From 94c70843e64ebe4d8e5bc711e81d488ffbabdaef Mon Sep 17 00:00:00 2001
From: Spencer Baugh <sbaugh@janestreet.com>
Date: Thu, 1 May 2025 14:27:30 -0400
Subject: [PATCH] Add load-path-filter-function and use it to optimize loading

When there are many directories on load-path, the part of load which
searches load-path can become very slow.  By filtering load-path up
front to only contain directories which are likely to contain the
searched-for file, load becomes much faster.

This can be set in early-init.el for maximum effect.

* lisp/startup.el (load-path-filter--cache)
(load-path-filter-cache-directory-files): Add.
* src/lread.c (Fload): Call load-path-filter-function.
(syms_of_lread): Add load-path-filter-function.

(cherry picked from commit e5218df144203ff1b5da3d46b7579b6455008ee7)
---
 lisp/startup.el | 33 +++++++++++++++++++++++++++++++++
 src/lread.c     | 20 ++++++++++++++++++--
 2 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/lisp/startup.el b/lisp/startup.el
index 0c0dffa32f6..29a04bef706 100644
--- a/lisp/startup.el
+++ b/lisp/startup.el
@@ -1097,6 +1097,39 @@ the `--debug-init' option to view a complete error backtrace."
 (defvar lisp-directory nil
   "Directory where Emacs's own *.el and *.elc Lisp files are installed.")
 
+(defvar load-path-filter--cache nil
+  "A cache used by `load-path-filter-cache-directory-files'.
+
+This is an alist.  The car of each entry is a list of load suffixes,
+such as returned by `get-load-suffixes'.  The cdr of each entry is a
+cons whose car is an optimized regex matching those suffixes at the end
+of a string, and whose cdr is a hashtable mapping directories to files
+in that directory which end with one of the suffixes.")
+
+(defun load-path-filter-cache-directory-files (path file suffixes)
+  "Filter PATH to only directories which might contain FILE with SUFFIXES.
+
+Doesn't filter if FILE is an absolute file name or if FILE is a relative
+file name with more than one component.
+
+Caches directory contents in `load-path-filter--cache'."
+  (if (file-name-directory file)
+      ;; FILE has more than one component, don't bother filtering.
+      path
+    (seq-filter
+     (let ((rx-and-ht
+            (with-memoization (alist-get suffixes load-path-filter--cache nil nil #'equal)
+              (cons
+               (concat (regexp-opt suffixes) "\\'")
+               (make-hash-table :test #'equal)))))
+       (lambda (dir)
+         (when (file-directory-p dir)
+           (try-completion
+            file
+            (with-memoization (gethash dir (cdr rx-and-ht))
+              (directory-files dir nil (car rx-and-ht) t))))))
+     path)))
+
 (defun command-line ()
   "A subroutine of `normal-top-level'.
 Amongst another things, it parses the command-line arguments."
diff --git a/src/lread.c b/src/lread.c
index 98cda8316ac..ed481c19721 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -1424,12 +1424,16 @@ Return t if the file exists and loads successfully.  */)
 	    suffixes = CALLN (Fappend, suffixes, Vload_file_rep_suffixes);
 	}
 
+      Lisp_Object load_path = Vload_path;
+      if (FUNCTIONP (Vload_path_filter_function))
+	load_path = calln (Vload_path_filter_function, load_path, file, suffixes);
+
 #if !defined USE_ANDROID_ASSETS
-      fd = openp (Vload_path, file, suffixes, &found, Qnil,
+      fd = openp (load_path, file, suffixes, &found, Qnil,
 		  load_prefer_newer, no_native, NULL);
 #else
       asset = NULL;
-      rc = openp (Vload_path, file, suffixes, &found, Qnil,
+      rc = openp (load_path, file, suffixes, &found, Qnil,
 		  load_prefer_newer, no_native, &asset);
       fd.fd = rc;
       fd.asset = asset;
@@ -6107,6 +6111,18 @@ where FILE is the filename of the eln file, including the .eln extension.
 through `require'.  */);
   load_no_native = false;
 
+  DEFVAR_LISP ("load-path-filter-function",
+	       Vload_path_filter_function,
+	       doc: /* Non-nil means to call this function to filter `load-path' for `load'.
+
+When load is called, this function is called with three arguments: the
+current value of `load-path' (a list of directories), the FILE argument
+to load, and the current load-suffixes.
+
+It should return a list of directories, which `load' will use instead of
+`load-path'.  */);
+  Vload_path_filter_function = Qnil;
+
   /* Vsource_directory was initialized in init_lread.  */
 
   DEFSYM (Qcurrent_load_list, "current-load-list");
-- 
2.39.5