Subj : Re: OO compilers and efficiency
To   : comp.programming
From : Rob Thorpe
Date : Mon Jul 25 2005 03:23 am

Jon Harrop wrote:
> Chris Dollin wrote:
> > Rob Thorpe wrote:
> >> 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.]
>
> I wrote a mini Mathematica implementation for Wolfram Research in OCaml (the
> relevant parts of the real Mathematica are written in C). My code turned
> out to be unintentionally robust when interpreting recursive Mathematica
> code because the OCaml compiler allocates mostly on the heap and uses very
> small stack frames. In contrast, the C allocated mainly on the stack,
> giving huge stack frames and quickly exhausting stack space.
>
> So, while stack allocation can run slightly faster, it is no good if it runs
> out of resources after only 4ms. So you should be trying to avoid stack
> allocation!

That's rather odd, did the system you were programming on have a
particularly small stack?  Or did the C code allocate lots of
unnecessary information on the stack?  Was it just the lack of GC
acting on the stack?

Most of the time you can put a similar amount of data in either the
stack or heap.

.