timing.cpp

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
 
#include <dispenso/timing.h>
 
#include <chrono>
#include <cmath>
 
#if defined(_MSC_VER)
#include <intrin.h>
#endif // _MSC_VER
 
#if defined(_WIN32)
#include <Windows.h>
#endif // _WIN32
 
#if defined(__MACH__)
#include <mach/mach.h>
#include <mach/mach_time.h>
#endif // __MACH__
 
namespace dispenso {
namespace {
#if defined(__x86_64__) || defined(_M_AMD64)
#define DISPENSO_HAS_TIMESTAMP
#if defined(_MSC_VER)
inline uint64_t rdtscp() {
  uint32_t ui;
  return __rdtscp(&ui);
}
 
#else
inline uint64_t rdtscp() {
  uint32_t lo, hi;
  __asm__ volatile("rdtscp"
                   : /* outputs */ "=a"(lo), "=d"(hi)
                   : /* no inputs */
                   : /* clobbers */ "%rcx");
  return (uint64_t)lo | (((uint64_t)hi) << 32);
}
#endif // OS
#elif (defined(__GNUC__) || defined(__clang__)) && defined(__aarch64__)
#define DISPENSO_HAS_TIMESTAMP
uint64_t rdtscp(void) {
  uint64_t val;
  __asm__ volatile("mrs %0, cntvct_el0" : "=r"(val));
  return val;
}
#endif // ARCH
} // namespace
 
#if defined(DISPENSO_HAS_TIMESTAMP)
 
static bool snapFreq(double& firstApprox) {
  switch (static_cast<int>(firstApprox)) {
    case 0:
      if (std::abs(int(firstApprox * 10.0)) <= 1) {
        firstApprox = 0.0;
        return true;
      }
      break;
    case 9:
      if (std::abs(int(firstApprox * 10.0) - 99) <= 1) {
        firstApprox = 10.0;
 
        return true;
      }
      break;
    case 3:
      if (std::abs(int(firstApprox * 10.0) - 33) <= 1) {
        firstApprox = 3.0 + 1.0 / 3.0;
        return true;
      }
      break;
    case 6:
      if (std::abs(int(firstApprox * 10.0) - 66) <= 1) {
        firstApprox = 6.0 + 2.0 / 3.0;
        return true;
      }
      break;
  }
  return false;
}
 
static double fallbackTicksPerSecond() {
  using namespace std::chrono_literals;
  constexpr double kChronoOverheadBias = 250e-9;
 
  auto baseStart = std::chrono::high_resolution_clock::now();
  auto start = rdtscp();
  std::this_thread::sleep_for(50ms);
  auto end = rdtscp();
  auto baseEnd = std::chrono::high_resolution_clock::now();
 
  auto base = std::chrono::duration<double>(baseEnd - baseStart).count() - kChronoOverheadBias;
  double firstApprox = (static_cast<double>(end - start)) / base;
 
  // Try to refine the approximation.  In some circumstances we can "snap" the frequency to a very
  // good guess that is off by less than one part in thousands.  Accuracy should already be quite
  // good in any case, but this allows us to improve in some cases.
 
  // Get first 3 digits
  firstApprox *= 1e-7;
 
  int firstInt = static_cast<int>(firstApprox);
  firstApprox -= firstInt;
 
  firstApprox *= 10.0;
 
  if (!snapFreq(firstApprox)) {
    int secondInt = static_cast<int>(firstApprox);
    firstApprox -= secondInt;
    firstApprox *= 10.0;
    snapFreq(firstApprox);
    firstApprox *= 0.1;
    firstApprox += secondInt;
  }
 
  firstApprox *= 0.1;
 
  firstApprox += firstInt;
  firstApprox *= 1e7;
  return firstApprox;
}
 
#if defined(__MACH__)
static double ticksPerSecond() {
  mach_timebase_info_data_t info;
  if (mach_timebase_info(&info) != KERN_SUCCESS) {
    return fallbackTicksPerSecond();
  }
  return 1e9 * static_cast<double>(info.denom) / static_cast<double>(info.numer);
}
#else
double ticksPerSecond() {
  return fallbackTicksPerSecond();
}
#endif
 
double getTime() {
  static double secondsPerTick = 1.0 / ticksPerSecond();
  static double startTime = static_cast<double>(rdtscp()) * secondsPerTick;
 
  double t = static_cast<double>(rdtscp()) * secondsPerTick;
  return t - startTime;
}
#else
double getTime() {
  static auto startTime = std::chrono::high_resolution_clock::now();
  auto cur = std::chrono::high_resolution_clock::now();
 
  return std::chrono::duration<double>(cur - startTime).count();
}
#endif // DISPENSO_HAS_TIMESTAMP
 
namespace {
// This should ensure that we initialize the time before main.
double g_dummyTime = getTime();
} // namespace
 
} // namespace dispenso