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[MachinePipeliner] Use RegisterClassInfo::getRegPressureSetLimit #119827

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Merged
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Dec 18, 2024
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[MachinePipeliner] Use RegisterClassInfo::getRegPressureSetLimit #119827

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43 changes: 1 addition & 42 deletions llvm/lib/CodeGen/MachinePipeliner.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1326,48 +1326,7 @@ class HighRegisterPressureDetector {
// Calculate the upper limit of each pressure set
void computePressureSetLimit(const RegisterClassInfo &RCI) {
for (unsigned PSet = 0; PSet < PSetNum; PSet++)
PressureSetLimit[PSet] = TRI->getRegPressureSetLimit(MF, PSet);

// We assume fixed registers, such as stack pointer, are already in use.
// Therefore subtracting the weight of the fixed registers from the limit of
// each pressure set in advance.
SmallDenseSet<Register, 8> FixedRegs;
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Not sure what a fixed register is, but it seems to not be the same as reserved. Seems like another hook with bad defaults

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Yeah, it is a bad API. Fixed registers are not reserved register... Fixed register are registers defined in FixedRegisters RegisterCategory by default...
The logic here seems to want to remove reserved registers @kasuga-fj.

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As I commented in #118787 (comment), I dared to replace it from RegisterClassInfo::getRegPressureSetLimit before (in #87312). This is because there is a duplication between the reserved registers handled inside RegisterClassInfo::getRegPressureSetLimit and the FixedRegs calculated here, so the Limit >= Weight assertion fails. If I remember correctly, in my case (AArch64), the $ffr triggers it. What I'd like to do here is to ignore registers that have "specific rolls" (e.g., stack pointer) and those where TargetRegisterInfo::isFixedRegister returns true looks appropriate in this case to me. Maybe I'm misunderstanding reserved and fixed registers, and there may be a better way to achieve what I want, but I'm not sure this change will work well.

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IMO isFixedRegister should be deleted. The intent seems to be isReserved || isConstantPhysREg

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Thanks for explanation! At least, after removing this part of code, the tests still pass? @kasuga-fj

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I ran tests and none of them failed. But that just means there are no correctness issues. This code is part of a heuristic decision, so performance degradation could happen in some cases (and I think the heuristics will be less accurate with this change).

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@wangpc-pp wangpc-pp Dec 13, 2024 •
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I think the heuristics will be less accurate with this change

I don't think so. These "fixed" registers are also "reserved" registers, and "reserved" registers contain runtime fixed registers like those are reserved via -ffixed-xx options.

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These "fixed" registers are also "reserved" registers,

Previously (where RegisterClassInfo::getRegPressureSetLimit was used here), #83437 caused the Limit >= Weight assertion to fail. This patch marks the $ffr register as reserved. I just tried to comment out the code that marks $ffr as reserved and run MachinePipeliner, and found that FixedRegs contains $ffr. It looks to me like there are some gaps between "reserved" and "fixed". Or maybe I misunderstood something (I don't know much about the LLVM API for registers).

for (const TargetRegisterClass *TRC : TRI->regclasses()) {
for (const MCPhysReg Reg : *TRC)
if (isFixedRegister(Reg))
FixedRegs.insert(Reg);
}

LLVM_DEBUG({
for (auto Reg : FixedRegs) {
dbgs() << printReg(Reg, TRI, 0, &MRI) << ": [";
for (MCRegUnit Unit : TRI->regunits(Reg)) {
const int *Sets = TRI->getRegUnitPressureSets(Unit);
for (; *Sets != -1; Sets++) {
dbgs() << TRI->getRegPressureSetName(*Sets) << ", ";
}
}
dbgs() << "]\n";
}
});

for (auto Reg : FixedRegs) {
LLVM_DEBUG(dbgs() << "fixed register: " << printReg(Reg, TRI, 0, &MRI)
<< "\n");
for (MCRegUnit Unit : TRI->regunits(Reg)) {
auto PSetIter = MRI.getPressureSets(Unit);
unsigned Weight = PSetIter.getWeight();
for (; PSetIter.isValid(); ++PSetIter) {
unsigned &Limit = PressureSetLimit[*PSetIter];
assert(
Limit >= Weight &&
"register pressure limit must be greater than or equal weight");
Limit -= Weight;
LLVM_DEBUG(dbgs() << "PSet=" << *PSetIter << " Limit=" << Limit
<< " (decreased by " << Weight << ")\n");
}
}
}
PressureSetLimit[PSet] = RCI.getRegPressureSetLimit(PSet);
}

// There are two patterns of last-use.
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