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// SPDX-License-Identifier: GPL-2.0
/*
* Use slice-by-8, which is the fastest variant.
*
* Calculate checksum 8 bytes at a time with a clever slicing algorithm.
* This is the fastest algorithm, but comes with a 8KiB lookup table.
* Most modern processors have enough cache to hold this table without
* thrashing the cache.
*
* The Linux kernel uses this as the default implementation "unless you
* have a good reason not to". The reason why Kconfig urges you to pick
* SLICEBY8 is because people challenged the assertion that we should
* always use slice by 8, so Darrick wrote a crc microbenchmark utility
* and ran it on as many machines as he could get his hands on to show
* that sb8 was the fastest.
*
* Every 64-bit machine (and most of the 32-bit ones too) saw the best
* results with sb8. Any machine with more than 4K of cache saw better
* results. The spreadsheet still exists today[1]; note that
* 'crc32-kern-le' corresponds to the slice by 4 algorithm which is the
* default unless CRC_LE_BITS is defined explicitly.
*
* FWIW, there are a handful of board defconfigs in the kernel that
* don't pick sliceby8. These are all embedded 32-bit mips/ppc systems
* with very small cache sizes which experience cache thrashing with the
* slice by 8 algorithm, and therefore chose to pick defaults that are
* saner for their particular board configuration. For nearly all of
* XFS' perceived userbase (which we assume are 32 and 64-bit machines
* with sufficiently large CPU cache and largeish storage devices) slice
* by 8 is the right choice.
*
* [1] https://goo.gl/0LSzsG ("crc32c_bench")
*/
#define CRC_LE_BITS 64
/*
* This is the CRC32c polynomial, as outlined by Castagnoli.
* x^32+x^28+x^27+x^26+x^25+x^23+x^22+x^20+x^19+x^18+x^14+x^13+x^11+x^10+x^9+
* x^8+x^6+x^0
*/
#define CRC32C_POLY_LE 0x82F63B78
/*
* Little-endian CRC computation. Used with serial bit streams sent
* lsbit-first. Be sure to use cpu_to_le32() to append the computed CRC.
*/
#if CRC_LE_BITS > 64 || CRC_LE_BITS < 1 || CRC_LE_BITS == 16 || \
CRC_LE_BITS & CRC_LE_BITS-1
# error "CRC_LE_BITS must be one of {1, 2, 4, 8, 32, 64}"
#endif
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