Classes, specializations, and statics

[Brian Goetz]

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 <brian.goetz at oracle.com>
>     To: Bjorn B Vardal/Ottawa/IBM at IBMCA,
>     valhalla-spec-experts at openjdk.java.net
>     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.
>
>