RFR: 8309502: RISC-V: String.indexOf intrinsic may produce misaligned memory loads [v2]

[Vladimir Kempik]

On Wed, 7 Jun 2023 15:56:27 GMT, Vladimir Kempik <vkempik at openjdk.org> wrote:

>> Please review this attempt to remove misaligned loads in String.indexOf intrinsic on RISC-V
>> 
>> Initialy found these misaligned loads when profiling finagle-http test from renaissance suite.
>> The majority of trp_lam events (about 66k per finagle-http round) came at line 706 (https://github.com/openjdk/jdk/pull/14320/files#diff-35eb1d2f1e2f0514dd46bd7fbad49ff2c87703d5a3041a6433956df00a3fe6e6L706)
>> The other two produced about 100 events combined.
>> Later I've found this can partially be reproduced with StringIndexOf.advancedWithMediumSub.
>> Numbers on hifive before and after applying the patch:
>> 
>> 
>> Benchmark                                                  Mode  Cnt       Score      Error  Units
>> StringIndexOf.advancedWithMediumSub                        avgt   25   47031.406 ±  144.005  ns/op
>> 
>> 
>> After:
>> 
>> Benchmark                                                 Mode  Cnt       Score     Error  Units
>> StringIndexOf.advancedWithMediumSub                       avgt   25    4256.830 ±  23.075  ns/op
>> 
>> 
>> Testing: tier1/tier2 is clean on hifive.
>
> Vladimir Kempik has updated the pull request incrementally with one additional commit since the last revision:
> 
>   make DO2 read by one character from memory per loop

Numbers on DO4 ( comparing 4 characters at once)
DO4:

hifive
Benchmark                                 Mode  Cnt     Score    Error  Units
before
StringIndexOf.advancedWithShortSub4Chars  avgt   25  69514.891 ± 128.730  ns/op
after
StringIndexOf.advancedWithShortSub4Chars  avgt   25  2481.448 ± 13.481  ns/op

thead

Benchmark                                 Mode  Cnt     Score    Error  Units
before
StringIndexOf.advancedWithShortSub4Chars  avgt   25  753.125 ? 2.859  ns/op
after
StringIndexOf.advancedWithShortSub4Chars  avgt   25  741.031 ? 9.075  ns/op

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

PR Comment: https://git.openjdk.org/jdk/pull/14320#issuecomment-1581288502