bug#41646: Startup in Windows is very slow when load-path contains many entries.

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  41646@debbugs.gnu.org
> Date: Fri, 14 Aug 2020 19:22:26 +0200
> 
> > That was a very good idea. The attached patch reduces time spent
> > in openp() during startup by 5 seconds, or 15%.
> 
> That's a huge speed-up indeed.
> 
> [...]
> 
> > -           fd = emacs_open (pfn, O_RDONLY, 0);
> > +                /*  In some systems (like Windows) finding out if a
> > +                    file exists is cheaper to do than actually opening
> > +                    it.  Only open the file when we are sure that it
> > +                    exists.  */
> > +#ifdef WINDOWSNT
> > +                if (faccessat (AT_FDCWD, pfn, R_OK, AT_EACCESS))
> > +                  fd = -1;
> > +                else
> > +#endif
> > +                  fd = emacs_open (pfn, O_RDONLY, 0);
> > +
> 
> This is a Windows-specific patch, so I have no opinion on it.  Eli?
> Does this make sense to you?

Not sure.  15% sounds too little for having OS-specific code in that
place.  I hoped it will be much more.