text: // Copyright 2022 PingCAP, Ltd.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #pragma once
  
  #include <common/defines.h>
  #include <string.h>
  
  /** memcpy function could work suboptimal if all the following conditions are met:
    * 1. Size of memory region is relatively small (approximately, under 50 bytes).
    * 2. Size of memory region is not known at compile-time.
    *
    * In that case, memcpy works suboptimal by following reasons:
    * 1. Function is not inlined.
    * 2. Much time/instructions are spend to process "tails" of data.
    *
    * There are cases when function could be implemented in more optimal way, with help of some assumptions.
    * One of that assumptions - ability to read and write some number of bytes after end of passed memory regions.
    * Under that assumption, it is possible not to implement difficult code to process tails of data and do copy always by big chunks.
    *
    * This case is typical, for example, when many small pieces of data are gathered to single contiguous piece of memory in a loop.
    * - because each next copy will overwrite excessive data after previous copy.
    *
    * Assumption that size of memory region is small enough allows us to not unroll the loop.
    * This is slower, when size of memory is actually big.
    *
    * Use with caution.
    */
  
  namespace detail
  {
  ALWAYS_INLINE inline void memcpySmallAllowReadWriteOverflow15Impl(char * __restrict dst, const char * __restrict src, ssize_t n)
  {
      while (n > 0)
      {
          tiflash_compiler_builtin_memcpy(dst, src, 16);
          dst += 16;
          src += 16;
          n -= 16;
      }
  }
  } // namespace detail
  
  /** Works under assumption, that it's possible to read up to 15 excessive bytes after end of 'src' region
    *  and to write any garbage into up to 15 bytes after end of 'dst' region.
    */
  __attribute__((always_inline)) inline void memcpySmallAllowReadWriteOverflow15(void * __restrict dst, const void * __restrict src, size_t n)
  {
      ::detail::memcpySmallAllowReadWriteOverflow15Impl(reinterpret_cast<char *>(dst), reinterpret_cast<const char *>(src), n);
  }
  