Re: [RFC] caar/cadr/cdar/cddr

[Dmitry Antipov]

On 07/12/2012 08:53 PM, Paul Eggert wrote:

> How widely?  I've always been leery about those
> functions, myself.

$ grep -nHR "(car (car (" lisp | wc -l
45
$ grep -nHR "(car (cdr (" lisp | wc -l
164
$ grep -nHR "(cdr (car (" lisp | wc -l
45
$ grep -nHR "(cdr (cdr (" lisp | wc -l
84

> Since this is a performance
> improvement, how much does performance improve?

Not yet because byte-opt.el should be able to optimize
(car (cdr X)) -> (cadr X) etc. Or, all Lisp sources
should be converted manually :-).

> Quite possibly they're a win in the C implementation,
> but I'm not sure they're worth adding bytecodes for.

Calling them via Funcall is too slow.

> We might want to save bytecodes for something more
> important.

Do we really runs out of bytecode range?

> Assuming they're needed, I vote for a clean
> implementation of Fcaar etc. rather than tricky
> ones involving loops.  Especially since the tricky
> loops are wrong and will infinite-loop in some
> cases.

This Fcaar is 6 insns smaller than the obvious CAR (CAR (x))
implementation :-).

Dmitry