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

[janus at gcc dot gnu.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: unassig...@gcc.gnu.org

        ReportedBy: ja...@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'