Avoid pop when not necessary, but might keep objects alive, let them escape

Signed-off-by: Stefan Marr <git@stefan-marr.de>

src/som/interpreter/frame.py

@@ -69,9 +69,15 @@ class Frame(object):
     def get_number_of_arguments(self):
         return self._method.get_number_of_arguments()
 
+    def top(self):
+        stack_pointer = jit.promote(self._stack_pointer)
+        assert 0 <= stack_pointer < len(self._stack)
+        return self._stack[stack_pointer]
+
     def pop(self):
         """ Pop an object from the expression stack and return it """
-        stack_pointer = self._stack_pointer
+        stack_pointer = jit.promote(self._stack_pointer)
+        assert 0 <= stack_pointer < len(self._stack)
         self._stack_pointer = stack_pointer - 1
         result = self._stack[stack_pointer]
         self._stack[stack_pointer] = None
@@ -81,6 +87,7 @@ class Frame(object):
     def push(self, value):
         """ Push an object onto the expression stack """
         stack_pointer = jit.promote(self._stack_pointer) + 1
+        assert 0 <= stack_pointer < len(self._stack)
         self._stack[stack_pointer] = value
         self._stack_pointer = stack_pointer
 

src/som/interpreter/interpreter.py

@@ -106,12 +106,12 @@ class Interpreter(object):
             receiver.send_does_not_understand(frame, signature, self._universe, self)
 
     def _do_return_local(self, frame):
-        return frame.pop()
+        return frame.top()
 
     @jit.unroll_safe
     def _do_return_non_local(self, frame):
         # get result from stack
-        result = frame.pop()
+        result = frame.top()
 
         # Compute the context for the non-local return
         context = frame.get_outer_context()
@@ -127,7 +127,7 @@ class Interpreter(object):
 
             # ... and execute the escapedBlock message instead
             sender.send_escaped_block(frame, block, self._universe, self)
-            return frame.pop()
+            return frame.top()
 
         raise ReturnException(result, context)