Subj : Re: OO compilers and efficiency
To   : comp.programming
From : Rob Thorpe
Date : Mon Jul 25 2005 02:23 am

Chris Dollin wrote:
> Rob Thorpe wrote:
>
> > Chris Dollin wrote:
>
> >> [And of course this isn't a property of Java particularly; isn't the
> >>  same true in Lisp, Smalltalk, Pop11, SML, OCaml, etc?]
> >
> > No, not all of them.
> >
> > Let's call the commonly used data structures of each of those languages
> > "things", for want of a better word.  In many of those languages the
> > compiler can (and will) allocate those things on the stack so long as
> > no pointer to them could possibly leave the context.
>
> In what way is it different for Java? If no pointer to a `new Spoo()`
> can leave the context, then why can't a Java compiler allocate the
> Spoo on the stack?
>
> [Here, the `compiler` is whatever post-processes the bytecode at
>  runtime, not the Java source -> classfile compiler.]
>
> I can see that the analysis for "no pointer could possibly leave
> the context" can be harder, much harder, because of polymorphism.
> So mayhap a Java compiler can stackallocate much less often for
> just this reason. Is that the claim?
>
> [Do you happen to have cites for any of Lisp, Smalltalk, Pop11, SML,
>  or OCaml doing stack-allocation, 'cos I'd like to chase them up.
>  I'm sure I remember reading stuff about Lisp compilers attempting
>  it; I'm equally - or more - sure that Pop11 doesn't do it.]

The question here is what languages can allocate more than just simple
data structures on the stack.  I had a look at the sources of some of
the languages you mention above.  I found: -

SML/OCaml - Only allocate simple things on the Stack
Pop11 - Couldn't find out about this one, I'll take your word on it.
Lisp - Allocates anything you want on the stack if the programmer
specifies "dynamic-extent" but will cannot do so automatically.
Smalltalk - Smalltalk "contexts" are held in a stack, but this makes
little odds since "new:" is not a language intrinsic, it's a class
method.

So, I've successfully proved myself wrong.

(Think of this as a mid-thread batting collapse.  I'll make it up in
the second test, promise.)

.