[PATCH 1/4] sim: Allow an overdue event to be descheduled

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

sim_events_deschedule() asserted that a non-empty event queue implies a
non-negative time from event.  That is not an invariant.
sim_events_slip() decrements the time from event unconditionally, so the
time can pass an event which is still queued.  update_time_from_event()
then computes a negative time from event for the head of the queue,
which is overdue rather than pending.  The next tick raises the pending
work flag and sim_events_process() consumes it, so nothing is lost.

An event handler which deschedules another overdue event triggers the
assertion.  The MIPS jmr3904 board reaches this with two timers running:
they queue events for the same time, a branch delay slot slips the time
past both, and the handler of the first deschedules the second.

Assert only the direction which holds: an empty queue gives a negative
time from event.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>
---
 sim/common/sim-events.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/sim/common/sim-events.c b/sim/common/sim-events.c
index f87f133efc3..e908c40f764 100644
--- a/sim/common/sim-events.c
+++ b/sim/common/sim-events.c
@@ -852,7 +852,11 @@ sim_events_deschedule (SIM_DESC sd,
 		       (dead->trace != NULL) ? dead->trace : ""));
 	      sim_events_free (sd, dead);
 	      update_time_from_event (sd);
-	      SIM_ASSERT ((events->time_from_event >= 0) == (events->queue != NULL));
+	      /* sim_events_slip() advances the time past an event
+		 which is still queued. The head of the queue is then
+		 overdue rather than pending.  */
+	      SIM_ASSERT (events->queue != NULL
+			  || events->time_from_event < 0);
 	      return;
 	    }
 	}
-- 
2.51.0