基准可变参数模板函数调用

Question

我使用 Quick bench（参见下面的 link）来衡量通过调用带有可变参数模板参数的函数解码缓冲区与通过调用不带可变参数包扩展的函数解码相同缓冲区之间的性能。

关于如何使可变参数实现与其他实现相提并论的任何想法？

基准测试的结果是（CPU 时间 / Noop 时间）的比率。负载未知的 AWS 机器池上的基准运行s。目的是在相同条件下对两个代码片段运行进行合理的比较。非可变参数模板函数的 CPU 时间为 5.9，可变参数实现为 21.3。编译器：优化级别为 O3 的 Clang 5.0。

#include <cstdint>
#include <cstring>
#include <string>
#include <type_traits>

namespace core { namespace decoder 
{
   class LittleEndian
   {
   public:
      LittleEndian(const LittleEndian&) = delete;
      LittleEndian& operator=(const LittleEndian&) = delete;

   public:
      constexpr LittleEndian(const std::uint8_t* buffer, size_t size) noexcept
      : m_buffer(buffer),
        m_size(size)
      {}

      constexpr bool decodeU8(
        size_t& offset, std::uint8_t& decodedValue) const noexcept
      {
        if (offset >= m_size)
            return false;

        decodedValue = m_buffer[offset];
        offset += sizeof(std::uint8_t);
        return true;
      }

      constexpr bool decodeU16(
        size_t& offset, std::uint16_t& decodedValue) const noexcept
      {
        if (offset + sizeof(std::uint16_t) > m_size)
            return false;

        const uint8_t b0 = m_buffer[offset], b1 = m_buffer[offset + 1];
        decodedValue = (b0 << 0) | (b1 << 8);
        offset += sizeof(std::uint16_t);
        return true;
      }

      constexpr bool decodeU32(
        size_t& offset, std::uint32_t& decodedValue) const noexcept
      {
        if (offset + sizeof(std::uint32_t) > m_size)
            return false;

        const uint8_t b0 = m_buffer[offset], b1 = m_buffer[offset + 1], b2 = m_buffer[offset + 2], b3 = m_buffer[offset + 3];
        decodedValue = (b0 << 0) | (b1 << 8) | (b2 << 16) | (b3 << 24);
        offset += sizeof(std::uint32_t);
        return true;
      }

      constexpr bool decodeU64(
        size_t& offset, std::uint64_t& decodedValue) const noexcept
      {
        if (offset + sizeof(std::uint64_t) > m_size)
            return false;

        const uint8_t b0 = m_buffer[offset], b1 = m_buffer[offset + 1],
            b2 = m_buffer[offset + 2], b3 = m_buffer[offset + 3],
            b4 = m_buffer[offset + 4], b5 = m_buffer[offset + 5],
            b6 = m_buffer[offset + 6], b7 = m_buffer[offset + 7];

        decodedValue = (static_cast<std::uint64_t>(b0) << 0) |
            (static_cast<std::uint64_t>(b1) << 8) |
            (static_cast<std::uint64_t>(b2) << 16) |
            (static_cast<std::uint64_t>(b3) << 24) |
            (static_cast<std::uint64_t>(b4) << 32) |
            (static_cast<std::uint64_t>(b5) << 40) |
            (static_cast<std::uint64_t>(b6) << 48) |
            (static_cast<std::uint64_t>(b7) << 56);

        offset += sizeof(std::uint64_t);
        return true;
      }

  private:
    const std::uint8_t* m_buffer;
    const size_t m_size;
  };

  template<typename EndianDecoderT>
  class ByteDecoder
  {
  public:
    ByteDecoder(const ByteDecoder&) = delete;
    ByteDecoder& operator=(const ByteDecoder&) = delete;

  public:
    constexpr ByteDecoder(const std::uint8_t* buffer, size_t size)
      : m_buffer(buffer),
        m_size(size),
        m_endianDecoder(buffer, size)
    {}

    template<typename ...Args>
    constexpr bool decode(size_t offset, Args&... args) const noexcept
    {
        bool success = true;

        using expand_type = int[];
        expand_type
        {
            ([&success] (auto result) noexcept
            {
                success = (!success || !result) ? false : true;

            } (decodeValue(offset, args)), 0)...
        };

        return success;

      }

      template<typename T>
      constexpr bool decode(size_t offset, T& decodedValue) const noexcept
      {
        return decodeValue(offset, decodedValue);
      }

   private:
    template<typename T>
    constexpr bool decodeValue(
        size_t &offset, T& decodedValue) const noexcept
    {
        if constexpr (std::is_same< std::decay_t<T>, std::uint8_t>::value)
            return m_endianDecoder.decodeU8(offset, decodedValue);

        if constexpr (std::is_same< std::decay_t<T>, std::uint16_t>::value)
            return m_endianDecoder.decodeU16(offset, decodedValue);

        if constexpr (std::is_same< std::decay_t<T>, std::uint32_t>::value)
            return m_endianDecoder.decodeU32(offset, decodedValue);

        if constexpr (std::is_same< std::decay_t<T>, std::uint64_t>::value)
            return m_endianDecoder.decodeU64(offset, decodedValue);

        if constexpr (std::is_same<char *, typename std::decay<T>::type>::value ||
                      std::is_same<char const *, typename std::decay<T>::type>::value)
            return decodeCHR(offset, decodedValue);

        return false;
      }

      template<size_t SIZE>
      constexpr bool decodeCHR(
        size_t &offset, char (&buffer)[SIZE]) const noexcept
      {
        if (offset + SIZE > m_size)
            return false;

        memset(&buffer[0], 0x00, sizeof(char) * SIZE);
        memcpy(&buffer[0], &m_buffer[offset], sizeof(char) * (std::min)(SIZE, std::extent<decltype(buffer)>::value - 1));
        offset += SIZE;
        return true;
      }

  private:
    const std::uint8_t* m_buffer;
    const size_t m_size;
    EndianDecoderT m_endianDecoder;
  };

}} // namespace core::decoder

static void NonVariadicDecoding(benchmark::State& state) {
// Code inside this loop is measured repeatedly
constexpr std::uint8_t littleEndian[] = { 0x0D, 0x0C, 0x84, 0x03, 0x00, 0x00, 'H', 'e', 'l', 'l', 'o', '[=11=]',  0x84, 0x03, 0x84, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
core::decoder::ByteDecoder<core::decoder::LittleEndian> decoder(littleEndian, sizeof(littleEndian));


for (auto _ : state) {

  size_t offset = 0;

  struct DecodedValue
  {
      std::uint16_t v1_U16;
      std::uint32_t v2_U32;
      char v3_CHR[6];
      std::uint16_t v4_U16;
      std::uint64_t v5_U64;
  };
  DecodedValue dv;

  decoder.decode(offset, dv.v1_U16);  
  offset += sizeof(dv.v1_U16);
  decoder.decode(offset, dv.v2_U32);
  offset += sizeof(dv.v2_U32);
  decoder.decode(offset, dv.v3_CHR);
  offset += sizeof(dv.v3_CHR);
  decoder.decode(offset, dv.v4_U16);
  offset += sizeof(dv.v4_U16);
  decoder.decode(offset, dv.v5_U64);

  benchmark::DoNotOptimize(dv);
  }
}
// Register the function as a benchmark
BENCHMARK(NonVariadicDecoding);

static void VariadicDecoding(benchmark::State& state) {
// Code before the loop is not measured
constexpr std::uint8_t littleEndian[] = { 0x0D, 0x0C, 0x84, 0x03, 0x00, 0x00, 'H', 'e', 'l', 'l', 'o', '[=12=]',  0x84, 0x03, 0x84, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
core::decoder::ByteDecoder<core::decoder::LittleEndian> decoder(littleEndian, sizeof(littleEndian));

for (auto _ : state) {

  struct DecodedValue
  {
      std::uint16_t v1_U16;
      std::uint32_t v2_U32;
      char v3_CHR[6];
      std::uint16_t v4_U16;
      std::uint64_t v5_U64;
  };

  DecodedValue dv;
  decoder.decode(0, dv.v1_U16, dv.v2_U32, dv.v3_CHR, dv.v4_U16, dv.v5_U64);
  benchmark::DoNotOptimize(dv);
  }
}
BENCHMARK(VariadicDecoding);

Quick-Bench

Answer 1

如果您对可变参数实现进行递归调用并使用完美转发，那么您可以获得更好的性能：

    template <typename Type>
    constexpr bool decode_impl(size_t offset, Type&& value) const noexcept
    {
      return decodeValue(offset, std::forward<Type>(value));
    }

    template <typename First, typename Second, typename... Other>
    constexpr bool decode_impl(size_t offset, First&& first, Second&& second, Other&&... others) const noexcept
    {
      return decode_impl(offset, std::forward<First>(first)) && decode_impl(offset, std::forward<Second>(second), std::forward<Other>(others)...);
    }

    template<typename ...Args>
    constexpr bool decode(size_t offset, Args&&... args) const noexcept
    {
      return decode_impl(offset, std::forward<Args>(args)...);
    }

See there.

基准可变参数模板函数调用

Benchmark variadic template function call

c++

performance

benchmarking

c++17

microbenchmark