[swift-evolution] Draft: Add @noescape and rethrows to ManagedBuffer API

Joe Groff jgroff at apple.com
Mon Feb 8 21:12:52 CST 2016

I don't expect @noescape to have much effect on optimization yet since we don't propagate that information to SIL at all.


> On Feb 8, 2016, at 7:11 PM, Arnold Schwaighofer via swift-evolution <swift-evolution at swift.org> wrote:
> Your benchmark code looks fine to me.
> You could put the individual tests into @inline(never) functions then it would be easier to look at what an individual test does in Xcode’s Instruments.
> It is true that small changes can trigger large swings in runtime performance as a seemingly small change can for example change inlining decisions or prevent ARC operation removal and then performance is widely different.
> Given that you only see a 3% regression in -O mode where we are not able to remove all the abstraction the performance numbers you report look good to me.
>> From: Károly Lőrentey via swift-evolution <swift-evolution-m3FHrko0VLzYtjvyW6yDsg at public.gmane.org>
>> Date: February 8, 2016 at 7:08:33 AM PST
>> To: Dave Abrahams <dabrahams-2kanFRK1NckAvxtiuMwx3w at public.gmane.org>
>> Cc: swift-evolution-m3FHrko0VLzYtjvyW6yDsg at public.gmane.org
>> Subject: Re: Draft: Add @noescape and rethrows to ManagedBuffer API
>>> On 2016-02-07, at 16:18, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>> On first look this seems to be a great idea.  Have you checked for
>>> performance impact?
>> Yes, although I found it challenging to create a good 
>> microbenchmark for this. Subtle changes in the benchmarking code
>> lead to large swings in runtime performance, which makes me question
>> the usefulness of my results.
>> Keeping that in mind, for a trivial ManagedBuffer subclass, I found 
>> that @noescape makes for a ~15-18% improvement when whole module 
>> optimization is disabled, or when the subclass is imported.
>> Throwing in the rethrows declarations reduces the improvement to 
>> ~9-13%, or (in the case of a particular subscript getter test) even 
>> reverses it, making the code ~3% slower.
>> The proposal has no discernible impact on ManagedBuffer subclasses
>> that the optimizer has full access to. (I.e., when they’re defined
>> in the same file as the code that’s using them, or in the same 
>> module with WMO.) Unoptimized code also seems unaffected by these 
>> changes.
>> My benchmarking code is on GitHub; feedback would be very welcome:
>>   https://github.com/lorentey/ManagedBuffer-Benchmark
>> -- 
>> Karoly
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