[Python-Dev] ANSI strict aliasing and Python

Martin v. L÷wis martin@v.loewis.de
18 Jul 2003 19:08:24 +0200


"Tim Peters" <tim.one@comcast.net> writes:

> This is strange (according to me).  The real point of adding that option
> would be to prevent bad code generation in the presence of pretty common
> non-standard C code, but I don't know why a compiler would complain about
> PyObject_IsTrue:
[...]
> 	if (v == Py_True)   THIS IS LINE 1565 FOR ME

Notice that a compiler is allowed to infer that the test is always
false. Py_True is

#define Py_True ((PyObject *) &_Py_TrueStruct)

where _Py_TrueStruct is of type PyIntObject. A PyObject* could never
ever possibly point to a PyIntObject. If you still compare the two,
you obviously assume the language is not standard C.

> The way in which Python fakes inheritance by hand means there are potential
> problems all over the place (just about everywhere we cast to or from
> PyObject*), but very likely very few real problems.  If the only ones gcc
> complains about involve Py_{True,False,None}, it's not being helpful.

These are the ones that gcc recognizes. It can and will generate bad
code all over the place.

Regards,
Martin