add vs lookup table

intostr.cpp

#include <chrono>
#include <cstdint>
#include <iostream>
#include <random>
#include <sstream>

using namespace std;

typedef std::chrono::microseconds time_unit;
static const std::string time_unit_suffix{"us"};

time_unit getTimeNow()
{
    auto t = chrono::high_resolution_clock::now();
    return std::chrono::duration_cast<time_unit>(t.time_since_epoch());
}

uint32_t lwan_test(uint64_t value, char *buffer)
{
    char *p = buffer + numeric_limits<uint64_t>::digits10 - 1;

    *p = '\0';
    do {
        *--p = (char) ('0' + value % 10);
    } while (value /= 10);

    return (unsigned int) (numeric_limits<uint64_t>::digits10 -
            (unsigned int) (p - buffer) - 1);
}

uint32_t lwan_lut_test(uint64_t value, char *buffer)
{
    char *p = buffer + numeric_limits<uint64_t>::digits10 - 1;

    *p = '\0';
    do {
        *--p = "0123456789"[value % 10];
    } while (value /= 10);

    return (unsigned int) (numeric_limits<uint64_t>::digits10 -
            (unsigned int) (p - buffer) - 1);
}

static void strreverse(char* begin, char* end)
{
    char aux;
    while (end > begin)
        aux = *end, *end-- = *begin, *begin++ = aux;
}

template <typename F>
time_unit test(const vector<uint64_t>& vector, F func)
{
    char buf[3 * sizeof(uint64_t)];
    int checksum = 0;

    auto start = getTimeNow();
    for (size_t run = 0; run < 50; run++) {
        for (uint64_t value : vector)
            checksum += func(value, buf);
    }
    auto end = getTimeNow();

    return end - start;
}

void print_results(std::string name, time_unit time)
{
    std::cout << name << ": took " << time.count() << ' ' << time_unit_suffix << '\n';
}

int main(int argc, char* argv[])
{
    mt19937 random_engine;
    vector<uint64_t> vector(1000000);

    random_engine.seed(0xdeadbeef);

    for (auto power = 100000000000L; power >= 1; power /= 10) {
        uniform_int_distribution<uint64_t> distribution(0, power);
        for (auto i = vector.size(); i != 0; i--)
            vector[i] = distribution(random_engine);

        cout << "For values in interval [0," << power << "]:\n";

        auto t1 = test(vector, lwan_test);
        auto t2 = test(vector, lwan_lut_test);

        print_results("lawn_test_add", t1);
        print_results("lwan_lut_test", t2);

        auto diff = (t2 - t1).count();
        auto percentageDiff = static_cast<double>(diff) / (t1 + t2).count();

        std::cout << "add was: " << diff << " " << time_unit_suffix << " ";
        std::cout << (diff < 0 ? "slower" : "faster");
        std::cout << " (" << 100 * percentageDiff << "% difference)\n";

        cout << "\n";
    }

    return 0;
}

output.txt

$ c++ --version
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
$ c++ inttostr.cpp -std=c++11 -O3 -o inttostr
$ ./inttostr 
For values in interval [0,100000000000]:
lawn_test_add: took 1180147 us
lwan_lut_test: took 1387113 us
add was: 206966 us faster (8.06175% difference)

For values in interval [0,10000000000]:
lawn_test_add: took 1100406 us
lwan_lut_test: took 1232800 us
add was: 132394 us faster (5.67434% difference)

For values in interval [0,1000000000]:
lawn_test_add: took 962111 us
lwan_lut_test: took 1039763 us
add was: 77652 us faster (3.87897% difference)

For values in interval [0,100000000]:
lawn_test_add: took 832761 us
lwan_lut_test: took 918867 us
add was: 86106 us faster (4.91577% difference)

For values in interval [0,10000000]:
lawn_test_add: took 759096 us
lwan_lut_test: took 822768 us
add was: 63672 us faster (4.02512% difference)

For values in interval [0,1000000]:
lawn_test_add: took 606999 us
lwan_lut_test: took 680095 us
add was: 73096 us faster (5.67915% difference)

For values in interval [0,100000]:
lawn_test_add: took 498602 us
lwan_lut_test: took 538036 us
add was: 39434 us faster (3.80403% difference)

For values in interval [0,10000]:
lawn_test_add: took 408179 us
lwan_lut_test: took 431154 us
add was: 22975 us faster (2.73729% difference)

For values in interval [0,1000]:
lawn_test_add: took 298869 us
lwan_lut_test: took 335538 us
add was: 36669 us faster (5.78004% difference)

For values in interval [0,100]:
lawn_test_add: took 228339 us
lwan_lut_test: took 225532 us
add was: -2807 us slower (-0.618458% difference)

For values in interval [0,10]:
lawn_test_add: took 133550 us
lwan_lut_test: took 136035 us
add was: 2485 us faster (0.921787% difference)

For values in interval [0,1]:
lawn_test_add: took 105890 us
lwan_lut_test: took 123110 us
add was: 17220 us faster (7.51965% difference)