xz/src/liblzma/check/crc32_fast.c

///////////////////////////////////////////////////////////////////////////////
//
/// \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 LZMA_API 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;
}