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

[Thomas Koenig <tkoenig@netcologne.de>]

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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.

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.

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diff --git a/gcc/fortran/invoke.texi b/gcc/fortran/invoke.texi
index 5679e2f2650..4f4950dad41 100644
--- a/gcc/fortran/invoke.texi
+++ b/gcc/fortran/invoke.texi
@@ -549,15 +549,16 @@ Fortran standard that includes all of the extensions supported by GNU
 Fortran, although warnings will be given for obsolete extensions not
 recommended for use in new code.  The @samp{legacy} value is
 equivalent but without the warnings for obsolete extensions, and may
-be useful for old non-standard programs.  The @samp{f95},
-@samp{f2003}, @samp{f2008}, and @samp{f2018} values specify strict
-conformance to the Fortran 95, Fortran 2003, Fortran 2008 and Fortran
-2018 standards, respectively; errors are given for all extensions
-beyond the relevant language standard, and warnings are given for the
-Fortran 77 features that are permitted but obsolescent in later
-standards. The deprecated option @samp{-std=f2008ts} acts as an alias for
-@samp{-std=f2018}. It is only present for backwards compatibility with
-earlier gfortran versions and should not be used any more.
+be useful for old non-standard programs.  It also sets
+@option{-fwrapv}.  The @samp{f95}, @samp{f2003}, @samp{f2008}, and
+@samp{f2018} values specify strict conformance to the Fortran 95,
+Fortran 2003, Fortran 2008 and Fortran 2018 standards, respectively;
+errors are given for all extensions beyond the relevant language
+standard, and warnings are given for the Fortran 77 features that are
+permitted but obsolescent in later standards. The deprecated option
+@samp{-std=f2008ts} acts as an alias for @samp{-std=f2018}. It is only
+present for backwards compatibility with earlier gfortran versions and
+should not be used any more.
 
 @opindex @code{ftest-forall-temp}
 @item -ftest-forall-temp
diff --git a/gcc/fortran/options.cc b/gcc/fortran/options.cc
index 27311961325..76166ac69aa 100644
--- 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: