RFR: 8331098: [Aarch64] Fix crash in Arrays.equals() intrinsic with -CCP [v2]

[Roman Kennke]

On Thu, 25 Apr 2024 17:52:10 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Remove excess whitespace
>>  - Avoid loading cnt2 on paths that don't need it
>
> src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 5730:
> 
>> 5728:       // main loop and don't need to compare it
>> 5729:       // explicitely ahead of the loop.
>> 5730:       cmp(cnt2, cnt1);
> 
> Why do we need this? Surely if the base isn't required to be aligned, then it might be aligned. So why can't we use the not-aligned version in all cases?

The current implementation assumes that the base (first array element) is aligned. In this case, the array length is *not* aligned (8 bytes mark, word, 4 bytes compressed-Klass*, 4 bytes length), that is why in this case we compare the length ahead of the main loop. With uncompressed Klass* (8 bytes mark-word, 8 bytes Klass*, 4 bytes length, ...) or Lilliput (8 bytes mark-word/Klass*, 4 bytes length, ...), the base is only 4-bytes-aligned, but we can start at the length and still enter the main-loop at an 8 bytes aligned address. As a bonus, that also compares the lengths and we can save a few instructions/branches for that.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18948#discussion_r1579936795