[llvm] [VPlan] Don't use the legacy cost model for loop conditions (PR #156864)
John Brawn via llvm-commits
llvm-commits at lists.llvm.org
Wed Sep 10 07:45:25 PDT 2025
================
@@ -1117,6 +1117,29 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,
return Ctx.TTI.getIndexedVectorInstrCostFromEnd(Instruction::ExtractElement,
VecTy, Ctx.CostKind, 0);
}
+ case VPInstruction::BranchOnCount: {
+ // If TC <= VF then this is just a branch.
+ // FIXME: Removing the branch happens in simplifyBranchConditionForVFAndUF
+ // where it checks TC <= VF * UF, but we don't know UF yet. This means in
+ // some cases we get a cost that's too high due to counting a cmp that
+ // later gets removed.
+ Value *TC = getParent()->getPlan()->getTripCount()->getUnderlyingValue();
+ ConstantInt *TCConst = dyn_cast_if_present<ConstantInt>(TC);
+ if (TCConst && TCConst->getValue().ule(VF.getKnownMinValue()))
+ return 0;
+ // Otherwise BranchOnCount generates ICmpEQ followed by a branch.
+ Type *ValTy = Ctx.Types.inferScalarType(getOperand(0));
+ return Ctx.TTI.getCmpSelInstrCost(Instruction::ICmp, ValTy,
+ CmpInst::makeCmpResultType(ValTy),
+ CmpInst::ICMP_EQ, Ctx.CostKind);
----------------
john-brawn-arm wrote:
I've deliberately avoided touching branch costs to keep the scope of this work as small as possible.
https://github.com/llvm/llvm-project/pull/156864
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