[DEBUGINFO] Do not output labels for empty macinfo sections.

[ABataev]

Summary

If there is no debug info for macros, do not emit labels for empty
macinfo sections.

[probinson]

test/DebugInfo/X86/empty_macinfo.ll

1

Adding -dwarf-version 4 here would make me happy (the .debug_macinfo section goes away in DWARF v5).

7

This depends on the order of sections, and can be made more robust pretty easily. Replace the second CHECK-NEXT with this:

; CHECK-NEXT: .section
; CHECK-NOT: .debug_macinfo

[ABataev]

test/DebugInfo/X86/empty_macinfo.ll

1

Ok

7

Ok, will do

[ABataev]

Updated patch

[probinson]

LGTM

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

void DwarfDebug::emitDebugMacinfo() {
   Asm->OutStreamer->SwitchSection(
       Asm->getObjFileLowering().getDwarfMacinfoSection());
 
   for (const auto &P : CUMap) {
     auto &TheCU = *P.second;
     auto *SkCU = TheCU.getSkeleton();
     DwarfCompileUnit &U = SkCU ? *SkCU : TheCU;
     auto *CUNode = cast<DICompileUnit>(P.first);
+    DIMacroNodeArray Macros = CUNode->getMacros();
+    if (!Macros.empty()) {
       Asm->OutStreamer->EmitLabel(U.getMacroLabelBegin());
-    handleMacroNodes(CUNode->getMacros(), U);
+      handleMacroNodes(Macros, U);
+    }
   }
   Asm->OutStreamer->AddComment("End Of Macro List Mark");
   Asm->EmitInt8(0);
 }
 
 // DWARF5 Experimental Separate Dwarf emitters.
 
 void DwarfDebug::initSkeletonUnit(const DwarfUnit &U, DIE &Die,

test/DebugInfo/X86/empty_macinfo.ll

+; RUN: llc -mtriple x86_64-pc-linux < %s  | FileCheck %s
+
+; Test that we don't pollute the start of the file with debug sections
+
+; CHECK:      .section .debug_macinfo,"",@progbits
+; CHECK-NEXT: .byte 0 # End Of Macro List Mark
+; CHECK-NEXT: .section .debug_pubnames,"",@progbits
+
+define void @f() !dbg !4 {
+  ret void, !dbg !9
+}
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!7, !8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "foo", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2, globals: !2, imports: !2)
+!1 = !DIFile(filename: "/foo/test.c", directory: "/foo")
+!2 = !{}
+!4 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !2)
+!5 = !DISubroutineType(types: !6)
+!6 = !{null}
+!7 = !{i32 2, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !DILocation(line: 1, column: 15, scope: !4)