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

[Shih-Po Hung 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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arcbbb wrote:

Although I prefer using VF explicitly, I don't see a correctness issue with the first approach in the current state.

First approach: Reverse does not carry a VF operand.
  1. EVL tailfolding must replace the Reverse recipe with a widen intrinsic recipe, since no VF is available.
 
Second approach: Reverse carries a VF operand.
1. EVL tailfolding can either replace VF with EVL (ensuring  vp.reverse is generated) , or replace the Reverse recipe with a widen intrinsic recipe as in the first approach.

The two approaches seem similar to me.

Even though the function is named `optimizeMaskToEVL`, EVL transform for both WidenLoadRecipe and ReverseRecipe seems necessary for correctness.

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