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

[Richard Biener <richard.guenther@gmail.com>]

> Am 10.03.2023 um 18:54 schrieb Thomas Koenig via Fortran <fortran@gcc.gnu.org>:
> 
> 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.

The „problem“ is latent forever, I’m not sure it’s good to amend the kitchen-sink std=legacy option with -fwrapv since that has quite some negative effects on optimization.

Richard 

> Best regards
> 
>    Thomas
> 
> Set -frapv if -std=legacy is set.
> 
> 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.
> 
> gcc/fortran/ChangeLog:
> 
>    PR fortran/109075
>    * options.cc (gfc_handle_option):  If -std=legacy is set,
>    also set -frwapv.
>    * invoke.texi: Document the change.
> <p1.diff>