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///////////////////////////////////////////////////////////////////////////////
//
/// \file crc32.c
/// \brief CRC32 calculation
//
// This code has been put into the public domain.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
//
///////////////////////////////////////////////////////////////////////////////
#include "check.h"
#include "crc_macros.h"
// If you make any changes, do some bench marking! Seemingly unrelated
// changes can very easily ruin the performance (and very probably is
// very compiler dependent).
extern uint32_t
lzma_crc32(const uint8_t *buf, size_t size, uint32_t crc)
{
crc = ~crc;
#ifdef WORDS_BIGENDIAN
crc = bswap_32(crc);
#endif
if (size > 8) {
// Fix the alignment, if needed. The if statement above
// ensures that this won't read past the end of buf[].
while ((uintptr_t)(buf) & 7) {
crc = lzma_crc32_table[0][*buf++ ^ A(crc)] ^ S8(crc);
--size;
}
// Calculate the position where to stop.
const uint8_t *const limit = buf + (size & ~(size_t)(7));
// Calculate how many bytes must be calculated separately
// before returning the result.
size &= (size_t)(7);
// Calculate the CRC32 using the slice-by-eight algorithm.
// It is explained in this document:
// http://www.intel.com/technology/comms/perfnet/download/CRC_generators.pdf
//
// The code below is different than the code in Intel's
// paper, but the principle is identical. This should be
// faster with GCC than Intel's code. This is tested only
// with GCC 3.4.6 and 4.1.2 on x86, so your results may vary.
//
// Using -Os and -fomit-frame-pointer seem to give the best
// results at least with GCC 4.1.2 on x86. It's sill far
// from the speed of hand-optimized assembler.
while (buf < limit) {
crc ^= *(uint32_t *)(buf);
buf += 4;
crc = lzma_crc32_table[7][A(crc)]
^ lzma_crc32_table[6][B(crc)]
^ lzma_crc32_table[5][C(crc)]
^ lzma_crc32_table[4][D(crc)];
const uint32_t tmp = *(uint32_t *)(buf);
buf += 4;
// It is critical for performance, that
// the crc variable is XORed between the
// two table-lookup pairs.
crc = lzma_crc32_table[3][A(tmp)]
^ lzma_crc32_table[2][B(tmp)]
^ crc
^ lzma_crc32_table[1][C(tmp)]
^ lzma_crc32_table[0][D(tmp)];
}
}
while (size-- != 0)
crc = lzma_crc32_table[0][*buf++ ^ A(crc)] ^ S8(crc);
#ifdef WORDS_BIGENDIAN
crc = bswap_32(crc);
#endif
return ~crc;
}
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