[llvm] [RISCV][CostModel] Estimate cost of llvm.vector.reduce.fmaximum/fminimum (PR #80697)

[Luke Lau via llvm-commits]

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@@ -1001,6 +1001,53 @@ RISCVTTIImpl::getMinMaxReductionCost(Intrinsic::ID IID, VectorType *Ty,
       return getArithmeticReductionCost(Instruction::And, Ty, FMF, CostKind);
   }
 
+  if (IID == Intrinsic::maximum || IID == Intrinsic::minimum) {
+    SmallVector<unsigned, 5> SplitOps;
+    SmallVector<unsigned, 3> Opcodes;
+    InstructionCost ExtraCost = 0;
+    switch (IID) {
+    case Intrinsic::maximum:
+      if (FMF.noNaNs()) {
+        SplitOps = {RISCV::VFMAX_VV};
+        Opcodes = {RISCV::VFREDMAX_VS, RISCV::VFMV_F_S};
+      } else {
+        SplitOps = {RISCV::VMFEQ_VV, RISCV::VMERGE_VVM, RISCV::VMFEQ_VV,
+                    RISCV::VMERGE_VVM, RISCV::VFMAX_VV};
+        Opcodes = {RISCV::VMFNE_VV, RISCV::VCPOP_M, RISCV::VFREDMAX_VS,
+                   RISCV::VFMV_F_S};
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lukel97 wrote:

Worth noting that we assume branches are predicted so `getCFInstrCost(Instruction::Br, CostKind)` will return 0 for throughput

https://github.com/llvm/llvm-project/pull/80697