sim: make Python Root object a singleton

Enforce that the Python Root SimObject is instantiated only
once.  The C++ Root object already panics if more than one is
created.  This change avoids the need to track what the root
object is, since it's available from Root.getInstance() (if it
exists).  It's now redundant to have the user pass the root
object to functions like instantiate(), checkpoint(), and
restoreCheckpoint(), so that arg is gone.  Users who use
configs/common/Simulate.py should not notice.

src/python/m5/SimObject.py

@@ -623,7 +623,7 @@ class SimObject(object):
 
     def path(self):
         if not self._parent:
-            return 'root'
+            return '(orphan)'
         ppath = self._parent.path()
         if ppath == 'root':
             return self._name

src/python/m5/simulate.py

@@ -39,13 +39,19 @@ from main import options
 import SimObject
 import ticks
 import objects
+from util import fatal
 
 # define a MaxTick parameter
 MaxTick = 2**63 - 1
 
 # The final hook to generate .ini files.  Called from the user script
 # once the config is built.
-def instantiate(root):
+def instantiate():
+    root = objects.Root.getInstance()
+
+    if not root:
+        fatal("Need to instantiate Root() before calling instantiate()")
+
     # we need to fix the global frequency
     ticks.fixGlobalFrequency()
 
@@ -136,7 +142,8 @@ def drain(root):
 def resume(root):
     root.resume()
 
-def checkpoint(root, dir):
+def checkpoint(dir):
+    root = objects.Root.getInstance()
     if not isinstance(root, objects.Root):
         raise TypeError, "Checkpoint must be called on a root object."
     doDrain(root)
@@ -144,7 +151,8 @@ def checkpoint(root, dir):
     internal.core.serializeAll(dir)
     resume(root)
 
-def restoreCheckpoint(root, dir):
+def restoreCheckpoint(dir):
+    root = objects.Root.getInstance()
     print "Restoring from checkpoint"
     internal.core.unserializeAll(dir)
     need_resume.append(root)