Misplaced decref due to PyDict_Next optimization

I just took a look at what was generated before the PyDict_Next optimization was added, and it looks like there was a temp variable used to store a PyList reference during iteration. That temp variable appears to be what the bogus Py_DECREF call is for.

The problem is the return statement (ReturnStatNode?). In analyse_expressions(), it retrieves the list of active temps:

        self.temps_in_use = env.temps_in_use()

and then frees that in its execution code:

        for entry in self.temps_in_use:
            code.put_var_decref_clear(entry)

Like some other tree transforms, the iter-dict optimisation changes the temp allocation, which leaves some temps unused later in the code, such as the one for the loop iterator in your example. The return statement isn't prepared for that. So the impact of this bug is actually larger than just the iter-dict transform.

Here's a shorter example with the same problem:

def spam(dict d):
    for elm in d:
        return False
    return True

spam(dict(test=2))

From what I can see, these two fixes it:

http://hg.cython.org/cython-devel/rev/15459e336874 http://hg.cython.org/cython-devel/rev/65990f108724

More temp resolution stuff should be seen in relation to #77.