Re: fast subshell

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

On Sun, Apr 1, 2018 at 8:59 AM, Ray Andrews <rayandrews@xxxxxxxxxxx> wrote:
>
>     local IFS=$'\n' # Must split on newlines only.
>     all_matches=( $( whence -mavS $@ ) )
>     IFS=$OLD_IFS
>
> (
>     IFS=$'\n' # Must split on newlines only.
>     all_matches=( $( whence -mavS $@ ) )
> )
>
> ... and the interesting thing is that the former executes a stress test in
> ca. 290 mS, but the latter in ca. 250 mS.  The difference isn't significant
> in the real world, but it is hard to understand, it seems strange that the
> subshell would be faster. Is this possible?

$(...) is always going to create one subshell.

How expensive a subshell is, may depend on several things:  How the
operating system implements process forking; how much memory your
current shell is using at the time; how busy the system otherwise is;
what kind of multithreading your processor supports; and so on.

It's quite possible that doing the memory management for the
assignment to all_matches in a newly forked process is faster than in
the original shell (perhaps because the subshell never needs to free
it again?).  There is also some overhead involved in saving/restoring
IFS with "local" which you are avoiding.

Also there's really no reason to do both "local IFS=..." and
"IFS=$OLD_IFS", the whole point of the "local" is to allow zsh to do
the save/restore of IFS for you.  If you only need the change of $IFS
for "whence" then you can do it with an anonymous function:

  (){ local IFS=$'\n'; all_matches=( $( whence -mavS $@ ) ) }

Or you can do it without changing IFS at all, like this:

  all_matches=( ${(f)"$( whence -mavS $@ )"} )