RFR: 8270848: Redundant unsafe opmask register allocation in some instruction patterns. [v2]

[Vladimir Kozlov]

On Fri, 30 Jul 2021 12:27:53 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> - Some string compare/array equality patters in x86_[32/64].ad files accept the temporary opmark register operands, instructions using these registers are guarded by target feature checks.
>> - Even if control path leading to these instructions is not chosen, RA will still allocate physical registers to these temporary operands.
>> - Pulling out the feature checks upto the instruction level can save redundant allocations which may effect spilling decisions.
>> - Also existing platform dependent routine Matcher::has_predicated_vector() check the existence of AVX512VL feature which is not required.
>
> Jatin Bhateja has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8270848: Review comments resolution.

Testing passed clean.

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PR: https://git.openjdk.java.net/jdk/pull/4813