Re: [Caml-list] OCaml and Boehm

[Jon Harrop <jon@ffconsultancy.com>]

On Saturday 11 April 2009 20:17:58 Ed Keith wrote:
> --- On Sat, 4/11/09, Jon Harrop <jon@ffconsultancy.com> wrote:
> > From: Jon Harrop <jon@ffconsultancy.com>
> > Subject: Re: [Caml-list] OCaml and Boehm
> > To: caml-list@yquem.inria.fr
> > Date: Saturday, April 11, 2009, 10:27 AM
> >
> > Also, don't forget that many people incorrectly claim that smart pointers
> > deallocate at the earliest possible point when, in fact, they typically
> > keep values alive longer than necessary.
>
> Could elaborate on this? I'm having a hard time envisioning a situation
> where GC could free memory that smart pointers would not free.

Smart pointers deallocate when values fall out of scope. GC deallocates when 
it runs and values are unreachable.

Consider:

  let () =
    let x = ..
    f x
    g()

The value "x" stays in scope to the end of the block so a smart pointer will 
not deallocate it. The GC may well run during "g", realise that "x" is 
unreachable and deallocate it.

Note that there are further unwanted side effects of smart pointers here. 
Specifically, having to keep "x" around until the end of scope increases 
register pressure and makes it more likely to values will be spilled, which 
is a substantial performance cost.

-- 
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e