llvm dev - Feb 2019 - How to get Greedy RA to not spill results of trivially rematerializable instructions

I have encountered a rather odd situation with Greedy where it will end up
spilling a register that was populated with a zero (with a trivially
rematerializable load-immediate instruction).
In fact, it spills 3 such values (LICM moves stuff out of a loop, register
coalescer replaces copies with load-immediates and then Greedy spills them).

I personally can't think of a situation where a spill (with a reload later
presumably) is better than simply rematerializing the value where it would
have otherwise been reloaded. To that end, would it be possible for Greedy
to simply duplicate the trivially rematerializable instruction at every
reload site? Perhaps this is something it would need to query the target
for? Perhaps Greedy would be able to call something like
TargetInstrInfo::rematerializeValue(MachineInstr &RematMI,
MachineBasicBlock::iterator InsertAt) or something along those lines?
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Do you have a reproducer?

That shouldn’t happen.
> On Feb 20, 2019, at 4:12 PM, Nemanja Ivanovic via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> I have encountered a rather odd situation with Greedy where it will end up
spilling a register that was populated with a zero (with a trivially
rematerializable load-immediate instruction).
> In fact, it spills 3 such values (LICM moves stuff out of a loop, register
coalescer replaces copies with load-immediates and then Greedy spills them).
> 
> I personally can't think of a situation where a spill (with a reload
later presumably) is better than simply rematerializing the value where it would
have otherwise been reloaded. To that end, would it be possible for Greedy to
simply duplicate the trivially rematerializable instruction at every reload
site? Perhaps this is something it would need to query the target for? Perhaps
Greedy would be able to call something like
TargetInstrInfo::rematerializeValue(MachineInstr &RematMI,
MachineBasicBlock::iterator InsertAt) or something along those lines?
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

I do have a reproducer, but it's not for the faint of heart :)
This is from a large and messy C file (Perlbench's regexec.c), reduced by
bugpoint down to 1050 lines of IR. Perhaps I can paste it on pastebin.

Just for fun, I added some debug dumps for machine instructions that spill
registers (i.e. return non-zero from MachineInstr::getFoldedSpillSize())
that are fed by load-immediates and kill that register. Then I bootstrapped
LLVM/Clang/compiler-rt with those dumps. Turns out there are 5692
occurrences of that. I might have more luck reducing one of those files.

On Wed, Feb 20, 2019 at 7:16 PM Quentin Colombet <qcolombet at apple.com>
wrote:
> Do you have a reproducer?
>
> That shouldn’t happen.
>
> > On Feb 20, 2019, at 4:12 PM, Nemanja Ivanovic via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > I have encountered a rather odd situation with Greedy where it will
end
> up spilling a register that was populated with a zero (with a trivially
> rematerializable load-immediate instruction).
> > In fact, it spills 3 such values (LICM moves stuff out of a loop,
> register coalescer replaces copies with load-immediates and then Greedy
> spills them).
> >
> > I personally can't think of a situation where a spill (with a
reload
> later presumably) is better than simply rematerializing the value where it
> would have otherwise been reloaded. To that end, would it be possible for
> Greedy to simply duplicate the trivially rematerializable instruction at
> every reload site? Perhaps this is something it would need to query the
> target for? Perhaps Greedy would be able to call something like
> TargetInstrInfo::rematerializeValue(MachineInstr &RematMI,
> MachineBasicBlock::iterator InsertAt) or something along those lines?
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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