Get TSC frequency on Windows

1_tsc_exact.cpp

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

#pragma intrinsic(__cpuid)

int main() {
    int id[4];
    __cpuid(id, 0x15);
    auto tscFreq = (__int64)id[2] * id[1] / id[0];
    printf("TSC frequency: %I64d Hz  (%I64d MHz)\n", tscFreq, tscFreq / 1000000ULL);
}

2_tsc_approx.cpp

#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[3] & 0x100) {
        printf("Host has invariant TSC\n");
    }

    __cpuid(id, 0x15);
    auto tscFreq = (__int64)id[2] * id[1] / id[0];
    printf("Calculated TSC frequency: %I64d Hz  (%I64d MHz)\n", tscFreq, tscFreq / 1000000ULL);

    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;
        }
    }
}