[Bug fortran/81344] Can't disable -ffpe-trap (or not documented)

["dominiq at lps dot ens.fr" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81344

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2017-07-07
     Ever confirmed|0                           |1

--- Comment #1 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
> Once I enable a floating point exception trap with "-ffpe-trap=whatever",
> it doesn't seem to be possible to disable it in the same command line
> (as it is possible with warnings, for instance. An empty list as in
> "-ffpe-trap=" is accepted, but it doesn't disable traps.

AFAIU gfc_handle_fpe_option in gcc/fortran/options.c, '-ffpe-trap=set1
-ffpe-trap=set2' is equivalent to '-ffpe-trap=set1,set2'.

If it helps, I can add a line in the manual.

Note that this is not true for '-ffpe-summary=set1 -ffpe-summary=set2' which
reports the exceptions for set2 only.

> This would be useful when using a tool like CMake, where it is convenient
> to set a default set of flags, and then override them if desired in specific
> files, rather.

(1) You cal always use '-ffpe-trap=set1,set2' for some files and
'-ffpe-trap=set1' for some other files.
(2) You can look at the code implementing -fcheck=no-diag and implement a
-ffpe-trap=no-execpt. IMO it does not worth the trouble.

The above have been tested with

! test the -ffpe-trap= and -ffpe-summary= options.
      program main
      real a,b,c
      a=1e-20
      b=a**2
      print *,b
      a=0
      b=0
      c=a/b
      print *,c
      c=1./b
      print *,c
      STOP
      end