[DebugInfo][NewGVN] Fix debug value loss

[Apochens]

Fix #147511

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-debuginfo

Author: Shan Huang (Apochens)

Changes

Fix #147511

---

Full diff: https://github.com/llvm/llvm-project/pull/147634.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/NewGVN.cpp (+1)
• (added) llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll (+43)

diff --git a/llvm/lib/Transforms/Scalar/NewGVN.cpp b/llvm/lib/Transforms/Scalar/NewGVN.cpp
index 7eeaaa0d99602..f101d3c126491 100644
--- a/llvm/lib/Transforms/Scalar/NewGVN.cpp
+++ b/llvm/lib/Transforms/Scalar/NewGVN.cpp
@@ -4076,6 +4076,7 @@ bool NewGVN::eliminateInstructions(Function &F) {
                 if (!match(DefI, m_Intrinsic<Intrinsic::ssa_copy>()))
                   patchReplacementInstruction(DefI, DominatingLeader);
 
+                salvageDebugInfo(*DefI);
                 markInstructionForDeletion(DefI);
               }
             }
diff --git a/llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll b/llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll
new file mode 100644
index 0000000000000..fe26cefb2803e
--- /dev/null
+++ b/llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll
@@ -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.
+
+
+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,
+;.

[dzhidzhoev]

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?

[Apochens]

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.

[dzhidzhoev]

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