winpthreads: Increase precision of CLOCK_REALTIME, add CLOCK_REALTIME_COARSE.

Use GetSystemTimePreciseAsFileTime() for CLOCK_REALTIME if available.
Always use GetSystemTimeAsFileTime() for CLOCK_REALTIME_COARSE.

Signed-off-by: Christian Franke <christian.franke@t-online.de>
Signed-off-by: Liu Hao <lh_mouse@126.com>

mingw-w64-libraries/winpthreads/include/pthread_time.h

diff --git a/mingw-w64-libraries/winpthreads/include/pthread_time.h b/mingw-w64-libraries/winpthreads/include/pthread_time.h
index 7a7538c..7818b11 100644
--- a/mingw-w64-libraries/winpthreads/include/pthread_time.h
+++ b/mingw-w64-libraries/winpthreads/include/pthread_time.h
@@ -70,6 +70,10 @@
 #define CLOCK_THREAD_CPUTIME_ID     3
 #endif
 
+#ifndef CLOCK_REALTIME_COARSE
+#define CLOCK_REALTIME_COARSE       4
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif

mingw-w64-libraries/winpthreads/src/clock.c

diff --git a/mingw-w64-libraries/winpthreads/src/clock.c b/mingw-w64-libraries/winpthreads/src/clock.c
index 5ad710b..ba983c9 100644
--- a/mingw-w64-libraries/winpthreads/src/clock.c
+++ b/mingw-w64-libraries/winpthreads/src/clock.c
@@ -31,6 +31,30 @@
     return 0;
 }
 
+typedef void (WINAPI * GetSystemTimeAsFileTime_t)(LPFILETIME);
+static GetSystemTimeAsFileTime_t GetSystemTimeAsFileTime_p /* = 0 */;
+
+static GetSystemTimeAsFileTime_t try_load_GetSystemPreciseTimeAsFileTime()
+{
+    /* Use GetSystemTimePreciseAsFileTime() if available (Windows 8 or later) */
+    GetSystemTimeAsFileTime_t get_time = (GetSystemTimeAsFileTime_t)(intptr_t)GetProcAddress(
+        GetModuleHandle ("kernel32.dll"),
+        "GetSystemTimePreciseAsFileTime"); /* <1us precision on Windows 10 */
+    if (get_time == NULL)
+        get_time = GetSystemTimeAsFileTime; /* >15ms precision on Windows 10 */
+    __atomic_store_n(&GetSystemTimeAsFileTime_p, get_time, __ATOMIC_RELAXED);
+    return get_time;
+}
+
+static WINPTHREADS_INLINE GetSystemTimeAsFileTime_t load_GetSystemTimeBestAsFileTime()
+{
+    GetSystemTimeAsFileTime_t get_time =
+        __atomic_load_n(&GetSystemTimeAsFileTime_p, __ATOMIC_RELAXED);
+    if (get_time == NULL)
+        get_time = try_load_GetSystemPreciseTimeAsFileTime();
+    return get_time;
+}
+
 /**
  * Get the resolution of the specified clock clock_id and
  * stores it in the struct timespec pointed to by res.
@@ -52,7 +76,13 @@
  */
 int clock_getres(clockid_t clock_id, struct timespec *res)
 {
-    switch(clock_id) {
+    clockid_t id = clock_id;
+
+    if (id == CLOCK_REALTIME && load_GetSystemTimeBestAsFileTime() == GetSystemTimeAsFileTime)
+        id = CLOCK_REALTIME_COARSE; /* GetSystemTimePreciseAsFileTime() not available */
+
+    switch(id) {
+    case CLOCK_REALTIME:
     case CLOCK_MONOTONIC:
         {
             LARGE_INTEGER pf;
@@ -68,7 +98,7 @@
             return 0;
         }
 
-    case CLOCK_REALTIME:
+    case CLOCK_REALTIME_COARSE:
     case CLOCK_PROCESS_CPUTIME_ID:
     case CLOCK_THREAD_CPUTIME_ID:
         {
@@ -118,6 +148,16 @@
     switch(clock_id) {
     case CLOCK_REALTIME:
         {
+            load_GetSystemTimeBestAsFileTime()(&ct.ft);
+            t = ct.u64 - DELTA_EPOCH_IN_100NS;
+            tp->tv_sec = t / POW10_7;
+            tp->tv_nsec = ((int) (t % POW10_7)) * 100;
+
+            return 0;
+        }
+
+    case CLOCK_REALTIME_COARSE:
+        {
             GetSystemTimeAsFileTime(&ct.ft);
             t = ct.u64 - DELTA_EPOCH_IN_100NS;
             tp->tv_sec = t / POW10_7;