[llvm] [LV] Support strided memory accesses with a stride of -1 (PR #128718)

[Mel Chen via llvm-commits]

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@@ -2126,8 +2126,13 @@ void VPVectorPointerRecipe::execute(VPTransformState &State) {
   Value *Ptr = State.get(getOperand(0), VPLane(0));
 
   Value *Increment = createStepForVF(Builder, IndexTy, State.VF, CurrentPart);
+  // TODO: Support non-unit-reverse strided accesses.
+  Value *Index =
+      Strided
+          ? Builder.CreateMul(Increment, ConstantInt::getSigned(IndexTy, -1))
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Mel-Chen wrote:

> If we want to support it for EVL tail folding further down the line would we also need to add an EVL + Strided load recipe?

@lukel97 I think we can pass VF into the new recipe, and replace VF with EVL, like VPVectorEndPointerRecipe did.

https://github.com/llvm/llvm-project/pull/128718