RFR: 8269559: AArch64: Implement string_compare intrinsic in SVE [v2]

[Nick Gasson]

On Mon, 23 Aug 2021 21:48:01 GMT, TatWai Chong <github.com+78814694+tatwaichong at openjdk.org> wrote:

>> This patch implements string_compare intrinsic in SVE.
>> It supports all LL, LU, UL and UU comparisons.
>> 
>> As we haven't found an existing benchmark to measure performance impact,
>> we created a benchmark derived from the test [1] for this evaluation.
>> This benchmark is attached to this patch.
>> 
>> Besides, remove the unused temporary register `vtmp3` from the existing
>> match rules for StrCmp.
>> 
>> The result below shows all varients can be benefited largely.
>> Command: make exploded-test TEST="micro:StringCompareToDifferentLength"
>> 
>> Benchmark    (size)  Mode  Cnt   Score Speedup Units
>> compareToLL      24  avgt   10      1.0x       ms/op
>> compareToLL      36  avgt   10      1.0x       ms/op
>> compareToLL      72  avgt   10      1.0x       ms/op
>> compareToLL     128  avgt   10      1.4x       ms/op
>> compareToLL     256  avgt   10      1.8x       ms/op
>> compareToLL     512  avgt   10      2.7x       ms/op
>> compareToLU      24  avgt   10      1.6x       ms/op
>> compareToLU      36  avgt   10      1.8x       ms/op
>> compareToLU      72  avgt   10      2.3x       ms/op
>> compareToLU     128  avgt   10      3.8x       ms/op
>> compareToLU     256  avgt   10      4.7x       ms/op
>> compareToLU     512  avgt   10      6.3x       ms/op
>> compareToUL      24  avgt   10      1.6x       ms/op
>> compareToUL      36  avgt   10      1.7x       ms/op
>> compareToUL      72  avgt   10      2.2x       ms/op
>> compareToUL     128  avgt   10      3.3x       ms/op
>> compareToUL     256  avgt   10      4.4x       ms/op
>> compareToUL     512  avgt   10      6.1x       ms/op
>> compareToUU      24  avgt   10      1.0x       ms/op
>> compareToUU      36  avgt   10      1.0x       ms/op
>> compareToUU      72  avgt   10      1.4x       ms/op
>> compareToUU     128  avgt   10      2.2x       ms/op
>> compareToUU     256  avgt   10      2.6x       ms/op
>> compareToUU     512  avgt   10      3.7x       ms/op
>> 
>> [1] https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/compiler/intrinsics/string/TestStringCompareToDifferentLength.java
>
> TatWai Chong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Restore the removal of vtmp3 (=V2) as it is still used by the non-SVE compare-long-strings stub.
>   
>   And remove the assertion in `string_compare` since it won't help as the registers
>   used in the stub are fixed.

This looks ok to me now but please wait to see if @theRealAph has any comments. I suppose the the short-string comparison code which is expanded at the call site could also benefit from SVE, if only to make it shorter.

-------------

Marked as reviewed by ngasson (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5129