debuginfo: Use symbol difference for CU length to simplify 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).

Reviewers: JDevlieghere, probinson, ABataev

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

llvm-svn: 348806

3 files changed, 16 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp
index 4e410bb49be..094c654943d 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp
@@ -46,6 +46,8 @@ void DwarfFile::emitUnit(DwarfUnit *TheU, bool UseOffsets) {
   TheU->emitHeader(UseOffsets);
 
   Asm->emitDwarfDIE(Die);
+
+  Asm->OutStreamer->EmitLabel(TheU->getEndLabel());
 }
 
 // Compute the size and offset for each DIE.
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index 2053395808f..90b91180c80 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -38,6 +38,7 @@
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetLoweringObjectFile.h"
+#include "llvm/Target/TargetMachine.h"
 #include <cassert>
 #include <cstdint>
 #include <string>
@@ -1553,7 +1554,17 @@ 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());
+  StringRef Prefix = isDwoUnit() ? "debug_info_dwo_" : "debug_info_";
+  MCSymbol *BeginLabel = Asm->createTempSymbol(Prefix + "start");
+  EndLabel = Asm->createTempSymbol(Prefix + "end");
+
+  // Use a label difference for the convenience of legible/easily modified
+  // assembly - except on NVPTX where label differences aren't supported.
+  if (Asm->TM.getTargetTriple().isNVPTX())
+    Asm->emitInt32(getHeaderSize() + getUnitDie().getSize());
+  else
+    Asm->EmitLabelDifference(EndLabel, BeginLabel, 4);
+  Asm->OutStreamer->EmitLabel(BeginLabel);
 
   Asm->OutStreamer->AddComment("DWARF version number");
   unsigned Version = DD->getDwarfVersion();
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h
index 860d1653184..ec67c4b1cda 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h
@@ -48,6 +48,7 @@ protected:
 
   /// Target of Dwarf emission.
   AsmPrinter *Asm;
+  MCSymbol *EndLabel;
 
   // Holders for some common dwarf information.
   DwarfDebug *DD;
@@ -82,6 +83,7 @@ protected:
 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; }