Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1194,6 +1194,7 @@
   PHINode *NewPN = PHINode::Create(I.getType(), PN->getNumIncomingValues());
   InsertNewInstBefore(NewPN, *PN);
   NewPN->takeName(PN);
+  NewPN->setDebugLoc(PN->getDebugLoc());
 
   // If we are going to have to insert a new computation, do so right before the
   // predecessor's terminator.
@@ -1222,6 +1223,10 @@
     replaceInstUsesWith(*User, NewPN);
     eraseInstFromFunction(*User);
   }
+
+  replaceAllDbgUsesWith(const_cast<PHINode &>(*PN),
+                        const_cast<PHINode &>(*NewPN),
+                        const_cast<PHINode &>(*PN), DT);
   return replaceInstUsesWith(I, NewPN);
 }
 
Index: llvm/test/DebugInfo/X86/instcombine-fold-cast-into-phi.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/X86/instcombine-fold-cast-into-phi.ll
@@ -0,0 +1,48 @@
+; RUN: opt -passes=instcombine -S %s | FileCheck %s
+
+;; Instcombine folds the trunc %x into the phi. Check it updates the phi's dbg
+;; user (otherwise the dbg use becomes poison after the original phi is
+;; deleted). Check the new phi inherits the DebugLoc.
+
+;; CHECK: %p.0 = phi i8 [ 1, %if.then ], [ 0, %entry ], !dbg ![[#]]
+;; CHECK: call void @llvm.dbg.value(metadata i8 %p.0, metadata ![[#]], metadata !DIExpression(DW_OP_LLVM_convert, 8, DW_ATE_signed, DW_OP_LLVM_convert, 32, DW_ATE_signed, DW_OP_stack_value)
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define linkonce_odr float @f(i1 %cond) {
+entry:
+  br i1 %cond, label %if.then, label %if.end
+
+if.then:                                          ; preds = entry
+  br label %if.end
+
+if.end:                                           ; preds = %if.then, %entry
+  %p.0 = phi i32 [ 1, %if.then ], [ 0, %entry ], !dbg !13
+  call void @llvm.dbg.value(metadata i32 %p.0, metadata !4, metadata !DIExpression()), !dbg !13
+  %x = trunc i32 %p.0 to i8
+  %callff = call float @ff(i8  %x)
+  ret float %callff
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+declare float @ff(float)
+attributes #0 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_11, file: !1, producer: "clang version 17.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, retainedTypes: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "btConvexTriangleMeshShape.cpp", directory: "/")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !DILocalVariable(name: "p", scope: !5, file: !6, line: 2057, type: !12)
+!5 = distinct !DILexicalBlock(scope: !7, file: !6, line: 2056, column: 4)
+!6 = !DIFile(filename: "reduce.cpp", directory: "/")
+!7 = distinct !DISubprogram(name: "diagonalize", linkageName: "_ZN11btMatrix3x311diagonalizeERS_fi", scope: !8, file: !6, line: 2054, type: !9, scopeLine: 2055, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, declaration: !11, retainedNodes: !2)
+!8 = distinct !DICompositeType(tag: DW_TAG_class_type, name: "btMatrix3x3", file: !6, line: 2050, size: 24, flags: DIFlagTypePassByValue, elements: !2, identifier: "_ZTS11btMatrix3x3")
+!9 = !DISubroutineType(types: !10)
+!10 = !{null}
+!11 = !DISubprogram(name: "diagonalize", linkageName: "_ZN11btMatrix3x311diagonalizeERS_fi", scope: !8, file: !6, line: 2054, type: !9, scopeLine: 2054, flags: DIFlagPublic | DIFlagPrototyped, spFlags: DISPFlagOptimized)
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !DILocation(line: 0, scope: !5)