The bug occurs when GCC emits a single instruction (stxsspx) for cast + memcpy.

I understand that the struct module is not affected because there is a condition branch (if) between the cast ("float x = ...") and the memcpy().

static int
np_float(char *p, PyObject *v, const formatdef *f)
{
    float x = (float)PyFloat_AsDouble(v);
    if (x == -1 && PyErr_Occurred()) {
        PyErr_SetString(StructError,
                        "required argument is not a float");
        return -1;
    }
    memcpy(p, (char *)&x, sizeof x);
    return 0;
}