[llvm] [LoopPeel] Remove known trip count restriction when peeling last. (PR #140792)

[Eli Friedman via llvm-commits]

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@@ -360,12 +357,17 @@ bool llvm::canPeelLastIteration(const Loop &L, ScalarEvolution &SE) {
 /// is known at the second-to-last.
 static bool shouldPeelLastIteration(Loop &L, CmpPredicate Pred,
                                     const SCEVAddRecExpr *LeftAR,
-                                    const SCEV *RightSCEV,
-                                    ScalarEvolution &SE) {
+                                    const SCEV *RightSCEV, ScalarEvolution &SE,
+                                    const TargetTransformInfo &TTI) {
   if (!canPeelLastIteration(L, SE))
     return false;
 
   const SCEV *BTC = SE.getBackedgeTakenCount(&L);
+  SCEVExpander Expander(SE, L.getHeader()->getDataLayout(), "loop-peel");
+  if (!SE.isKnownNonZero(BTC) && Expander.isHighCostExpansion(BTC, &L, SCEVCheapExpansionBudget, &TTI,
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efriedma-quic wrote:

Should this use getExitCount() instead of getBackedgeTakenCount()?  Peeling shouldn't care if there's an early exit from some other block.

https://github.com/llvm/llvm-project/pull/140792