Vitja Makarov, 23.05.2011 11:24:
2011/5/23 Stefan Behnel:
Vitja Makarov, 23.05.2011 10:50:
2011/5/23 Stefan Behnel:
Vitja Makarov, 23.05.2011 10:13:
With live variable analysis that should be easy to save/restore only
active variables at the yield point.
"Active" in the sense of "modified", I suppose? That's what I was
expecting.
Active means that variable value will be used. In my example after
'print a' a isn't used anymore.
That's not correct then. In a generator, a modified value must be kept alive
over a yield, even if it is no longer used afterwards.
We can safely reduce the write-back code to modified values, but we cannot
reduce it to values to that will be used later on.
I'm not sure how to get modified variables list at yield point. Now I
only know which assignments reach yield point.
But then you already know which variables were assigned to, right? That's
all you need.
For instance:
for i in args:
print i
this code will be translated into:
PyObject *i = NULL;
for (;;)
{
tmp = next();
if (!tmp) break;
Pyx_XDECREF(i);
i = tmp;
print(i);
}
using active variables information this could be translated into:
PyObject *i = NULL;
for (;;)
{
tmp = next();
if (!tmp) break;
i = tmp;
print(i);
Pyx_DECREF(i);
}
That's not correct, though. Python semantics dictate that 'i' must keep its
value until the end of the function or until it's being reassigned to,
whatever comes first. Remember that objects can have deallocators in Python.
That must not be called at an undefined point.
The only thing that can safely be special cased is None. It's common in
Python code to set a variable to None when the value is worth being
deallocated (e.g. a large data structure). Cython can optimise this as I
indicated above.
Ohh, I see that variable references couldn't be simply removed.
Unused result reference removal seems safe to me:
a = foo() # a will be assigned to NULL here
print
No, same thing: the reference that 'a' holds here may be important, so we
cannot just drop it at an arbitrary point. There's (likely) a reason the
user chose to use a variable, instead of just letting the return value fall
off the shelve silently.
Stefan
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