[LV] Mark checks as never succeeding for high cost cutoff.

When GeneratedRTChecks::create bails out due to exceeding the cost
threshold, no runtime checks are generated and we must not proceed
assuming checks have been generated.

Mark the checks as never succeeding, to make sure we don't try to
vectorize assuming the runtime checks hold. This fixes a case where we
previously incorrectly vectorized assuming runtime checks had been
generated when forcing vectorization via metadate.

Fixes the mis-compile mentioned in
#166247 (comment)

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1827,8 +1827,12 @@ class GeneratedRTChecks {
     // profile info.
     CostTooHigh =
         LAI.getNumRuntimePointerChecks() > VectorizeMemoryCheckThreshold;
-    if (CostTooHigh)
+    if (CostTooHigh) {
+      // Mark runtime checks as never succeeding when they exceed the threshold.
+      MemRuntimeCheckCond = ConstantInt::getTrue(L->getHeader()->getContext());
+      SCEVCheckCond = ConstantInt::getTrue(L->getHeader()->getContext());
       return;
+    }
 
     BasicBlock *LoopHeader = L->getHeader();
     BasicBlock *Preheader = L->getLoopPreheader();

llvm/test/Transforms/LoopVectorize/runtime-check-threshold-with-force-metadata.ll

@@ -2,29 +2,23 @@
 ; RUN: opt -p loop-vectorize -vectorize-memory-check-threshold=0 -S %s | FileCheck --check-prefix=LIMIT0 %s
 ; RUN: opt -p loop-vectorize -vectorize-memory-check-threshold=1 -S %s | FileCheck --check-prefix=LIMIT1 %s
 
-; FIXME: Currently this miscompiles with -vectorize-memory-check-threshold=0;
-; no runtime check is generated even though one is needed and !noalias
-; annotations are added.
+; Make sure we do not incorrectly vectorize with -vectorize-memory-check-threshold=0;
+; no runtime check is generated and the loop should not be vectorized.
 define i16 @runtime_checks_needed(ptr %src, ptr %dst) {
 ; LIMIT0-LABEL: define i16 @runtime_checks_needed(
 ; LIMIT0-SAME: ptr [[SRC:%.*]], ptr [[DST:%.*]]) {
-; LIMIT0-NEXT:  [[ENTRY:.*:]]
-; LIMIT0-NEXT:    br label %[[VECTOR_PH:.*]]
-; LIMIT0:       [[VECTOR_PH]]:
-; LIMIT0-NEXT:    [[TMP0:%.*]] = load i16, ptr [[SRC]], align 1, !alias.scope [[META0:![0-9]+]]
-; LIMIT0-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <2 x i16> poison, i16 [[TMP0]], i64 0
-; LIMIT0-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <2 x i16> [[BROADCAST_SPLATINSERT]], <2 x i16> poison, <2 x i32> zeroinitializer
-; LIMIT0-NEXT:    br label %[[VECTOR_BODY:.*]]
-; LIMIT0:       [[VECTOR_BODY]]:
-; LIMIT0-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; LIMIT0-NEXT:  [[ENTRY:.*]]:
+; LIMIT0-NEXT:    br label %[[LOOP:.*]]
+; LIMIT0:       [[LOOP]]:
+; LIMIT0-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[INDEX_NEXT:%.*]], %[[LOOP]] ]
+; LIMIT0-NEXT:    [[L:%.*]] = load i16, ptr [[SRC]], align 1
 ; LIMIT0-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i16, ptr [[DST]], i64 [[INDEX]]
-; LIMIT0-NEXT:    store <2 x i16> [[BROADCAST_SPLAT]], ptr [[TMP1]], align 1, !alias.scope [[META3:![0-9]+]], !noalias [[META0]]
-; LIMIT0-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
+; LIMIT0-NEXT:    store i16 [[L]], ptr [[TMP1]], align 1
+; LIMIT0-NEXT:    [[INDEX_NEXT]] = add nuw nsw i64 [[INDEX]], 1
 ; LIMIT0-NEXT:    [[TMP2:%.*]] = icmp eq i64 [[INDEX_NEXT]], 1000
-; LIMIT0-NEXT:    br i1 [[TMP2]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
-; LIMIT0:       [[MIDDLE_BLOCK]]:
-; LIMIT0-NEXT:    br label %[[EXIT:.*]]
+; LIMIT0-NEXT:    br i1 [[TMP2]], label %[[EXIT:.*]], label %[[LOOP]], !llvm.loop [[LOOP0:![0-9]+]]
 ; LIMIT0:       [[EXIT]]:
+; LIMIT0-NEXT:    [[TMP0:%.*]] = phi i16 [ [[L]], %[[LOOP]] ]
 ; LIMIT0-NEXT:    ret i16 [[TMP0]]
 ;
 ; LIMIT1-LABEL: define i16 @runtime_checks_needed(
@@ -88,14 +82,9 @@ exit:
 !3 = !{!"llvm.loop.vectorize.enable", i1 true}
 
 ;.
-; LIMIT0: [[META0]] = !{[[META1:![0-9]+]]}
-; LIMIT0: [[META1]] = distinct !{[[META1]], [[META2:![0-9]+]]}
-; LIMIT0: [[META2]] = distinct !{[[META2]], !"LVerDomain"}
-; LIMIT0: [[META3]] = !{[[META4:![0-9]+]]}
-; LIMIT0: [[META4]] = distinct !{[[META4]], [[META2]]}
-; LIMIT0: [[LOOP5]] = distinct !{[[LOOP5]], [[META6:![0-9]+]], [[META7:![0-9]+]]}
-; LIMIT0: [[META6]] = !{!"llvm.loop.isvectorized", i32 1}
-; LIMIT0: [[META7]] = !{!"llvm.loop.unroll.runtime.disable"}
+; LIMIT0: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
+; LIMIT0: [[META1]] = !{!"llvm.loop.vectorize.width", i32 2}
+; LIMIT0: [[META2]] = !{!"llvm.loop.vectorize.enable", i1 true}
 ;.
 ; LIMIT1: [[META0]] = !{[[META1:![0-9]+]]}
 ; LIMIT1: [[META1]] = distinct !{[[META1]], [[META2:![0-9]+]]}