[llvm] [RISCV][CostModel] Remove inst cost of cmp inst in cmp-select sequence. (PR #91158)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Elvis Wang (ElvisWang123)

<details>
<summary>Changes</summary>

Cmp and select instructinos will lower to SELECT_CC and lower to PseudoCCMOVGPR which will generate a conditional branch inst and a move inst. Remove the instruction cost of the cmp instruction because of the generated branch instruction has 0 cost in the cost model.

---
Full diff: https://github.com/llvm/llvm-project/pull/91158.diff


3 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp (+8) 
- (added) llvm/test/Analysis/CostModel/RISCV/cmp-select.ll (+14) 
- (modified) llvm/test/Transforms/LoopVectorize/RISCV/strided-accesses.ll (+1-1) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
index ce26e61880fd05..885697e48b59ad 100644
--- a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
@@ -1436,6 +1436,14 @@ 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. Remove
+  // the cost of the cmp instruction because the estimated cost of
+  // branch instruction is 0.
+  if (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);
diff --git a/llvm/test/Analysis/CostModel/RISCV/cmp-select.ll b/llvm/test/Analysis/CostModel/RISCV/cmp-select.ll
new file mode 100644
index 00000000000000..6c88467afe6351
--- /dev/null
+++ b/llvm/test/Analysis/CostModel/RISCV/cmp-select.ll
@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py
+; 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 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
+;
+  %cmp1 = icmp slt i64 0, 1
+  %select1 = select i1 %cmp1, i32 5, i32 4
+  ret void
+}
diff --git a/llvm/test/Transforms/LoopVectorize/RISCV/strided-accesses.ll b/llvm/test/Transforms/LoopVectorize/RISCV/strided-accesses.ll
index 12fdf2149daf47..af62aad84d6be3 100644
--- a/llvm/test/Transforms/LoopVectorize/RISCV/strided-accesses.ll
+++ b/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:

``````````

</details>


https://github.com/llvm/llvm-project/pull/91158