[llvm] [LAA,Loads] Use loop guards and max BTC if needed when checking deref. (PR #155672)

[Florian Hahn via llvm-commits]

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@@ -263,23 +260,36 @@ evaluatePtrAddRecAtMaxBTCWillNotWrap(const SCEVAddRecExpr *AR,
       SE.getMinusSCEV(AR->getStart(), StartPtr), WiderTy);
 
   const SCEV *OffsetAtLastIter =
-      mulSCEVOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE, CtxI);
-  if (!OffsetAtLastIter)
-    return false;
+      mulSCEVOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE);
+  if (!OffsetAtLastIter) {
+    // Re-try with constant max backedge-taken count if using the symbolic one
+    // failed.
+    MaxBTC = SE.getNoopOrZeroExtend(
+        SE.getConstantMaxBackedgeTakenCount(AR->getLoop()), WiderTy);
+    OffsetAtLastIter =
+        mulSCEVOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE);
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fhahn wrote:

Unfortunately that's not enough in some cases, for example https://github.com/llvm/llvm-project/blob/main/llvm/test/Transforms/LoopVectorize/single_early_exit.ll#L333 where the trip count of the inner loop depends on an induction from the outer loop

https://github.com/llvm/llvm-project/pull/155672