[llvm] [RISCV][TTI] Add shuffle costing for masked slide lowering (PR #128537)

[Luke Lau via llvm-commits]

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@@ -474,6 +474,64 @@ costShuffleViaVRegSplitting(RISCVTTIImpl &TTI, MVT LegalVT,
   return InstructionCost::getInvalid();
 }
 
+InstructionCost RISCVTTIImpl::getSlideCost(FixedVectorType *Tp,
+                                           ArrayRef<int> Mask,
+                                           TTI::TargetCostKind CostKind) {
+  // Avoid missing masks
+  if (Mask.size() <= 2)
+    return InstructionCost::getInvalid();
+
+  int NumElts = Tp->getNumElements();
+  std::pair<InstructionCost, MVT> LT = getTypeLegalizationCost(Tp);
+  // Avoid scalarization cases
+  if (!LT.second.isFixedLengthVector())
+    return InstructionCost::getInvalid();
+
+  // Requires moving elements between parts, which requires additional
+  // unmodeled instructions.
+  if (LT.first != 1)
+    return InstructionCost::getInvalid();
+
+  auto getSlideOpcode = [&](int SlideAmt) {
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lukel97 wrote:

Nit, capital letter for variable names?

https://github.com/llvm/llvm-project/pull/128537