RFR: 8342601: AArch64: Micro-optimize bit shift in copy_memory [v3]

[Dean Long]

On Mon, 21 Oct 2024 16:03:23 GMT, Chad Rakoczy <duke at openjdk.org> wrote:

>> [JDK-8342601](https://bugs.openjdk.org/browse/JDK-8342601)
>> 
>> Fix minor inefficiency in `copy_memory` by adding check before doing bit shift to see if we are able to do a move instruction instead. Change is low risk because of the low complexity of the change
>> 
>> Ran array copy and tier 1 on aarch64 machine
>> 
>> Test summary
>> ==============================
>>    TEST                                              TOTAL  PASS  FAIL ERROR   
>>    jtreg:test/hotspot/jtreg/compiler/arraycopy          49    49     0     0   
>> ==============================
>> 
>> ==============================
>> Test summary
>> ==============================
>>    TEST                                              TOTAL  PASS  FAIL ERROR   
>>    jtreg:test/hotspot/jtreg:tier1                     2591  2591     0     0   
>>    jtreg:test/jdk:tier1                               2436  2436     0     0   
>>    jtreg:test/langtools:tier1                         4577  4577     0     0   
>>    jtreg:test/jaxp:tier1                                 0     0     0     0   
>>    jtreg:test/lib-test:tier1                            34    34     0     0   
>> ==============================
>
> Chad Rakoczy has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update check

If I had known about aarch64 optimizations like Neoverse 0 latency moves when I first looked at this, I would have approved it, so some code comments hinting at why we are doing this not-completely-obvious optimization would be appreciated.

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Marked as reviewed by dlong (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21589#pullrequestreview-2383215877