On Thu, 13 Nov 2025 19:12:41 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 two > additional commits since the last revision: > > - Second try to fix alignment for native segments > > Introducing a helper function as suggested by JornVernee to decide on > the proper alignment based on the segment size. > > This work was sponsored by: The FreeBSD Foundation > > Co-authored-by: JornVernee > - Test that native segments don't overlap > > This work was sponsored by: The FreeBSD Foundation The VM (which the JDK interfaces with malloc through) guarantees at-most 16 byte alignment from `malloc`, so any alignment less than that is also going to be fine. We should, as always, test this, but I don't think that anything will break on the VM side with this change. ------------- PR Comment: https://git.openjdk.org/jdk/pull/28235#issuecomment-3532178966
