Re: Garbage Collection in Ada

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Lars replies to me

"
> it is interesting to note that the mixture of
> two such fiddles can sometimes go astray (as is the case with conservative
> GC, and manual virtual origin mucking)

If you were someone other than Prof. Dewar I'd be tempted to say: "No,
you are confused " ;-) User code is not the same thing as the runtime
system provided by the implementation. The GC should be provided by the
impementation."


I reply to the reply:

But this is, so far, a fantasy. The only kind of GC provided by standard
implementations, at least in my experience, is proper accurate GC, that is
of course embedded into the compiler or operating system or both.

You can certainly imagine a system that provides conservative garbage
collection as a standard part of a compiler, or even just a standard
system library. But I don't know of any such commercial products, do you?

So in practice, a conservative garbage collector is just a bit of application
code which you get from somewhere and include in your application. It also
happens to be application code that is fiddling with C semantics. Hence my
comment definitely does apply!

Actually, as I have noted before, I am dubious about real applications
depending on conservative GC, so I can't see it being an integral part
of a language implementation. For example, I can see putting a conservative
collector interface into some user library for GNAT, but not into the main
language library.

Actually I think that Geert is indeed working on interfacing the HJB 
CGC to GNAT (that's why he complained about the possibility of us moving
to use virtual origins, something we definitely intend to do at some point,
so presumably this conservative GC would stop working if we did that, unless
it got more clever (more clever is possible here, but difficult, i.e. the
scan of memory could recognize in addition to simple direct pointers, possible
instances of virtual origin descriptors, they have a perfetly well defined
format.