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

[Lutz Schmidt]

On Mon, 10 Jun 2024 13:06:32 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Amit Kumar has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   removes warmup code, changes time unit and iteration number
>
> src/hotspot/cpu/s390/macroAssembler_s390.cpp line 5816:
> 
>> 5814: 
>> 5815:     assert(r_tmp != noreg, "temp register required for popcnt, for machines < z15");
>> 5816:     assert_different_registers(r_dst, r_tmp); // if r_src is same as r_tmp, it should be fine
> 
> Please move these assertions out of the if block. We want to ensure this for testing purposes at all times, so that even if we test on Z15 we expect it to run on others.

@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

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PR Review Comment: https://git.openjdk.org/jdk/pull/19509#discussion_r1633235453