Optimizer (-O3) Disabling IEEE Exception Handling

[Damian McGuckin]

Hi,

I hope this has not been addressed before but my search returned nothing
the same. I found vaguely related stuff but not this.

I am talking is straightforward optimization, i.e. -O3 (say), which I read 
the manual page to say, does not affect IEEE 754 stuff at all.

The following code

#include <fenv.h>
#include <stdio.h>

int bug()
{
        double z;
        int test;

        feclearexcept(FE_ALL_EXCEPT);
        printf("Except %x\n", fetestexcept(FE_ALL_EXCEPT));
        z = 1.0 - Tiny;
        test = fetestexcept(FE_ALL_EXCEPT);
        printf("TEST says %x @%.9e should be %x\n", test, z, FE_INEXACT);

        return(0);
}

int main()
{
        bug();
        return(0);
}

When compiled as just

        gcc bug.c -lm

correctly raised the FP_INEXACT (0x20) flag. It produces

        Except 0
        TEST says 20 @1.000000000e+00 should be 20

where the first line notes that it starts cleared.

When compiled as

        gcc -O3 bug.c -lm

or even
        g++ bug.c

The second line produced says

        TEST says 0 @1.000000000e+00 should be 20

There is no exception raised.

What did I do wrong. The -O flags is not supposed to affect that.

Does it need some extra flag? Do I need a much newer version?

The version is 4.8.5-16 (on CentOS 7.4)

Thanks - Damian

Pacific Engineering Systems International, 277-279 Broadway, Glebe NSW 2037
Ph:+61-2-8571-0847 .. Fx:+61-2-9692-9623 | unsolicited email not wanted here
Views & opinions here are mine and not those of any past or present employer