[PATCH] D55281: debuginfo: Use symbol difference for CU length to simplify assembly reading/editing

[David Blaikie via Phabricator via llvm-commits]

dblaikie created this revision.
dblaikie added reviewers: aprantl, JDevlieghere, probinson, ABataev.
Herald added a subscriber: llvm-commits.

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).

This does cause a few tests to fail currently - DebugInfo/X86/sections_as_references.ll (which was testing that there were no labels in the unit header & now there's one label there) and a few NVPTX ones (these might be a real issue - because NVPTX seems to have some strong limitations on what sort of assembly it can cope with - and maybe these label differences wouldn't be acceptable there, so I've added Alexey to this review to get his perspective on that part)


Repository:
  rL LLVM

https://reviews.llvm.org/D55281

Files:
  lib/CodeGen/AsmPrinter/DwarfFile.cpp
  lib/CodeGen/AsmPrinter/DwarfUnit.cpp
  lib/CodeGen/AsmPrinter/DwarfUnit.h


Index: lib/CodeGen/AsmPrinter/DwarfUnit.h
===================================================================
--- lib/CodeGen/AsmPrinter/DwarfUnit.h
+++ lib/CodeGen/AsmPrinter/DwarfUnit.h
@@ -48,6 +48,7 @@
 
   /// Target of Dwarf emission.
   AsmPrinter *Asm;
+  MCSymbol *EndLabel;
 
   // Holders for some common dwarf information.
   DwarfDebug *DD;
@@ -82,6 +83,7 @@
 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; }
 
Index: lib/CodeGen/AsmPrinter/DwarfUnit.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1553,7 +1553,11 @@
 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");
+  Asm->EmitLabelDifference(EndLabel, BeginLabel, 4);
+  Asm->OutStreamer->EmitLabel(BeginLabel);
 
   Asm->OutStreamer->AddComment("DWARF version number");
   unsigned Version = DD->getDwarfVersion();
Index: lib/CodeGen/AsmPrinter/DwarfFile.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/DwarfFile.cpp
+++ lib/CodeGen/AsmPrinter/DwarfFile.cpp
@@ -46,6 +46,8 @@
   TheU->emitHeader(UseOffsets);
 
   Asm->emitDwarfDIE(Die);
+
+  Asm->OutStreamer->EmitLabel(TheU->getEndLabel());
 }
 
 // Compute the size and offset for each DIE.


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