On Thu, 13 Nov 2025 14:59:53 GMT, Jorn Vernee <[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 > > I think what Maurizio is suggesting is probably the most flexible. We can > assume that e.g. a 4 byte allocation is at least 4 byte aligned, and an 8 > byte allocation is also at least 8 bytes aligned (which implies 4 byte > alignment as well), up to a value equal to `alignof(max_align_t)`, which we > currently assume to be 16 (though, we could have a native method that > actually returns `alignof(max_align_t)`). > >> Doesn't this assume that all malloc implementations follow power of 2 >> pattern of arena sizes: 8, 16, 32, 64 and pointer alignments between min and >> max? malloc could also be implemented skipping some of those intermediate >> sizes. e.g. 16, 64, 256. > > If an 8 byte value is allocated in a 16 byte arena, I assume it is 16 byte > aligned, which implies 8 byte alignment. I've pushed a new version now, by adding a helper function as suggested by @JornVernee, but if you want I can have another go with @mcimadamore's suggestion as well. ------------- PR Comment: https://git.openjdk.org/jdk/pull/28235#issuecomment-3529336594
