bug#18410: Use SAFE_ALLOCA etc. to avoid unbounded stack allocation.

[Dmitry Antipov]

On 09/05/2014 10:08 AM, Paul Eggert wrote:

> Attached is a patch to fix the unbounded alloca calls that I found when 
> auditing the Emacs source.
> I'm sending this to bug-gnu-emacs to give Eli a heads-up, as some of the fixes 
> affect Windows code.
> This patch is relative to Emacs trunk bzr 117822.

Code like:

USE_SAFE_ALLOCA;                                                    |-
ptrdiff_t count = SPECPDL_INDEX ();                    |-           |
...                                                    | inner bind | outer bind
Lisp_Object result = unbind_to (count, Fsome_func ()); |-           |
SAFE_FREE ();                                                       |-
return result;

looks suboptimal because it calls unbind_to twice.  May be we need 
SAFE_FREE_RETURN,
somewhat similar to RETURN_UNGCPRO?  I.e. we should be able to say:

USE_SAFE_ALLOCA;
ptrdiff_t count = SPECPDL_INDEX ();
...
SAFE_FREE_RETURN (Fsome_func ());

Minor note: why specbind can't return previous binding level?
To avoid extra typing, someone can write:

ptrdiff_t count = specbind (Qsome_var, Qnil);

instead of:

ptrdiff_t count = SPECPDL_INDEX ();
specbind (Qsome_var, Qnil);

Dmitry