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

[Florian Hahn via llvm-commits]

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@@ -2438,12 +2438,29 @@ 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();
+    // TODO: Emit unconditional branch to vector preheader instead of
+    // conditional branch with known condition.
+    const SCEV *TripCountSCEV = SE.applyLoopGuards(SE.getSCEV(Count), OrigLoop);
+    // Check if the trip count is < the step.
+    if (SE.isKnownPredicate(P, TripCountSCEV, SE.getSCEV(Step))) {
+      // TODO: Ensure step is at most the trip count when determining max VF and
+      // UF, w/o tail folding.
+      CheckMinIters = Builder.getTrue();
+    } else if (!SE.isKnownPredicate(CmpInst::getInversePredicate(P),
+                                    TripCountSCEV, SE.getSCEV(Step))) {
+      // Generate the minimum iteration check only if we cannot prove the
+      // check is known to be true, or known to be false
+      CheckMinIters = Builder.CreateICmp(P, Count, Step, "min.iters.check");
+
+      // else step is known to be smaller than trip count, use CheckMinIters
+      // preset to false.
+    }
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fhahn wrote:

adjusted, thanks!

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