[DebugInfo][Reassociate] Preserve DebugLocs when reassociating subs (llvm#114226)

In NegateValue in Reassociate, we return the negation of an existing
value in order to break a subtract into an negate + add, potentially
creating a new instruction to perform the negation, but we neglect to
propagate the DebugLoc of the sub being replaced to the negate
instruction if one is created. This patch adds that propagation.

Found using llvm#107279.

Author: SLTozer

llvm/lib/Transforms/Scalar/Reassociate.cpp

@@ -874,6 +874,8 @@ static Value *NegateValue(Value *V, Instruction *BI,
   // negation.
   Instruction *NewNeg =
       CreateNeg(V, V->getName() + ".neg", BI->getIterator(), BI);
+  // NewNeg is generated to potentially replace BI, so use its DebugLoc.
+  NewNeg->setDebugLoc(BI->getDebugLoc());
   ToRedo.insert(NewNeg);
   return NewNeg;
 }

llvm/test/Transforms/Reassociate/preserve-debugloc.ll

@@ -0,0 +1,38 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+;; Tests that we preserve DebugLocs through reassociation of sub instructions.
+; RUN: opt < %s -passes=reassociate -S | FileCheck %s
+
+define void @foo(i64 %0) {
+; CHECK-LABEL: define void @foo(
+; CHECK-SAME: i64 [[TMP0:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[DOTNEG:%.*]] = sub i64 0, [[TMP0]], !dbg [[DBG3:![0-9]+]]
+; CHECK-NEXT:    [[ADD_I_I:%.*]] = add i64 [[DOTNEG]], 1
+; CHECK-NEXT:    store i64 [[ADD_I_I]], ptr null, align 8
+; CHECK-NEXT:    ret void
+;
+entry:
+  %sub5.i.i = sub i64 1, %0, !dbg !4
+  %add.i.i = add i64 %sub5.i.i, 0
+  store i64 %add.i.i, ptr null, align 8
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 20.0.0git", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "test.cpp", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !DILocation(line: 212, column: 25, scope: !5)
+!5 = distinct !DISubprogram(name: "foo", scope: !0, file: !1, line: 161, type: !6, scopeLine: 162, unit: !0, retainedNodes: !2)
+!6 = distinct !DISubroutineType(types: !2)
+;.
+; CHECK: [[META0:![0-9]+]] = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: [[META1:![0-9]+]], producer: "{{.*}}clang version {{.*}}", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+; CHECK: [[META1]] = !DIFile(filename: "test.cpp", directory: {{.*}})
+; CHECK: [[DBG3]] = !DILocation(line: 212, column: 25, scope: [[META4:![0-9]+]])
+; CHECK: [[META4]] = distinct !DISubprogram(name: "foo", scope: [[META0]], file: [[META1]], line: 161, type: [[META5:![0-9]+]], scopeLine: 162, spFlags: DISPFlagDefinition, unit: [[META0]], retainedNodes: [[META6:![0-9]+]])
+; CHECK: [[META5]] = distinct !DISubroutineType(types: [[META6]])
+; CHECK: [[META6]] = !{}
+;.