[llvm] [VPlan] Extract reverse operation for reverse accesses (PR #146525)

[Luke Lau via llvm-commits]

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@@ -917,6 +918,8 @@ Value *VPInstruction::generate(VPTransformState &State) {
 
     return Res;
   }
+  case VPInstruction::Reverse:
+    return Builder.CreateVectorReverse(State.get(getOperand(0)), "reverse");
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lukel97 wrote:

> VPInstruction::Reverse itself doesn't appear to rely on the runtime VF.

But the fact that we need to transform a VPInstruction::Reverse to a experimental.vp.reverse for it to be correct with EVL tail folding, to me feels that it actually does depend on the runtime VF. 

I.e. VPInstruction::Reverse should really always be a experimental.vp.reverse that we just happen to optimise to a plain `Builder.CreateVectorReverse` when the VF operand = Plan.getVF

https://github.com/llvm/llvm-project/pull/146525