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5e78fcbf2e
Using the aligned methods requires more care to ensure that the address really is aligned, so it's nicer if the aligned methods are prefixed. The next commit will remove the unaligned_ prefix from the unaligned methods which in liblzma are used in more places than the aligned ones.
82 lines
2.2 KiB
C
82 lines
2.2 KiB
C
///////////////////////////////////////////////////////////////////////////////
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//
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/// \file crc32.c
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/// \brief CRC32 calculation
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///
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/// Calculate the CRC32 using the slice-by-eight algorithm.
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/// It is explained in this document:
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/// http://www.intel.com/technology/comms/perfnet/download/CRC_generators.pdf
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/// The code in this file is not the same as in Intel's paper, but
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/// the basic principle is identical.
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//
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// Author: Lasse Collin
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//
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// This file has been put into the public domain.
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// You can do whatever you want with this file.
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//
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///////////////////////////////////////////////////////////////////////////////
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#include "check.h"
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#include "crc_macros.h"
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// If you make any changes, do some benchmarking! Seemingly unrelated
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// changes can very easily ruin the performance (and very probably is
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// very compiler dependent).
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extern LZMA_API(uint32_t)
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lzma_crc32(const uint8_t *buf, size_t size, uint32_t crc)
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{
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crc = ~crc;
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#ifdef WORDS_BIGENDIAN
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crc = bswap32(crc);
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#endif
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if (size > 8) {
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// Fix the alignment, if needed. The if statement above
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// ensures that this won't read past the end of buf[].
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while ((uintptr_t)(buf) & 7) {
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crc = lzma_crc32_table[0][*buf++ ^ A(crc)] ^ S8(crc);
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--size;
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}
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// Calculate the position where to stop.
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const uint8_t *const limit = buf + (size & ~(size_t)(7));
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// Calculate how many bytes must be calculated separately
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// before returning the result.
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size &= (size_t)(7);
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// Calculate the CRC32 using the slice-by-eight algorithm.
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while (buf < limit) {
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crc ^= aligned_read32ne(buf);
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buf += 4;
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crc = lzma_crc32_table[7][A(crc)]
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^ lzma_crc32_table[6][B(crc)]
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^ lzma_crc32_table[5][C(crc)]
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^ lzma_crc32_table[4][D(crc)];
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const uint32_t tmp = aligned_read32ne(buf);
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buf += 4;
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// At least with some compilers, it is critical for
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// performance, that the crc variable is XORed
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// between the two table-lookup pairs.
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crc = lzma_crc32_table[3][A(tmp)]
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^ lzma_crc32_table[2][B(tmp)]
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^ crc
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^ lzma_crc32_table[1][C(tmp)]
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^ lzma_crc32_table[0][D(tmp)];
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}
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}
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while (size-- != 0)
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crc = lzma_crc32_table[0][*buf++ ^ A(crc)] ^ S8(crc);
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#ifdef WORDS_BIGENDIAN
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crc = bswap32(crc);
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#endif
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return ~crc;
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}
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