RFR: 8279676: Dubious YMM register clearing in x86_64 arraycopy stubs

[Dean Long]

On Mon, 10 Jan 2022 16:59:18 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See discussion in the bug.
> 
> Additional testing, TR3970X (Zen 2, AVX2):
>  - [x] Linux x86_64 fastdebug `hotspot_compiler_arraycopy`
>  - [x] Linux x86_64 fastdebug `tier1`
>  - [x] Linux x86_64 fastdebug `tier2`
>  - [x] Linux x86_64 fastdebug `tier3`
> 
> Additional testing, i5-11500 (Rocket Lake, AVX-512):
>  - [x] Linux x86_64 fastdebug `hotspot_compiler_arraycopy`
>  - [x] Linux x86_64 fastdebug `tier1`
>  - [x] Linux x86_64 fastdebug `tier2`
>  - [x] Linux x86_64 fastdebug `tier3`
> 
> Targeted arraycopy microbenchmarks find a barely noticeable improvement on TR 3970X and i5-11500 with UseAVX=2.

We have new JCK test failures in java.lang.Character tests (see JDK-8280544).  I'm wondering if this change could be the cause.

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PR: https://git.openjdk.java.net/jdk/pull/7016