[Numpy-discussion] Problems testing the floating point flags

[Charles R Harris]

On Sat, Nov 13, 2010 at 7:41 PM, Charles R Harris <charlesr.harris at gmail.com
> wrote:

> Hi All,
>
> This is in reference to numpy ticket #1671<http://projects.scipy.org/numpy/ticket/1671>and the comments on pull
> request 13 <https://github.com/numpy/numpy/pull/13>. The original problem
> was that the gcc compiler was reordering the instructions so that the
> floating point flags were tested before the computation that needed to be
> checked. The compiler couldn't know that the flags were being tested in the
> original code because it didn't know that about PyUFunc_getfperr(), although
> the fact that floating point computations have side effects should probably
> have limited any code reordering given that unknown. However, even when the
> macro using the glibc function fetestexcept was used the problem persisted.
> This is a known bug <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29186>against
> gcc >= 4.1 that hasn't been addressed in the last four years and it seems
> unlikely that it will be fixed anytime soon. The upshot is that there is no
> reliable way to check the floating point flags using either PyUFunc_getfperr
> or the macro UFUNC_CHECK_STATUS.
>
> Enter the ugly workarounds. If a floating point operation produces a value,
> it is possible to pass a void pointer to that value as a dummy argument to a
> flag checking routine which will force the value to be computed before the
> function is called. There are other games that can be played with volatile
> but I am not convinced that they are robust or portable across compilers. An
> added complication is that PyUFunc_getfperr is part of the numpy API and the
> macro UFUNC_CHECK_STATUS is public so if we add workarounds they need new
> names. There is also the question if we want to expose them. In any case,
> suggestions as to names and approaches are welcome. And if anyone has a
> better solution it would be great to hear it.
>
>
Another possible solution is like so:

static __attribute__ ((noinline)) int
fpecheck(int *status)
{
    *status = PyUFunc_getfperr();
    return 0;
}

static __attribute__ ((noinline)) int
fpeclear(int *status)
{
    PyUFunc_clearfperr();
    return 0;
}

int myfunc(void)
{
    int status;

   fpeclear(&status);
    do {
        stuff;
    } while (fpecheck(&status));
    return status;
}

Chuck
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