bug#35316: 26.2; Emacs lags in c++-mode buffer when editing with iedit-mode on

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> Reproducing steps:
> 1. emacs -Q
> 2. Eval code: (add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/“))
> 3. install the package iedit (version from melpa)
> 4. open the attachment c++ file, and goto line 262 and column 17. cursor
> will be on word “subsession"
> 5. M-x narrow-to-defun
> 6. M-x iedit-mode
> 7. M-x widen
> 8. You will see the lag when inputting chars.

This seems to be a good use case for my syntax-propertize patch.
I just tried it on your test case and while there is still a slight slow
down, it seemed to be much less problematic.

BEWARE: it likely introduces bugs.


        Stefan


diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
index 49268c4482..31f5ecdfdb 100644
--- a/lisp/progmodes/cc-mode.el
+++ b/lisp/progmodes/cc-mode.el
@@ -506,6 +506,8 @@ c-just-done-before-change
 ;; and `after-change-functions'.  Note that this variable is not set when
 ;; `c-before-change' is invoked by a change to text properties.
 
+(defvar c--use-syntax-propertize t)
+
 (defun c-basic-common-init (mode default-style)
   "Do the necessary initialization for the syntax handling routines
 and the line breaking/filling code.  Intended to be used by other
@@ -648,12 +650,17 @@ c-basic-common-init
 
   ;; Install the functions that ensure that various internal caches
   ;; don't become invalid due to buffer changes.
-  (when (featurep 'xemacs)
-    (make-local-hook 'before-change-functions)
-    (make-local-hook 'after-change-functions))
-  (add-hook 'before-change-functions 'c-before-change nil t)
-  (setq c-just-done-before-change nil)
-  (add-hook 'after-change-functions 'c-after-change nil t)
+  (if c--use-syntax-propertize
+      (setq-local syntax-propertize-function
+		  (lambda (start end)
+		    (c-before-change start (point-max))
+		    (c-after-change start end (- end start))))
+    (when (featurep 'xemacs)
+      (make-local-hook 'before-change-functions)
+      (make-local-hook 'after-change-functions))
+    (add-hook 'before-change-functions 'c-before-change nil t)
+    (setq c-just-done-before-change nil)
+    (add-hook 'after-change-functions 'c-after-change nil t))
   (when (boundp 'font-lock-extend-after-change-region-function)
     (set (make-local-variable 'font-lock-extend-after-change-region-function)
          'c-extend-after-change-region))) ; Currently (2009-05) used by all
@@ -711,15 +718,17 @@ c-common-init
     (widen)
     (setq c-new-BEG (point-min))
     (setq c-new-END (point-max))
-    (save-excursion
-      (let (before-change-functions after-change-functions)
-	(mapc (lambda (fn)
-		(funcall fn (point-min) (point-max)))
-	      c-get-state-before-change-functions)
-	(mapc (lambda (fn)
-		(funcall fn (point-min) (point-max)
-			 (- (point-max) (point-min))))
-	      c-before-font-lock-functions))))
+    (unless c--use-syntax-propertize
+      (save-excursion
+	(let (before-change-functions after-change-functions)
+	  (mapc (lambda (fn)
+		  (funcall fn (point-min) (point-max)))
+		c-get-state-before-change-functions)
+	  (mapc (lambda (fn)
+		  (funcall fn (point-min) (point-max)
+			   (- (point-max) (point-min))))
+		c-before-font-lock-functions)
+	  ))))
 
   (set (make-local-variable 'outline-regexp) "[^#\n\^M]")
   (set (make-local-variable 'outline-level) 'c-outline-level)
@@ -1954,6 +1963,12 @@ c-font-lock-fontify-region
   ;;
   ;; Type a space in the first blank line, and the fontification of the next
   ;; line was fouled up by context fontification.
+  (when c--use-syntax-propertize
+    ;; This should also update c-new-END and c-new-BEG.
+    (syntax-propertize end)
+    ;; FIXME: Apparently `c-new-END' may be left unchanged to a stale value,
+    ;; presumably when the buffer gets truncated.
+    (if (> c-new-END (point-max)) (setq c-new-END (point-max))))
   (let (new-beg new-end new-region case-fold-search)
     (if (and c-in-after-change-fontification
 	     (< beg c-new-END) (> end c-new-BEG))
@@ -1992,7 +2007,8 @@ c-font-lock-fontify-region
 (defun c-after-font-lock-init ()
   ;; Put on `font-lock-mode-hook'.  This function ensures our after-change
   ;; function will get executed before the font-lock one.
-  (when (memq #'c-after-change after-change-functions)
+  (when (and c--use-syntax-propertize
+	     (memq #'c-after-change after-change-functions))
     (remove-hook 'after-change-functions #'c-after-change t)
     (add-hook 'after-change-functions #'c-after-change nil t)))
 
@@ -2046,11 +2062,14 @@ c-extend-after-change-region
   (when (eq font-lock-support-mode 'jit-lock-mode)
     (save-restriction
       (widen)
+      ;; FIXME: This presumes that c-new-BEG and c-new-END have been set
+      ;; I guess from the before-change-function.
       (c-save-buffer-state () ; Protect the undo-list from put-text-property.
 	(if (< c-new-BEG beg)
 	    (put-text-property c-new-BEG beg 'fontified nil))
 	(if (> c-new-END end)
-	    (put-text-property end c-new-END 'fontified nil)))))
+	    (put-text-property end (min c-new-END (point-max))
+			       'fontified nil)))))
   (cons c-new-BEG c-new-END))
 
 ;; Emacs < 22 and XEmacs