[llvm] [AArch64] Add costs for ST3 and ST4 instructions, modelled as store(shuffle). (PR #87934)

[Simon Pilgrim via llvm-commits]

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@@ -3815,18 +3815,30 @@ InstructionCost AArch64TTIImpl::getSpliceCost(VectorType *Tp, int Index) {
   return LegalizationCost * LT.first;
 }
 
-InstructionCost AArch64TTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
-                                               VectorType *Tp,
-                                               ArrayRef<int> Mask,
-                                               TTI::TargetCostKind CostKind,
-                                               int Index, VectorType *SubTp,
-                                               ArrayRef<const Value *> Args) {
+InstructionCost AArch64TTIImpl::getShuffleCost(
+    TTI::ShuffleKind Kind, VectorType *Tp, ArrayRef<int> Mask,
+    TTI::TargetCostKind CostKind, int Index, VectorType *SubTp,
+    ArrayRef<const Value *> Args, const Instruction *CxtI) {
   std::pair<InstructionCost, MVT> LT = getTypeLegalizationCost(Tp);
+
   // If we have a Mask, and the LT is being legalized somehow, split the Mask
   // into smaller vectors and sum the cost of each shuffle.
   if (!Mask.empty() && isa<FixedVectorType>(Tp) && LT.second.isVector() &&
       Tp->getScalarSizeInBits() == LT.second.getScalarSizeInBits() &&
       Mask.size() > LT.second.getVectorNumElements() && !Index && !SubTp) {
+
+    // Check for ST3/ST4 instructions, which are represented in llvm IR as
+    // store(interleaving-shuffle). The shuffle cost could potentially be free,
+    // but we model it with a cost of LT.first so that LD3/LD3 have a higher
+    // cost than just the store.
+    if ((ShuffleVectorInst::isInterleaveMask(
+             Mask, 4, Tp->getElementCount().getKnownMinValue() * 2) ||
+         ShuffleVectorInst::isInterleaveMask(
+             Mask, 3, Tp->getElementCount().getKnownMinValue() * 2)) &&
+        !ShuffleVectorInst::isZeroEltSplatMask(
+            Mask, Tp->getElementCount().getKnownMinValue()))
+      return LT.first;
----------------
RKSimon wrote:

Where do you use CxtI to detect a store?

https://github.com/llvm/llvm-project/pull/87934