vm: speed up nano-count primitive on Windows

diff --git a/vm/os-windows-nt.cpp b/vm/os-windows-nt.cpp
index 4fea294a12da583fb82ff55565c83a67f636113d..97cd2146afe50b54b5dc3910ea65a36574a521dc 100755 (executable)
--- a/vm/os-windows-nt.cpp
+++ b/vm/os-windows-nt.cpp
@@ -18,17 +18,24 @@ u64 system_micros()
 
 u64 nano_count()
 {
-       LARGE_INTEGER count;
-       LARGE_INTEGER frequency;
+       static double scale_factor;
+
        static u32 hi = 0;
        static u32 lo = 0;
-       BOOL ret;
-       ret = QueryPerformanceCounter(&count);
+
+       LARGE_INTEGER count;
+       BOOL ret = QueryPerformanceCounter(&count);
        if(ret == 0)
                fatal_error("QueryPerformanceCounter", 0);
-       ret = QueryPerformanceFrequency(&frequency);
-       if(ret == 0)
-               fatal_error("QueryPerformanceFrequency", 0);
+
+       if(scale_factor == 0.0)
+       {
+               LARGE_INTEGER frequency;
+               BOOL ret = QueryPerformanceFrequency(&frequency);
+               if(ret == 0)
+                       fatal_error("QueryPerformanceFrequency", 0);
+               scale_factor = (1000000000.0 / frequency.QuadPart);
+       }
 
 #ifdef FACTOR_64
        hi = count.HighPart;
@@ -40,7 +47,7 @@ u64 nano_count()
 #endif
        lo = count.LowPart;
 
-       return (u64)((((u64)hi << 32) | (u64)lo)*(1000000000.0/frequency.QuadPart));
+       return (u64)((((u64)hi << 32) | (u64)lo) * scale_factor);
 }
 
 void sleep_nanos(u64 nsec)