Re: robustness vs. conservative GC

[Fergus Henderson <fjh at cs dot mu dot oz dot au>]

On 30-Jul-2001, Tom Lord <lord@regexps.com> wrote:
> 
> 2. In the face of an aggressively optimizing C compiler, one that
>    sometimes disguises pointers, conservative GC can fail to protect
>    some live objects, resulting in program failures.  This is a 
>    famous limitation of conservative GC.
> 
>    Sticking to a specific compiler and options known not to disguise
>    pointers avoids the bug, but is a fragile solution.

I agree.  It would be better to have a specific compiler option, called say
`-fdont-hide-pointers' or `-fconservative-gc', which guarantees that the
compiler won't perform any such optimizations.

Does anyone know of any current GCC optimizations which might hide pointers?

> In my experience, both bugs (#1 and #2) are likely to eventually show
> up in deployed systems,

If you have specific experience of #2 occurring in deployed systems,
I'd be interested in hearing about that.

> Fergus Henderson made the claim that an exact GC implemented without
> compiler support is likely to perform worse than one implemented with
> compiler support.  I don't know what "likely" means in that claim, but
> I don't see any a priori reason to believe there will necessarily be
> performance problems.

Any scheme for doing exact GC without cooperation from the compiler
back-end needs to store pointers in a known location on the stack, rather
than in registers, so that they can be updated when objects are moved.
This denies the back-end compiler freedom to put such pointers in
registers.  Register allocation obviously has a big effect on performance.

That's my a priori reason.

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.