Unsafe making too strong assumptions about array alignment?

[Remi Forax]

----- Original Message -----
> From: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
> To: "Andrew Haley" <aph-open at littlepinkcloud.com>, "panama-dev" <panama-dev at openjdk.org>, "Vladimir Ivanov"
> <vladimir.x.ivanov at oracle.com>
> Sent: Monday, May 26, 2025 2:23:59 PM
> Subject: Re: Unsafe making too strong assumptions about array alignment?

>>
>> might perform a misaligned store. The obvious response is that such
>> systems must not allocate objects at less than the maximum (presumably
>> 64-bit) alignment.
> 
> Yes and no.
> 
> IIUC, `offset` here is not the base address of the array -- but the
> offset at which array elements start. And such offset can indeed be only
> 4-byte aligned in some cases.
> 
> For instance, if I do this using jshell on a JDK 22 x86 build, I get:
> 
> ```
> jshell> Unsafe.ARRAY_SHORT_BASE_OFFSET
> $2 ==> 12
> ```
> 
> The same query returns 16 on a regular x64 JVM.

This is the header size, 4 (class word) + 8 (mark word) + 4 (array.length) = 16.

It should change with Java 25 and -XX:+UseCompactObjectHeaders [1]

regards,
Rémi

[1] https://openjdk.org/jeps/519