m_walltime Function

   FUNCTION m_walltime() RESULT(wt)
      !! returns time from a real-time clock, protected against rolling
      !! early/easily
      !! @note
      !! same implementation for all machines.
      !! might still roll, if not called multiple times per count_max/count_rate

#if defined(__LIBXSMM)
      USE libxsmm, ONLY: libxsmm_timer_tick, libxsmm_timer_duration
#endif
      REAL(KIND=dp)                                      :: wt

#if defined(__LIBXSMM)
      wt = libxsmm_timer_duration(0_int_8, libxsmm_timer_tick())
#else

      INTEGER(KIND=int_8)                                :: count
      INTEGER(KIND=int_8), SAVE                          :: count_max, count_rate, cycles = -1, &
                                                            last_count
!$    IF (.FALSE.) THEN
! count lies in [0,count_max] and increases monotonically

         IF (cycles == -1) THEN ! get parameters of system_clock and initialise
            CALL SYSTEM_CLOCK(count_rate=count_rate, count_max=count_max)
            cycles = 0
            last_count = 0
         END IF

         CALL SYSTEM_CLOCK(count=count)

         ! protect against non-standard cases where time might be non-monotonous,
         ! but it is unlikely that the clock cycled (e.g. underlying system clock adjustments)
         ! i.e. if count is smaller than last_count by only a small fraction of count_max,
         ! we use last_count instead
         ! if count is smaller, we assume that the clock cycled.
         IF (count < last_count) THEN
            IF (last_count - count < count_max/100) THEN
               count = last_count
            ELSE
               cycles = cycles + 1
            END IF
         END IF

         ! keep track of our history
         last_count = count

         wt = (REAL(count, KIND=dp) + REAL(cycles, KIND=dp)*(1.0_dp + REAL(count_max, KIND=dp))) &
              /REAL(count_rate, KIND=dp)
!$    ELSE
!$       wt = OMP_GET_WTIME()
!$    END IF
#endif
   END FUNCTION m_walltime