bug#55636: 27.2; etags performance fix when working with very big TAGS files

[Eli Zaretskii <eliz@gnu.org>]

> Cc: jurgen.de-backer.ext@eurocontrol.int, stef.van-vlierberghe@eurocontrol.int,
>  philippe.waroquiers@eurocontrol.int
> Date: Wed, 25 May 2022 16:04:04 +0000
> From:  Jurgen De Backer via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> We implemented a fix for making etags searching faster in our custom
> emacs build.
> In our project, sometimes a search could take more than 10 seconds.
> The fix is to expand filenames only if they are relative:
> 
> (defun expand-if-relative (a-file)
> (if (file-name-absolute-p a-file)
>     a-file
>   (expand-file-name a-file)
>   )
> )
> 
> and then use this function in the mapcar statement inside (defun
> tags-table-including ...), replacing expand-file-name:
>   ....
>   (if (member this-file (mapcar #'expand-if-relative
>                                           (tags-table-files)))
>                 ;; Found it.
>                 (setq found tables)
>                 )
>   ....

Thanks.  How much speedup does this bring in your use cases?  From 10
sec down to how long?

> Looking at expand-file-name doc, it does 2 things:
>   * make file name absolute
>   * and canonicalize it (removes the xxxx/.. dir components,
>        the . dir components, the double slashes).

It actually does more, like resolve the ~/ etc.

> We are wondering if the standard C function in emacs could do
> a fast "do nothing" when the file name is absolute and has
> nothing to cannonicalize.

I think testing for "nothing to canonicalize" is as complex as
canonicalizing the file name.