RFR: 8277617: Adjust AVX3Threshold for copy/fill stubs [v4]

[Jatin Bhateja]

On Thu, 25 Nov 2021 05:08:42 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Sandhya Viswanathan has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Override threshold only if flag is default
>
> src/hotspot/cpu/x86/vm_version_x86.cpp line 1893:
> 
>> 1891:     return AVX3Threshold;
>> 1892:   }
>> 1893: }
> 
> I am somewhat concerned about the overhead of evaluating this each time it is used. I realize these will only be startup costs while generating the stubs, not part of the stubs themselves, but it still may be a startup impact. Can you run a startup benchmark to see if there is any problem?
> 
> I was also thinking the more direct formulation would just be:
> ```return (is_intel_family_core() && supports_serialize() && FLAG_IS_DEFAULT(AVX3Threshold)) ? 0 : AVX3Threshold;```

Hi  @sviswa7 agree with @dholmes-ora , instead of calling multiple times in a stub can we not call it only once per stub? Since stubs are assembled once and not relocated hence it should be okay to call this method only once for stubs which are going to benefit form this change.

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PR: https://git.openjdk.java.net/jdk/pull/6512