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Revision 1.15 by root, Thu May 26 23:26:29 2011 UTC vs.
Revision 1.76 by root, Mon Jan 20 13:13:56 2020 UTC

…		…
3	=head2 ABOUT LIBECB	3	=head2 ABOUT LIBECB
4		4
5	Libecb is currently a simple header file that doesn't require any	5	Libecb is currently a simple header file that doesn't require any
6	configuration to use or include in your project.	6	configuration to use or include in your project.
7		7
8	It's part of the e-suite of libraries, other memembers of which include	8	It's part of the e-suite of libraries, other members of which include
9	libev and libeio.	9	libev and libeio.
10		10
11	Its homepage can be found here:	11	Its homepage can be found here:
12		12
13	http://software.schmorp.de/pkg/libecb	13	http://software.schmorp.de/pkg/libecb
14		14
15	It mainly provides a number of wrappers around GCC built-ins, together	15	It mainly provides a number of wrappers around GCC built-ins, together
16	with replacement functions for other compilers. In addition to this,	16	with replacement functions for other compilers. In addition to this,
17	it provides a number of other lowlevel C utilities, such endienness	17	it provides a number of other lowlevel C utilities, such as endianness
18	detection, byte swapping or bit rotations.	18	detection, byte swapping or bit rotations.
		19
		20	Or in other words, things that should be built into any standard C system,
		21	but aren't, implemented as efficient as possible with GCC, and still
		22	correct with other compilers.
19		23
20	More might come.	24	More might come.
21		25
22	=head2 ABOUT THE HEADER	26	=head2 ABOUT THE HEADER
23		27
…		…
27	#include <ecb.h>	31	#include <ecb.h>
28		32
29	The header should work fine for both C and C++ compilation, and gives you	33	The header should work fine for both C and C++ compilation, and gives you
30	all of F<inttypes.h> in addition to the ECB symbols.	34	all of F<inttypes.h> in addition to the ECB symbols.
31		35
32	There are currently no objetc files to link to - future versions might	36	There are currently no object files to link to - future versions might
33	come with an (optional) object code library to link against, to reduce	37	come with an (optional) object code library to link against, to reduce
34	code size or gain access to additional features.	38	code size or gain access to additional features.
35		39
36	It also currently includes everything from F<inttypes.h>.	40	It also currently includes everything from F<inttypes.h>.
37		41
…		…
50	only a generic name is used (C<expr>, C<cond>, C<value> and so on), then	54	only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
51	the corresponding function relies on C to implement the correct types, and	55	the corresponding function relies on C to implement the correct types, and
52	is usually implemented as a macro. Specifically, a "bool" in this manual	56	is usually implemented as a macro. Specifically, a "bool" in this manual
53	refers to any kind of boolean value, not a specific type.	57	refers to any kind of boolean value, not a specific type.
54		58
		59	=head2 TYPES / TYPE SUPPORT
		60
		61	ecb.h makes sure that the following types are defined (in the expected way):
		62
		63	int8_t uint8_
		64	int16_t uint16_t
		65	int32_t uint32_
		66	int64_t uint64_t
		67	int_fast8_t uint_fast8_t
		68	int_fast16_t uint_fast16_t
		69	int_fast32_t uint_fast32_t
		70	int_fast64_t uint_fast64_t
		71	intptr_t uintptr_t
		72
		73	The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
		74	platform (currently C<4> or C<8>) and can be used in preprocessor
		75	expressions.
		76
		77	For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>.
		78
		79	=head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
		80
		81	All the following symbols expand to an expression that can be tested in
		82	preprocessor instructions as well as treated as a boolean (use C<!!> to
		83	ensure it's either C<0> or C<1> if you need that).
		84
		85	=over 4
		86
		87	=item ECB_C
		88
		89	True if the implementation defines the C<__STDC__> macro to a true value,
		90	while not claiming to be C++.
		91
		92	=item ECB_C99
		93
		94	True if the implementation claims to be compliant to C99 (ISO/IEC
		95	9899:1999) or any later version, while not claiming to be C++.
		96
		97	Note that later versions (ECB_C11) remove core features again (for
		98	example, variable length arrays).
		99
		100	=item ECB_C11, ECB_C17
		101
		102	True if the implementation claims to be compliant to C11/C17 (ISO/IEC
		103	9899:2011, :20187) or any later version, while not claiming to be C++.
		104
		105	=item ECB_CPP
		106
		107	True if the implementation defines the C<__cplusplus__> macro to a true
		108	value, which is typically true for C++ compilers.
		109
		110	=item ECB_CPP11, ECB_CPP14, ECB_CPP17
		111
		112	True if the implementation claims to be compliant to C++11/C++14/C++17
		113	(ISO/IEC 14882:2011, :2014, :2017) or any later version.
		114
		115	=item ECB_GCC_VERSION (major, minor)
		116
		117	Expands to a true value (suitable for testing in by the preprocessor)
		118	if the compiler used is GNU C and the version is the given version, or
		119	higher.
		120
		121	This macro tries to return false on compilers that claim to be GCC
		122	compatible but aren't.
		123
		124	=item ECB_EXTERN_C
		125
		126	Expands to C<extern "C"> in C++, and a simple C<extern> in C.
		127
		128	This can be used to declare a single external C function:
		129
		130	ECB_EXTERN_C int printf (const char *format, ...);
		131
		132	=item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
		133
		134	These two macros can be used to wrap multiple C<extern "C"> definitions -
		135	they expand to nothing in C.
		136
		137	They are most useful in header files:
		138
		139	ECB_EXTERN_C_BEG
		140
		141	int mycfun1 (int x);
		142	int mycfun2 (int x);
		143
		144	ECB_EXTERN_C_END
		145
		146	=item ECB_STDFP
		147
		148	If this evaluates to a true value (suitable for testing in by the
		149	preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
		150	and double/binary64 representations internally I<and> the endianness of
		151	both types match the endianness of C<uint32_t> and C<uint64_t>.
		152
		153	This means you can just copy the bits of a C<float> (or C<double>) to an
		154	C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
		155	without having to think about format or endianness.
		156
		157	This is true for basically all modern platforms, although F<ecb.h> might
		158	not be able to deduce this correctly everywhere and might err on the safe
		159	side.
		160
		161	=item ECB_AMD64, ECB_AMD64_X32
		162
		163	These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
		164	ABI, respectively, and undefined elsewhere.
		165
		166	The designers of the new X32 ABI for some inexplicable reason decided to
		167	make it look exactly like amd64, even though it's completely incompatible
		168	to that ABI, breaking about every piece of software that assumed that
		169	C<__x86_64> stands for, well, the x86-64 ABI, making these macros
		170	necessary.
		171
		172	=back
		173
		174	=head2 MACRO TRICKERY
		175
		176	=over 4
		177
		178	=item ECB_CONCAT (a, b)
		179
		180	Expands any macros in C<a> and C<b>, then concatenates the result to form
		181	a single token. This is mainly useful to form identifiers from components,
		182	e.g.:
		183
		184	#define S1 str
		185	#define S2 cpy
		186
		187	ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
		188
		189	=item ECB_STRINGIFY (arg)
		190
		191	Expands any macros in C<arg> and returns the stringified version of
		192	it. This is mainly useful to get the contents of a macro in string form,
		193	e.g.:
		194
		195	#define SQL_LIMIT 100
		196	sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
		197
		198	=item ECB_STRINGIFY_EXPR (expr)
		199
		200	Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
		201	is a valid expression. This is useful to catch typos or cases where the
		202	macro isn't available:
		203
		204	#include <errno.h>
		205
		206	ECB_STRINGIFY (EDOM); // "33" (on my system at least)
		207	ECB_STRINGIFY_EXPR (EDOM); // "33"
		208
		209	// now imagine we had a typo:
		210
		211	ECB_STRINGIFY (EDAM); // "EDAM"
		212	ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
		213
		214	=back
		215
55	=head2 GCC ATTRIBUTES	216	=head2 ATTRIBUTES
56		217
57	blabla where to put, what others	218	A major part of libecb deals with additional attributes that can be
		219	assigned to functions, variables and sometimes even types - much like
		220	C<const> or C<volatile> in C. They are implemented using either GCC
		221	attributes or other compiler/language specific features. Attributes
		222	declarations must be put before the whole declaration:
58		223
59	=over 4	224	ecb_const int mysqrt (int a);
		225	ecb_unused int i;
60		226
61	=item ecb_attribute ((attrs...))	227	=over 4
62
63	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
64	nothing on other compilers, so the effect is that only GCC sees these.
65
66	Example: use the C<deprecated> attribute on a function.
67
68	ecb_attribute((__deprecated__)) void
69	do_not_use_me_anymore (void);
70		228
71	=item ecb_unused	229	=item ecb_unused
72		230
73	Marks a function or a variable as "unused", which simply suppresses a	231	Marks a function or a variable as "unused", which simply suppresses a
74	warning by GCC when it detects it as unused. This is useful when you e.g.	232	warning by GCC when it detects it as unused. This is useful when you e.g.
75	declare a variable but do not always use it:	233	declare a variable but do not always use it:
76		234
77	{	235	{
78	int var ecb_unused;	236	ecb_unused int var;
79		237
80	#ifdef SOMECONDITION	238	#ifdef SOMECONDITION
81	var = ...;	239	var = ...;
82	return var;	240	return var;
83	#else	241	#else
84	return 0;	242	return 0;
85	#endif	243	#endif
86	}	244	}
87		245
		246	=item ecb_deprecated
		247
		248	Similar to C<ecb_unused>, but marks a function, variable or type as
		249	deprecated. This makes some compilers warn when the type is used.
		250
		251	=item ecb_deprecated_message (message)
		252
		253	Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
		254	used instead of a generic depreciation message when the object is being
		255	used.
		256
		257	=item ecb_inline
		258
		259	Expands either to (a compiler-specific equivalent of) C<static inline> or
		260	to just C<static>, if inline isn't supported. It should be used to declare
		261	functions that should be inlined, for code size or speed reasons.
		262
		263	Example: inline this function, it surely will reduce codesize.
		264
		265	ecb_inline int
		266	negmul (int a, int b)
		267	{
		268	return - (a * b);
		269	}
		270
88	=item ecb_noinline	271	=item ecb_noinline
89		272
90	Prevent a function from being inlined - it might be optimised away, but	273	Prevents a function from being inlined - it might be optimised away, but
91	not inlined into other functions. This is useful if you know your function	274	not inlined into other functions. This is useful if you know your function
92	is rarely called and large enough for inlining not to be helpful.	275	is rarely called and large enough for inlining not to be helpful.
93		276
94	=item ecb_noreturn	277	=item ecb_noreturn
95		278
		279	Marks a function as "not returning, ever". Some typical functions that
		280	don't return are C<exit> or C<abort> (which really works hard to not
		281	return), and now you can make your own:
		282
		283	ecb_noreturn void
		284	my_abort (const char *errline)
		285	{
		286	puts (errline);
		287	abort ();
		288	}
		289
		290	In this case, the compiler would probably be smart enough to deduce it on
		291	its own, so this is mainly useful for declarations.
		292
		293	=item ecb_restrict
		294
		295	Expands to the C<restrict> keyword or equivalent on compilers that support
		296	them, and to nothing on others. Must be specified on a pointer type or
		297	an array index to indicate that the memory doesn't alias with any other
		298	restricted pointer in the same scope.
		299
		300	Example: multiply a vector, and allow the compiler to parallelise the
		301	loop, because it knows it doesn't overwrite input values.
		302
		303	void
		304	multiply (ecb_restrict float *src,
		305	ecb_restrict float *dst,
		306	int len, float factor)
		307	{
		308	int i;
		309
		310	for (i = 0; i < len; ++i)
		311	dst [i] = src [i] * factor;
		312	}
		313
96	=item ecb_const	314	=item ecb_const
97		315
		316	Declares that the function only depends on the values of its arguments,
		317	much like a mathematical function. It specifically does not read or write
		318	any memory any arguments might point to, global variables, or call any
		319	non-const functions. It also must not have any side effects.
		320
		321	Such a function can be optimised much more aggressively by the compiler -
		322	for example, multiple calls with the same arguments can be optimised into
		323	a single call, which wouldn't be possible if the compiler would have to
		324	expect any side effects.
		325
		326	It is best suited for functions in the sense of mathematical functions,
		327	such as a function returning the square root of its input argument.
		328
		329	Not suited would be a function that calculates the hash of some memory
		330	area you pass in, prints some messages or looks at a global variable to
		331	decide on rounding.
		332
		333	See C<ecb_pure> for a slightly less restrictive class of functions.
		334
98	=item ecb_pure	335	=item ecb_pure
99		336
		337	Similar to C<ecb_const>, declares a function that has no side
		338	effects. Unlike C<ecb_const>, the function is allowed to examine global
		339	variables and any other memory areas (such as the ones passed to it via
		340	pointers).
		341
		342	While these functions cannot be optimised as aggressively as C<ecb_const>
		343	functions, they can still be optimised away in many occasions, and the
		344	compiler has more freedom in moving calls to them around.
		345
		346	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		347	function that calculates the MD5 sum of some input and updates some MD5
		348	state passed as argument would I<NOT> be pure, however, as it would modify
		349	some memory area that is not the return value.
		350
100	=item ecb_hot	351	=item ecb_hot
101		352
		353	This declares a function as "hot" with regards to the cache - the function
		354	is used so often, that it is very beneficial to keep it in the cache if
		355	possible.
		356
		357	The compiler reacts by trying to place hot functions near to each other in
		358	memory.
		359
		360	Whether a function is hot or not often depends on the whole program,
		361	and less on the function itself. C<ecb_cold> is likely more useful in
		362	practise.
		363
102	=item ecb_cold	364	=item ecb_cold
103		365
		366	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		367	the cache, or in other words, this function is not called often, or not at
		368	speed-critical times, and keeping it in the cache might be a waste of said
		369	cache.
		370
		371	In addition to placing cold functions together (or at least away from hot
		372	functions), this knowledge can be used in other ways, for example, the
		373	function will be optimised for size, as opposed to speed, and codepaths
		374	leading to calls to those functions can automatically be marked as if
		375	C<ecb_expect_false> had been used to reach them.
		376
		377	Good examples for such functions would be error reporting functions, or
		378	functions only called in exceptional or rare cases.
		379
104	=item ecb_artificial	380	=item ecb_artificial
105		381
		382	Declares the function as "artificial", in this case meaning that this
		383	function is not really meant to be a function, but more like an accessor
		384	- many methods in C++ classes are mere accessor functions, and having a
		385	crash reported in such a method, or single-stepping through them, is not
		386	usually so helpful, especially when it's inlined to just a few instructions.
		387
		388	Marking them as artificial will instruct the debugger about just this,
		389	leading to happier debugging and thus happier lives.
		390
		391	Example: in some kind of smart-pointer class, mark the pointer accessor as
		392	artificial, so that the whole class acts more like a pointer and less like
		393	some C++ abstraction monster.
		394
		395	template<typename T>
		396	struct my_smart_ptr
		397	{
		398	T *value;
		399
		400	ecb_artificial
		401	operator T *()
		402	{
		403	return value;
		404	}
		405	};
		406
106	=back	407	=back
107		408
108	=head2 OPTIMISATION HINTS	409	=head2 OPTIMISATION HINTS
109		410
110	=over 4	411	=over 4
111		412
		413	=item ECB_OPTIMIZE_SIZE
		414
		415	Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
		416	can also be defined before including F<ecb.h>, in which case it will be
		417	unchanged.
		418
112	=item bool ecb_is_constant(expr)	419	=item bool ecb_is_constant (expr)
113		420
114	Returns true iff the expression can be deduced to be a compile-time	421	Returns true iff the expression can be deduced to be a compile-time
115	constant, and false otherwise.	422	constant, and false otherwise.
116		423
117	For example, when you have a C<rndm16> function that returns a 16 bit	424	For example, when you have a C<rndm16> function that returns a 16 bit
…		…
135	return is_constant (n) && !(n & (n - 1))	442	return is_constant (n) && !(n & (n - 1))
136	? rndm16 () & (num - 1)	443	? rndm16 () & (num - 1)
137	: (n * (uint32_t)rndm16 ()) >> 16;	444	: (n * (uint32_t)rndm16 ()) >> 16;
138	}	445	}
139		446
140	=item bool ecb_expect (expr, value)	447	=item ecb_expect (expr, value)
141		448
142	Evaluates C<expr> and returns it. In addition, it tells the compiler that	449	Evaluates C<expr> and returns it. In addition, it tells the compiler that
143	the C<expr> evaluates to C<value> a lot, which can be used for static	450	the C<expr> evaluates to C<value> a lot, which can be used for static
144	branch optimisations.	451	branch optimisations.
145		452
146	Usually, you want to use the more intuitive C<ecb_likely> and	453	Usually, you want to use the more intuitive C<ecb_expect_true> and
147	C<ecb_unlikely> functions instead.	454	C<ecb_expect_false> functions instead.
148		455
		456	=item bool ecb_expect_true (cond)
		457
149	=item bool ecb_likely (cond)	458	=item bool ecb_expect_false (cond)
150
151	=item bool ecb_unlikely (cond)
152		459
153	These two functions expect a expression that is true or false and return	460	These two functions expect a expression that is true or false and return
154	C<1> or C<0>, respectively, so when used in the condition of an C<if> or	461	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
155	other conditional statement, it will not change the program:	462	other conditional statement, it will not change the program:
156		463
157	/* these two do the same thing */	464	/* these two do the same thing */
158	if (some_condition) ...;	465	if (some_condition) ...;
159	if (ecb_likely (some_condition)) ...;	466	if (ecb_expect_true (some_condition)) ...;
160		467
161	However, by using C<ecb_likely>, you tell the compiler that the condition	468	However, by using C<ecb_expect_true>, you tell the compiler that the
162	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be	469	condition is likely to be true (and for C<ecb_expect_false>, that it is
163	true).	470	unlikely to be true).
164		471
165	For example, when you check for a null pointer and expect this to be a	472	For example, when you check for a null pointer and expect this to be a
166	rare, exceptional, case, then use C<ecb_unlikely>:	473	rare, exceptional, case, then use C<ecb_expect_false>:
167		474
168	void my_free (void *ptr)	475	void my_free (void *ptr)
169	{	476	{
170	if (ecb_unlikely (ptr == 0))	477	if (ecb_expect_false (ptr == 0))
171	return;	478	return;
172	}	479	}
173		480
174	Consequent use of these functions to mark away exceptional cases or to	481	Consequent use of these functions to mark away exceptional cases or to
175	tell the compiler what the hot path through a function is can increase	482	tell the compiler what the hot path through a function is can increase
176	performance considerably.	483	performance considerably.
		484
		485	You might know these functions under the name C<likely> and C<unlikely>
		486	- while these are common aliases, we find that the expect name is easier
		487	to understand when quickly skimming code. If you wish, you can use
		488	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		489	C<ecb_expect_false> - these are simply aliases.
177		490
178	A very good example is in a function that reserves more space for some	491	A very good example is in a function that reserves more space for some
179	memory block (for example, inside an implementation of a string stream) -	492	memory block (for example, inside an implementation of a string stream) -
180	each time something is added, you have to check for a buffer overrun, but	493	each time something is added, you have to check for a buffer overrun, but
181	you expect that most checks will turn out to be false:	494	you expect that most checks will turn out to be false:
182		495
183	/* make sure we have "size" extra room in our buffer */	496	/* make sure we have "size" extra room in our buffer */
184	ecb_inline void	497	ecb_inline void
185	reserve (int size)	498	reserve (int size)
186	{	499	{
187	if (ecb_unlikely (current + size > end))	500	if (ecb_expect_false (current + size > end))
188	real_reserve_method (size); /* presumably noinline */	501	real_reserve_method (size); /* presumably noinline */
189	}	502	}
190		503
191	=item bool ecb_assume (cond)	504	=item ecb_assume (cond)
192		505
193	Try to tell the compiler that some condition is true, even if it's not	506	Tries to tell the compiler that some condition is true, even if it's not
194	obvious.	507	obvious. This is not a function, but a statement: it cannot be used in
		508	another expression.
195		509
196	This can be used to teach the compiler about invariants or other	510	This can be used to teach the compiler about invariants or other
197	conditions that might improve code generation, but which are impossible to	511	conditions that might improve code generation, but which are impossible to
198	deduce form the code itself.	512	deduce form the code itself.
199		513
200	For example, the example reservation function from the C<ecb_unlikely>	514	For example, the example reservation function from the C<ecb_expect_false>
201	description could be written thus (only C<ecb_assume> was added):	515	description could be written thus (only C<ecb_assume> was added):
202		516
203	ecb_inline void	517	ecb_inline void
204	reserve (int size)	518	reserve (int size)
205	{	519	{
206	if (ecb_unlikely (current + size > end))	520	if (ecb_expect_false (current + size > end))
207	real_reserve_method (size); /* presumably noinline */	521	real_reserve_method (size); /* presumably noinline */
208		522
209	ecb_assume (current + size <= end);	523	ecb_assume (current + size <= end);
210	}	524	}
211		525
…		…
216		530
217	Then the compiler I<might> be able to optimise out the second call	531	Then the compiler I<might> be able to optimise out the second call
218	completely, as it knows that C<< current + 1 > end >> is false and the	532	completely, as it knows that C<< current + 1 > end >> is false and the
219	call will never be executed.	533	call will never be executed.
220		534
221	=item bool ecb_unreachable ()	535	=item ecb_unreachable ()
222		536
223	This function does nothing itself, except tell the compiler that it will	537	This function does nothing itself, except tell the compiler that it will
224	never be executed. Apart from suppressing a warning in some cases, this	538	never be executed. Apart from suppressing a warning in some cases, this
225	function can be used to implement C<ecb_assume> or similar functions.	539	function can be used to implement C<ecb_assume> or similar functionality.
226		540
227	=item bool ecb_prefetch (addr, rw, locality)	541	=item ecb_prefetch (addr, rw, locality)
228		542
229	Tells the compiler to try to prefetch memory at the given C<addr>ess	543	Tells the compiler to try to prefetch memory at the given C<addr>ess
230	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of	544	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
231	C<0> means that there will only be one access later, C<3> means that	545	C<0> means that there will only be one access later, C<3> means that
232	the data will likely be accessed very often, and values in between mean	546	the data will likely be accessed very often, and values in between mean
233	something... in between. The memory pointed to by the address does not	547	something... in between. The memory pointed to by the address does not
234	need to be accessible (it could be a null pointer for example), but C<rw>	548	need to be accessible (it could be a null pointer for example), but C<rw>
235	and C<locality> must be compile-time constants.	549	and C<locality> must be compile-time constants.
236		550
		551	This is a statement, not a function: you cannot use it as part of an
		552	expression.
		553
237	An obvious way to use this is to prefetch some data far away, in a big	554	An obvious way to use this is to prefetch some data far away, in a big
238	array you loop over. This prefetches memory some 128 array elements later,	555	array you loop over. This prefetches memory some 128 array elements later,
239	in the hope that it will be ready when the CPU arrives at that location.	556	in the hope that it will be ready when the CPU arrives at that location.
240		557
241	int sum = 0;	558	int sum = 0;
…		…
260	After processing the node, (part of) the next node might already be in	577	After processing the node, (part of) the next node might already be in
261	cache.	578	cache.
262		579
263	=back	580	=back
264		581
265	=head2 BIT FIDDLING / BITSTUFFS	582	=head2 BIT FIDDLING / BIT WIZARDRY
266		583
267	=over 4	584	=over 4
268		585
269	=item bool ecb_big_endian ()	586	=item bool ecb_big_endian ()
270		587
…		…
272		589
273	These two functions return true if the byte order is big endian	590	These two functions return true if the byte order is big endian
274	(most-significant byte first) or little endian (least-significant byte	591	(most-significant byte first) or little endian (least-significant byte
275	first) respectively.	592	first) respectively.
276		593
		594	On systems that are neither, their return values are unspecified.
		595
277	=item int ecb_ctz32 (uint32_t x)	596	=item int ecb_ctz32 (uint32_t x)
278		597
		598	=item int ecb_ctz64 (uint64_t x)
		599
279	Returns the index of the least significant bit set in C<x> (or	600	Returns the index of the least significant bit set in C<x> (or
280	equivalently the number of bits set to 0 before the least significant	601	equivalently the number of bits set to 0 before the least significant bit
281	bit set), starting from 0. If C<x> is 0 the result is undefined. A	602	set), starting from 0. If C<x> is 0 the result is undefined.
282	common use case is to compute the integer binary logarithm, i.e.,	603
283	floor(log2(n)). For example:	604	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		605
		606	For example:
284		607
285	ecb_ctz32 (3) = 0	608	ecb_ctz32 (3) = 0
286	ecb_ctz32 (6) = 1	609	ecb_ctz32 (6) = 1
287		610
		611	=item bool ecb_is_pot32 (uint32_t x)
		612
		613	=item bool ecb_is_pot64 (uint32_t x)
		614
		615	Returns true iff C<x> is a power of two or C<x == 0>.
		616
		617	For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
		618
		619	=item int ecb_ld32 (uint32_t x)
		620
		621	=item int ecb_ld64 (uint64_t x)
		622
		623	Returns the index of the most significant bit set in C<x>, or the number
		624	of digits the number requires in binary (so that C<< 2**ld <= x <
		625	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		626	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		627	example to see how many bits a certain number requires to be encoded.
		628
		629	This function is similar to the "count leading zero bits" function, except
		630	that that one returns how many zero bits are "in front" of the number (in
		631	the given data type), while C<ecb_ld> returns how many bits the number
		632	itself requires.
		633
		634	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		635
288	=item int ecb_popcount32 (uint32_t x)	636	=item int ecb_popcount32 (uint32_t x)
289		637
		638	=item int ecb_popcount64 (uint64_t x)
		639
290	Returns the number of bits set to 1 in C<x>. For example:	640	Returns the number of bits set to 1 in C<x>.
		641
		642	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		643
		644	For example:
291		645
292	ecb_popcount32 (7) = 3	646	ecb_popcount32 (7) = 3
293	ecb_popcount32 (255) = 8	647	ecb_popcount32 (255) = 8
294		648
		649	=item uint8_t ecb_bitrev8 (uint8_t x)
		650
		651	=item uint16_t ecb_bitrev16 (uint16_t x)
		652
		653	=item uint32_t ecb_bitrev32 (uint32_t x)
		654
		655	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		656	and so on.
		657
		658	Example:
		659
		660	ecb_bitrev8 (0xa7) = 0xea
		661	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		662
295	=item uint32_t ecb_bswap16 (uint32_t x)	663	=item uint32_t ecb_bswap16 (uint32_t x)
296		664
297	=item uint32_t ecb_bswap32 (uint32_t x)	665	=item uint32_t ecb_bswap32 (uint32_t x)
298		666
		667	=item uint64_t ecb_bswap64 (uint64_t x)
		668
299	These two functions return the value of the 16-bit (32-bit) variable	669	These functions return the value of the 16-bit (32-bit, 64-bit) value
300	C<x> after reversing the order of bytes.	670	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		671	C<ecb_bswap32>).
		672
		673	=item T ecb_bswap (T x) [C++]
		674
		675	For C++, an additional generic bswap function is provided. It supports
		676	C<uint8_t>, C<uint16_t>, C<uint32_t> and C<uint64_t>.
		677
		678	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		679
		680	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		681
		682	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		683
		684	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		685
		686	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		687
		688	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
301		689
302	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	690	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
303		691
304	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	692	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
305		693
306	These two functions return the value of C<x> after shifting all the bits	694	These two families of functions return the value of C<x> after rotating
307	by C<count> positions to the right or left respectively.	695	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		696	(C<ecb_rotl>).
		697
		698	Current GCC versions understand these functions and usually compile them
		699	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		700	x86).
		701
		702	=back
		703
		704	=head2 HOST ENDIANNESS CONVERSION
		705
		706	=over 4
		707
		708	=item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v)
		709
		710	=item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v)
		711
		712	=item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v)
		713
		714	=item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v)
		715
		716	=item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v)
		717
		718	=item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v)
		719
		720	Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order.
		721
		722	The naming convention is C<ecb_>(C<be>\|C<le>)C<_u>C<16\|32\|64>C<_to_host>,
		723	where be and le stand for big endian and little endian, respectively.
		724
		725	=item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v)
		726
		727	=item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v)
		728
		729	=item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v)
		730
		731	=item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v)
		732
		733	=item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v)
		734
		735	=item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v)
		736
		737	Like above, but converts I<from> host byte order to the specified
		738	endianness.
		739
		740	=back
		741
		742	In C++ the following additional functions are supported:
		743
		744	=over 4
		745
		746	=item T ecb_be_to_host (T v)
		747
		748	=item T ecb_le_to_host (T v)
		749
		750	=item T ecb_host_to_be (T v)
		751
		752	=item T ecb_host_to_le (T v)
		753
		754	These work like their C counterparts, above, but use templates for the
		755	type, which make them useful in generic code.
		756
		757	C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>
		758	(so unlike their C counterparts, there is a version for C<uint8_t>, which
		759	again can be useful in generic code).
		760
		761	=head2 UNALIGNED LOAD/STORE
		762
		763	These function load or store unaligned multi-byte values.
		764
		765	=over 4
		766
		767	=item uint_fast16_t ecb_peek_u16_u (const void *ptr)
		768
		769	=item uint_fast32_t ecb_peek_u32_u (const void *ptr)
		770
		771	=item uint_fast64_t ecb_peek_u64_u (const void *ptr)
		772
		773	These functions load an unaligned, unsigned 16, 32 or 64 bit value from
		774	memory.
		775
		776	=item uint_fast16_t ecb_peek_be_u16_u (const void *ptr)
		777
		778	=item uint_fast32_t ecb_peek_be_u32_u (const void *ptr)
		779
		780	=item uint_fast64_t ecb_peek_be_u64_u (const void *ptr)
		781
		782	=item uint_fast16_t ecb_peek_le_u16_u (const void *ptr)
		783
		784	=item uint_fast32_t ecb_peek_le_u32_u (const void *ptr)
		785
		786	=item uint_fast64_t ecb_peek_le_u64_u (const void *ptr)
		787
		788	Like above, but additionally convert from big endian (C<be>) or little
		789	endian (C<le>) byte order to host byte order while doing so.
		790
		791	=item ecb_poke_u16_u (void *ptr, uint16_t v)
		792
		793	=item ecb_poke_u32_u (void *ptr, uint32_t v)
		794
		795	=item ecb_poke_u64_u (void *ptr, uint64_t v)
		796
		797	These functions store an unaligned, unsigned 16, 32 or 64 bit value to
		798	memory.
		799
		800	=item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v)
		801
		802	=item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v)
		803
		804	=item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v)
		805
		806	=item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v)
		807
		808	=item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v)
		809
		810	=item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v)
		811
		812	Like above, but additionally convert from host byte order to big endian
		813	(C<be>) or little endian (C<le>) byte order while doing so.
		814
		815	=back
		816
		817	In C++ the following additional functions are supported:
		818
		819	=over 4
		820
		821	=item T ecb_peek (const void *ptr)
		822
		823	=item T ecb_peek_be (const void *ptr)
		824
		825	=item T ecb_peek_le (const void *ptr)
		826
		827	=item T ecb_peek_u (const void *ptr)
		828
		829	=item T ecb_peek_be_u (const void *ptr)
		830
		831	=item T ecb_peek_le_u (const void *ptr)
		832
		833	Similarly to their C counterparts, these functions load an unsigned 8, 16,
		834	32 or 64 bit value from memory, with optional conversion from big/little
		835	endian.
		836
		837	Since the type cannot be deduced, it has top be specified explicitly, e.g.
		838
		839	uint_fast16_t v = ecb_peek<uint16_t> (ptr);
		840
		841	C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
		842
		843	Unlike their C counterparts, these functions support 8 bit quantities
		844	(C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
		845	all of which hopefully makes them more useful in generic code.
		846
		847	=item ecb_poke (void *ptr, T v)
		848
		849	=item ecb_poke_be (void *ptr, T v)
		850
		851	=item ecb_poke_le (void *ptr, T v)
		852
		853	=item ecb_poke_u (void *ptr, T v)
		854
		855	=item ecb_poke_be_u (void *ptr, T v)
		856
		857	=item ecb_poke_le_u (void *ptr, T v)
		858
		859	Again, similarly to their C counterparts, these functions store an
		860	unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to
		861	big/little endian.
		862
		863	C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
		864
		865	Unlike their C counterparts, these functions support 8 bit quantities
		866	(C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
		867	all of which hopefully makes them more useful in generic code.
		868
		869	=back
		870
		871	=head2 FLOATING POINT FIDDLING
		872
		873	=over 4
		874
		875	=item ECB_INFINITY [-UECB_NO_LIBM]
		876
		877	Evaluates to positive infinity if supported by the platform, otherwise to
		878	a truly huge number.
		879
		880	=item ECB_NAN [-UECB_NO_LIBM]
		881
		882	Evaluates to a quiet NAN if supported by the platform, otherwise to
		883	C<ECB_INFINITY>.
		884
		885	=item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
		886
		887	Same as C<ldexpf>, but always available.
		888
		889	=item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
		890
		891	=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
		892
		893	=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
		894
		895	These functions each take an argument in the native C<float> or C<double>
		896	type and return the IEEE 754 bit representation of it (binary16/half,
		897	binary32/single or binary64/double precision).
		898
		899	The bit representation is just as IEEE 754 defines it, i.e. the sign bit
		900	will be the most significant bit, followed by exponent and mantissa.
		901
		902	This function should work even when the native floating point format isn't
		903	IEEE compliant, of course at a speed and code size penalty, and of course
		904	also within reasonable limits (it tries to convert NaNs, infinities and
		905	denormals, but will likely convert negative zero to positive zero).
		906
		907	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		908	be able to optimise away this function completely.
		909
		910	These functions can be helpful when serialising floats to the network - you
		911	can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
		912
		913	Another use for these functions is to manipulate floating point values
		914	directly.
		915
		916	Silly example: toggle the sign bit of a float.
		917
		918	/* On gcc-4.7 on amd64, */
		919	/* this results in a single add instruction to toggle the bit, and 4 extra */
		920	/* instructions to move the float value to an integer register and back. */
		921
		922	x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
		923
		924	=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
		925
		926	=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
		927
		928	=item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
		929
		930	The reverse operation of the previous function - takes the bit
		931	representation of an IEEE binary16, binary32 or binary64 number (half,
		932	single or double precision) and converts it to the native C<float> or
		933	C<double> format.
		934
		935	This function should work even when the native floating point format isn't
		936	IEEE compliant, of course at a speed and code size penalty, and of course
		937	also within reasonable limits (it tries to convert normals and denormals,
		938	and might be lucky for infinities, and with extraordinary luck, also for
		939	negative zero).
		940
		941	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		942	be able to optimise away this function completely.
		943
		944	=item uint16_t ecb_binary32_to_binary16 (uint32_t x)
		945
		946	=item uint32_t ecb_binary16_to_binary32 (uint16_t x)
		947
		948	Convert a IEEE binary32/single precision to binary16/half format, and vice
		949	versa, handling all details (round-to-nearest-even, subnormals, infinity
		950	and NaNs) correctly.
		951
		952	These are functions are available under C<-DECB_NO_LIBM>, since
		953	they do not rely on the platform floating point format. The
		954	C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
		955	usually what you want.
308		956
309	=back	957	=back
310		958
311	=head2 ARITHMETIC	959	=head2 ARITHMETIC
312		960
313	=over 4	961	=over 4
314		962
315	=item x = ecb_mod (m, n)	963	=item x = ecb_mod (m, n)
316		964
317	Returns the positive remainder of the modulo operation between C<m> and	965	Returns C<m> modulo C<n>, which is the same as the positive remainder
		966	of the division operation between C<m> and C<n>, using floored
318	C<n>. Unlike the C moduloe operator C<%>, this function ensures that the	967	division. Unlike the C remainder operator C<%>, this function ensures that
319	return value is always positive).	968	the return value is always positive and that the two numbers I<m> and
		969	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		970	C<ecb_mod> implements the mathematical modulo operation, which is missing
		971	in the language.
320		972
321	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	973	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
322	negatable, that is, both C<m> and C<-m> must be representable in its	974	negatable, that is, both C<m> and C<-m> must be representable in its
323	type.	975	type (this typically excludes the minimum signed integer value, the same
		976	limitation as for C</> and C<%> in C).
		977
		978	Current GCC versions compile this into an efficient branchless sequence on
		979	almost all CPUs.
		980
		981	For example, when you want to rotate forward through the members of an
		982	array for increasing C<m> (which might be negative), then you should use
		983	C<ecb_mod>, as the C<%> operator might give either negative results, or
		984	change direction for negative values:
		985
		986	for (m = -100; m <= 100; ++m)
		987	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		988
		989	=item x = ecb_div_rd (val, div)
		990
		991	=item x = ecb_div_ru (val, div)
		992
		993	Returns C<val> divided by C<div> rounded down or up, respectively.
		994	C<val> and C<div> must have integer types and C<div> must be strictly
		995	positive. Note that these functions are implemented with macros in C
		996	and with function templates in C++.
324		997
325	=back	998	=back
326		999
327	=head2 UTILITY	1000	=head2 UTILITY
328		1001
329	=over 4	1002	=over 4
330		1003
331	=item element_count = ecb_array_length (name) [MACRO]	1004	=item element_count = ecb_array_length (name)
332		1005
333	Returns the number of elements in the array C<name>. For example:	1006	Returns the number of elements in the array C<name>. For example:
334		1007
335	int primes[] = { 2, 3, 5, 7, 11 };	1008	int primes[] = { 2, 3, 5, 7, 11 };
336	int sum = 0;	1009	int sum = 0;
…		…
338	for (i = 0; i < ecb_array_length (primes); i++)	1011	for (i = 0; i < ecb_array_length (primes); i++)
339	sum += primes [i];	1012	sum += primes [i];
340		1013
341	=back	1014	=back
342		1015
		1016	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
343		1017
		1018	These symbols need to be defined before including F<ecb.h> the first time.
		1019
		1020	=over 4
		1021
		1022	=item ECB_NO_THREADS
		1023
		1024	If F<ecb.h> is never used from multiple threads, then this symbol can
		1025	be defined, in which case memory fences (and similar constructs) are
		1026	completely removed, leading to more efficient code and fewer dependencies.
		1027
		1028	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		1029
		1030	=item ECB_NO_SMP
		1031
		1032	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		1033	multiple threads, but never concurrently (e.g. if the system the program
		1034	runs on has only a single CPU with a single core, no hyperthreading and so
		1035	on), then this symbol can be defined, leading to more efficient code and
		1036	fewer dependencies.
		1037
		1038	=item ECB_NO_LIBM
		1039
		1040	When defined to C<1>, do not export any functions that might introduce
		1041	dependencies on the math library (usually called F<-lm>) - these are
		1042	marked with [-UECB_NO_LIBM].
		1043
		1044	=back
		1045
		1046	=head1 UNDOCUMENTED FUNCTIONALITY
		1047
		1048	F<ecb.h> is full of undocumented functionality as well, some of which is
		1049	intended to be internal-use only, some of which we forgot to document, and
		1050	some of which we hide because we are not sure we will keep the interface
		1051	stable.
		1052
		1053	While you are welcome to rummage around and use whatever you find useful
		1054	(we can't stop you), keep in mind that we will change undocumented
		1055	functionality in incompatible ways without thinking twice, while we are
		1056	considerably more conservative with documented things.
		1057
		1058	=head1 AUTHORS
		1059
		1060	C<libecb> is designed and maintained by:
		1061
		1062	Emanuele Giaquinta <e.giaquinta@glauco.it>
		1063	Marc Alexander Lehmann <schmorp@schmorp.de>
		1064
		1065

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Comparing libecb/ecb.pod (file contents): Revision 1.15 by root, Thu May 26 23:26:29 2011 UTC vs. Revision 1.76 by root, Mon Jan 20 13:13:56 2020 UTC

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Comparing libecb/ecb.pod (file contents):
Revision 1.15 by root, Thu May 26 23:26:29 2011 UTC vs.
Revision 1.76 by root, Mon Jan 20 13:13:56 2020 UTC