Unsafe making too strong assumptions about array alignment?

[Andrew Haley]

On 5/27/25 23:30, some-java-user-99206970363698485155 at vodafonemail.de wrote:
> Yes, that is what I meant. But I am not sure if compact object headers
> would be really a problem. I think in that case for example
> ARRAY_BYTE_BASE_OFFSET would simply be < 16,

Probably, but again there are no hard guarantees.

> A few days ago I accidentally only responded to Maurizio instead of the
> mailing list. I will repeat my message here:
> 
> My concern here is that the code in `putLongUnaligned` decides if it is
> an unaligned store just based on the `offset`, and I am wondering if
> that information alone is really enough to tell that.

No. It isn't.

> Because if it determines that the store is aligned it delegates to
> `putLong` and that requires that the store is aligned, if that is not
> actually the case it is undefined behavior (if I understand it correctly).
> 
> So this means the JVM must guarantee that the physical address of the
> array is aligned by 8 to make sure `putLongUnaligned` for a byte[] is
> actually safe?

Indeed so. My belief is that any JVM running on a machine which has both 
strict alignment and 8-byte stores will 8-align all objects (or never 
use 8-byte-wide stores) but there's no guarantee of that. It's quite 
conceivable that a JVM could be written that only 8-aligns objects with 
8-byte-wide fields, 4-aligning everything else.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671