[llvm-dev] How to get Greedy RA to not spill results of trivially rematerializable instructions

[Nemanja Ivanovic via 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?
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
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>
>
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