MEM: Pass the ports from Python to C++ using the Swig params

This patch adds basic information about the ports in the parameter
classes to be passed from the Python world to the corresponding C++
object. Currently, the only information passed is the number of
connected peers, which for a Port is either 0 or 1, and for a
VectorPort reflects the size of the VectorPort. The default port of
the bus had to be renamed to avoid using the name "default" as a field
in the parameter class. It is possible to extend the Swig'ed
information further and add e.g. a pair with a description and size.

src/python/m5/params.py

@@ -1349,6 +1349,11 @@ class PortRef(object):
     def __str__(self):
         return '%s.%s' % (self.simobj, self.name)
 
+    def __len__(self):
+        # Return the number of connected ports, i.e. 0 is we have no
+        # peer and 1 if we do.
+        return int(self.peer != None)
+
     # for config.ini, print peer's name (not ours)
     def ini_str(self):
         return str(self.peer)
@@ -1462,6 +1467,11 @@ class VectorPortRef(object):
     def __str__(self):
         return '%s.%s[:]' % (self.simobj, self.name)
 
+    def __len__(self):
+        # Return the number of connected peers, corresponding the the
+        # length of the elements.
+        return len(self.elements)
+
     # for config.ini, print peer's name (not ours)
     def ini_str(self):
         return ' '.join([el.ini_str() for el in self.elements])
@@ -1525,6 +1535,17 @@ class Port(object):
     def connect(self, simobj, ref):
         self.makeRef(simobj).connect(ref)
 
+    # No need for any pre-declarations at the moment as we merely rely
+    # on an unsigned int.
+    def cxx_predecls(self, code):
+        pass
+
+    # Declare an unsigned int with the same name as the port, that
+    # will eventually hold the number of connected ports (and thus the
+    # number of elements for a VectorPort).
+    def cxx_decl(self, code):
+        code('unsigned int port_${{self.name}}_connection_count;')
+
 class MasterPort(Port):
     # MasterPort("description")
     def __init__(self, *args):