RFR: 8331558: AArch64: optimize integer remainder [v2]

[Jin Guojie]

On Mon, 6 May 2024 09:30:45 GMT, Eric Liu <eliu at openjdk.org> wrote:

>> Jin Guojie has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
>> 
>>  - Merge branch 'openjdk:master' into dev
>>  - Update vm_version_aarch64.hpp
>>  - 8331558: AArch64: optimize integer remainder
>>    
>>    On some Arm processors, a separate multiply/subtract is actually faster than the combined instruction.
>>  - 8331556: AArch64: CPU_Model support for Neoverse N1/N2/V1/V2
>>    
>>    Add full platform coverage for Neoverse variants in vm_version.?pp
>
> src/hotspot/cpu/aarch64/macroAssembler_aarch64.hpp line 447:
> 
>> 445:   inline void msub(Register Rd, Register Rn, Register Rm, Register Ra) {
>> 446:     if (VM_Version::supports_a53mac() && Ra != zr)
>> 447:       nop();
> 
> It was in JDK-8079203 [1] for the first time. May I ask what's the specials on a53mac?
> 
> [1] https://github.com/openjdk/jdk/commit/a65f9f95894e22ce2fd160024ce46f6aaa6c8bd3

This code entered the JDK in 2015. Frankly, I have no idea why an extra nop is needed on CPUs with the a53mac feature. 
Perhaps the author of patch a65f9f9589, enevill at openjdk.org, could explain?

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PR Review Comment: https://git.openjdk.org/jdk/pull/19093#discussion_r1590798890