[DebugInfo] Keep the DWARF64 flag in the module metadata

[ikudrin]

Summary

This allows the option to affect the LTO output. Module::Max helps to generate debug info for all modules in the same format.

[dblaikie]

@aprantl - mind having a look at this/signing off as well, if you're OK with it? Since it's a core metadata change, wouldn't mind some extra eyes on it.

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

396

Once this patch is accepted, it's probably worth going back and removing the MCOption? (I don't think we have MCOptions for other debug related module metadata, like the DWARF version?)

[ikudrin]

Thank you, @dblaikie!

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

396

Exactly for the DWARF version, there is a similar option, please take a look at line 388.
As for the DWARF64 option here, it only simplifies testing with tools like llc, opt, etc. Originally, MCTargetOptions::Dwarf64 was aimed to adjust generating in the assembler, but the corresponding command-line flag is visible in other tools, too. Do you think it is worth it to move the command line option from the common library code to specific tools?

[dblaikie]

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

396

Oh, nah, that's fine to keep around like the DWARF version for LLVM testing purposes (easier to write a test with a single .ll file and different RUN lines to test both 32 and 64 behavior, without having to have two IR Files, or rewrite the IR file to switch its format, etc)

clang/lib/CodeGen/CodeGenModule.cpp

if (Context.getTargetInfo().getTriple().getArch() == llvm::Triple::x86)
     getModule().addModuleFlag(llvm::Module::Error, "NumRegisterParameters",
                               CodeGenOpts.NumRegisterParameters);
 
   if (CodeGenOpts.DwarfVersion) {
     getModule().addModuleFlag(llvm::Module::Max, "Dwarf Version",
                               CodeGenOpts.DwarfVersion);
   }
 
+  if (CodeGenOpts.Dwarf64)
+    getModule().addModuleFlag(llvm::Module::Max, "DWARF64", 1);
+
   if (Context.getLangOpts().SemanticInterposition)
     // Require various optimization to respect semantic interposition.
     getModule().setSemanticInterposition(1);
 
   if (CodeGenOpts.EmitCodeView) {
     // Indicate that we want CodeView in the metadata.
     getModule().addModuleFlag(llvm::Module::Warning, "CodeView", 1);
   }

clang/test/CodeGen/dwarf-format.c

+// RUN: %clang -target x86_64-linux-gnu -g -S -emit-llvm -o - %s | \
+// RUN:   FileCheck %s --check-prefix=NODWARF64
+// RUN: %clang -target x86_64-linux-gnu -g -gdwarf64 -S -emit-llvm -o - %s | \
+// RUN:   FileCheck %s --check-prefix=DWARF64
+// RUN: %clang -target x86_64-linux-gnu -g -gdwarf64 -gdwarf32 -S -emit-llvm -o - %s | \
+// RUN:   FileCheck %s --check-prefix=NODWARF64
+
+// DWARF64: !{i32 7, !"DWARF64", i32 1}
+// NODWARF64-NOT: !"DWARF64"
+
+int main (void) {
+  return 0;
+}

llvm/include/llvm/IR/Module.h

/// @{
 
   /// Returns the Number of Register ParametersDwarf Version by checking
   /// module flags.
   unsigned getNumberRegisterParameters() const;
 
   /// Returns the Dwarf Version by checking module flags.
   unsigned getDwarfVersion() const;
 
+  /// Returns the DWARF format by checking module flags.
+  bool isDwarf64() const;
+
   /// Returns the CodeView Version by checking module flags.
   /// Returns zero if not present in module.
   unsigned getCodeViewFlag() const;
 
 /// @}
 /// @name Utility functions for querying and setting PIC level
 /// @{
 

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

DwarfDebug::DwarfDebug(AsmPrinter *A)
 
   unsigned DwarfVersionNumber = Asm->TM.Options.MCOptions.DwarfVersion;
   unsigned DwarfVersion = DwarfVersionNumber ? DwarfVersionNumber
                                     : MMI->getModule()->getDwarfVersion();
   // Use dwarf 4 by default if nothing is requested. For NVPTX, use dwarf 2.
   DwarfVersion =
       TT.isNVPTX() ? 2 : (DwarfVersion ? DwarfVersion : dwarf::DWARF_VERSION);
 
-  bool Dwarf64 = Asm->TM.Options.MCOptions.Dwarf64 &&
-                 DwarfVersion >= 3 &&   // DWARF64 was introduced in DWARFv3.
-                 TT.isArch64Bit() &&    // DWARF64 requires 64-bit relocations.
-                 TT.isOSBinFormatELF(); // Support only ELF for now.
+  bool Dwarf64 =
+      (Asm->TM.Options.MCOptions.Dwarf64 || MMI->getModule()->isDwarf64()) &&
 
   UseRangesSection = !NoDwarfRangesSection && !TT.isNVPTX();
 
   // Use sections as references. Force for NVPTX.
   if (DwarfSectionsAsReferences == Default)
     UseSectionsAsReferences = TT.isNVPTX();
   else
     UseSectionsAsReferences = DwarfSectionsAsReferences == Enable;

llvm/lib/IR/Module.cpp

 
 unsigned Module::getDwarfVersion() const {
   auto *Val = cast_or_null<ConstantAsMetadata>(getModuleFlag("Dwarf Version"));
   if (!Val)
     return 0;
   return cast<ConstantInt>(Val->getValue())->getZExtValue();
 }
 
+bool Module::isDwarf64() const {
+  auto *Val = cast_or_null<ConstantAsMetadata>(getModuleFlag("DWARF64"));
+  return Val && cast<ConstantInt>(Val->getValue())->isOne();
+}
+
 unsigned Module::getCodeViewFlag() const {
   auto *Val = cast_or_null<ConstantAsMetadata>(getModuleFlag("CodeView"));
   if (!Val)
     return 0;
   return cast<ConstantInt>(Val->getValue())->getZExtValue();
 }
 
 unsigned Module::getInstructionCount() {

llvm/test/DebugInfo/X86/dwarf64-module-flag.ll

+; This checks that the debug info is generated in the 64-bit format if the
+; module has the corresponding flag.
+
+; RUN: llc -mtriple=x86_64 -filetype=obj %s -o %t
+; RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s
+
+; CHECK:      Compile Unit: {{.*}} format = DWARF64
+; CHECK:      debug_line[
+; CHECK-NEXT: Line table prologue:
+; CHECK-NEXT:   total_length:
+; CHECK-NEXT:     format: DWARF64
+
+; IR generated and reduced from:
+; $ cat foo.c
+; int foo;
+; $ clang -g -gdwarf64 -S -emit-llvm foo.c -o foo.ll
+
+target triple = "x86_64-unknown-linux-gnu"
+
+@foo = dso_local global i32 0, align 4, !dbg !0
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9, !10}
+!llvm.ident = !{!11}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "foo", scope: !2, file: !3, line: 1, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 12.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "foo.c", directory: "/tmp")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 7, !"Dwarf Version", i32 4}
+!8 = !{i32 7, !"DWARF64", i32 1}
+!9 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{i32 1, !"wchar_size", i32 4}
+!11 = !{!"clang version 13.0.0"}