[BDCE] does not salvage/invalidate debuginfo when rewriting sext as zext

[bjope]

Bugzilla Link	47296
Version	trunk
OS	Linux
Blocks	#38116
CC	@DMG862,@jmorse,@jdm

Extended Description

The patch from https://reviews.llvm.org/D60413 (commit 7bf1683)
teached bit-tracking dead code elimination to rewrite:

%6 = sext i32 %5 to i64, !dbg !​26
call void @​llvm.dbg.value(metadata i64 %6, metadata !​18, metadata !DIExpression()), !dbg !​27

as

%6 = zext i32 %5 to i64, !dbg !​26
call void @​llvm.dbg.value(metadata i64 %6, metadata !​18, metadata !DIExpression()), !dbg !​27

when no uses of %6 depended on the most significant bits.

However, since the metadata references in the dbg.value intrinsics aren't dealt with in any special way, a debugger would print the wrong value for variable !​18 (showing a zero extended value instead of a sign extended value).

I figure that it would be more correct to invalidate the debug-uses (presenting an undef value), or somehow magically "salvage" the correct value by some fancy DIExpression based on %5.

Here is an example with full IR (compile with "opt -bdce"):

;-----------------------------------------------------------------------------
; ModuleID = './example.ll'
source_filename = "./example.ll"

; Function Attrs: nounwind uwtable
define i32 @​foo(i32 %0, i32 %1, i32* %2, i32* %3) #​0 !dbg !​7 {
%5 = add nsw i32 %0, 2, !dbg !​25
%6 = sext i32 %5 to i64, !dbg !​26
call void @​llvm.dbg.value(metadata i64 %6, metadata !​18, metadata !DIExpression()), !dbg !​27
%7 = icmp sgt i32 %1, 0, !dbg !​28
br i1 %7, label %8, label %11, !dbg !​29

8: ; preds = %4
%9 = or i64 %6, 5, !dbg !​30
call void @​llvm.dbg.value(metadata i64 %9, metadata !​20, metadata !DIExpression()), !dbg !​31
%10 = trunc i64 %9 to i32, !dbg !​32
store i32 %10, i32* %2, align 4, !dbg !​33, !tbaa !​34
br label %14, !dbg !​38

11: ; preds = %4
%12 = add nsw i64 %6, 3, !dbg !​39
call void @​llvm.dbg.value(metadata i64 %12, metadata !​23, metadata !DIExpression()), !dbg !​40
%13 = trunc i64 %12 to i32, !dbg !​41
store i32 %13, i32* %3, align 4, !dbg !​42, !tbaa !​34
br label %14

14: ; preds = %11, %8
ret i32 undef, !dbg !​43
}

; Function Attrs: nounwind readnone speculatable willreturn
declare void @​llvm.dbg.value(metadata, metadata, metadata) #​1

attributes #​0 = { nounwind uwtable }
attributes #​1 = { nounwind readnone speculatable willreturn }

!llvm.dbg.cu = !{#0}
!llvm.module.flags = !{#3, !​4, !​5}
!llvm.ident = !{#6}

!​0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !​1, producer: "clang version 12.0.0 (https://github.com/llvm/llvm-project.git 2b37174)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !​2, splitDebugInlining: false, nameTableKind: None)
!​1 = !DIFile(filename: "example.c", directory: "/home/ce")
!​2 = !{}
!​3 = !{i32 7, !"Dwarf Version", i32 4}
!​4 = !{i32 2, !"Debug Info Version", i32 3}
!​5 = !{i32 1, !"wchar_size", i32 4}
!​6 = !{!"clang version 12.0.0 (https://github.com/llvm/llvm-project.git 2b37174)"}
!​7 = distinct !DISubprogram(name: "foo", scope: !​8, file: !​8, line: 2, type: !​9, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !​0, retainedNodes: !​13)
!​8 = !DIFile(filename: "", directory: "/home/ce")
!​9 = !DISubroutineType(types: !​10)
!​10 = !{#11, !​11, !​11, !​12, !​12}
!​11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!​12 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !​11, size: 64)
!​13 = !{#14, !​15, !​16, !​17, !​18, !​20, !​23}
!​14 = !DILocalVariable(name: "X", arg: 1, scope: !​7, file: !​8, line: 2, type: !​11)
!​15 = !DILocalVariable(name: "Y", arg: 2, scope: !​7, file: !​8, line: 2, type: !​11)
!​16 = !DILocalVariable(name: "Z", arg: 3, scope: !​7, file: !​8, line: 2, type: !​12)
!​17 = !DILocalVariable(name: "W", arg: 4, scope: !​7, file: !​8, line: 2, type: !​12)
!​18 = !DILocalVariable(name: "LX", scope: !​7, file: !​8, line: 3, type: !​19)
!​19 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
!​20 = !DILocalVariable(name: "T", scope: !​21, file: !​8, line: 5, type: !​19)
!​21 = distinct !DILexicalBlock(scope: !​22, file: !​8, line: 4, column: 16)
!​22 = distinct !DILexicalBlock(scope: !​7, file: !​8, line: 4, column: 9)
!​23 = !DILocalVariable(name: "T", scope: !​24, file: !​8, line: 8, type: !​19)
!​24 = distinct !DILexicalBlock(scope: !​22, file: !​8, line: 7, column: 12)
!​25 = !DILocation(line: 3, column: 17, scope: !​7)
!​26 = !DILocation(line: 3, column: 15, scope: !​7)
!​27 = !DILocation(line: 0, scope: !​7)
!​28 = !DILocation(line: 4, column: 11, scope: !​22)
!​29 = !DILocation(line: 4, column: 9, scope: !​7)
!​30 = !DILocation(line: 5, column: 21, scope: !​21)
!​31 = !DILocation(line: 0, scope: !​21)
!​32 = !DILocation(line: 6, column: 13, scope: !​21)
!​33 = !DILocation(line: 6, column: 11, scope: !​21)
!​34 = !{#35, !​35, i64 0}
!​35 = !{!"int", !​36, i64 0}
!​36 = !{!"omnipotent char", !​37, i64 0}
!​37 = !{!"Simple C/C++ TBAA"}
!​38 = !DILocation(line: 7, column: 5, scope: !​21)
!​39 = !DILocation(line: 8, column: 21, scope: !​24)
!​40 = !DILocation(line: 0, scope: !​24)
!​41 = !DILocation(line: 9, column: 13, scope: !​24)
!​42 = !DILocation(line: 9, column: 11, scope: !​24)
!​43 = !DILocation(line: 11, column: 1, scope: !​7)
;-----------------------------------------------------------------------------