Get approximate TSC frequency on Windows

tsc.cpp

#include <stdio.h>
#include <intrin.h>

#pragma intrinsic(__rdtsc)

#include <Windows.h>

int main() {
    LARGE_INTEGER freq;
    QueryPerformanceFrequency(&freq);
    printf("QPC Frequency: %I64d\n", freq.QuadPart);
    
    LARGE_INTEGER start;
    LARGE_INTEGER now;
    unsigned __int64 first = __rdtsc();
    QueryPerformanceCounter(&start);
    for (;;) {
        unsigned __int64 i = __rdtsc();
        QueryPerformanceCounter(&now);
        printf_s("\r%I64d ticks", i);
        if (now.QuadPart - start.QuadPart >= freq.QuadPart) {
            printf_s("\rMeasured frequency: %I64d\n", i - first);
            start = now;
            first = i;
        }
    }
}

tsc2.cpp

// A more complete and accurate measurement
#include <stdio.h>
#include <intrin.h>

#pragma intrinsic(__rdtsc)
#pragma intrinsic(__cpuid)

#include <Windows.h>

int main() {
    LARGE_INTEGER freqQPC;
    QueryPerformanceFrequency(&freqQPC);
    printf("QPC Frequency: %I64d\n", freqQPC.QuadPart);

    int id[4];
    __cpuid(id, 0x80000007);
    if (id[4] & 0x100) {
        printf("Host has invariant TSC\n");
    }

    LARGE_INTEGER startQPC;
    LARGE_INTEGER prevQPC;
    QueryPerformanceCounter(&startQPC);
    prevQPC = startQPC;
    unsigned __int64 startTSC = __rdtsc();
    unsigned __int64 prevTSC = startTSC;
    for (;;) {
        LARGE_INTEGER nowQPC;
        QueryPerformanceCounter(&nowQPC);
        if (nowQPC.QuadPart - prevQPC.QuadPart >= freqQPC.QuadPart/3) {
            unsigned __int64 nowTSC = __rdtsc();
            
            unsigned __int64 deltaTSC = (nowTSC - prevTSC);
            LONGLONG deltaQPC = (nowQPC.QuadPart - prevQPC.QuadPart);
            
            unsigned __int64 freqComputed = deltaTSC * freqQPC.QuadPart / deltaQPC;
            unsigned __int64 freqMHz = freqComputed / 1000000ULL + (freqComputed % 1000000ULL >= 500000ULL ? 1 : 0);

            unsigned __int64 deltaStartTSC = (nowTSC - startTSC);
            LONGLONG deltaStartQPC = (nowQPC.QuadPart - startQPC.QuadPart);

            unsigned __int64 freqAvgComputed = deltaStartTSC * freqQPC.QuadPart / deltaStartQPC;
            unsigned __int64 freqAvgMHz = freqAvgComputed / 1000000ULL + (freqAvgComputed % 1000000ULL >= 500000ULL ? 1 : 0);
            printf_s("Measured frequency: %I64d Hz  (%I64d MHz)    Average: %I64d Hz  (%I64d MHz)\n", freqComputed, freqMHz, freqAvgComputed, freqAvgMHz);

            prevQPC = nowQPC;
            prevTSC = nowTSC;
        }
    }
}