[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I have idea on how to fix the FENV_ACCESS issue (for gimple), basically you add
an extra virtual use op for float gimples and virtual define for functions and
inline-asm. Now I am not going to implement this but I want to record it
somewhere.

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[pavel.morozkin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

Pavel M <pavel.morozkin at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pavel.morozkin at gmail dot com

--- Comment #12 from Pavel M <pavel.morozkin at gmail dot com> ---
As far as I understand, support of #pragma STDC xxx is required by the C
standard for hosted implementation. If so, then it is worth reaching
conformance to increase the priority of this issue. It is logical for the end
users to expect the support of  #pragma STDC xxx. Now they see:
t888.c:3: warning: ignoring ‘#pragma STDC FP_CONTRACT’ [-Wunknown-pragmas]
t888.c:4: warning: ignoring ‘#pragma STDC FENV_ACCESS’ [-Wunknown-pragmas]
t888.c:5: warning: ignoring ‘#pragma STDC CX_LIMITED_RANGE’ [-Wunknown-pragmas]
Invoked with: gcc t888.c -c -std=c11 -Wall.

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

--- Comment #13 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
If the default state of these pragmas (at least with options like -std=c11) is
respectively "off", "on", "off", then changing the pragma state can safely be
ignored by the implementation, as implementations are not obliged to optimize
as allowed by the new pragma state. Thus, in such a case, the warnings are
useless.

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[pavel.morozkin at gmail dot com]

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

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

--- Comment #15 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Pavel M from comment #14)
> The "warning: ignoring '#pragma STDC FENV_ACCESS' [-Wunknown-pragmas]"
> probably needs to be generated by default

Getting the warning on "#pragma STDC FENV_ACCESS OFF" is useless. The issue
comes from the fact that the FENV_ACCESS is OFF by default and you want to set
it to ON; there should be a warning in this case, but only in this case.
Concerning the other pragmas, this also depends on their default state.

For instance, since commit 6dbe09581a1349498f6b4546f68cb9fd3205e120[*], the
STDC FP_CONTRACT pragma is set to OFF by default when a standard mode is
provided (e.g. -std=c17), so that the warning on this pragma is not needed in
this case (whether the user sets it to ON or OFF).

[*]
https://gcc.gnu.org/git/?p=gcc.git&a=commit;h=6dbe09581a1349498f6b4546f68cb9fd3205e120

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[pavel.morozkin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

--- Comment #16 from Pavel M <pavel.morozkin at gmail dot com> ---
Note: The #pragma STDC FENV_ACCESS is unknown and ignored (leading to FP
issues), however, the __STDC_IEC_559__ is defined to 1. Confused.

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

--- Comment #17 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Pavel M from comment #16)
> Note: The #pragma STDC FENV_ACCESS is unknown and ignored (leading to FP
> issues), however, the __STDC_IEC_559__ is defined to 1. Confused.

Yes, and IMHO, with __STDC_IEC_559__ defined to 1, this is a much more severe
issue. This PR should not be regarded as an enhancement, but as a real bug
(this would be an enhancement only if when __STDC_IEC_559__ is defined to 1,
there would be a lack of optimization).

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|enhancement                 |normal

--- Comment #18 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Vincent Lefèvre from comment #17)
> (In reply to Pavel M from comment #16)
> > Note: The #pragma STDC FENV_ACCESS is unknown and ignored (leading to FP
> > issues), however, the __STDC_IEC_559__ is defined to 1. Confused.
> 
> Yes, and IMHO, with __STDC_IEC_559__ defined to 1, this is a much more
> severe issue. This PR should not be regarded as an enhancement, but as a
> real bug

OK, I put the importance back to "normal"

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[rlcamp.pdx at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

Campbell <rlcamp.pdx at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rlcamp.pdx at gmail dot com

--- Comment #19 from Campbell <rlcamp.pdx at gmail dot com> ---
Regardless of the wider issue, it would be nice to get STDC CX_LIMITED_RANGE
implemented. Is there something specific blocking this?

[Bug c/20785] Pragma STDC * (C99 FP) unimplemented

[rguenth at gcc dot gnu dot org]

------- Comment #6 from rguenth at gcc dot gnu dot org  2009-01-30 07:19 -------
*** Bug 39036 has been marked as a duplicate of this bug. ***


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tydeman at tybor dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785