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

[Jatin Bhateja]

On Fri, 16 Jul 2021 15:51:27 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.

Hi @vnkozlov , can you kindly review the patch.

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