Reduce the size of the kbd-buffer GC root

* src/igc.c (root_create_kbd_buffer): New.
(root_create_main_thread): Use it.
(scan_kbd_buffer_ambig): New. The actual scan function.
* src/keyboard.c (init_keyboard): Move the GC root creation code to
root_create_kbd_buffer in igc.c.
(prev_kbd_event): New helper.
* src/keyboard.h (next_kbd_event, prev_kbd_event): Make them available
to igc.

src/igc.c

@@ -60,6 +60,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>. */
 #include "thread.h"
 #include "treesit.h"
 #include "termchar.h"
+#include "keyboard.h"
 #ifdef HAVE_WINDOW_SYSTEM
 #include TERM_HEADER
 #endif /* HAVE_WINDOW_SYSTEM */
@@ -1725,6 +1726,32 @@ scan_tty_list (mps_ss_t ss, void *start, void *end, void *closure)
   return MPS_RES_OK;
 }
 
+static mps_res_t
+scan_kbd_buffer_ambig (mps_ss_t ss, void *start, void *end, void *closure)
+{
+  igc_assert (start == kbd_buffer);
+  igc_assert (end == kbd_buffer + ARRAYELTS (kbd_buffer));
+
+  /* Instead of tracing the entire kbd_buffer, only scan the region from
+     kbd_fetch_ptr - 1 to kbd_store_ptr + 1.  The -1/+1 is supposed to
+     cover the cases where MPS stops the mutator while those pointers
+     are being updated. */
+
+  union buffered_input_event *fetch = prev_kbd_event (kbd_fetch_ptr);
+  union buffered_input_event *store = next_kbd_event (kbd_store_ptr);
+
+  if (fetch <= store - 2)
+    return scan_ambig (ss, fetch, store, closure);
+  else
+    {
+      mps_res_t res
+	= scan_ambig (ss, fetch, kbd_buffer + KBD_BUFFER_SIZE, closure);
+      if (res == MPS_RES_OK)
+	res = scan_ambig (ss, kbd_buffer, store, closure);
+      return res;
+    }
+}
+
 /***********************************************************************
 			 Default pad, fwd, ...
  ***********************************************************************/
@@ -2905,6 +2932,14 @@ root_create_tty_list (struct igc *gc)
 		     scan_tty_list, "tty-list");
 }
 
+static void
+root_create_kbd_buffer (struct igc *gc)
+{
+  root_create (gc, kbd_buffer, kbd_buffer + ARRAYELTS (kbd_buffer),
+	       mps_rank_ambig (), scan_kbd_buffer_ambig, NULL,
+	       true, "kbd-buffer");
+}
+
 static void
 root_create_main_thread (struct igc *gc)
 {
@@ -4934,6 +4969,7 @@ make_igc (void)
   root_create_main_thread (gc);
   root_create_exact_ptr (gc, &current_thread);
   root_create_exact_ptr (gc, &all_threads);
+  root_create_kbd_buffer (gc);
 
   enable_messages (gc, true);
   return gc;

src/keyboard.c

@@ -423,12 +423,18 @@ static void do_async_work (void);
 
 /* Advance or retreat a buffered input event pointer.  */
 
-static union buffered_input_event *
+union buffered_input_event *
 next_kbd_event (union buffered_input_event *ptr)
 {
   return ptr == kbd_buffer + KBD_BUFFER_SIZE - 1 ? kbd_buffer : ptr + 1;
 }
 
+union buffered_input_event *
+prev_kbd_event (union buffered_input_event *ptr)
+{
+  return ptr == kbd_buffer ? kbd_buffer + KBD_BUFFER_SIZE - 1 : ptr - 1;
+}
+
 /* Like EVENT_START, but assume EVENT is an event.
    This pacifies gcc -Wnull-dereference, which might otherwise
    complain about earlier checks that EVENT is indeed an event.  */
@@ -12892,10 +12898,6 @@ delete_kboard (KBOARD *kb)
 void
 init_keyboard (void)
 {
-#ifdef HAVE_MPS
-  igc_root_create_ambig (kbd_buffer, kbd_buffer + ARRAYELTS (kbd_buffer),
-			 "kbd-buffer");
-#endif
   /* This is correct before outermost invocation of the editor loop.  */
   command_loop_level = -1;
   quit_char = Ctl ('g');

src/keyboard.h

@@ -533,6 +533,9 @@ extern char *dev_tty;
 /* Initial value for dev_tty.  */
 extern char const DEV_TTY[];
 
+union buffered_input_event *next_kbd_event (union buffered_input_event *);
+union buffered_input_event *prev_kbd_event (union buffered_input_event *);
+
 INLINE_HEADER_END
 
 enum async_work_type