[llvm] [LV][TTI] Calculate cost of extracting last index in a scalable vector (PR #144086)

[David Green via llvm-commits]

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@@ -3711,12 +3711,24 @@ InstructionCost AArch64TTIImpl::getCFInstrCost(unsigned Opcode,
 }
 
 InstructionCost AArch64TTIImpl::getVectorInstrCostHelper(
-    unsigned Opcode, Type *Val, TTI::TargetCostKind CostKind, unsigned Index,
+    unsigned Opcode, Type *Val, TTI::TargetCostKind CostKind, int Index,
     bool HasRealUse, const Instruction *I, Value *Scalar,
     ArrayRef<std::tuple<Value *, User *, int>> ScalarUserAndIdx) const {
   assert(Val->isVectorTy() && "This must be a vector type");
 
-  if (Index != -1U) {
+  if (Index == TargetTransformInfo::LastIndex) {
+    if (isa<ScalableVectorType>(Val)) {
+      // This typically requires both while and lastb instructions in order
+      // to extract the last element. If this is in a loop the while
+      // instruction can at least be hoisted out, although it will consume a
+      // predicate register. The cost should be more expensive than the base
+      // extract cost, which is 2 for most CPUs.
+      return CostKind == TTI::TCK_CodeSize ? 2 : 3;
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davemgreen wrote:

Use the base vector instruction cost directly? `ST->getVectorInsertExtractBaseCost() + 1`

https://github.com/llvm/llvm-project/pull/144086