RFR: 8371637: allocateNativeInternal sometimes return incorrectly aligned memory

[Per Minborg]

On Tue, 11 Nov 2025 14:11:28 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

Would it make sense to add `OperatingSystem.BSD` to consolidate any such predicates?

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PR Comment: https://git.openjdk.org/jdk/pull/28235#issuecomment-3520634175