<chrono>: Cache QueryPerformanceFrequency() (#646)

stl/src/xtime.cpp

@@ -4,6 +4,7 @@
 // xtime functions
 
 #include "awint.h"
+#include <atomic>
 #include <stdlib.h>
 #include <time.h>
 #include <xtimec.h>
@@ -87,9 +88,15 @@ _CRTIMP2_PURE long long __cdecl _Query_perf_counter() { // get current value of
 }
 
 _CRTIMP2_PURE long long __cdecl _Query_perf_frequency() { // get frequency of performance counter
-    LARGE_INTEGER li;
-    QueryPerformanceFrequency(&li); // always succeeds
-    return li.QuadPart;
+    static std::atomic<long long> freq_cached{0};
+    long long freq = freq_cached.load(std::memory_order_relaxed);
+    if (freq == 0) {
+        LARGE_INTEGER li;
+        QueryPerformanceFrequency(&li); // always succeeds
+        freq = li.QuadPart; // doesn't change after system boot
+        freq_cached.store(freq, std::memory_order_relaxed);
+    }
+    return freq;
 }
 
 _END_EXTERN_C