32 bits header and alignment slack

[forax at univ-mlv.fr]

----- Original Message -----
> From: "Roman Kennke" <rkennke at amazon.de>
> To: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "lilliput-dev" <lilliput-dev at openjdk.org>
> Sent: Monday, March 4, 2024 12:59:48 PM
> Subject: Re: 32 bits header and alignment slack

> 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!

Thinking a little more about it.
The aligment gap is not necessary at the end of the object, a j.l.Long with a 32 bits header has a 32 bits gap in between the mark/class word and the field containing the value.

> 
> Roman

Rémi

> 
> 
> 
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