[llvm] [RISCV][CostModel] Updates reduction and shuffle cost (PR #77342)

[Shih-Po Hung via llvm-commits]

================
@@ -1392,20 +1426,58 @@ RISCVTTIImpl::getArithmeticReductionCost(unsigned Opcode, VectorType *Ty,
     return BaseT::getArithmeticReductionCost(Opcode, Ty, FMF, CostKind);
 
   std::pair<InstructionCost, MVT> LT = getTypeLegalizationCost(Ty);
-  if (Ty->getElementType()->isIntegerTy(1))
+  SmallVector<unsigned, 3> Opcodes;
+  Type *ElementTy = Ty->getElementType();
+  if (ElementTy->isIntegerTy(1)) {
     // vcpop sequences, see vreduction-mask.ll
-    return (LT.first - 1) + (ISD == ISD::AND ? 3 : 2);
+    if (ISD == ISD::AND) {
+      Opcodes = {RISCV::VMNAND_MM, RISCV::VCPOP_M};
----------------
arcbbb wrote:

You've made a good observation about the capacity and size here.
For clarity, in the context of SmallVector the capacity of Opcodes is 3, and the current size is 2 here.
ArrayRef is constructed from the size of the SmallVector, so I think the extra capacity will not be used.
I was based on the logic in
```
    /// Construct an ArrayRef from a SmallVector. This is templated in order to
    /// avoid instantiating SmallVectorTemplateCommon<T> whenever we
    /// copy-construct an ArrayRef.
    template<typename U>
    /*implicit*/ ArrayRef(const SmallVectorTemplateCommon<T, U> &Vec)
      : Data(Vec.data()), Length(Vec.size()) {
    }
```

https://github.com/llvm/llvm-project/pull/77342