Branchless conversion of a hex character to its value

hex_character_to_binary_benchmark.cpp

// Requires google's benchmark library: https://github.com/google/benchmark

#include <benchmark/benchmark.h>

#include <cstdint>

constexpr auto k_samples_count = 1000000u;

uint8_t random_char()
{
  constexpr char chars[] = "0123456789abcdefABCDEF";
  return chars[rand() % 22u];
}

char to_value_with_branches(uint8_t c)
{
  if (c >= 'a') {
    c -= 'a' - ':';
  } else if (c >= 'A') {
    c -= 'A' - ':';
  }

  return c - '0';
}

static void OldRustyCalculations(benchmark::State& state)
{
  srand(0);
  while (state.KeepRunning()) {
    for (auto i = 0u; i < k_samples_count; ++i) {
      const auto c = to_value_with_branches(random_char());
      benchmark::DoNotOptimize(c);
    }
  }
}
BENCHMARK(OldRustyCalculations);

char to_value_branchless(uint8_t c)
{
  constexpr uint8_t to_add[] = { 0, 9 };
  const bool is_alpha_char = (c & 0b01000000u);
  return (c & 0x0fu) + to_add[is_alpha_char];
}

static void ShinyBranchless(benchmark::State& state)
{
  srand(0);
  while (state.KeepRunning()) {
    for (auto i = 0u; i < k_samples_count; ++i) {
      const auto c = to_value_branchless(random_char());
      benchmark::DoNotOptimize(c);
    }
  }
}
BENCHMARK(ShinyBranchless);

BENCHMARK_MAIN();