New

[Peter Levart]

On 03/29/10, Mark Thornton wrote:
> Howard Lovatt wrote:
> > 3. The main argument for not using new is that the compiler can 'lift' the
> > lambda to be static if it doesn't access any instance fields or local
> > variables. However this type of optimization has a poor history in Java,
> > think String interning and copying of static fields.

I never had any trouble as a consequence of interned Strings. What do you mean by "copying of static fields"?

> > In Java a JVM
> > optimization has proven much more successful than a compiler optimization.
> > Hence using new, which would allow a JVM optimization but prevent a compiler
> > optimization, is the best choice.
> >   
> On the other hand the JVM optimisations (escape analysis) to reduce heap 
> allocation have been "coming soon" for more years than I care to remember.
> 

But it has arrived, hasn't it? It's in JDK 1.6.0_14 (check -XX:+DoEscapeAnalysis)...

The thing to note is that such escape analysis is only effective if it can positively determine that an object instance (constructed with new) can not escape local computation. If someone wanted to use escape analysis to optimize instantiation of lambdas then such analysis could only be effective if the instantiation of lambda and invocation of it was peformed as part of the same local computation. For example (in term's of Lowatt's syntax):

{
    #<int()> lambda = new #<int()>(42);
    assert 42 == lambda.();
}

I think that such usages are useless (why creating a lambda in order to only invoke it afterwards).

Other usages involve passing lambda instance to some method. In those cases escape analysis can't help if those methods are not aggresively inlined. I think that relying on such situations would render optimization in case of lambdas practically non-effective.

I think that specifying lambda identity by not specifying it is so far the most promissing direction to enable optimizations. This unfortunately conflicts with "new" semantics but I don't find this as a drawback for specifying a usefull syntax that doesn't include "new".

Regards, Peter