[llvm] [VPlan] Extract reverse mask from reverse accesses (PR #155579)

[Luke Lau via llvm-commits]

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@@ -263,6 +263,18 @@ void VPRecipeBase::moveBefore(VPBasicBlock &BB,
 }
 
 InstructionCost VPRecipeBase::cost(ElementCount VF, VPCostContext &Ctx) {
+  // Skip the reverse operation cost for the mask.
+  // TODO: Remove this once redundant mask reverse operations can be eliminated
+  // by VPlanTransforms::cse before cost computation.
+  auto SkipReverseMask = [this]() {
+    if (!match(this,
+               m_CombineOr(m_Reverse(m_VPValue()),
+                           m_Intrinsic<Intrinsic::experimental_vp_reverse>())))
+      return false;
+    auto *Def = cast<VPSingleDefRecipe>(this);
+    auto *Mem = dyn_cast_or_null<VPWidenMemoryRecipe>(Def->getSingleUser());
+    return Mem && Mem->getMask() == Def;
+  };
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lukel97 wrote:

For the vp.reverse case, could we just have added a check for `if (Ty == i1 && ID == experimental_vp_reverse) return 0;` in VPWidenIntrinsicRecipe::computeCost? 

That check would only be temporary and would go away once we either remove vp.reverse, or start costing the mask in the legacy cost model. 

https://github.com/llvm/llvm-project/pull/155579