RFR: 8294865: x86: Improve the code generation of MulVB and MulVL [v3]

[Quan Anh Mai]

On Thu, 13 Oct 2022 18:09:31 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> Quan Anh Mai has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:
>> 
>>  - Merge branch 'master' into improveMulVB
>>  - refactor conditions
>>  - add vmulB for 8 bytes
>>  - Merge branch 'master' into improveMulVB
>>  - Merge branch 'master' into improveMulVB
>>  - Merge branch 'master' into improveMulVB
>>  - fix
>>  - mulV
>
> src/hotspot/cpu/x86/matcher_x86.hpp line 195:
> 
>> 193:         return 0;
>> 194:       case Op_MulVB:
>> 195:         return 7;
> 
> Why do you unconditionally return `7` here? Is it because AVX512 doesn't feature a vector instruction to multiply byte vectors?

Yes AFAIK there is no instruction to perform byte vector multiplication directly on x86.

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PR: https://git.openjdk.org/jdk/pull/10571