systemc: Store timed notifications in a list instead of a set.

This has three advantages. First, the data structure doesn't have to
try to keep track of whether or not an event is already listed there.
Second, it's easier to delete an item by storing an iterator for it
when it gets inserted. Third, the ordering of events is not dependent
on the arbitrary ordering of the set, it's bsaed on the fixed order
the events get added to the list.

One part of this change makes ScEvent-s keep track of what list they're
on, and handle their own insertion and deletion when they're
scheduled or descheduled. A side effect of that is that it's no longer
safe to simply use a range based for loop to loop over all of an
ScEvent and deschedule all its events or to run then (which deschedules
them internally once they execute).

That can be avoided by looping until the list is empty, and operating
on the first element. As the first element is processed and removed
from the list, the next element will become first and will get picked
up in the next iteration.

Change-Id: Icad51a63f153297c88e65f85d22ac721e6c571d8
Reviewed-on: https://gem5-review.googlesource.com/c/12456
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

3 files changed, 42 insertions, 31 deletions

diff --git a/src/systemc/core/sched_event.hh b/src/systemc/core/sched_event.hh
index 9aefb7440..ca5521574 100644
--- a/src/systemc/core/sched_event.hh
+++ b/src/systemc/core/sched_event.hh
@@ -31,37 +31,54 @@
 #define __SYSTEMC_CORE_SCHED_EVENT_HH__
 
 #include <functional>
+#include <list>
 
 #include "base/types.hh"
 
 namespace sc_gem5
 {
 
+class ScEvent;
+
+typedef std::list<ScEvent *> ScEvents;
+
 class ScEvent
 {
   private:
     std::function<void()> work;
     Tick _when;
-    bool _scheduled;
+    ScEvents *_events;
+    ScEvents::iterator _it;
 
     friend class Scheduler;
 
     void
-    schedule(Tick w)
+    schedule(ScEvents &events, Tick w)
     {
         when(w);
-        _scheduled = true;
+        assert(!scheduled());
+        _events = &events;
+        _events->push_back(this);
+        _it = _events->end();
+        _it--;
     }
 
-    void deschedule() { _scheduled = false; }
+    void
+    deschedule()
+    {
+        assert(scheduled());
+        _events->erase(_it);
+        _events = nullptr;
+    }
   public:
     ScEvent(std::function<void()> work) :
-        work(work), _when(MaxTick), _scheduled(false)
+        work(work), _when(MaxTick), _events(nullptr)
     {}
 
     ~ScEvent();
 
-    bool scheduled() { return _scheduled; }
+    bool scheduled() { return _events != nullptr; }
+    ScEvents *scheduledOn() { return _events; }
 
     void when(Tick w) { _when = w; }
     Tick when() { return _when; }
diff --git a/src/systemc/core/scheduler.cc b/src/systemc/core/scheduler.cc
index e2d8e62f4..d5081c667 100644
--- a/src/systemc/core/scheduler.cc
+++ b/src/systemc/core/scheduler.cc
@@ -61,15 +61,14 @@ void
 Scheduler::clear()
 {
     // Delta notifications.
-    for (auto &e: deltas)
-        e->deschedule();
-    deltas.clear();
+    while (!deltas.empty())
+        deltas.front()->deschedule();
 
     // Timed notifications.
     for (auto &tsp: timeSlots) {
         TimeSlot *&ts = tsp.second;
-        for (auto &e: ts->events)
-            e->deschedule();
+        while (!ts->events.empty())
+            ts->events.front()->deschedule();
         deschedule(ts);
     }
     timeSlots.clear();
@@ -115,9 +114,8 @@ Scheduler::initPhase()
 
     update();
 
-    for (auto &e: deltas)
-        e->run();
-    deltas.clear();
+    while (!deltas.empty())
+        deltas.front()->run();
 
     for (auto ets: eventsToSchedule)
         eq->schedule(ets.first, ets.second);
@@ -281,9 +279,8 @@ Scheduler::runReady()
     update();
 
     // The delta phase.
-    for (auto &e: deltas)
-        e->run();
-    deltas.clear();
+    while (!deltas.empty())
+        deltas.front()->run();
 
     if (!runToTime && starved())
         scheduleStarvationEvent();
diff --git a/src/systemc/core/scheduler.hh b/src/systemc/core/scheduler.hh
index 2bee0b090..924cfb29e 100644
--- a/src/systemc/core/scheduler.hh
+++ b/src/systemc/core/scheduler.hh
@@ -145,7 +145,7 @@ typedef NodeList<Channel> ChannelList;
 class Scheduler
 {
   public:
-    typedef std::set<ScEvent *> ScEvents;
+    typedef std::list<ScEvent *> ScEvents;
 
     class TimeSlot : public ::Event
     {
@@ -226,11 +226,9 @@ class Scheduler
         if (tick < getCurTick())
             tick = getCurTick();
 
-        event->schedule(tick);
-
         // Delta notification/timeout.
         if (delay.value() == 0) {
-            deltas.insert(event);
+            event->schedule(deltas, tick);
             scheduleReadyEvent();
             return;
         }
@@ -241,19 +239,18 @@ class Scheduler
             ts = new TimeSlot;
             schedule(ts, tick);
         }
-        ts->events.insert(event);
+        event->schedule(ts->events, tick);
     }
 
     // For descheduling delayed/timed notifications/timeouts.
     void
     deschedule(ScEvent *event)
     {
-        if (event->when() == getCurTick()) {
-            // Attempt to remove from delta notifications.
-            if (deltas.erase(event) == 1) {
-                event->deschedule();
-                return;
-            }
+        ScEvents *on = event->scheduledOn();
+
+        if (on == &deltas) {
+            event->deschedule();
+            return;
         }
 
         // Timed notification/timeout.
@@ -262,7 +259,7 @@ class Scheduler
                 "Descheduling event at time with no events.");
         TimeSlot *ts = tsit->second;
         ScEvents &events = ts->events;
-        assert(events.erase(event));
+        assert(on == &events);
         event->deschedule();
 
         // If no more events are happening at this time slot, get rid of it.
@@ -424,8 +421,8 @@ extern Scheduler scheduler;
 inline void
 Scheduler::TimeSlot::process()
 {
-    for (auto &e: events)
-        e->run();
+    while (!events.empty())
+        events.front()->run();
     scheduler.completeTimeSlot(this);
 }