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

[Shih-Po Hung via llvm-commits]

================
@@ -531,9 +529,12 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
     if (LT.second.isFixedLengthVector())
       // vrsub.vi has a 5 bit immediate field, otherwise an li suffices
       LenCost = isInt<5>(LT.second.getVectorNumElements() - 1) ? 0 : 1;
-    // FIXME: replace the constant `2` below with cost of {VID_V,VRSUB_VX}
+    unsigned Opcodes[] = {RISCV::VID_V, RISCV::VRSUB_VX, RISCV::VRGATHER_VV};
+    if (LT.second.isFixedLengthVector() &&
+        isInt<5>(LT.second.getVectorNumElements() - 1))
+      Opcodes[1] = RISCV::VRSUB_VI;
----------------
arcbbb wrote:

Yes, the cost for both is the same.

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