Sharing the markword (aka Valhalla's markword use)

[Remi Forax]

----- Original Message -----
> From: "Roman Kennke" <rkennke at amazon.de>
> To: "Dan Heidinga" <dan.heidinga at oracle.com>
> Cc: "Thomas Stüfe" <thomas.stuefe at gmail.com>, "valhalla-dev" <valhalla-dev at openjdk.org>, "lilliput-dev"
> <lilliput-dev at openjdk.org>
> Sent: Wednesday, March 6, 2024 7:47:09 PM
> Subject: Re: Sharing the markword (aka Valhalla's markword use)

>> 
>> We have a plan how we can make the class-pointer much smaller. Based on the
>> observation that most workloads only require a few 10000 classes, let’s say 15
>> bits is ‘enough’ and not wasteful. Workloads that require more often generate
>> those classes, which often don’t have many instances (e.g. only 1). In order to
>> address arbitrary many classes, we would require one more bit that indicates
>> that the class-pointer should be loaded from a dedicated field - this could be
>> compressed 32 bits, or uncompressed 64 bits, or somehow be combined with the 15
>> class-bits from the header
> 
> I shall add that this is possible because we know at class-loading time whether
> or not we’re going to exceed the class-addressing-limit, and reserve the extra
> Klass* field at class-loading time, during field layout, and allocate all
> affected instances with that extra field right from the start, no stunts needed
> like with the compact I-hash stuff.

and

> Using those 16 class-bits, and assuming 2(hash)+4(Valhalla)+4(age)+3(locking), 
> we’d still have a reserve of 3 bits. We could use them for more classes, or
> reserve them for future use. It does not seem unreasonable to make the
> class-bits even smaller, in case we ever need more bits.

If you have 2 bits for the hash, you need the compact identity hash optimization too.
So splitting the classpointer to save more bits is a third optimization.

Liliput 64 uses compressed classpointer (22 bits) and lightweight locking,
Liliput 32 uses compact identity hash (2 bits) with storage inflation on demand (plus all the above),
Liliput 16 uses split classpointer (plus all the above). 

> 
> Roman
> 

Rémi

> 
> 
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