[Python-Dev] Speed up function calls

[Neal Norwitz]

I added a patch to SF:  http://python.org/sf/1107887

I would like feedback on whether the approach is desirable.

The patch adds a new method type (flags) METH_ARGS that is used in
PyMethodDef. METH_ARGS means the min and max # of arguments are
specified in the PyMethodDef by adding 2 new fields. This information
can be used in ceval to
call the method. No tuple packing/unpacking is required since the C
stack is used.

The benefits are:
 * faster function calls
 * simplify function call machinery by removing METH_NOARGS, METH_O,
and possibly METH_VARARGS
 * more introspection info for C functions (ie, min/max arg count)
(not implemented)

The drawbacks are:
 * the defn of the MethodDef (# args) is separate from the function defn
 * potentially more error prone to write C methods???

I've measured between 13-22% speed improvement (debug build on
Operton) when doing simple tests like:

     ./python ./Lib/timeit.py -v 'pow(3, 5)'

I think the difference tends to be fairly constant at about .3 usec per loop.

Here's a portion of the patch to show the difference between conventions:

-builtin_filter(PyObject *self, PyObject *args)
+builtin_filter(PyObject *self, PyObject *func, PyObject *seq)
 {
-       PyObject *func, *seq, *result, *it, *arg;
+       PyObject *result, *it, *arg;
        int len;   /* guess for result list size */
        register int j;

-       if (!PyArg_UnpackTuple(args, "filter", 2, 2, &func, &seq))
-               return NULL;
-

# the are no other changes between METH_O and METH_ARGS
-       {"abs",         builtin_abs,        METH_O, abs_doc},
+       {"abs",         builtin_abs,        METH_ARGS, abs_doc, 1, 1},

-       {"filter",      builtin_filter,     METH_VARARGS, filter_doc},
+       {"filter",      builtin_filter,     METH_ARGS, filter_doc, 2, 2},

Neal