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.

1 files changed, 18 insertions, 2 deletions

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");