[llvm] [DebugInfo][LoopUnroll] Preserve DebugLocs on optimized cond branches (PR #114225)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-debuginfo

Author: Stephen Tozer (SLTozer)

<details>
<summary>Changes</summary>

This patch fixes a simple error where as part of loop unrolling we optimize conditional loop-exiting branches into unconditional branches when we know that they will or won't exit the loop, but does not propagate the source location of the original branch to the new one.

---
Full diff: https://github.com/llvm/llvm-project/pull/114225.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/LoopUnroll.cpp (+2-1) 
- (added) llvm/test/Transforms/LoopUnroll/preserve-branch-debuglocs.ll (+52) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index b90addcef69e6e..4cbbac07915fbe 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -883,7 +883,8 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
     DeadSucc->removePredecessor(Src, /* KeepOneInputPHIs */ true);
 
     // Replace the conditional branch with an unconditional one.
-    BranchInst::Create(Dest, Term->getIterator());
+    auto *BI = BranchInst::Create(Dest, Term->getIterator());
+    BI->setDebugLoc(Term->getDebugLoc());
     Term->eraseFromParent();
 
     DTUpdates.emplace_back(DominatorTree::Delete, Src, DeadSucc);
diff --git a/llvm/test/Transforms/LoopUnroll/preserve-branch-debuglocs.ll b/llvm/test/Transforms/LoopUnroll/preserve-branch-debuglocs.ll
new file mode 100644
index 00000000000000..623fb68c784fe7
--- /dev/null
+++ b/llvm/test/Transforms/LoopUnroll/preserve-branch-debuglocs.ll
@@ -0,0 +1,52 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals all --version 5
+; RUN: opt < %s -passes=loop-unroll -S | FileCheck %s
+; RUN: opt < %s -passes=loop-unroll-full -S | FileCheck %s
+
+;; Loop Unrolling should preserve the DebugLocs of conditional branches that get
+;; optimized into unconditional branches.
+
+define i32 @_ZeqRK4int3S1_() {
+; CHECK-LABEL: define i32 @_ZeqRK4int3S1_() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
+; CHECK:       [[FOR_COND:.*]]:
+; CHECK-NEXT:    br label %[[CLEANUP:.*]], !dbg [[DBG3:![0-9]+]]
+; CHECK:       [[FOR_BODY]]:
+; CHECK-NEXT:    br i1 false, label %[[FOR_COND]], label %[[CLEANUP]]
+; CHECK:       [[CLEANUP]]:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  br label %for.body
+
+for.cond:                                         ; preds = %for.body
+  br i1 false, label %cleanup, label %for.body, !dbg !3
+
+for.body:                                         ; preds = %for.cond, %entry
+  br i1 false, label %for.cond, label %cleanup
+
+cleanup:                                          ; preds = %for.body, %for.cond
+  ret i32 0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 20.0.0git", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug)
+!1 = !DIFile(filename: "test.cpp", directory: "/tmp")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = !DILocation(line: 304, column: 2, scope: !4)
+!4 = distinct !DILexicalBlock(scope: !5, file: !1, line: 304, column: 2)
+!5 = distinct !DISubprogram(name: "operator==", linkageName: "_ZeqRK4int3S1_", scope: !1, file: !1, line: 302, type: !6, scopeLine: 303, spFlags: DISPFlagDefinition, unit: !0)
+!6 = distinct !DISubroutineType(types: !7)
+!7 = !{}
+;.
+; CHECK: [[META0:![0-9]+]] = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: [[META1:![0-9]+]], producer: "{{.*}}clang version {{.*}}", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug)
+; CHECK: [[META1]] = !DIFile(filename: "test.cpp", directory: {{.*}})
+; CHECK: [[META2:![0-9]+]] = !{i32 2, !"Debug Info Version", i32 3}
+; CHECK: [[DBG3]] = !DILocation(line: 304, column: 2, scope: [[META4:![0-9]+]])
+; CHECK: [[META4]] = distinct !DILexicalBlock(scope: [[META5:![0-9]+]], file: [[META1]], line: 304, column: 2)
+; CHECK: [[META5]] = distinct !DISubprogram(name: "operator==", linkageName: "_ZeqRK4int3S1_", scope: [[META1]], file: [[META1]], line: 302, type: [[META6:![0-9]+]], scopeLine: 303, spFlags: DISPFlagDefinition, unit: [[META0]])
+; CHECK: [[META6]] = distinct !DISubroutineType(types: [[META7:![0-9]+]])
+; CHECK: [[META7]] = !{}
+;.

``````````

</details>


https://github.com/llvm/llvm-project/pull/114225