RFR: JDK-8320448 Accelerate IndexOf using AVX2 [v7]

[Scott Gibbons]

On Tue, 16 Jan 2024 22:27:52 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Scott Gibbons has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 22 commits:
>> 
>>  - Merge branch 'openjdk:master' into indexof
>>  - Merge branch 'openjdk:master' into indexof
>>  - Addressing review comments.
>>  - Fix for JDK-8321599
>>  - Support UU IndexOf
>>  - Only use optimization when EnableX86ECoreOpts is true
>>  - Fix whitespace
>>  - Merge branch 'openjdk:master' into indexof
>>  - Comments; added exhaustive-ish test
>>  - Subtracting 0x10 twice.
>>  - ... and 12 more: https://git.openjdk.org/jdk/compare/8e12053e...3e58d0c2
>
> src/hotspot/cpu/x86/stubGenerator_x86_64.cpp line 4111:
> 
>> 4109:   if ((UseAVX == 2) && EnableX86ECoreOpts && VM_Version::supports_avx2()) {
>> 4110:     StubRoutines::_string_indexof = generate_string_indexof();
>> 4111:   }
> 
> What motivation for this extensive new code only for avx2? 30% is nice (for some cases) but it is enabled only for AVX2 and not for avx512 which all modern x86 CPUs have so the code will not be used.
> 
> Or it is typo?

This is acceleration for AVX2, replacing the pcmpestri instruction which is microcoded on E-cores and causes significant performance impact.  I am working on a pared-down implementation and should update this PR in a couple of days.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16753#discussion_r1454217437