[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 1.85 by root, Mon Jan 20 21:13:38 2020 UTC

…		…
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 many compiler built-ins,
16	with replacement functions for other compilers. In addition to this,	16	together with replacement functions for other compilers. In addition
17	it provides a number of other lowlevel C utilities, such as endianness	17	to this, it provides a number of other lowlevel C utilities, such as
18	detection, byte swapping or bit rotations.	18	endianness 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
21	system, but aren't.	21	system, but aren't, implemented as efficient as possible with GCC (clang,
		22	msvc...), and still 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
		59	=head2 TYPES / TYPE SUPPORT
		60
		61	ecb.h makes sure that the following types are defined (in the expected way):
		62
		63	int8_t uint8_
		64	int16_t uint16_t
		65	int32_t uint32_
		66	int64_t uint64_t
		67	int_fast8_t uint_fast8_t
		68	int_fast16_t uint_fast16_t
		69	int_fast32_t uint_fast32_t
		70	int_fast64_t uint_fast64_t
		71	intptr_t uintptr_t
		72
		73	The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
		74	platform (currently C<4> or C<8>) and can be used in preprocessor
		75	expressions.
		76
		77	For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>.
		78
		79	=head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
		80
		81	All the following symbols expand to an expression that can be tested in
		82	preprocessor instructions as well as treated as a boolean (use C<!!> to
		83	ensure it's either C<0> or C<1> if you need that).
		84
		85	=over 4
		86
		87	=item ECB_C
		88
		89	True if the implementation defines the C<__STDC__> macro to a true value,
		90	while not claiming to be C++, i..e C, but not C++.
		91
		92	=item ECB_C99
		93
		94	True if the implementation claims to be compliant to C99 (ISO/IEC
		95	9899:1999) or any later version, while not claiming to be C++.
		96
		97	Note that later versions (ECB_C11) remove core features again (for
		98	example, variable length arrays).
		99
		100	=item ECB_C11, ECB_C17
		101
		102	True if the implementation claims to be compliant to C11/C17 (ISO/IEC
		103	9899:2011, :20187) or any later version, while not claiming to be C++.
		104
		105	=item ECB_CPP
		106
		107	True if the implementation defines the C<__cplusplus__> macro to a true
		108	value, which is typically true for C++ compilers.
		109
		110	=item ECB_CPP11, ECB_CPP14, ECB_CPP17
		111
		112	True if the implementation claims to be compliant to C++11/C++14/C++17
		113	(ISO/IEC 14882:2011, :2014, :2017) or any later version.
		114
		115	Note that many C++20 features will likely have their own feature test
		116	macros (see e.g. L<http://eel.is/c++draft/cpp.predefined#1.8>).
		117
		118	=item ECB_OPTIMIZE_SIZE
		119
		120	Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
		121	can also be defined before including F<ecb.h>, in which case it will be
		122	unchanged.
		123
		124	=item ECB_GCC_VERSION (major, minor)
		125
		126	Expands to a true value (suitable for testing by the preprocessor) if the
		127	compiler used is GNU C and the version is the given version, or higher.
		128
		129	This macro tries to return false on compilers that claim to be GCC
		130	compatible but aren't.
		131
		132	=item ECB_EXTERN_C
		133
		134	Expands to C<extern "C"> in C++, and a simple C<extern> in C.
		135
		136	This can be used to declare a single external C function:
		137
		138	ECB_EXTERN_C int printf (const char *format, ...);
		139
		140	=item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
		141
		142	These two macros can be used to wrap multiple C<extern "C"> definitions -
		143	they expand to nothing in C.
		144
		145	They are most useful in header files:
		146
		147	ECB_EXTERN_C_BEG
		148
		149	int mycfun1 (int x);
		150	int mycfun2 (int x);
		151
		152	ECB_EXTERN_C_END
		153
		154	=item ECB_STDFP
		155
		156	If this evaluates to a true value (suitable for testing by the
		157	preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
		158	and double/binary64 representations internally I<and> the endianness of
		159	both types match the endianness of C<uint32_t> and C<uint64_t>.
		160
		161	This means you can just copy the bits of a C<float> (or C<double>) to an
		162	C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
		163	without having to think about format or endianness.
		164
		165	This is true for basically all modern platforms, although F<ecb.h> might
		166	not be able to deduce this correctly everywhere and might err on the safe
		167	side.
		168
		169	=item ECB_AMD64, ECB_AMD64_X32
		170
		171	These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
		172	ABI, respectively, and undefined elsewhere.
		173
		174	The designers of the new X32 ABI for some inexplicable reason decided to
		175	make it look exactly like amd64, even though it's completely incompatible
		176	to that ABI, breaking about every piece of software that assumed that
		177	C<__x86_64> stands for, well, the x86-64 ABI, making these macros
		178	necessary.
		179
		180	=back
		181
		182	=head2 MACRO TRICKERY
		183
		184	=over 4
		185
		186	=item ECB_CONCAT (a, b)
		187
		188	Expands any macros in C<a> and C<b>, then concatenates the result to form
		189	a single token. This is mainly useful to form identifiers from components,
		190	e.g.:
		191
		192	#define S1 str
		193	#define S2 cpy
		194
		195	ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
		196
		197	=item ECB_STRINGIFY (arg)
		198
		199	Expands any macros in C<arg> and returns the stringified version of
		200	it. This is mainly useful to get the contents of a macro in string form,
		201	e.g.:
		202
		203	#define SQL_LIMIT 100
		204	sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
		205
		206	=item ECB_STRINGIFY_EXPR (expr)
		207
		208	Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
		209	is a valid expression. This is useful to catch typos or cases where the
		210	macro isn't available:
		211
		212	#include <errno.h>
		213
		214	ECB_STRINGIFY (EDOM); // "33" (on my system at least)
		215	ECB_STRINGIFY_EXPR (EDOM); // "33"
		216
		217	// now imagine we had a typo:
		218
		219	ECB_STRINGIFY (EDAM); // "EDAM"
		220	ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
		221
		222	=back
		223
58	=head2 GCC ATTRIBUTES	224	=head2 ATTRIBUTES
59		225
60	blabla where to put, what others	226	A major part of libecb deals with additional attributes that can be
		227	assigned to functions, variables and sometimes even types - much like
		228	C<const> or C<volatile> in C. They are implemented using either GCC
		229	attributes or other compiler/language specific features. Attributes
		230	declarations must be put before the whole declaration:
61		231
62	=over 4	232	ecb_const int mysqrt (int a);
		233	ecb_unused int i;
63		234
64	=item ecb_attribute ((attrs...))	235	=over 4
65
66	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
67	nothing on other compilers, so the effect is that only GCC sees these.
68
69	Example: use the C<deprecated> attribute on a function.
70
71	ecb_attribute((__deprecated__)) void
72	do_not_use_me_anymore (void);
73		236
74	=item ecb_unused	237	=item ecb_unused
75		238
76	Marks a function or a variable as "unused", which simply suppresses a	239	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.	240	warning by the compiler when it detects it as unused. This is useful when
78	declare a variable but do not always use it:	241	you e.g. declare a variable but do not always use it:
79		242
80	{	243	{
81	int var ecb_unused;	244	ecb_unused int var;
82		245
83	#ifdef SOMECONDITION	246	#ifdef SOMECONDITION
84	var = ...;	247	var = ...;
85	return var;	248	return var;
86	#else	249	#else
87	return 0;	250	return 0;
88	#endif	251	#endif
89	}	252	}
90		253
		254	=item ecb_deprecated
		255
		256	Similar to C<ecb_unused>, but marks a function, variable or type as
		257	deprecated. This makes some compilers warn when the type is used.
		258
		259	=item ecb_deprecated_message (message)
		260
		261	Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
		262	used instead of a generic depreciation message when the object is being
		263	used.
		264
		265	=item ecb_inline
		266
		267	Expands either to (a compiler-specific equivalent of) C<static inline> or
		268	to just C<static>, if inline isn't supported. It should be used to declare
		269	functions that should be inlined, for code size or speed reasons.
		270
		271	Example: inline this function, it surely will reduce codesize.
		272
		273	ecb_inline int
		274	negmul (int a, int b)
		275	{
		276	return - (a * b);
		277	}
		278
91	=item ecb_noinline	279	=item ecb_noinline
92		280
93	Prevent a function from being inlined - it might be optimised away, but	281	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	282	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.	283	is rarely called and large enough for inlining not to be helpful.
96		284
97	=item ecb_noreturn	285	=item ecb_noreturn
98		286
…		…
105	{	293	{
106	puts (errline);	294	puts (errline);
107	abort ();	295	abort ();
108	}	296	}
109		297
110	In this case, the compiler would probbaly be smart enough to decude it on	298	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.	299	its own, so this is mainly useful for declarations.
		300
		301	=item ecb_restrict
		302
		303	Expands to the C<restrict> keyword or equivalent on compilers that support
		304	them, and to nothing on others. Must be specified on a pointer type or
		305	an array index to indicate that the memory doesn't alias with any other
		306	restricted pointer in the same scope.
		307
		308	Example: multiply a vector, and allow the compiler to parallelise the
		309	loop, because it knows it doesn't overwrite input values.
		310
		311	void
		312	multiply (ecb_restrict float *src,
		313	ecb_restrict float *dst,
		314	int len, float factor)
		315	{
		316	int i;
		317
		318	for (i = 0; i < len; ++i)
		319	dst [i] = src [i] * factor;
		320	}
112		321
113	=item ecb_const	322	=item ecb_const
114		323
115	Declares that the function only depends on the values of it's arguments,	324	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	325	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	326	any memory any arguments might point to, global variables, or call any
118	non-const functions. It also must not have any side effects.	327	non-const functions. It also must not have any side effects.
119		328
120	Such a function can be optimised much more aggressively by the compiler -	329	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	330	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	331	a single call, which wouldn't be possible if the compiler would have to
123	expect any side effects.	332	expect any side effects.
124		333
125	It is best suited for functions in the sense of mathematical functions,	334	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.	335	such as a function returning the square root of its input argument.
127		336
128	Not suited would be a function that calculates the hash of some memory	337	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	338	area you pass in, prints some messages or looks at a global variable to
130	decide on rounding.	339	decide on rounding.
131		340
…		…
154	possible.	363	possible.
155		364
156	The compiler reacts by trying to place hot functions near to each other in	365	The compiler reacts by trying to place hot functions near to each other in
157	memory.	366	memory.
158		367
159	Whether a function is hot or not often depend son the whole program,	368	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	369	and less on the function itself. C<ecb_cold> is likely more useful in
161	practise.	370	practise.
162		371
163	=item ecb_cold	372	=item ecb_cold
164		373
…		…
169		378
170	In addition to placing cold functions together (or at least away from hot	379	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	380	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	381	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	382	leading to calls to those functions can automatically be marked as if
174	C<ecb_unlikel> had been used to reach them.	383	C<ecb_expect_false> had been used to reach them.
175		384
176	Good examples for such functions would be error reporting functions, or	385	Good examples for such functions would be error reporting functions, or
177	functions only called in exceptional or rare cases.	386	functions only called in exceptional or rare cases.
178		387
179	=item ecb_artificial	388	=item ecb_artificial
180		389
181	Declares the function as "artificial", in this case meaning that this	390	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	391	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	392	- 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	393	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.	394	usually so helpful, especially when it's inlined to just a few instructions.
186		395
187	Marking them as artificial will instruct the debugger about just this,	396	Marking them as artificial will instruct the debugger about just this,
…		…
207		416
208	=head2 OPTIMISATION HINTS	417	=head2 OPTIMISATION HINTS
209		418
210	=over 4	419	=over 4
211		420
212	=item bool ecb_is_constant(expr)	421	=item bool ecb_is_constant (expr)
213		422
214	Returns true iff the expression can be deduced to be a compile-time	423	Returns true iff the expression can be deduced to be a compile-time
215	constant, and false otherwise.	424	constant, and false otherwise.
216		425
217	For example, when you have a C<rndm16> function that returns a 16 bit	426	For example, when you have a C<rndm16> function that returns a 16 bit
…		…
235	return is_constant (n) && !(n & (n - 1))	444	return is_constant (n) && !(n & (n - 1))
236	? rndm16 () & (num - 1)	445	? rndm16 () & (num - 1)
237	: (n * (uint32_t)rndm16 ()) >> 16;	446	: (n * (uint32_t)rndm16 ()) >> 16;
238	}	447	}
239		448
240	=item bool ecb_expect (expr, value)	449	=item ecb_expect (expr, value)
241		450
242	Evaluates C<expr> and returns it. In addition, it tells the compiler that	451	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	452	the C<expr> evaluates to C<value> a lot, which can be used for static
244	branch optimisations.	453	branch optimisations.
245		454
246	Usually, you want to use the more intuitive C<ecb_likely> and	455	Usually, you want to use the more intuitive C<ecb_expect_true> and
247	C<ecb_unlikely> functions instead.	456	C<ecb_expect_false> functions instead.
248		457
		458	=item bool ecb_expect_true (cond)
		459
249	=item bool ecb_likely (cond)	460	=item bool ecb_expect_false (cond)
250
251	=item bool ecb_unlikely (cond)
252		461
253	These two functions expect a expression that is true or false and return	462	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	463	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:	464	other conditional statement, it will not change the program:
256		465
257	/* these two do the same thing */	466	/* these two do the same thing */
258	if (some_condition) ...;	467	if (some_condition) ...;
259	if (ecb_likely (some_condition)) ...;	468	if (ecb_expect_true (some_condition)) ...;
260		469
261	However, by using C<ecb_likely>, you tell the compiler that the condition	470	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	471	condition is likely to be true (and for C<ecb_expect_false>, that it is
263	true).	472	unlikely to be true).
264		473
265	For example, when you check for a null pointer and expect this to be a	474	For example, when you check for a null pointer and expect this to be a
266	rare, exceptional, case, then use C<ecb_unlikely>:	475	rare, exceptional, case, then use C<ecb_expect_false>:
267		476
268	void my_free (void *ptr)	477	void my_free (void *ptr)
269	{	478	{
270	if (ecb_unlikely (ptr == 0))	479	if (ecb_expect_false (ptr == 0))
271	return;	480	return;
272	}	481	}
273		482
274	Consequent use of these functions to mark away exceptional cases or to	483	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	484	tell the compiler what the hot path through a function is can increase
276	performance considerably.	485	performance considerably.
		486
		487	You might know these functions under the name C<likely> and C<unlikely>
		488	- while these are common aliases, we find that the expect name is easier
		489	to understand when quickly skimming code. If you wish, you can use
		490	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		491	C<ecb_expect_false> - these are simply aliases.
277		492
278	A very good example is in a function that reserves more space for some	493	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) -	494	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	495	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:	496	you expect that most checks will turn out to be false:
282		497
283	/* make sure we have "size" extra room in our buffer */	498	/* make sure we have "size" extra room in our buffer */
284	ecb_inline void	499	ecb_inline void
285	reserve (int size)	500	reserve (int size)
286	{	501	{
287	if (ecb_unlikely (current + size > end))	502	if (ecb_expect_false (current + size > end))
288	real_reserve_method (size); /* presumably noinline */	503	real_reserve_method (size); /* presumably noinline */
289	}	504	}
290		505
291	=item bool ecb_assume (cond)	506	=item ecb_assume (cond)
292		507
293	Try to tell the compiler that some condition is true, even if it's not	508	Tries to tell the compiler that some condition is true, even if it's not
294	obvious.	509	obvious. This is not a function, but a statement: it cannot be used in
		510	another expression.
295		511
296	This can be used to teach the compiler about invariants or other	512	This can be used to teach the compiler about invariants or other
297	conditions that might improve code generation, but which are impossible to	513	conditions that might improve code generation, but which are impossible to
298	deduce form the code itself.	514	deduce form the code itself.
299		515
300	For example, the example reservation function from the C<ecb_unlikely>	516	For example, the example reservation function from the C<ecb_expect_false>
301	description could be written thus (only C<ecb_assume> was added):	517	description could be written thus (only C<ecb_assume> was added):
302		518
303	ecb_inline void	519	ecb_inline void
304	reserve (int size)	520	reserve (int size)
305	{	521	{
306	if (ecb_unlikely (current + size > end))	522	if (ecb_expect_false (current + size > end))
307	real_reserve_method (size); /* presumably noinline */	523	real_reserve_method (size); /* presumably noinline */
308		524
309	ecb_assume (current + size <= end);	525	ecb_assume (current + size <= end);
310	}	526	}
311		527
…		…
316		532
317	Then the compiler I<might> be able to optimise out the second call	533	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	534	completely, as it knows that C<< current + 1 > end >> is false and the
319	call will never be executed.	535	call will never be executed.
320		536
321	=item bool ecb_unreachable ()	537	=item ecb_unreachable ()
322		538
323	This function does nothing itself, except tell the compiler that it will	539	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	540	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.	541	function can be used to implement C<ecb_assume> or similar functionality.
326		542
327	=item bool ecb_prefetch (addr, rw, locality)	543	=item ecb_prefetch (addr, rw, locality)
328		544
329	Tells the compiler to try to prefetch memory at the given C<addr>ess	545	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	546	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	547	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	548	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	549	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>	550	need to be accessible (it could be a null pointer for example), but C<rw>
335	and C<locality> must be compile-time constants.	551	and C<locality> must be compile-time constants.
336		552
		553	This is a statement, not a function: you cannot use it as part of an
		554	expression.
		555
337	An obvious way to use this is to prefetch some data far away, in a big	556	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,	557	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.	558	in the hope that it will be ready when the CPU arrives at that location.
340		559
341	int sum = 0;	560	int sum = 0;
…		…
360	After processing the node, (part of) the next node might already be in	579	After processing the node, (part of) the next node might already be in
361	cache.	580	cache.
362		581
363	=back	582	=back
364		583
365	=head2 BIT FIDDLING / BITSTUFFS	584	=head2 BIT FIDDLING / BIT WIZARDRY
366		585
367	=over 4	586	=over 4
368		587
369	=item bool ecb_big_endian ()	588	=item bool ecb_big_endian ()
370		589
…		…
372		591
373	These two functions return true if the byte order is big endian	592	These two functions return true if the byte order is big endian
374	(most-significant byte first) or little endian (least-significant byte	593	(most-significant byte first) or little endian (least-significant byte
375	first) respectively.	594	first) respectively.
376		595
		596	On systems that are neither, their return values are unspecified.
		597
377	=item int ecb_ctz32 (uint32_t x)	598	=item int ecb_ctz32 (uint32_t x)
378		599
		600	=item int ecb_ctz64 (uint64_t x)
		601
		602	=item int ecb_ctz (T x) [C++]
		603
379	Returns the index of the least significant bit set in C<x> (or	604	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	605	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	606	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.,	607
383	floor(log2(n)). For example:	608	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		609
		610	The overloaded C++ C<ecb_ctz> function supports C<uint8_t>, C<uint16_t>,
		611	C<uint32_t> and C<uint64_t> types.
		612
		613	For example:
384		614
385	ecb_ctz32 (3) = 0	615	ecb_ctz32 (3) = 0
386	ecb_ctz32 (6) = 1	616	ecb_ctz32 (6) = 1
387		617
		618	=item bool ecb_is_pot32 (uint32_t x)
		619
		620	=item bool ecb_is_pot64 (uint32_t x)
		621
		622	=item bool ecb_is_pot (T x) [C++]
		623
		624	Returns true iff C<x> is a power of two or C<x == 0>.
		625
		626	For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
		627
		628	The overloaded C++ C<ecb_is_pot> function supports C<uint8_t>, C<uint16_t>,
		629	C<uint32_t> and C<uint64_t> types.
		630
		631	=item int ecb_ld32 (uint32_t x)
		632
		633	=item int ecb_ld64 (uint64_t x)
		634
		635	=item int ecb_ld64 (T x) [C++]
		636
		637	Returns the index of the most significant bit set in C<x>, or the number
		638	of digits the number requires in binary (so that C<< 2**ld <= x <
		639	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		640	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		641	example to see how many bits a certain number requires to be encoded.
		642
		643	This function is similar to the "count leading zero bits" function, except
		644	that that one returns how many zero bits are "in front" of the number (in
		645	the given data type), while C<ecb_ld> returns how many bits the number
		646	itself requires.
		647
		648	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		649
		650	The overloaded C++ C<ecb_ld> function supports C<uint8_t>, C<uint16_t>,
		651	C<uint32_t> and C<uint64_t> types.
		652
388	=item int ecb_popcount32 (uint32_t x)	653	=item int ecb_popcount32 (uint32_t x)
389		654
		655	=item int ecb_popcount64 (uint64_t x)
		656
		657	=item int ecb_popcount (T x) [C++]
		658
390	Returns the number of bits set to 1 in C<x>. For example:	659	Returns the number of bits set to 1 in C<x>.
		660
		661	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		662
		663	The overloaded C++ C<ecb_popcount> function supports C<uint8_t>, C<uint16_t>,
		664	C<uint32_t> and C<uint64_t> types.
		665
		666	For example:
391		667
392	ecb_popcount32 (7) = 3	668	ecb_popcount32 (7) = 3
393	ecb_popcount32 (255) = 8	669	ecb_popcount32 (255) = 8
394		670
		671	=item uint8_t ecb_bitrev8 (uint8_t x)
		672
		673	=item uint16_t ecb_bitrev16 (uint16_t x)
		674
		675	=item uint32_t ecb_bitrev32 (uint32_t x)
		676
		677	=item T ecb_bitrev (T x) [C++]
		678
		679	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		680	and so on.
		681
		682	The overloaded C++ C<ecb_bitrev> function supports C<uint8_t>, C<uint16_t> and C<uint32_t> types.
		683
		684	Example:
		685
		686	ecb_bitrev8 (0xa7) = 0xea
		687	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		688
		689	=item T ecb_bitrev (T x) [C++]
		690
		691	Overloaded C++ bitrev function.
		692
		693	C<T> must be one of C<uint8_t>, C<uint16_t> or C<uint32_t>.
		694
395	=item uint32_t ecb_bswap16 (uint32_t x)	695	=item uint32_t ecb_bswap16 (uint32_t x)
396		696
397	=item uint32_t ecb_bswap32 (uint32_t x)	697	=item uint32_t ecb_bswap32 (uint32_t x)
398		698
		699	=item uint64_t ecb_bswap64 (uint64_t x)
		700
		701	=item T ecb_bswap (T x)
		702
399	These two functions return the value of the 16-bit (32-bit) variable	703	These functions return the value of the 16-bit (32-bit, 64-bit) value
400	C<x> after reversing the order of bytes.	704	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		705	C<ecb_bswap32>).
		706
		707	The overloaded C++ C<ecb_bswap> function supports C<uint8_t>, C<uint16_t>,
		708	C<uint32_t> and C<uint64_t> types.
		709
		710	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		711
		712	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		713
		714	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		715
		716	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		717
		718	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		719
		720	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
401		721
402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	722	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
403		723
404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	724	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
405		725
406	These two functions return the value of C<x> after shifting all the bits	726	These two families of functions return the value of C<x> after rotating
407	by C<count> positions to the right or left respectively.	727	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		728	(C<ecb_rotl>).
		729
		730	Current GCC/clang versions understand these functions and usually compile
		731	them to "optimal" code (e.g. a single C<rol> or a combination of C<shld>
		732	on x86).
		733
		734	=item T ecb_rotl (T x, unsigned int count) [C++]
		735
		736	=item T ecb_rotr (T x, unsigned int count) [C++]
		737
		738	Overloaded C++ rotl/rotr functions.
		739
		740	C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
408		741
409	=back	742	=back
410		743
		744	=head2 HOST ENDIANNESS CONVERSION
		745
		746	=over 4
		747
		748	=item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v)
		749
		750	=item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v)
		751
		752	=item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v)
		753
		754	=item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v)
		755
		756	=item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v)
		757
		758	=item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v)
		759
		760	Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order.
		761
		762	The naming convention is C<ecb_>(C<be>\|C<le>)C<_u>C<16\|32\|64>C<_to_host>,
		763	where C<be> and C<le> stand for big endian and little endian, respectively.
		764
		765	=item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v)
		766
		767	=item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v)
		768
		769	=item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v)
		770
		771	=item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v)
		772
		773	=item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v)
		774
		775	=item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v)
		776
		777	Like above, but converts I<from> host byte order to the specified
		778	endianness.
		779
		780	=back
		781
		782	In C++ the following additional template functions are supported:
		783
		784	=over 4
		785
		786	=item T ecb_be_to_host (T v)
		787
		788	=item T ecb_le_to_host (T v)
		789
		790	=item T ecb_host_to_be (T v)
		791
		792	=item T ecb_host_to_le (T v)
		793
		794	These functions work like their C counterparts, above, but use templates,
		795	which make them useful in generic code.
		796
		797	C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>
		798	(so unlike their C counterparts, there is a version for C<uint8_t>, which
		799	again can be useful in generic code).
		800
		801	=head2 UNALIGNED LOAD/STORE
		802
		803	These function load or store unaligned multi-byte values.
		804
		805	=over 4
		806
		807	=item uint_fast16_t ecb_peek_u16_u (const void *ptr)
		808
		809	=item uint_fast32_t ecb_peek_u32_u (const void *ptr)
		810
		811	=item uint_fast64_t ecb_peek_u64_u (const void *ptr)
		812
		813	These functions load an unaligned, unsigned 16, 32 or 64 bit value from
		814	memory.
		815
		816	=item uint_fast16_t ecb_peek_be_u16_u (const void *ptr)
		817
		818	=item uint_fast32_t ecb_peek_be_u32_u (const void *ptr)
		819
		820	=item uint_fast64_t ecb_peek_be_u64_u (const void *ptr)
		821
		822	=item uint_fast16_t ecb_peek_le_u16_u (const void *ptr)
		823
		824	=item uint_fast32_t ecb_peek_le_u32_u (const void *ptr)
		825
		826	=item uint_fast64_t ecb_peek_le_u64_u (const void *ptr)
		827
		828	Like above, but additionally convert from big endian (C<be>) or little
		829	endian (C<le>) byte order to host byte order while doing so.
		830
		831	=item ecb_poke_u16_u (void *ptr, uint16_t v)
		832
		833	=item ecb_poke_u32_u (void *ptr, uint32_t v)
		834
		835	=item ecb_poke_u64_u (void *ptr, uint64_t v)
		836
		837	These functions store an unaligned, unsigned 16, 32 or 64 bit value to
		838	memory.
		839
		840	=item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v)
		841
		842	=item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v)
		843
		844	=item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v)
		845
		846	=item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v)
		847
		848	=item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v)
		849
		850	=item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v)
		851
		852	Like above, but additionally convert from host byte order to big endian
		853	(C<be>) or little endian (C<le>) byte order while doing so.
		854
		855	=back
		856
		857	In C++ the following additional template functions are supported:
		858
		859	=over 4
		860
		861	=item T ecb_peek<T> (const void *ptr)
		862
		863	=item T ecb_peek_be<T> (const void *ptr)
		864
		865	=item T ecb_peek_le<T> (const void *ptr)
		866
		867	=item T ecb_peek_u<T> (const void *ptr)
		868
		869	=item T ecb_peek_be_u<T> (const void *ptr)
		870
		871	=item T ecb_peek_le_u<T> (const void *ptr)
		872
		873	Similarly to their C counterparts, these functions load an unsigned 8, 16,
		874	32 or 64 bit value from memory, with optional conversion from big/little
		875	endian.
		876
		877	Since the type cannot be deduced, it has to be specified explicitly, e.g.
		878
		879	uint_fast16_t v = ecb_peek<uint16_t> (ptr);
		880
		881	C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
		882
		883	Unlike their C counterparts, these functions support 8 bit quantities
		884	(C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
		885	all of which hopefully makes them more useful in generic code.
		886
		887	=item ecb_poke (void *ptr, T v)
		888
		889	=item ecb_poke_be (void *ptr, T v)
		890
		891	=item ecb_poke_le (void *ptr, T v)
		892
		893	=item ecb_poke_u (void *ptr, T v)
		894
		895	=item ecb_poke_be_u (void *ptr, T v)
		896
		897	=item ecb_poke_le_u (void *ptr, T v)
		898
		899	Again, similarly to their C counterparts, these functions store an
		900	unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to
		901	big/little endian.
		902
		903	C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
		904
		905	Unlike their C counterparts, these functions support 8 bit quantities
		906	(C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
		907	all of which hopefully makes them more useful in generic code.
		908
		909	=back
		910
		911	=head2 FLOATING POINT FIDDLING
		912
		913	=over 4
		914
		915	=item ECB_INFINITY [-UECB_NO_LIBM]
		916
		917	Evaluates to positive infinity if supported by the platform, otherwise to
		918	a truly huge number.
		919
		920	=item ECB_NAN [-UECB_NO_LIBM]
		921
		922	Evaluates to a quiet NAN if supported by the platform, otherwise to
		923	C<ECB_INFINITY>.
		924
		925	=item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
		926
		927	Same as C<ldexpf>, but always available.
		928
		929	=item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
		930
		931	=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
		932
		933	=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
		934
		935	These functions each take an argument in the native C<float> or C<double>
		936	type and return the IEEE 754 bit representation of it (binary16/half,
		937	binary32/single or binary64/double precision).
		938
		939	The bit representation is just as IEEE 754 defines it, i.e. the sign bit
		940	will be the most significant bit, followed by exponent and mantissa.
		941
		942	This function should work even when the native floating point format isn't
		943	IEEE compliant, of course at a speed and code size penalty, and of course
		944	also within reasonable limits (it tries to convert NaNs, infinities and
		945	denormals, but will likely convert negative zero to positive zero).
		946
		947	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		948	be able to optimise away this function completely.
		949
		950	These functions can be helpful when serialising floats to the network - you
		951	can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
		952
		953	Another use for these functions is to manipulate floating point values
		954	directly.
		955
		956	Silly example: toggle the sign bit of a float.
		957
		958	/* On gcc-4.7 on amd64, */
		959	/* this results in a single add instruction to toggle the bit, and 4 extra */
		960	/* instructions to move the float value to an integer register and back. */
		961
		962	x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
		963
		964	=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
		965
		966	=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
		967
		968	=item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
		969
		970	The reverse operation of the previous function - takes the bit
		971	representation of an IEEE binary16, binary32 or binary64 number (half,
		972	single or double precision) and converts it to the native C<float> or
		973	C<double> format.
		974
		975	This function should work even when the native floating point format isn't
		976	IEEE compliant, of course at a speed and code size penalty, and of course
		977	also within reasonable limits (it tries to convert normals and denormals,
		978	and might be lucky for infinities, and with extraordinary luck, also for
		979	negative zero).
		980
		981	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		982	be able to optimise away this function completely.
		983
		984	=item uint16_t ecb_binary32_to_binary16 (uint32_t x)
		985
		986	=item uint32_t ecb_binary16_to_binary32 (uint16_t x)
		987
		988	Convert a IEEE binary32/single precision to binary16/half format, and vice
		989	versa, handling all details (round-to-nearest-even, subnormals, infinity
		990	and NaNs) correctly.
		991
		992	These are functions are available under C<-DECB_NO_LIBM>, since
		993	they do not rely on the platform floating point format. The
		994	C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
		995	usually what you want.
		996
		997	=back
		998
411	=head2 ARITHMETIC	999	=head2 ARITHMETIC
412		1000
413	=over 4	1001	=over 4
414		1002
415	=item x = ecb_mod (m, n)	1003	=item x = ecb_mod (m, n)
416		1004
417	Returns the positive remainder of the modulo operation between C<m> and	1005	Returns C<m> modulo C<n>, which is the same as the positive remainder
		1006	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	1007	division. Unlike the C remainder operator C<%>, this function ensures that
419	return value is always positive).	1008	the return value is always positive and that the two numbers I<m> and
		1009	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		1010	C<ecb_mod> implements the mathematical modulo operation, which is missing
		1011	in the language.
420		1012
421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	1013	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	1014	negatable, that is, both C<m> and C<-m> must be representable in its
423	type.	1015	type (this typically excludes the minimum signed integer value, the same
		1016	limitation as for C</> and C<%> in C).
		1017
		1018	Current GCC/clang versions compile this into an efficient branchless
		1019	sequence on almost all CPUs.
		1020
		1021	For example, when you want to rotate forward through the members of an
		1022	array for increasing C<m> (which might be negative), then you should use
		1023	C<ecb_mod>, as the C<%> operator might give either negative results, or
		1024	change direction for negative values:
		1025
		1026	for (m = -100; m <= 100; ++m)
		1027	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		1028
		1029	=item x = ecb_div_rd (val, div)
		1030
		1031	=item x = ecb_div_ru (val, div)
		1032
		1033	Returns C<val> divided by C<div> rounded down or up, respectively.
		1034	C<val> and C<div> must have integer types and C<div> must be strictly
		1035	positive. Note that these functions are implemented with macros in C
		1036	and with function templates in C++.
424		1037
425	=back	1038	=back
426		1039
427	=head2 UTILITY	1040	=head2 UTILITY
428		1041
429	=over 4	1042	=over 4
430		1043
431	=item element_count = ecb_array_length (name) [MACRO]	1044	=item element_count = ecb_array_length (name)
432		1045
433	Returns the number of elements in the array C<name>. For example:	1046	Returns the number of elements in the array C<name>. For example:
434		1047
435	int primes[] = { 2, 3, 5, 7, 11 };	1048	int primes[] = { 2, 3, 5, 7, 11 };
436	int sum = 0;	1049	int sum = 0;
…		…
438	for (i = 0; i < ecb_array_length (primes); i++)	1051	for (i = 0; i < ecb_array_length (primes); i++)
439	sum += primes [i];	1052	sum += primes [i];
440		1053
441	=back	1054	=back
442		1055
		1056	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
443		1057
		1058	These symbols need to be defined before including F<ecb.h> the first time.
		1059
		1060	=over 4
		1061
		1062	=item ECB_NO_THREADS
		1063
		1064	If F<ecb.h> is never used from multiple threads, then this symbol can
		1065	be defined, in which case memory fences (and similar constructs) are
		1066	completely removed, leading to more efficient code and fewer dependencies.
		1067
		1068	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		1069
		1070	=item ECB_NO_SMP
		1071
		1072	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		1073	multiple threads, but never concurrently (e.g. if the system the program
		1074	runs on has only a single CPU with a single core, no hyperthreading and so
		1075	on), then this symbol can be defined, leading to more efficient code and
		1076	fewer dependencies.
		1077
		1078	=item ECB_NO_LIBM
		1079
		1080	When defined to C<1>, do not export any functions that might introduce
		1081	dependencies on the math library (usually called F<-lm>) - these are
		1082	marked with [-UECB_NO_LIBM].
		1083
		1084	=back
		1085
		1086	=head1 UNDOCUMENTED FUNCTIONALITY
		1087
		1088	F<ecb.h> is full of undocumented functionality as well, some of which is
		1089	intended to be internal-use only, some of which we forgot to document, and
		1090	some of which we hide because we are not sure we will keep the interface
		1091	stable.
		1092
		1093	While you are welcome to rummage around and use whatever you find useful
		1094	(we can't stop you), keep in mind that we will change undocumented
		1095	functionality in incompatible ways without thinking twice, while we are
		1096	considerably more conservative with documented things.
		1097
		1098	=head1 AUTHORS
		1099
		1100	C<libecb> is designed and maintained by:
		1101
		1102	Emanuele Giaquinta <e.giaquinta@glauco.it>
		1103	Marc Alexander Lehmann <schmorp@schmorp.de>
		1104
		1105

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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 1.85 by root, Mon Jan 20 21:13:38 2020 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 1.85 by root, Mon Jan 20 21:13:38 2020 UTC