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

Mon Feb 13 10:56:02 PST 2017

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.
>
>
>
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