RFR: 8268231: Aarch64: Use ldp in intrinsics for String.compareTo [v2]

[Andrew Haley]

On Mon, 12 Jul 2021 09:14:25 GMT, Wang Huang <whuang at openjdk.org> wrote:

>> Dear all, 
>>     Can you do me a favor to review this patch. This patch use `ldp` to implement String.compareTo.
>>    
>> * We add a JMH test case 
>> * Here is the result of this test case
>>  
>> Benchmark	                       |(size)| Mode| Cnt|Score | Error	 |Units	
>> ---------------------------------|------|-----|----|------|--------|-----
>> StringCompare.compareLL       |  64  | avgt| 5  |7.992 | ±	0.005|us/op
>> StringCompare.compareLL       |  72  | avgt| 5  |15.029| ±	0.006|us/op
>> StringCompare.compareLL       |  80  | avgt| 5  |14.655| ±	0.011|us/op
>> StringCompare.compareLL       |  91  | avgt| 5  |16.363| ±	0.12 |us/op
>> StringCompare.compareLL       |  101 | avgt| 5  |16.966| ±	0.007|us/op
>> StringCompare.compareLL       |  121 | avgt| 5  |19.276| ±	0.006|us/op
>> StringCompare.compareLL       |  181 | avgt| 5  |19.002| ±	0.417|us/op
>> StringCompare.compareLL       |  256 | avgt| 5  |24.707| ±	0.041|us/op
>> StringCompare.compareLLWithLdp|  64  | avgt| 5  |8.001	| ±	0.121|us/op
>> StringCompare.compareLLWithLdp|  72  | avgt| 5  |11.573| ±	0.003|us/op
>> StringCompare.compareLLWithLdp|  80  | avgt| 5  |6.861 | ±	0.004|us/op
>> StringCompare.compareLLWithLdp|  91  | avgt| 5  |12.774| ±	0.201|us/op
>> StringCompare.compareLLWithLdp|  101 | avgt| 5  |8.691 | ±	0.004|us/op
>> StringCompare.compareLLWithLdp|  121 | avgt| 5  |11.091| ±	1.342|us/op
>> StringCompare.compareLLWithLdp|  181 | avgt| 5  |14.64 | ±	0.581|us/op
>> StringCompare.compareLLWithLdp|  256 | avgt| 5  |25.879| ±	1.775|us/op
>> StringCompare.compareUU       |  64  | avgt| 5  |13.476| ±	0.01 |us/op
>> StringCompare.compareUU       |  72  | avgt| 5  |15.078| ±	0.006|us/op
>> StringCompare.compareUU       |  80  | avgt| 5  |23.512| ±	0.011|us/op
>> StringCompare.compareUU       |  91  | avgt| 5  |24.284| ±	0.008|us/op
>> StringCompare.compareUU       |  101 | avgt| 5  |20.707| ±	0.017|us/op
>> StringCompare.compareUU       |  121 | avgt| 5  |29.302| ±	0.011|us/op
>> StringCompare.compareUU       |  181 | avgt| 5  |39.31	| ±	0.016|us/op
>> StringCompare.compareUU       |  256 | avgt| 5  |54.592| ±	0.392|us/op
>> StringCompare.compareUUWithLdp|  64  | avgt| 5  |16.389| ±	0.008|us/op
>> StringCompare.compareUUWithLdp|  72  | avgt| 5  |10.71 | ±	0.158|us/op
>> StringCompare.compareUUWithLdp|  80  | avgt| 5  |11.488| ±	0.024|us/op
>> StringCompare.compareUUWithLdp|  91  | avgt| 5  |13.412| ±	0.006|us/op
>> StringCompare.compareUUWithLdp|  101 | avgt| 5  |16.245| ±	0.434|us/op
>> StringCompare.compareUUWithLdp|  121 | avgt| 5  |16.597| ±	0.016|us/op
>> StringCompare.compareUUWithLdp|  181 | avgt| 5  |27.373| ±	0.017|us/op
>> StringCompare.compareUUWithLdp|  256 | avgt| 5  |41.74 | ±	3.5	 |us/op
>> 
>> From this table, we can see that in most cases, our patch is better than old one.
>> 
>> Thank you for your review. Any suggestions are welcome.
>
> Wang Huang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   draft of refactor

And with longer strings, M1 and ThunderX2:


Benchmark                                       (diff_pos)  (size)  Mode  Cnt   Score   Error  Units
StringCompare.compareLLDiffStrings                    1023    1024  avgt    3  50.849 ± 0.087  us/op
StringCompare.compareLLDiffStringsWithLdp             1023    1024  avgt    3  23.676 ± 0.015  us/op
StringCompare.compareLLDiffStringsWithRefactor        1023    1024  avgt    3  28.967 ± 0.168  us/op


StringCompare.compareLLDiffStrings                    1023    1024  avgt    3  98.681 ± 0.026  us/op
StringCompare.compareLLDiffStringsWithLdp             1023    1024  avgt    3  82.576 ± 0.656  us/op
StringCompare.compareLLDiffStringsWithRefactor        1023    1024  avgt    3  98.801 ± 0.321  us/op

LDP wins on M1 here, but on ThunderX2 it makes almost no difference at all. And how often are we comparing such long strings?
I don't know what to think, really. It seems that we're near to a place where we're optimizing for micro-architecture, and I don't want to see that here. On the other hand, using LDP is not worse anywhere, so we should allow it.

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PR: https://git.openjdk.java.net/jdk/pull/4722