Re: Why have a #if .... #else .... #endif construct in Emacs Lisp, when we could make the existing code DTRT unchanged?

[Alan Mackenzie]

Hello, Richard.

On Mon, Sep 04, 2023 at 20:30:25 -0400, Richard Stallman wrote:
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>   > How about making the byte compiler recognize the construct
 
>   >  (if (< emacs-major-version NUMBER) ...)

>   > and do this optimization on it?

> People seem not to have considered this seriously, ....

It was actually the first thing I considered when the original problem
occurred to me.  We have a similar optimisation in place for (featurep
xemacs).  But then, the number of ways emacs-major-version (to say
nothing of emacs-minor-version) can be used is large (starting with <,
<=, >, >=, =, /=, eq, eql, equal, and extending to predicates involving
e-minor-v), that it would be at least somewhat difficult to apply in
practice.

> .... but I have not seen any serious discussion of a drawback.

I outlined some drawbacks in my post of yesterday evening (Monday
2022-09-04), European time.

> What flaw or drawback do people see in it?

The above.  Plus, some (possibly most) of the predicates we want to
optimise out are expressed in terms of present/absent features rather
than numeric version numbers.  There are  just 45 occurrences of
emacs-major-version in the Lisp sources, over 2000 occurrences of boundp
and fboundp.  We could convert (some of) those 45 to use static-if in
less time than it would take to amend the interpreter and byte-compiler
to handler emacs-major-version specially.

> It should optimize the existing the existing code with no change at
> all.  Isn't that just perfect?

Sadly, no it's not.  It would only do part of the job which is to be
done.  It would make some parts of Emacs more complicated.


> -- 
> Dr Richard Stallman (https://stallman.org)
> Chief GNUisance of the GNU Project (https://gnu.org)
> Founder, Free Software Foundation (https://fsf.org)
> Internet Hall-of-Famer (https://internethalloffame.org)

-- 
Alan Mackenzie (Nuremberg, Germany).