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It also works on E2K as it supports these intrinsics. On x86-64 runtime detection is used so the code keeps working on older processors too. A CLMUL-only build can be done by using -msse4.1 -mpclmul in CFLAGS and this will reduce the library size since the generic implementation and its 8 KiB lookup table will be omitted. On 32-bit x86 this isn't used by default for now because by default on 32-bit x86 the separate assembly file crc64_x86.S is used. If --disable-assembler is used then this new CLMUL code is used the same way as on 64-bit x86. However, a CLMUL-only build (-msse4.1 -mpclmul) won't omit the 8 KiB lookup table on 32-bit x86 due to a currently-missing check for disabled assembler usage. The configure.ac check should be such that the code won't be built if something in the toolchain doesn't support it but --disable-clmul-crc option can be used to unconditionally disable this feature. CLMUL speeds up decompression of files that have compressed very well (assuming CRC64 is used as a check type). It is know that the CLMUL code is significantly slower than the generic code for tiny inputs (especially 1-8 bytes but up to 16 bytes). If that is a real-world problem then there is already a commented-out variant that uses the generic version for small inputs. Thanks to Ilya Kurdyukov for the original patch which was derived from a white paper from Intel [1] (published in 2009) and public domain code from [2] (released in 2016). [1] https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/fast-crc-computation-generic-polynomials-pclmulqdq-paper.pdf [2] https://github.com/rawrunprotected/crc
35 lines
1.1 KiB
C
35 lines
1.1 KiB
C
///////////////////////////////////////////////////////////////////////////////
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//
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/// \file crc64_table.c
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/// \brief Precalculated CRC64 table with correct endianness
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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 "common.h"
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// FIXME: Compared to crc64_fast.c this has to check for __x86_64__ too
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// so that in 32-bit builds crc64_x86.S won't break due to a missing table.
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#if (defined(__x86_64__) && defined(__SSSE3__) \
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&& defined(__SSE4_1__) && defined(__PCLMUL__)) \
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|| (defined(__e2k__) && __iset__ >= 6)
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// No table needed but something has to be exported to keep some toolchains
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// happy. Also use a declaration to silence compiler warnings.
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extern const char lzma_crc64_dummy;
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const char lzma_crc64_dummy;
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#else
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// Having the declaration here silences clang -Wmissing-variable-declarations.
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extern const uint64_t lzma_crc64_table[4][256];
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# if defined(WORDS_BIGENDIAN)
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# include "crc64_table_be.h"
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# else
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# include "crc64_table_le.h"
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# endif
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#endif
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