RFR: 8354674: AArch64: Intrinsify Unsafe::setMemory [v2]

[Andrew Dinn]

On Fri, 9 May 2025 15:39:35 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> This intrinsic is generally faster than the current implementation for Panama segment operations for all writes larger than about 8 bytes in size, increasing to more than 2* the performance on larger memory blocks on Graviton 2, between "panama" (C2 generated, what we use now) and "unsafe" (this intrinsic).
>> 
>> 
>> Benchmark                       (aligned)  (size)  Mode  Cnt     Score    Error  Units
>> MemorySegmentFillUnsafe.panama       true  262143  avgt   10  7295.638 ±  0.422  ns/op
>> MemorySegmentFillUnsafe.panama      false  262143  avgt   10  8345.300 ± 80.161  ns/op
>> MemorySegmentFillUnsafe.unsafe       true  262143  avgt   10  2930.594 ±  0.180  ns/op
>> MemorySegmentFillUnsafe.unsafe      false  262143  avgt   10  3136.828 ±  0.232  ns/op
>
> Andrew Haley has updated the pull request incrementally with one additional commit since the last revision:
> 
>   generate_unsafecopy_common_error_exit

src/hotspot/cpu/aarch64/stubGenerator_aarch64.cpp line 2611:

> 2609: 
> 2610:       __ subs(count, count, 64);
> 2611:       __ add(dest, dest, 64);

This add could be elided by employing a post-increment on dest in each of the two writes above, saving on code size. Is there a reason to prefer the add?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25147#discussion_r2084225622