Re: Scrolling huge buffers and cc-mode

[Alan Mackenzie <acm@muc.de>]

Hello, Dmitry,

On Wed, Dec 19, 2012 at 11:20:55AM +0400, Dmitry Antipov wrote:
> It looks like the reason of slow scrolling is how the C++ namespaces are handled.

> Basically my test file is:

> namespace X {
> namespace Y {

> /* 13K lines of code */

> } }

> /* middle-point */

> namespace X {                      <==============================================
> namespace Y {

> /* 13K lines of code */

> } }

> When scrolling reaches middle-point, everything hangs, most probably because
> we need to scan huge regions (first and second namespace blocks). When I remove
> all namespace definitions (with matched '}'), scrolling works much faster.

What is happening is that (c-beginning-of-statement-1 ...) is being
called repeatedly on the <===== line.
o - This moves to the beginning of "namespace X {", 
o - then calls (scan-sexps (point) -1),
o - which has to scan over the 13k lines of code.  This is slow.

The solution appears to be to give the backward search a search limit.
Could you try out the following patch please, and let me know if it works
OK:




diff -r ce04d3763229 cc-fonts.el
--- a/cc-fonts.el	Thu Dec 20 20:11:22 2012 +0000
+++ b/cc-fonts.el	Thu Dec 20 21:58:50 2012 +0000
@@ -1558,6 +1558,7 @@
   ;; prevent a repeat invocation.  See elisp/lispref page "Search-based
   ;; Fontification".
   (let* ((paren-state (c-parse-state))
+	 (decl-search-lim (c-determine-limit 1000))
 	 decl-context in-typedef ps-elt)
     ;; Are we in any nested struct/union/class/etc. braces?
     (while paren-state
@@ -1566,7 +1567,7 @@
       (when (and (atom ps-elt)
 		 (eq (char-after ps-elt) ?\{))
 	(goto-char ps-elt)
-	(setq decl-context (c-beginning-of-decl-1)
+	(setq decl-context (c-beginning-of-decl-1 decl-search-lim)
 	      in-typedef (looking-at c-typedef-key))
 	(if in-typedef (c-forward-token-2))
 	(when (and c-opt-block-decls-with-vars-key




> Dmitry

-- 
Alan Mackenzie (Nuremberg, Germany).