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

Mon Feb 13 14:16:46 PST 2017

Thanks for the inputs.

On Mon, Feb 13, 2017 at 1:59 PM, Gerolf Hoflehner <ghoflehner at apple.com>
wrote:

> For threshold tuning I would like to see a little more insight than only
> running the numbers, eg. what is the reason for the biggest gain + biggest
> loss? Maybe another optimization could be improved and the unrolling factor
> is just hiding a weakness (and fix that would be more beneficial).
>

I analyzed the perf improvements on 2 google internal applications. The
benefits are simply from reduced dynamic instructions resulted from the
unrolled loop.

>
> For unrolling specifically I agree with Hal that the hooks should be
> target specific. Actually, I go further and think they should be uArch
> specific. I have no data or prove but would not be surprised to see a wider
> variety of numbers when the thresholds are tested on a wide range of x86
> machines.
>

I could provide more data on different x86 uArchs if necessary.

>
> My first thought also was along the lines of Matthias: do it at a higher
> opt level e.g. O3 or possibly revisit/start thinking about O4.
>

That was my first thought too. But after a second thought, if the code size
increase is small, and there is no data to show performance regression, I
could not think of a reason to block it from O2. Additionally, it does not
make sense to have the same threshold for full unroll and partial unroll
even in O2 (as reasoned in early posts of this thread). That's why I
started the patch and RFC to try to push this change to O2.

Thanks,
Dehao

>
> Gerolf
>
>
>
> On Feb 13, 2017, at 10:56 AM, Chandler Carruth via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> FWIW, I'm good with the updated data, but I'd really like at least someone
> from Apple and someone from ARM to chime in here... CC-ing random people in
> the hope it helps...
>
> On Mon, Feb 13, 2017 at 8:30 AM Dehao Chen via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Thanks for the comment. The performance experiments were performed on
>> Intel Sandybridge. Updated this info to the patch description.
>>
>> Dehao
>> On Sun, Feb 12, 2017 at 8:24 AM, Sanjay Patel <spatel at rotateright.com>
>> wrote:
>>
>> Since we can override the settings, I have no objections.
>>
>> I still think it would be good to document here and in the review/commit
>> message which CPU model was used to acquire the experimental data. That
>> could be useful to anyone that comes along later and wants to reproduce
>> and/or compare to the original, motivating data.
>>
>>
>> On Fri, Feb 10, 2017 at 4:53 PM, Dehao Chen <dehao at google.com> wrote:
>>
>> Thanks Hal, could you help approve https://reviews.llvm.org/D28368?
>>
>> I'll hold off until early Tuesday in case other people have more concerns.
>>
>> Thanks,
>> Dehao
>>
>> On Fri, Feb 10, 2017 at 3:23 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>>
>>
>> On 02/10/2017 05:21 PM, Dehao Chen wrote:
>>
>> Thanks every for the comments.
>>
>> Do we have a decision here?
>>
>>
>> You're good to go as far as I'm concerned.
>>
>>  -Hal
>>
>>
>> Dehao
>>
>> On Tue, Feb 7, 2017 at 10:24 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>>
>>
>> On 02/07/2017 05:29 PM, Sanjay Patel via llvm-dev wrote:
>>
>> Sorry if I missed it, but what machine/CPU are you using to collect the
>> perf numbers?
>>
>> I am concerned that what may be a win on a CPU that keeps a couple of
>> hundred instructions in-flight and has many MB of caches will not hold for
>> a small core.
>>
>>
>> In my experience, unrolling tends to help weaker cores even more than
>> stronger ones because it allows the instruction scheduler more
>> opportunities to hide latency. Obviously, instruction-cache pressure is an
>> important consideration, but the code size changes here seems small.
>>
>>
>> Is the proposed change universal? Is there a way to undo it?
>>
>>
>> All of the unrolling thresholds should be target-adjustable using the
>> TTI::getUnrollingPreferences hook.
>>
>>  -Hal
>>
>>
>>
>> On Tue, Feb 7, 2017 at 3:26 PM, Dehao Chen via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> 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
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170213/308d46f0/attachment-0001.html>