[Python-Dev] Compiler warnings

["Martin v. Löwis"]

Tim Peters wrote:
>>I noticed a few compiler warnings, when I compile Python on my amd64 with
>>gcc 4.0.3:
>>
>>Objects/longobject.c: In function 'PyLong_AsDouble':
>>Objects/longobject.c:655: warning: 'e' may be used uninitialized in this function
> 
> 
> Well, that's pretty bizarre.  There's _obviously_ no way to get to a
> reference to `e` without going through
> 
> 	x = _PyLong_AsScaledDouble(vv, &e);
> 
> first.  That isn't a useful warning.

It inlines the function to make this determination. Now, it's not true
that e can be uninitialized then, but there the gcc logic fails:

If you take the

        if (vv == NULL || !PyLong_Check(vv)) {
                PyErr_BadInternalCall();
                return -1;
        }

case in _PyLong_AsScaledDouble, *exponent won't be initialized. Then,
in PyLong_AsDouble, with

        x = _PyLong_AsScaledDouble(vv, &e);
        if (x == -1.0 && PyErr_Occurred())
                return -1.0;

it looks like the return would not be taken if PyErr_Occurred returns
false. Of course, it won't, but that is difficult to analyse.

> I don't know.  Is this version of gcc broken in some way relative to
> other gcc versions, or newer, or ... ?  We certainly don't want to see
> warnings under gcc, since it's heavily used, but I'm not clear on why
> other versions of gcc aren't producing these warnings (or are they,
> and people have been ignoring that?).

gcc 4 does inlining in far more cases now.

Regards,
Martin