Obtain RDTSC frequency on Win32 and Linux

tscqpc.h

// SPDX-FileCopyrightText: © 2022 Phillip Trudeau-Tavara <[email protected]>
// SPDX-License-Identifier: 0BSD

// https://hero.handmade.network/forums/code-discussion/t/7485-queryperformancefrequency_returning_10mhz_bug/2
// https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/timers#partition-reference-tsc-mechanism

#include <stdbool.h>
#include <stdint.h>

#define WIN32_LEAN_AND_MEAN
#define VC_EXTRALEAN
#define NOMINMAX
#include <Windows.h>
#include <intrin.h>

static inline uint64_t get_rdtsc_freq(void) {

    // Cache the answer so that multiple calls never take the slow path more than once
    static uint64_t tsc_freq = 0;
    if (tsc_freq) {
        return tsc_freq;
    }

    // Fast path: Load kernel-mapped memory page
    HMODULE ntdll = LoadLibraryA("ntdll.dll");
    if (ntdll) {

        int (*NtQuerySystemInformation)(int, void *, unsigned int, unsigned int *) =
            (int (*)(int, void *, unsigned int, unsigned int *))GetProcAddress(ntdll, "NtQuerySystemInformation");
        if (NtQuerySystemInformation) {

            volatile uint64_t *hypervisor_shared_page = NULL;
            unsigned int size = 0;

            // SystemHypervisorSharedPageInformation == 0xc5
            int result = (NtQuerySystemInformation)(0xc5, (void *)&hypervisor_shared_page, sizeof(hypervisor_shared_page), &size);

            // success
            if (size == sizeof(hypervisor_shared_page) && result >= 0) {
                // docs say ReferenceTime = ((VirtualTsc * TscScale) >> 64)
                //      set ReferenceTime = 10000000 = 1 second @ 10MHz, solve for VirtualTsc
                //       =>    VirtualTsc = 10000000 / (TscScale >> 64)
                tsc_freq = (10000000ull << 32) / (hypervisor_shared_page[1] >> 32);
                // If your build configuration supports 128 bit arithmetic, do this:
                // tsc_freq = ((unsigned __int128)10000000ull << (unsigned __int128)64ull) / hypervisor_shared_page[1];
            }
        }
        FreeLibrary(ntdll);
    }

    // Slow path
    if (!tsc_freq) {

        // Get time before sleep
        uint64_t qpc_begin = 0; QueryPerformanceCounter((LARGE_INTEGER *)&qpc_begin);
        uint64_t tsc_begin = __rdtsc();

        Sleep(2);

        // Get time after sleep
        uint64_t qpc_end = qpc_begin + 1; QueryPerformanceCounter((LARGE_INTEGER *)&qpc_end);
        uint64_t tsc_end = __rdtsc();

        // Do the math to extrapolate the RDTSC ticks elapsed in 1 second
        uint64_t qpc_freq = 0; QueryPerformanceFrequency((LARGE_INTEGER *)&qpc_freq);
        tsc_freq = (tsc_end - tsc_begin) * qpc_freq / (qpc_end - qpc_begin);
    }

    // Failure case
    if (!tsc_freq) {
        tsc_freq = 1000000000;
    }

    return tsc_freq;
}

#include <stdio.h>
int main() {
    printf("Calling get_rdtsc_freq() 1,000 times...\n");
    uint64_t start = __rdtsc();
    uint64_t freq = 0;
    for (int i = 0; i < 1000; i++) {
        freq = get_rdtsc_freq();
    }
    uint64_t end = __rdtsc();
    printf("...took %f milliseconds.\n", (end - start) * 1000.0 / freq);
    printf("RDTSC frequency is %lu (%.2f GHz).\n", (unsigned long)freq, freq * 0.000000001);
}

tsctimespec.h

// SPDX-FileCopyrightText: © 2022 Phillip Trudeau-Tavara <[email protected]>
// SPDX-License-Identifier: 0BSD

// https://linux.die.net/man/2/perf_event_open
// https://stackoverflow.com/a/57835630

#include <stdbool.h>
#include <stdint.h>

#include <sys/mman.h>
#include <linux/perf_event.h>
#include <time.h>
#include <unistd.h>
#include <x86intrin.h>

static inline uint64_t get_rdtsc_freq(void) {

    // Cache the answer so that multiple calls never take the slow path more than once
    static uint64_t tsc_freq = 0;
    if (tsc_freq) {
        return tsc_freq;
    }

    // Fast path: Load kernel-mapped memory page
    struct perf_event_attr pe = {0};
    pe.type = PERF_TYPE_HARDWARE;
    pe.size = sizeof(struct perf_event_attr);
    pe.config = PERF_COUNT_HW_INSTRUCTIONS;
    pe.disabled = 1;
    pe.exclude_kernel = 1;
    pe.exclude_hv = 1;

    // __NR_perf_event_open == 298 (on x86_64)
    int fd = syscall(298, &pe, 0, -1, -1, 0);
    if (fd != -1) {

        struct perf_event_mmap_page *pc = (struct perf_event_mmap_page *)mmap(NULL, 4096, PROT_READ, MAP_SHARED, fd, 0);
        if (pc) {

            // success
            if (pc->cap_user_time == 1) {
                // docs say nanoseconds = (tsc * time_mult) >> time_shift
                //      set nanoseconds = 1000000000 = 1 second in nanoseconds, solve for tsc
                //       =>         tsc = 1000000000 / (time_mult >> time_shift)
                tsc_freq = (1000000000ull << (pc->time_shift / 2)) / (pc->time_mult >> (pc->time_shift - pc->time_shift / 2));
                // If your build configuration supports 128 bit arithmetic, do this:
                // tsc_freq = ((__uint128_t)1000000000ull << (__uint128_t)pc->time_shift) / pc->time_mult;
            }
            munmap(pc, 4096);
        }
        close(fd);
    }

    // Slow path
    if (!tsc_freq) {

        // Get time before sleep
        uint64_t nsc_begin = 0; { struct timespec t; if (!clock_gettime(CLOCK_MONOTONIC_RAW, &t)) nsc_begin = (uint64_t)t.tv_sec * 1000000000ull + t.tv_nsec; }
        uint64_t tsc_begin = __rdtsc();

        usleep(10000); // 10ms gives ~4.5 digits of precision - the longer you sleep, the more precise you get

        // Get time after sleep
        uint64_t nsc_end = nsc_begin + 1; { struct timespec t; if (!clock_gettime(CLOCK_MONOTONIC_RAW, &t)) nsc_end = (uint64_t)t.tv_sec * 1000000000ull + t.tv_nsec; }
        uint64_t tsc_end = __rdtsc();

        // Do the math to extrapolate the RDTSC ticks elapsed in 1 second
        tsc_freq = (tsc_end - tsc_begin) * 1000000000 / (nsc_end - nsc_begin);
    }

    // Failure case
    if (!tsc_freq) {
        tsc_freq = 1000000000;
    }

    return tsc_freq;
}

#include <stdio.h>
int main() {
    printf("Calling get_rdtsc_freq() 1,000 times...\n");
    uint64_t start = __rdtsc();
    uint64_t freq = 0;
    for (int i = 0; i < 1000; i++) {
        freq = get_rdtsc_freq();
    }
    uint64_t end = __rdtsc();
    printf("...took %f milliseconds.\n", (end - start) * 1000.0 / freq);
    printf("RDTSC frequency is %lu (%.2f GHz).\n", (unsigned long)freq, freq * 0.000000001);
}

Awesome. This literally dropped right as I was about to look more into this stuff for my own profiler. It was meant to be

@bogez57 Same, i was really wondering how to do this. Thanks.