[llvm] [LV] Use SCEV to check if minimum iteration check is known. (PR #111310)

[Florian Hahn via llvm-commits]

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@@ -2454,8 +2471,16 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
         ConstantInt::get(CountTy, cast<IntegerType>(CountTy)->getMask());
     Value *LHS = Builder.CreateSub(MaxUIntTripCount, Count);
 
+    Value *Step = CreateStep();
+    ScalarEvolution &SE = *PSE.getSE();
+    // Check if we can prove that the trip count is >= the step.
+    const SCEV *TripCountSCEV = SE.applyLoopGuards(SE.getSCEV(LHS), OrigLoop);
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fhahn wrote:

It shouldn't have any side-effects (it may lead to a SCEV being cached for `LHS`, but that shouldn't impact anything unless there's a bug in SCEV invalidation).

Wrapped everything related in `#ifndef NDEBUG`, thanks!

https://github.com/llvm/llvm-project/pull/111310