turned off force_ftz.c, which creates numerous portability issues, an…

…d turned FIX_UNDERFLOW_PROBLEM to .false. by default (not needed any more on modern processors). Also added a comment in flags.guess about -Werror=no-unused-function

flags.guess

@@ -103,6 +103,10 @@ case $my_FC in
         # GNU gfortran
         #
         # optimization report: "-fopt-info" or very verbose: "-fopt-info-all"
+#
+# with some versions of gfortran/gcc you make get errors about unused functions when compiling with the options below;
+# if so, either change -Wunused to -Wunused -Werror=no-unused-function, or remove -Wunused, or remove -Werror
+#
         DEF_FFLAGS="-std=f2003 -fimplicit-none -frange-check -fmax-errors=10 -pedantic -pedantic-errors -Waliasing -Wampersand -Wcharacter-truncation -Wline-truncation -Wsurprising -Wno-tabs -Wunderflow -ffpe-trap=invalid,zero,overflow -Wunused -Werror" # -mcmodel=medium
         OPT_FFLAGS="-O3 -finline-functions"
         DEBUG_FFLAGS="-g -O0 -ggdb -fbacktrace -fbounds-check"

setup/constants.h.in

@@ -356,9 +356,9 @@
 !----------- do not modify anything below -------------
 !------------------------------------------------------
 
-! on some processors (e.g. some Intel chips) it is necessary to suppress underflows
-! by using a small initial field instead of zero
-  logical, parameter :: FIX_UNDERFLOW_PROBLEM = .true.
+!! DK DK August 2018: on modern processors this does not happen any more,
+!! DK DK August 2018: and thus no need to purposely lose accuracy to avoid underflows; thus turning it off by default
+  logical, parameter :: FIX_UNDERFLOW_PROBLEM = .false. ! .true.
 
 ! some useful constants
   double precision, parameter :: PI = 3.141592653589793d0

src/shared/force_ftz.c

@@ -1,5 +1,5 @@
 /*
- !=====================================================================
+ !========================================================================
  !
  !               S p e c f e m 3 D  V e r s i o n  3 . 0
  !               ---------------------------------------
@@ -17,14 +17,16 @@
  !
  ! This program is distributed in the hope that it will be useful,
  ! but WITHOUT ANY WARRANTY; without even the implied warranty of
- ! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ ! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  ! GNU General Public License for more details.
  !
  ! You should have received a copy of the GNU General Public License along
  ! with this program; if not, write to the Free Software Foundation, Inc.,
  ! 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  !
- !=====================================================================
+ ! The full text of the license is available in file "LICENSE".
+ !
+ !========================================================================
  */
 
 /* Dimitri Komatitsch, University of Toulouse, May 2011: */
@@ -69,6 +71,10 @@
 void
 FC_FUNC_(force_ftz,FORCE_FTZ)()
 {
+
+// DK DK Aug 2018: not needed anymore these days, and we had numerous portability issues with this routine, thus turning it off
+#undef FORCE_FTZ
+
 #ifdef __GNUC__
 #ifdef FORCE_FTZ
   unsigned int x;