[llvm-dev] (RFC) Adjusting default loop fully unroll threshold

Tue Feb 7 14:26:57 PST 2017

Ping... with the updated code size impact data, any more comments? Any more
data that would be interesting to collect?

Thanks,
Dehao

On Thu, Feb 2, 2017 at 2:07 PM, Dehao Chen <dehao at google.com> wrote:

> Here is the code size impact for clang, chrome and 24 google internal
> benchmarks (name omited, 14 15 16 are encoding/decoding benchmarks similar
> as h264). There are 2 columns, for threshold 300 and 450 respectively.
>
> I also tested the llvm test suite. Changing the threshold to 300/450 does
> not affect code gen for any binary in the test suite.
>
>
>
> 300 450
> clang 0.30% 0.63%
> chrome 0.00% 0.00%
> 1 0.27% 0.67%
> 2 0.44% 0.93%
> 3 0.44% 0.93%
> 4 0.26% 0.53%
> 5 0.74% 2.21%
> 6 0.74% 2.21%
> 7 0.74% 2.21%
> 8 0.46% 1.05%
> 9 0.35% 0.86%
> 10 0.35% 0.86%
> 11 0.40% 0.83%
> 12 0.32% 0.65%
> 13 0.31% 0.64%
> 14 4.52% 8.23%
> 15 9.90% 19.38%
> 16 9.90% 19.38%
> 17 0.68% 1.97%
> 18 0.21% 0.48%
> 19 0.99% 3.44%
> 20 0.19% 0.46%
> 21 0.57% 1.62%
> 22 0.37% 1.05%
> 23 0.78% 1.30%
> 24 0.51% 1.54%
>
> On Wed, Feb 1, 2017 at 6:08 PM, Mikhail Zolotukhin via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On Feb 1, 2017, at 4:57 PM, Xinliang David Li via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> clang, chrome, and some internal large apps are good candidates for size
>> metrics.
>>
>> I'd also add the standard LLVM testsuite just because it's the suite
>> everyone in the community can use.
>>
>> Michael
>>
>>
>> David
>>
>> On Wed, Feb 1, 2017 at 4:47 PM, Chandler Carruth via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> I had suggested having size metrics from somewhat larger applications
>>> such as Chrome, Webkit, or Firefox; clang itself; and maybe some of our
>>> internal binaries with rough size brackets?
>>>
>>> On Wed, Feb 1, 2017 at 4:33 PM Dehao Chen <dehao at google.com> wrote:
>>>
>>>> With the new data points, any comments on whether this can justify
>>>> setting fully inline threshold to 300 (or any other number) in O2? I can
>>>> collect more data points if it's helpful.
>>>>
>>>> Thanks,
>>>> Dehao
>>>>
>>>> On Tue, Jan 31, 2017 at 3:20 PM, Dehao Chen <dehao at google.com> wrote:
>>>>
>>>> Recollected the data from trunk head with stddev data and more
>>>> threshold data points attached:
>>>>
>>>> Performance:
>>>>
>>>> stddev/mean 300 450 600 750
>>>> 403 0.37% 0.11% 0.11% 0.09% 0.79%
>>>> 433 0.14% 0.51% 0.25% -0.63% -0.29%
>>>> 445 0.08% 0.48% 0.89% 0.12% 0.83%
>>>> 447 0.16% 3.50% 2.69% 3.66% 3.59%
>>>> 453 0.11% 1.49% 0.45% -0.07% 0.78%
>>>> 464 0.17% 0.75% 1.80% 1.86% 1.54%
>>>> Code size:
>>>>
>>>> 300 450 600 750
>>>> 403 0.56% 2.41% 2.74% 3.75%
>>>> 433 0.96% 2.84% 4.19% 4.87%
>>>> 445 2.16% 3.62% 4.48% 5.88%
>>>> 447 2.96% 5.09% 6.74% 8.89%
>>>> 453 0.94% 1.67% 2.73% 2.96%
>>>> 464 8.02% 13.50% 20.51% 26.59%
>>>> Compile time is proportional in the experiments and more noisy, so I
>>>> did not include it.
>>>>
>>>> We have >2% speedup on some google internal benchmarks when switching
>>>> the threshold from 150 to 300.
>>>>
>>>> Dehao
>>>>
>>>> On Mon, Jan 30, 2017 at 5:06 PM, Chandler Carruth <chandlerc at google.com
>>>> > wrote:
>>>>
>>>> On Mon, Jan 30, 2017 at 4:59 PM Mehdi Amini <mehdi.amini at apple.com>
>>>> wrote:
>>>>
>>>>
>>>>
>>>> Another question is about PGO integration: is it already hooked there?
>>>> Should we have a more aggressive threshold in a hot function? (Assuming
>>>> we’re willing to spend some binary size there but not on the cold path).
>>>>
>>>>
>>>> I would even wire the *unrolling* the other way: just suppress
>>>> unrolling in cold paths to save binary size. rolled loops seem like a
>>>> generally good thing in cold code unless they are having some larger impact
>>>> (IE, the loop itself is more expensive than the unrolled form).
>>>>
>>>>
>>>>
>>>> Agree that we could suppress unrolling in cold path to save code size.
>>>> But that's orthogonal with the propose here. This proposal focuses on O2
>>>> performance: shall we have different (higher) fully unroll threshold than
>>>> dynamic/partial unroll.
>>>>
>>>>
>>>> I agree that this is (to some extent) orthogonal, and it makes sense to
>>>> me to differentiate the threshold for full unroll and the dynamic/partial
>>>> case.
>>>>
>>>>
>>>> There is one issue that makes these not orthogonal.
>>>>
>>>> If even *static* profile hints will reduce some of the code size
>>>> increase caused by higher unrolling thresholds for non-cold code, we should
>>>> factor that into the tradeoff in picking where the threshold goes.
>>>>
>>>> However, getting PGO into the full unroller is currently challenging
>>>> outside of the new pass manager. We already have some unfortunate hacks
>>>> around this in LoopUnswitch that are making the port of it to the new PM
>>>> more annoying.
>>>>
>>>>
>>>>
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170207/2b02f874/attachment-0001.html>