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

[Mel Chen via llvm-commits]

================
@@ -3341,3 +3363,34 @@ void VPlanTransforms::addBranchWeightToMiddleTerminator(
       MDB.createBranchWeights({1, VectorStep - 1}, /*IsExpected=*/false);
   MiddleTerm->addMetadata(LLVMContext::MD_prof, BranchWeights);
 }
+
+void VPlanTransforms::adjustRecipesForReverseAccesses(VPlan &Plan) {
----------------
Mel-Chen wrote:

Reverse access can indeed be pattern-matched.
1392a872f0b69310340088d6323f1ae3735838c6
I’ve separated out the reverse mask, but this is not easy as we thought. :(
In addition to affecting the cost model since the cost of the reverse mask is currently not computed. In EVL lowering, it also require extra handling for the reverse mask.

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