RFR: 8371637: allocateNativeInternal sometimes return incorrectly aligned memory [v4]

[Kurt Miller]

On Sun, 16 Nov 2025 18:09:47 GMT, Harald Eilertsen <haraldei at openjdk.org> wrote:

>> `jdk.internal.foreign.SegmentFactories::allocateNativeInternal` assumes that the underlying implementation of malloc aligns allocations on 16 byte boundaries for 64 bit platforms, and 8 byte boundaries on 32 bit platforms. So for any allocation where the requested alignment is less than or equal to this default alignment it makes no adjustment.
>> 
>> However, this assumption does not hold for all allocators. Specifically jemallc, used by libc on FreeBSD will align small allocations on 8 or 4 byte boundaries, respectively. This causes allocateNativeInternal to sometimes return memory that is not properly aligned when the requested alignment is exactly 16 bytes.
>> 
>> To make sure we honour the requested alignment when it exaclty matches the quantum as defined by MAX_MALLOC_ALIGN, this patch ensures that we adjust the alignment also in this case.
>> 
>> This should make no difference for platforms where malloc allready aligns on the quantum, except for a few unnecessary trivial calculations.
>> 
>> This work was sponsored by: The FreeBSD Foundation
>
> Harald Eilertsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix calculation of allocationSize when byteAlignment < MAX_MALLOC_ALIGN
>   
>   This work was sponsored by: The FreeBSD Foundation

I believe this is the most concise way to support weak alignment allocators:
https://github.com/bsdkurt/jdk/commit/bb45e82a8772e50e321f07af37470fc4360b0adf

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PR Comment: https://git.openjdk.org/jdk/pull/28235#issuecomment-3543039999