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PHIElimination: add target hook to control reuse. #163604

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PHIElimination: add target hook to control reuse. #163604

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a4bb8a1
PHIElimination: add target hook to control reuse.
jgu222 Oct 15, 2025
9ead58d
Use common register class
jgu222 Oct 26, 2025
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13 changes: 12 additions & 1 deletion llvm/lib/CodeGen/PHIElimination.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -377,10 +377,21 @@ void PHIEliminationImpl::LowerPHINode(MachineBasicBlock &MBB,
// typically those created by tail duplication. Typically, an identical PHI
// node can't occur, so avoid hashing/storing such PHIs, which is somewhat
// expensive.

// canReuse() checks if two phis of the same rhs have the common sub class
// for their lhs's and allow reuse if so. (useful for some GPU targets)
auto canReuse = [](Register Reg0, Register Reg1, MachineFunction &MF) {
auto &MRI = MF.getRegInfo();
auto *RC0 = MRI.getRegClass(Reg0);
auto *RC1 = MRI.getRegClass(Reg1);
return MF.getSubtarget().getRegisterInfo()->getCommonSubClass(RC0, RC1) !=
nullptr;
};

Register *Entry = nullptr;
if (AllEdgesCritical)
Entry = &LoweredPHIs[MPhi];
if (Entry && *Entry) {
if (Entry && *Entry && canReuse(*Entry, DestReg, MF)) {
// An identical PHI node was already lowered. Reuse the incoming register.
IncomingReg = *Entry;
reusedIncoming = true;
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