bug#65250: 30.0.50; "C-h f" is much slower on the master branch

[Eli Zaretskii]

> Cc: 65250@debbugs.gnu.org
> Date: Mon, 14 Aug 2023 15:19:16 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
>    'configure -C --prefix=/d/usr --with-wide-int
>    --enable-checking=yes,glyphs 'CFLAGS=-O0 -gdwarf-4 -g3''
> 
> Perhaps --enable-checking makes the difference?
> 
> If even that doesn't show the problem, just time the above and compare
> with Emacs 29: it's possible that the command is much faster on your
> system, but the question is it significantly slower than Emacs 29?

It sounds like the problem is the packages that Emacs needs to load
when comp-function-type-spec is called.  If I set force-load-messages
to t before invoking "C-h f", I see this in the *Messages* buffer:

  Loading help-fns...
  Loading cl-lib...
  Loading cl-loaddefs...done
  Loading cl-lib...done
  Loading help-mode...done
  Loading radix-tree...done
  Loading help-fns...done
  Loading thingatpt...done
  Loading dictionary...
  Loading dictionary-connection...done
  Loading external-completion...done
  Loading dictionary...done
  Loading lisp/emacs-lisp/comp.el (source)...
  Loading bytecomp...done
  Loading cl-extra...done
  Loading cl-macs...
  Loading gv...done
  Loading cl-macs...done
  Loading cl-seq...done
  Loading rx...done
  Loading subr-x...done
  Loading warnings...
  Loading icons...done
  Loading warnings...done
  Loading lisp/emacs-lisp/comp-cstr.el (source)...
  Loading pcase...done
  Loading lisp/emacs-lisp/comp-cstr.el (source)...done
  Loading derived...done
  Loading lisp/emacs-lisp/comp.el (source)...done
  Loading shortdoc...
  Loading text-property-search...done
  Loading shortdoc...done

Note the loading of comp.el and comp-cstr.el -- we load their source
files, not the *.elc files.  That's because in a build without native
compilation these two files are not byte-compiled.  I think loading of
these files, especially of comp.el, in source form is what slows down
the command.

I'm guessing your build was with native compilation?  Because in such
a build the "C-h f" command is indeed fast, especially after the
requisite *.el files are all native-compiled (i.e. starting from the
second Emacs invocation after the build).

So I think the patch I presented in my original report is exactly what
is needed here: the problem only happens in builds without
native-compilation, and in that case there's no reason whatsoever to
call comp-function-type-spec.  (And builds from a release tarball will
not see that problem, since the tarball comes with byte-compiled
comp.el and comp-cstr.el.)

Do you agree?