[Bug fortran/56919] New: [4.6/4.7/4.8/4.9 Regression] Wrong result for SYSTEM_CLOCK on Cygwin

["burnus at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56919

             Bug #: 56919
           Summary: [4.6/4.7/4.8/4.9 Regression] Wrong result for
                    SYSTEM_CLOCK on Cygwin
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org
                CC: janus@gcc.gnu.org, jb@gcc.gnu.org


The regression has been caused by Janne's patch, Rev.
http://gcc.gnu.org/r169449 - which was checked in 2011-01-31  (= GCC 4.6)


See Angelo Graziosi's bug report at
http://gcc.gnu.org/ml/fortran/2013-04/msg00085.html


The problem is that Cygwin does not support CLOCK_MONOTONIC; using it will
return always return 0.

Note that Cygwin not only defines CLOCK_MONOTONIC but since Cygwin-1.7.6 also 
"_POSIX_MONOTONIC_CLOCK 200112L" [1]. Thus, _POSIX_MONOTONIC_CLOCK [2] can also
not be used.


Currently, libgfortran uses (cf.libgfortran/intrinsics/system_clock.c):


/* POSIX states that CLOCK_REALTIME must be present if clock_gettime
   is available, others are optional.  */
#ifdef CLOCK_MONOTONIC
#define GF_CLOCK_MONOTONIC CLOCK_MONOTONIC
#else
#define GF_CLOCK_MONOTONIC CLOCK_REALTIME
#endif

#ifdef HAVE_CLOCK_GETTIME
  err = clock_gettime (GF_CLOCK_MONOTONIC, &ts);
#else
#if defined(HAVE_CLOCK_GETTIME_LIBRT) && SUPPORTS_WEAK && GTHREAD_USE_WEAK
  if (weak_gettime)
    err = clock_gettime (GF_CLOCK_MONOTONIC, &ts);
#endif
  err = gf_gettime (secs, nanosecs);
#endif


And gf_gettime (time_1.h) uses: gettimeofday - or if not available:
"clock_gettime (CLOCK_REALTIME" - or a simple "time(NULL)".


Nick suggests to simply call gf_gettime unconditionally. [3]


[1] Cf. http://cygwin.com/ml/cygwin-announce/2010-08/msg00019.html ,
http://sourceware.org/ml/newlib/2010/msg00454.html
[2] http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap02.html
[3] http://gcc.gnu.org/ml/fortran/2013-04/msg00086.html