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.