RFR: 8268229: Aarch64: Use Neon in intrinsics for String.equals [v3]

Wed Mar 30 08:20:47 UTC 2022

On Fri, 2 Jul 2021 09:55:59 GMT, Wang Huang <whuang at openjdk.org> wrote:

>> Wang Huang has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   unroll when small string sizes
>
>> _Mailing list message from [Andrew Haley](mailto:aph at redhat.com) on [hotspot-dev](mailto:hotspot-dev at mail.openjdk.java.net):_
>> 
>> I had to make some changes to the benchmark to get accurate timing, because
>> it is swamped by JMH overhead for very small strings.
>> 
>> It should be clear from my patch what I did. The most important part is
>> to run the test code in a loop, or you won't see small effects. We're
>> trying to measure something that only takes a few nanoseconds.
>> 
>> This is what I see, Apple M1, two equal strings:
>> 
>> Old:
>> 
>> StringEquals.equal 8 avgt 5 0.948 ? 0.001 us/op
>> StringEquals.equal 11 avgt 5 0.948 ? 0.004 us/op
>> StringEquals.equal 16 avgt 5 0.948 ? 0.001 us/op
>> StringEquals.equal 22 avgt 5 1.260 ? 0.002 us/op
>> StringEquals.equal 32 avgt 5 1.886 ? 0.001 us/op
>> StringEquals.equal 45 avgt 5 2.514 ? 0.001 us/op
>> StringEquals.equal 64 avgt 5 3.141 ? 0.003 us/op
>> StringEquals.equal 91 avgt 5 4.395 ? 0.002 us/op
>> StringEquals.equal 121 avgt 5 5.653 ? 0.014 us/op
>> StringEquals.equal 181 avgt 5 8.011 ? 0.010 us/op
>> StringEquals.equal 256 avgt 5 11.433 ? 0.014 us/op
>> StringEquals.equal 512 avgt 5 23.005 ? 0.124 us/op
>> StringEquals.equal 1024 avgt 5 49.185 ? 0.032 us/op
>> 
>> Your patch:
>> 
>> Benchmark (size) Mode Cnt Score Error Units
>> StringEquals.equal 8 avgt 5 1.574 ? 0.001 us/op
>> StringEquals.equal 11 avgt 5 1.734 ? 0.004 us/op
>> StringEquals.equal 16 avgt 5 1.888 ? 0.002 us/op
>> StringEquals.equal 22 avgt 5 1.892 ? 0.003 us/op
>> StringEquals.equal 32 avgt 5 2.517 ? 0.003 us/op
>> StringEquals.equal 45 avgt 5 2.988 ? 0.002 us/op
>> StringEquals.equal 64 avgt 5 2.517 ? 0.003 us/op
>> StringEquals.equal 91 avgt 5 8.659 ? 0.007 us/op
>> StringEquals.equal 121 avgt 5 5.649 ? 0.007 us/op
>> StringEquals.equal 181 avgt 5 6.050 ? 0.009 us/op
>> StringEquals.equal 256 avgt 5 7.088 ? 0.016 us/op
>> StringEquals.equal 512 avgt 5 14.163 ? 0.018 us/op
>> StringEquals.equal 1024 avgt 5 29.998 ? 0.052 us/op
>> 
>> As you can see, we're looking at regressions all the way up to size=45,
>> with something very odd happening at size=91. Finally the vectorized
>> code starts to pull ahead at size=181.
>> 
>> A few things:
>> 
>> You should never be executing the TAIL unless the string is really
>> short. Just do one pair of unaligned loads at the end to finish.
>> 
>> Please don't use aliases for rscratch1 and rscratch2. Calling them tmp1
>> and tmp2 doesn't help the reader.
>> 
>> So: please make sure the smaller strings are at least as good as
>> they are now. Remember strings are usually short, so we can tolerate
>> no regressions with the smaller sizes.
>> 
>> I don't think that Neon does any good here. This is what I get by rewriting
>> (just) the stub with scalar registers, in the attached patch:
>> 
>> Benchmark (size) Mode Cnt Score Error Units
>> StringEquals.equal 8 avgt 5 1.574 ? 0.004 us/op
>> StringEquals.equal 11 avgt 5 1.734 ? 0.003 us/op
>> StringEquals.equal 16 avgt 5 1.888 ? 0.002 us/op
>> StringEquals.equal 22 avgt 5 1.891 ? 0.003 us/op
>> StringEquals.equal 32 avgt 5 2.517 ? 0.001 us/op
>> StringEquals.equal 45 avgt 5 2.988 ? 0.002 us/op
>> StringEquals.equal 64 avgt 5 2.595 ? 0.004 us/op
>> StringEquals.equal 91 avgt 5 4.083 ? 0.006 us/op
>> StringEquals.equal 121 avgt 5 5.432 ? 0.006 us/op
>> StringEquals.equal 181 avgt 5 6.292 ? 0.009 us/op
>> StringEquals.equal 256 avgt 5 7.232 ? 0.008 us/op
>> StringEquals.equal 512 avgt 5 13.304 ? 0.012 us/op
>> StringEquals.equal 1024 avgt 5 25.537 ? 0.012 us/op
>> 
>> I use an editor with automatic indentation, as do many people, so
>> I inserted brackets in the right places in the assembly code.
>> 
>> --
>> Andrew Haley (he/him)
>> Java Platform Lead Engineer
>> Red Hat UK Ltd. <https://www.redhat.com>
>> https://keybase.io/andrewhaley
>> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
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> 
> @theRealAph  Thank you for your suggestion. It's my fault that the JMH I used is not accurate. I changed my codes and re-tested under your JMH:
> 
> Before opt：
> Benchmark |(size)| Mode| Cnt | Score| Error |Units
> -------------------|------|-----|-----|-------|---------|-----
> StringEquals.equal | 8| avgt| 5 | 2.334|? 0.012 |us/op
> StringEquals.equal | 11| avgt| 5 | 2.335|? 0.012 |us/op
> StringEquals.equal | 16| avgt| 5 | 2.334|? 0.011 |us/op
> StringEquals.equal | 22| avgt| 5 | 3.414|? 0.422 |us/op
> StringEquals.equal | 32| avgt| 5 | 3.890|? 0.004 |us/op
> StringEquals.equal | 45| avgt| 5 | 5.610|? 0.023 |us/op
> StringEquals.equal | 64| avgt| 5 | 7.215|? 0.009 |us/op
> StringEquals.equal | 91| avgt| 5 | 12.305|? 1.716 |us/op
> StringEquals.equal | 121| avgt| 5 | 14.891|? 0.085 |us/op
> StringEquals.equal | 181| avgt| 5 | 21.502|? 0.050 |us/op
> StringEquals.equal | 256| avgt| 5 | 29.968|? 0.155 |us/op
> StringEquals.equal | 512| avgt| 5 | 59.414|? 2.341 |us/op
> StringEquals.equal | 1024| avgt| 5 |118.365|? 20.794 |us/op
> 
> After opt：
> Benchmark |(size)| Mode| Cnt | Score| Error| Units
> -------------------|------|-----|-----|------|-------|------
> StringEquals.equal | 8| avgt| 5 | 2.333|? 0.003| us/op
> StringEquals.equal | 11| avgt| 5 | 2.333|? 0.001| us/op
> StringEquals.equal | 16| avgt| 5 | 2.332|? 0.002| us/op
> StringEquals.equal | 22| avgt| 5 | 3.265|? 0.404| us/op
> StringEquals.equal | 32| avgt| 5 | 3.875|? 0.002| us/op
> StringEquals.equal | 45| avgt| 5 | 5.793|? 0.331| us/op
> StringEquals.equal | 64| avgt| 5 | 6.730|? 0.054| us/op
> StringEquals.equal | 91| avgt| 5 | 8.611|? 0.075| us/op
> StringEquals.equal | 121| avgt| 5 |10.041|? 0.042| us/op
> StringEquals.equal | 181| avgt| 5 |13.968|? 0.653| us/op
> StringEquals.equal | 256| avgt| 5 |19.199|? 1.227| us/op
> StringEquals.equal | 512| avgt| 5 |39.508|? 1.784| us/op
> StringEquals.equal | 1024| avgt| 5 |77.883|? 1.290| us/op

@Wanghuang-Huawei any plans to re-open and fix this?

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PR: https://git.openjdk.java.net/jdk/pull/4423