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[DebugInfo][NewGVN] Salvage debug values of trivially dead instructions #149304

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Merged
merged 2 commits into from
Jul 21, 2025
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[DebugInfo][NewGVN] Salvage debug values of trivially dead instructions #149304

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083e9d7
fix 147634
Apochens Jul 17, 2025
680cafd
refine the regression test
Apochens Jul 17, 2025
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1 change: 1 addition & 0 deletions llvm/lib/Transforms/Scalar/NewGVN.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3044,6 +3044,7 @@ std::pair<unsigned, unsigned> NewGVN::assignDFSNumbers(BasicBlock *B,
if (isInstructionTriviallyDead(&I, TLI)) {
InstrDFS[&I] = 0;
LLVM_DEBUG(dbgs() << "Skipping trivially dead instruction " << I << "\n");
salvageDebugInfo(I);
markInstructionForDeletion(&I);
continue;
}
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58 changes: 58 additions & 0 deletions llvm/test/Transforms/NewGVN/salvage-trivially-dead-inst.ll
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@@ -0,0 +1,58 @@
; RUN: opt -passes=newgvn -S %s | FileCheck %s

; Check that assignDFSNumbers() in NewGVN salvages the debug values of the
; trivially dead instructions that are marked for deletion.

; CHECK: #dbg_value(i8 %tmp, [[META11:![0-9]+]], !DIExpression(DW_OP_constu, 8, DW_OP_eq, DW_OP_stack_value), [[META26:![0-9]+]])
; CHECK: [[META11]] = !DILocalVariable(name: "2"
; CHECK: [[META26]] = !DILocation(line: 3

define void @test13() !dbg !5 {
bb:
br label %bb1

bb1:
%tmp = load i8, ptr null, align 1
%tmp2 = icmp eq i8 %tmp, 8, !dbg !26
#dbg_value(i1 %tmp2, !11, !DIExpression(), !26)
br label %bb3

bb3:
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@OCHyams OCHyams Jul 17, 2025

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Do we need all the instructions below, or is the following sufficient to exercise the behaviour?

define void @test13() !dbg !5 {
entry:
  %tmp = load i8, ptr null, align 1
  %tmp2 = icmp eq i8 %tmp, 8, !dbg !26
    #dbg_value(i1 %tmp2, !11, !DIExpression(), !26)
  ret void
}

If you do need the extra instructions, I think you'll want to replace the undef with poison.

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@Apochens Apochens Jul 17, 2025

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Thanks. I think this concise test case is enough.

%tmp4 = phi ptr [ null, %bb1 ], [ %tmp6, %bb3 ]
%tmp5 = phi i32 [ undef, %bb1 ], [ %tmp9, %bb3 ]
%tmp6 = getelementptr i8, ptr %tmp4, i64 1
%tmp7 = load i8, ptr %tmp4, align 1
%tmp8 = sext i8 %tmp7 to i32
%tmp9 = mul i32 %tmp5, %tmp8
%tmp10 = load i8, ptr %tmp6, align 1
%tmp11 = icmp eq i8 %tmp10, 0
br i1 %tmp11, label %bb12, label %bb3

bb12:
%tmp13 = phi i32 [ %tmp9, %bb3 ]
%tmp14 = icmp eq i32 %tmp13, 0
br i1 %tmp14, label %bb1, label %bb15

bb15:
call void (...) @bar()
br label %bb1
}

declare void @bar(...)

!llvm.dbg.cu = !{!0}
!llvm.debugify = !{!2, !3}
!llvm.module.flags = !{!4}

!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
!1 = !DIFile(filename: "/app/example.ll", directory: "/")
!2 = !{i32 18}
!3 = !{i32 12}
!4 = !{i32 2, !"Debug Info Version", i32 3}
!5 = distinct !DISubprogram(name: "test13", linkageName: "test13", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
!6 = !DISubroutineType(types: !7)
!7 = !{}
!8 = !{!11}
!10 = !DIBasicType(name: "ty8", size: 8, encoding: DW_ATE_unsigned)
!11 = !DILocalVariable(name: "2", scope: !5, file: !1, line: 3, type: !10)
!26 = !DILocation(line: 3, column: 1, scope: !5)
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