bug#17168: 24.3.50; Segfault at mark_object

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> Anyway, I'd like to get a fix into emacs-24 soon so we can make sure
> we've fixed the GC bug.

I installed a fix into emacs-24, which lets all symbols be uninterned.

> Are you vetoing the general approach used in this patch?

No: I think disallowing unintern is a good idea, but not for emacs-24.

Indeed, as it turns out, the only non-pure objects referenced from pure
space are symbols and distinguishing uninterned from interned reduces
the number of such "pinned" objects from about 10K to about 250.

Rather than scan all symbols to find the pinned ones, the code
I installed into emacs-24 keeps a pointer to the first symbol_block
that contains a pinned symbol.  This way we only scan about 15K symbols
at the beginning of every GC cycle to mark those 10K pinned symbols.
Compared to keeping a vector of 10K object, this seems like
a good tradeoff.

For trunk, we could disallow uninterning pinned symbols, at which point
it's worth the trouble to build a vector of those 250 remaining
pinned symbols.


        Stefan