[llvm] [LoopVectorize] Add support for reverse loops in isDereferenceableAndAlignedInLoop (PR #96752)

[David Sherwood via llvm-commits]

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@@ -318,11 +321,24 @@ bool llvm::isDereferenceableAndAlignedInLoop(LoadInst *LI, Loop *L,
       // TODO: generalize if a case found which warrants
       if (Offset->getAPInt().urem(Alignment.value()) != 0)
         return false;
+      if (StepIsNegative) {
+        // In the last iteration of the loop the address we access we will be
+        // lower than the first by (TC - 1) * Step. So we need to make sure
+        // that there is enough room in Offset to accomodate this.
+        APInt SubOffset = (TC - 1) * AbsStep;
+        if (Offset->getAPInt().ult(SubOffset))
+          return false;
+        // We can safely use the new base because the decrementing pointer is
+        // always guaranteed to be >= new base. The total access size needs to
+        // take into account the start offset and the loaded element size.
+        AccessSize = Offset->getAPInt() + EltSize;
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david-arm wrote:

So I originally thought about splitting off the change in a separate patch, but I couldn't see any way of testing it by itself because the overflow checks catch this by accident. If you want I can split it off into a separate PR - there just won't be any tests for it.

https://github.com/llvm/llvm-project/pull/96752