[llvm] LoopVectorize: guard appending InstsToScalarize; fix bug (PR #88720)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Ramkumar Ramachandra (artagnon)

<details>
<summary>Changes</summary>

In the process of collecting instructions to scalarize, LoopVectorize uses faulty reasoning whereby it also adds instructions that will be scalar after vectorization. If an instruction satisfies isScalarAfterVectorization() for the given VF, it should not be appended to InstsToScalarize. Add this extra guard, fixing a crash.

Fixes #<!-- -->55096.

---
Full diff: https://github.com/llvm/llvm-project/pull/88720.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Vectorize/LoopVectorize.cpp (+2-1) 
- (modified) llvm/test/Transforms/LoopVectorize/X86/pr55096-scalarize-add.ll (+48-8) 


``````````diff
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 5535cc55e93216..edc7b89fd96f1b 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5815,7 +5815,8 @@ void LoopVectorizationCostModel::collectInstsToScalarize(ElementCount VF) {
     if (!blockNeedsPredicationForAnyReason(BB))
       continue;
     for (Instruction &I : *BB)
-      if (isScalarWithPredication(&I, VF)) {
+      if (isScalarWithPredication(&I, VF) &&
+          !isScalarAfterVectorization(&I, VF)) {
         ScalarCostsTy ScalarCosts;
         // Do not apply discount if scalable, because that would lead to
         // invalid scalarization costs.
diff --git a/llvm/test/Transforms/LoopVectorize/X86/pr55096-scalarize-add.ll b/llvm/test/Transforms/LoopVectorize/X86/pr55096-scalarize-add.ll
index d75ab660461ecc..cc820f2a8f2a05 100644
--- a/llvm/test/Transforms/LoopVectorize/X86/pr55096-scalarize-add.ll
+++ b/llvm/test/Transforms/LoopVectorize/X86/pr55096-scalarize-add.ll
@@ -1,13 +1,48 @@
-; RUN: opt -passes=loop-vectorize -force-vector-width=2 -force-vector-interleave=1 -S %s | FileCheck %s
-
-; REQUIRES: asserts
-; XFAIL: *
-
-target triple = "x86_64-apple-macosx"
-
-; CHECK: vector.body
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -mtriple=x86_64-apple-macosx -passes=loop-vectorize,simplifycfg,dce -force-vector-width=2 -force-vector-interleave=1 -S %s | FileCheck %s
 
 define void @test_pr55096(i64 %c, ptr %p) {
+; CHECK-LABEL: define void @test_pr55096(
+; CHECK-SAME: i64 [[C:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <2 x i64> poison, i64 [[C]], i64 0
+; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <2 x i64> [[BROADCAST_SPLATINSERT]], <2 x i64> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
+; CHECK:       vector.body:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH:%.*]] ], [ [[INDEX_NEXT:%.*]], [[PRED_STORE_CONTINUE3:%.*]] ]
+; CHECK-NEXT:    [[VEC_IND:%.*]] = phi <2 x i64> [ <i64 122, i64 123>, [[VECTOR_PH]] ], [ [[VEC_IND_NEXT:%.*]], [[PRED_STORE_CONTINUE3]] ]
+; CHECK-NEXT:    [[DOTCAST:%.*]] = trunc i64 [[INDEX]] to i16
+; CHECK-NEXT:    [[TMP0:%.*]] = mul i16 [[DOTCAST]], 2008
+; CHECK-NEXT:    [[OFFSET_IDX:%.*]] = add i16 6229, [[TMP0]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <2 x i64> [[VEC_IND]], [[BROADCAST_SPLAT]]
+; CHECK-NEXT:    [[TMP2:%.*]] = xor <2 x i1> [[TMP1]], <i1 true, i1 true>
+; CHECK-NEXT:    [[TMP3:%.*]] = extractelement <2 x i1> [[TMP2]], i32 0
+; CHECK-NEXT:    br i1 [[TMP3]], label [[PRED_STORE_IF:%.*]], label [[PRED_STORE_CONTINUE:%.*]]
+; CHECK:       pred.store.if:
+; CHECK-NEXT:    [[TMP4:%.*]] = add i16 [[OFFSET_IDX]], 0
+; CHECK-NEXT:    [[TMP5:%.*]] = add i16 [[TMP4]], 2008
+; CHECK-NEXT:    [[TMP6:%.*]] = udiv i16 4943, [[TMP5]]
+; CHECK-NEXT:    [[TMP7:%.*]] = getelementptr inbounds i16, ptr [[P]], i16 [[TMP6]]
+; CHECK-NEXT:    store i16 0, ptr [[TMP7]], align 2
+; CHECK-NEXT:    br label [[PRED_STORE_CONTINUE]]
+; CHECK:       pred.store.continue:
+; CHECK-NEXT:    [[TMP9:%.*]] = extractelement <2 x i1> [[TMP2]], i32 1
+; CHECK-NEXT:    br i1 [[TMP9]], label [[PRED_STORE_IF2:%.*]], label [[PRED_STORE_CONTINUE3]]
+; CHECK:       pred.store.if2:
+; CHECK-NEXT:    [[TMP10:%.*]] = add i16 [[OFFSET_IDX]], 2008
+; CHECK-NEXT:    [[TMP11:%.*]] = add i16 [[TMP10]], 2008
+; CHECK-NEXT:    [[TMP12:%.*]] = udiv i16 4943, [[TMP11]]
+; CHECK-NEXT:    [[TMP13:%.*]] = getelementptr inbounds i16, ptr [[P]], i16 [[TMP12]]
+; CHECK-NEXT:    store i16 0, ptr [[TMP13]], align 2
+; CHECK-NEXT:    br label [[PRED_STORE_CONTINUE3]]
+; CHECK:       pred.store.continue3:
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
+; CHECK-NEXT:    [[VEC_IND_NEXT]] = add <2 x i64> [[VEC_IND]], <i64 2, i64 2>
+; CHECK-NEXT:    [[TMP15:%.*]] = icmp eq i64 [[INDEX_NEXT]], 340
+; CHECK-NEXT:    br i1 [[TMP15]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
 entry:
   br label %loop.header
 
@@ -32,3 +67,8 @@ loop.latch:
 exit:
   ret void
 }
+;.
+; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
+; CHECK: [[META1]] = !{!"llvm.loop.isvectorized", i32 1}
+; CHECK: [[META2]] = !{!"llvm.loop.unroll.runtime.disable"}
+;.

``````````

</details>


https://github.com/llvm/llvm-project/pull/88720