[llvm-dev] [GlobalISel][AArch64] Toward flipping the switch for O0: Please give it a try!

Tue May 30 13:57:08 PDT 2017

Hi Diana,

I’ve actually gone ahead and pushed the fix as I was able to produce a small reproducer.

This is r304244

Let me know if you encounter any other problem.

Cheers,
-Quentin
> On May 30, 2017, at 7:42 AM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Thanks Diana.
> 
> That is indeed the assumption in the code and this is obviously wrong.
> 
> Could you try the attached patch?
> 
> (I haven’t even tried to compile it though)
> 
> Cheers,
> -Quentin
> <localizer_tentative_fix.diff>
>> On May 30, 2017, at 6:56 AM, Diana Picus <diana.picus at linaro.org> wrote:
>> 
>> Hi Quentin,
>> 
>> I've attached a reproducer for the problem.
>> 
>> I've described what I think the problem is in the file, but the short
>> version is that the localizer shouldn't assume that the iteration
>> order for the uses corresponds to the logical order of instructions in
>> a basic block (we're now localizing before the first use that we find,
>> but that may be later in the basic block, so we'd end up with uses
>> before the def).
>> 
>> I'm not sure it's possible to test this without running a couple of
>> passes. You might be able to trigger it only with reg bank select +
>> localize, but I haven't tried. Using only the localizer would mean
>> that the iteration order for the uses would be the order in which
>> they're read in, so you wouldn't have this problem.
>> 
>> Hope that helps,
>> Diana
>> 
>> 
>> On 29 May 2017 at 10:06, Diana Picus <diana.picus at linaro.org> wrote:
>>> Thanks Quentin, it's in progress now, I'll let you know how it goes.
>>> 
>>> Cheers,
>>> Diana
>>> 
>>> On 27 May 2017 at 03:36, Quentin Colombet <qcolombet at apple.com> wrote:
>>>> Hi Kristof,
>>>> 
>>>> I’ve pushed the localizer in r304051 and added it in the AArch64 O0 pipeline
>>>> in r304052.
>>>> 
>>>> I let Diana investigate the seg fault she was seeing.
>>>> 
>>>> @Diana, let me know if you need help.
>>>> 
>>>> Cheers,
>>>> -Quentin
>>>> 
>>>> On May 25, 2017, at 1:53 PM, Quentin Colombet via llvm-dev
>>>> <llvm-dev at lists.llvm.org> wrote:
>>>> 
>>>> Hi Kristof,
>>>> 
>>>> On May 25, 2017, at 2:09 AM, Kristof Beyls <kristof.beyls at arm.com> wrote:
>>>> 
>>>> 
>>>> On 24 May 2017, at 22:01, Quentin Colombet <qcolombet at apple.com> wrote:
>>>> 
>>>> Hi Kristof,
>>>> 
>>>> Thanks for going back so fast!
>>>> 
>>>> On May 24, 2017, at 12:57 PM, Kristof Beyls <kristof.beyls at arm.com> wrote:
>>>> 
>>>> 
>>>> On 24 May 2017, at 19:31, Quentin Colombet <qcolombet at apple.com> wrote:
>>>> 
>>>> Hi Kristof,
>>>> 
>>>> Thanks for the measurements.
>>>> 
>>>> On May 24, 2017, at 6:00 AM, Kristof Beyls <kristof.beyls at arm.com> wrote:
>>>> 
>>>> 
>>>> - Comparing against -O0 without globalisel but with the above regalloc
>>>> options: 5.6% performance drop, 1% code size drop.
>>>> 
>>>> In summary, the measurements indicate some good improvements.
>>>> I also haven't measure the impact on compile time.
>>>> 
>>>> 
>>>> Do you have a mean to make this measurement?
>>>> Ahmed did a bunch of compile time measurements on our side and I wanted to
>>>> see if I need to put him on the hook again :).
>>>> 
>>>> 
>>>> I did a quick setup with CTMark (part of the test-suite). I ran each of
>>>> * '-O0 -g',
>>>> * '-O0 -g -mllvm -global-isel=true -mllvm -global-isel-abort=0', and
>>>> * '-O0 -g -mllvm -global-isel=true -mllvm -global-isel-abort=0 -mllvm
>>>> -optimize-regalloc -mllvm -regalloc=greedy'
>>>> 5 times, cross-compiling from X86 to AArch64, and took the median measured
>>>> compile times.
>>>> In summary, I see GlobalISel having a compile time that's 3.5% higher than
>>>> the current -O0 default.
>>>> With enabling the greedy register allocator, this increases to 28%.
>>>> 28% is probably too high?
>>>> 
>>>> 
>>>> I think it is yes.
>>>> I have attached a quick hack to the greedy allocator to feature a fast mode.
>>>> Could you give it a try?
>>>> 
>>>> To enable the fast mode, please use (-mllvm) -regalloc-greedy-fast=true
>>>> (default is false).
>>>> 
>>>> 
>>>> I'm afraid it doesn't seem to save much compile time. On geomean, I see
>>>> about 26% compile time increase against the current -O0 default (compared to
>>>> 28% increase for regalloc greedy without your patch).
>>>> 
>>>> 
>>>> Interesting, I guess a lot of time is spent in the coalescer. Could you give
>>>> a try with -join-liveintervals=false?
>>>> 
>>>> 
>>>> With adding -join-liveintervals=false, I see the compile time increase going
>>>> up to 28% again.
>>>> 
>>>> 
>>>> Heh, I am mildly surprised we hand much more live-ranges to the allocator
>>>> when we do that.
>>>> 
>>>> 
>>>> 
>>>> Do you know where the time is spent (-time-passes)?
>>>> 
>>>> 
>>>> I'm afraid I won't have time to have a closer look in the next couple of
>>>> days - I don't know where the time is spent at the moment.
>>>> 
>>>> 
>>>> Fair enough, will investigate later.
>>>> 
>>>> 
>>>> 
>>>> Anyhow, fixing all of those, although this is I think the right approach,
>>>> will take time, so we can go with the localizer.
>>>> 
>>>> 
>>>> Right, I don't understand the register allocator well enough to know if that
>>>> compile time overhead can be fixed, while still getting the necessary
>>>> codegen benefits the greedy allocator gives.
>>>> Is there any specific help you're looking for with getting the localizer
>>>> work well enough for production use?
>>>> 
>>>> 
>>>> I’ll clean-up the WIP patch for the localizer, then you guys can fix the bug
>>>> that you found.
>>>> 
>>>> I’ll do that tomorrow.
>>>> 
>>>> Cheers,
>>>> -Quentin
>>>> 
>>>> 
>>>> Thanks,
>>>> 
>>>> Kristof
>>>> 
>>>> 
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>>>> 
>>>> 
>> <localizer-mo-order.mir>
> 
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