Index: flang/runtime/time-intrinsic.cpp
===================================================================
--- flang/runtime/time-intrinsic.cpp
+++ flang/runtime/time-intrinsic.cpp
@@ -107,6 +107,57 @@
   return std::min(std::numeric_limits<std::clock_t>::max(),
       std::numeric_limits<count_t>::max());
 }
+
+constexpr count_t NSECS_PER_SEC = 1'000'000'000;
+
+// POSIX implementation using clock_gettime. This is only enabled if
+// clock_gettime is available.
+template <typename T = int, typename U = struct timespec>
+count_t getSystemClockCount(preferred_implementation,
+    // We need some dummy parameters to pass to decltype(clock_gettime).
+    T ClockId = 0, U *Timespec = nullptr,
+    decltype(clock_gettime(ClockId, Timespec)) *Enabled = nullptr) {
+#if defined CLOCK_THREAD_CPUTIME_ID
+#define CLOCKID CLOCK_THREAD_CPUTIME_ID
+#elif defined CLOCK_PROCESS_CPUTIME_ID
+#define CLOCKID CLOCK_PROCESS_CPUTIME_ID
+#elif defined CLOCK_MONOTONIC
+#define CLOCKID CLOCK_MONOTONIC
+#else
+#define CLOCKID CLOCK_REALTIME
+#endif
+  struct timespec tspec;
+  if (clock_gettime(CLOCKID, &tspec) != 0) {
+    // Return -HUGE() to represent failure.
+    return -std::numeric_limits<count_t>::max();
+  }
+
+  // Wrap around to avoid overflows.
+  count_t max_secs = std::numeric_limits<count_t>::max() / NSECS_PER_SEC;
+  count_t wrapped_secs = tspec.tv_sec % max_secs;
+
+  // At this point, wrapped_secs < max_secs, and max_secs has already been
+  // truncated by the division. Therefore, we should still have enough room to
+  // add tv_nsec, since it is < NSECS_PER_SEC.
+  return tspec.tv_nsec + wrapped_secs * NSECS_PER_SEC;
+}
+
+template <typename T = int, typename U = struct timespec>
+count_t getSystemClockCountRate(preferred_implementation,
+    // We need some dummy parameters to pass to decltype(clock_gettime).
+    T ClockId = 0, U *Timespec = nullptr,
+    decltype(clock_gettime(ClockId, Timespec)) *Enabled = nullptr) {
+  return NSECS_PER_SEC;
+}
+
+template <typename T = int, typename U = struct timespec>
+count_t getSystemClockCountMax(preferred_implementation,
+    // We need some dummy parameters to pass to decltype(clock_gettime).
+    T ClockId = 0, U *Timespec = nullptr,
+    decltype(clock_gettime(ClockId, Timespec)) *Enabled = nullptr) {
+  count_t max_secs = std::numeric_limits<count_t>::max() / NSECS_PER_SEC;
+  return max_secs * NSECS_PER_SEC - 1;
+}
 } // anonymous namespace
 
 namespace Fortran::runtime {