How do I force 0.0/0.0 to return zero instead of NaN in MIPSPro C compiler?

Question

As the question states, I am using the MIPSPRo C compiler, and I have an operation that will return NaN for some data sets where both the numerator and denom are zero. How do I keep this from happening?

Answer 1

On SGI systems with the MIPSPro compiler, you can set the handling of various floating point exceptions with great precision using the facilities in sigfpe.h. As it happens, the division of zero by zero is one such case:

#include <stdio.h>
#include <sigfpe.h>

int main (void) {
    float x = 0.0f;
    (void) printf("default %f / %f = %f\n", x, x, (x / x));
    invalidop_results_[_ZERO_DIV_ZERO] = _ZERO;
    handle_sigfpes(_ON, _EN_INVALID, 0, 0, 0);
    (void) printf("handled %f / %f = %f\n", x, x, (x / x));
    return 0;
}

In use:

arkku@seven:~/test$ cc -version
MIPSpro Compilers: Version 7.3.1.3m
arkku@seven:~/test$ cc -o sigfpe sigfpe.c -lfpe
arkku@seven:~/test$ ./sigfpe
default 0.000000 / 0.000000 = nan0x7ffffe00
handled 0.000000 / 0.000000 = 0.000000

As you can see, setting the _ZERO_DIV_ZERO result changes the outcome of the same division. Likewise you can handle regular division by zero (e.g. if you don't want infinity as the result).

Of course, none of this is standard; it would be more portable to check for NaN after each division and even better to check for zeros before. C99 offers some control over the floating point environment in fenv.h, but I don't think anything suitable for this is available. In any case my old MIPSPro doesn't support C99.