[LLVM] Support to emit extern variables debuginfo with "-fstandalone-debug"

[Jac1494]

Summary

To emit debuginfo for declaration of global extern variable with option "-fstandalone-debug" this patch is created. It is contain change related to llvm code and for clang code patch is available at (https://reviews.llvm.org/D71451)

[dblaikie]

This'll need a test case

[Jac1494]

Test case Added

[Jac1494]

DW_AT_linkage_name attribute support added for global variable declaration.

[vsk]

Please fill out the 'Summary' field in the Phab review so the context is easier to pick up for reviewers.

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

172

Why is a linkage name needed on a global variable declaration? Could you leave a comment that explains this?

173

Please clang-format your diffs so that they conform to llvm's coding standards (https://www.llvm.org/docs/CodingStandards.html#source-code-formatting)

186

How is this source change covered by your test? If not, could you please add test coverage?

llvm/test/DebugInfo/Generic/debug-info-extern-variable-declaration.ll

31

Could you reduce this test case by deleting unnecessary attributes?

[Jac1494]

clang format source file added ,and removed unnecessary attribute from test case.

[Jac1494]

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

172

Currently linkage is add for global variable from method addLocationAttribute ,But after this change for declaration of global variable location will be not Created,Because of that linkage name is not missing for variable "None" and "__default_lock_policy".I have analysed clang binary from that i have seen this cases.That's why i have added linkage name here.

173

Updated Source code is added.

186

Yes, debug-info-extern-variable-declaration.ll test case will cover this source changes.

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

if (auto *SDMDecl = GV->getStaticDataMemberDeclaration()) {
     if (!GV->isLocalToUnit())
       addFlag(*VariableDIE, dwarf::DW_AT_external);
 
     // Add line number info.
     addSourceLine(*VariableDIE, GV);
   }
 
   if (!GV->isDefinition())
+  {
     addFlag(*VariableDIE, dwarf::DW_AT_declaration);
+
+    if (DD->useAllLinkageNames())
+    addLinkageName(*VariableDIE, GV->getLinkageName());
+  }
   else
     addGlobalName(GV->getName(), *VariableDIE, DeclContext);
 
   if (uint32_t AlignInBytes = GV->getAlignInBytes())
     addUInt(*VariableDIE, dwarf::DW_AT_alignment, dwarf::DW_FORM_udata,
             AlignInBytes);
 
   if (MDTuple *TP = GV->getTemplateParams())
     addTemplateParams(*VariableDIE, DINodeArray(TP));
 
   // Add location.
+  if (GV->isDefinition())
   addLocationAttribute(VariableDIE, GV, GlobalExprs);
 
   return VariableDIE;
 }
 
 void DwarfCompileUnit::addLocationAttribute(
     DIE *VariableDIE, const DIGlobalVariable *GV, ArrayRef<GlobalExpr> GlobalExprs) {
   bool addToAccelTable = false;
   DIELoc *Loc = nullptr;

llvm/test/DebugInfo/Generic/debug-info-extern-variable-declaration.ll

+; RUN: %llc_dwarf -O0 -filetype=obj < %s | llvm-dwarfdump -v -debug-info - | FileCheck %s
+; Check extern variable debuginfo , declaration attribute
+
+; CHECK: DW_TAG_variable 
+; CHECK:   DW_AT_name {{.*}} "i"
+; CHECK:   DW_AT_declaration
+; CHECK-NOT:  DW_AT_location
+
+; CHECK: DW_TAG_variable
+; CHECK:   DW_AT_name {{.*}} "j"
+; CHECK:   DW_AT_declaration
+; CHECK-NOT:  DW_AT_location
+
+
+; ModuleID = 'debug-info-extern-variable-declaration.c'
+source_filename = "debug-info-extern-variable-declaration.c"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@i = external dso_local global i32, align 4, !dbg !0
+@j = external dso_local global i32, align 4, !dbg !6
+; Function Attrs: noinline nounwind optnone uwtable
+define dso_local i32 @foo() #0 !dbg !13 {
+entry:
+  %0 = load i32, i32* @i, align 4, !dbg !16
+  %1 = load i32, i32* @j, align 4, !dbg !17
+  %add = add nsw i32 %0, %1, !dbg !18
+  ret i32 %add, !dbg !19
+}
+
+attributes #0 = { noinline nounwind optnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!9, !10, !11}
+!llvm.ident = !{!12}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "i", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: false)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 10.0.0 (https://github.com/llvm/llvm-project.git bbcf1c3496ce2bd1ed87e8fb15ad896e279633ce)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "debug-info-extern-variable-declaration.c", directory: "/home/bft/Jaydeep/llvm-project/build_debug_fix/bin")
+!4 = !{}
+!5 = !{!0, !6}
+!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
+!7 = distinct !DIGlobalVariable(name: "j", scope: !2, file: !3, line: 3, type: !8, isLocal: false, isDefinition: false)
+!8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!9 = !{i32 7, !"Dwarf Version", i32 4}
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = !{i32 1, !"wchar_size", i32 4}
+!12 = !{!"clang version 10.0.0 (https://github.com/llvm/llvm-project.git bbcf1c3496ce2bd1ed87e8fb15ad896e279633ce)"}
+!13 = distinct !DISubprogram(name: "foo", scope: !3, file: !3, line: 2, type: !14, scopeLine: 2, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !4)
+!14 = !DISubroutineType(types: !15)
+!15 = !{!8}
+!16 = !DILocation(line: 4, column: 10, scope: !13)
+!17 = !DILocation(line: 4, column: 12, scope: !13)
+!18 = !DILocation(line: 4, column: 11, scope: !13)
+!19 = !DILocation(line: 4, column: 3, scope: !13)