119aecb955
Before this patch, the value of DW_AT_bit_offset, used for bitfields before DWARF version 4, was always emitted as an unsigned integer using the form DW_FORM_data<n>. If the value was originally a signed integer, for instance in the case of negative offsets, it was up to debug information consumers to re-cast it to a signed integer. This is problematic since the burden of deciding if the value should be read as signed or unsigned was put onto the debug info consumers: the DWARF specification doesn't define DW_AT_bit_offset's underlying type. If a debugger decided to interpret this attribute in the form data<n> as unsigned, then negative offsets would be completely broken. The DWARF specification version 3 mentions in the Data Representation section, page 127: > If one of the DW_FORM_data<n> forms is used to represent a signed or unsigned integer, it can be hard for a consumer to discover the context necessary to determine which interpretation is intended. Producers are therefore strongly encouraged to use DW_FORM_sdata or DW_FORM_udata for signed and unsigned integers respectively, rather than DW_FORM_data<n>. Therefore, the proposal is to use DW_FORM_sdata, which is explicitly signed. This is an indication to consumers that the offset must be parsed unambiguously as a signed integer. Finally, gcc already uses DW_FORM_sdata for negative offsets, fixing the potential ambiguity altogether. This patch mimics gcc's behaviour by emitting negative values of DW_AT_bit_offset using the DW_FORM_sdata form. This eliminates any potential misinterpretation. One could argue that all values should use DW_FORM_sdata, but for the sake of parity with gcc, it is safe to restrict the change to negative values.
45 lines
2.0 KiB
LLVM
45 lines
2.0 KiB
LLVM
; RUN: llc -mtriple=thumbv7-apple-ios -O0 -filetype=obj -o %t.o %s
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; RUN: llvm-dwarfdump -v -debug-info %t.o | FileCheck %s
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;
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; Generated from:
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; struct {
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; char c;
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; int : 4;
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; int reserved : 28;
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; } a;
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;
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; CHECK: DW_TAG_member
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; CHECK: DW_AT_name {{.*}} "reserved"
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; CHECK: DW_AT_byte_size {{.*}} (0x04)
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; CHECK: DW_AT_bit_size {{.*}} (0x1c)
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; CHECK: DW_AT_bit_offset {{.*}} (-8)
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; CHECK: DW_AT_data_member_location {{.*}} (DW_OP_plus_uconst 0x0)
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%struct.anon = type { i8, [5 x i8] }
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@a = common global %struct.anon zeroinitializer, align 1, !dbg !0
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!llvm.dbg.cu = !{!2}
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!llvm.module.flags = !{!12, !13, !14, !15, !16}
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!llvm.ident = !{!17}
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!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
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!1 = !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 5, type: !6, isLocal: false, isDefinition: true)
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!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 3.7.0 (trunk 240548) (llvm/trunk 240554)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, retainedTypes: !4, globals: !5, imports: !4)
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!3 = !DIFile(filename: "test.i", directory: "/")
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!4 = !{}
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!5 = !{!0}
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!6 = !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 1, size: 48, align: 8, elements: !7)
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!7 = !{!8, !10}
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!8 = !DIDerivedType(tag: DW_TAG_member, name: "c", scope: !6, file: !3, line: 2, baseType: !9, size: 8, align: 8)
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!9 = !DIBasicType(name: "char", size: 8, align: 8, encoding: DW_ATE_signed_char)
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!10 = !DIDerivedType(tag: DW_TAG_member, name: "reserved", scope: !6, file: !3, line: 4, baseType: !11, size: 28, align: 32, offset: 12, flags: DIFlagBitField)
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!11 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
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!12 = !{i32 2, !"Dwarf Version", i32 2}
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!13 = !{i32 2, !"Debug Info Version", i32 3}
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!14 = !{i32 1, !"wchar_size", i32 4}
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!15 = !{i32 1, !"min_enum_size", i32 4}
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!16 = !{i32 1, !"PIC Level", i32 2}
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!17 = !{!"clang version 3.7.0 (trunk 240548) (llvm/trunk 240554)"}
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