text: // Protocol Buffers - Google's data interchange format
  // Copyright 2014 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  // copyright notice, this list of conditions and the following disclaimer
  // in the documentation and/or other materials provided with the
  // distribution.
  //     * Neither the name of Google Inc. nor the names of its
  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  // Fast memory copying and comparison routines.
  //   strings::fastmemcmp_inlined() replaces memcmp()
  //   strings::memcpy_inlined() replaces memcpy()
  //   strings::memeq(a, b, n) replaces memcmp(a, b, n) == 0
  //
  // strings::*_inlined() routines are inline versions of the
  // routines exported by this module.  Sometimes using the inlined
  // versions is faster.  Measure before using the inlined versions.
  //
  // Performance measurement:
  //   strings::fastmemcmp_inlined
  //     Analysis: memcmp, fastmemcmp_inlined, fastmemcmp
  //     2012-01-30
  
  #ifndef GOOGLE_PROTOBUF_STUBS_FASTMEM_H_
  #define GOOGLE_PROTOBUF_STUBS_FASTMEM_H_
  
  #include <stddef.h>
  #include <stdio.h>
  #include <string.h>
  
  #include <protobuf/stubs/common.h>
  
  namespace google {
  namespace protobuf {
  namespace internal {
  
  // Return true if the n bytes at a equal the n bytes at b.
  // The regions are allowed to overlap.
  //
  // The performance is similar to the performance memcmp(), but faster for
  // moderately-sized inputs, or inputs that share a common prefix and differ
  // somewhere in their last 8 bytes. Further optimizations can be added later
  // if it makes sense to do so.:w
  inline bool memeq(const char* a, const char* b, size_t n) {
    size_t n_rounded_down = n & ~static_cast<size_t>(7);
    if (GOOGLE_PREDICT_FALSE(n_rounded_down == 0)) {  // n <= 7
      return memcmp(a, b, n) == 0;
    }
    // n >= 8
    uint64 u = GOOGLE_UNALIGNED_LOAD64(a) ^ GOOGLE_UNALIGNED_LOAD64(b);
    uint64 v = GOOGLE_UNALIGNED_LOAD64(a + n - 8) ^ GOOGLE_UNALIGNED_LOAD64(b + n - 8);
    if ((u | v) != 0) {  // The first or last 8 bytes differ.
      return false;
    }
    a += 8;
    b += 8;
    n = n_rounded_down - 8;
    if (n > 128) {
      // As of 2012, memcmp on x86-64 uses a big unrolled loop with SSE2
      // instructions, and while we could try to do something faster, it
      // doesn't seem worth pursuing.
      return memcmp(a, b, n) == 0;
    }
    for (; n >= 16; n -= 16) {
      uint64 x = GOOGLE_UNALIGNED_LOAD64(a) ^ GOOGLE_UNALIGNED_LOAD64(b);
      uint64 y = GOOGLE_UNALIGNED_LOAD64(a + 8) ^ GOOGLE_UNALIGNED_LOAD64(b + 8);
      if ((x | y) != 0) {
        return false;
      }
      a += 16;
      b += 16;
    }
    // n must be 0 or 8 now because it was a multiple of 8 at the top of the loop.
    return n == 0 || GOOGLE_UNALIGNED_LOAD64(a) == GOOGLE_UNALIGNED_LOAD64(b);
  }
  
  inline int fastmemcmp_inlined(const char *a, const char *b, size_t n) {
    if (n >= 64) {
      return memcmp(a, b, n);
    }
    const char* a_limit = a + n;
    while (a + sizeof(uint64) <= a_limit &&
           GOOGLE_UNALIGNED_LOAD64(a) == GOOGLE_UNALIGNED_LOAD64(b)) {
      a += sizeof(uint64);
      b += sizeof(uint64);
    }
    if (a + sizeof(uint32) <= a_limit &&
        GOOGLE_UNALIGNED_LOAD32(a) == GOOGLE_UNALIGNED_LOAD32(b)) {
      a += sizeof(uint32);
      b += sizeof(uint32);
    }
    while (a < a_limit) {
      int d = static_cast<uint32>(*a++) - static_cast<uint32>(*b++);
      if (d) return d;
    }
    return 0;
  }
  
  // The standard memcpy operation is slow for variable small sizes.
  // This implementation inlines the optimal realization for sizes 1 to 16.
  // To avoid code bloat don't use it in case of not performance-critical spots,
  // nor when you don't expect very frequent values of size <= 16.
  inline void memcpy_inlined(char *dst, const char *src, size_t size) {
    // Compiler inlines code with minimal amount of data movement when third
    // parameter of memcpy is a constant.
    switch (size) {
      case  1: memcpy(dst, src, 1); break;
      case  2: memcpy(dst, src, 2); break;
      case  3: memcpy(dst, src, 3); break;
      case  4: memcpy(dst, src, 4); break;
      case  5: memcpy(dst, src, 5); break;
      case  6: memcpy(dst, src, 6); break;
      case  7: memcpy(dst, src, 7); break;
      case  8: memcpy(dst, src, 8); break;
      case  9: memcpy(dst, src, 9); break;
      case 10: memcpy(dst, src, 10); break;
      case 11: memcpy(dst, src, 11); break;
      case 12: memcpy(dst, src, 12); break;
      case 13: memcpy(dst, src, 13); break;
      case 14: memcpy(dst, src, 14); break;
      case 15: memcpy(dst, src, 15); break;
      case 16: memcpy(dst, src, 16); break;
      default: memcpy(dst, src, size); break;
    }
  }
  
  }  // namespace internal
  }  // namespace protobuf
  }  // namespace google
  
  #endif  // GOOGLE_PROTOBUF_STUBS_FASTMEM_H_
  