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

Nemanja Ivanovic via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 20 18:53:16 PST 2019

Finally managed to reduce this to something manageable:

On line 40 of the output, we have a load-immediate to put zero into R3.
Then we spill that value on the next line. And as far as I can tell, we
reload it on line 97 before the call to getValueAsBit().

On Wed, Feb 20, 2019 at 7:33 PM Nemanja Ivanovic <nemanja.i.ibm at gmail.com>

> 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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