Re: eval proposals

[Paul Eggert]

Dmitry Antipov wrote:

> 1) Protection against C stack overflow caused by enormously huge
>     `max-lisp-eval-depth'.

Can't we rely on the virtual memory hardware to have a guard page, so 
that Emacs doesn't need to check for stack overflow in software?  That 
is, Emacs could trap the SIGSEGV and do the right thing.

Perhaps we do need a software check on some platforms, but in that case 
'stack_guard' should be a static function, not a macro.  Also, it does 
some computation that could be done once, so that checking for stack 
overflow is just a compare-and-branch.

> 2) Capability to shrink specpdl stack if it becomes too large.

Multiplying (specpdl_ptr - specpdl) by SPECPDL_SHRINK_FACTOR could have 
problems if the multiplication overflows.  Choose a power-of-two 
SPECPDL_SHRINK_FACTOR so that you can do the overflow test quickly by 
division (unsigned right shift) instead.