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Optimizes message memmoves (#501)
Changes read_buffer to always use powers of two when growing in size Optimizes message_reader to avoid memmoves when the data to move is big close #462
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‎include/boost/mysql/impl/internal/next_power_of_two.hpp‎

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//
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// Copyright (c) 2026 Vladislav Soulgard (vsoulgard at gmail dot com)
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//
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// Distributed under the Boost Software License, Version 1.0. (See accompanying
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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#ifndef BOOST_MYSQL_IMPL_INTERNAL_NEXT_POWER_OF_TWO_HPP
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#define BOOST_MYSQL_IMPL_INTERNAL_NEXT_POWER_OF_TWO_HPP
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#include <boost/assert.hpp>
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#include <type_traits>
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#include <limits>
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namespace boost {
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namespace mysql {
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namespace detail {
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template<int Shift, class UnsignedInt, bool Continue>
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struct recursive_shift_or;
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// Apply shift and continue to the next power of 2
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template<int Shift, class UnsignedInt>
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struct recursive_shift_or<Shift, UnsignedInt, true>
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{
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static void apply(UnsignedInt& n)
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{
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n |= n >> Shift;
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recursive_shift_or<Shift * 2, UnsignedInt, (Shift * 2 < sizeof(UnsignedInt) * 8)>::apply(n);
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}
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};
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// Stop recursion when shift exceeds type width
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template<int Shift, class UnsignedInt>
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struct recursive_shift_or<Shift, UnsignedInt, false>
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{
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static void apply(UnsignedInt&) {}
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};
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// Returns the smallest power of two greater than or equal to n.
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// Precondition: n must not exceed the largest power of two that fits
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// in UnsignedInt. For example:
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// - uint8_t: n <= 128 (2^7)
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// - uint16_t: n <= 32768 (2^15)
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// - uint32_t: n <= 2147483648 (2^31)
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// - uint64_t: n <= 9223372036854775808 (2^63)
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//
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// Passing a larger value results in undefined behavior (overflow).
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// In debug builds, this is caught by BOOST_ASSERT.
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template<class UnsignedInt>
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UnsignedInt next_power_of_two(UnsignedInt n) noexcept
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{
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static_assert(std::is_unsigned<UnsignedInt>::value, "");
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// Assert overflow (if value is bigger than maximum power)
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BOOST_ASSERT(!(n > (std::numeric_limits<UnsignedInt>::max() >> 1) + 1));
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if (n == 0) return 1;
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n--;
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// Fill all lower bits
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recursive_shift_or<1, UnsignedInt, (1 < sizeof(UnsignedInt) * 8)>::apply(n);
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return n + 1;
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}
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} // namespace detail
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} // namespace mysql
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} // namespace boost
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#endif

‎include/boost/mysql/impl/internal/sansio/message_reader.hpp‎

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@@ -93,7 +93,17 @@ class message_reader
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BOOST_ATTRIBUTE_NODISCARD
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error_code prepare_buffer()
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{
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buffer_.remove_reserved();
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constexpr std::size_t small_move_threshold = 1024;
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const std::size_t occupied_space = buffer_.pending_size() + buffer_.current_message_size();
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// Compact the buffer (remove reserved area) if one of the following holds:
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// 1. The cost of memmove is low: active data (current_message + pending)
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// is small enough to make the copy cheap.
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// 2. Compaction could prevent a reallocation.
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if (occupied_space <= small_move_threshold ||
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(state_.required_size > buffer_.free_size()))
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{
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buffer_.remove_reserved();
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}
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auto ec = buffer_.grow_to_fit(state_.required_size);
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if (ec)
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return ec;

‎include/boost/mysql/impl/internal/sansio/read_buffer.hpp‎

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#include <boost/mysql/client_errc.hpp>
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#include <boost/mysql/error_code.hpp>
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#include <boost/mysql/impl/internal/next_power_of_two.hpp>
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#include <boost/assert.hpp>
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#include <boost/config.hpp>
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#include <boost/core/span.hpp>
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}
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// Makes sure the free size is at least n bytes long; resizes the buffer if required
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// Buffer grows to power of two, unless limited by max_size
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BOOST_ATTRIBUTE_NODISCARD
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error_code grow_to_fit(std::size_t n)
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{
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if (free_size() < n)
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{
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std::size_t new_size = buffer_.size() + n - free_size();
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std::size_t required_size = buffer_.size() + n - free_size();
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std::size_t new_size = next_power_of_two<std::size_t>(required_size);
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if (new_size > max_size_)
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return client_errc::max_buffer_size_exceeded;
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{
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new_size = required_size;
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if (new_size > max_size_)
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return client_errc::max_buffer_size_exceeded;
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}
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buffer_.resize(new_size);
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}
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return error_code();

‎test/unit/CMakeLists.txt‎

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test/impl/dt_to_string.cpp
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test/impl/ssl_context_with_default.cpp
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test/impl/next_power_of_two.cpp
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test/impl/variant_stream.cpp
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test/spotchecks/connection_use_after_move.cpp

‎test/unit/Jamfile‎

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test/impl/dt_to_string.cpp
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test/impl/ssl_context_with_default.cpp
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test/impl/next_power_of_two.cpp
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test/impl/variant_stream.cpp
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test/spotchecks/connection_use_after_move.cpp

‎test/unit/test/impl/next_power_of_two.cpp‎

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//
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// Copyright (c) 2026 Vladislav Soulgard (vsoulgard at gmail dot com)
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//
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// Distributed under the Boost Software License, Version 1.0. (See accompanying
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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#include <boost/mysql/impl/internal/next_power_of_two.hpp>
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#include <boost/test/unit_test.hpp>
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#include <cstdint>
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using namespace boost::mysql::detail;
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BOOST_AUTO_TEST_SUITE(test_next_power_of_two)
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BOOST_AUTO_TEST_CASE(basic)
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{
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// n = 0 (special case)
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BOOST_TEST(next_power_of_two<std::size_t>(0u) == 1u);
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// n is already power of two
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BOOST_TEST(next_power_of_two<std::size_t>(1u) == 1u);
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BOOST_TEST(next_power_of_two<std::size_t>(2u) == 2u);
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BOOST_TEST(next_power_of_two<std::size_t>(4u) == 4u);
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BOOST_TEST(next_power_of_two<std::size_t>(8u) == 8u);
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BOOST_TEST(next_power_of_two<std::size_t>(16u) == 16u);
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BOOST_TEST(next_power_of_two<std::size_t>(32u) == 32u);
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BOOST_TEST(next_power_of_two<std::size_t>(64u) == 64u);
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BOOST_TEST(next_power_of_two<std::size_t>(128u) == 128u);
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// n just below power of two
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BOOST_TEST(next_power_of_two<std::size_t>(3u) == 4u);
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BOOST_TEST(next_power_of_two<std::size_t>(7u) == 8u);
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BOOST_TEST(next_power_of_two<std::size_t>(15u) == 16u);
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BOOST_TEST(next_power_of_two<std::size_t>(31u) == 32u);
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BOOST_TEST(next_power_of_two<std::size_t>(63u) == 64u);
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BOOST_TEST(next_power_of_two<std::size_t>(127u) == 128u);
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// n just above power of two
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BOOST_TEST(next_power_of_two<std::size_t>(5u) == 8u);
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BOOST_TEST(next_power_of_two<std::size_t>(9u) == 16u);
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BOOST_TEST(next_power_of_two<std::size_t>(17u) == 32u);
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BOOST_TEST(next_power_of_two<std::size_t>(33u) == 64u);
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BOOST_TEST(next_power_of_two<std::size_t>(65u) == 128u);
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BOOST_TEST(next_power_of_two<std::size_t>(129u) == 256u);
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// n is random value
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BOOST_TEST(next_power_of_two<std::size_t>(6u) == 8u);
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BOOST_TEST(next_power_of_two<std::size_t>(13u) == 16u);
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BOOST_TEST(next_power_of_two<std::size_t>(21u) == 32u);
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BOOST_TEST(next_power_of_two<std::size_t>(45u) == 64u);
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BOOST_TEST(next_power_of_two<std::size_t>(89u) == 128u);
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BOOST_TEST(next_power_of_two<std::size_t>(200u) == 256u);
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BOOST_TEST(next_power_of_two<std::size_t>(300u) == 512u);
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BOOST_TEST(next_power_of_two<std::size_t>(400u) == 512u);
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BOOST_TEST(next_power_of_two<std::size_t>(505u) == 512u);
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BOOST_TEST(next_power_of_two<std::size_t>(888u) == 1024u);
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}
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BOOST_AUTO_TEST_CASE(different_types)
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{
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// uint8_t
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BOOST_TEST(next_power_of_two<std::uint8_t>(0u) == 1u);
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BOOST_TEST(next_power_of_two<std::uint8_t>(62u) == 64u);
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BOOST_TEST(next_power_of_two<std::uint8_t>(100u) == 128u);
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BOOST_TEST(next_power_of_two<std::uint8_t>(128u) == 128u);
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// uint16_t
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BOOST_TEST(next_power_of_two<std::uint16_t>(0u) == 1u);
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BOOST_TEST(next_power_of_two<std::uint16_t>(1000u) == 1024u);
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BOOST_TEST(next_power_of_two<std::uint16_t>(16383u) == 16384u);
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BOOST_TEST(next_power_of_two<std::uint16_t>(32768u) == 32768u);
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// uint32_t
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BOOST_TEST(next_power_of_two<std::uint32_t>(0u) == 1u);
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BOOST_TEST(next_power_of_two<std::uint32_t>(100000u) == 131072u);
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BOOST_TEST(next_power_of_two<std::uint32_t>(1u << 30) == 1u << 30);
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BOOST_TEST(next_power_of_two<std::uint32_t>((1u << 30) + 1) == 1u << 31);
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// uint64_t
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BOOST_TEST(next_power_of_two<std::uint64_t>(0u) == 1u);
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BOOST_TEST(next_power_of_two<std::uint64_t>(1ull << 40) == 1ull << 40);
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BOOST_TEST(next_power_of_two<std::uint64_t>((1ull << 40) + 1) == 1ull << 41);
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BOOST_TEST(next_power_of_two<std::uint64_t>(1ull << 62) == 1ull << 62);
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BOOST_TEST(next_power_of_two<std::uint64_t>((1ull << 62) + 1) == 1ull << 63);
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}
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BOOST_AUTO_TEST_SUITE_END()

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