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.

src/liblzma/check/crc32_fast.c

@@ -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)

src/liblzma/check/crc64_fast.c

@@ -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)