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liblzma: Refactor CRC comments.

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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 2023-10-14 13:23:23 +08:00
parent 8c0f9376f5
commit 96b663f67c
2 changed files with 53 additions and 72 deletions
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64
src/liblzma/check/crc32_fast.c
View file

@ -2,25 +2,6 @@
//
/// \file crc32.c
/// \brief CRC32 calculation
///
/// There are two methods in this file.
/// crc32_generic uses the slice-by-eight algorithm.
/// It is explained in this document:
/// http://www.intel.com/technology/comms/perfnet/download/CRC_generators.pdf
/// The code in this file is not the same as in Intel's paper, but
/// the basic principle is identical.
///
/// crc32_clmul uses 32/64-bit x86 SSSE3, SSE4.1, and CLMUL instructions.
/// It was derived from
/// https://www.researchgate.net/publication/263424619_Fast_CRC_computation
/// and the public domain code from https://github.com/rawrunprotected/crc
/// (URLs were checked on 2023-09-29).
///
/// FIXME: Builds for 32-bit x86 use crc32_x86.S by default instead
/// of this file and thus CLMUL version isn't available on 32-bit x86
/// unless configured with --disable-assembler. Even then the lookup table
/// isn't omitted in crc32_table.c since it doesn't know that assembly
/// code has been disabled.
//
// Authors: Lasse Collin
// Ilya Kurdyukov
@ -100,6 +81,38 @@ crc32_generic(const uint8_t *buf, size_t size, uint32_t crc)
#endif
#if defined(CRC_GENERIC) && defined(CRC_CLMUL)
//////////////////////////
// Function dispatching //
//////////////////////////
// If both the generic and CLMUL implementations are built, then the
// function to use is selected at runtime since system running the
// binary may not have the CLMUL instructions.
// The three dispatch methods in order of priority:
//
// 1. Indirect function (ifunc). This method is slightly more efficient
// than the constructor method because it will change the entry in the
// Procedure Linkage Table (PLT) for the function either at load time or
// at the first call. This avoids having to call the function through a
// function pointer and will treat the function call like a regular call
// through the PLT. ifuncs are created by using
// __attribute__((__ifunc__("resolver"))) on a function which has no
// body. The "resolver" is the name of the function that chooses at
// runtime which implementation to use.
//
// 2. Constructor. This method uses __attribute__((__constructor__)) to
// set crc32_func at load time. This avoids extra computation (and any
// unlikely threading bugs) on the first call to lzma_crc32() to decide
// which implementation should be used.
//
// 3. First Call Resolution. On the very first call to lzma_crc32(), the
// call will be directed to crc32_dispatch() instead. This will set the
// appropriate implementation function and will not be called again.
// This method does not use any kind of locking but is safe because if
// multiple threads run the dispatcher simultaneously then they will all
// set crc32_func to the same value.
typedef uint32_t (*crc32_func_type)(
const uint8_t *buf, size_t size, uint32_t crc);
@ -111,6 +124,9 @@ typedef uint32_t (*crc32_func_type)(
# pragma GCC diagnostic ignored "-Wunused-function"
#endif
// This resolver is shared between all three dispatch methods. It serves as
// the ifunc resolver if ifunc is supported, otherwise it is called as a
// regular function by the constructor or first call resolution methods.
static crc32_func_type
crc32_resolve(void)
{
@ -124,9 +140,11 @@ crc32_resolve(void)
#ifndef HAVE_FUNC_ATTRIBUTE_IFUNC
#ifdef HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR
// Constructor method.
# define CRC32_SET_FUNC_ATTR __attribute__((__constructor__))
static crc32_func_type crc32_func;
#else
// First Call Resolution method.
# define CRC32_SET_FUNC_ATTR
static uint32_t crc32_dispatch(const uint8_t *buf, size_t size, uint32_t crc);
static crc32_func_type crc32_func = &crc32_dispatch;
@ -190,6 +208,14 @@ lzma_crc32(const uint8_t *buf, size_t size, uint32_t crc)
return crc32_generic(buf, size, crc);
#endif
/*
#ifndef HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR
// See crc32_dispatch(). This would be the alternative which uses
// locking and doesn't use crc32_dispatch(). Note that on Windows
// this method needs Vista threads.
mythread_once(crc64_set_func);
#endif
*/
return crc32_func(buf, size, crc);
#elif defined(CRC_CLMUL)

61
src/liblzma/check/crc64_fast.c
View file

@ -2,23 +2,6 @@
//
/// \file crc64.c
/// \brief CRC64 calculation
///
/// There are two methods in this file. crc64_generic uses the
/// the slice-by-four algorithm. This is the same idea that is
/// used in crc32_fast.c, but for CRC64 we use only four tables
/// instead of eight to avoid increasing CPU cache usage.
///
/// crc64_clmul uses 32/64-bit x86 SSSE3, SSE4.1, and CLMUL instructions.
/// It was derived from
/// https://www.researchgate.net/publication/263424619_Fast_CRC_computation
/// and the public domain code from https://github.com/rawrunprotected/crc
/// (URLs were checked on 2023-09-29).
///
/// FIXME: Builds for 32-bit x86 use crc64_x86.S by default instead
/// of this file and thus CLMUL version isn't available on 32-bit x86
/// unless configured with --disable-assembler. Even then the lookup table
/// isn't omitted in crc64_table.c since it doesn't know that assembly
/// code has been disabled.
//
// Authors: Lasse Collin
// Ilya Kurdyukov
@ -93,12 +76,16 @@ crc64_generic(const uint8_t *buf, size_t size, uint64_t crc)
#endif
#if defined(CRC_GENERIC) && defined(CRC_CLMUL)
//////////////////////////
// Function dispatching //
//////////////////////////
// If both the generic and CLMUL implementations are usable, then the
// function that is used is selected at runtime. See crc32_fast.c.
typedef uint64_t (*crc64_func_type)(
const uint8_t *buf, size_t size, uint64_t crc);
// Clang 16.0.0 and older has a bug where it marks the ifunc resolver
// function as unused since it is static and never used outside of
// __attribute__((__ifunc__())).
#if defined(HAVE_FUNC_ATTRIBUTE_IFUNC) && defined(__clang__)
# pragma GCC diagnostic push
# pragma GCC diagnostic ignored "-Wunused-function"
@ -139,13 +126,6 @@ crc64_set_func(void)
static uint64_t
crc64_dispatch(const uint8_t *buf, size_t size, uint64_t crc)
{
// When __attribute__((__ifunc__(...))) and
// __attribute__((__constructor__)) isn't supported, set the
// function pointer without any locking. If multiple threads run
// the detection code in parallel, they will all end up setting
// the pointer to the same value. This avoids the use of
// mythread_once() on every call to lzma_crc64() but this likely
// isn't strictly standards compliant. Let's change it if it breaks.
crc64_set_func();
return crc64_func(buf, size, crc);
}
@ -163,36 +143,11 @@ extern LZMA_API(uint64_t)
lzma_crc64(const uint8_t *buf, size_t size, uint64_t crc)
{
#if defined(CRC_GENERIC) && defined(CRC_CLMUL)
// If CLMUL is available, it is the best for non-tiny inputs,
// being over twice as fast as the generic slice-by-four version.
// However, for size <= 16 it's different. In the extreme case
// of size == 1 the generic version can be five times faster.
// At size >= 8 the CLMUL starts to become reasonable. It
// varies depending on the alignment of buf too.
//
// The above doesn't include the overhead of mythread_once().
// At least on x86-64 GNU/Linux, pthread_once() is very fast but
// it still makes lzma_crc64(buf, 1, crc) 50-100 % slower. When
// size reaches 12-16 bytes the overhead becomes negligible.
//
// So using the generic version for size <= 16 may give better
// performance with tiny inputs but if such inputs happen rarely
// it's not so obvious because then the lookup table of the
// generic version may not be in the processor cache.
#ifdef CRC_USE_GENERIC_FOR_SMALL_INPUTS
if (size <= 16)
return crc64_generic(buf, size, crc);
#endif
/*
#ifndef HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR
// See crc64_dispatch(). This would be the alternative which uses
// locking and doesn't use crc64_dispatch(). Note that on Windows
// this method needs Vista threads.
mythread_once(crc64_set_func);
#endif
*/
return crc64_func(buf, size, crc);
#elif defined(CRC_CLMUL)