use C++11 functionality to determine wall time

src/platform.cpp

@@ -106,6 +106,10 @@ static const zip_info &find_zip_type(const std::string &file)
 
 /* ------------------------------------------------------------------ */
 
+// set reference time stamp during executable/library init.
+// should provide better resolution than using epoch, if the system clock supports it.
+static auto initial_time = std::chrono::steady_clock::now();
+
 using namespace LAMMPS_NS;
 
 // get CPU time
@@ -156,22 +160,7 @@ double platform::cputime()
 ------------------------------------------------------------------------ */
 double platform::walltime()
 {
-  double wtime;
-
-#if defined(_WIN32)
-
-  wtime = GetTickCount64() * 0.001;
-
-#else
-
-  struct timeval tv;
-
-  gettimeofday(&tv, nullptr);
-  wtime = 1.0 * tv.tv_sec + 1.0e-6 * tv.tv_usec;
-
-#endif
-
-  return wtime;
+  return std::chrono::duration<double>(std::chrono::steady_clock::now() - initial_time).count();
 }
 
 /* ----------------------------------------------------------------------

src/platform.h

@@ -27,25 +27,30 @@ namespace platform {
   /*! Return the consumed CPU time for the current process in seconds
    *
    * This is a wrapper around the POSIX function getrusage() and its Windows equivalent.
-   * It is to be used in a similar fashion than MPI_Wtime().
+   * It is to be used in a similar fashion than MPI_Wtime(). Its resolution may
+   * be rather low so it can only be trusted when observing processes consuming at
+   * seconds or more of CPU time.
    *
-   *  \return used CPU time in second */
+   *  \return used CPU time in seconds */
 
   double cputime();
 
   /*! Return the wall clock state for the current process in seconds
    *
-   * This is a wrapper around the gettimeofday() function  and its Windows equivalent.
-   * It is to be used in a similar fashion than MPI_Wtime().
+   * This this clock is counting continuous time is initialized during
+   * Load of the executable/library. Its absolute value must be considered
+   * arbitrary and thus elapsed wall times are measured in taking differences.
+   * It is therefore to be used in a similar fashion as MPI_Wtime() but
+   * has a different offset, usually leading to better resolution.
    *
-   *  \return wall clock time in second */
+   *  \return wall clock time in seconds */
 
   double walltime();
 
-
   /*! Suspend execution for a microsecond interval
    *
    * This emulates the usleep(3) BSD function call also mentioned in POSIX.1-2001.
+   * This is not a precise delay; it may be longer, but not shorter.
    *
    * \param  usec  length of delay in microseconds */