[DebugInfo] Fix for adding "returns cxx udt" option to functions in CodeView.

[akhuang]

Summary

This change adds DIFlagNonTrivial to forward declarations of
DICompositeType. It adds the flag to nontrivial types and types with
unknown triviality.

It fixes adding the "CxxReturnUdt" flag to functions inconsistently,
since it is added based on whether the return type is marked NonTrivial, and
that changes if the return type was a forward declaration.

continues the discussion at https://reviews.llvm.org/D75215

Bug: https://bugs.llvm.org/show_bug.cgi?id=44785

[rnk]

LGTM!

clang/lib/CodeGen/CGDebugInfo.cpp

if (llvm::DIType *T = getTypeOrNull(CGM.getContext().getRecordType(RD)))
     return cast<llvm::DICompositeType>(T);
   llvm::DIFile *DefUnit = getOrCreateFile(RD->getLocation());
   unsigned Line = getLineNumber(RD->getLocation());
   StringRef RDName = getClassName(RD);
 
   uint64_t Size = 0;
   uint32_t Align = 0;
 
+  llvm::DINode::DIFlags Flags = llvm::DINode::FlagFwdDecl;
+
+  // Add flag to nontrivial forward declarations. To be consistent with MSVC,
+  // add the flag if a record has no definition because we don't know whether
+  // it will be trivial or not.
+  if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD))
+    if (!CXXRD->hasDefinition() ||
+        (CXXRD->hasDefinition() && !CXXRD->isTrivial()))
+      Flags |= llvm::DINode::FlagNonTrivial;
+
   // Create the type.
   SmallString<256> Identifier = getTypeIdentifier(Ty, CGM, TheCU);
   llvm::DICompositeType *RetTy = DBuilder.createReplaceableCompositeType(
-      getTagForRecord(RD), RDName, Ctx, DefUnit, Line, 0, Size, Align,
-      llvm::DINode::FlagFwdDecl, Identifier);
+      getTagForRecord(RD), RDName, Ctx, DefUnit, Line, 0, Size, Align, Flags,
+      Identifier);
   if (CGM.getCodeGenOpts().DebugFwdTemplateParams)
     if (auto *TSpecial = dyn_cast<ClassTemplateSpecializationDecl>(RD))
       DBuilder.replaceArrays(RetTy, llvm::DINodeArray(),
                              CollectCXXTemplateParams(TSpecial, DefUnit));
   ReplaceMap.emplace_back(
       std::piecewise_construct, std::make_tuple(Ty),
       std::make_tuple(static_cast<llvm::Metadata *>(RetTy)));
   return RetTy;

clang/test/CodeGenCXX/debug-info-composite-triviality-fwd-decl.cpp

+// RUN: %clang_cc1 -emit-llvm -gcodeview -debug-info-kind=limited -x c %s -o - | FileCheck %s --check-prefix CHECK-C
+// RUN: %clang_cc1 -emit-llvm -gcodeview -debug-info-kind=limited -x c++ %s -o - | FileCheck %s --check-prefix CHECK-CXX
+//
+// Test for DIFlagNonTrivial on forward declared DICompositeTypes.
+
+struct Incomplete;
+struct Incomplete (*func_ptr)() = 0;
+// CHECK-C: !DICompositeType({{.*}}name: "Incomplete"
+// CHECK-C-NOT: DIFlagNonTrivial
+// CHECK-CXX: !DICompositeType({{.*}}name: "Incomplete"
+// CHECK-CXX-SAME: DIFlagNonTrivial