Re: Garbage Collection in Ada

[Hans-Juergen Boehm <boehm@mti.sgi.com>]

Rick Hudson wrote:
> 
> bobduff@world.std.com (Robert A Duff) writes:
> >
> > In article <3265BD97.41C6@mti.sgi.com>,
> > Hans-Juergen Boehm  <boehm@mti.sgi.com> wrote:
> > >For systems I've seen it does inhibit some optimizations, ar least to
> > >the extent that proper use of a conservative collector inhibits
> > >optimizations.  Especially in multithreaded systems, there must be a
> > >recognizable pointer to every accessible object at every program point
> > >at which a GC might occur.  Often this is between every pair of
> > >instructions.  In a nonconservative system you could generate
> > >arbitrarily complex tables to allow reconstruction of those pointers.
> 
> You could but you don't have to. If GC is triggered only by an allocate then
> one could roll forward the threads to a 'gc' point. This greatly reduces the
> amount of table information needed.
> 

This works well if you can reliably and reasonably quickly advance
threads to the next GC point without forcing them to execute code at the
GC point when they don't need to stop.  In some systems that's easy.  In
some that's hard.  The thread systems I've used can't easily do that.

-- 
Hans-Juergen Boehm
boehm@mti.sgi.com