bug#30539: 26.0; `char-displayable-p' is much slower in Emacs 25 and 26

[Noam Postavsky <npostavs@gmail.com>]

Drew Adams <drew.adams@oracle.com> writes:

> Can someone please confirm that they can repro this problem?

I can reproduce on a Windows 10 box.  It looks like something was being
cached before, where now it's not.  E.g., try the following function
(char-names as defined in your throw-mule-bug-2.el).  In Emacs 24,
there's only one slow call.

(defun my-test-each-char ()
  (interactive)
  (view-echo-area-messages)
  (pcase-dolist (`(,name . ,ch) char-names)
    (read-char (format "continue? (next: %s)" name))
    (let ((t0 (current-time))
          dt displayable)
      (setq displayable (char-displayable-p ch))
      (setq dt (subtract-time (current-time) t0))
      (message "%s display:%s (%fs)" name displayable (float-time dt)))))

Doing (setq inhibit-compacting-font-caches t) brings back reasonable
performance.

I can't reproduce on my GNU/Linux box, although that may just be due to
different fonts installed.  In particular, char-displayable-p never gave
me nil.