RFR: 8300253: Introduce AArch64 nzcv accessors

[Andrew Haley]

On Thu, 19 Jan 2023 07:03:39 GMT, Hao Sun <haosun at openjdk.org> wrote:

>> The bits for the instructions are correct. I presume there'll be a train of patches for ZGC generational to necessitate it.
>
> I suggest adding one comment here. How about:
> 
> // macro instructions for accessing and updating 
> // PSTATE.{N,Z,C,V} fields at EL0
> //
> // NZCV:  op1 = 011
> //        CRn = 0100
> //        CRm = 0010
> //        op2 = 000

As a general rule, we don't add commentary that duplicates information in the Architecture Reference Manual. In this case, it's obvious that `MRS <Xt>, NZCV` is intended, and the information above is on Page C5-689 of ARM DDI 0487H.a

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PR: https://git.openjdk.org/jdk/pull/12038