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[Dwarf] Emit DW_AT_bit_size for non-byte-sized types #137118

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[Dwarf] Emit DW_AT_bit_size for non-byte-sized types #137118

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8d37b69
[InstCombine] Add pre-commit tests. NFC.
dtcxzyw Apr 23, 2025
0eb30b7
[InstCombine] Update debug value uses in `freelyInvertAllUsersOf`
dtcxzyw Apr 23, 2025
037aa20
[Dwarf] Emit `DW_AT_bit_size` for non-byte-sized types
dtcxzyw Apr 24, 2025
db3c8d6
[Dwarf] Remove local dir. NFC.
dtcxzyw Apr 24, 2025
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[Dwarf] Emit DW_AT_bit_size for non-byte-sized types
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@dtcxzyw
dtcxzyw committed Apr 24, 2025
commit 037aa2015dae1dc0f400b67effc98b060a6b25be
4 changes: 4 additions & 0 deletions llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
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Expand Up @@ -1749,6 +1749,10 @@ void DwarfCompileUnit::createBaseTypeDIEs() {
// Round up to smallest number of bytes that contains this number of bits.
addUInt(Die, dwarf::DW_AT_byte_size, std::nullopt,
divideCeil(Btr.BitSize, 8));
// If the size is not a multiple of 8 (e.g., boolean), add the bit size
// field.
if (Btr.BitSize % 8 != 0)
addUInt(Die, dwarf::DW_AT_bit_size, std::nullopt, Btr.BitSize);
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DWARF seems to say that either the bit- or byte-size should be specified. So rather than emit both sizes, I think it's preferable to emit just the bit size if it is not a multiple of 8. See DWARF 5 section 5.1 "Base Type Entries":

A base type entry has a DW_AT_byte_size attribute or a
DW_AT_bit_size attribute whose integer constant value (see Section 2.21 on
page 56) is the amount of storage needed to hold a value of the type.

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I was looking at this again and there's some later text about a scenario where DW_AT_byte_size is used for the object size, but then the actual data is described with DW_AT_bit_size plus DW_AT_data_bit_offset. So while it's not totally correct to say that the base type must have one or the other, it's still the case that this particular patch should probably do that. IMO.


Btr.Die = &Die;
}
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43 changes: 43 additions & 0 deletions llvm/test/DebugInfo/X86/convert-bool.ll
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@@ -0,0 +1,43 @@
; RUN: llc -mtriple=x86_64 -dwarf-version=5 -filetype=obj -O0 < %s | llvm-dwarfdump - | FileCheck %s

; CHECK: DW_TAG_base_type
; CHECK-NEXT: DW_AT_name ("DW_ATE_unsigned_1")
; CHECK-NEXT: DW_AT_encoding (DW_ATE_unsigned)
; CHECK-NEXT: DW_AT_byte_size (0x01)
; CHECK-NEXT: DW_AT_bit_size (0x01)

define void @main() !dbg !18 {
entry:
#dbg_value(i1 false, !22, !DIExpression(DW_OP_not, DW_OP_LLVM_convert, 1, DW_ATE_unsigned, DW_OP_LLVM_convert, 32, DW_ATE_unsigned, DW_OP_stack_value), !23)
ret void, !dbg !24
}

!llvm.dbg.cu = !{!0}
!llvm.module.flags = !{!10, !11, !12, !13, !14, !15, !16}
!llvm.ident = !{!17}

!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 21.0.0git", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !2, splitDebugInlining: false, nameTableKind: None)
!1 = !DIFile(filename: "test.c", directory: "/home/dtcxzyw/WorkSpace/Projects/compilers/LLVM/llvm-build", checksumkind: CSK_MD5, checksum: "100bdbce655d729c24c7c0e8523a58ae")
!2 = !{!3, !6, !8}
!3 = !DIGlobalVariableExpression(var: !4, expr: !DIExpression())
!4 = distinct !DIGlobalVariable(name: "a", scope: !0, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true)
!5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
!7 = distinct !DIGlobalVariable(name: "b", scope: !0, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true)
!8 = !DIGlobalVariableExpression(var: !9, expr: !DIExpression())
!9 = distinct !DIGlobalVariable(name: "c", scope: !0, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true)
!10 = !{i32 7, !"Dwarf Version", i32 5}
!11 = !{i32 2, !"Debug Info Version", i32 3}
!12 = !{i32 1, !"wchar_size", i32 4}
!13 = !{i32 8, !"PIC Level", i32 2}
!14 = !{i32 7, !"PIE Level", i32 2}
!15 = !{i32 7, !"uwtable", i32 2}
!16 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
!17 = !{!"clang version 21.0.0git"}
!18 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 3, type: !19, scopeLine: 3, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !21)
!19 = !DISubroutineType(types: !20)
!20 = !{null}
!21 = !{!22}
!22 = !DILocalVariable(name: "l_4516", scope: !18, file: !1, line: 4, type: !5)
!23 = !DILocation(line: 0, scope: !18)
!24 = !DILocation(line: 9, column: 1, scope: !18)
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