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[DebugInfo][NewGVN] Fix debug value loss #147634

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Merged
merged 4 commits into from
Jul 21, 2025
Merged

[DebugInfo][NewGVN] Fix debug value loss #147634

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95cb833
fix #147511
Apochens Jul 9, 2025
4dc155a
replacing dbg uses instead of salvaging
Apochens Jul 13, 2025
f657571
refine the test comment
Apochens Jul 13, 2025
9162ff7
format
Apochens Jul 13, 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 @@ -4076,6 +4076,7 @@ bool NewGVN::eliminateInstructions(Function &F) {
if (!match(DefI, m_Intrinsic<Intrinsic::ssa_copy>()))
patchReplacementInstruction(DefI, DominatingLeader);

salvageDebugInfo(*DefI);
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Should this be moved inside the loop

llvm-project/llvm/lib/Transforms/Scalar/NewGVN.cpp

Line 3527 in 95cb833

for (Instruction *ToErase : InstructionsToErase) {
to salvage more instructions?

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That would be a solution, but that would introduce some unnecessary computation overhead of salvaging. For example, salvaging the store instructions in InstructionsToErase is unnecessary:

llvm-project/llvm/lib/Transforms/Scalar/NewGVN.cpp

Lines 4194 to 4196 in 6c8c836

LLVM_DEBUG(dbgs() << "Marking dead store " << *Member
<< " that is dominated by " << *Leader << "\n");
markInstructionForDeletion(Member);

So, I didn't place this salvage into the loop but place it just before where the salvageable instructions are marked for deletion. But if this unnecessary compulation is acceptable, I can move this salvage into the loop.

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Thank you. I'm fine with the current version if extra salvaging calls are redundant.

markInstructionForDeletion(DefI);
}
}
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43 changes: 43 additions & 0 deletions llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll
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@@ -0,0 +1,43 @@
; RUN: opt -S -passes=newgvn %s | FileCheck %s

; Check that eliminateInstruction() salvages the debug value of `Def` (`DefI`)
; which is marked for deletion.
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The comment has a chance of going stale, referring to the LLVM source. I think function name is ok but instead of LLVM source variables, it's probably better to describe what's happening in terms of the IR in the test.

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Thanks, I have refined it in the latest commit.



define void @binop(i32 %x, i32 %y) !dbg !5 {
; CHECK: #dbg_value(!DIArgList(i32 %y, i32 %x), [[META11:![0-9]+]], !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_LLVM_arg, 1, DW_OP_plus, DW_OP_stack_value), [[META13:![0-9]+]])
;
%add1 = add i32 %x, %y, !dbg !12
#dbg_value(i32 %add1, !9, !DIExpression(), !12)
%add2 = add i32 %y, %x, !dbg !13
#dbg_value(i32 %add2, !11, !DIExpression(), !13)
call void @use(i32 %add1, i32 %add2), !dbg !14
ret void, !dbg !15
}

declare void @use(i32, i32)

!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 4}
!3 = !{i32 2}
!4 = !{i32 2, !"Debug Info Version", i32 3}
!5 = distinct !DISubprogram(name: "binop", linkageName: "binop", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
!6 = !DISubroutineType(types: !7)
!7 = !{}
!8 = !{!9, !11}
!9 = !DILocalVariable(name: "1", scope: !5, file: !1, line: 1, type: !10)
!10 = !DIBasicType(name: "ty32", size: 32, encoding: DW_ATE_unsigned)
!11 = !DILocalVariable(name: "2", scope: !5, file: !1, line: 2, type: !10)
!12 = !DILocation(line: 1, column: 1, scope: !5)
!13 = !DILocation(line: 2, column: 1, scope: !5)
!14 = !DILocation(line: 3, column: 1, scope: !5)
!15 = !DILocation(line: 4, column: 1, scope: !5)
;.
; CHECK: [[META11]] = !DILocalVariable(name: "2",
; CHECK: [[META13]] = !DILocation(line: 2,
;.