[Bug fortran/55548] New: SYSTEM_CLOCK with integer(8) provides nanosecond resolution, but only microsecond precision (without -lrt)

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55548

             Bug #: 55548
           Summary: SYSTEM_CLOCK with integer(8) provides nanosecond
                    resolution, but only microsecond precision (without
                    -lrt)
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: janus@gcc.gnu.org


Simple test case:

  integer(8) :: t, rate, cmax
  call system_clock(t, rate, cmax)
  print *, t, rate, cmax
end

When compiling this without any special flags (and in particular without -lrt),
this gives a rate of 1000000000 (corresponding to 1 nanosecond), but the values
of t are only precise to 1 microsecond (the last three digits are always zero).
This is on x86_64-unknown-linux-gnu (Linux 3.4.11, glibc 2.15).

I am aware that linking with -lrt (which is mentioned in the docu) solves this
problem and makes SYSTEM_CLOCK yield values which indeed have nanosecond
precision.

However, the precision claimed by the COUNT_RATE argument should better match
the actual precision (also with default flags!).


Possible solutions:
1) Use a nanosecond COUNT_RATE only when -lrt is given, and microsecond
otherwise.
2) Always use microsecond with integer(8), and nanosecond with integer(16).


Using SYSTEM_CLOCK with integer(16) arguments currently results in:
sysclock.f90:(.text+0x455): undefined reference to `_gfortran_system_clock_16'