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

[Mikhail Zolotukhin via llvm-dev]

> 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 <mailto: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 <mailto: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 <mailto: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 <mailto:chandlerc at google.com>> wrote:
> On Mon, Jan 30, 2017 at 4:59 PM Mehdi Amini <mehdi.amini at apple.com <mailto: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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