crc32_fast.c

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//
//
//  Author:     Lasse Collin
//
//  This file has been put into the public domain.
//  You can do whatever you want with this file.
//
 
#include "check.h"
#include "crc_macros.h"
 
 
// If you make any changes, do some benchmarking! Seemingly unrelated
// changes can very easily ruin the performance (and very probably is
// very compiler dependent).
extern LZMA_API(uint32_t)
lzma_crc32(const uint8_t *buf, size_t size, uint32_t crc)
{
    crc = ~crc;
 
#ifdef WORDS_BIGENDIAN
    crc = bswap32(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.
        while (buf < limit) {
            crc ^= aligned_read32ne(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 = aligned_read32ne(buf);
            buf += 4;
 
            // At least with some compilers, 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 = bswap32(crc);
#endif
 
    return ~crc;
}