[AIE2] Fix pathological case where greedy reloads directly from a CR register #211
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gbossu
requested review from
abhinay-anubola,
abnikant,
andcarminati,
khallouh,
konstantinschwarz,
martien-de-jong,
SagarMaheshwari99 and
stephenneuendorffer
as code owners
| const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo(); | ||
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| // Clear previous cache, reserved registers can change for each function | ||
| SuitableRCForRA.clear(); |
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I can suggest that we don't need to clean this cache always (from the compile time perspective). You can encapsulate this map with a BitVector representing the current set or reserved regs in a standalone class/struct (you can access the reserved registers with BitVector getReservedRegs from TRI).
We can have, in the end something like:
SuitableRCForRA.clearIfDiverges(TRI) -> just clean if we have different sets here.
This is important as greedy feels free to fold a lot of operations into spill/reload instructions as long as the register class allows it In our case, we do not want to fold %0:ercr = LD_reload ... $crSat = COPY %0 into: $crSat = LD_reload ... as this makes generating spill code harder. We would need to bounce through a new GPR anyway.
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It turns out greedy does quite a lot of "operation folding" to get more compact spill code.
It will turn
into
This is problematic because we cannot directly reload into a
$crSatregister. I actually expected such folding to be illegal, because after all,$crSatis a reserved register. And usually, reserved registers just mean "don't you dare optimize me". But it's also true that this register is part ofeRCR(which it needs to be in for some of our copy-propagation extensions to kick in), so I guess what greedy does is fair game.I could change
const TargetRegisterClass *canFoldCopy(...)to actually never fold into reserved registers, but that causes one X86 test to fail. At this point, I'd rather just constraineRCRtoeRbefore greedy, because it's anyway what we want: the classes are equivalent in terms of allocatable registers, andeRis easy to spill.No QoR impact, which is expected, but you never know.