On Fri, 14 Nov 2025 13:13:00 GMT, Harald Eilertsen <[email protected]> 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. ------------- PR Comment: https://git.openjdk.org/jdk/pull/28235#issuecomment-3538900093
