Numpy: Improvement in PyArray_FromArray

Call-graph of Numpy scalar array addition, shows that get_ufunc_arguments contribute to almost 18% in cumulative time. 

Callgraph for x = np.asarray(1.0). 

Problem

Tracing the execution path get_ufunc_arguments under for x = numpy.asarray(1.0); x + x , which flow as PyArrayFromAny, PyArrayFromArray, PyArrayCanCastArrayTo, can_cast_scalar_to.

In PyArrayFromArray, I find out if argument newtype is Null, it get value of oldtype. So, technically it check if casting is possible for same type which is useless.

 
    oldtype = PyArray_DESCR(arr);
    if (newtype == NULL) {
        newtype = oldtype;
        Py_INCREF(oldtype);
    }

Improvement

It is better to bypass this for newtype=Null and flag=0. As I did below,

 
    oldtype = PyArray_DESCR(arr);
    if (newtype == NULL) {
        /* Check if object is of array with Null newtype. 
         * If so return it directly instead of checking for casting.
         */
        if (flags == 0) {
            Py_INCREF(arr);
            return (PyObject *)arr;
        }
        newtype = oldtype;
        Py_INCREF(oldtype);
    }

This brings down the cumulative time contribution significantly from 17.9% to 3.1%.

Callgraph for x = np.asarray(1.0), but after improvement

Pull request for this is #3509.