//
/// \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
#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);
# 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)
{
#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;
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)
//
/// \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
#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"
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);
}
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)
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