Speed up ls-lisp

This speeds up Dired by 25% in large directories.
* lisp/ls-lisp.el (ls-lisp--time-locale): New defvar.
(ls-lisp-format-time): calculate the locale for formatting times
only once and cache the value in 'ls-lisp--time-locale'.
(Bug#44273)

1 files changed, 7 insertions, 2 deletions

diff --git a/lisp/ls-lisp.el b/lisp/ls-lisp.el
index 8851522bbdb..e2646209313 100644
--- a/lisp/ls-lisp.el
+++ b/lisp/ls-lisp.el
@@ -836,6 +836,9 @@ Return nil if no time switch found."
         ((memq ?t switches) 5)          ; last modtime
         ((memq ?u switches) 4)))        ; last access
 
+(defvar ls-lisp--time-locale nil
+  "Locale to be used for formatting file times.")
+
 (defun ls-lisp-format-time (file-attr time-index)
   "Format time for file with attributes FILE-ATTR according to TIME-INDEX.
 Use the same method as ls to decide whether to show time-of-day or year,
@@ -851,11 +854,13 @@ All ls time options, namely c, t and u, are handled."
     (condition-case nil
         ;; Use traditional time format in the C or POSIX locale,
         ;; ISO-style time format otherwise, so columns line up.
-        (let ((locale system-time-locale))
+        (let ((locale (or system-time-locale ls-lisp--time-locale)))
           (if (not locale)
               (let ((vars '("LC_ALL" "LC_TIME" "LANG")))
                 (while (and vars (not (setq locale (getenv (car vars)))))
-                  (setq vars (cdr vars)))))
+                  (setq vars (cdr vars)))
+                ;; Cache the locale for next calls.
+                (setq ls-lisp--time-locale (or locale "C"))))
           (if (member locale '("C" "POSIX"))
               (setq locale nil))
           (format-time-string