[llvm] [RISCV][TTI] Scale the cost of ICmp with LMUL (PR #88235)

[Philip Reames via llvm-commits]

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@@ -1375,14 +1375,15 @@ InstructionCost RISCVTTIImpl::getCmpSelInstrCost(unsigned Opcode, Type *ValTy,
                           LT.second, CostKind);
   }
 
-  if ((Opcode == Instruction::ICmp || Opcode == Instruction::FCmp) &&
-      ValTy->isVectorTy()) {
-    std::pair<InstructionCost, MVT> LT = getTypeLegalizationCost(ValTy);
-
-    // Support natively.
-    if (CmpInst::isIntPredicate(VecPred))
-      return LT.first * 1;
+  if ((Opcode == Instruction::ICmp) && ValTy->isVectorTy() &&
+      CmpInst::isIntPredicate(VecPred)) {
+    // Use VMSLT_VV to represent VMSEQ, VMSNE, VMSLTU, VMSLEU, VMSLT, VMSLE
+    // provided they incur the same cost across all implementations
+    return LT.first *
+           getRISCVInstructionCost(RISCV::VMSLT_VV, LT.second, CostKind);
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preames wrote:

We can split these apart if needed, starting with the assumption they're all the same cost seems entirely reasonable to me and simplifies the code significantly. 

https://github.com/llvm/llvm-project/pull/88235