Re: Conservative GC isn't safe

[Daniel Colascione <dancol@dancol.org>]

On 11/28/2016 11:37 AM, Eli Zaretskii wrote:
>> From: Daniel Colascione <dancol@dancol.org>
>> Date: Mon, 28 Nov 2016 11:18:32 -0800
>> Cc: emacs-devel@gnu.org
>>
>> Let me ask again: we already have all the runtime data we need for more
>> conservative GC. Where is the resistance to the idea coming from?
>
> I already answered that up-thread: it will be dead code, and thus will
> likely do the wrong thing if it ever runs.

We'll always have stray pointers to object interiors that will exercise 
these code paths. I've broken enough of them recently enough to know.

> I also suggested what to do instead: add assertions that express what
> we believe should never happen.  Stefan says doing that is unlikely to
> be justified by the dangers, but if we think so, then we shouldn't be
> afraid of the problem in the first place.  If we are, then adding
> assertions is the way to go.

It's not possible to assert, statically or dynamically, that we don't 
have this problem. If we could, we wouldn't need conservative GC at all, 
since we'd know the exact locations of all pointers.