[DebugInfo][LoopVectorizer] Avoid dropping !dbg in optimizeForVFAndUF (#114243)

Prior to this patch, optimizeForVFAndUF may optimize the conditional
branch for a VPBasicblock to have a constant condition, but
unnecessarily drops the DILocation attachment when it does so; this
patch changes it to preserve the DILocation.

Author: SLTozer

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -691,9 +691,9 @@ void VPlanTransforms::optimizeForVFAndUF(VPlan &Plan, ElementCount BestVF,
     return;
 
   LLVMContext &Ctx = SE.getContext();
-  auto *BOC =
-      new VPInstruction(VPInstruction::BranchOnCond,
-                        {Plan.getOrAddLiveIn(ConstantInt::getTrue(Ctx))});
+  auto *BOC = new VPInstruction(
+      VPInstruction::BranchOnCond,
+      {Plan.getOrAddLiveIn(ConstantInt::getTrue(Ctx))}, Term->getDebugLoc());
 
   SmallVector<VPValue *> PossiblyDead(Term->operands());
   Term->eraseFromParent();

llvm/test/Transforms/LoopVectorize/debugloc-optimize-vfuf-term.ll

@@ -0,0 +1,79 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S < %s -passes=loop-vectorize -force-vector-width=2 | FileCheck %s
+;; Tests that when we simplify the loop-exit branch condition, we preserve the
+;; !dbg attachment for all loop-exit branches.
+
+define i32 @foo(ptr %p) {
+; CHECK-LABEL: define i32 @foo(
+; CHECK-SAME: ptr [[P:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
+; CHECK:       [[VECTOR_PH]]:
+; CHECK-NEXT:    br label %[[VECTOR_BODY:.*]]
+; CHECK:       [[VECTOR_BODY]]:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ], !dbg [[DBG3:![0-9]+]]
+; CHECK-NEXT:    store i8 0, ptr [[P]], align 1, !dbg [[DBG7:![0-9]+]]
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2, !dbg [[DBG3]]
+; CHECK-NEXT:    br i1 true, label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !dbg [[DBG3]], !llvm.loop [[LOOP8:![0-9]+]]
+; CHECK:       [[MIDDLE_BLOCK]]:
+; CHECK-NEXT:    br i1 true, label %[[EXIT:.*]], label %[[SCALAR_PH]], !dbg [[DBG11:![0-9]+]]
+; CHECK:       [[SCALAR_PH]]:
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 2, %[[MIDDLE_BLOCK]] ], [ 0, %[[ENTRY]] ], !dbg [[DBG3]]
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], %[[LOOP]] ], [ [[BC_RESUME_VAL]], %[[SCALAR_PH]] ], !dbg [[DBG3]]
+; CHECK-NEXT:    [[CONV:%.*]] = trunc i64 0 to i8, !dbg [[DBG12:![0-9]+]]
+; CHECK-NEXT:    store i8 [[CONV]], ptr [[P]], align 1, !dbg [[DBG7]]
+; CHECK-NEXT:    [[IV_NEXT]] = add i64 [[IV]], 1, !dbg [[DBG13:![0-9]+]]
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[IV]], 1, !dbg [[DBG14:![0-9]+]]
+; CHECK-NEXT:    br i1 [[EXITCOND]], label %[[EXIT]], label %[[LOOP]], !dbg [[DBG11]], !llvm.loop [[LOOP15:![0-9]+]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  br label %loop
+
+loop:                                          ; preds = %loop, %entry
+  %iv = phi i64 [ %iv.next, %loop ], [ 0, %entry ], !dbg !3
+  %conv = trunc i64 0 to i8, !dbg !4
+  store i8 %conv, ptr %p, align 1, !dbg !5
+  %iv.next = add i64 %iv, 1, !dbg !6
+  %exitcond = icmp eq i64 %iv, 1, !dbg !7
+  br i1 %exitcond, label %exit, label %loop, !dbg !8
+
+exit:                              ; preds = %loop
+  ret i32 0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 20.0.0git")
+!1 = !DIFile(filename: "test.cpp", directory: "/")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = !DILocation(line: 4, scope: !9)
+!4 = !DILocation(line: 5, scope: !9)
+!5 = !DILocation(line: 6, scope: !9)
+!6 = !DILocation(line: 7, scope: !9)
+!7 = !DILocation(line: 8, scope: !9)
+!8 = !DILocation(line: 9, scope: !9)
+!9 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 11, type: !10, unit: !0, retainedNodes: !11)
+!10 = distinct !DISubroutineType(types: !11)
+!11 = !{}
+;.
+; CHECK: [[META0:![0-9]+]] = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: [[META1:![0-9]+]], producer: "{{.*}}clang version {{.*}}", isOptimized: false, runtimeVersion: 0, emissionKind: NoDebug)
+; CHECK: [[META1]] = !DIFile(filename: "test.cpp", directory: {{.*}})
+; CHECK: [[DBG3]] = !DILocation(line: 4, scope: [[META4:![0-9]+]])
+; CHECK: [[META4]] = distinct !DISubprogram(name: "foo", scope: [[META1]], file: [[META1]], line: 11, type: [[META5:![0-9]+]], spFlags: DISPFlagDefinition, unit: [[META0]], retainedNodes: [[META6:![0-9]+]])
+; CHECK: [[META5]] = distinct !DISubroutineType(types: [[META6]])
+; CHECK: [[META6]] = !{}
+; CHECK: [[DBG7]] = !DILocation(line: 6, scope: [[META4]])
+; CHECK: [[LOOP8]] = distinct !{[[LOOP8]], [[META9:![0-9]+]], [[META10:![0-9]+]]}
+; CHECK: [[META9]] = !{!"llvm.loop.isvectorized", i32 1}
+; CHECK: [[META10]] = !{!"llvm.loop.unroll.runtime.disable"}
+; CHECK: [[DBG11]] = !DILocation(line: 9, scope: [[META4]])
+; CHECK: [[DBG12]] = !DILocation(line: 5, scope: [[META4]])
+; CHECK: [[DBG13]] = !DILocation(line: 7, scope: [[META4]])
+; CHECK: [[DBG14]] = !DILocation(line: 8, scope: [[META4]])
+; CHECK: [[LOOP15]] = distinct !{[[LOOP15]], [[META10]], [[META9]]}
+;.