[DebugInfo] Keep the DWARF64 flag in the module metadata

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

Differential Revision: https://reviews.llvm.org/D96597

clang/lib/CodeGen/CodeGenModule.cpp

@@ -533,6 +533,9 @@ void CodeGenModule::Release() {
                               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);

clang/test/CodeGen/dwarf-format.c

@@ -0,0 +1,13 @@
+// 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

@@ -805,6 +805,9 @@ public:
   /// 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;

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -392,10 +392,11 @@ DwarfDebug::DwarfDebug(AsmPrinter *A)
   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()) &&
+      DwarfVersion >= 3 &&   // DWARF64 was introduced in DWARFv3.
+      TT.isArch64Bit() &&    // DWARF64 requires 64-bit relocations.
+      TT.isOSBinFormatELF(); // Support only ELF for now.
 
   UseRangesSection = !NoDwarfRangesSection && !TT.isNVPTX();
 

llvm/lib/IR/Module.cpp

@@ -509,6 +509,11 @@ unsigned Module::getDwarfVersion() const {
   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)

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

@@ -0,0 +1,37 @@
+; 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"}