[llvm] [LoopVectorize] Add cost of generating tail-folding mask to the loop (PR #130565)

[Florian Hahn via llvm-commits]

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@@ -5610,6 +5610,31 @@ InstructionCost LoopVectorizationCostModel::expectedCost(ElementCount VF) {
     Cost += BlockCost;
   }
 
+#ifndef NDEBUG
+  // TODO: We're effectively having to duplicate the code from
+  // VPInstruction::computeCost, which is ugly. This isn't meant to be a fully
+  // accurate representation of the cost of tail-folding - it exists purely to
+  // stop asserts firing when the legacy cost doesn't match the VPlan cost.
+  if (!VF.isScalar() && foldTailByMasking()) {
+    TailFoldingStyle Style = getTailFoldingStyle();
+    LLVMContext &Context = TheLoop->getHeader()->getContext();
+    Type *I1Ty = IntegerType::getInt1Ty(Context);
+    Type *IndTy = Legal->getWidestInductionType();
+    TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput;
+    if (Style == TailFoldingStyle::DataWithEVL) {
+      Type *I32Ty = IntegerType::getInt32Ty(Context);
+      IntrinsicCostAttributes Attrs(Intrinsic::experimental_get_vector_length,
+                                    I32Ty, {IndTy, I32Ty, I1Ty});
+      Cost += TTI.getIntrinsicInstrCost(Attrs, CostKind);
+    } else if (useActiveLaneMask(Style)) {
+      VectorType *RetTy = VectorType::get(I1Ty, VF);
+      IntrinsicCostAttributes Attrs(Intrinsic::get_active_lane_mask, RetTy,
+                                    {IndTy, IndTy});
+      Cost += TTI.getIntrinsicInstrCost(Attrs, CostKind);
----------------
fhahn wrote:

I am wondering if it would be cleaner to handle this in the caller of `expectedCost`, where we have the VPlans available. With that, we could just iterate over all recipes in the loop region and compute the costs for ActiveLaneMask/EVL using the VPlan-based cost model and add them to the cost returned by `expectedCost`?

This might be more scalable for future use-cases.

https://github.com/llvm/llvm-project/pull/130565