RFR: 8371637: allocateNativeInternal sometimes return incorrectly aligned memory [v3]

[Kurt Miller]

On Fri, 14 Nov 2025 13:13:00 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:
> 
>   OS agnostic fix for alignment of native segments
>   
>   Only align up the requested memory if the requested alignment is larget
>   than max alignment provided by malloc, or if the requested size is not a
>   multiple of the alignment size.
>   
>   This work was sponsored by: The FreeBSD Foundation
>   
>   Co-authored-by: mcimadamore

There are now two cases where we need to adjust `allocationSize` to add support for the weak-alignment environment. The current case where `byteAlignment > MAX_MALLOC_ALIGN` and now when `byteAlignment <= MAX_MALLOC_ALIGN && alignedSize < byteAlignment`. The calculation of `allocationSize`  in the first case should stay the same. For the second case increasing `allocationSize` to match `byteAlignment` will guarantee the returned pointer will be aligned to  `byteAlignment` or greater.

Edited to fix typo.

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PR Comment: https://git.openjdk.org/jdk/pull/28235#issuecomment-3538900093