[DebugInfo] Fix emitting offsets to CUs with -dwarf-sections-as-references=Enable.

[ikudrin]

Summary

The size of the field depends on the DWARF format, not the address size of the target.

[jhenderson]

LGTM. I'm rather surprised this hasn't been picked up before now, given it would generate invalid debug information. Perhaps this indicates the option is never used?

[ikudrin]

The problematic code path is performed only when generating .debug_pub* or .debug_aranges sections when the option is set to Enabled, but as it was stated in D43943, where the option was added, it was intended only for cases, when these sections are omitted/ignored. Nevertheless, the common code should be correct as its usage might be extended in the future.

[jhenderson]

In D82311#2108415, @ikudrin wrote:

The problematic code path is performed only when generating .debug_pub* or .debug_aranges sections when the option is set to Enabled, but as it was stated in D43943, where the option was added, it was intended only for cases, when these sections are omitted/ignored. Nevertheless, the common code should be correct as its usage might be extended in the future.

Great, thanks for the explanation!

llvm/lib/CodeGen/AsmPrinter/AsmPrinterDwarf.cpp

if (MAI->doesDwarfUseRelocationsAcrossSections()) {
     return;
   }
 
   // Just emit the offset directly; no need for symbol math.
   emitInt32(S.Offset);
 }
 
 void AsmPrinter::emitDwarfOffset(const MCSymbol *Label, uint64_t Offset) const {
-  emitLabelPlusOffset(Label, Offset, MAI->getCodePointerSize());
+  // TODO: Support DWARF64
+  emitLabelPlusOffset(Label, Offset, 4);
 }
 
 void AsmPrinter::emitCallSiteOffset(const MCSymbol *Hi, const MCSymbol *Lo,
                                     unsigned Encoding) const {
   // The least significant 3 bits specify the width of the encoding
   if ((Encoding & 0x7) == dwarf::DW_EH_PE_uleb128)
     emitLabelDifferenceAsULEB128(Hi, Lo);
   else

llvm/test/DebugInfo/X86/sections-as-references-cu-offset.ll

+; Test that references to CU are 4 bytes long when the target is 64-bit
+; and -dwarf-sections-as-references=Enable is specified.
+
+; RUN: llc -filetype=asm -mtriple=x86_64-linux-gnu -dwarf-sections-as-references=Enable < %s \
+; RUN:   | FileCheck %s
+
+; CHECK: .section .debug_pubnames
+; CHECK: .long .debug_info      # Offset of Compilation Unit Info
+; CHECK: .section .debug_pubtypes
+; CHECK: .long .debug_info      # Offset of Compilation Unit Info
+
+%struct.foo = type { i8 }
+
+@f = global %struct.foo zeroinitializer, align 1, !dbg !0
+
+!llvm.dbg.cu = !{!5}
+!llvm.module.flags = !{!8}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = !DIGlobalVariable(name: "f", scope: null, file: !2, line: 2, type: !3, isLocal: false, isDefinition: true)
+!2 = !DIFile(filename: "tu1.cpp", directory: "/dir")
+!3 = !DICompositeType(tag: DW_TAG_structure_type, name: "foo", file: !2, line: 1, size: 8, align: 8, elements: !4, identifier: "_ZTS3foo")
+!4 = !{}
+!5 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !2, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, retainedTypes: !6, globals: !7, imports: !4)
+!6 = !{!3}
+!7 = !{!0}
+!8 = !{i32 1, !"Debug Info Version", i32 3}