Fix debbugs#16971

* lisp/emacs-lisp/lisp-mode.el (lisp-mode-variables): Set
`comment-use-syntax' to t to avoid the unnecessary runtime check.
Set `comment-start-skip' to a simpler value that doesn't try to
check if the semicolon is escaped (this is handled by
`syntax-ppss' now). 

* lisp/progmodes/scheme.el (scheme-mode-variables): Same.

lisp/ChangeLog

+2014-03-17  Dmitry Gutov  <dgutov@yandex.ru>
+
+	* emacs-lisp/lisp-mode.el (lisp-mode-variables): Set
+	`comment-use-syntax' to t to avoid the unnecessary runtime check.
+	Set `comment-start-skip' to a simpler value that doesn't try to
+	check if the semicolon is escaped (this is handled by
+	`syntax-ppss' now).  (Bug#16971)
+
+	* progmodes/scheme.el (scheme-mode-variables): Same.
+
 2014-03-16  Martin Rudalics  <rudalics@gmx.at>
 
 	Fix behavior of with-temp-buffer-window (Bug#16816, Bug#17007).

lisp/emacs-lisp/lisp-mode.el

@@ -449,15 +449,10 @@ font-lock keywords will not be case sensitive."
   (setq-local outline-level 'lisp-outline-level)
   (setq-local add-log-current-defun-function #'lisp-current-defun-name)
   (setq-local comment-start ";")
-  ;; Look within the line for a ; following an even number of backslashes
-  ;; after either a non-backslash or the line beginning.
-  (setq-local comment-start-skip "\\(\\(^\\|[^\\\\\n]\\)\\(\\\\\\\\\\)*\\);+ *")
-  ;; Font lock mode uses this only when it KNOWS a comment is starting.
-  (setq-local font-lock-comment-start-skip ";+ *")
+  (setq-local comment-start-skip ";+ *")
   (setq-local comment-add 1)		;default to `;;' in comment-region
   (setq-local comment-column 40)
-  ;; Don't get confused by `;' in doc strings when paragraph-filling.
-  (setq-local comment-use-global-state t)
+  (setq-local comment-use-syntax t)
   (setq-local imenu-generic-expression lisp-imenu-generic-expression)
   (setq-local multibyte-syntax-as-symbol t)
   ;; (setq-local syntax-begin-function 'beginning-of-defun)  ;;Bug#16247.

lisp/progmodes/scheme.el

@@ -140,11 +140,8 @@
   (setq-local add-log-current-defun-function #'lisp-current-defun-name)
   (setq-local comment-start ";")
   (setq-local comment-add 1)
-  ;; Look within the line for a ; following an even number of backslashes
-  ;; after either a non-backslash or the line beginning.
-  (setq-local comment-start-skip
-	      "\\(\\(^\\|[^\\\\\n]\\)\\(\\\\\\\\\\)*\\);+[ \t]*")
-  (setq-local font-lock-comment-start-skip ";+ *")
+  (setq-local comment-start-skip ";+[ \t]*")
+  (setq-local comment-use-syntax t)
   (setq-local comment-column 40)
   (setq-local parse-sexp-ignore-comments t)
   (setq-local lisp-indent-function 'scheme-indent-function)