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…		…
		1	=head1 LIBECB - e-C-Builtins
		2
1	=head1 LIBECB	3	=head2 ABOUT LIBECB
2		4
3	You suck, we don't(tm)	5	Libecb is currently a simple header file that doesn't require any
		6	configuration to use or include in your project.
		7
		8	It's part of the e-suite of libraries, other members of which include
		9	libev and libeio.
		10
		11	Its homepage can be found here:
		12
		13	http://software.schmorp.de/pkg/libecb
		14
		15	It mainly provides a number of wrappers around GCC built-ins, together
		16	with replacement functions for other compilers. In addition to this,
		17	it provides a number of other lowlevel C utilities, such as endianness
		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.
		23
		24	More might come.
4		25
5	=head2 ABOUT THE HEADER	26	=head2 ABOUT THE HEADER
6		27
7	- how to include it	28	At the moment, all you have to do is copy F<ecb.h> somewhere where your
8	- it includes inttypes.h	29	compiler can find it and include it:
9	- no .a
10	- whats a bool
11	- function mean macro or function
12	- macro means untyped
13		30
14	=head2 GCC ATTRIBUTES	31	#include <ecb.h>
15		32
16	blabla where to put, what others	33	The header should work fine for both C and C++ compilation, and gives you
		34	all of F<inttypes.h> in addition to the ECB symbols.
		35
		36	There are currently no object files to link to - future versions might
		37	come with an (optional) object code library to link against, to reduce
		38	code size or gain access to additional features.
		39
		40	It also currently includes everything from F<inttypes.h>.
		41
		42	=head2 ABOUT THIS MANUAL / CONVENTIONS
		43
		44	This manual mainly describes each (public) function available after
		45	including the F<ecb.h> header. The header might define other symbols than
		46	these, but these are not part of the public API, and not supported in any
		47	way.
		48
		49	When the manual mentions a "function" then this could be defined either as
		50	as inline function, a macro, or an external symbol.
		51
		52	When functions use a concrete standard type, such as C<int> or
		53	C<uint32_t>, then the corresponding function works only with that type. If
		54	only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
		55	the corresponding function relies on C to implement the correct types, and
		56	is usually implemented as a macro. Specifically, a "bool" in this manual
		57	refers to any kind of boolean value, not a specific type.
		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_t int16_t uint16_t
		64	int32_t uint32_t int64_t uint64_t
		65	intptr_t uintptr_t
		66
		67	The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
		68	platform (currently C<4> or C<8>) and can be used in preprocessor
		69	expressions.
		70
		71	For C<ptrdiff_t> and C<size_t> use C<stddef.h>.
		72
		73	=head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
		74
		75	All the following symbols expand to an expression that can be tested in
		76	preprocessor instructions as well as treated as a boolean (use C<!!> to
		77	ensure it's either C<0> or C<1> if you need that).
17		78
18	=over 4	79	=over 4
19		80
20	=item ecb_attribute ((attrs...))	81	=item ECB_C
21		82
22	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and	83	True if the implementation defines the C<__STDC__> macro to a true value,
23	to nothing on other compilers, so the effect is that only GCC sees these.	84	while not claiming to be C++.
		85
		86	=item ECB_C99
		87
		88	True if the implementation claims to be compliant to C99 (ISO/IEC
		89	9899:1999) or any later version, while not claiming to be C++.
		90
		91	Note that later versions (ECB_C11) remove core features again (for
		92	example, variable length arrays).
		93
		94	=item ECB_C11
		95
		96	True if the implementation claims to be compliant to C11 (ISO/IEC
		97	9899:2011) or any later version, while not claiming to be C++.
		98
		99	=item ECB_CPP
		100
		101	True if the implementation defines the C<__cplusplus__> macro to a true
		102	value, which is typically true for C++ compilers.
		103
		104	=item ECB_CPP11
		105
		106	True if the implementation claims to be compliant to ISO/IEC 14882:2011
		107	(C++11) or any later version.
		108
		109	=item ECB_GCC_VERSION (major, minor)
		110
		111	Expands to a true value (suitable for testing in by the preprocessor)
		112	if the compiler used is GNU C and the version is the given version, or
		113	higher.
		114
		115	This macro tries to return false on compilers that claim to be GCC
		116	compatible but aren't.
		117
		118	=item ECB_EXTERN_C
		119
		120	Expands to C<extern "C"> in C++, and a simple C<extern> in C.
		121
		122	This can be used to declare a single external C function:
		123
		124	ECB_EXTERN_C int printf (const char *format, ...);
		125
		126	=item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
		127
		128	These two macros can be used to wrap multiple C<extern "C"> definitions -
		129	they expand to nothing in C.
		130
		131	They are most useful in header files:
		132
		133	ECB_EXTERN_C_BEG
		134
		135	int mycfun1 (int x);
		136	int mycfun2 (int x);
		137
		138	ECB_EXTERN_C_END
		139
		140	=item ECB_STDFP
		141
		142	If this evaluates to a true value (suitable for testing in by the
		143	preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
		144	and double/binary64 representations internally I<and> the endianness of
		145	both types match the endianness of C<uint32_t> and C<uint64_t>.
		146
		147	This means you can just copy the bits of a C<float> (or C<double>) to an
		148	C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
		149	without having to think about format or endianness.
		150
		151	This is true for basically all modern platforms, although F<ecb.h> might
		152	not be able to deduce this correctly everywhere and might err on the safe
		153	side.
		154
		155	=item ECB_AMD64, ECB_AMD64_X32
		156
		157	These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
		158	ABI, respectively, and undefined elsewhere.
		159
		160	The designers of the new X32 ABI for some inexplicable reason decided to
		161	make it look exactly like amd64, even though it's completely incompatible
		162	to that ABI, breaking about every piece of software that assumed that
		163	C<__x86_64> stands for, well, the x86-64 ABI, making these macros
		164	necessary.
		165
		166	=back
		167
		168	=head2 MACRO TRICKERY
		169
		170	=over 4
		171
		172	=item ECB_CONCAT (a, b)
		173
		174	Expands any macros in C<a> and C<b>, then concatenates the result to form
		175	a single token. This is mainly useful to form identifiers from components,
		176	e.g.:
		177
		178	#define S1 str
		179	#define S2 cpy
		180
		181	ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
		182
		183	=item ECB_STRINGIFY (arg)
		184
		185	Expands any macros in C<arg> and returns the stringified version of
		186	it. This is mainly useful to get the contents of a macro in string form,
		187	e.g.:
		188
		189	#define SQL_LIMIT 100
		190	sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
		191
		192	=item ECB_STRINGIFY_EXPR (expr)
		193
		194	Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
		195	is a valid expression. This is useful to catch typos or cases where the
		196	macro isn't available:
		197
		198	#include <errno.h>
		199
		200	ECB_STRINGIFY (EDOM); // "33" (on my system at least)
		201	ECB_STRINGIFY_EXPR (EDOM); // "33"
		202
		203	// now imagine we had a typo:
		204
		205	ECB_STRINGIFY (EDAM); // "EDAM"
		206	ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
		207
		208	=back
		209
		210	=head2 ATTRIBUTES
		211
		212	A major part of libecb deals with additional attributes that can be
		213	assigned to functions, variables and sometimes even types - much like
		214	C<const> or C<volatile> in C. They are implemented using either GCC
		215	attributes or other compiler/language specific features. Attributes
		216	declarations must be put before the whole declaration:
		217
		218	ecb_const int mysqrt (int a);
		219	ecb_unused int i;
		220
		221	=over 4
24		222
25	=item ecb_unused	223	=item ecb_unused
26		224
27	Marks a function or a variable as "unused", which simply suppresses a	225	Marks a function or a variable as "unused", which simply suppresses a
28	warning by GCC when it detects it as unused. This is useful when you e.g.	226	warning by GCC when it detects it as unused. This is useful when you e.g.
29	declare a variable but do not always use it:	227	declare a variable but do not always use it:
30		228
		229	{
		230	ecb_unused int var;
		231
		232	#ifdef SOMECONDITION
		233	var = ...;
		234	return var;
		235	#else
		236	return 0;
		237	#endif
		238	}
		239
		240	=item ecb_deprecated
		241
		242	Similar to C<ecb_unused>, but marks a function, variable or type as
		243	deprecated. This makes some compilers warn when the type is used.
		244
		245	=item ecb_deprecated_message (message)
		246
		247	Same as C<ecb_deprecated>, but if possible, supply a diagnostic that is
		248	used instead of a generic depreciation message when the object is being
		249	used.
		250
		251	=item ecb_inline
		252
		253	Expands either to C<static inline> or to just C<static>, if inline
		254	isn't supported. It should be used to declare functions that should be
		255	inlined, for code size or speed reasons.
		256
		257	Example: inline this function, it surely will reduce codesize.
		258
		259	ecb_inline int
		260	negmul (int a, int b)
31	{	261	{
32	int var ecb_unused;	262	return - (a * b);
33
34	#ifdef SOMECONDITION
35	var = ...;
36	return var;
37	#else
38	return 0;
39	#endif
40	}	263	}
41		264
42	=item ecb_noinline	265	=item ecb_noinline
43		266
44	Prevent a function from being inlined - it might be optimised away, but	267	Prevent a function from being inlined - it might be optimised away, but
45	not inlined into other functions. This is useful if you know your function	268	not inlined into other functions. This is useful if you know your function
46	is rarely called and large enough for inlining not to be helpful.	269	is rarely called and large enough for inlining not to be helpful.
47		270
48	=item ecb_noreturn	271	=item ecb_noreturn
49		272
		273	Marks a function as "not returning, ever". Some typical functions that
		274	don't return are C<exit> or C<abort> (which really works hard to not
		275	return), and now you can make your own:
		276
		277	ecb_noreturn void
		278	my_abort (const char *errline)
		279	{
		280	puts (errline);
		281	abort ();
		282	}
		283
		284	In this case, the compiler would probably be smart enough to deduce it on
		285	its own, so this is mainly useful for declarations.
		286
		287	=item ecb_restrict
		288
		289	Expands to the C<restrict> keyword or equivalent on compilers that support
		290	them, and to nothing on others. Must be specified on a pointer type or
		291	an array index to indicate that the memory doesn't alias with any other
		292	restricted pointer in the same scope.
		293
		294	Example: multiply a vector, and allow the compiler to parallelise the
		295	loop, because it knows it doesn't overwrite input values.
		296
		297	void
		298	multiply (ecb_restrict float *src,
		299	ecb_restrict float *dst,
		300	int len, float factor)
		301	{
		302	int i;
		303
		304	for (i = 0; i < len; ++i)
		305	dst [i] = src [i] * factor;
		306	}
		307
50	=item ecb_const	308	=item ecb_const
51		309
		310	Declares that the function only depends on the values of its arguments,
		311	much like a mathematical function. It specifically does not read or write
		312	any memory any arguments might point to, global variables, or call any
		313	non-const functions. It also must not have any side effects.
		314
		315	Such a function can be optimised much more aggressively by the compiler -
		316	for example, multiple calls with the same arguments can be optimised into
		317	a single call, which wouldn't be possible if the compiler would have to
		318	expect any side effects.
		319
		320	It is best suited for functions in the sense of mathematical functions,
		321	such as a function returning the square root of its input argument.
		322
		323	Not suited would be a function that calculates the hash of some memory
		324	area you pass in, prints some messages or looks at a global variable to
		325	decide on rounding.
		326
		327	See C<ecb_pure> for a slightly less restrictive class of functions.
		328
52	=item ecb_pure	329	=item ecb_pure
53		330
		331	Similar to C<ecb_const>, declares a function that has no side
		332	effects. Unlike C<ecb_const>, the function is allowed to examine global
		333	variables and any other memory areas (such as the ones passed to it via
		334	pointers).
		335
		336	While these functions cannot be optimised as aggressively as C<ecb_const>
		337	functions, they can still be optimised away in many occasions, and the
		338	compiler has more freedom in moving calls to them around.
		339
		340	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		341	function that calculates the MD5 sum of some input and updates some MD5
		342	state passed as argument would I<NOT> be pure, however, as it would modify
		343	some memory area that is not the return value.
		344
54	=item ecb_hot	345	=item ecb_hot
55		346
		347	This declares a function as "hot" with regards to the cache - the function
		348	is used so often, that it is very beneficial to keep it in the cache if
		349	possible.
		350
		351	The compiler reacts by trying to place hot functions near to each other in
		352	memory.
		353
		354	Whether a function is hot or not often depends on the whole program,
		355	and less on the function itself. C<ecb_cold> is likely more useful in
		356	practise.
		357
56	=item ecb_cold	358	=item ecb_cold
57		359
		360	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		361	the cache, or in other words, this function is not called often, or not at
		362	speed-critical times, and keeping it in the cache might be a waste of said
		363	cache.
		364
		365	In addition to placing cold functions together (or at least away from hot
		366	functions), this knowledge can be used in other ways, for example, the
		367	function will be optimised for size, as opposed to speed, and codepaths
		368	leading to calls to those functions can automatically be marked as if
		369	C<ecb_expect_false> had been used to reach them.
		370
		371	Good examples for such functions would be error reporting functions, or
		372	functions only called in exceptional or rare cases.
		373
58	=item ecb_artificial	374	=item ecb_artificial
59		375
		376	Declares the function as "artificial", in this case meaning that this
		377	function is not really meant to be a function, but more like an accessor
		378	- many methods in C++ classes are mere accessor functions, and having a
		379	crash reported in such a method, or single-stepping through them, is not
		380	usually so helpful, especially when it's inlined to just a few instructions.
		381
		382	Marking them as artificial will instruct the debugger about just this,
		383	leading to happier debugging and thus happier lives.
		384
		385	Example: in some kind of smart-pointer class, mark the pointer accessor as
		386	artificial, so that the whole class acts more like a pointer and less like
		387	some C++ abstraction monster.
		388
		389	template<typename T>
		390	struct my_smart_ptr
		391	{
		392	T *value;
		393
		394	ecb_artificial
		395	operator T *()
		396	{
		397	return value;
		398	}
		399	};
		400
60	=back	401	=back
61		402
62	=head2 OPTIMISATION HINTS	403	=head2 OPTIMISATION HINTS
63		404
64	=over 4	405	=over 4
65		406
66	=item bool ecb_is_constant(expr) [MACRO]	407	=item bool ecb_is_constant (expr)
67		408
68	Returns true iff the expression can be deduced to be a compile-time	409	Returns true iff the expression can be deduced to be a compile-time
69	constant, and false otherwise.	410	constant, and false otherwise.
70		411
71	For example, when you have a C<rndm16> function that returns a 16 bit	412	For example, when you have a C<rndm16> function that returns a 16 bit
89	return is_constant (n) && !(n & (n - 1))	430	return is_constant (n) && !(n & (n - 1))
90	? rndm16 () & (num - 1)	431	? rndm16 () & (num - 1)
91	: (n * (uint32_t)rndm16 ()) >> 16;	432	: (n * (uint32_t)rndm16 ()) >> 16;
92	}	433	}
93		434
94	=item bool ecb_expect (expr, value) [MACRO]	435	=item ecb_expect (expr, value)
95		436
96	Evaluates C<expr> and returns it. In addition, it tells the compiler that	437	Evaluates C<expr> and returns it. In addition, it tells the compiler that
97	the C<expr> evaluates to C<value> a lot, which can be used for static	438	the C<expr> evaluates to C<value> a lot, which can be used for static
98	branch optimisations.	439	branch optimisations.
99		440
100	Usually, you want to use the more intuitive C<ecb_likely> and	441	Usually, you want to use the more intuitive C<ecb_expect_true> and
101	C<ecb_unlikely> functions instead.	442	C<ecb_expect_false> functions instead.
102		443
103	=item bool ecb_likely (bool) [MACRO]	444	=item bool ecb_expect_true (cond)
104		445
105	=item bool ecb_unlikely (bool) [MACRO]	446	=item bool ecb_expect_false (cond)
106		447
107	These two functions expect a expression that is true or false and return	448	These two functions expect a expression that is true or false and return
108	C<1> or C<0>, respectively, so when used in the condition of an C<if> or	449	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
109	other conditional statement, it will not change the program:	450	other conditional statement, it will not change the program:
110		451
111	/* these two do the same thing */	452	/* these two do the same thing */
112	if (some_condition) ...;	453	if (some_condition) ...;
113	if (ecb_likely (some_condition)) ...;	454	if (ecb_expect_true (some_condition)) ...;
114		455
115	However, by using C<ecb_likely>, you tell the compiler that the condition	456	However, by using C<ecb_expect_true>, you tell the compiler that the
116	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be	457	condition is likely to be true (and for C<ecb_expect_false>, that it is
117	true).	458	unlikely to be true).
118		459
119	For example, when you check for a null pointer and expect this to be a	460	For example, when you check for a null pointer and expect this to be a
120	rare, exceptional, case, then use C<ecb_unlikely>:	461	rare, exceptional, case, then use C<ecb_expect_false>:
121		462
122	void my_free (void *ptr)	463	void my_free (void *ptr)
123	{	464	{
124	if (ecb_unlikely (ptr == 0))	465	if (ecb_expect_false (ptr == 0))
125	return;	466	return;
126	}	467	}
127		468
128	Consequent use of these functions to mark away exceptional cases or to	469	Consequent use of these functions to mark away exceptional cases or to
129	tell the compiler what the hot path through a function is can increase	470	tell the compiler what the hot path through a function is can increase
130	performance considerably.	471	performance considerably.
		472
		473	You might know these functions under the name C<likely> and C<unlikely>
		474	- while these are common aliases, we find that the expect name is easier
		475	to understand when quickly skimming code. If you wish, you can use
		476	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		477	C<ecb_expect_false> - these are simply aliases.
131		478
132	A very good example is in a function that reserves more space for some	479	A very good example is in a function that reserves more space for some
133	memory block (for example, inside an implementation of a string stream) -	480	memory block (for example, inside an implementation of a string stream) -
134	each time something is added, you have to check for a buffer overrun, but	481	each time something is added, you have to check for a buffer overrun, but
135	you expect that most checks will turn out to be false:	482	you expect that most checks will turn out to be false:
136		483
137	/* make sure we have "size" extra room in our buffer */	484	/* make sure we have "size" extra room in our buffer */
138	ecb_inline void	485	ecb_inline void
139	reserve (int size)	486	reserve (int size)
140	{	487	{
141	if (ecb_unlikely (current + size > end))	488	if (ecb_expect_false (current + size > end))
142	real_reserve_method (size); /* presumably noinline */	489	real_reserve_method (size); /* presumably noinline */
143	}	490	}
144		491
145	=item bool ecb_assume (cond) [MACRO]	492	=item ecb_assume (cond)
146		493
147	Try to tell the compiler that some condition is true, even if it's not	494	Try to tell the compiler that some condition is true, even if it's not
148	obvious.	495	obvious.
149		496
150	This can be used to teach the compiler about invariants or other	497	This can be used to teach the compiler about invariants or other
151	conditions that might improve code generation, but which are impossible to	498	conditions that might improve code generation, but which are impossible to
152	deduce form the code itself.	499	deduce form the code itself.
153		500
154	For example, the example reservation function from the C<ecb_unlikely>	501	For example, the example reservation function from the C<ecb_expect_false>
155	description could be written thus (only C<ecb_assume> was added):	502	description could be written thus (only C<ecb_assume> was added):
156		503
157	ecb_inline void	504	ecb_inline void
158	reserve (int size)	505	reserve (int size)
159	{	506	{
160	if (ecb_unlikely (current + size > end))	507	if (ecb_expect_false (current + size > end))
161	real_reserve_method (size); /* presumably noinline */	508	real_reserve_method (size); /* presumably noinline */
162		509
163	ecb_assume (current + size <= end);	510	ecb_assume (current + size <= end);
164	}	511	}
165		512
…		…
170		517
171	Then the compiler I<might> be able to optimise out the second call	518	Then the compiler I<might> be able to optimise out the second call
172	completely, as it knows that C<< current + 1 > end >> is false and the	519	completely, as it knows that C<< current + 1 > end >> is false and the
173	call will never be executed.	520	call will never be executed.
174		521
175	=item bool ecb_unreachable ()	522	=item ecb_unreachable ()
176		523
177	This function does nothing itself, except tell the compiler that it will	524	This function does nothing itself, except tell the compiler that it will
178	never be executed. Apart from suppressing a warning in some cases, this	525	never be executed. Apart from suppressing a warning in some cases, this
179	function can be used to implement C<ecb_assume> or similar functions.	526	function can be used to implement C<ecb_assume> or similar functions.
180		527
181	=item bool ecb_prefetch (addr, rw, locality) [MACRO]	528	=item ecb_prefetch (addr, rw, locality)
182		529
183	Tells the compiler to try to prefetch memory at the given C<addr>ess	530	Tells the compiler to try to prefetch memory at the given C<addr>ess
184	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of	531	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
185	C<0> means that there will only be one access later, C<3> means that	532	C<0> means that there will only be one access later, C<3> means that
186	the data will likely be accessed very often, and values in between mean	533	the data will likely be accessed very often, and values in between mean
…		…
214	After processing the node, (part of) the next node might already be in	561	After processing the node, (part of) the next node might already be in
215	cache.	562	cache.
216		563
217	=back	564	=back
218		565
219	=head2 BIT FIDDLING / BITSTUFFS	566	=head2 BIT FIDDLING / BIT WIZARDRY
220		567
221	=over 4	568	=over 4
222		569
223	=item bool ecb_big_endian ()	570	=item bool ecb_big_endian ()
224		571
…		…
226		573
227	These two functions return true if the byte order is big endian	574	These two functions return true if the byte order is big endian
228	(most-significant byte first) or little endian (least-significant byte	575	(most-significant byte first) or little endian (least-significant byte
229	first) respectively.	576	first) respectively.
230		577
		578	On systems that are neither, their return values are unspecified.
		579
231	=item int ecb_ctz32 (uint32_t x)	580	=item int ecb_ctz32 (uint32_t x)
232		581
		582	=item int ecb_ctz64 (uint64_t x)
		583
233	Returns the index of the least significant bit set in C<x> (or	584	Returns the index of the least significant bit set in C<x> (or
234	equivalently the number of bits set to 0 before the least significant	585	equivalently the number of bits set to 0 before the least significant bit
235	bit set), starting from 0. If C<x> is 0 the result is undefined. A	586	set), starting from 0. If C<x> is 0 the result is undefined.
236	common use case is to compute the integer binary logarithm, i.e.,
237	floor(log2(n)). For example:
238		587
		588	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		589
		590	For example:
		591
239	ecb_ctz32(3) = 1	592	ecb_ctz32 (3) = 0
240	ecb_ctz32(6) = 2	593	ecb_ctz32 (6) = 1
		594
		595	=item bool ecb_is_pot32 (uint32_t x)
		596
		597	=item bool ecb_is_pot64 (uint32_t x)
		598
		599	Return true iff C<x> is a power of two or C<x == 0>.
		600
		601	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		602
		603	=item int ecb_ld32 (uint32_t x)
		604
		605	=item int ecb_ld64 (uint64_t x)
		606
		607	Returns the index of the most significant bit set in C<x>, or the number
		608	of digits the number requires in binary (so that C<< 2**ld <= x <
		609	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		610	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		611	example to see how many bits a certain number requires to be encoded.
		612
		613	This function is similar to the "count leading zero bits" function, except
		614	that that one returns how many zero bits are "in front" of the number (in
		615	the given data type), while C<ecb_ld> returns how many bits the number
		616	itself requires.
		617
		618	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
241		619
242	=item int ecb_popcount32 (uint32_t x)	620	=item int ecb_popcount32 (uint32_t x)
243		621
		622	=item int ecb_popcount64 (uint64_t x)
		623
244	Returns the number of bits set to 1 in C<x>. For example:	624	Returns the number of bits set to 1 in C<x>.
245		625
		626	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		627
		628	For example:
		629
246	ecb_popcount32(7) = 3	630	ecb_popcount32 (7) = 3
247	ecb_popcount32(255) = 8	631	ecb_popcount32 (255) = 8
		632
		633	=item uint8_t ecb_bitrev8 (uint8_t x)
		634
		635	=item uint16_t ecb_bitrev16 (uint16_t x)
		636
		637	=item uint32_t ecb_bitrev32 (uint32_t x)
		638
		639	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		640	and so on.
		641
		642	Example:
		643
		644	ecb_bitrev8 (0xa7) = 0xea
		645	ecb_bitrev32 (0xffcc4411) = 0x882233ff
248		646
249	=item uint32_t ecb_bswap16 (uint32_t x)	647	=item uint32_t ecb_bswap16 (uint32_t x)
250		648
251	=item uint32_t ecb_bswap32 (uint32_t x)	649	=item uint32_t ecb_bswap32 (uint32_t x)
252		650
		651	=item uint64_t ecb_bswap64 (uint64_t x)
		652
		653	These functions return the value of the 16-bit (32-bit, 64-bit) value
		654	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		655	C<ecb_bswap32>).
		656
		657	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		658
		659	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		660
		661	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		662
		663	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		664
		665	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		666
		667	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
		668
253	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	669	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
254		670
255	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	671	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
256		672
257	These two functions return the value of C<x> after shifting all the bits	673	These two families of functions return the value of C<x> after rotating
258	by C<count> positions to the right or left respectively.	674	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		675	(C<ecb_rotl>).
		676
		677	Current GCC versions understand these functions and usually compile them
		678	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		679	x86).
259		680
260	=back	681	=back
261		682
		683	=head2 FLOATING POINT FIDDLING
		684
		685	=over 4
		686
		687	=item ECB_INFINITY
		688
		689	Evaluates to positive infinity if supported by the platform, otherwise to
		690	a truly huge number.
		691
		692	=item ECB_NAN
		693
		694	Evaluates to a quiet NAN if supported by the platform, otherwise to
		695	C<ECB_INFINITY>.
		696
		697	=item float ecb_ldexpf (float x, int exp)
		698
		699	Same as C<ldexpf>, but always available.
		700
		701	=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
		702
		703	=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
		704
		705	These functions each take an argument in the native C<float> or C<double>
		706	type and return the IEEE 754 bit representation of it.
		707
		708	The bit representation is just as IEEE 754 defines it, i.e. the sign bit
		709	will be the most significant bit, followed by exponent and mantissa.
		710
		711	This function should work even when the native floating point format isn't
		712	IEEE compliant, of course at a speed and code size penalty, and of course
		713	also within reasonable limits (it tries to convert NaNs, infinities and
		714	denormals, but will likely convert negative zero to positive zero).
		715
		716	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		717	be able to optimise away this function completely.
		718
		719	These functions can be helpful when serialising floats to the network - you
		720	can serialise the return value like a normal uint32_t/uint64_t.
		721
		722	Another use for these functions is to manipulate floating point values
		723	directly.
		724
		725	Silly example: toggle the sign bit of a float.
		726
		727	/* On gcc-4.7 on amd64, */
		728	/* this results in a single add instruction to toggle the bit, and 4 extra */
		729	/* instructions to move the float value to an integer register and back. */
		730
		731	x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
		732
		733	=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
		734
		735	=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
		736
		737	=item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
		738
		739	The reverse operation of the previous function - takes the bit
		740	representation of an IEEE binary16, binary32 or binary64 number and
		741	converts it to the native C<float> or C<double> format.
		742
		743	This function should work even when the native floating point format isn't
		744	IEEE compliant, of course at a speed and code size penalty, and of course
		745	also within reasonable limits (it tries to convert normals and denormals,
		746	and might be lucky for infinities, and with extraordinary luck, also for
		747	negative zero).
		748
		749	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		750	be able to optimise away this function completely.
		751
		752	=back
		753
262	=head2 ARITHMETIC	754	=head2 ARITHMETIC
263		755
264	=over 4	756	=over 4
265		757
266	=item x = ecb_mod (m, n) [MACRO]	758	=item x = ecb_mod (m, n)
267		759
268	Returns the positive remainder of the modulo operation between C<m>	760	Returns C<m> modulo C<n>, which is the same as the positive remainder
269	and C<n>.	761	of the division operation between C<m> and C<n>, using floored
		762	division. Unlike the C remainder operator C<%>, this function ensures that
		763	the return value is always positive and that the two numbers I<m> and
		764	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		765	C<ecb_mod> implements the mathematical modulo operation, which is missing
		766	in the language.
		767
		768	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
		769	negatable, that is, both C<m> and C<-m> must be representable in its
		770	type (this typically excludes the minimum signed integer value, the same
		771	limitation as for C</> and C<%> in C).
		772
		773	Current GCC versions compile this into an efficient branchless sequence on
		774	almost all CPUs.
		775
		776	For example, when you want to rotate forward through the members of an
		777	array for increasing C<m> (which might be negative), then you should use
		778	C<ecb_mod>, as the C<%> operator might give either negative results, or
		779	change direction for negative values:
		780
		781	for (m = -100; m <= 100; ++m)
		782	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		783
		784	=item x = ecb_div_rd (val, div)
		785
		786	=item x = ecb_div_ru (val, div)
		787
		788	Returns C<val> divided by C<div> rounded down or up, respectively.
		789	C<val> and C<div> must have integer types and C<div> must be strictly
		790	positive. Note that these functions are implemented with macros in C
		791	and with function templates in C++.
270		792
271	=back	793	=back
272		794
273	=head2 UTILITY	795	=head2 UTILITY
274		796
275	=over 4	797	=over 4
276		798
277	=item element_count = ecb_array_length (name) [MACRO]	799	=item element_count = ecb_array_length (name)
		800
		801	Returns the number of elements in the array C<name>. For example:
		802
		803	int primes[] = { 2, 3, 5, 7, 11 };
		804	int sum = 0;
		805
		806	for (i = 0; i < ecb_array_length (primes); i++)
		807	sum += primes [i];
278		808
279	=back	809	=back
280		810
		811	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
281		812
		813	These symbols need to be defined before including F<ecb.h> the first time.
		814
		815	=over 4
		816
		817	=item ECB_NO_THREADS
		818
		819	If F<ecb.h> is never used from multiple threads, then this symbol can
		820	be defined, in which case memory fences (and similar constructs) are
		821	completely removed, leading to more efficient code and fewer dependencies.
		822
		823	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		824
		825	=item ECB_NO_SMP
		826
		827	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		828	multiple threads, but never concurrently (e.g. if the system the program
		829	runs on has only a single CPU with a single core, no hyperthreading and so
		830	on), then this symbol can be defined, leading to more efficient code and
		831	fewer dependencies.
		832
		833	=item ECB_NO_LIBM
		834
		835	When defined to C<1>, do not export any functions that might introduce
		836	dependencies on the math library (usually called F<-lm>) - these are
		837	marked with [-UECB_NO_LIBM].
		838
		839	=back
		840
		841

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Comparing cvsroot/libecb/ecb.pod (file contents):
Revision 1.11 by sf-exg, Thu May 26 21:18:52 2011 UTC vs.
Revision 1.64 by root, Wed Feb 18 20:48:59 2015 UTC