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Revision 1.62 by root, Wed Feb 18 20:29:27 2015 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
55	=head2 GCC ATTRIBUTES	59	=head2 TYPES / TYPE SUPPORT
56		60
57	blabla where to put, what others	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).
58		78
59	=over 4	79	=over 4
60		80
61	=item ecb_attribute ((attrs...))	81	=item ECB_C
62		82
63	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to	83	True if the implementation defines the C<__STDC__> macro to a true value,
64	nothing on other compilers, so the effect is that only GCC sees these.	84	while not claiming to be C++.
65		85
66	Example: use the C<deprecated> attribute on a function.	86	=item ECB_C99
67		87
68	ecb_attribute((__deprecated__)) void	88	True if the implementation claims to be compliant to C99 (ISO/IEC
69	do_not_use_me_anymore (void);	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	=back
		193
		194	=head2 ATTRIBUTES
		195
		196	A major part of libecb deals with additional attributes that can be
		197	assigned to functions, variables and sometimes even types - much like
		198	C<const> or C<volatile> in C. They are implemented using either GCC
		199	attributes or other compiler/language specific features. Attributes
		200	declarations must be put before the whole declaration:
		201
		202	ecb_const int mysqrt (int a);
		203	ecb_unused int i;
		204
		205	=over 4
70		206
71	=item ecb_unused	207	=item ecb_unused
72		208
73	Marks a function or a variable as "unused", which simply suppresses a	209	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.	210	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:	211	declare a variable but do not always use it:
76		212
77	{	213	{
78	int var ecb_unused;	214	ecb_unused int var;
79		215
80	#ifdef SOMECONDITION	216	#ifdef SOMECONDITION
81	var = ...;	217	var = ...;
82	return var;	218	return var;
83	#else	219	#else
84	return 0;	220	return 0;
85	#endif	221	#endif
86	}	222	}
87		223
		224	=item ecb_deprecated
		225
		226	Similar to C<ecb_unused>, but marks a function, variable or type as
		227	deprecated. This makes some compilers warn when the type is used.
		228
		229	=item ecb_deprecated_message (message)
		230
		231	Same as C<ecb_deprecated>, but if possible, supply a diagnostic that is
		232	used instead of a generic depreciation message when the object is being
		233	used.
		234
		235	=item ecb_inline
		236
		237	Expands either to C<static inline> or to just C<static>, if inline
		238	isn't supported. It should be used to declare functions that should be
		239	inlined, for code size or speed reasons.
		240
		241	Example: inline this function, it surely will reduce codesize.
		242
		243	ecb_inline int
		244	negmul (int a, int b)
		245	{
		246	return - (a * b);
		247	}
		248
88	=item ecb_noinline	249	=item ecb_noinline
89		250
90	Prevent a function from being inlined - it might be optimised away, but	251	Prevent 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	252	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.	253	is rarely called and large enough for inlining not to be helpful.
93		254
94	=item ecb_noreturn	255	=item ecb_noreturn
95		256
		257	Marks a function as "not returning, ever". Some typical functions that
		258	don't return are C<exit> or C<abort> (which really works hard to not
		259	return), and now you can make your own:
		260
		261	ecb_noreturn void
		262	my_abort (const char *errline)
		263	{
		264	puts (errline);
		265	abort ();
		266	}
		267
		268	In this case, the compiler would probably be smart enough to deduce it on
		269	its own, so this is mainly useful for declarations.
		270
		271	=item ecb_restrict
		272
		273	Expands to the C<restrict> keyword or equivalent on compilers that support
		274	them, and to nothing on others. Must be specified on a pointer type or
		275	an array index to indicate that the memory doesn't alias with any other
		276	restricted pointer in the same scope.
		277
		278	Example: multiply a vector, and allow the compiler to parallelise the
		279	loop, because it knows it doesn't overwrite input values.
		280
		281	void
		282	multiply (ecb_restrict float *src,
		283	ecb_restrict float *dst,
		284	int len, float factor)
		285	{
		286	int i;
		287
		288	for (i = 0; i < len; ++i)
		289	dst [i] = src [i] * factor;
		290	}
		291
96	=item ecb_const	292	=item ecb_const
97		293
		294	Declares that the function only depends on the values of its arguments,
		295	much like a mathematical function. It specifically does not read or write
		296	any memory any arguments might point to, global variables, or call any
		297	non-const functions. It also must not have any side effects.
		298
		299	Such a function can be optimised much more aggressively by the compiler -
		300	for example, multiple calls with the same arguments can be optimised into
		301	a single call, which wouldn't be possible if the compiler would have to
		302	expect any side effects.
		303
		304	It is best suited for functions in the sense of mathematical functions,
		305	such as a function returning the square root of its input argument.
		306
		307	Not suited would be a function that calculates the hash of some memory
		308	area you pass in, prints some messages or looks at a global variable to
		309	decide on rounding.
		310
		311	See C<ecb_pure> for a slightly less restrictive class of functions.
		312
98	=item ecb_pure	313	=item ecb_pure
99		314
		315	Similar to C<ecb_const>, declares a function that has no side
		316	effects. Unlike C<ecb_const>, the function is allowed to examine global
		317	variables and any other memory areas (such as the ones passed to it via
		318	pointers).
		319
		320	While these functions cannot be optimised as aggressively as C<ecb_const>
		321	functions, they can still be optimised away in many occasions, and the
		322	compiler has more freedom in moving calls to them around.
		323
		324	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		325	function that calculates the MD5 sum of some input and updates some MD5
		326	state passed as argument would I<NOT> be pure, however, as it would modify
		327	some memory area that is not the return value.
		328
100	=item ecb_hot	329	=item ecb_hot
101		330
		331	This declares a function as "hot" with regards to the cache - the function
		332	is used so often, that it is very beneficial to keep it in the cache if
		333	possible.
		334
		335	The compiler reacts by trying to place hot functions near to each other in
		336	memory.
		337
		338	Whether a function is hot or not often depends on the whole program,
		339	and less on the function itself. C<ecb_cold> is likely more useful in
		340	practise.
		341
102	=item ecb_cold	342	=item ecb_cold
103		343
		344	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		345	the cache, or in other words, this function is not called often, or not at
		346	speed-critical times, and keeping it in the cache might be a waste of said
		347	cache.
		348
		349	In addition to placing cold functions together (or at least away from hot
		350	functions), this knowledge can be used in other ways, for example, the
		351	function will be optimised for size, as opposed to speed, and codepaths
		352	leading to calls to those functions can automatically be marked as if
		353	C<ecb_expect_false> had been used to reach them.
		354
		355	Good examples for such functions would be error reporting functions, or
		356	functions only called in exceptional or rare cases.
		357
104	=item ecb_artificial	358	=item ecb_artificial
105		359
		360	Declares the function as "artificial", in this case meaning that this
		361	function is not really meant to be a function, but more like an accessor
		362	- many methods in C++ classes are mere accessor functions, and having a
		363	crash reported in such a method, or single-stepping through them, is not
		364	usually so helpful, especially when it's inlined to just a few instructions.
		365
		366	Marking them as artificial will instruct the debugger about just this,
		367	leading to happier debugging and thus happier lives.
		368
		369	Example: in some kind of smart-pointer class, mark the pointer accessor as
		370	artificial, so that the whole class acts more like a pointer and less like
		371	some C++ abstraction monster.
		372
		373	template<typename T>
		374	struct my_smart_ptr
		375	{
		376	T *value;
		377
		378	ecb_artificial
		379	operator T *()
		380	{
		381	return value;
		382	}
		383	};
		384
106	=back	385	=back
107		386
108	=head2 OPTIMISATION HINTS	387	=head2 OPTIMISATION HINTS
109		388
110	=over 4	389	=over 4
111		390
112	=item bool ecb_is_constant(expr)	391	=item bool ecb_is_constant (expr)
113		392
114	Returns true iff the expression can be deduced to be a compile-time	393	Returns true iff the expression can be deduced to be a compile-time
115	constant, and false otherwise.	394	constant, and false otherwise.
116		395
117	For example, when you have a C<rndm16> function that returns a 16 bit	396	For example, when you have a C<rndm16> function that returns a 16 bit
…		…
135	return is_constant (n) && !(n & (n - 1))	414	return is_constant (n) && !(n & (n - 1))
136	? rndm16 () & (num - 1)	415	? rndm16 () & (num - 1)
137	: (n * (uint32_t)rndm16 ()) >> 16;	416	: (n * (uint32_t)rndm16 ()) >> 16;
138	}	417	}
139		418
140	=item bool ecb_expect (expr, value)	419	=item ecb_expect (expr, value)
141		420
142	Evaluates C<expr> and returns it. In addition, it tells the compiler that	421	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	422	the C<expr> evaluates to C<value> a lot, which can be used for static
144	branch optimisations.	423	branch optimisations.
145		424
146	Usually, you want to use the more intuitive C<ecb_likely> and	425	Usually, you want to use the more intuitive C<ecb_expect_true> and
147	C<ecb_unlikely> functions instead.	426	C<ecb_expect_false> functions instead.
148		427
		428	=item bool ecb_expect_true (cond)
		429
149	=item bool ecb_likely (cond)	430	=item bool ecb_expect_false (cond)
150
151	=item bool ecb_unlikely (cond)
152		431
153	These two functions expect a expression that is true or false and return	432	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	433	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:	434	other conditional statement, it will not change the program:
156		435
157	/* these two do the same thing */	436	/* these two do the same thing */
158	if (some_condition) ...;	437	if (some_condition) ...;
159	if (ecb_likely (some_condition)) ...;	438	if (ecb_expect_true (some_condition)) ...;
160		439
161	However, by using C<ecb_likely>, you tell the compiler that the condition	440	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	441	condition is likely to be true (and for C<ecb_expect_false>, that it is
163	true).	442	unlikely to be true).
164		443
165	For example, when you check for a null pointer and expect this to be a	444	For example, when you check for a null pointer and expect this to be a
166	rare, exceptional, case, then use C<ecb_unlikely>:	445	rare, exceptional, case, then use C<ecb_expect_false>:
167		446
168	void my_free (void *ptr)	447	void my_free (void *ptr)
169	{	448	{
170	if (ecb_unlikely (ptr == 0))	449	if (ecb_expect_false (ptr == 0))
171	return;	450	return;
172	}	451	}
173		452
174	Consequent use of these functions to mark away exceptional cases or to	453	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	454	tell the compiler what the hot path through a function is can increase
176	performance considerably.	455	performance considerably.
		456
		457	You might know these functions under the name C<likely> and C<unlikely>
		458	- while these are common aliases, we find that the expect name is easier
		459	to understand when quickly skimming code. If you wish, you can use
		460	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		461	C<ecb_expect_false> - these are simply aliases.
177		462
178	A very good example is in a function that reserves more space for some	463	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) -	464	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	465	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:	466	you expect that most checks will turn out to be false:
182		467
183	/* make sure we have "size" extra room in our buffer */	468	/* make sure we have "size" extra room in our buffer */
184	ecb_inline void	469	ecb_inline void
185	reserve (int size)	470	reserve (int size)
186	{	471	{
187	if (ecb_unlikely (current + size > end))	472	if (ecb_expect_false (current + size > end))
188	real_reserve_method (size); /* presumably noinline */	473	real_reserve_method (size); /* presumably noinline */
189	}	474	}
190		475
191	=item bool ecb_assume (cond)	476	=item ecb_assume (cond)
192		477
193	Try to tell the compiler that some condition is true, even if it's not	478	Try to tell the compiler that some condition is true, even if it's not
194	obvious.	479	obvious.
195		480
196	This can be used to teach the compiler about invariants or other	481	This can be used to teach the compiler about invariants or other
197	conditions that might improve code generation, but which are impossible to	482	conditions that might improve code generation, but which are impossible to
198	deduce form the code itself.	483	deduce form the code itself.
199		484
200	For example, the example reservation function from the C<ecb_unlikely>	485	For example, the example reservation function from the C<ecb_expect_false>
201	description could be written thus (only C<ecb_assume> was added):	486	description could be written thus (only C<ecb_assume> was added):
202		487
203	ecb_inline void	488	ecb_inline void
204	reserve (int size)	489	reserve (int size)
205	{	490	{
206	if (ecb_unlikely (current + size > end))	491	if (ecb_expect_false (current + size > end))
207	real_reserve_method (size); /* presumably noinline */	492	real_reserve_method (size); /* presumably noinline */
208		493
209	ecb_assume (current + size <= end);	494	ecb_assume (current + size <= end);
210	}	495	}
211		496
…		…
216		501
217	Then the compiler I<might> be able to optimise out the second call	502	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	503	completely, as it knows that C<< current + 1 > end >> is false and the
219	call will never be executed.	504	call will never be executed.
220		505
221	=item bool ecb_unreachable ()	506	=item ecb_unreachable ()
222		507
223	This function does nothing itself, except tell the compiler that it will	508	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	509	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.	510	function can be used to implement C<ecb_assume> or similar functions.
226		511
227	=item bool ecb_prefetch (addr, rw, locality)	512	=item ecb_prefetch (addr, rw, locality)
228		513
229	Tells the compiler to try to prefetch memory at the given C<addr>ess	514	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	515	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	516	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	517	the data will likely be accessed very often, and values in between mean
…		…
260	After processing the node, (part of) the next node might already be in	545	After processing the node, (part of) the next node might already be in
261	cache.	546	cache.
262		547
263	=back	548	=back
264		549
265	=head2 BIT FIDDLING / BITSTUFFS	550	=head2 BIT FIDDLING / BIT WIZARDRY
266		551
267	=over 4	552	=over 4
268		553
269	=item bool ecb_big_endian ()	554	=item bool ecb_big_endian ()
270		555
…		…
272		557
273	These two functions return true if the byte order is big endian	558	These two functions return true if the byte order is big endian
274	(most-significant byte first) or little endian (least-significant byte	559	(most-significant byte first) or little endian (least-significant byte
275	first) respectively.	560	first) respectively.
276		561
		562	On systems that are neither, their return values are unspecified.
		563
277	=item int ecb_ctz32 (uint32_t x)	564	=item int ecb_ctz32 (uint32_t x)
278		565
		566	=item int ecb_ctz64 (uint64_t x)
		567
279	Returns the index of the least significant bit set in C<x> (or	568	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	569	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	570	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.,	571
283	floor(log2(n)). For example:	572	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		573
		574	For example:
284		575
285	ecb_ctz32 (3) = 0	576	ecb_ctz32 (3) = 0
286	ecb_ctz32 (6) = 1	577	ecb_ctz32 (6) = 1
287		578
		579	=item bool ecb_is_pot32 (uint32_t x)
		580
		581	=item bool ecb_is_pot64 (uint32_t x)
		582
		583	Return true iff C<x> is a power of two or C<x == 0>.
		584
		585	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		586
		587	=item int ecb_ld32 (uint32_t x)
		588
		589	=item int ecb_ld64 (uint64_t x)
		590
		591	Returns the index of the most significant bit set in C<x>, or the number
		592	of digits the number requires in binary (so that C<< 2**ld <= x <
		593	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		594	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		595	example to see how many bits a certain number requires to be encoded.
		596
		597	This function is similar to the "count leading zero bits" function, except
		598	that that one returns how many zero bits are "in front" of the number (in
		599	the given data type), while C<ecb_ld> returns how many bits the number
		600	itself requires.
		601
		602	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		603
288	=item int ecb_popcount32 (uint32_t x)	604	=item int ecb_popcount32 (uint32_t x)
289		605
		606	=item int ecb_popcount64 (uint64_t x)
		607
290	Returns the number of bits set to 1 in C<x>. For example:	608	Returns the number of bits set to 1 in C<x>.
		609
		610	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		611
		612	For example:
291		613
292	ecb_popcount32 (7) = 3	614	ecb_popcount32 (7) = 3
293	ecb_popcount32 (255) = 8	615	ecb_popcount32 (255) = 8
294		616
		617	=item uint8_t ecb_bitrev8 (uint8_t x)
		618
		619	=item uint16_t ecb_bitrev16 (uint16_t x)
		620
		621	=item uint32_t ecb_bitrev32 (uint32_t x)
		622
		623	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		624	and so on.
		625
		626	Example:
		627
		628	ecb_bitrev8 (0xa7) = 0xea
		629	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		630
295	=item uint32_t ecb_bswap16 (uint32_t x)	631	=item uint32_t ecb_bswap16 (uint32_t x)
296		632
297	=item uint32_t ecb_bswap32 (uint32_t x)	633	=item uint32_t ecb_bswap32 (uint32_t x)
298		634
		635	=item uint64_t ecb_bswap64 (uint64_t x)
		636
299	These two functions return the value of the 16-bit (32-bit) variable	637	These functions return the value of the 16-bit (32-bit, 64-bit) value
300	C<x> after reversing the order of bytes.	638	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		639	C<ecb_bswap32>).
		640
		641	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		642
		643	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		644
		645	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		646
		647	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		648
		649	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		650
		651	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
301		652
302	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	653	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
303		654
304	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	655	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
305		656
306	These two functions return the value of C<x> after shifting all the bits	657	These two families of functions return the value of C<x> after rotating
307	by C<count> positions to the right or left respectively.	658	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		659	(C<ecb_rotl>).
		660
		661	Current GCC versions understand these functions and usually compile them
		662	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		663	x86).
308		664
309	=back	665	=back
310		666
		667	=head2 FLOATING POINT FIDDLING
		668
		669	=over 4
		670
		671	=item ECB_INFINITY
		672
		673	Evaluates to positive infinity if supported by the platform, otherwise to
		674	a truly huge number.
		675
		676	=item ECB_NON
		677
		678	Evaluates to a quiet NAN if supported by the platform, otherwise to
		679	C<ECB_INFINITY>.
		680
		681	=item float ecb_ldexpf (float x, int exp)
		682
		683	Same as C<ldexpf>, but always available.
		684
		685	=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
		686
		687	=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
		688
		689	These functions each take an argument in the native C<float> or C<double>
		690	type and return the IEEE 754 bit representation of it.
		691
		692	The bit representation is just as IEEE 754 defines it, i.e. the sign bit
		693	will be the most significant bit, followed by exponent and mantissa.
		694
		695	This function should work even when the native floating point format isn't
		696	IEEE compliant, of course at a speed and code size penalty, and of course
		697	also within reasonable limits (it tries to convert NaNs, infinities and
		698	denormals, but will likely convert negative zero to positive zero).
		699
		700	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		701	be able to optimise away this function completely.
		702
		703	These functions can be helpful when serialising floats to the network - you
		704	can serialise the return value like a normal uint32_t/uint64_t.
		705
		706	Another use for these functions is to manipulate floating point values
		707	directly.
		708
		709	Silly example: toggle the sign bit of a float.
		710
		711	/* On gcc-4.7 on amd64, */
		712	/* this results in a single add instruction to toggle the bit, and 4 extra */
		713	/* instructions to move the float value to an integer register and back. */
		714
		715	x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
		716
		717	=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
		718
		719	=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
		720
		721	=item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
		722
		723	The reverse operation of the previous function - takes the bit
		724	representation of an IEEE binary16, binary32 or binary64 number and
		725	converts it to the native C<float> or C<double> format.
		726
		727	This function should work even when the native floating point format isn't
		728	IEEE compliant, of course at a speed and code size penalty, and of course
		729	also within reasonable limits (it tries to convert normals and denormals,
		730	and might be lucky for infinities, and with extraordinary luck, also for
		731	negative zero).
		732
		733	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		734	be able to optimise away this function completely.
		735
		736	=back
		737
311	=head2 ARITHMETIC	738	=head2 ARITHMETIC
312		739
313	=over 4	740	=over 4
314		741
315	=item x = ecb_mod (m, n)	742	=item x = ecb_mod (m, n)
316		743
317	Returns the positive remainder of the modulo operation between C<m> and	744	Returns C<m> modulo C<n>, which is the same as the positive remainder
		745	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	746	division. Unlike the C remainder operator C<%>, this function ensures that
319	return value is always positive).	747	the return value is always positive and that the two numbers I<m> and
		748	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		749	C<ecb_mod> implements the mathematical modulo operation, which is missing
		750	in the language.
320		751
321	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	752	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	753	negatable, that is, both C<m> and C<-m> must be representable in its
323	type.	754	type (this typically excludes the minimum signed integer value, the same
		755	limitation as for C</> and C<%> in C).
		756
		757	Current GCC versions compile this into an efficient branchless sequence on
		758	almost all CPUs.
		759
		760	For example, when you want to rotate forward through the members of an
		761	array for increasing C<m> (which might be negative), then you should use
		762	C<ecb_mod>, as the C<%> operator might give either negative results, or
		763	change direction for negative values:
		764
		765	for (m = -100; m <= 100; ++m)
		766	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		767
		768	=item x = ecb_div_rd (val, div)
		769
		770	=item x = ecb_div_ru (val, div)
		771
		772	Returns C<val> divided by C<div> rounded down or up, respectively.
		773	C<val> and C<div> must have integer types and C<div> must be strictly
		774	positive. Note that these functions are implemented with macros in C
		775	and with function templates in C++.
324		776
325	=back	777	=back
326		778
327	=head2 UTILITY	779	=head2 UTILITY
328		780
329	=over 4	781	=over 4
330		782
331	=item element_count = ecb_array_length (name) [MACRO]	783	=item element_count = ecb_array_length (name)
332		784
333	Returns the number of elements in the array C<name>. For example:	785	Returns the number of elements in the array C<name>. For example:
334		786
335	int primes[] = { 2, 3, 5, 7, 11 };	787	int primes[] = { 2, 3, 5, 7, 11 };
336	int sum = 0;	788	int sum = 0;
…		…
338	for (i = 0; i < ecb_array_length (primes); i++)	790	for (i = 0; i < ecb_array_length (primes); i++)
339	sum += primes [i];	791	sum += primes [i];
340		792
341	=back	793	=back
342		794
		795	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
343		796
		797	These symbols need to be defined before including F<ecb.h> the first time.
		798
		799	=over 4
		800
		801	=item ECB_NO_THREADS
		802
		803	If F<ecb.h> is never used from multiple threads, then this symbol can
		804	be defined, in which case memory fences (and similar constructs) are
		805	completely removed, leading to more efficient code and fewer dependencies.
		806
		807	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		808
		809	=item ECB_NO_SMP
		810
		811	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		812	multiple threads, but never concurrently (e.g. if the system the program
		813	runs on has only a single CPU with a single core, no hyperthreading and so
		814	on), then this symbol can be defined, leading to more efficient code and
		815	fewer dependencies.
		816
		817	=item ECB_NO_LIBM
		818
		819	When defined to C<1>, do not export any functions that might introduce
		820	dependencies on the math library (usually called F<-lm>) - these are
		821	marked with [-UECB_NO_LIBM].
		822
		823	=back
		824
		825

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Revision 1.15 by root, Thu May 26 23:26:29 2011 UTC vs.
Revision 1.62 by root, Wed Feb 18 20:29:27 2015 UTC