bug#41029: Improve ido-switch-buffer performance when many buffers are open

[Arnold Noronha <arnold@tdrhq.com>]

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Many of the buffer locals (in particular I think it's the
SYMBOL_FORWARDED types --- my terminoly may be off), appear to be O(n)
to bind, where n is the number of live buffers.

I tried to see if I can optimize that binding process directly, but
couldn't figure out a way to do it without changing current behavior,
so I opted to just optimize ido-switch-buffer directly.

After this patch, the performance is quite bearable at ~6000 buffers,
but still not ideal.

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From 3bc4eb78c365f4b4a387451f0b55264076a97c4b Mon Sep 17 00:00:00 2001
From: Arnold Noronha <arnold@tdrhq.com>
Date: Sat, 2 May 2020 08:30:47 -0700
Subject: [PATCH] Avoid binding CASE-FOLD-SEARCH in tight loop

IDO-IGNORE-ITEM-P is called for every buffer when switching
buffers. However, that function binds CASE-FOLD-SEARCH, which
appears to be an O(n) operation where n is (length (buffer-list)).
So we avoid binding it if we know we don't need to as a workaround.

To test this, this this:

(loop for i from 0 to 6000
      do
      (find-file (format "/tmp/xy%d.txt" i)))

Then try switching buffers with and without this patch.
---
 lisp/ido.el | 67 ++++++++++++++++++++++++++++-------------------------
 1 file changed, 36 insertions(+), 31 deletions(-)

diff --git a/lisp/ido.el b/lisp/ido.el
index 81883402ad..d7a24ae1d7 100644
--- a/lisp/ido.el
+++ b/lisp/ido.el
@@ -3408,7 +3408,7 @@ ido-make-merged-file-list
 (defun ido-make-buffer-list-1 (&optional frame visible)
   "Return list of non-ignored buffer names."
   (delq nil
-	(mapcar
+       (mapcar
 	 (lambda (x)
 	   (let ((name (buffer-name x)))
 	     (if (not (or (ido-ignore-item-p name ido-ignore-buffers) (member name visible)))
@@ -3823,36 +3823,41 @@ ido-ignore-item-p
       (and
        ido-process-ignore-lists re-list
        (save-match-data
-	 (let ((ext-list (and ignore-ext ido-ignore-extensions
-			      completion-ignored-extensions))
-	       (case-fold-search ido-case-fold)
-	       ignorep nextstr
-	       (flen (length name)) slen)
-	   (while ext-list
-	     (setq nextstr (car ext-list))
-	     (if (cond
-		  ((stringp nextstr)
-		   (and (>= flen (setq slen (length nextstr)))
-			(string-equal (substring name (- flen slen)) nextstr)))
-		  ((functionp nextstr) (funcall nextstr name))
-		  (t nil))
-		 (setq ignorep t
-		       ext-list nil
-		       re-list nil)
-	       (setq ext-list (cdr ext-list))))
-	   (while re-list
-	     (setq nextstr (car re-list))
-	     (if (cond
-		  ((stringp nextstr) (string-match nextstr name))
-		  ((functionp nextstr) (funcall nextstr name))
-		  (t nil))
-		 (setq ignorep t
-		       re-list nil)
-	       (setq re-list (cdr re-list))))
-	   ;; return the result
-	   (if ignorep
-	       (setq ido-ignored-list (cons name ido-ignored-list)))
-	   ignorep)))))
+         (flet ((inner ()
+                       (let ((ext-list (and ignore-ext ido-ignore-extensions
+			                    completion-ignored-extensions))
+	                     ignorep nextstr
+	                     (flen (length name)) slen)
+	                 (while ext-list
+	                   (setq nextstr (car ext-list))
+	                   (if (cond
+		                ((stringp nextstr)
+		                 (and (>= flen (setq slen (length nextstr)))
+			              (string-equal (substring name (- flen slen)) nextstr)))
+		                ((functionp nextstr) (funcall nextstr name))
+		                (t nil))
+		               (setq ignorep t
+		                     ext-list nil
+		                     re-list nil)
+	                     (setq ext-list (cdr ext-list))))
+	                 (while re-list
+	                   (setq nextstr (car re-list))
+	                   (if (cond
+		                ((stringp nextstr) (string-match nextstr name))
+		                ((functionp nextstr) (funcall nextstr name))
+		                (t nil))
+		               (setq ignorep t
+		                     re-list nil)
+	                     (setq re-list (cdr re-list))))
+	                 ;; return the result
+	                 (if ignorep
+	                     (setq ido-ignored-list (cons name ido-ignored-list)))
+	                 ignorep)))
+
+           (if (eq case-fold-search ido-case-fold)
+               (inner)
+             (let ((case-fold-search ido-case-fold))
+               (inner))))))))
 
 ;; Private variable used by `ido-word-matching-substring'.
 (defvar ido-change-word-sub)
-- 
2.20.1