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

[Luke Lau via llvm-commits]

================
@@ -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)) {
----------------
lukel97 wrote:

Yeah, this is the review comment that led to the assert being added: https://github.com/llvm/llvm-project/pull/111310#discussion_r1795113769 cc @ayalz 

I'm not sure if that assert always holds though, but I could be missing something. `isIndvarOverflowCheckKnownFalse` doesn't have the information from SCEVs nor the loop guards, which seems to be where the discrepancy between it and the assert lies. We could maybe instead move the SCEV stuff into it?

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