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
1.9 KiB
LLVM
45 lines
1.9 KiB
LLVM
; RUN: llc < %s -mtriple=nvptx64-nvidia-cuda | FileCheck %s
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; RUN: %if ptxas %{ llc < %s -mtriple=nvptx64-nvidia-cuda | %ptxas-verify %}
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; Produced at -O0 from:
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; struct {
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; char : 3;
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; char a : 6;
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; } __attribute__((__packed__)) b;
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; Note that DWARF 2 counts bit offsets backwards from the high end of
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; the storage unit to the high end of the bit field.
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; CHECK: .section .debug_info
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; CHECK: .b8 3 // Abbrev {{.*}} DW_TAG_structure_type
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; CHECK: .b8 3 // DW_AT_decl_line
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; CHECK-NEXT: .b8 1 // DW_AT_byte_size
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; CHECK-NEXT: .b8 6 // DW_AT_bit_size
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; Negative offset must be encoded as an unsigned integer.
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; CHECK-NEXT: .b8 127 // DW_AT_bit_offset
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; CHECK-NEXT: .b8 2 // DW_AT_data_member_location
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%struct.anon = type { i16 }
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@b = global %struct.anon zeroinitializer, align 1, !dbg !0
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!llvm.dbg.cu = !{!2}
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!llvm.module.flags = !{!10, !11, !12, !13}
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!llvm.ident = !{!14}
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!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
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!1 = distinct !DIGlobalVariable(name: "b", scope: !2, file: !3, line: 4, type: !6, isLocal: false, isDefinition: true)
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!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 11.0.0 ", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None, sysroot: "/")
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!3 = !DIFile(filename: "repro.c", directory: "/tmp")
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!4 = !{}
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!5 = !{!0}
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!6 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 1, size: 16, elements: !7)
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!7 = !{!8}
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!8 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !6, file: !3, line: 3, baseType: !9, size: 6, offset: 3, flags: DIFlagBitField, extraData: i64 0)
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!9 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
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!10 = !{i32 7, !"Dwarf Version", i32 2}
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!11 = !{i32 2, !"Debug Info Version", i32 3}
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!12 = !{i32 1, !"wchar_size", i32 4}
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!13 = !{i32 7, !"PIC Level", i32 2}
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!14 = !{!"clang version 11.0.0 "}
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