/src/xz/src/liblzma/common/memcmplen.h

Source
// SPDX-License-Identifier: 0BSD

///////////////////////////////////////////////////////////////////////////////
//
/// \file       memcmplen.h
/// \brief      Optimized comparison of two buffers
//
//  Author:     Lasse Collin
//
///////////////////////////////////////////////////////////////////////////////

#ifndef LZMA_MEMCMPLEN_H
#define LZMA_MEMCMPLEN_H

#include "common.h"

#ifdef HAVE_IMMINTRIN_H
# include <immintrin.h>
#endif

// Only include <intrin.h> if it is needed. The header is only needed
// on Windows when using an MSVC compatible compiler. The Intel compiler
// can use the intrinsics without the header file.
#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
    && defined(_MSC_VER) \
    && (defined(_M_X64) \
      || defined(_M_ARM64) || defined(_M_ARM64EC)) \
    && !defined(__INTEL_COMPILER)
# include <intrin.h>
#endif


/// Find out how many equal bytes the two buffers have.
///
/// \param      buf1    First buffer
/// \param      buf2    Second buffer
/// \param      len     How many bytes have already been compared and will
///                     be assumed to match
/// \param      limit   How many bytes to compare at most, including the
///                     already-compared bytes. This must be significantly
///                     smaller than UINT32_MAX to avoid integer overflows.
///                     Up to LZMA_MEMCMPLEN_EXTRA bytes may be read past
///                     the specified limit from both buf1 and buf2.
///
/// \return     Number of equal bytes in the buffers is returned.
///             This is always at least len and at most limit.
///
/// \note       LZMA_MEMCMPLEN_EXTRA defines how many extra bytes may be read.
///             It's rounded up to 2^n. This extra amount needs to be
///             allocated in the buffers being used. It needs to be
///             initialized too to keep Valgrind quiet.
static lzma_always_inline uint32_t
lzma_memcmplen(const uint8_t *buf1, const uint8_t *buf2,
    uint32_t len, uint32_t limit)
{
  assert(len <= limit);
  assert(limit <= UINT32_MAX / 2);

#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
    && (((TUKLIB_GNUC_REQ(3, 4) || defined(__clang__)) \
        && SIZE_MAX == UINT64_MAX) \
      || (defined(__INTEL_COMPILER) && defined(__x86_64__)) \
      || (defined(__INTEL_COMPILER) && defined(_M_X64)) \
      || (defined(_MSC_VER) && (defined(_M_X64) \
        || defined(_M_ARM64) || defined(_M_ARM64EC))))
  // This is only for x86-64 and ARM64 for now. This might be fine on
  // other 64-bit processors too.
  //
  // Reasons to use subtraction instead of xor:
  //
  //   - On some x86-64 processors (Intel Sandy Bridge to Tiger Lake),
  //     sub+jz and sub+jnz can be fused but xor+jz or xor+jnz cannot.
  //     Thus using subtraction has potential to be a tiny amount faster
  //     since the code checks if the quotient is non-zero.
  //
  //   - Some processors (Intel Pentium 4) used to have more ALU
  //     resources for add/sub instructions than and/or/xor.
  //
  // The processor info is based on Agner Fog's microarchitecture.pdf
  // version 2023-05-26. https://www.agner.org/optimize/
#define LZMA_MEMCMPLEN_EXTRA 8
  while (len < limit) {
# ifdef WORDS_BIGENDIAN
    const uint64_t x = read64ne(buf1 + len) ^ read64ne(buf2 + len);
# else
    const uint64_t x = read64ne(buf1 + len) - read64ne(buf2 + len);
# endif
    if (x != 0) {
  // MSVC or Intel C compiler on Windows
# if defined(_MSC_VER) || defined(__INTEL_COMPILER)
      unsigned long tmp;
      _BitScanForward64(&tmp, x);
      len += (uint32_t)tmp >> 3;
  // GCC, Clang, or Intel C compiler
# elif defined(WORDS_BIGENDIAN)
      len += (uint32_t)__builtin_clzll(x) >> 3;
# else
      len += (uint32_t)__builtin_ctzll(x) >> 3;
# endif
      return my_min(len, limit);
    }

    len += 8;
  }

  return limit;

#elif defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
    && defined(HAVE__MM_MOVEMASK_EPI8) \
    && (defined(__SSE2__) \
      || (defined(_MSC_VER) && defined(_M_IX86_FP) \
        && _M_IX86_FP >= 2))
  // NOTE: This will use 128-bit unaligned access which
  // TUKLIB_FAST_UNALIGNED_ACCESS wasn't meant to permit,
  // but it's convenient here since this is x86-only.
  //
  // SSE2 version for 32-bit and 64-bit x86. On x86-64 the above
  // version is sometimes significantly faster and sometimes
  // slightly slower than this SSE2 version, so this SSE2
  // version isn't used on x86-64.
# define LZMA_MEMCMPLEN_EXTRA 16
  while (len < limit) {
    const uint32_t x = 0xFFFF ^ (uint32_t)_mm_movemask_epi8(
      _mm_cmpeq_epi8(
      _mm_loadu_si128((const __m128i *)(buf1 + len)),
      _mm_loadu_si128((const __m128i *)(buf2 + len))));

    if (x != 0) {
      len += ctz32(x);
      return my_min(len, limit);
    }

    len += 16;
  }

  return limit;

#elif defined(TUKLIB_FAST_UNALIGNED_ACCESS) && !defined(WORDS_BIGENDIAN)
  // Generic 32-bit little endian method
# define LZMA_MEMCMPLEN_EXTRA 4
  while (len < limit) {
    uint32_t x = read32ne(buf1 + len) - read32ne(buf2 + len);
    if (x != 0) {
      if ((x & 0xFFFF) == 0) {
        len += 2;
        x >>= 16;
      }

      if ((x & 0xFF) == 0)
        ++len;

      return my_min(len, limit);
    }

    len += 4;
  }

  return limit;

#elif defined(TUKLIB_FAST_UNALIGNED_ACCESS) && defined(WORDS_BIGENDIAN)
  // Generic 32-bit big endian method
# define LZMA_MEMCMPLEN_EXTRA 4
  while (len < limit) {
    uint32_t x = read32ne(buf1 + len) ^ read32ne(buf2 + len);
    if (x != 0) {
      if ((x & 0xFFFF0000) == 0) {
        len += 2;
        x <<= 16;
      }

      if ((x & 0xFF000000) == 0)
        ++len;

      return my_min(len, limit);
    }

    len += 4;
  }

  return limit;

#else
  // Simple portable version that doesn't use unaligned access.
# define LZMA_MEMCMPLEN_EXTRA 0
  while (len < limit && buf1[len] == buf2[len])
    ++len;

  return len;
#endif
}

#endif

Coverage Report

Created: 2026-10-28 07:26