Speed up lookup in redisplay--variables

* lisp/frame.el (redisplay--variables): Make it a hash-table.

* src/xdisp.c (maybe_set_redisplay): Access redisplay--variables
as a hash-table.  This speeds up this function by an order of
magnitude: where previously a setq was slowed down by 100% by
introducing the maybe_set_redisplay test, it is now only 5%
slower.
(syms_of_xdisp) <redisplay--variables>: Doc fix.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index dbc2d840d44..30dfac55601 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -623,7 +623,8 @@ bset_update_mode_line (struct buffer *b)
 void
 maybe_set_redisplay (Lisp_Object symbol)
 {
-  if (!NILP (Fassoc_string (symbol, Vredisplay__variables, Qnil)))
+  if (HASH_TABLE_P (Vredisplay__variables)
+      && hash_lookup (XHASH_TABLE (Vredisplay__variables), symbol, NULL) >= 0)
     {
       bset_update_mode_line (current_buffer);
       current_buffer->prevent_redisplay_optimizations_p = true;
@@ -31478,7 +31479,7 @@ display table takes effect; in this case, Emacs does not consult
   Vredisplay__mode_lines_cause = Fmake_hash_table (0, NULL);
 
   DEFVAR_LISP ("redisplay--variables", Vredisplay__variables,
-     doc: /* A list of variables changes to which trigger a thorough redisplay.  */);
+     doc: /* A hash-table of variables changing which triggers a thorough redisplay.  */);
   Vredisplay__variables = Qnil;
 }