[DebugInfo][Reassociate] Propagate source locs when factoring add->mul

[SLTozer]

As part of reassociating add instructions, we may factorize some of the adds and produce a mul instruction; this patch propagates the source location of the reassociated tree of instructions to the new mul.

Found using #107279.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-debuginfo

Author: Stephen Tozer (SLTozer)

Changes

As part of reassociating add instructions, we may factorize some of the adds and produce a mul instruction; this patch propagates the source location of the reassociated tree of instructions to the new mul.

Found using #107279.

---

Full diff: https://github.com/llvm/llvm-project/pull/134829.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/Reassociate.cpp (+1)
• (added) llvm/test/Transforms/Reassociate/debugloc-factoring-add.ll (+43)

diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp
index f9aef064641d8..0abb68c6a3964 100644
--- a/llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -1519,6 +1519,7 @@ Value *ReassociatePass::OptimizeAdd(Instruction *I,
       Constant *C = Ty->isIntOrIntVectorTy() ?
         ConstantInt::get(Ty, NumFound) : ConstantFP::get(Ty, NumFound);
       Instruction *Mul = CreateMul(TheOp, C, "factor", I->getIterator(), I);
+      Mul->setDebugLoc(I->getDebugLoc());
 
       // Now that we have inserted a multiply, optimize it. This allows us to
       // handle cases that require multiple factoring steps, such as this:
diff --git a/llvm/test/Transforms/Reassociate/debugloc-factoring-add.ll b/llvm/test/Transforms/Reassociate/debugloc-factoring-add.ll
new file mode 100644
index 0000000000000..9c507f998b5e7
--- /dev/null
+++ b/llvm/test/Transforms/Reassociate/debugloc-factoring-add.ll
@@ -0,0 +1,43 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -p=reassociate -S | FileCheck %s
+
+;; Ensure that when we factorize part of a tree of adds into a multiply, we
+;; propagate the debug loc of the tree to the mul.
+
+define fastcc void @ham(i32 %arg) !dbg !10 {
+; CHECK-LABEL: define fastcc void @ham(
+; CHECK-SAME: i32 [[ARG:%.*]]) !dbg [[DBG4:![0-9]+]] {
+; CHECK-NEXT:  [[BB:.*:]]
+; CHECK-NEXT:    [[FACTOR:%.*]] = mul i32 [[ARG]], 2, !dbg [[DBG7:![0-9]+]]
+; CHECK-NEXT:    [[ADD1:%.*]] = add i32 [[FACTOR]], 1, !dbg [[DBG7]]
+; CHECK-NEXT:    store i32 [[ADD1]], ptr null, align 4
+; CHECK-NEXT:    ret void
+;
+bb:
+  %add = add i32 %arg, 1
+  %add1 = add i32 %arg, %add, !dbg !4
+  store i32 %add1, ptr null, align 4
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 20.0.0git", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2, globals: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "debugloc-factoring-add.c", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !DILocation(line: 10, column: 1, scope: !10)
+!6 = !DIFile(filename: "debugloc-factoring-add.c", directory: "/tmp")
+!10 = distinct !DISubprogram(name: "ham", scope: !6, file: !6, line: 10, type: !11, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!11 = distinct !DISubroutineType(types: !12)
+!12 = !{null}
+;.
+; CHECK: [[META0:![0-9]+]] = distinct !DICompileUnit(language: DW_LANG_C11, file: [[META1:![0-9]+]], producer: "{{.*}}clang version {{.*}}", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: [[META2:![0-9]+]], retainedTypes: [[META2]], globals: [[META2]], splitDebugInlining: false, nameTableKind: None)
+; CHECK: [[META1]] = !DIFile(filename: "debugloc-factoring-add.c", directory: {{.*}})
+; CHECK: [[META2]] = !{}
+; CHECK: [[DBG4]] = distinct !DISubprogram(name: "ham", scope: [[META1]], file: [[META1]], line: 10, type: [[META5:![0-9]+]], flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: [[META0]], retainedNodes: [[META2]])
+; CHECK: [[META5]] = distinct !DISubroutineType(types: [[META6:![0-9]+]])
+; CHECK: [[META6]] = !{null}
+; CHECK: [[DBG7]] = !DILocation(line: 10, column: 1, scope: [[DBG4]])
+;.