Re: [patch, Fortran] Enable -fwrapv for -std=legacy

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Mar 10, 2023 at 06:54:10PM +0100, Thomas Koenig via Gcc-patches wrote:
> Hello world, here's the patch that was discussed.
> 
> Regression-tested. OK for trunk?
> 
> Since this appeared only in gcc13, I see no need for a backport.
> I will also document this in the changes file.
> 
> Best regards
> 
> 	Thomas
> 
> Set -frapv if -std=legacy is set.

s/frapv/fwrapv/

> Fortran legacy codes sometimes contain linear congruential
> seudorandom number generators.  These generators implicitly depend
> on wrapping behavior on integer overflow, which is illegal Fortran,
> but the best they could to at the time.
> 
> A gcc13 change exposed this in rnflow, part of the Polyhedron
> benchmark, with -O3.  Rather than "regress" on such code, this patch
> enables -fwrapv if -std=legacy is enabled.  This allows the benchmark
> to run successfully, and presumably lots of other code as well.

I think it certainly shouldn't overwrite it, it can adjust the default,
but if user asks for -std=legacy -fno-wrapv or
-fno-wrapv -std=legacy, it should honor that.

> gcc/fortran/ChangeLog:
> 
> 	PR fortran/109075
> 	* options.cc (gfc_handle_option):  If -std=legacy is set,

s/  / /

> 	also set -frwapv.

s/rwapv/wrapv/

> 	* invoke.texi: Document the change.

> --- a/gcc/fortran/options.cc
> +++ b/gcc/fortran/options.cc
> @@ -797,6 +797,8 @@ gfc_handle_option (size_t scode, const char *arg, HOST_WIDE_INT value,
>      case OPT_std_legacy:
>        set_default_std_flags ();
>        gfc_option.warn_std = 0;
> +      /* -std=legacy implies -fwapv, but the user can override it.  */
> +      flag_wrapv = 1;
>        break;
>  
>      case OPT_fshort_enums:

So, I think it should be done later, after option processing, say
in gfc_post_options if it is possible to determine if -std=legacy
has been specified (and not say -std=legacy -std=f2018 etc.),
and using SET_OPTION_IF_UNSET.

	Jakub