[ViewVC] Diff of: cvs/libecb/ecb.pod

Comparing libecb/ecb.pod (file contents):
Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC vs.
Revision 1.68 by root, Fri Feb 20 16:50:08 2015 UTC

…		…
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 as endianness	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		19
20	Or in other words, things that should be built-in into any standard C	20	Or in other words, things that should be built into any standard C system,
21	system, but aren't.	21	but aren't, implemented as efficient as possible with GCC, and still
		22	correct with other compilers.
22		23
23	More might come.	24	More might come.
24		25
25	=head2 ABOUT THE HEADER	26	=head2 ABOUT THE HEADER
26		27
…		…
53	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
54	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
55	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
56	refers to any kind of boolean value, not a specific type.	57	refers to any kind of boolean value, not a specific type.
57		58
58	=head2 GCC ATTRIBUTES	59	=head2 TYPES / TYPE SUPPORT
59		60
60	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).
61		78
62	=over 4	79	=over 4
63		80
64	=item ecb_attribute ((attrs...))	81	=item ECB_C
65		82
66	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,
67	nothing on other compilers, so the effect is that only GCC sees these.	84	while not claiming to be C++.
68		85
69	Example: use the C<deprecated> attribute on a function.	86	=item ECB_C99
70		87
71	ecb_attribute((__deprecated__)) void	88	True if the implementation claims to be compliant to C99 (ISO/IEC
72	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	=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
73		222
74	=item ecb_unused	223	=item ecb_unused
75		224
76	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
77	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.
78	declare a variable but do not always use it:	227	declare a variable but do not always use it:
79		228
80	{	229	{
81	int var ecb_unused;	230	ecb_unused int var;
82		231
83	#ifdef SOMECONDITION	232	#ifdef SOMECONDITION
84	var = ...;	233	var = ...;
85	return var;	234	return var;
86	#else	235	#else
87	return 0;	236	return 0;
88	#endif	237	#endif
89	}	238	}
90		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, the specified diagnostic 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)
		261	{
		262	return - (a * b);
		263	}
		264
91	=item ecb_noinline	265	=item ecb_noinline
92		266
93	Prevent a function from being inlined - it might be optimised away, but	267	Prevents a function from being inlined - it might be optimised away, but
94	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
95	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.
96		270
97	=item ecb_noreturn	271	=item ecb_noreturn
98		272
…		…
105	{	279	{
106	puts (errline);	280	puts (errline);
107	abort ();	281	abort ();
108	}	282	}
109		283
110	In this case, the compiler would probbaly be smart enough to decude it on	284	In this case, the compiler would probably be smart enough to deduce it on
111	it's own, so this is mainly useful for declarations.	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	}
112		307
113	=item ecb_const	308	=item ecb_const
114		309
115	Declares that the function only depends on the values of it's arguments,	310	Declares that the function only depends on the values of its arguments,
116	much like a mathematical function. It specifically does not read or write	311	much like a mathematical function. It specifically does not read or write
117	any memory any arguments might point to, global variables, or call any	312	any memory any arguments might point to, global variables, or call any
118	non-const functions. It also must not have any side effects.	313	non-const functions. It also must not have any side effects.
119		314
120	Such a function can be optimised much more aggressively by the compiler -	315	Such a function can be optimised much more aggressively by the compiler -
121	for example, multiple calls with the same arguments can be optimised into	316	for example, multiple calls with the same arguments can be optimised into
122	a single call, which wouldn't be possible if the compiler would have to	317	a single call, which wouldn't be possible if the compiler would have to
123	expect any side effects.	318	expect any side effects.
124		319
125	It is best suited for functions in the sense of mathematical functions,	320	It is best suited for functions in the sense of mathematical functions,
126	such as a function return the square root of its input argument.	321	such as a function returning the square root of its input argument.
127		322
128	Not suited would be a function that calculates the hash of some memory	323	Not suited would be a function that calculates the hash of some memory
129	area you pass in, prints some messages or looks at a global variable to	324	area you pass in, prints some messages or looks at a global variable to
130	decide on rounding.	325	decide on rounding.
131		326
…		…
154	possible.	349	possible.
155		350
156	The compiler reacts by trying to place hot functions near to each other in	351	The compiler reacts by trying to place hot functions near to each other in
157	memory.	352	memory.
158		353
159	Whether a function is hot or not often depend son the whole program,	354	Whether a function is hot or not often depends on the whole program,
160	and less on the function itself. C<ecb_cold> is likely more useful in	355	and less on the function itself. C<ecb_cold> is likely more useful in
161	practise.	356	practise.
162		357
163	=item ecb_cold	358	=item ecb_cold
164		359
…		…
169		364
170	In addition to placing cold functions together (or at least away from hot	365	In addition to placing cold functions together (or at least away from hot
171	functions), this knowledge can be used in other ways, for example, the	366	functions), this knowledge can be used in other ways, for example, the
172	function will be optimised for size, as opposed to speed, and codepaths	367	function will be optimised for size, as opposed to speed, and codepaths
173	leading to calls to those functions can automatically be marked as if	368	leading to calls to those functions can automatically be marked as if
174	C<ecb_unlikel> had been used to reach them.	369	C<ecb_expect_false> had been used to reach them.
175		370
176	Good examples for such functions would be error reporting functions, or	371	Good examples for such functions would be error reporting functions, or
177	functions only called in exceptional or rare cases.	372	functions only called in exceptional or rare cases.
178		373
179	=item ecb_artificial	374	=item ecb_artificial
180		375
181	Declares the function as "artificial", in this case meaning that this	376	Declares the function as "artificial", in this case meaning that this
182	function is not really mean to be a function, but more like an accessor	377	function is not really meant to be a function, but more like an accessor
183	- many methods in C++ classes are mere accessor functions, and having a	378	- many methods in C++ classes are mere accessor functions, and having a
184	crash reported in such a method, or single-stepping through them, is not	379	crash reported in such a method, or single-stepping through them, is not
185	usually so helpful, especially when it's inlined to just a few instructions.	380	usually so helpful, especially when it's inlined to just a few instructions.
186		381
187	Marking them as artificial will instruct the debugger about just this,	382	Marking them as artificial will instruct the debugger about just this,
…		…
207		402
208	=head2 OPTIMISATION HINTS	403	=head2 OPTIMISATION HINTS
209		404
210	=over 4	405	=over 4
211		406
212	=item bool ecb_is_constant(expr)	407	=item bool ecb_is_constant (expr)
213		408
214	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
215	constant, and false otherwise.	410	constant, and false otherwise.
216		411
217	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
…		…
235	return is_constant (n) && !(n & (n - 1))	430	return is_constant (n) && !(n & (n - 1))
236	? rndm16 () & (num - 1)	431	? rndm16 () & (num - 1)
237	: (n * (uint32_t)rndm16 ()) >> 16;	432	: (n * (uint32_t)rndm16 ()) >> 16;
238	}	433	}
239		434
240	=item bool ecb_expect (expr, value)	435	=item ecb_expect (expr, value)
241		436
242	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
243	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
244	branch optimisations.	439	branch optimisations.
245		440
246	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
247	C<ecb_unlikely> functions instead.	442	C<ecb_expect_false> functions instead.
248		443
		444	=item bool ecb_expect_true (cond)
		445
249	=item bool ecb_likely (cond)	446	=item bool ecb_expect_false (cond)
250
251	=item bool ecb_unlikely (cond)
252		447
253	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
254	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
255	other conditional statement, it will not change the program:	450	other conditional statement, it will not change the program:
256		451
257	/* these two do the same thing */	452	/* these two do the same thing */
258	if (some_condition) ...;	453	if (some_condition) ...;
259	if (ecb_likely (some_condition)) ...;	454	if (ecb_expect_true (some_condition)) ...;
260		455
261	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
262	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
263	true).	458	unlikely to be true).
264		459
265	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
266	rare, exceptional, case, then use C<ecb_unlikely>:	461	rare, exceptional, case, then use C<ecb_expect_false>:
267		462
268	void my_free (void *ptr)	463	void my_free (void *ptr)
269	{	464	{
270	if (ecb_unlikely (ptr == 0))	465	if (ecb_expect_false (ptr == 0))
271	return;	466	return;
272	}	467	}
273		468
274	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
275	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
276	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.
277		478
278	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
279	memory block (for example, inside an implementation of a string stream) -	480	memory block (for example, inside an implementation of a string stream) -
280	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
281	you expect that most checks will turn out to be false:	482	you expect that most checks will turn out to be false:
282		483
283	/* make sure we have "size" extra room in our buffer */	484	/* make sure we have "size" extra room in our buffer */
284	ecb_inline void	485	ecb_inline void
285	reserve (int size)	486	reserve (int size)
286	{	487	{
287	if (ecb_unlikely (current + size > end))	488	if (ecb_expect_false (current + size > end))
288	real_reserve_method (size); /* presumably noinline */	489	real_reserve_method (size); /* presumably noinline */
289	}	490	}
290		491
291	=item bool ecb_assume (cond)	492	=item ecb_assume (cond)
292		493
293	Try to tell the compiler that some condition is true, even if it's not	494	Tries to tell the compiler that some condition is true, even if it's not
294	obvious.	495	obvious. This is not a function, but a statement: it cannot be used in
		496	another expression.
295		497
296	This can be used to teach the compiler about invariants or other	498	This can be used to teach the compiler about invariants or other
297	conditions that might improve code generation, but which are impossible to	499	conditions that might improve code generation, but which are impossible to
298	deduce form the code itself.	500	deduce form the code itself.
299		501
300	For example, the example reservation function from the C<ecb_unlikely>	502	For example, the example reservation function from the C<ecb_expect_false>
301	description could be written thus (only C<ecb_assume> was added):	503	description could be written thus (only C<ecb_assume> was added):
302		504
303	ecb_inline void	505	ecb_inline void
304	reserve (int size)	506	reserve (int size)
305	{	507	{
306	if (ecb_unlikely (current + size > end))	508	if (ecb_expect_false (current + size > end))
307	real_reserve_method (size); /* presumably noinline */	509	real_reserve_method (size); /* presumably noinline */
308		510
309	ecb_assume (current + size <= end);	511	ecb_assume (current + size <= end);
310	}	512	}
311		513
…		…
316		518
317	Then the compiler I<might> be able to optimise out the second call	519	Then the compiler I<might> be able to optimise out the second call
318	completely, as it knows that C<< current + 1 > end >> is false and the	520	completely, as it knows that C<< current + 1 > end >> is false and the
319	call will never be executed.	521	call will never be executed.
320		522
321	=item bool ecb_unreachable ()	523	=item ecb_unreachable ()
322		524
323	This function does nothing itself, except tell the compiler that it will	525	This function does nothing itself, except tell the compiler that it will
324	never be executed. Apart from suppressing a warning in some cases, this	526	never be executed. Apart from suppressing a warning in some cases, this
325	function can be used to implement C<ecb_assume> or similar functions.	527	function can be used to implement C<ecb_assume> or similar functionality.
326		528
327	=item bool ecb_prefetch (addr, rw, locality)	529	=item ecb_prefetch (addr, rw, locality)
328		530
329	Tells the compiler to try to prefetch memory at the given C<addr>ess	531	Tells the compiler to try to prefetch memory at the given C<addr>ess
330	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of	532	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
331	C<0> means that there will only be one access later, C<3> means that	533	C<0> means that there will only be one access later, C<3> means that
332	the data will likely be accessed very often, and values in between mean	534	the data will likely be accessed very often, and values in between mean
333	something... in between. The memory pointed to by the address does not	535	something... in between. The memory pointed to by the address does not
334	need to be accessible (it could be a null pointer for example), but C<rw>	536	need to be accessible (it could be a null pointer for example), but C<rw>
335	and C<locality> must be compile-time constants.	537	and C<locality> must be compile-time constants.
336		538
		539	This is a statement, not a function: you cannot use it as part of an
		540	expression.
		541
337	An obvious way to use this is to prefetch some data far away, in a big	542	An obvious way to use this is to prefetch some data far away, in a big
338	array you loop over. This prefetches memory some 128 array elements later,	543	array you loop over. This prefetches memory some 128 array elements later,
339	in the hope that it will be ready when the CPU arrives at that location.	544	in the hope that it will be ready when the CPU arrives at that location.
340		545
341	int sum = 0;	546	int sum = 0;
…		…
360	After processing the node, (part of) the next node might already be in	565	After processing the node, (part of) the next node might already be in
361	cache.	566	cache.
362		567
363	=back	568	=back
364		569
365	=head2 BIT FIDDLING / BITSTUFFS	570	=head2 BIT FIDDLING / BIT WIZARDRY
366		571
367	=over 4	572	=over 4
368		573
369	=item bool ecb_big_endian ()	574	=item bool ecb_big_endian ()
370		575
…		…
372		577
373	These two functions return true if the byte order is big endian	578	These two functions return true if the byte order is big endian
374	(most-significant byte first) or little endian (least-significant byte	579	(most-significant byte first) or little endian (least-significant byte
375	first) respectively.	580	first) respectively.
376		581
		582	On systems that are neither, their return values are unspecified.
		583
377	=item int ecb_ctz32 (uint32_t x)	584	=item int ecb_ctz32 (uint32_t x)
378		585
		586	=item int ecb_ctz64 (uint64_t x)
		587
379	Returns the index of the least significant bit set in C<x> (or	588	Returns the index of the least significant bit set in C<x> (or
380	equivalently the number of bits set to 0 before the least significant	589	equivalently the number of bits set to 0 before the least significant bit
381	bit set), starting from 0. If C<x> is 0 the result is undefined. A	590	set), starting from 0. If C<x> is 0 the result is undefined.
382	common use case is to compute the integer binary logarithm, i.e.,	591
383	floor(log2(n)). For example:	592	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		593
		594	For example:
384		595
385	ecb_ctz32 (3) = 0	596	ecb_ctz32 (3) = 0
386	ecb_ctz32 (6) = 1	597	ecb_ctz32 (6) = 1
387		598
		599	=item bool ecb_is_pot32 (uint32_t x)
		600
		601	=item bool ecb_is_pot64 (uint32_t x)
		602
		603	Returns true iff C<x> is a power of two or C<x == 0>.
		604
		605	For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
		606
		607	=item int ecb_ld32 (uint32_t x)
		608
		609	=item int ecb_ld64 (uint64_t x)
		610
		611	Returns the index of the most significant bit set in C<x>, or the number
		612	of digits the number requires in binary (so that C<< 2**ld <= x <
		613	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		614	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		615	example to see how many bits a certain number requires to be encoded.
		616
		617	This function is similar to the "count leading zero bits" function, except
		618	that that one returns how many zero bits are "in front" of the number (in
		619	the given data type), while C<ecb_ld> returns how many bits the number
		620	itself requires.
		621
		622	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		623
388	=item int ecb_popcount32 (uint32_t x)	624	=item int ecb_popcount32 (uint32_t x)
389		625
		626	=item int ecb_popcount64 (uint64_t x)
		627
390	Returns the number of bits set to 1 in C<x>. For example:	628	Returns the number of bits set to 1 in C<x>.
		629
		630	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		631
		632	For example:
391		633
392	ecb_popcount32 (7) = 3	634	ecb_popcount32 (7) = 3
393	ecb_popcount32 (255) = 8	635	ecb_popcount32 (255) = 8
394		636
		637	=item uint8_t ecb_bitrev8 (uint8_t x)
		638
		639	=item uint16_t ecb_bitrev16 (uint16_t x)
		640
		641	=item uint32_t ecb_bitrev32 (uint32_t x)
		642
		643	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		644	and so on.
		645
		646	Example:
		647
		648	ecb_bitrev8 (0xa7) = 0xea
		649	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		650
395	=item uint32_t ecb_bswap16 (uint32_t x)	651	=item uint32_t ecb_bswap16 (uint32_t x)
396		652
397	=item uint32_t ecb_bswap32 (uint32_t x)	653	=item uint32_t ecb_bswap32 (uint32_t x)
398		654
		655	=item uint64_t ecb_bswap64 (uint64_t x)
		656
399	These two functions return the value of the 16-bit (32-bit) variable	657	These functions return the value of the 16-bit (32-bit, 64-bit) value
400	C<x> after reversing the order of bytes.	658	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		659	C<ecb_bswap32>).
		660
		661	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		662
		663	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		664
		665	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		666
		667	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		668
		669	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		670
		671	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
401		672
402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	673	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
403		674
404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	675	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
405		676
406	These two functions return the value of C<x> after shifting all the bits	677	These two families of functions return the value of C<x> after rotating
407	by C<count> positions to the right or left respectively.	678	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		679	(C<ecb_rotl>).
		680
		681	Current GCC versions understand these functions and usually compile them
		682	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		683	x86).
408		684
409	=back	685	=back
410		686
		687	=head2 FLOATING POINT FIDDLING
		688
		689	=over 4
		690
		691	=item ECB_INFINITY
		692
		693	Evaluates to positive infinity if supported by the platform, otherwise to
		694	a truly huge number.
		695
		696	=item ECB_NAN
		697
		698	Evaluates to a quiet NAN if supported by the platform, otherwise to
		699	C<ECB_INFINITY>.
		700
		701	=item float ecb_ldexpf (float x, int exp)
		702
		703	Same as C<ldexpf>, but always available.
		704
		705	=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
		706
		707	=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
		708
		709	These functions each take an argument in the native C<float> or C<double>
		710	type and return the IEEE 754 bit representation of it.
		711
		712	The bit representation is just as IEEE 754 defines it, i.e. the sign bit
		713	will be the most significant bit, followed by exponent and mantissa.
		714
		715	This function should work even when the native floating point format isn't
		716	IEEE compliant, of course at a speed and code size penalty, and of course
		717	also within reasonable limits (it tries to convert NaNs, infinities and
		718	denormals, but will likely convert negative zero to positive zero).
		719
		720	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		721	be able to optimise away this function completely.
		722
		723	These functions can be helpful when serialising floats to the network - you
		724	can serialise the return value like a normal uint32_t/uint64_t.
		725
		726	Another use for these functions is to manipulate floating point values
		727	directly.
		728
		729	Silly example: toggle the sign bit of a float.
		730
		731	/* On gcc-4.7 on amd64, */
		732	/* this results in a single add instruction to toggle the bit, and 4 extra */
		733	/* instructions to move the float value to an integer register and back. */
		734
		735	x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
		736
		737	=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
		738
		739	=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
		740
		741	=item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
		742
		743	The reverse operation of the previous function - takes the bit
		744	representation of an IEEE binary16, binary32 or binary64 number and
		745	converts it to the native C<float> or C<double> format.
		746
		747	This function should work even when the native floating point format isn't
		748	IEEE compliant, of course at a speed and code size penalty, and of course
		749	also within reasonable limits (it tries to convert normals and denormals,
		750	and might be lucky for infinities, and with extraordinary luck, also for
		751	negative zero).
		752
		753	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		754	be able to optimise away this function completely.
		755
		756	=back
		757
411	=head2 ARITHMETIC	758	=head2 ARITHMETIC
412		759
413	=over 4	760	=over 4
414		761
415	=item x = ecb_mod (m, n)	762	=item x = ecb_mod (m, n)
416		763
417	Returns the positive remainder of the modulo operation between C<m> and	764	Returns C<m> modulo C<n>, which is the same as the positive remainder
		765	of the division operation between C<m> and C<n>, using floored
418	C<n>. Unlike the C modulo operator C<%>, this function ensures that the	766	division. Unlike the C remainder operator C<%>, this function ensures that
419	return value is always positive).	767	the return value is always positive and that the two numbers I<m> and
		768	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		769	C<ecb_mod> implements the mathematical modulo operation, which is missing
		770	in the language.
420		771
421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	772	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
422	negatable, that is, both C<m> and C<-m> must be representable in its	773	negatable, that is, both C<m> and C<-m> must be representable in its
423	type.	774	type (this typically excludes the minimum signed integer value, the same
		775	limitation as for C</> and C<%> in C).
		776
		777	Current GCC versions compile this into an efficient branchless sequence on
		778	almost all CPUs.
		779
		780	For example, when you want to rotate forward through the members of an
		781	array for increasing C<m> (which might be negative), then you should use
		782	C<ecb_mod>, as the C<%> operator might give either negative results, or
		783	change direction for negative values:
		784
		785	for (m = -100; m <= 100; ++m)
		786	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		787
		788	=item x = ecb_div_rd (val, div)
		789
		790	=item x = ecb_div_ru (val, div)
		791
		792	Returns C<val> divided by C<div> rounded down or up, respectively.
		793	C<val> and C<div> must have integer types and C<div> must be strictly
		794	positive. Note that these functions are implemented with macros in C
		795	and with function templates in C++.
424		796
425	=back	797	=back
426		798
427	=head2 UTILITY	799	=head2 UTILITY
428		800
429	=over 4	801	=over 4
430		802
431	=item element_count = ecb_array_length (name) [MACRO]	803	=item element_count = ecb_array_length (name)
432		804
433	Returns the number of elements in the array C<name>. For example:	805	Returns the number of elements in the array C<name>. For example:
434		806
435	int primes[] = { 2, 3, 5, 7, 11 };	807	int primes[] = { 2, 3, 5, 7, 11 };
436	int sum = 0;	808	int sum = 0;
…		…
438	for (i = 0; i < ecb_array_length (primes); i++)	810	for (i = 0; i < ecb_array_length (primes); i++)
439	sum += primes [i];	811	sum += primes [i];
440		812
441	=back	813	=back
442		814
		815	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
443		816
		817	These symbols need to be defined before including F<ecb.h> the first time.
		818
		819	=over 4
		820
		821	=item ECB_NO_THREADS
		822
		823	If F<ecb.h> is never used from multiple threads, then this symbol can
		824	be defined, in which case memory fences (and similar constructs) are
		825	completely removed, leading to more efficient code and fewer dependencies.
		826
		827	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		828
		829	=item ECB_NO_SMP
		830
		831	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		832	multiple threads, but never concurrently (e.g. if the system the program
		833	runs on has only a single CPU with a single core, no hyperthreading and so
		834	on), then this symbol can be defined, leading to more efficient code and
		835	fewer dependencies.
		836
		837	=item ECB_NO_LIBM
		838
		839	When defined to C<1>, do not export any functions that might introduce
		840	dependencies on the math library (usually called F<-lm>) - these are
		841	marked with [-UECB_NO_LIBM].
		842
		843	=head1 UNDOCUMENTED FUNCTIONALITY
		844
		845	F<ecb.h> is full of undocumented functionality as well, some of which is
		846	intended to be internal-use only, some of which we forgot to document, and
		847	some of which we hide because we are not sure we will keep the interface
		848	stable.
		849
		850	While you are welcome to rummage around and use whatever you find useful
		851	(we can't stop you), keep in mind that we will change undocumented
		852	functionality in incompatible ways without thinking twice, while we are
		853	considerably more conservative with documented things.
		854
		855	=head1 AUTHORS
		856
		857	C<libecb> is designed and maintained by:
		858
		859	Emanuele Giaquinta <e.giaquinta@glauco.it>
		860	Marc Alexander Lehmann <schmorp@schmorp.de>
		861
		862	=back
		863
		864

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Comparing libecb/ecb.pod (file contents): Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC vs. Revision 1.68 by root, Fri Feb 20 16:50:08 2015 UTC

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Comparing libecb/ecb.pod (file contents):
Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC vs.
Revision 1.68 by root, Fri Feb 20 16:50:08 2015 UTC