2024-08-17  David Ponce  <da_vid@orange.fr>

	Tweak the implementation of string-pixel-width to run faster and
	use less memory.

	* subr-x.el (string--pixel-width-buffer): New variable and function.
	(string--pixel-width): Use it.  Prefer `remove-text-properties' to
	`propertize' to avoid creating a new string on each call.


diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index 058c06bc5f6..1f564fa5628 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -336,6 +336,23 @@ named-let
       (cl-labels ((,name ,fargs . ,body)) #',name)
       . ,aargs)))
 
+(defvar string--pixel-width-buffer nil)
+
+(defsubst string--pixel-width-buffer ()
+  "Get internal buffer used to calculate the pixel width of a string."
+  ;; Keeping a work buffer around is more efficient than creating a
+  ;; new temporary buffer on each call to `string-pixel-width'.
+  (if (buffer-live-p string--pixel-width-buffer)
+      string--pixel-width-buffer
+    (with-current-buffer (get-buffer-create " *string-pixel-width*" t)
+      ;; If `display-line-numbers' is enabled in internal buffers
+      ;; (e.g. globally), it breaks width calculation (bug#59311).
+      ;; Disable line-prefix and wrap-prefix, for the same reason.
+      ;; Set all these variables only one time here: they
+      ;; automatically become buffer-local when set.
+      (setq display-line-numbers nil line-prefix nil wrap-prefix nil)
+      (setq string--pixel-width-buffer (current-buffer)))))
+
 ;;;###autoload
 (defun string-pixel-width (string &optional buffer)
   "Return the width of STRING in pixels.
@@ -344,22 +361,18 @@ string-pixel-width
   (declare (important-return-value t))
   (if (zerop (length string))
       0
-    ;; Keeping a work buffer around is more efficient than creating a
-    ;; new temporary buffer.
-    (with-current-buffer (get-buffer-create " *string-pixel-width*")
-      ;; If `display-line-numbers' is enabled in internal buffers
-      ;; (e.g. globally), it breaks width calculation (bug#59311)
-      (setq-local display-line-numbers nil)
-      (delete-region (point-min) (point-max))
-      ;; Disable line-prefix and wrap-prefix, for the same reason.
-      (setq line-prefix nil
-	    wrap-prefix nil)
+    (with-current-buffer (string--pixel-width-buffer)
+      (erase-buffer)
       (if buffer
           (setq-local face-remapping-alist
                       (with-current-buffer buffer
                         face-remapping-alist))
         (kill-local-variable 'face-remapping-alist))
-      (insert (propertize string 'line-prefix nil 'wrap-prefix nil))
+      (insert string)
+      ;; Prefer `remove-text-properties' to `propertize' to avoid
+      ;; creating a new string on each call.
+      (remove-text-properties
+       (point-min) (point-max) '(line-prefix nil wrap-prefix nil))
       (car (buffer-text-pixel-size nil nil t)))))
 
 ;;;###autoload