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[VPlan] Use predicate in VPInstruction::computeCost for selects. #170278

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Merged
fhahn merged 3 commits into from
Dec 3, 2025
Merged

[VPlan] Use predicate in VPInstruction::computeCost for selects. #170278

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f3f7679
[LV] Add test for cost of select VPInstruction.
fhahn Dec 2, 2025
d7fa21f
[VPlan] Use predicate in VPInstruction::computeCost for selects.
fhahn Dec 2, 2025
8049e0c
Merge branch 'main' into vpinst-use-select-predicate-compute-cost
fhahn Dec 3, 2025
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5 changes: 2 additions & 3 deletions llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1007,9 +1007,8 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,

switch (getOpcode()) {
case Instruction::Select: {
// TODO: It may be possible to improve this by analyzing where the
// condition operand comes from.
CmpInst::Predicate Pred = CmpInst::BAD_ICMP_PREDICATE;
llvm::CmpPredicate Pred = CmpInst::BAD_ICMP_PREDICATE;
match(getOperand(0), m_Cmp(Pred, m_VPValue(), m_VPValue()));
auto *CondTy = Ctx.Types.inferScalarType(getOperand(0));
auto *VecTy = Ctx.Types.inferScalarType(getOperand(1));
if (!vputils::onlyFirstLaneUsed(this)) {
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23 changes: 23 additions & 0 deletions llvm/test/Transforms/LoopVectorize/AArch64/select-costs.ll
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Expand Up @@ -73,3 +73,26 @@ exit:
%1 = select i1 %all.off, i32 1, i32 %0
ret i32 %1
}

define i32 @select_vpinst_for_tail_folding(i8 %n) {
; CHECK: LV: Checking a loop in 'select_vpinst_for_tail_folding'
; CHECK: Cost of 1 for VF 2: EMIT vp<{{.+}}> = select vp<{{.+}}>, ir<%red.next>, ir<%red>
; CHECK: Cost of 1 for VF 4: EMIT vp<{{.+}}> = select vp<{{.+}}>, ir<%red.next>, ir<%red>
; CHECK: LV: Selecting VF: 4

entry:
%c = icmp ne i8 %n, 0
%ext = zext i1 %c to i32
br label %loop

loop:
%iv = phi i32 [ %ext, %entry ], [ %iv.next, %loop ]
%red = phi i32 [ 0, %entry ], [ %red.next, %loop ]
%iv.next = add i32 %iv, 1
%red.next = mul i32 %red, %iv
%ec = icmp eq i32 %iv, 12
br i1 %ec, label %exit, label %loop

exit:
ret i32 %red.next
}