Prefault comparison

.cpp

#include <benchmark/benchmark.h>
#include <absl/container/flat_hash_map.h>

#define BENCH_SIZES ->Arg(1000)->Arg(1000000)->Arg(10000000)

template <typename Map>
void fill_map(Map& map, const std::uint64_t size) {
    for (std::uint64_t i = 0 ; i < size; ++i) {
        map.try_emplace(i, i*2);
    }
}

template <typename Map>
std::uint64_t sum_map(Map& map) {
    std::uint64_t sum = 0;
    for (auto&& [k,v]: map) {
        sum += k + 2 * v;
    }

    return sum;
}

static void with_reserve(benchmark::State& state) {
    const std::size_t test_size = state.range(0);
    for (auto __attribute__((unused)) _ : state) {
        state.PauseTiming();
        absl::flat_hash_map<std::uint64_t, std::uint64_t> map;
        map.reserve(test_size);

        state.ResumeTiming();

        fill_map(map, test_size);
        benchmark::DoNotOptimize(sum_map(map));
    }
}
BENCHMARK(with_reserve)
BENCH_SIZES;

static void no_reserve(benchmark::State& state) {
    const std::size_t test_size = state.range(0);
    for (auto __attribute__((unused)) _ : state) {
        state.PauseTiming();
        absl::flat_hash_map<std::uint64_t, std::uint64_t> map;

        state.ResumeTiming();

        fill_map(map, test_size);
        benchmark::DoNotOptimize(sum_map(map));
    }
}
BENCHMARK(no_reserve)
BENCH_SIZES;

int main(int argc, char** argv) {
    benchmark::Initialize(&argc, argv);
    benchmark::RunSpecifiedBenchmarks();
}