我不熟悉使用Google Benchmark，当在本地与Quick-Bench.com上运行代码时，运行相同的Benchmark（下图）会收到不同的结果，它使用C++检索本地时间。两次我都使用了gcc8.2和-o3。

为什么在本地运行和在Quick-Bench.com上运行的结果差别很大？哪个是正确的？

#include <benchmark/benchmark.h>
#include <ctime>      
#include <sys/time.h> 
#include <chrono>     


static void BM_ctime(benchmark::State& state) {
  unsigned long long count = 0;

  for (auto _ : state) {
    std::time_t sec = std::time(0);  

    benchmark::DoNotOptimize(count += sec);
  }
}

BENCHMARK(BM_ctime);


static void BM_sysTime(benchmark::State& state) {
  unsigned long long count = 0;

  for (auto _ : state) {
    unsigned long sec = time(NULL);

    benchmark::DoNotOptimize(count += sec);
  }
}

BENCHMARK(BM_sysTime);


static void BM_chronoMilliseconds(benchmark::State& state) {
  unsigned long long count = 0;

  for (auto _ : state) {
    unsigned long long ms = std::chrono::duration_cast<std::chrono::milliseconds>(
      std::chrono::system_clock::now().time_since_epoch()
    ).count();

    benchmark::DoNotOptimize(count += ms);
  }
}

BENCHMARK(BM_chronoMilliseconds);

static void BM_chronoSececonds(benchmark::State& state) {
  unsigned long long count = 0;

  for (auto _ : state) {
    unsigned long long sec = std::chrono::duration_cast<std::chrono::seconds>(
      std::chrono::system_clock::now().time_since_epoch()
    ).count();

    benchmark::DoNotOptimize(count += sec);
  }
}

BENCHMARK(BM_chronoSececonds);

在当地产生了以下结果:

-------------------------------------------------------------
Benchmark                      Time           CPU Iterations
-------------------------------------------------------------
BM_ctime                     183 ns        175 ns    4082013
BM_sysTime                   197 ns        179 ns    4004829
BM_chronoMilliseconds         37 ns         36 ns   19092506
BM_chronoSececonds            37 ns         36 ns   19057991

QuickBench结果: