[llvm] [DebugInfo][LoopStrengthReduce] Salvage the debug value of the dead cmp instruction (PR #147241)

[Shan Huang via llvm-commits]

https://github.com/Apochens created https://github.com/llvm/llvm-project/pull/147241

Fix #147238 

>From c59ec5d8b8750cfc7d11b56deab293837d201d02 Mon Sep 17 00:00:00 2001
From: Apochens <apowenq at gmail.com>
Date: Mon, 7 Jul 2025 06:13:51 +0000
Subject: [PATCH] fix #147238

---
 llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp        | 4 +++-
 .../LoopStrengthReduce/optimizemax_debugloc.ll           | 9 +++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index 786e4516ace05..7b631dbb16571 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -2559,8 +2559,10 @@ ICmpInst *LSRInstance::OptimizeMax(ICmpInst *Cond, IVStrideUse* &CondUse) {
   Instruction *Cmp = cast<Instruction>(Sel->getOperand(0));
   Cond->eraseFromParent();
   Sel->eraseFromParent();
-  if (Cmp->use_empty())
+  if (Cmp->use_empty()) {
+    salvageDebugInfo(*Cmp);
     Cmp->eraseFromParent();
+  }
   return NewCond;
 }
 
diff --git a/llvm/test/Transforms/LoopStrengthReduce/optimizemax_debugloc.ll b/llvm/test/Transforms/LoopStrengthReduce/optimizemax_debugloc.ll
index d2d5550c3d68f..eabcb0e1cb81c 100644
--- a/llvm/test/Transforms/LoopStrengthReduce/optimizemax_debugloc.ll
+++ b/llvm/test/Transforms/LoopStrengthReduce/optimizemax_debugloc.ll
@@ -1,7 +1,16 @@
 ; RUN: opt < %s -loop-reduce -S 2>&1 | FileCheck %s
+
 ;; This test case checks that whether the new icmp instruction preserves
 ;; the debug location of the original instruction for %exitcond
+
+;; This test case also checks that the debug value of the dead icmp
+;; instruction is salvaged.
+
+; CHECK:   #dbg_value(i32 %n, ![[META1:[0-9]+]], !DIExpression(DW_OP_lit0, DW_OP_eq, DW_OP_stack_value), ![[META2:[0-9]+]])
 ; CHECK: icmp uge i32 %indvar.next, %n, !dbg ![[DBGLOC:[0-9]+]]
+
+; CHECK: ![[META1]] = !DILocalVariable(name: "1",
+; CHECK: ![[META2]] = !DILocation(line: 1, column: 1,
 ; CHECK: ![[DBGLOC]] = !DILocation(line: 6, column: 1, scope
 
 ; ModuleID = 'simplified-dbg.bc'