bug#36032: 27.0.50; Massive GC when shift-selecting text in HELLO

[martin rudalics <rudalics@gmx.at>]

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 > To reproduce:
 >
 >    emacs -Q
 >    C-h h
 >    C-SPC
 >    C-f C-f C-f ....
 >
 > You will see a ~0.5 sec delay after each C-f, before its effect of
 > extending the region is visible.  The reason is GC after every
 > keystroke, and it seems to be caused by the changes in commit 479f51a.
 > That commit caused mode-line-default-help-echo to be called each time
 > Emacs needs to redisplay the mode line, and that seems to produce lots
 > of garbage, at least with the HELLO file.

Here the delays are considerably larger (one second at least) and
there is at least one GC after each keystroke, sometimes two.  But I
can't confirm the remainder of what you say.  Attached find two small
files.

Loading the first one (show-gc.el) with emacs -Q gives approximately
the same behaviors with Emacs 26.2.50 (which does not have the
'mode-line-default-help-echo' function) and Emacs 27.0.50 - at least
one GC per keystroke here.

Loading the second one (show-gc-he.el) with emacs -Q (Emacs 27.0.50
only) shows that there is one call of 'mode-line-default-help-echo'
per keystroke and I doubt that one such call can produce that much
garbage.

Also, customizing the option 'mode-line-default-help-echo' to nil does
not make any performance difference here.

All tested with 64-bit MSYS2 builds on Windows 10.  If you get
different results please tell me.  Also I have not installed the
Harfbuzz branch so OMMV.

 > If this function cannot be optimized to produce less garbage, could we
 > perhaps somehow cache the value of the help-echo and not recalculate
 > it unless something really changed?

This is certainly a good idea.  But I doubt it would have any impact
in the case at hand.

martin

[-- Attachment #2: show-gc.el --]
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(defvar gc 0)
(add-hook 'post-gc-hook (lambda () (setq gc (1+ gc))))
(defun show-gc () (message "gc: %s" gc))
(add-hook 'post-command-hook 'show-gc)

[-- Attachment #3: show-gc-he.el --]
[-- Type: text/plain, Size: 1936 bytes --]

(defvar gc 0)
(defvar he 0)
(add-hook 'post-gc-hook (lambda () (setq gc (1+ gc))))
(defun show-gc-he () (message "gc: %s ... he: %s" gc he))
(add-hook 'post-command-hook 'show-gc-he)

(defun mode-line-default-help-echo (window)
  "Return default help echo text for WINDOW's mode line."
  (let* ((frame (window-frame window))
         (line-1a
          ;; Show text to select window only if the window is not
          ;; selected.
          (not (eq window (frame-selected-window frame))))
         (line-1b
          ;; Show text to drag mode line if either the window is not
          ;; at the bottom of its frame or the minibuffer window of
          ;; this frame can be resized.  This matches a corresponding
          ;; check in `mouse-drag-mode-line'.
          (or (not (window-at-side-p window 'bottom))
              (let ((mini-window (minibuffer-window frame)))
                (and (eq frame (window-frame mini-window))
                     (or (minibuffer-window-active-p mini-window)
                         (not resize-mini-windows))))))
         (line-2
          ;; Show text make window occupy the whole frame
          ;; only if it doesn't already do that.
          (not (eq window (frame-root-window frame))))
         (line-3
          ;; Show text to delete window only if that's possible.
          (not (eq window (frame-root-window frame)))))
    (setq he (1+ he))
    (when (or line-1a line-1b line-2 line-3)
      (concat
       (when (or line-1a line-1b)
         (concat
          "mouse-1: "
          (when line-1a "Select window")
          (when line-1b
            (if line-1a " (drag to resize)" "Drag to resize"))
          (when (or line-2 line-3) "\n")))
       (when line-2
         (concat
          "mouse-2: Make window occupy whole frame"
          (when line-3 "\n")))
       (when line-3
         "mouse-3: Remove window from frame")))))