Recommit r348806: DebugInfo: Use symbol difference for CU length to s…

…implify assembly reading/editing

Mucking about simplifying a test case ( https://reviews.llvm.org/D55261 ) I stumbled across something I've hit before - that LLVM's (GCC's does too, FWIW) assembly output includes a hardcode length for a DWARF unit in its header. Instead we could emit a label difference - making the assembly easier to read/edit (though potentially at a slight (I haven't tried to observe it) performance cost of delaying/sinking the length computation into the MC layer).

Fix: Predicated all the changes (including creating the labels, even if they aren't used/needed) behind the NVPTX useSectionsAsReferences, avoiding emitting labels in NVPTX where ptxas can't parse them.

Reviewers: JDevlieghere, probinson, ABataev

Differential Revision: https://reviews.llvm.org/D55281

llvm-svn: 349430

llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp

@@ -47,6 +47,9 @@ void DwarfFile::emitUnit(DwarfUnit *TheU, bool UseOffsets) {
   Asm->OutStreamer->SwitchSection(S);
   TheU->emitHeader(UseOffsets);
   Asm->emitDwarfDIE(TheU->getUnitDie());
+
+  if (MCSymbol *EndLabel = TheU->getEndLabel())
+    Asm->OutStreamer->EmitLabel(EndLabel);
 }
 
 // Compute the size and offset for each DIE.

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

@@ -1553,7 +1553,14 @@ DIE *DwarfUnit::getOrCreateStaticMemberDIE(const DIDerivedType *DT) {
 void DwarfUnit::emitCommonHeader(bool UseOffsets, dwarf::UnitType UT) {
   // Emit size of content not including length itself
   Asm->OutStreamer->AddComment("Length of Unit");
-  Asm->emitInt32(getHeaderSize() + getUnitDie().getSize());
+  if (!DD->useSectionsAsReferences()) {
+    StringRef Prefix = isDwoUnit() ? "debug_info_dwo_" : "debug_info_";
+    MCSymbol *BeginLabel = Asm->createTempSymbol(Prefix + "start");
+    EndLabel = Asm->createTempSymbol(Prefix + "end");
+    Asm->EmitLabelDifference(EndLabel, BeginLabel, 4);
+    Asm->OutStreamer->EmitLabel(BeginLabel);
+  } else
+    Asm->emitInt32(getHeaderSize() + getUnitDie().getSize());
 
   Asm->OutStreamer->AddComment("DWARF version number");
   unsigned Version = DD->getDwarfVersion();

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h

@@ -49,6 +49,9 @@ class DwarfUnit : public DIEUnit {
   /// Target of Dwarf emission.
   AsmPrinter *Asm;
 
+  /// Emitted at the end of the CU and used to compute the CU Length field.
+  MCSymbol *EndLabel = nullptr;
+
   // Holders for some common dwarf information.
   DwarfDebug *DD;
   DwarfFile *DU;
@@ -82,6 +85,7 @@ class DwarfUnit : public DIEUnit {
 public:
   // Accessors.
   AsmPrinter* getAsmPrinter() const { return Asm; }
+  MCSymbol *getEndLabel() const { return EndLabel; }
   uint16_t getLanguage() const { return CUNode->getSourceLanguage(); }
   const DICompileUnit *getCUNode() const { return CUNode; }
 

llvm/test/DebugInfo/X86/length_symbol_difference.ll

@@ -0,0 +1,30 @@
+; RUN: llc -filetype=asm -O0 -mtriple=x86_64-linux-gnu < %s | FileCheck %s
+
+; CHECK:      .long   .Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
+; CHECK-NEXT: .Ldebug_info_start0:
+; CHECK-NOT:  .byte   0
+; CHECK:      .byte   0                       # End Of Children Mark
+; CHECK-NEXT: .Ldebug_info_end0:
+; CHECK-NEXT: .section
+
+
+define dso_local void @_Z2f1v() !dbg !7 {
+entry:
+  ret void, !dbg !10
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 8.0.0 (trunk 349394) (llvm/trunk 349377)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "foo.cpp", directory: "/usr/local/google/home/blaikie/dev/scratch")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 8.0.0 (trunk 349394) (llvm/trunk 349377)"}
+!7 = distinct !DISubprogram(name: "f1", linkageName: "_Z2f1v", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null}
+!10 = !DILocation(line: 1, column: 12, scope: !7)