liblzma: Refactor CRC comments.
A detailed description of the three dispatch methods was added. Also, duplicated comments now only appear in crc32_fast.c or were removed from both crc32_fast.c and crc64_fast.c if they appeared in crc_clmul.c.
This commit is contained in:
Jia Tan
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8c0f9376f5
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96b663f67c
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@ -2,25 +2,6 @@
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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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/// There are two methods in this file.
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/// crc32_generic uses 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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/// crc32_clmul uses 32/64-bit x86 SSSE3, SSE4.1, and CLMUL instructions.
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/// It was derived from
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/// https://www.researchgate.net/publication/263424619_Fast_CRC_computation
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/// and the public domain code from https://github.com/rawrunprotected/crc
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/// (URLs were checked on 2023-09-29).
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///
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/// FIXME: Builds for 32-bit x86 use crc32_x86.S by default instead
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/// of this file and thus CLMUL version isn't available on 32-bit x86
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/// unless configured with --disable-assembler. Even then the lookup table
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/// isn't omitted in crc32_table.c since it doesn't know that assembly
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/// code has been disabled.
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//
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// Authors: Lasse Collin
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// Ilya Kurdyukov
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@ -100,6 +81,38 @@ crc32_generic(const uint8_t *buf, size_t size, uint32_t crc)
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#endif
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#if defined(CRC_GENERIC) && defined(CRC_CLMUL)
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//////////////////////////
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// Function dispatching //
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//////////////////////////
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// If both the generic and CLMUL implementations are built, then the
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// function to use is selected at runtime since system running the
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// binary may not have the CLMUL instructions.
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// The three dispatch methods in order of priority:
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//
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// 1. Indirect function (ifunc). This method is slightly more efficient
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// than the constructor method because it will change the entry in the
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// Procedure Linkage Table (PLT) for the function either at load time or
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// at the first call. This avoids having to call the function through a
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// function pointer and will treat the function call like a regular call
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// through the PLT. ifuncs are created by using
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// __attribute__((__ifunc__("resolver"))) on a function which has no
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// body. The "resolver" is the name of the function that chooses at
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// runtime which implementation to use.
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//
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// 2. Constructor. This method uses __attribute__((__constructor__)) to
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// set crc32_func at load time. This avoids extra computation (and any
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// unlikely threading bugs) on the first call to lzma_crc32() to decide
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// which implementation should be used.
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//
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// 3. First Call Resolution. On the very first call to lzma_crc32(), the
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// call will be directed to crc32_dispatch() instead. This will set the
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// appropriate implementation function and will not be called again.
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// This method does not use any kind of locking but is safe because if
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// multiple threads run the dispatcher simultaneously then they will all
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// set crc32_func to the same value.
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typedef uint32_t (*crc32_func_type)(
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const uint8_t *buf, size_t size, uint32_t crc);
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@ -111,6 +124,9 @@ typedef uint32_t (*crc32_func_type)(
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# pragma GCC diagnostic ignored "-Wunused-function"
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#endif
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// This resolver is shared between all three dispatch methods. It serves as
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// the ifunc resolver if ifunc is supported, otherwise it is called as a
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// regular function by the constructor or first call resolution methods.
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static crc32_func_type
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crc32_resolve(void)
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{
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@ -124,9 +140,11 @@ crc32_resolve(void)
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#ifndef HAVE_FUNC_ATTRIBUTE_IFUNC
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#ifdef HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR
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// Constructor method.
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# define CRC32_SET_FUNC_ATTR __attribute__((__constructor__))
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static crc32_func_type crc32_func;
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#else
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// First Call Resolution method.
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# define CRC32_SET_FUNC_ATTR
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static uint32_t crc32_dispatch(const uint8_t *buf, size_t size, uint32_t crc);
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static crc32_func_type crc32_func = &crc32_dispatch;
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@ -190,6 +208,14 @@ lzma_crc32(const uint8_t *buf, size_t size, uint32_t crc)
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return crc32_generic(buf, size, crc);
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#endif
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/*
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#ifndef HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR
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// See crc32_dispatch(). This would be the alternative which uses
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// locking and doesn't use crc32_dispatch(). Note that on Windows
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// this method needs Vista threads.
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mythread_once(crc64_set_func);
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#endif
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*/
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return crc32_func(buf, size, crc);
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#elif defined(CRC_CLMUL)
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@ -2,23 +2,6 @@
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//
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/// \file crc64.c
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/// \brief CRC64 calculation
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///
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/// There are two methods in this file. crc64_generic uses the
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/// the slice-by-four algorithm. This is the same idea that is
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/// used in crc32_fast.c, but for CRC64 we use only four tables
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/// instead of eight to avoid increasing CPU cache usage.
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///
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/// crc64_clmul uses 32/64-bit x86 SSSE3, SSE4.1, and CLMUL instructions.
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/// It was derived from
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/// https://www.researchgate.net/publication/263424619_Fast_CRC_computation
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/// and the public domain code from https://github.com/rawrunprotected/crc
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/// (URLs were checked on 2023-09-29).
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///
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/// FIXME: Builds for 32-bit x86 use crc64_x86.S by default instead
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/// of this file and thus CLMUL version isn't available on 32-bit x86
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/// unless configured with --disable-assembler. Even then the lookup table
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/// isn't omitted in crc64_table.c since it doesn't know that assembly
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/// code has been disabled.
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//
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// Authors: Lasse Collin
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// Ilya Kurdyukov
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@ -93,12 +76,16 @@ crc64_generic(const uint8_t *buf, size_t size, uint64_t crc)
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#endif
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#if defined(CRC_GENERIC) && defined(CRC_CLMUL)
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//////////////////////////
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// Function dispatching //
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//////////////////////////
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// If both the generic and CLMUL implementations are usable, then the
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// function that is used is selected at runtime. See crc32_fast.c.
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typedef uint64_t (*crc64_func_type)(
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const uint8_t *buf, size_t size, uint64_t crc);
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// Clang 16.0.0 and older has a bug where it marks the ifunc resolver
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// function as unused since it is static and never used outside of
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// __attribute__((__ifunc__())).
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#if defined(HAVE_FUNC_ATTRIBUTE_IFUNC) && defined(__clang__)
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# pragma GCC diagnostic push
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# pragma GCC diagnostic ignored "-Wunused-function"
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@ -139,13 +126,6 @@ crc64_set_func(void)
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static uint64_t
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crc64_dispatch(const uint8_t *buf, size_t size, uint64_t crc)
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{
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// When __attribute__((__ifunc__(...))) and
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// __attribute__((__constructor__)) isn't supported, set the
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// function pointer without any locking. If multiple threads run
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// the detection code in parallel, they will all end up setting
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// the pointer to the same value. This avoids the use of
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// mythread_once() on every call to lzma_crc64() but this likely
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// isn't strictly standards compliant. Let's change it if it breaks.
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crc64_set_func();
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return crc64_func(buf, size, crc);
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}
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@ -163,36 +143,11 @@ extern LZMA_API(uint64_t)
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lzma_crc64(const uint8_t *buf, size_t size, uint64_t crc)
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{
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#if defined(CRC_GENERIC) && defined(CRC_CLMUL)
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// If CLMUL is available, it is the best for non-tiny inputs,
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// being over twice as fast as the generic slice-by-four version.
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// However, for size <= 16 it's different. In the extreme case
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// of size == 1 the generic version can be five times faster.
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// At size >= 8 the CLMUL starts to become reasonable. It
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// varies depending on the alignment of buf too.
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//
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// The above doesn't include the overhead of mythread_once().
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// At least on x86-64 GNU/Linux, pthread_once() is very fast but
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// it still makes lzma_crc64(buf, 1, crc) 50-100 % slower. When
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// size reaches 12-16 bytes the overhead becomes negligible.
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//
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// So using the generic version for size <= 16 may give better
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// performance with tiny inputs but if such inputs happen rarely
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// it's not so obvious because then the lookup table of the
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// generic version may not be in the processor cache.
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#ifdef CRC_USE_GENERIC_FOR_SMALL_INPUTS
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if (size <= 16)
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return crc64_generic(buf, size, crc);
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#endif
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/*
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#ifndef HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR
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// See crc64_dispatch(). This would be the alternative which uses
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// locking and doesn't use crc64_dispatch(). Note that on Windows
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// this method needs Vista threads.
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mythread_once(crc64_set_func);
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#endif
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*/
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return crc64_func(buf, size, crc);
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#elif defined(CRC_CLMUL)
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