From 89b40650da20b70e32872e0bb08bac48ea251781 Mon Sep 17 00:00:00 2001
From: Stefan Monnier <monnier@iro.umontreal.ca>
Date: Thu, 19 Mar 2026 14:50:21 -0400
Subject: [PATCH] diff-mode.el: Don't recompute the position of Git
 preamble/footer

On some profiles (after disabling syntax and refined
fontification), this was taking >90% of the time to fontify a buffer.

* lisp/vc/diff-mode.el (diff--git-preamble-overlay)
(diff--git-footer-overlay): New variables.
(diff--git-preamble-end, diff--git-footer-start): Use them to cache
the result.
---
 lisp/vc/diff-mode.el | 46 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 37 insertions(+), 9 deletions(-)

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 86095780f67..8a432a7f618 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -535,18 +535,46 @@ use the face `diff-removed' for removed lines, and the face
       (group (any "<-"))
       (group (zero-or-more nonl) "\n")))
 
+(defvar-local diff--git-preamble-overlay nil)
+(defvar-local diff--git-footer-overlay nil)
+
 (defun diff--git-preamble-end ()
-  (save-excursion
-    (goto-char (point-min))
-    (re-search-forward "^diff --git .+ .+$" nil t)
-    (forward-line 2)
-    (point)))
+  (unless (and diff--git-preamble-overlay
+               (overlay-buffer diff--git-preamble-overlay))
+    (save-excursion
+      (goto-char (point-min))
+      (let ((found (re-search-forward "^diff --git .+ .+$" nil 'move)))
+        (forward-line 2)
+        (if diff--git-preamble-overlay
+            (move-overlay diff--git-preamble-overlay (point-min) (point))
+          (let ((ol (make-overlay (point-min) (point))))
+            (overlay-put ol 'modification-hooks
+                         (lambda (&rest _) (delete-overlay ol)))
+            (overlay-put ol 'evaporate t)
+            (setq diff--git-preamble-overlay ol)))
+        (overlay-put diff--git-preamble-overlay 'diff--found found))))
+  (if (not (overlay-get diff--git-preamble-overlay 'diff--found))
+      (point-min)
+    (overlay-end diff--git-preamble-overlay)))
 
 (defun diff--git-footer-start ()
-  (save-excursion
-    (goto-char (point-max))
-    (re-search-backward "^-- $" nil t)
-    (point)))
+  (unless (and diff--git-footer-overlay
+               (overlay-buffer diff--git-footer-overlay))
+    (save-excursion
+      (goto-char (point-max))
+      (let ((found (re-search-backward "\n-- $" nil 'move)))
+        (if diff--git-footer-overlay
+            (move-overlay diff--git-footer-overlay
+                          (match-beginning 0) (point-max))
+          (let ((ol (make-overlay (match-beginning 0) (point-max) nil t t)))
+            (overlay-put ol 'modification-hooks
+                         (lambda (&rest _) (delete-overlay ol)))
+            (overlay-put ol 'evaporate t)
+            (setq diff--git-footer-overlay ol)))
+        (overlay-put diff--git-footer-overlay 'diff--found found))))
+  (if (not (overlay-get diff--git-footer-overlay 'diff--found))
+      (point-max)
+    (1+ (overlay-start diff--git-footer-overlay))))
 
 (defun diff--indicator-matcher-helper (limit regexp)
   "Fontify added/removed lines from point to LIMIT using REGEXP.