32 bits header and alignment slack

[Kennke, Roman]

Hello Remi,

> Hello,
> I was try to read the code corresponding to the compact identity hashcode
>  https://wiki.openjdk.org/display/lilliput/Compact+Identity+Hashcode
> 
> But I did not see any mention of the fact that due to object alignment, there is maybe already an empty 32 bits space at the end of the object.
> By example, with a java.lang.Integer and a 64 bits header, due to alignment to 64 bits, there is 32 bits of space which is not used after the value of the Integer
> 
>  j.l.Integer layout
>  mark word / 64 bits
>  value / 32 bits
>  empty / 32 bits
> 
> In that case, when System.identityHashCode() is called, it could use that space directly instead of waiting the GC to move the object to another location of the mostly same size.
> 
> Obviously, there is a good chance that this is exactly what the VM currently does and I've just missed that part :)

I have not called it out separately, but it’s mentioned in side-sentences, like:

"The first call to System.identityHashCode() transitions the hash-bits from the 00 state to the 01 state, *if there is no sufficient alignment gap at the end of the object,* and return the computed i-hash.”

The current prototype does it that way.
https://github.com/openjdk/lilliput/pull/137

I’ll write it down more specifically.

Thanks for pointing out!

Roman



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