[DebugInfo] Make DWARF ignore sizes on forward declared class types.

[akhuang]

Summary

Make sure the sizes for forward declared classes aren't emitted in
DWARF.

This comes before https://reviews.llvm.org/D87062, which adds sizes to
all classes with definitions.

Bug: https://bugs.llvm.org/show_bug.cgi?id=47338

[dblaikie]

Looks good - thanks!

llvm/test/DebugInfo/X86/struct-fwd-decl.ll

17–22

Thinking about it, perhaps this'd be easier written as:

; CHECK: DW_TAG_structure_type
; CHECK-NOT: DW_AT_byte_size
; CHECK: {{NULL|DW_TAG}}

(I realize, ultimately, this isn't far off just "CHECK-NOT: DW_AT_byte_size" in general, but I think it's probably a good balance)

[akhuang]

llvm/test/DebugInfo/X86/struct-fwd-decl.ll

17–22

sure, this seems reasonable

[akhuang]

update test

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

void DwarfUnit::constructTypeDIE(DIE &Buffer, const DICompositeType *CTy) {
   // Add name if not anonymous or intermediate type.
   if (!Name.empty())
     addString(Buffer, dwarf::DW_AT_name, Name);
 
   if (Tag == dwarf::DW_TAG_enumeration_type ||
       Tag == dwarf::DW_TAG_class_type || Tag == dwarf::DW_TAG_structure_type ||
       Tag == dwarf::DW_TAG_union_type) {
     // Add size if non-zero (derived types might be zero-sized.)
+    // Ignore the size if it's a non-enum forward decl.
     // TODO: Do we care about size for enum forward declarations?
-    if (Size)
+    if (Size &&
+        (!CTy->isForwardDecl() || Tag == dwarf::DW_TAG_enumeration_type))
       addUInt(Buffer, dwarf::DW_AT_byte_size, None, Size);
     else if (!CTy->isForwardDecl())
       // Add zero size if it is not a forward declaration.
       addUInt(Buffer, dwarf::DW_AT_byte_size, None, 0);
 
     // If we're a forward decl, say so.
     if (CTy->isForwardDecl())
       addFlag(Buffer, dwarf::DW_AT_declaration);

llvm/test/DebugInfo/X86/struct-fwd-decl.ll

+; RUN: llc -O0 -mtriple=x86_64-unknown-linux %s -o %t -filetype=obj
+; RUN: llvm-dwarfdump -debug-info %t | FileCheck %s
+; Test that size is not emitted for class declarations in DWARF, even if it exists.
+
+@s = global i16 0, align 2, !dbg !0
+
+!llvm.dbg.cu = !{!4}
+!llvm.module.flags = !{!7}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = !DIGlobalVariable(name: "s", scope: null, file: !2, line: 2, type: !3, isLocal: false, isDefinition: true)
+!2 = !DIFile(filename: "foo.cpp", directory: "/tmp")
+!3 = !DICompositeType(tag: DW_TAG_structure_type, name: "S", file: !2, line: 1, size: 16, align: 16, flags: DIFlagFwdDecl)
+!4 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !2, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, retainedTypes: !5, globals: !6, imports: !5)
+!5 = !{}
+; CHECK: DW_TAG_structure_type
+; CHECK-NEXT: DW_AT_name
+; CHECK-NOT: DW_AT_byte_size
+; CHECK: {{NULL|DW_TAG}}
+!6 = !{!0}
+!7 = !{i32 1, !"Debug Info Version", i32 3}