[issue1621] Do not assume signed integer overflow behavior

[Neal Norwitz]

Neal Norwitz added the comment:

I just added -Wstrict-overflow to the code in sysmodule.c
(sys_getframe) and tried with gcc 4.2.1.  It doesn't warn.  I wonder
if 4.3 is more picky or warns when it shouldn't?

Unless if I changed the code so it doesn't work:

typedef struct {int ref;} PyObject;
typedef struct { PyObject* f_back; } PyFrameObject;
int PyArg_ParseTuple(PyObject*, const char*, int*);

PyObject *
sys_getframe(PyFrameObject *f, PyObject *self, PyObject *args)
{
        int depth = -1;

        if (!PyArg_ParseTuple(args, "|i:_getframe", &depth))
                return 0;

        while (depth > 0 && f != 0) {
                f = (PyFrameObject*)f->f_back;
                --depth;
        }
        return (PyObject*)f;
}

Compiled with:
gcc-4.2.1-glibc-2.3.2/x86_64-unknown-linux-gnu/bin/x86_64-unknown-linux-gnu-gcc
-Wstrict-overflow -c xx.c

produced no warnings.  This is not a stock gcc 4.2.1, so that could
also be an issue.  Did I run it correctly.  Is there anything else I
need to do?  If you run the code above with gcc 4.3, does it produce a
warning?

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