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

Tue Feb 7 15:29:27 PST 2017

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.

Is the proposed change universal? Is there a way to undo it?

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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>>
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