangDeclId`.
+  Map<InstId, ClangDeclId> inst_id_to_clang_decl_id_;
+};
 
 }  // namespace Carbon::SemIR
 

toolchain/sem_ir/class.h

@@ -5,6 +5,7 @@
 #ifndef CARBON_TOOLCHAIN_SEM_IR_CLASS_H_
 #define CARBON_TOOLCHAIN_SEM_IR_CLASS_H_
 
+#include "common/map.h"
 #include "toolchain/base/value_store.h"
 #include "toolchain/sem_ir/entity_with_params_base.h"
 #include "toolchain/sem_ir/ids.h"
@@ -34,6 +35,9 @@ struct ClassFields {
   // Whether this class or any base class has at least one virtual function.
   bool is_dynamic = false;
 
+  // Whether the class's fields have been exported to C++.
+  bool fields_exported = false;
+
   // The following members are set at the `{` of the class definition.
 
   // The class scope.

toolchain/sem_ir/file.h

@@ -381,7 +381,7 @@ class File : public Printable<File> {
   // 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`.
-  ClangDeclStore clang_decls_;
+  ClangDeclStore clang_decls_ = ClangDeclStore(check_ir_id());
 
   // All instructions. The first entries will always be the singleton
   // instructions.