[llvm] [LV] Use SCEV to check if IV overflow check is known (PR #115705)

[David Sherwood via llvm-commits]

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@@ -2491,17 +2491,19 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
     Value *LHS = Builder.CreateSub(MaxUIntTripCount, Count);
 
     Value *Step = CreateStep();
-#ifndef NDEBUG
     ScalarEvolution &SE = *PSE.getSE();
     const SCEV *TC2OverflowSCEV = SE.applyLoopGuards(SE.getSCEV(LHS), OrigLoop);
-    assert(
-        !isIndvarOverflowCheckKnownFalse(Cost, VF * UF) &&
-        !SE.isKnownPredicate(CmpInst::getInversePredicate(ICmpInst::ICMP_ULT),
-                             TC2OverflowSCEV, SE.getSCEV(Step)) &&
-        "unexpectedly proved overflow check to be known");
-#endif
-    // Don't execute the vector loop if (UMax - n) < (VF * UF).
-    CheckMinIters = Builder.CreateICmp(ICmpInst::ICMP_ULT, LHS, Step);
+    const SCEV *StepSCEV = SE.getSCEV(Step);
+
+    // Check if (UMax - n) < (VF * UF).
+    if (SE.isKnownPredicate(ICmpInst::ICMP_ULT, TC2OverflowSCEV, StepSCEV)) {
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david-arm wrote:

The patch to add the NDEBUG code landed only recently - https://github.com/llvm/llvm-project/pull/111310 by @fhahn. It looks like this assert was added for a reason - is it possible that `isIndvarOverflowCheckKnownFalse` is returning the incorrect answer for the EVL case?

https://github.com/llvm/llvm-project/pull/115705