[llvm] [DebugInfo][NewGVN] Salvage debug values of trivially dead instructions (PR #149304)

[Shan Huang via llvm-commits]

https://github.com/Apochens created https://github.com/llvm/llvm-project/pull/149304

fix #147634

>From 083e9d71ec9c91568fceaf4f7a29cebca7750f8c Mon Sep 17 00:00:00 2001
From: Apochens <apowenq at gmail.com>
Date: Thu, 17 Jul 2025 13:09:11 +0000
Subject: [PATCH] fix 147634

---
 llvm/lib/Transforms/Scalar/NewGVN.cpp         |  1 +
 .../NewGVN/salvage-trivially-dead-inst.ll     | 58 +++++++++++++++++++
 2 files changed, 59 insertions(+)
 create mode 100644 llvm/test/Transforms/NewGVN/salvage-trivially-dead-inst.ll

diff --git a/llvm/lib/Transforms/Scalar/NewGVN.cpp b/llvm/lib/Transforms/Scalar/NewGVN.cpp
index 7eeaaa0d99602..17c4fd9c2aae9 100644
--- a/llvm/lib/Transforms/Scalar/NewGVN.cpp
+++ b/llvm/lib/Transforms/Scalar/NewGVN.cpp
@@ -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;
     }
diff --git a/llvm/test/Transforms/NewGVN/salvage-trivially-dead-inst.ll b/llvm/test/Transforms/NewGVN/salvage-trivially-dead-inst.ll
new file mode 100644
index 0000000000000..1845cf6f0852c
--- /dev/null
+++ b/llvm/test/Transforms/NewGVN/salvage-trivially-dead-inst.ll
@@ -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:
+  %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)
\ No newline at end of file