RFR: 8371637: allocateNativeInternal sometimes return incorrectly aligned memory [v5]

[ExE Boss]

On Wed, 19 Nov 2025 10:54:42 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:
> 
>   use suggested solution by bsdkurt

src/java.base/share/classes/jdk/internal/foreign/SegmentFactories.java line 215:

> 213:             result = Utils.alignUp(allocationBase, byteAlignment);
> 214:         } else {
> 215:             allocationSize = (alignedSize < byteAlignment) ? byteAlignment : alignedSize;

This can probably use [`Math​::max​(long, long)`], which is an intrinsic[^1].
Suggestion:

            allocationSize = Math.max(alignedSize, byteAlignment);


[^1]: For whatever reason, [`StrictMath​::max​(long, long)`] is the only overload which doesn’t have `@IntrinsicCandidate`, even though it’s just a simple call to `Math​::max​(long, long)`, and so gets intrisified anyway after inlining; same with [`StrictMath​::min​(long, long)`] and [`Math​::min​(long, long)`].

[`Math​::max​(long, long)`]: https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Math.html#max%28long%2Clong%29
[`Math​::min​(long, long)`]: https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Math.html#min%28long%2Clong%29
[`StrictMath​::max​(long, long)`]: https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/StrictMath.html#max%28long%2Clong%29
[`StrictMath​::min​(long, long)`]: https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/StrictMath.html#min%28long%2Clong%29

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PR Review Comment: https://git.openjdk.org/jdk/pull/28235#discussion_r2541810615