Speed up display of long lines under 'truncate-lines'

* src/xdisp.c (partial_line_height): Return zero for long and
truncated lines.
(fast_move_it_horizontally): New function.
(hscroll_window_tree, display_line): Use
'fast_move_it_horizontally' in preference to
'move_it_in_display_line_to', when dealing with long and truncated
lines.
(redisplay_internal): Optimize "optimization 3" for long and
truncated lines.
* src/buffer.c (syms_of_buffer) <large-hscroll-threshold>: New
variable.

* etc/NEWS: Announce 'large-hscroll-threshold'.

1 files changed, 18 insertions, 0 deletions

diff --git a/src/buffer.c b/src/buffer.c
index e5601af5051..6ab516d5f51 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -6442,6 +6442,24 @@ If nil, these display shortcuts will always remain disabled.
 There is no reason to change that value except for debugging purposes.  */);
   XSETFASTINT (Vlong_line_threshold, 10000);
 
+  DEFVAR_INT ("large-hscroll-threshold", large_hscroll_threshold,
+    doc: /* Horizontal scroll of truncated lines above which to use redisplay shortcuts.
+
+The value should be a positive integer.
+
+Shortcuts in the display code intended to speed up redisplay for long
+and truncated lines will automatically be enabled when a line's
+horizontal scroll amount is or about to become larger than the value
+of this variable.
+
+This variable has effect only in buffers which contain one or more
+lines whose length is above `long-line-threshold', which see.
+To disable redisplay shortcuts for long truncated line, set this
+variable to `most-positive-fixnum'.
+
+There is no reason to change that value except for debugging purposes.  */);
+  large_hscroll_threshold = 10000;
+
   defsubr (&Sbuffer_live_p);
   defsubr (&Sbuffer_list);
   defsubr (&Sget_buffer);