text: /*
   *            Copyright Andrey Semashev 2013.
   * Distributed under the Boost Software License, Version 1.0.
   *    (See accompanying file LICENSE_1_0.txt or copy at
   *          http://www.boost.org/LICENSE_1_0.txt)
   */
  /*!
   * \file   uuid/detail/uuid_x86.hpp
   *
   * \brief  This header contains optimized SSE implementation of \c boost::uuid operations.
   */
  
  #ifndef BOOST_UUID_DETAIL_UUID_X86_HPP_INCLUDED_
  #define BOOST_UUID_DETAIL_UUID_X86_HPP_INCLUDED_
  
  // MSVC does not always have immintrin.h (at least, not up to MSVC 10), so include the appropriate header for each instruction set
  #if defined(BOOST_UUID_USE_SSE41)
  #include <smmintrin.h>
  #elif defined(BOOST_UUID_USE_SSE3)
  #include <pmmintrin.h>
  #else
  #include <emmintrin.h>
  #endif
  
  #if defined(BOOST_MSVC) && defined(_M_X64) && !defined(BOOST_UUID_USE_SSE3) && (BOOST_MSVC < 1900 /* Fixed in Visual Studio 2015 */ )
  // At least MSVC 9 (VS2008) and 12 (VS2013) have an optimizer bug that sometimes results in incorrect SIMD code
  // generated in Release x64 mode. In particular, it affects operator==, where the compiler sometimes generates
  // pcmpeqd with a memory opereand instead of movdqu followed by pcmpeqd. The problem is that uuid can be
  // not aligned to 16 bytes and pcmpeqd causes alignment violation in this case. We cannot be sure that other
  // MSVC versions are not affected so we apply the workaround for all versions, except VS2015 on up where
  // the bug has been fixed.
  //
  // https://svn.boost.org/trac/boost/ticket/8509#comment:3
  // https://connect.microsoft.com/VisualStudio/feedbackdetail/view/981648#tabs
  #define BOOST_UUID_DETAIL_MSVC_BUG981648
  #if BOOST_MSVC >= 1600
  extern "C" void _ReadWriteBarrier(void);
  #pragma intrinsic(_ReadWriteBarrier)
  #endif
  #endif
  
  namespace boost {
  namespace uuids {
  namespace detail {
  
  BOOST_FORCEINLINE __m128i load_unaligned_si128(const uint8_t* p) BOOST_NOEXCEPT
  {
  #if defined(BOOST_UUID_USE_SSE3)
      return _mm_lddqu_si128(reinterpret_cast< const __m128i* >(p));
  #elif !defined(BOOST_UUID_DETAIL_MSVC_BUG981648)
      return _mm_loadu_si128(reinterpret_cast< const __m128i* >(p));
  #elif defined(BOOST_MSVC) && BOOST_MSVC >= 1600
      __m128i mm = _mm_loadu_si128(reinterpret_cast< const __m128i* >(p));
      // Make sure this load doesn't get merged with the subsequent instructions
      _ReadWriteBarrier();
      return mm;
  #else
      // VS2008 x64 doesn't respect _ReadWriteBarrier above, so we have to generate this crippled code to load unaligned data
      return _mm_unpacklo_epi64(_mm_loadl_epi64(reinterpret_cast< const __m128i* >(p)), _mm_loadl_epi64(reinterpret_cast< const __m128i* >(p + 8)));
  #endif
  }
  
  } // namespace detail
  
  inline bool uuid::is_nil() const BOOST_NOEXCEPT
  {
      __m128i mm = uuids::detail::load_unaligned_si128(data);
  #if defined(BOOST_UUID_USE_SSE41)
      return _mm_test_all_zeros(mm, mm) != 0;
  #else
      mm = _mm_cmpeq_epi32(mm, _mm_setzero_si128());
      return _mm_movemask_epi8(mm) == 0xFFFF;
  #endif
  }
  
  inline void uuid::swap(uuid& rhs) BOOST_NOEXCEPT
  {
      __m128i mm_this = uuids::detail::load_unaligned_si128(data);
      __m128i mm_rhs = uuids::detail::load_unaligned_si128(rhs.data);
      _mm_storeu_si128(reinterpret_cast< __m128i* >(rhs.data), mm_this);
      _mm_storeu_si128(reinterpret_cast< __m128i* >(data), mm_rhs);
  }
  
  inline bool operator== (uuid const& lhs, uuid const& rhs) BOOST_NOEXCEPT
  {
      __m128i mm_left = uuids::detail::load_unaligned_si128(lhs.data);
      __m128i mm_right = uuids::detail::load_unaligned_si128(rhs.data);
  
  #if defined(BOOST_UUID_USE_SSE41)
      __m128i mm = _mm_xor_si128(mm_left, mm_right);
      return _mm_test_all_zeros(mm, mm) != 0;
  #else
      __m128i mm_cmp = _mm_cmpeq_epi32(mm_left, mm_right);
      return _mm_movemask_epi8(mm_cmp) == 0xFFFF;
  #endif
  }
  
  inline bool operator< (uuid const& lhs, uuid const& rhs) BOOST_NOEXCEPT
  {
      __m128i mm_left = uuids::detail::load_unaligned_si128(lhs.data);
      __m128i mm_right = uuids::detail::load_unaligned_si128(rhs.data);
  
      // To emulate lexicographical_compare behavior we have to perform two comparisons - the forward and reverse one.
      // Then we know which bytes are equivalent and which ones are different, and for those different the comparison results
      // will be opposite. Then we'll be able to find the first differing comparison result (for both forward and reverse ways),
      // and depending on which way it is for, this will be the result of the operation. There are a few notes to consider:
      //
      // 1. Due to little endian byte order the first bytes go into the lower part of the xmm registers,
      //    so the comparison results in the least significant bits will actually be the most signigicant for the final operation result.
      //    This means we have to determine which of the comparison results have the least significant bit on, and this is achieved with
      //    the "(x - 1) ^ x" trick.
      // 2. Because there is only signed comparison in SSE/AVX, we have to invert byte comparison results whenever signs of the corresponding
      //    bytes are different. I.e. in signed comparison it's -1 < 1, but in unsigned it is the opposite (255 > 1). To do that we XOR left and right,
      //    making the most significant bit of each byte 1 if the signs are different, and later apply this mask with another XOR to the comparison results.
      // 3. pcmpgtw compares for "greater" relation, so we swap the arguments to get what we need.
  
      const __m128i mm_signs_mask = _mm_xor_si128(mm_left, mm_right);
  
      __m128i mm_cmp = _mm_cmpgt_epi8(mm_right, mm_left), mm_rcmp = _mm_cmpgt_epi8(mm_left, mm_right);
  
      mm_cmp = _mm_xor_si128(mm_signs_mask, mm_cmp);
      mm_rcmp = _mm_xor_si128(mm_signs_mask, mm_rcmp);
  
      uint32_t cmp = static_cast< uint32_t >(_mm_movemask_epi8(mm_cmp)), rcmp = static_cast< uint32_t >(_mm_movemask_epi8(mm_rcmp));
  
      cmp = (cmp - 1u) ^ cmp;
      rcmp = (rcmp - 1u) ^ rcmp;
  
      return cmp < rcmp;
  }
  
  } // namespace uuids
  } // namespace boost
  
  #endif // BOOST_UUID_DETAIL_UUID_X86_HPP_INCLUDED_
  