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

[Samuel Bronson]

On Jul 12, 2012, at 8:12 PM, Stefan Monnier 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
>
> Static counts might not be related to dynamic counts and don't  
> indicate
> much either in terms of actual performance gains.
>
> FWIW the current cXXr implementation using `defsubst' is not very
> efficient because they're written in subr.el which does not use  
> lexical
> scoping, so the bytecodes that get inlined by the compiler include an
> unneeded "varref x".  So you'd already get better performance by  
> simply
> moving these definitions to a file using lexical-binding.

Hmm. I see what you mean:

(disassemble '(lambda (x) (caar (cadr x))) (current-buffer))

nil
byte code:
  args: (x)
0       varref    x
1       dup     
2       varbind   x
3       cdr     
4       car     
5       unbind    1
6       dup     
7       varbind   x
8       car     
9       car     
10      unbind    1
11      return  

Surely the bytecode compiler can be made to do better with this; it's  
currently much more efficient to use car and cdr directly:

(disassemble '(lambda (x) (car (car (car (cdr x))))) (current-buffer))
nil
byte code:
  args: (x)
0       varref    x
1       cdr     
2       car     
3       car     
4       car     
5       return