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

[David Sherwood via llvm-commits]

================
@@ -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;
----------------
david-arm wrote:

> Would something like the below work? I might be missing something, but it looks like the overflow check should trigger there?

Yep, that's exactly the test I tried and the offset produced was a 64-bit APInt. I'll try again with your IR just in case you've written it differently to me.

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