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

Comparing libecb/ecb.pod (file contents):
Revision 1.16 by sf-exg, Thu May 26 23:32:41 2011 UTC vs.
Revision 1.75 by root, Sat Dec 28 08:01:05 2019 UTC

…		…
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 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
		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
…		…
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>/C<cstddef>.
		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, ECB_C17
		95
		96	True if the implementation claims to be compliant to C11/C17 (ISO/IEC
		97	9899:2011, :20187) 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, ECB_CPP14, ECB_CPP17
		105
		106	True if the implementation claims to be compliant to C++11/C++14/C++17
		107	(ISO/IEC 14882:2011, :2014, :2017) 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
70		222
71	=item ecb_unused	223	=item ecb_unused
72		224
73	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
74	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.
75	declare a variable but do not always use it:	227	declare a variable but do not always use it:
76		228
77	{	229	{
78	int var ecb_unused;	230	ecb_unused int var;
79		231
80	#ifdef SOMECONDITION	232	#ifdef SOMECONDITION
81	var = ...;	233	var = ...;
82	return var;	234	return var;
83	#else	235	#else
84	return 0;	236	return 0;
85	#endif	237	#endif
86	}	238	}
87		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 (a compiler-specific equivalent of) C<static inline> or
		254	to just C<static>, if inline isn't supported. It should be used to declare
		255	functions that should be 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
88	=item ecb_noinline	265	=item ecb_noinline
89		266
90	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
91	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
92	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.
93		270
94	=item ecb_noreturn	271	=item ecb_noreturn
95		272
		273	Marks a function as "not returning, ever". Some typical functions that
		274	don't return are C<exit> or C<abort> (which really works hard to not
		275	return), and now you can make your own:
		276
		277	ecb_noreturn void
		278	my_abort (const char *errline)
		279	{
		280	puts (errline);
		281	abort ();
		282	}
		283
		284	In this case, the compiler would probably be smart enough to deduce it on
		285	its own, so this is mainly useful for declarations.
		286
		287	=item ecb_restrict
		288
		289	Expands to the C<restrict> keyword or equivalent on compilers that support
		290	them, and to nothing on others. Must be specified on a pointer type or
		291	an array index to indicate that the memory doesn't alias with any other
		292	restricted pointer in the same scope.
		293
		294	Example: multiply a vector, and allow the compiler to parallelise the
		295	loop, because it knows it doesn't overwrite input values.
		296
		297	void
		298	multiply (ecb_restrict float *src,
		299	ecb_restrict float *dst,
		300	int len, float factor)
		301	{
		302	int i;
		303
		304	for (i = 0; i < len; ++i)
		305	dst [i] = src [i] * factor;
		306	}
		307
96	=item ecb_const	308	=item ecb_const
97		309
		310	Declares that the function only depends on the values of its arguments,
		311	much like a mathematical function. It specifically does not read or write
		312	any memory any arguments might point to, global variables, or call any
		313	non-const functions. It also must not have any side effects.
		314
		315	Such a function can be optimised much more aggressively by the compiler -
		316	for example, multiple calls with the same arguments can be optimised into
		317	a single call, which wouldn't be possible if the compiler would have to
		318	expect any side effects.
		319
		320	It is best suited for functions in the sense of mathematical functions,
		321	such as a function returning the square root of its input argument.
		322
		323	Not suited would be a function that calculates the hash of some memory
		324	area you pass in, prints some messages or looks at a global variable to
		325	decide on rounding.
		326
		327	See C<ecb_pure> for a slightly less restrictive class of functions.
		328
98	=item ecb_pure	329	=item ecb_pure
99		330
		331	Similar to C<ecb_const>, declares a function that has no side
		332	effects. Unlike C<ecb_const>, the function is allowed to examine global
		333	variables and any other memory areas (such as the ones passed to it via
		334	pointers).
		335
		336	While these functions cannot be optimised as aggressively as C<ecb_const>
		337	functions, they can still be optimised away in many occasions, and the
		338	compiler has more freedom in moving calls to them around.
		339
		340	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		341	function that calculates the MD5 sum of some input and updates some MD5
		342	state passed as argument would I<NOT> be pure, however, as it would modify
		343	some memory area that is not the return value.
		344
100	=item ecb_hot	345	=item ecb_hot
101		346
		347	This declares a function as "hot" with regards to the cache - the function
		348	is used so often, that it is very beneficial to keep it in the cache if
		349	possible.
		350
		351	The compiler reacts by trying to place hot functions near to each other in
		352	memory.
		353
		354	Whether a function is hot or not often depends on the whole program,
		355	and less on the function itself. C<ecb_cold> is likely more useful in
		356	practise.
		357
102	=item ecb_cold	358	=item ecb_cold
103		359
		360	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		361	the cache, or in other words, this function is not called often, or not at
		362	speed-critical times, and keeping it in the cache might be a waste of said
		363	cache.
		364
		365	In addition to placing cold functions together (or at least away from hot
		366	functions), this knowledge can be used in other ways, for example, the
		367	function will be optimised for size, as opposed to speed, and codepaths
		368	leading to calls to those functions can automatically be marked as if
		369	C<ecb_expect_false> had been used to reach them.
		370
		371	Good examples for such functions would be error reporting functions, or
		372	functions only called in exceptional or rare cases.
		373
104	=item ecb_artificial	374	=item ecb_artificial
105		375
		376	Declares the function as "artificial", in this case meaning that this
		377	function is not really meant to be a function, but more like an accessor
		378	- many methods in C++ classes are mere accessor functions, and having a
		379	crash reported in such a method, or single-stepping through them, is not
		380	usually so helpful, especially when it's inlined to just a few instructions.
		381
		382	Marking them as artificial will instruct the debugger about just this,
		383	leading to happier debugging and thus happier lives.
		384
		385	Example: in some kind of smart-pointer class, mark the pointer accessor as
		386	artificial, so that the whole class acts more like a pointer and less like
		387	some C++ abstraction monster.
		388
		389	template<typename T>
		390	struct my_smart_ptr
		391	{
		392	T *value;
		393
		394	ecb_artificial
		395	operator T *()
		396	{
		397	return value;
		398	}
		399	};
		400
106	=back	401	=back
107		402
108	=head2 OPTIMISATION HINTS	403	=head2 OPTIMISATION HINTS
109		404
110	=over 4	405	=over 4
111		406
		407	=item ECB_OPTIMIZE_SIZE
		408
		409	Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
		410	can also be defined before including F<ecb.h>, in which case it will be
		411	unchanged.
		412
112	=item bool ecb_is_constant(expr)	413	=item bool ecb_is_constant (expr)
113		414
114	Returns true iff the expression can be deduced to be a compile-time	415	Returns true iff the expression can be deduced to be a compile-time
115	constant, and false otherwise.	416	constant, and false otherwise.
116		417
117	For example, when you have a C<rndm16> function that returns a 16 bit	418	For example, when you have a C<rndm16> function that returns a 16 bit
…		…
135	return is_constant (n) && !(n & (n - 1))	436	return is_constant (n) && !(n & (n - 1))
136	? rndm16 () & (num - 1)	437	? rndm16 () & (num - 1)
137	: (n * (uint32_t)rndm16 ()) >> 16;	438	: (n * (uint32_t)rndm16 ()) >> 16;
138	}	439	}
139		440
140	=item bool ecb_expect (expr, value)	441	=item ecb_expect (expr, value)
141		442
142	Evaluates C<expr> and returns it. In addition, it tells the compiler that	443	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	444	the C<expr> evaluates to C<value> a lot, which can be used for static
144	branch optimisations.	445	branch optimisations.
145		446
146	Usually, you want to use the more intuitive C<ecb_likely> and	447	Usually, you want to use the more intuitive C<ecb_expect_true> and
147	C<ecb_unlikely> functions instead.	448	C<ecb_expect_false> functions instead.
148		449
		450	=item bool ecb_expect_true (cond)
		451
149	=item bool ecb_likely (cond)	452	=item bool ecb_expect_false (cond)
150
151	=item bool ecb_unlikely (cond)
152		453
153	These two functions expect a expression that is true or false and return	454	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	455	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:	456	other conditional statement, it will not change the program:
156		457
157	/* these two do the same thing */	458	/* these two do the same thing */
158	if (some_condition) ...;	459	if (some_condition) ...;
159	if (ecb_likely (some_condition)) ...;	460	if (ecb_expect_true (some_condition)) ...;
160		461
161	However, by using C<ecb_likely>, you tell the compiler that the condition	462	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	463	condition is likely to be true (and for C<ecb_expect_false>, that it is
163	true).	464	unlikely to be true).
164		465
165	For example, when you check for a null pointer and expect this to be a	466	For example, when you check for a null pointer and expect this to be a
166	rare, exceptional, case, then use C<ecb_unlikely>:	467	rare, exceptional, case, then use C<ecb_expect_false>:
167		468
168	void my_free (void *ptr)	469	void my_free (void *ptr)
169	{	470	{
170	if (ecb_unlikely (ptr == 0))	471	if (ecb_expect_false (ptr == 0))
171	return;	472	return;
172	}	473	}
173		474
174	Consequent use of these functions to mark away exceptional cases or to	475	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	476	tell the compiler what the hot path through a function is can increase
176	performance considerably.	477	performance considerably.
		478
		479	You might know these functions under the name C<likely> and C<unlikely>
		480	- while these are common aliases, we find that the expect name is easier
		481	to understand when quickly skimming code. If you wish, you can use
		482	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		483	C<ecb_expect_false> - these are simply aliases.
177		484
178	A very good example is in a function that reserves more space for some	485	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) -	486	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	487	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:	488	you expect that most checks will turn out to be false:
182		489
183	/* make sure we have "size" extra room in our buffer */	490	/* make sure we have "size" extra room in our buffer */
184	ecb_inline void	491	ecb_inline void
185	reserve (int size)	492	reserve (int size)
186	{	493	{
187	if (ecb_unlikely (current + size > end))	494	if (ecb_expect_false (current + size > end))
188	real_reserve_method (size); /* presumably noinline */	495	real_reserve_method (size); /* presumably noinline */
189	}	496	}
190		497
191	=item bool ecb_assume (cond)	498	=item ecb_assume (cond)
192		499
193	Try to tell the compiler that some condition is true, even if it's not	500	Tries to tell the compiler that some condition is true, even if it's not
194	obvious.	501	obvious. This is not a function, but a statement: it cannot be used in
		502	another expression.
195		503
196	This can be used to teach the compiler about invariants or other	504	This can be used to teach the compiler about invariants or other
197	conditions that might improve code generation, but which are impossible to	505	conditions that might improve code generation, but which are impossible to
198	deduce form the code itself.	506	deduce form the code itself.
199		507
200	For example, the example reservation function from the C<ecb_unlikely>	508	For example, the example reservation function from the C<ecb_expect_false>
201	description could be written thus (only C<ecb_assume> was added):	509	description could be written thus (only C<ecb_assume> was added):
202		510
203	ecb_inline void	511	ecb_inline void
204	reserve (int size)	512	reserve (int size)
205	{	513	{
206	if (ecb_unlikely (current + size > end))	514	if (ecb_expect_false (current + size > end))
207	real_reserve_method (size); /* presumably noinline */	515	real_reserve_method (size); /* presumably noinline */
208		516
209	ecb_assume (current + size <= end);	517	ecb_assume (current + size <= end);
210	}	518	}
211		519
…		…
216		524
217	Then the compiler I<might> be able to optimise out the second call	525	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	526	completely, as it knows that C<< current + 1 > end >> is false and the
219	call will never be executed.	527	call will never be executed.
220		528
221	=item bool ecb_unreachable ()	529	=item ecb_unreachable ()
222		530
223	This function does nothing itself, except tell the compiler that it will	531	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	532	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.	533	function can be used to implement C<ecb_assume> or similar functionality.
226		534
227	=item bool ecb_prefetch (addr, rw, locality)	535	=item ecb_prefetch (addr, rw, locality)
228		536
229	Tells the compiler to try to prefetch memory at the given C<addr>ess	537	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	538	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	539	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	540	the data will likely be accessed very often, and values in between mean
233	something... in between. The memory pointed to by the address does not	541	something... in between. The memory pointed to by the address does not
234	need to be accessible (it could be a null pointer for example), but C<rw>	542	need to be accessible (it could be a null pointer for example), but C<rw>
235	and C<locality> must be compile-time constants.	543	and C<locality> must be compile-time constants.
236		544
		545	This is a statement, not a function: you cannot use it as part of an
		546	expression.
		547
237	An obvious way to use this is to prefetch some data far away, in a big	548	An obvious way to use this is to prefetch some data far away, in a big
238	array you loop over. This prefetches memory some 128 array elements later,	549	array you loop over. This prefetches memory some 128 array elements later,
239	in the hope that it will be ready when the CPU arrives at that location.	550	in the hope that it will be ready when the CPU arrives at that location.
240		551
241	int sum = 0;	552	int sum = 0;
…		…
260	After processing the node, (part of) the next node might already be in	571	After processing the node, (part of) the next node might already be in
261	cache.	572	cache.
262		573
263	=back	574	=back
264		575
265	=head2 BIT FIDDLING / BITSTUFFS	576	=head2 BIT FIDDLING / BIT WIZARDRY
266		577
267	=over 4	578	=over 4
268		579
269	=item bool ecb_big_endian ()	580	=item bool ecb_big_endian ()
270		581
…		…
272		583
273	These two functions return true if the byte order is big endian	584	These two functions return true if the byte order is big endian
274	(most-significant byte first) or little endian (least-significant byte	585	(most-significant byte first) or little endian (least-significant byte
275	first) respectively.	586	first) respectively.
276		587
		588	On systems that are neither, their return values are unspecified.
		589
277	=item int ecb_ctz32 (uint32_t x)	590	=item int ecb_ctz32 (uint32_t x)
278		591
		592	=item int ecb_ctz64 (uint64_t x)
		593
279	Returns the index of the least significant bit set in C<x> (or	594	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	595	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	596	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.,	597
283	floor(log2(n)). For example:	598	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		599
		600	For example:
284		601
285	ecb_ctz32 (3) = 0	602	ecb_ctz32 (3) = 0
286	ecb_ctz32 (6) = 1	603	ecb_ctz32 (6) = 1
287		604
		605	=item bool ecb_is_pot32 (uint32_t x)
		606
		607	=item bool ecb_is_pot64 (uint32_t x)
		608
		609	Returns true iff C<x> is a power of two or C<x == 0>.
		610
		611	For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
		612
		613	=item int ecb_ld32 (uint32_t x)
		614
		615	=item int ecb_ld64 (uint64_t x)
		616
		617	Returns the index of the most significant bit set in C<x>, or the number
		618	of digits the number requires in binary (so that C<< 2**ld <= x <
		619	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		620	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		621	example to see how many bits a certain number requires to be encoded.
		622
		623	This function is similar to the "count leading zero bits" function, except
		624	that that one returns how many zero bits are "in front" of the number (in
		625	the given data type), while C<ecb_ld> returns how many bits the number
		626	itself requires.
		627
		628	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		629
288	=item int ecb_popcount32 (uint32_t x)	630	=item int ecb_popcount32 (uint32_t x)
289		631
		632	=item int ecb_popcount64 (uint64_t x)
		633
290	Returns the number of bits set to 1 in C<x>. For example:	634	Returns the number of bits set to 1 in C<x>.
		635
		636	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		637
		638	For example:
291		639
292	ecb_popcount32 (7) = 3	640	ecb_popcount32 (7) = 3
293	ecb_popcount32 (255) = 8	641	ecb_popcount32 (255) = 8
294		642
		643	=item uint8_t ecb_bitrev8 (uint8_t x)
		644
		645	=item uint16_t ecb_bitrev16 (uint16_t x)
		646
		647	=item uint32_t ecb_bitrev32 (uint32_t x)
		648
		649	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		650	and so on.
		651
		652	Example:
		653
		654	ecb_bitrev8 (0xa7) = 0xea
		655	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		656
295	=item uint32_t ecb_bswap16 (uint32_t x)	657	=item uint32_t ecb_bswap16 (uint32_t x)
296		658
297	=item uint32_t ecb_bswap32 (uint32_t x)	659	=item uint32_t ecb_bswap32 (uint32_t x)
298		660
		661	=item uint64_t ecb_bswap64 (uint64_t x)
		662
299	These two functions return the value of the 16-bit (32-bit) variable	663	These functions return the value of the 16-bit (32-bit, 64-bit) value
300	C<x> after reversing the order of bytes.	664	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		665	C<ecb_bswap32>).
		666
		667	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		668
		669	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		670
		671	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		672
		673	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		674
		675	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		676
		677	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
301		678
302	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	679	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
303		680
304	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	681	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
305		682
306	These two functions return the value of C<x> after shifting all the bits	683	These two families of functions return the value of C<x> after rotating
307	by C<count> positions to the right or left respectively.	684	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		685	(C<ecb_rotl>).
		686
		687	Current GCC versions understand these functions and usually compile them
		688	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		689	x86).
308		690
309	=back	691	=back
310		692
		693	=head2 FLOATING POINT FIDDLING
		694
		695	=over 4
		696
		697	=item ECB_INFINITY [-UECB_NO_LIBM]
		698
		699	Evaluates to positive infinity if supported by the platform, otherwise to
		700	a truly huge number.
		701
		702	=item ECB_NAN [-UECB_NO_LIBM]
		703
		704	Evaluates to a quiet NAN if supported by the platform, otherwise to
		705	C<ECB_INFINITY>.
		706
		707	=item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
		708
		709	Same as C<ldexpf>, but always available.
		710
		711	=item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
		712
		713	=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
		714
		715	=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
		716
		717	These functions each take an argument in the native C<float> or C<double>
		718	type and return the IEEE 754 bit representation of it (binary16/half,
		719	binary32/single or binary64/double precision).
		720
		721	The bit representation is just as IEEE 754 defines it, i.e. the sign bit
		722	will be the most significant bit, followed by exponent and mantissa.
		723
		724	This function should work even when the native floating point format isn't
		725	IEEE compliant, of course at a speed and code size penalty, and of course
		726	also within reasonable limits (it tries to convert NaNs, infinities and
		727	denormals, but will likely convert negative zero to positive zero).
		728
		729	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		730	be able to optimise away this function completely.
		731
		732	These functions can be helpful when serialising floats to the network - you
		733	can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
		734
		735	Another use for these functions is to manipulate floating point values
		736	directly.
		737
		738	Silly example: toggle the sign bit of a float.
		739
		740	/* On gcc-4.7 on amd64, */
		741	/* this results in a single add instruction to toggle the bit, and 4 extra */
		742	/* instructions to move the float value to an integer register and back. */
		743
		744	x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
		745
		746	=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
		747
		748	=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
		749
		750	=item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
		751
		752	The reverse operation of the previous function - takes the bit
		753	representation of an IEEE binary16, binary32 or binary64 number (half,
		754	single or double precision) and converts it to the native C<float> or
		755	C<double> format.
		756
		757	This function should work even when the native floating point format isn't
		758	IEEE compliant, of course at a speed and code size penalty, and of course
		759	also within reasonable limits (it tries to convert normals and denormals,
		760	and might be lucky for infinities, and with extraordinary luck, also for
		761	negative zero).
		762
		763	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		764	be able to optimise away this function completely.
		765
		766	=item uint16_t ecb_binary32_to_binary16 (uint32_t x)
		767
		768	=item uint32_t ecb_binary16_to_binary32 (uint16_t x)
		769
		770	Convert a IEEE binary32/single precision to binary16/half format, and vice
		771	versa, handling all details (round-to-nearest-even, subnormals, infinity
		772	and NaNs) correctly.
		773
		774	These are functions are available under C<-DECB_NO_LIBM>, since
		775	they do not rely on the platform floating point format. The
		776	C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
		777	usually what you want.
		778
		779	=back
		780
311	=head2 ARITHMETIC	781	=head2 ARITHMETIC
312		782
313	=over 4	783	=over 4
314		784
315	=item x = ecb_mod (m, n)	785	=item x = ecb_mod (m, n)
316		786
317	Returns the positive remainder of the modulo operation between C<m> and	787	Returns C<m> modulo C<n>, which is the same as the positive remainder
		788	of the division operation between C<m> and C<n>, using floored
318	C<n>. Unlike the C modulo operator C<%>, this function ensures that the	789	division. Unlike the C remainder operator C<%>, this function ensures that
319	return value is always positive).	790	the return value is always positive and that the two numbers I<m> and
		791	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		792	C<ecb_mod> implements the mathematical modulo operation, which is missing
		793	in the language.
320		794
321	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	795	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	796	negatable, that is, both C<m> and C<-m> must be representable in its
323	type.	797	type (this typically excludes the minimum signed integer value, the same
		798	limitation as for C</> and C<%> in C).
		799
		800	Current GCC versions compile this into an efficient branchless sequence on
		801	almost all CPUs.
		802
		803	For example, when you want to rotate forward through the members of an
		804	array for increasing C<m> (which might be negative), then you should use
		805	C<ecb_mod>, as the C<%> operator might give either negative results, or
		806	change direction for negative values:
		807
		808	for (m = -100; m <= 100; ++m)
		809	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		810
		811	=item x = ecb_div_rd (val, div)
		812
		813	=item x = ecb_div_ru (val, div)
		814
		815	Returns C<val> divided by C<div> rounded down or up, respectively.
		816	C<val> and C<div> must have integer types and C<div> must be strictly
		817	positive. Note that these functions are implemented with macros in C
		818	and with function templates in C++.
324		819
325	=back	820	=back
326		821
327	=head2 UTILITY	822	=head2 UTILITY
328		823
329	=over 4	824	=over 4
330		825
331	=item element_count = ecb_array_length (name) [MACRO]	826	=item element_count = ecb_array_length (name)
332		827
333	Returns the number of elements in the array C<name>. For example:	828	Returns the number of elements in the array C<name>. For example:
334		829
335	int primes[] = { 2, 3, 5, 7, 11 };	830	int primes[] = { 2, 3, 5, 7, 11 };
336	int sum = 0;	831	int sum = 0;
…		…
338	for (i = 0; i < ecb_array_length (primes); i++)	833	for (i = 0; i < ecb_array_length (primes); i++)
339	sum += primes [i];	834	sum += primes [i];
340		835
341	=back	836	=back
342		837
		838	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
343		839
		840	These symbols need to be defined before including F<ecb.h> the first time.
		841
		842	=over 4
		843
		844	=item ECB_NO_THREADS
		845
		846	If F<ecb.h> is never used from multiple threads, then this symbol can
		847	be defined, in which case memory fences (and similar constructs) are
		848	completely removed, leading to more efficient code and fewer dependencies.
		849
		850	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		851
		852	=item ECB_NO_SMP
		853
		854	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		855	multiple threads, but never concurrently (e.g. if the system the program
		856	runs on has only a single CPU with a single core, no hyperthreading and so
		857	on), then this symbol can be defined, leading to more efficient code and
		858	fewer dependencies.
		859
		860	=item ECB_NO_LIBM
		861
		862	When defined to C<1>, do not export any functions that might introduce
		863	dependencies on the math library (usually called F<-lm>) - these are
		864	marked with [-UECB_NO_LIBM].
		865
		866	=back
		867
		868	=head1 UNDOCUMENTED FUNCTIONALITY
		869
		870	F<ecb.h> is full of undocumented functionality as well, some of which is
		871	intended to be internal-use only, some of which we forgot to document, and
		872	some of which we hide because we are not sure we will keep the interface
		873	stable.
		874
		875	While you are welcome to rummage around and use whatever you find useful
		876	(we can't stop you), keep in mind that we will change undocumented
		877	functionality in incompatible ways without thinking twice, while we are
		878	considerably more conservative with documented things.
		879
		880	=head1 AUTHORS
		881
		882	C<libecb> is designed and maintained by:
		883
		884	Emanuele Giaquinta <e.giaquinta@glauco.it>
		885	Marc Alexander Lehmann <schmorp@schmorp.de>
		886
		887

Diff Legend

-–
+Removed lines
-+
+Added lines
-<
+Changed lines
->
+Changed lines

Comparing libecb/ecb.pod (file contents): Revision 1.16 by sf-exg, Thu May 26 23:32:41 2011 UTC vs. Revision 1.75 by root, Sat Dec 28 08:01:05 2019 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.16 by sf-exg, Thu May 26 23:32:41 2011 UTC vs.
Revision 1.75 by root, Sat Dec 28 08:01:05 2019 UTC