[llvm] [DebugInfo][NewGVN] Fix debug value loss (PR #147634)

[Shan Huang via llvm-commits]

https://github.com/Apochens updated https://github.com/llvm/llvm-project/pull/147634

>From 95cb8336d1598780d66eeb85d8fcb1c9a433a459 Mon Sep 17 00:00:00 2001
From: Apochens <apowenq at gmail.com>
Date: Wed, 9 Jul 2025 01:52:23 +0000
Subject: [PATCH 1/2] fix #147511

---
 llvm/lib/Transforms/Scalar/NewGVN.cpp         |  1 +
 .../NewGVN/salvage-eliminate-instruction.ll   | 43 +++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll

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,
+;.

>From 4dc155a09ba32cc4ada50622fd97f0722226377e Mon Sep 17 00:00:00 2001
From: Apochens <apowenq at gmail.com>
Date: Sun, 13 Jul 2025 04:47:02 +0000
Subject: [PATCH 2/2] replacing dbg uses instead of salvaging

---
 llvm/lib/Transforms/Scalar/NewGVN.cpp                 | 11 ++++++++++-
 .../NewGVN/salvage-eliminate-instruction.ll           |  5 ++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/NewGVN.cpp b/llvm/lib/Transforms/Scalar/NewGVN.cpp
index f101d3c126491..5e6316fe25ad8 100644
--- a/llvm/lib/Transforms/Scalar/NewGVN.cpp
+++ b/llvm/lib/Transforms/Scalar/NewGVN.cpp
@@ -82,6 +82,7 @@
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/InstrTypes.h"
@@ -4076,7 +4077,15 @@ bool NewGVN::eliminateInstructions(Function &F) {
                 if (!match(DefI, m_Intrinsic<Intrinsic::ssa_copy>()))
                   patchReplacementInstruction(DefI, DominatingLeader);
 
-                salvageDebugInfo(*DefI);
+                SmallVector<DbgVariableIntrinsic *> DbgUsers;
+                SmallVector<DbgVariableRecord *> DVRUsers;
+                findDbgUsers(DbgUsers, DefI, &DVRUsers);
+
+                for (auto *DVI: DbgUsers)
+                  DVI->replaceVariableLocationOp(DefI, DominatingLeader);
+                for (auto *DVR: DVRUsers)
+                  DVR->replaceVariableLocationOp(DefI, DominatingLeader);
+
                 markInstructionForDeletion(DefI);
               }
             }
diff --git a/llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll b/llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll
index fe26cefb2803e..a2073d0a153e2 100644
--- a/llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll
+++ b/llvm/test/Transforms/NewGVN/salvage-eliminate-instruction.ll
@@ -5,7 +5,8 @@
 
 
 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]+]])
+; CHECK:      #dbg_value(i32 %add1, [[META9:![0-9]+]], !DIExpression(), [[META12:![0-9]+]])
+; CHECK-NEXT: #dbg_value(i32 %add1, [[META11:![0-9]+]], !DIExpression(), [[META13:![0-9]+]])
 ;
   %add1 = add i32 %x, %y, !dbg !12
     #dbg_value(i32 %add1, !9, !DIExpression(), !12)
@@ -38,6 +39,8 @@ declare void @use(i32, i32)
 !14 = !DILocation(line: 3, column: 1, scope: !5)
 !15 = !DILocation(line: 4, column: 1, scope: !5)
 ;.
+; CHECK: [[META9]] = !DILocalVariable(name: "1",
 ; CHECK: [[META11]] = !DILocalVariable(name: "2",
+; CHECK: [[META12]] = !DILocation(line: 1,
 ; CHECK: [[META13]] = !DILocation(line: 2,
 ;.