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

Quentin Colombet via llvm-dev llvm-dev at lists.llvm.org
Thu Feb 21 15:53:39 PST 2019

Hi Nemanja,

I haven’t looked in the compiler, but just from the output assembly, this is more complicated than plain rematerialization.

Unless I read the assembly wrong, what we spill is not a simple constant but two different values based on some condition.
Basically, we are looking at code that looks like this:
If <...> 
  R3 = some value
  R3 = cst
= R3

When we spill R3, we don’t know if we are going to get the if or else part of R3. Furthermore, if R3 was a constant on both paths, we would need to emit a select-like instruction to pick the right value, which is definitely not simple remarterialization.

Let me know if I miss something, otherwise I feel there isn’t much we need to do here.

> On Feb 20, 2019, at 9:19 PM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Thanks for the reduced test case, I’ll try to take a look by the end of the week.
>> On Feb 20, 2019, at 6:53 PM, Nemanja Ivanovic <nemanja.i.ibm at gmail.com <mailto:nemanja.i.ibm at gmail.com>> wrote:
>> Finally managed to reduce this to something manageable: https://godbolt.org/z/Hw529k <https://godbolt.org/z/Hw529k>
>> 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 <mailto:nemanja.i.ibm at gmail.com>> wrote:
>> 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 <mailto: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 <mailto: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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