Re: [Emacs-diffs] emacs-25 3eb93c0: Rely on conservative stack scanning to find "emacs_value"s

[Paul Eggert]

On 03/29/2016 03:07 AM, Philipp Stephani wrote:
> - From a user's perspective, it removes NULL as a sentinel value to 
> signal an error.

It's not clear that this feature is worth the hassle of supporting it. 
As Stefan mentioned, one can easily use Qnil etc. as sentinels.

> - From a developer perspective, it adds low-level complexity, 
> bit-twiddling, and possibly undefined behavior to a module that should 
> only contain glue code between two interfaces.

The old approach was also complicated; its data structures were more 
complex and typically a bit slower, and it had more lines of code and it 
was typically more of a pain to debug at the C level. The new approach's 
use of undefined bit-twiddling isn't bad, given that even the old 
approach bit-twiddled with abandon by calling XCAR, XCDR and the like.

If the main objection here is seeing bit operations like "<<" in 
emacs-module.c, we could move uses of these operations from 
emacs-module.c to lisp.h so that emacs-module.c could be more abstract. 
Rearrange the deck chairs, so to speak.