[llvm] [RISCV][CostModel] Estimate cost of Extract/InsertElement with non-constant index when vector instructions are not available (PR #67334)

[Philip Reames via llvm-commits]

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@@ -1456,6 +1456,24 @@ InstructionCost RISCVTTIImpl::getVectorInstrCost(unsigned Opcode, Type *Val,
       Opcode != Instruction::InsertElement)
     return BaseT::getVectorInstrCost(Opcode, Val, CostKind, Index, Op0, Op1);
 
+  // Extract/InsertElement with non-constant index is very costly without
+  // V instructions; estimate cost of loads/stores sequence via the stack:
+  // ExtractElement cost: store vector to stack, load scalar;
+  // InsertElement cost: store vector to stack, store scalar, load vector.
+  if (Index == -1U && !ST->hasVInstructions()) {
+    auto *VecTy = cast<VectorType>(Val);
+    auto *ElemTy = VecTy->getElementType();
+    auto NumElems = VecTy->getElementCount().getKnownMinValue();
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preames wrote:

This will produce a significant under-estimate on cost for scalable vectors as the number elements will be very low.  See getEstimatedVLFor.

https://github.com/llvm/llvm-project/pull/67334