RFR: 8333382: [s390x] Enhance popcnt Instruction to use Z15 facilities [v4]

[Andrew Haley]

On Mon, 10 Jun 2024 13:35:29 GMT, Amit Kumar <amitkumar at openjdk.org> wrote:

>> @theRealAph,
>> I understand your argument. Nonetheless, I'm not happy with this request. The match rules in s390.ad have been designed such that, in case the more potent instruction is available, the register allocator does not need to provide a temp register.
>> Regards, Lutz
>
> I agree with Lutz. I have added a separate match rule which will be applicable only for machines `Z15 onwards`. Which will not pass `r_tmp` and it will be considered `noreg` will not satisfy first assert requirement.
> 
>> so that even if we test on Z15 we expect it to run on others.
> 
> `else` block will never run on `Z15 or Z16` machines.

I see. I wasn't so much thinking about the match rules, but anywhere else that needs to use popcount. While usages from the ad file are undoubtedly safe, anywhere else must provide a register to use or have some sort of conditionals at the point pop_count_int is used.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19509#discussion_r1633292332