[DebugInfo] Fix to ctor homing to ignore classes with trivial ctors.

[akhuang]

Summary

Previously ctor homing was omitting debug info for classes if they
have both trival and nontrivial constructors, but we should only omit debug
info if the class doesn't have any trivial constructors.

Also change to use completeUnusedClass so the debug info is added to the
retained types list.

bug: https://bugs.llvm.org/show_bug.cgi?id=46537

[dblaikie]

I think it'd be good to separate these two issues/patches.

In part because I'm curious whether, even with a trivial ctor - if we did the retained types thing - would that be enough?

[akhuang]

In D84870#2182463, @dblaikie wrote:

I think it'd be good to separate these two issues/patches.

In part because I'm curious whether, even with a trivial ctor - if we did the retained types thing - would that be enough?

I think just the retained types thing wouldn't help in this case because the trivial ctor is not emitted? I tried it and the struct is still fwd declared. I could split out the retained types change, though.

[akhuang]

remove change to add class types to retained types list,

also fix the test case to be a struct that previously didn't
get complete debug info.

[dblaikie]

In D84870#2183096, @akhuang wrote:

remove change to add class types to retained types list,

also fix the test case to be a struct that previously didn't
get complete debug info.

Ah, I see what you mean. Perhaps the implementation can/should be changed to trigger on source uses of the ctor rather than only DWARF uses? Though I suppose that'd have problems with the ctor being used in unused code. (like the "requires complete type" optimization does - the type can be required to be complete in unused code)

Sounds good!

clang/lib/CodeGen/CGDebugInfo.cpp

static bool shouldOmitDefinition(codegenoptions::DebugInfoKind DebugKind,
   // another, but it is the current behavior of LimitedDebugInfo.
   if (CXXDecl->hasDefinition() && CXXDecl->isDynamicClass() &&
       !isClassOrMethodDLLImport(CXXDecl))
     return true;
 
   // In constructor debug mode, only emit debug info for a class when its
   // constructor is emitted. Skip this optimization if the class or any of
   // its methods are marked dllimport.
+  //
+  // This applies to classes that don't have any trivial constructors and have
+  // at least one constructor.
   if (DebugKind == codegenoptions::DebugInfoConstructor &&
       !CXXDecl->isLambda() && !CXXDecl->hasConstexprNonCopyMoveConstructor() &&
-      !isClassOrMethodDLLImport(CXXDecl))
+      !isClassOrMethodDLLImport(CXXDecl)) {
+    if (CXXDecl->ctors().empty())
+      return false;
     for (const auto *Ctor : CXXDecl->ctors())
-      if (Ctor->isUserProvided())
-        return true;
+      if (Ctor->isTrivial() && !Ctor->isCopyOrMoveConstructor())
+        return false;
+  }
 
   TemplateSpecializationKind Spec = TSK_Undeclared;
   if (const auto *SD = dyn_cast<ClassTemplateSpecializationDecl>(RD))
     Spec = SD->getSpecializationKind();
 
   if (Spec == TSK_ExplicitInstantiationDeclaration &&
       hasExplicitMemberDefinition(CXXDecl->method_begin(),
                                   CXXDecl->method_end()))

clang/test/CodeGenCXX/debug-info-limited-ctor.cpp

struct D {
   D();
 };
 D::D() {}
 
 // CHECK-DAG: !DICompositeType(tag: DW_TAG_structure_type, name: "E"{{.*}}DIFlagTypePassByValue
 struct E {
   constexpr E(){};
 } TestE;
+
+// CHECK-DAG: !DICompositeType(tag: DW_TAG_structure_type, name: "F"{{.*}}DIFlagTypePassByValue
+struct F {
+  F() = default;
+  F(int) {}
+  int i;
+} TestF;