RFR: 8351994: Enable Extended EVEX to REX2/REX demotion when src and dst are the same [v34]

[Sandhya Viswanathan]

On Wed, 21 May 2025 23:35:39 GMT, Srinivas Vamsi Parasa <sparasa at openjdk.org> wrote:

>> Intel APX NDD instructions are encoded using EVEX encoding. The goal of this PR is to enable optimized instruction encoding for Intel APX NDD instructions when the non-destructive destination is same as the first source.
>> 
>> For example:
>> 
>> `eaddl r18, r18, r25` can be encoded as `addl r18, r25` using APX REX2 encoding
>> `eaddl r2, r2, r7` can be encoded as `addl r2, r7` using non-APX legacy encoding
>
> Srinivas Vamsi Parasa has updated the pull request incrementally with five additional commits since the last revision:
> 
>  - refactor to use is_P6_or_later()
>  - rename byte1 to opcode_byte
>  - rename evex_opcode_prefix_and_encode as emit_eevex_or_demote
>  - rename evex to eevex in method names
>  - reset swap=false as default

Changes look good to me.

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Marked as reviewed by sviswanathan (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24431#pullrequestreview-2859399896