[Python-Dev] Why a STACKADJ(-1) in UNARY_NOT on ceval.c?

[Guido van Rossum]

It's to make up for the lack of SET_TOP in that path.

On Thu, Feb 5, 2009 at 10:19 AM, Cesare Di Mauro
<cesare.dimauro at a-tono.com> wrote:
> Looking at the UNARY_NOT case in ceval.c:
>
>                case UNARY_NOT:
>                        v = TOP();
>                        err = PyObject_IsTrue(v);
>                        Py_DECREF(v);
>                        if (err == 0) {
>                                Py_INCREF(Py_True);
>                                SET_TOP(Py_True);
>                                continue;
>                        }
>                        else if (err > 0) {
>                                Py_INCREF(Py_False);
>                                SET_TOP(Py_False);
>                                err = 0;
>                                continue;
>                        }
>                        STACKADJ(-1);
>                        break;
>
>
> I don't understand why there's a STACKADJ(-1) at its end. Looking at the
> code, we know that if the CPU arrives to the STACKADJ, it's because of an
> error condition in the PyObject_IsTrue that sets err to a < 0 value, so
> exiting the big switch statement, an error will be raised.
>
> So the question is, why there's the need to skip the top stack PyObject?
> It's a different behaviour comparing it to the all other unary operators.
> For example:
>
>                case UNARY_NEGATIVE:
>                        v = TOP();
>                        x = PyNumber_Negative(v);
>                        Py_DECREF(v);
>                        SET_TOP(x);
>                        if (x != NULL) continue;
>                        break;
>
> There's no STACKADJ instruction on errors.
>
> Can someone explain it?
>
> Thanks a lot
>
> Cesare
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)