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

[via llvm-commits]

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@@ -2440,12 +2440,20 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
   };
 
   TailFoldingStyle Style = Cost->getTailFoldingStyle();
-  if (Style == TailFoldingStyle::None)
-    CheckMinIters =
-        Builder.CreateICmp(P, Count, CreateStep(), "min.iters.check");
-  else if (VF.isScalable() &&
-           !isIndvarOverflowCheckKnownFalse(Cost, VF, UF) &&
-           Style != TailFoldingStyle::DataAndControlFlowWithoutRuntimeCheck) {
+  if (Style == TailFoldingStyle::None) {
+    Value *Step = CreateStep();
+    ScalarEvolution &SE = *PSE.getSE();
+    // Check if we can prove that the trip count is >= the step.
+    const SCEV *TripCountSCEV = SE.getSCEV(Count);
+    if (SE.isKnownPredicate(CmpInst::getInversePredicate(P),
+                            SE.applyLoopGuards(TripCountSCEV, OrigLoop),
+                            SE.getSCEV(Step)))
+      CheckMinIters = Builder.getFalse();
+    else
+      CheckMinIters = Builder.CreateICmp(P, Count, Step, "min.iters.check");
+  } else if (VF.isScalable() &&
+             !isIndvarOverflowCheckKnownFalse(Cost, VF, UF) &&
+             Style != TailFoldingStyle::DataAndControlFlowWithoutRuntimeCheck) {
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ayalz wrote:

The runtime comparison introduced below checks for overflow, in case the overflow check is not known (to be false) at compile time. Perhaps worth asserting that this predicate is indeed unknown to SCEV.

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