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

[Vladimir Ivanov]

On Tue, 16 Mar 2021 19:13:09 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> src/hotspot/cpu/x86/x86_64.ad line 3383:
>> 
>>> 3381: %}
>>> 3382: 
>>> 3383: operand kReg()
>> 
>> Should `kReg` operand exclude `k0` and `k1`?
>
> 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.

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