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///////////////////////////////////////////////////////////////////////////////
//
/// \file crc32.c
/// \brief CRC32 calculation
//
// Authors: Lasse Collin
// Ilya Kurdyukov
// Hans Jansen
//
// This file has been put into the public domain.
// You can do whatever you want with this file.
//
///////////////////////////////////////////////////////////////////////////////
#include "check.h"
#include "crc_common.h"
#ifdef CRC_GENERIC
///////////////////
// Generic CRC32 //
///////////////////
static uint32_t
crc32_generic(const uint8_t *buf, size_t size, uint32_t crc)
{
crc = ~crc;
#ifdef WORDS_BIGENDIAN
crc = bswap32(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.
while (buf < limit) {
crc ^= aligned_read32ne(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 = aligned_read32ne(buf);
buf += 4;
// At least with some compilers, 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 = bswap32(crc);
#endif
return ~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);
// 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"
#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)
{
return lzma_is_clmul_supported() ? &lzma_crc32_clmul : &crc32_generic;
}
#if defined(HAVE_FUNC_ATTRIBUTE_IFUNC) && defined(__clang__)
# pragma GCC diagnostic pop
#endif
#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;
#endif
CRC32_SET_FUNC_ATTR
static void
crc32_set_func(void)
{
crc32_func = crc32_resolve();
return;
}
#ifndef HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR
static uint32_t
crc32_dispatch(const uint8_t *buf, size_t size, uint32_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_crc32() but this likely
// isn't strictly standards compliant. Let's change it if it breaks.
crc32_set_func();
return crc32_func(buf, size, crc);
}
#endif
#endif
#endif
#ifdef CRC_USE_IFUNC
extern LZMA_API(uint32_t)
lzma_crc32(const uint8_t *buf, size_t size, uint32_t crc)
__attribute__((__ifunc__("crc32_resolve")));
#else
extern LZMA_API(uint32_t)
lzma_crc32(const uint8_t *buf, size_t size, uint32_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_crc32(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 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)
return lzma_crc32_clmul(buf, size, crc);
#else
return crc32_generic(buf, size, crc);
#endif
}
#endif
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