systemc: Change how signal based resets work.

The previous implementation used the value changed event to track when
signals changed value, but there were a couple problems with this
approach. First, this piggybacked on the sensitivity mechanism in some
ways, but diverged in others. The sensitivity didn't notify a process
when it was satisfied like other sensitivity types would, and it also
ignored whether the process was disabled.

Second, the value_changed_event is notified by a signal instance as a
delta notification, but reset signals are supposed to act immediately.
That means they should happen before all delta notifications, or in
other words all delta notifications should see the reset status of a
given process. That's particularly important in the case of wait(int n)
where setting the reset clears the reset count, and the count is
checked when determining whether or not to wake up a process when its
sensitivity is satisfied, potentially by a delta notification.

Third, by removing the middle man and not trying to repurpose the
sensitivity mechanism, the code gets simpler and easier to understand.

Change-Id: I0d05d11437291d368b060f6a45a207813615f113
Reviewed-on: https://gem5-review.googlesource.com/c/13294
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

2 files changed, 27 insertions, 0 deletions

diff --git a/src/systemc/ext/channel/sc_signal.hh b/src/systemc/ext/channel/sc_signal.hh
index e4300343d..3b2765db5 100644
--- a/src/systemc/ext/channel/sc_signal.hh
+++ b/src/systemc/ext/channel/sc_signal.hh
@@ -51,6 +51,7 @@ namespace sc_gem5
 {
 
 class Process;
+class Reset;
 
 class ScSignalBase : public sc_core::sc_prim_channel
 {
@@ -80,6 +81,10 @@ class ScSignalBaseBinary : public ScSignalBase
   protected:
     ScSignalBaseBinary(const char *_name);
 
+    mutable std::vector<sc_gem5::Reset *> _resets;
+    void _signalReset(sc_gem5::Reset *reset);
+    void _signalReset();
+
     const sc_core::sc_event &posedgeEvent() const;
     const sc_core::sc_event &negedgeEvent() const;
 
@@ -352,6 +357,7 @@ class sc_signal<bool, WRITER_POLICY> :
             return;
 
         this->m_cur_val = this->m_new_val;
+        this->_signalReset();
         this->_signalChange();
         if (this->m_cur_val) {
             this->_posStamp = ::sc_gem5::getChangeStamp();
@@ -363,6 +369,13 @@ class sc_signal<bool, WRITER_POLICY> :
     }
 
   private:
+    bool
+    _addReset(sc_gem5::Reset *reset) const
+    {
+        this->_resets.push_back(reset);
+        return true;
+    }
+
     // Disabled
     sc_signal(const sc_signal<bool, WRITER_POLICY> &);
 };
diff --git a/src/systemc/ext/channel/sc_signal_in_if.hh b/src/systemc/ext/channel/sc_signal_in_if.hh
index 6fac35860..008516495 100644
--- a/src/systemc/ext/channel/sc_signal_in_if.hh
+++ b/src/systemc/ext/channel/sc_signal_in_if.hh
@@ -39,6 +39,13 @@ class sc_logic;
 
 };
 
+namespace sc_gem5
+{
+
+class Reset;
+
+} // namespace sc_gem5
+
 namespace sc_core
 {
 
@@ -83,6 +90,13 @@ class sc_signal_in_if<bool> : virtual public sc_interface
     sc_signal_in_if() : sc_interface() {}
 
   private:
+    friend class sc_gem5::Reset;
+    virtual bool
+    _addReset(sc_gem5::Reset *reset) const
+    {
+        return false;
+    }
+
     // Disabled
     sc_signal_in_if(const sc_signal_in_if<bool> &) : sc_interface() {}
     sc_signal_in_if<bool> &