Re: mark_stack () vs GCPROn

[Dmitry Antipov]

On 12/06/2012 01:00 PM, Sergey Mozgovoy wrote:

> Emacs has a `mark_stack' function in alloc.c, which looks for (potential)
> Lisp_Objects located on the current C stack.  Does it mean that GCPROn
> mechanism is not necessary for local Lisp_Object variables now ?

If stack marking is supported, then yes in general; but GCPROs are also
used for debugging. This is controlled by GC_MARK_STACK in lisp.h.

> It is quite clear that gcprolist is still necessary for static Lisp_Objects.

IIUC you mix staticpro and GCPRO.

> What are the relationships between these 2 approaches for marking objects ?

In short, GCPRO is faster because you don't need to check whether the word
in memory is a Lisp_Object. But stack marking is much more useful because
you don't need to check whether C code calls Feval (and so potentially
Fgarbage_collect) and so you don't worry about protecting local Lisp_Objects.

BTW, the more important distinction is that the GCPRO-assisted collection
is exact: you always know where the Lisp_Objects are, and mark them. Stack
marking is conservative, e.g. treats everything which looks like
the valid Lisp_Object as Lisp_Object. For example, if you have Lisp_Object
at 0x12345678 and this object is not accessible from other objects, but
there is an integer value 0x12345678 somewhere on C stack, the object will
be marked. This way the collector doesn't reclaim some dead objects, which
is impossible if GCPRO is used.

Dmitry