Emit the DW_AT_type for a C++ static member definition if it is more specific than the one in its DW_AT_specification.

[aprantl]

Summary

If a static member is an array, the translation unit containing the member definition may have a more specific type (including its length) than TUs only seeing the class declaration. This patch adds a DW_AT_type to the member's DW_TAG_variable in addition to the DW_AT_specification in these cases. The member type in the DW_AT_specification still shows the more generic type (without the length) to avoid defeating type uniquing.

The DWARF standard discourages “duplicating” a DW_AT_type in a member variable definition but doesn’t explicitly forbid it.
Having the more specific type (with the array length) available is what allows the debugger to print the contents of a static array member variable.

rdar://problem/28706946

[probinson]

I think it makes sense to put the more explicit type on the definition. Same deal as putting the more explicit return type on the definition of an 'auto' member function.

test/DebugInfo/X86/static_member_array.ll

22

The subrange must be a child of the array_type, so you should also CHECK-NOT: NULL here.

32

CHECK-NOT: NULL here too

34

No DW_TAG or NULL between the subrange_type and the count.

35

Don't need this CHECK: NULL at the end.

[dblaikie]

Looks good - Paul's suggestions sound right too.

[aprantl]

Thanks! r286302.

lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

DIE *DwarfCompileUnit::getOrCreateGlobalVariableDIE(
   DIScope *DeclContext;
   if (auto *SDMDecl = GV->getStaticDataMemberDeclaration()) {
     DeclContext = resolve(SDMDecl->getScope());
     assert(SDMDecl->isStaticMember() && "Expected static member decl");
     assert(GV->isDefinition());
     // We need the declaration DIE that is in the static member's class.
     DIE *VariableSpecDIE = getOrCreateStaticMemberDIE(SDMDecl);
     addDIEEntry(*VariableDIE, dwarf::DW_AT_specification, *VariableSpecDIE);
+    // If the global variable's type is different from the one in the class
+    // member type, assume that it's more specific and also emit it.
+    if (GTy != DD->resolve(SDMDecl->getBaseType()))
+      addType(*VariableDIE, GTy);
   } else {
     DeclContext = GV->getScope();
     // Add name and type.
     addString(*VariableDIE, dwarf::DW_AT_name, GV->getDisplayName());
     addType(*VariableDIE, GTy);
 
     // Add scoping info.
     if (!GV->isLocalToUnit())

test/DebugInfo/X86/static_member_array.ll

+; RUN: %llc_dwarf %s -filetype=obj -o - | llvm-dwarfdump -debug-dump=info - | FileCheck %s
+; Generated from:
+;
+; struct A {
+;   static int fully_specified;
+;   static int smem[];
+; };
+;  
+; int A::fully_specified;
+; int A::smem[] = { 0, 1, 2, 3 };
+;
+; CHECK: DW_TAG_variable
+; CHECK-NEXT: DW_AT_specification{{.*}}"fully_specified"
+; CHECK-NOT:  DW_AT_type
+; CHECK: DW_TAG_structure_type
+; CHECK: DW_TAG_member
+; CHECK: DW_TAG_member
+; CHECK-NEXT:   DW_AT_name {{.*}} "smem"
+; CHECK-NEXT:   DW_AT_type {{.*}} {0x[[GENERIC:[0-9]+]]}
+;
+; CHECK: 0x[[GENERIC]]: DW_TAG_array_type
+; CHECK-NOT:  DW_TAG
+; CHECK: DW_TAG_subrange_type
+; CHECK-NOT:  DW_AT_count
+; CHECK:  NULL
+;
+; CHECK: DW_TAG_variable
+; CHECK-NEXT: DW_AT_specification {{.*}}"smem"
+; CHECK-NEXT: DW_AT_type {{.*}} {0x[[SPECIFIC:[0-9]+]]}
+;
+; CHECK: 0x[[SPECIFIC]]: DW_TAG_array_type
+; CHECK-NOT:  DW_TAG
+; CHECK: DW_TAG_subrange_type
+; CHECK:  DW_AT_count {{.*}} (0x04)
+; CHECK:  NULL
+
+source_filename = "static_member_array.cpp"
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.12.0"
+
+@_ZN1A15fully_specifiedE = global i32 0, align 4, !dbg !0
+@_ZN1A4smemE = global [4 x i32] [i32 0, i32 1, i32 2, i32 3], align 16, !dbg !5
+
+!llvm.dbg.cu = !{!1}
+!llvm.module.flags = !{!17, !18, !19}
+!llvm.ident = !{!20}
+
+!0 = distinct !DIGlobalVariable(name: "fully_specified", linkageName: "_ZN1A15fully_specifiedE", scope: !1, file: !2, line: 7, type: !7, isLocal: false, isDefinition: true, declaration: !13)
+!1 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !2, producer: "clang version 4.0.0 (trunk 286129) (llvm/trunk 286128)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !3, globals: !4)
+!2 = !DIFile(filename: "static_member_array.cpp", directory: "/Volumes/Data/radar/28706946")
+!3 = !{}
+!4 = !{!0, !5}
+!5 = distinct !DIGlobalVariable(name: "smem", linkageName: "_ZN1A4smemE", scope: !1, file: !2, line: 8, type: !6, isLocal: false, isDefinition: true, declaration: !10)
+!6 = !DICompositeType(tag: DW_TAG_array_type, baseType: !7, size: 128, elements: !8)
+!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!8 = !{!9}
+!9 = !DISubrange(count: 4)
+!10 = !DIDerivedType(tag: DW_TAG_member, name: "smem", scope: !11, file: !2, line: 4, baseType: !14, flags: DIFlagStaticMember)
+!11 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "A", file: !2, line: 1, size: 8, elements: !12, identifier: "_ZTS1A")
+!12 = !{!13, !10}
+!13 = !DIDerivedType(tag: DW_TAG_member, name: "fully_specified", scope: !11, file: !2, line: 3, baseType: !7, flags: DIFlagStaticMember)
+!14 = !DICompositeType(tag: DW_TAG_array_type, baseType: !7, elements: !15)
+!15 = !{!16}
+!16 = !DISubrange(count: -1)
+!17 = !{i32 2, !"Dwarf Version", i32 4}
+!18 = !{i32 2, !"Debug Info Version", i32 3}
+!19 = !{i32 1, !"PIC Level", i32 2}
+!20 = !{!"clang version 4.0.0 (trunk 286129) (llvm/trunk 286128)"}