https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785
--- Comment #14 from Pavel M <pavel.morozkin at gmail dot com> ---
To: Vincent Lefèvre
Re: the warnings are useless.
The "warning: ignoring '#pragma STDC FENV_ACCESS' [-Wunknown-pragmas]" probably
needs to be generated by default (i.e. not with -Wall) because now gcc silently
miscompiles the following (though unusual) code:
/* t0.c */
#include <stdio.h>
#include <fenv.h>
#include <float.h>
#pragma STDC FENV_ACCESS ON
int main( void )
{
volatile double d1 = DBL_MAX;
volatile double d2 = DBL_MAX;
#ifdef __STDC__
printf("__STDC__ %d\n", __STDC__);
#endif
#ifdef __STDC_VERSION__
printf("__STDC_VERSION__ %ld\n", __STDC_VERSION__);
#endif
#ifdef __STDC_IEC_559__
printf("__STDC_IEC_559__ %d\n", __STDC_IEC_559__);
#endif
if (feclearexcept( FE_ALL_EXCEPT ) != 0)
{
printf("error: feclearexcept( FE_ALL_EXCEPT ) failed\n");
return 1;
}
( void )( d1 * d2 );
if (fetestexcept( FE_OVERFLOW ) == 0)
{
printf("error: no FE_OVERFLOW is raised\n");
return 2;
}
return 0;
}
Invocation: gcc t0.c -std=c11 -pedantic -lm
<nothing>
Execution: ./a.out
Expected output:
__STDC__ 1
__STDC_VERSION__ 201710
__STDC_IEC_559__ 1
Actual output:
__STDC__ 1
__STDC_VERSION__ 201710
__STDC_IEC_559__ 1
error: no FE_OVERFLOW is raised
