Status of Anno Class loader with constant patches?

[John Rose]

On Dec 28, 2010, at 1:53 PM, Rémi Forax wrote:

> We, the JSR292 EG, think that most of the VMs will be able to optimize this.

Thanks for answering Mark's question so fully, Remi.

"Live constants" are definitely one of the use cases that invokedynamic is designed for.

The design is a little indirect:  You bind a call site permanently to a K combinator, which always returns your desired value.

(The K combinator is K(x) := lambda(){x}.  See http://en.wikipedia.org/wiki/SKI_combinator_calculus .)

But the design is also general:  An invokedynamic call site can take arguments.

This opens the door to "almost constant" values.  Many languages feature templated value constructors, in which most of the constructor expression is a constant template, with little bits of inserted data.

   Shell:  "foo $x bar 12"
   Lisp:  `(foo ,x bar 12)
   JavaScript:  {foo: x, bar: 12}
   Java+Lambda: {PrintStream out -> out.println("foo "+x+" bar 12")}

In all the above cases, a variable value x is stuffed into the middle of a mostly-constant value (string, quasiquoted list, object literal, closure, respectively).

Invokedynamic allows you a hook to pass the constant parts of the template as bootstrap method parameters, and the non-constant parts as normal stacked invocation parameters.  The bootstrap method is responsible for spinning a factor method which will take the non-constant parts and build the requested patterned value.  There are lots of interesting language-specific optimizations possible here.

From this viewpoint, a "live constant" is simply a zero-argument templated value, and the K combinator is its constructor.

Best wishes,
-- John