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

[Shih-Po Hung via llvm-commits]

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@@ -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};
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arcbbb wrote:

Maybe I overlooked something, here are the three cost sequences I am considering:
1. scalable vector
   LenCost:3 from { csrr a0, vlenb, srli a0, a0, 3, addi a0, a0, -1 }
   GatherCost from {vid.v, vrsub.vx, vrgather.vv}
2. fixed-length vector, imm > 5bit
    LenCost:1 from { li }
    GatherCost from {vid.v, vrsub.vx, vrgather.vv}
3. fixed-length vector, imm <= 5 bit
    LenCost:0 from { }
    GatherCost from {vid.v, vrsub.vi, vrgather.vv}

Could you help me correct this?

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