Set Frame stack pointer in the constructor

Avoids need to call reset on it, and might help RPython to figure out that frame does not escape (making it more functional like this often helped in the past)
Signed-off-by: Stefan Marr <git@stefan-marr.de>

src/som/interpreter/frame.py

@@ -15,11 +15,11 @@ class Frame(object):
         
     _immutable_fields_ = ["_method", "_context", "_stack"]
 
-    def __init__(self, nilObject, num_elements, method, context, previous_frame):
+    def __init__(self, num_elements, method, context, previous_frame):
         self._method         = method
         self._context        = context
         self._stack          = [None] * num_elements
-        self._stack_pointer  = 0
+        self._stack_pointer  = self._get_initial_stack_pointer()
         self._previous_frame = previous_frame
     
     def get_previous_frame(self):
@@ -88,7 +88,10 @@ class Frame(object):
         """ Set the stack pointer to its initial value thereby clearing
             the stack """
         # arguments are stored in front of local variables
-        self._stack_pointer = (self.get_number_of_arguments() +
+        self._stack_pointer = self._get_initial_stack_pointer()
+
+    def _get_initial_stack_pointer(self):
+        return (self.get_number_of_arguments() +
                 self.get_method().get_number_of_locals().get_embedded_integer() - 1)
 
     def get_stack_element(self, index):

src/som/vm/universe.py

@@ -338,7 +338,8 @@ class Universe(object):
         result.set_class(class_class)
         return result
 
-    def new_frame(self, previous_frame, method, context):
+    @staticmethod
+    def new_frame(previous_frame, method, context):
         # Compute the maximum number of stack locations (including arguments,
         # locals and extra buffer to support doesNotUnderstand) and set the
         # number of indexable fields accordingly
@@ -346,9 +347,7 @@ class Universe(object):
                   method.get_number_of_locals().get_embedded_integer() +
                   method.get_maximum_number_of_stack_elements().get_embedded_integer() + 2)
 
-        result = Frame(self.nilObject, length, method, context, previous_frame)
-        result.reset_stack_pointer()
-        return result
+        return Frame(length, method, context, previous_frame)
 
     def new_method(self, signature, num_bytecodes, literals,
                    num_locals, maximum_number_of_stack_elements):