[llvm] [VPlan] Fix convertToPhisToBlends folding non poison blend to poison (PR #180686)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-vectorizers

Author: Luke Lau (lukel97)

<details>
<summary>Changes</summary>

This fixes a miscompile in #<!-- -->180005 where we didn't check that the first incoming value isn't poison.

We should use the first non-poison incoming value if it exists, or just poison if all the incoming values are poison.

---
Full diff: https://github.com/llvm/llvm-project/pull/180686.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Vectorize/VPlanPredicator.cpp (+6-4) 
- (modified) llvm/test/Transforms/LoopVectorize/uniform-blend.ll (+85) 


``````````diff
diff --git a/llvm/lib/Transforms/Vectorize/VPlanPredicator.cpp b/llvm/lib/Transforms/Vectorize/VPlanPredicator.cpp
index 216efaf4b2fe9..dbc2e71c785ee 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanPredicator.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanPredicator.cpp
@@ -241,10 +241,12 @@ void VPPredicator::convertPhisToBlends(VPBasicBlock *VPBB) {
     // be duplications since this is a simple recursive scan, but future
     // optimizations will clean it up.
 
-    if (all_equal(make_filter_range(PhiR->incoming_values(), [](VPValue *V) {
-          return !match(V, m_Poison());
-        }))) {
-      PhiR->replaceAllUsesWith(PhiR->getIncomingValue(0));
+    auto NotPoison = make_filter_range(PhiR->incoming_values(), [](VPValue *V) {
+      return !match(V, m_Poison());
+    });
+    if (all_equal(NotPoison)) {
+      PhiR->replaceAllUsesWith(NotPoison.empty() ? PhiR->getIncomingValue(0)
+                                                 : *NotPoison.begin());
       PhiR->eraseFromParent();
       continue;
     }
diff --git a/llvm/test/Transforms/LoopVectorize/uniform-blend.ll b/llvm/test/Transforms/LoopVectorize/uniform-blend.ll
index 38bdd782257a5..126846456bb10 100644
--- a/llvm/test/Transforms/LoopVectorize/uniform-blend.ll
+++ b/llvm/test/Transforms/LoopVectorize/uniform-blend.ll
@@ -291,6 +291,89 @@ exit:
   ret void
 }
 
+define void @blend_poison_first_argument(ptr %p, i1 %c) {
+; CHECK-LABEL: define void @blend_poison_first_argument(
+; CHECK-SAME: ptr [[P:%.*]], i1 [[C:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br 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]] ]
+; CHECK-NEXT:    [[VEC_IND:%.*]] = phi <4 x i64> [ <i64 0, i64 1, i64 2, i64 3>, %[[VECTOR_PH]] ], [ [[VEC_IND_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i64, ptr [[P]], i64 [[INDEX]]
+; CHECK-NEXT:    store <4 x i64> [[VEC_IND]], ptr [[TMP0]], align 4
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
+; CHECK-NEXT:    [[VEC_IND_NEXT]] = add nuw nsw <4 x i64> [[VEC_IND]], splat (i64 4)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i64 [[INDEX_NEXT]], 32
+; CHECK-NEXT:    br i1 [[TMP1]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
+; CHECK:       [[MIDDLE_BLOCK]]:
+; CHECK-NEXT:    br label %[[EXIT:.*]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %latch ]
+  br i1 %c, label %if, label %latch
+
+if:
+  br label %latch
+
+latch:
+  %blend = phi i64 [ %iv, %loop ], [ poison, %if ]
+  %gep = getelementptr i64, ptr %p, i64 %iv
+  store i64 %blend, ptr %gep
+  %iv.next = add nuw nsw i64 %iv, 1
+  %ec = icmp ult i64 %iv, 31
+  br i1 %ec, label %loop, label %exit
+
+exit:
+  ret void
+}
+
+define void @blend_all_poison(ptr %p, i1 %c) {
+; CHECK-LABEL: define void @blend_all_poison(
+; CHECK-SAME: ptr [[P:%.*]], i1 [[C:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br 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]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i64, ptr [[P]], i64 [[INDEX]]
+; CHECK-NEXT:    store <4 x i64> poison, ptr [[TMP0]], align 4
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i64 [[INDEX_NEXT]], 32
+; CHECK-NEXT:    br i1 [[TMP1]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP7:![0-9]+]]
+; CHECK:       [[MIDDLE_BLOCK]]:
+; CHECK-NEXT:    br label %[[EXIT:.*]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %latch ]
+  br i1 %c, label %if, label %latch
+
+if:
+  br label %latch
+
+latch:
+  %blend = phi i64 [ poison, %loop ], [ poison, %if ]
+  %gep = getelementptr i64, ptr %p, i64 %iv
+  store i64 %blend, ptr %gep
+  %iv.next = add nuw nsw i64 %iv, 1
+  %ec = icmp ult i64 %iv, 31
+  br i1 %ec, label %loop, label %exit
+
+exit:
+  ret void
+}
 
 ;.
 ; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
@@ -299,4 +382,6 @@ exit:
 ; CHECK: [[LOOP3]] = distinct !{[[LOOP3]], [[META1]], [[META2]]}
 ; CHECK: [[LOOP4]] = distinct !{[[LOOP4]], [[META1]], [[META2]]}
 ; CHECK: [[LOOP5]] = distinct !{[[LOOP5]], [[META1]], [[META2]]}
+; CHECK: [[LOOP6]] = distinct !{[[LOOP6]], [[META1]], [[META2]]}
+; CHECK: [[LOOP7]] = distinct !{[[LOOP7]], [[META1]], [[META2]]}
 ;.

``````````

</details>


https://github.com/llvm/llvm-project/pull/180686