Draft JEP Announcement: "Computed Constants"

[Dan Heidinga]

On Fri, Jul 28, 2023 at 2:35 PM John Rose <john.r.rose at oracle.com> wrote:

> On 28 Jul 2023, at 8:53, Dan Heidinga wrote:
>
> > … users now need to
> > understand if the cost of the lambda generation (inflation?  Not sure
> what
> > the right term is for converting a lambda expression to a functional
> > interface instance) will be more or less than the cost of the computation
> > they want to store in a ComputedConstant if they want to realize a
> startup
> > benefit from this change.
>
> Yes, this problem of scaling is raised by the very existence of a better
> alternative to <clinit>:  Can it beat <clinit> robustly or must it be used
> sparingly, because the language and VM don’t optimize it very well?
>
> I would like to take CC’s as they are proposed here, and then expand their
> mission.  (…After some shallow name changes: package should be java.lang
> and s/ofList/list/ or s/ofList/of/.)
>
> After that there are things we can do to reduce their footprint (in cycles
> and memory, and in user experience), so as to make them much more
> attractive as an everyday alternative to <clinit>-based fields.


I'm with you on that goal.


> Such “things” might be condensers (as Brian also mentions),


As much as I want condensers (please, let's get condensers asap!), I'm
concerned that features which depend on condensers to get good performance
will live in an unstable state unless we can make running condensers so
cheap and routine and natural that doing so fits in developers workflows -
especially when debugging from within their IDEs.  Features that are
painful (and I'm not saying CC's are) at development time, even if great at
deployment time, tend to have lower adoption and aren't "sticky" as devs
avoid them.

Unscrambling <clinit> is an essential part of the work Leyden needs to do
and CCs are a great step forward on that path.  Now the JVM needs to find a
way to routinely optimize them so even uncondensed uses are an obvious
choice for developers.



> or maybe language and JVM support.  In any case, the goal would be to help
> users make a clean escape from the clutches of <clinit>, for the sake of
> static tools.
>
> This will be a followup exercise to CC’s.  I’ve written some thoughts
> about that here, FTR:
>
> https://cr.openjdk.org/~jrose/leyden/after-computed-constants.html
>
> In short:  I think the sweet spot for a condenser target here is not an
> array of MHs as Brian suggests, but rather a single static method with a
> string-switch, and a framework for invoking it reflectively from a public
> BSM of some sort.
>
>
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