RFR: 8262355: Support for AVX-512 opmask register allocation. [v8]

[Jatin Bhateja]

On Tue, 16 Mar 2021 21:14:23 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> k0  - signifies all true mask.
>
> Yes, k0 is a special register. 
> But is it even correct to pass k0 as a temp register? (`kReg` is extensively used for TEMPs.) 
> According to the documentation, many instructions accepting k-registers #UD fault on k0.

That is correct, k0 is being removed from register mask corresponding to kReg operand during register mask initializations, please refer to reg_mask_init () routine in x86_64.ad

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PR: https://git.openjdk.java.net/jdk/pull/2768