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Merge #11646: Require a steady clock for bench with at least micro precision

620bae3 Require a steady clock for bench with at least micro precision (Matt Corallo)

Pull request description:

  Using a non-steady high_precision_clock by default is definitely not what we want, and in practice steady_clock has more than enough precision. Should double-check that travis passes on this one to make sure we actually have at least microsecond precision on all platforms.

Tree-SHA512: 54a4af3b6addca9897e8ab04694f9461343691b475ca3ed2368595c37520612e284969be94a8ee3d7c66d16532f7bb16b6ad80284cbc153653e8ef2d56696e9d
0.16
Wladimir J. van der Laan 7 years ago
parent
commit
fe503e118f
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  1. 3
      src/bench/bench.cpp
  2. 6
      src/bench/bench.h

3
src/bench/bench.cpp

@ -23,6 +23,9 @@ void
benchmark::BenchRunner::RunAll(benchmark::duration elapsedTimeForOne) benchmark::BenchRunner::RunAll(benchmark::duration elapsedTimeForOne)
{ {
perf_init(); perf_init();
if (std::ratio_less_equal<benchmark::clock::period, std::micro>::value) {
std::cerr << "WARNING: Clock precision is worse than microsecond - benchmarks may be less accurate!\n";
}
std::cout << "#Benchmark" << "," << "count" << "," << "min(ns)" << "," << "max(ns)" << "," << "average(ns)" << "," std::cout << "#Benchmark" << "," << "count" << "," << "min(ns)" << "," << "max(ns)" << "," << "average(ns)" << ","
<< "min_cycles" << "," << "max_cycles" << "," << "average_cycles" << "\n"; << "min_cycles" << "," << "max_cycles" << "," << "average_cycles" << "\n";

6
src/bench/bench.h

@ -37,13 +37,11 @@ BENCHMARK(CODE_TO_TIME);
*/ */
namespace benchmark { namespace benchmark {
// On many systems, the high_resolution_clock offers no better resolution than the steady_clock. // In case high_resolution_clock is steady, prefer that, otherwise use steady_clock.
// If that's the case, prefer the steady_clock.
struct best_clock { struct best_clock {
using hi_res_clock = std::chrono::high_resolution_clock; using hi_res_clock = std::chrono::high_resolution_clock;
using steady_clock = std::chrono::steady_clock; using steady_clock = std::chrono::steady_clock;
static constexpr bool steady_is_high_res = std::ratio_less_equal<steady_clock::period, hi_res_clock::period>::value; using type = std::conditional<hi_res_clock::is_steady, hi_res_clock, steady_clock>::type;
using type = std::conditional<steady_is_high_res, steady_clock, hi_res_clock>::type;
}; };
using clock = best_clock::type; using clock = best_clock::type;
using time_point = clock::time_point; using time_point = clock::time_point;

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