I think there's two ways to think about "specialized statics" at the VM
level; how you think about it conditions which way you're inclined to go.
Way 1: This is a new thing; we used to have placements for "static" and
"instance", and now we have three placements: static, instance, and
specialization. We add a new bit (ACC_SPECIES), and define new bytecode
behaviors for members with the SPECIES bit set.
Way 2: There's still only two placements, static and instance;
per-specialization static is just the natural reinterpretation of
"static" in the face of a many-to-one relationship between source
classes and runtime classes. What we used to think of static really is
the weird case, which we can model as "new static, but restricted (using
"where" restriction) to the all-erased specialization."
Both ways are 100% compatible with existing generic sources and
classfiles. Way 2 (which seems more natural to me, and also means we
don't have to invent a new thing at the VM level) exploits the
requirement that Constant_Class[Foo] and Constant_ParamType[Foo, erased]
*must* describe the same runtime entity.
With Way 2, we still (probably) have a new concept at the language
level, which governs whether the so-described member is a member of all
parameterizations, or only the all-erased one, but this gets compiled
away. This matches up with our treatment of erasure; it is the choice
of the language compiler to choose erased generics or reified, and Java
chooses to erase some parameterizations but not all.
.NET chooses Way 2 in the extreme; static members are not shared across
specializations at all, since they have no notion of erasure.
Compatibility wouldn't let us go that far; existing statics are shared
across all erased parameterizations, so they would need to remain so.
On 3/18/2016 5:01 PM, Bjorn B Vardal wrote:
> I wonder if it's not better to have a class like ThreadLocal or
ClassValue that
> represents a constant that can be different depending on the
specialization.
That solution seems possible - we could implement
specialization-specific statics as a static Map<Class<?>, Foo<?>>,
where the type parameter is the key and "specialized static" is the
value. However, it would be slower than compiling to static access.
Are there other use cases that make specialized statics necessary? So
far we have empty collection, which can be implemented using the map.
State of the Specialization from Dec 2014 mentioned layers - are
specialized statics a step along the path to layers? Will a
SpecializedValue map for specialized statics look like an
afterthought if we go the rest of the way to layers?
--
Bjørn Vårdal
----- Original message -----
From: Remi Forax <[email protected]>
To: Valhalla Expert Group Observers
<[email protected]>
Cc: Bjorn B Vardal/Ottawa/IBM@IBMCA, Brian Goetz
<[email protected]>
Subject: Re: Classes, specializations, and statics
Date: Tue, Feb 23, 2016 2:57 PM
I wonder if it's not better to have a class like ThreadLocal or
ClassValue that represents a constant that can be different
depending on the specialization.
Rémi
----- Mail original -----
> De: "Brian Goetz" <[email protected]>
> À: "Bjorn B Vardal" <[email protected]>
> Cc: [email protected]
> Envoyé: Mardi 23 Février 2016 01:23:16
> Objet: Re: Classes, specializations, and statics
>
> It's possible that there could be multiple "ssinit" methods, each
> restricted to specific parameterizations (just like any other
restricted
> method), but in general, the "ssinit" method can be specialized just
> like any other method. So what I envision (in the absence of
> initialization of conditional members) is possibly two such
methods; one
> that is specializable (corresponding to _SS members) and one that is
> not, restricted to the erased parameterization (corresponding to
> traditional statics.)
>
>
>
> On 2/22/2016 4:11 PM, Bjorn B Vardal wrote:
> > I think we're on the same page regarding specialized <clinit>.
> > - The JVM will be handed multiple <clinit> partial methods,
and the
> > specializer will take care of selecting the appropriate
<clinit> for
> > each specialization.
> > - The erased <clinit> will contain the non-specialized static
> > initialization code, which ensures that it only runs once.
> > - The erased <clinit> will always run before the first
specialization
> > <clinit>.
> > - The Java syntax is still up for discussion.
> > > I think this is mostly a matter of coming up with the right
syntax,
> > which makes it clear that statics can be per-class or
> > per-specialization. There are a whole pile of related
> > specialization-related syntax issues, I'll try to get them all
in one
> > place.
> > I don't think the problem will be to make it clear that
statics can be
> > per-class or per-specialization, but rather why some
parameterizations
> > (which to the user are synonymous with specializations) don't
appear
> > to have specialized statics. Do we want to put erasure in the
face of
> > users like this? It seems better to let the users deal purely with
> > parameterizations, and we let specialization and erasure be
> > implementation details.
> > --
> > Bjørn Vårdal
> >
> > ----- Original message -----
> > From: Brian Goetz <[email protected]>
> > To: Bjorn B Vardal/Ottawa/IBM@IBMCA,
> > [email protected]
> > Cc:
> > Subject: Re: Classes, specializations, and statics
> > Date: Thu, Feb 18, 2016 7:55 PM
> >
> >
> >> Based on the example above, I think we need to be more
explicit
> >> about how the <clinit> method is handled.
> >> There are really two different sets of statics that need
to be
> >> handled by the class initialization:
> >> A) common statics (shared across all instantiations)
> >> B) specialized statics
> >> In addition to the statics, there is also common (and maybe
> >> specialized?) code that is run as part of <clinit>.
> >
> > There is a reasonable model to collapse these back into one
> > concept; treat "common statics" as specialized statics on the
> > all-erased parameterization, with a <where> clause that
restricts
> > them to that parameterization. Not clear whether we
actually want
> > to represent it that way or not, but its a useful mental model
> > that doesn't require the creation of a third thing. (Since
> > Class[Foo] and ParamType[Foo,erased*] describe the same class,
> > this is also fully binary compatible with existing classes.)
> >
> > Which means we can do a similar thing with <clinit>, if we
want.
> > I'll wave my hands because we've not yet talked much about
> > conditional members, but it basically looks like this:
> >
> > <where T*=erased*>
> > <init>() { /* common static init code */
> > /* specializable init code */ }
> >
> > <init>() { /* specializable init code */ }
> >
> > Or not.
> >> Where will the initialization code for both kinds of
statics be?
> >> The existing <clinit> method?
> >
> > We have two choices:
> > - have a new <sclinit> block that gets run once per
> > specialization, and keep <clinit>
> > - merge the two as above, exploiting planned support for
> > conditional members
> >
> > Either way, as you say, we have to ensure that the common init
> > runs exactly once.
> >> When using *static, are we only discussing {get,put}? Or
is this
> >> also proposing invokestatic changes to allow specialized
static
> >> methods?
> >
> > Methods too.
> >> All of the technical details aside, is this something we
really
> >> want to expose to the users? They're going to have a
hard time
> >> understanding why Foo<int> (or Foo<ValueType) gets
specialized
> >> statics while Foo<String> & Foo<Bar> share the erased
version.
> >
> > I think this is mostly a matter of coming up with the right
> > syntax, which makes it clear that statics can be per-class or
> > per-specialization. There are a whole pile of related
> > specialization-related syntax issues, I'll try to get them
all in
> > one place.
> >
> >
>
>
