bug#17168: 24.3.50; Segfault at mark_object

[Daniel Colascione]

On 04/07/2014 09:28 AM, Stefan Monnier wrote:
>> 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.

Thanks. I'll install my change in trunk. How should we prevent your
change merging into trunk?

> 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.

It's unfortunate that we still have to mark Vobarray even though we're
separately marking most of the symbols it contains, but I suppose it
doesn't matter all that much: because we've already marked most of the
symbols in the interned symbol chains, we'll short-circuit in
mark_object anyway.

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