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// Copyright 2013 Alex Reece.
// A cross platform monotonic timer.
#include <unistd.h>
#define NANOS_PER_SECF 1000000000.0
#define USECS_PER_SEC 1000000
// If we have it, use clock_gettime and CLOCK_MONOTONIC.
#include <time.h>
double monotonic_seconds() {
struct timespec time;
// Note: Make sure to link with -lrt to define clock_gettime.
clock_gettime(CLOCK_MONOTONIC, &time);
return ((double) time.tv_sec) + ((double) time.tv_nsec / (NANOS_PER_SECF));
#elif defined(__APPLE__)
// If we don't have CLOCK_MONOTONIC, we might be on a Mac. There we instead
// use mach_absolute_time().
#include <mach/mach_time.h>
static mach_timebase_info_data_t info;
static void __attribute__((constructor)) init_info() {
double monotonic_seconds() {
uint64_t time = mach_absolute_time();
double dtime = (double) time;
dtime *= (double) info.numer;
dtime /= (double) info.denom;
return dtime / NANOS_PER_SECF;
#elif defined(_MSC_VER)
// On Windows, use QueryPerformanceCounter and QueryPerformanceFrequency.
#include <windows.h>
static double PCFreq = 0.0;
// According to, this will
// make this function a constructor.
// TODO(awreece) Actually attempt to compile on windows.
static void __cdecl init_pcfreq();
__declspec(allocate(".CRT$XCU")) void (__cdecl*init_pcfreq_)() = init_pcfreq;
static void __cdecl init_pcfreq() {
// Accoring to, this will
// properly initialize the QueryPerformanceCounter.
int has_qpc = QueryPerformanceFrequency(&li);
PCFreq = ((double) li.QuadPart) / 1000.0;
double monotonic_seconds() {
return ((double) li.QuadPart) / PCFreq;
// Fall back to rdtsc. The reason we don't use clock() is this scary message
// from the man page:
// "On several other implementations, the value returned by clock() also
// includes the times of any children whose status has been collected via
// wait(2) (or another wait-type call)."
// Also, clock() only has microsecond accuracy.
// This whitepaper offered excellent advice on how to use rdtscp for
// profiling:
// Unfortunately, we can't follow its advice exactly with our semantics,
// so we're just going to use rdtscp with cpuid.
// Note that rdtscp will only be available on new processors.
#include <stdint.h>
static inline uint64_t rdtsc() {
uint32_t hi, lo;
asm volatile("rdtscp\n"
"movl %%edx, %0\n"
"movl %%eax, %1\n"
: "=r" (hi), "=r" (lo) : : "%rax", "%rbx", "%rcx", "%rdx");
return (((uint64_t)hi) << 32) | (uint64_t)lo;
static uint64_t rdtsc_per_sec = 0;
static void __attribute__((constructor)) init_rdtsc_per_sec() {
uint64_t before, after;
before = rdtsc();
after = rdtsc();
rdtsc_per_sec = after - before;
double monotonic_seconds() {
return (double) rdtsc() / (double) rdtsc_per_sec;