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

[Diana Picus via llvm-dev]

On 12 June 2017 at 18:54, Diana Picus <diana.picus at linaro.org> wrote:

> Hi all,
>
> I added a buildbot [1] running the test-suite with -O0 -global-isel. It
> runs into the same 2 timeouts that I reported previously on this thread
> (paq8p and scimark2). It would be nice to make it green before flipping the
> switch.
>
>
I did some more investigations on a machine similar to the one running the
buildbot. For paq8p and scimark2, I get these results for O0:

PAQ8p:
Fast isel: 666.344
Global isel: 731.384

SciMark2-C:
Fast isel: 463.908
Global isel: 496.22

The current timeout is 500s (so in this particular case we didn't hit it
for scimark2, and it ran successfully to completion). I don't think the
difference between FastISel and GlobalISel is too atrocious, so I would
propose increasing the timeout for these 2 benchmarks. I'm not sure if we
can do this on a per-bot basis, but I see some precedent for setting custom
timeout thresholds for various benchmarks on different architectures
(sometimes with comments that it's done so we can run O0 on that particular
benchmark).

Something along these lines works:
https://reviews.llvm.org/differential/diff/102547/

What do you guys think about this approach?

Thanks,
Diana

PS: The buildbot is using the Makefiles because that's what our other
AArch64 test-suite bots use. Moving all of them to CMake is a transition
for another time.


> At the moment, it lives in an internal buildmaster that I've setup for
> this purpose. If we fix it and it proves to be stable for a week or two,
> I'll move it to the public master.
>
> Cheers,
> Diana
>
> [1] http://master2.llvm.validation.linaro.org/builders/
> clang-cmake-aarch64-global-isel
>
>
> On 6 June 2017 at 19:11, Quentin Colombet <qcolombet at apple.com> wrote:
>
>> Thanks Kristof.
>>
>> Sounds like we'll need to investigate though I'd say it is not blocking
>> the switch.
>>
>> At this point I think everybody is on board to flip the switch.
>> @Eric, how does that sound to you?
>>
>> Thanks,
>> Q
>>
>> Le 1 juin 2017 à 07:46, Kristof Beyls <Kristof.Beyls at arm.com> a écrit :
>>
>>
>> On 31 May 2017, at 17:07, Quentin Colombet <qcolombet at apple.com> wrote:
>>
>>
>> Latest comparisons on my side, after picking up r304244, i.e. the correct
>> Localizer pass.
>> * CTMark compile time, comparing "-O0 -g" vs '-O0 -g -mllvm
>> -global-isel=true -mllvm -global-isel-abort=0': about 6% increase with
>> globalisel. This was about 3.5% before the Localizer pass landed.
>>
>>
>> That one is surprising too. I wouldn’t have expected this pass to show up
>> in the compile time profile. At least not to this extend.
>> What is the biggest offender?
>>
>>
>> Hmmm. So I took the 3.5% compile time overhead from my last measurement
>> before the localizer landed, from around 24th of May.
>> When using -ftime-report, I see the Localizer pass typically taking very
>> roughly about 1% of compile time.
>> Maybe another part of GlobalISel became a bit slower since I did that
>> 3.5% measurement?
>> Or maybe the Localizer pass changes the structure of the program so that
>> another later pass gets a different compile time profile?
>> Basically, I'd have to do more experiments to figure that one out.
>>
>> As far as where time is spent in the gisel-passes itself, on average, I
>> saw the following on the latest CTMark experiment I ran:
>> Avg compile time spent in IRTranslator: 4.61%
>> Avg compile time spent in InstructionSelect: 7.51%
>> Avg compile time spent in Legalizer: 1.06%
>> Avg compile time spent in Localizer: 0.76%
>> Avg compile time spent in RegBankSelect: 2.12%
>>
>>
>> * My usual performance benchmarking run: 8.5% slow-down. This was about
>> 9.5% before the Localizer pass landed, so a slight improvement.
>> * Code size: 3.14% larger. This was about 2.8% before the Localizer pass
>> landed, so a slight regression.
>>
>>
>> That one is surprising. Do you have an idea of what is happening?
>> Alternatively if you can point me to the biggest offender, I can have a
>> look.
>>
>>
>> So the biggest offenders on the mem_bytes metric in LNT are:
>> O0 -g O0 -g gisel-with-localizer O0 -g gisel-without-localizer
>> SingleSource/Benchmarks/Misc/perlin 14272 14640 18344 25.95%
>> SingleSource/Benchmarks/Dhrystone/dry 16560 17144 20160 18.21%
>> SingleSource/Benchmarks/Stanford/QueensProfile 13912 14192 15136 6.79%
>> MultiSource/Benchmarks/Trimaran/netbench-url/netbench-url 71400 72272
>> 75504 4.53%
>>
>> I haven't had time to investigate what exact changes make the code size
>> go up that much with the localizer pass in those cases...
>>
>>
>> The only thing I can think of is that we duplicate constants that are
>> expensive to materialize. If that’s the case, we were discussing with Ahmed
>> an alternative to the localizer pass that would operate during
>> InstructionSelect so may be worth pursuing.
>>
>>
>>
>
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