[RISCV][CostModel] Remove inst cost of cmp inst in cmp-select sequence.

Cmp and select instructinos will lower to SELECT_CC and lower to PseudoCCMOVGPR
which will generate a conditional branch inst and a move inst.
The cost of (cmp + select) = (0 + select inst cost).

Author: ElvisWang123

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -1436,6 +1436,13 @@ InstructionCost RISCVTTIImpl::getCmpSelInstrCost(unsigned Opcode, Type *ValTy,
     }
   }
 
+  // The cmp + select instructions will lower to SELECT_CC and lower to
+  // PseudoCCMOVGPR which will generate a conditional branch + mv. The
+  // cost of (cmp + select) will be (0 + select inst cost).
+  if (I && isa<CmpInst>(I) && I->hasOneUser() &&
+      isa<SelectInst>(I->user_back()) && I->user_back()->getOperand(0) == I)
+    return 0;
+
   // TODO: Add cost for scalar type.
 
   return BaseT::getCmpSelInstrCost(Opcode, ValTy, CondTy, VecPred, CostKind, I);

llvm/test/Analysis/CostModel/RISCV/cmp-select.ll

@@ -2,8 +2,9 @@
 ; RUN: opt < %s -mtriple=riscv64  -passes="print<cost-model>" -cost-kind=throughput 2>&1 -disable-output | FileCheck %s --check-prefixes=CHECK
 
 define void @cmp-select() {
+;
 ; CHECK-LABEL: 'cmp-select'
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %cmp1 = icmp slt i64 0, 1
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %cmp1 = icmp slt i64 0, 1
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %select1 = select i1 %cmp1, i32 5, i32 4
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: ret void
 ;

llvm/test/Transforms/LoopVectorize/RISCV/strided-accesses.ll

@@ -699,7 +699,7 @@ define void @double_stride_ptr_iv(ptr %p, ptr %p2, i64 %stride) {
 ; STRIDED-NEXT:  entry:
 ; STRIDED-NEXT:    [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
 ; STRIDED-NEXT:    [[TMP1:%.*]] = mul i64 [[TMP0]], 4
-; STRIDED-NEXT:    [[TMP2:%.*]] = call i64 @llvm.umax.i64(i64 32, i64 [[TMP1]])
+; STRIDED-NEXT:    [[TMP2:%.*]] = call i64 @llvm.umax.i64(i64 24, i64 [[TMP1]])
 ; STRIDED-NEXT:    [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1024, [[TMP2]]
 ; STRIDED-NEXT:    br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_MEMCHECK:%.*]]
 ; STRIDED:       vector.memcheck: