[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 1.74 by root, Mon Nov 19 00:27:38 2018 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
112	=item bool ecb_is_constant(expr)	407	=item bool ecb_is_constant (expr)
113		408
114	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
115	constant, and false otherwise.	410	constant, and false otherwise.
116		411
117	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
…		…
135	return is_constant (n) && !(n & (n - 1))	430	return is_constant (n) && !(n & (n - 1))
136	? rndm16 () & (num - 1)	431	? rndm16 () & (num - 1)
137	: (n * (uint32_t)rndm16 ()) >> 16;	432	: (n * (uint32_t)rndm16 ()) >> 16;
138	}	433	}
139		434
140	=item bool ecb_expect (expr, value)	435	=item ecb_expect (expr, value)
141		436
142	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
143	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
144	branch optimisations.	439	branch optimisations.
145		440
146	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
147	C<ecb_unlikely> functions instead.	442	C<ecb_expect_false> functions instead.
148		443
		444	=item bool ecb_expect_true (cond)
		445
149	=item bool ecb_likely (cond)	446	=item bool ecb_expect_false (cond)
150
151	=item bool ecb_unlikely (cond)
152		447
153	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
154	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
155	other conditional statement, it will not change the program:	450	other conditional statement, it will not change the program:
156		451
157	/* these two do the same thing */	452	/* these two do the same thing */
158	if (some_condition) ...;	453	if (some_condition) ...;
159	if (ecb_likely (some_condition)) ...;	454	if (ecb_expect_true (some_condition)) ...;
160		455
161	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
162	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
163	true).	458	unlikely to be true).
164		459
165	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
166	rare, exceptional, case, then use C<ecb_unlikely>:	461	rare, exceptional, case, then use C<ecb_expect_false>:
167		462
168	void my_free (void *ptr)	463	void my_free (void *ptr)
169	{	464	{
170	if (ecb_unlikely (ptr == 0))	465	if (ecb_expect_false (ptr == 0))
171	return;	466	return;
172	}	467	}
173		468
174	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
175	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
176	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.
177		478
178	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
179	memory block (for example, inside an implementation of a string stream) -	480	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	481	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:	482	you expect that most checks will turn out to be false:
182		483
183	/* make sure we have "size" extra room in our buffer */	484	/* make sure we have "size" extra room in our buffer */
184	ecb_inline void	485	ecb_inline void
185	reserve (int size)	486	reserve (int size)
186	{	487	{
187	if (ecb_unlikely (current + size > end))	488	if (ecb_expect_false (current + size > end))
188	real_reserve_method (size); /* presumably noinline */	489	real_reserve_method (size); /* presumably noinline */
189	}	490	}
190		491
191	=item bool ecb_assume (cond)	492	=item ecb_assume (cond)
192		493
193	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
194	obvious.	495	obvious. This is not a function, but a statement: it cannot be used in
		496	another expression.
195		497
196	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
197	conditions that might improve code generation, but which are impossible to	499	conditions that might improve code generation, but which are impossible to
198	deduce form the code itself.	500	deduce form the code itself.
199		501
200	For example, the example reservation function from the C<ecb_unlikely>	502	For example, the example reservation function from the C<ecb_expect_false>
201	description could be written thus (only C<ecb_assume> was added):	503	description could be written thus (only C<ecb_assume> was added):
202		504
203	ecb_inline void	505	ecb_inline void
204	reserve (int size)	506	reserve (int size)
205	{	507	{
206	if (ecb_unlikely (current + size > end))	508	if (ecb_expect_false (current + size > end))
207	real_reserve_method (size); /* presumably noinline */	509	real_reserve_method (size); /* presumably noinline */
208		510
209	ecb_assume (current + size <= end);	511	ecb_assume (current + size <= end);
210	}	512	}
211		513
…		…
216		518
217	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
218	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
219	call will never be executed.	521	call will never be executed.
220		522
221	=item bool ecb_unreachable ()	523	=item ecb_unreachable ()
222		524
223	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
224	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
225	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.
226		528
227	=item bool ecb_prefetch (addr, rw, locality)	529	=item ecb_prefetch (addr, rw, locality)
228		530
229	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
230	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
231	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
232	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
233	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
234	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>
235	and C<locality> must be compile-time constants.	537	and C<locality> must be compile-time constants.
236		538
		539	This is a statement, not a function: you cannot use it as part of an
		540	expression.
		541
237	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
238	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,
239	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.
240		545
241	int sum = 0;	546	int sum = 0;
…		…
260	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
261	cache.	566	cache.
262		567
263	=back	568	=back
264		569
265	=head2 BIT FIDDLING / BITSTUFFS	570	=head2 BIT FIDDLING / BIT WIZARDRY
266		571
267	=over 4	572	=over 4
268		573
269	=item bool ecb_big_endian ()	574	=item bool ecb_big_endian ()
270		575
…		…
272		577
273	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
274	(most-significant byte first) or little endian (least-significant byte	579	(most-significant byte first) or little endian (least-significant byte
275	first) respectively.	580	first) respectively.
276		581
		582	On systems that are neither, their return values are unspecified.
		583
277	=item int ecb_ctz32 (uint32_t x)	584	=item int ecb_ctz32 (uint32_t x)
278		585
		586	=item int ecb_ctz64 (uint64_t x)
		587
279	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
280	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
281	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.
282	common use case is to compute the integer binary logarithm, i.e.,	591
283	floor(log2(n)). For example:	592	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		593
		594	For example:
284		595
285	ecb_ctz32 (3) = 0	596	ecb_ctz32 (3) = 0
286	ecb_ctz32 (6) = 1	597	ecb_ctz32 (6) = 1
287		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
288	=item int ecb_popcount32 (uint32_t x)	624	=item int ecb_popcount32 (uint32_t x)
289		625
		626	=item int ecb_popcount64 (uint64_t x)
		627
290	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:
291		633
292	ecb_popcount32 (7) = 3	634	ecb_popcount32 (7) = 3
293	ecb_popcount32 (255) = 8	635	ecb_popcount32 (255) = 8
294		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
295	=item uint32_t ecb_bswap16 (uint32_t x)	651	=item uint32_t ecb_bswap16 (uint32_t x)
296		652
297	=item uint32_t ecb_bswap32 (uint32_t x)	653	=item uint32_t ecb_bswap32 (uint32_t x)
298		654
		655	=item uint64_t ecb_bswap64 (uint64_t x)
		656
299	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
300	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)
301		672
302	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	673	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
303		674
304	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	675	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
305		676
306	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
307	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).
308		684
309	=back	685	=back
310		686
		687	=head2 FLOATING POINT FIDDLING
		688
		689	=over 4
		690
		691	=item ECB_INFINITY [-UECB_NO_LIBM]
		692
		693	Evaluates to positive infinity if supported by the platform, otherwise to
		694	a truly huge number.
		695
		696	=item ECB_NAN [-UECB_NO_LIBM]
		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) [-UECB_NO_LIBM]
		702
		703	Same as C<ldexpf>, but always available.
		704
		705	=item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
		706
		707	=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
		708
		709	=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
		710
		711	These functions each take an argument in the native C<float> or C<double>
		712	type and return the IEEE 754 bit representation of it (binary16/half,
		713	binary32/single or binary64/double precision).
		714
		715	The bit representation is just as IEEE 754 defines it, i.e. the sign bit
		716	will be the most significant bit, followed by exponent and mantissa.
		717
		718	This function should work even when the native floating point format isn't
		719	IEEE compliant, of course at a speed and code size penalty, and of course
		720	also within reasonable limits (it tries to convert NaNs, infinities and
		721	denormals, but will likely convert negative zero to positive zero).
		722
		723	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		724	be able to optimise away this function completely.
		725
		726	These functions can be helpful when serialising floats to the network - you
		727	can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
		728
		729	Another use for these functions is to manipulate floating point values
		730	directly.
		731
		732	Silly example: toggle the sign bit of a float.
		733
		734	/* On gcc-4.7 on amd64, */
		735	/* this results in a single add instruction to toggle the bit, and 4 extra */
		736	/* instructions to move the float value to an integer register and back. */
		737
		738	x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
		739
		740	=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
		741
		742	=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
		743
		744	=item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
		745
		746	The reverse operation of the previous function - takes the bit
		747	representation of an IEEE binary16, binary32 or binary64 number (half,
		748	single or double precision) and converts it to the native C<float> or
		749	C<double> format.
		750
		751	This function should work even when the native floating point format isn't
		752	IEEE compliant, of course at a speed and code size penalty, and of course
		753	also within reasonable limits (it tries to convert normals and denormals,
		754	and might be lucky for infinities, and with extraordinary luck, also for
		755	negative zero).
		756
		757	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		758	be able to optimise away this function completely.
		759
		760	=item uint16_t ecb_binary32_to_binary16 (uint32_t x)
		761
		762	=item uint32_t ecb_binary16_to_binary32 (uint16_t x)
		763
		764	Convert a IEEE binary32/single precision to binary16/half format, and vice
		765	versa, handling all details (round-to-nearest-even, subnormals, infinity
		766	and NaNs) correctly.
		767
		768	These are functions are available under C<-DECB_NO_LIBM>, since
		769	they do not rely on the platform floating point format. The
		770	C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
		771	usually what you want.
		772
		773	=back
		774
311	=head2 ARITHMETIC	775	=head2 ARITHMETIC
312		776
313	=over 4	777	=over 4
314		778
315	=item x = ecb_mod (m, n)	779	=item x = ecb_mod (m, n)
316		780
317	Returns the positive remainder of the modulo operation between C<m> and	781	Returns C<m> modulo C<n>, which is the same as the positive remainder
		782	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	783	division. Unlike the C remainder operator C<%>, this function ensures that
319	return value is always positive).	784	the return value is always positive and that the two numbers I<m> and
		785	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		786	C<ecb_mod> implements the mathematical modulo operation, which is missing
		787	in the language.
320		788
321	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	789	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	790	negatable, that is, both C<m> and C<-m> must be representable in its
323	type.	791	type (this typically excludes the minimum signed integer value, the same
		792	limitation as for C</> and C<%> in C).
		793
		794	Current GCC versions compile this into an efficient branchless sequence on
		795	almost all CPUs.
		796
		797	For example, when you want to rotate forward through the members of an
		798	array for increasing C<m> (which might be negative), then you should use
		799	C<ecb_mod>, as the C<%> operator might give either negative results, or
		800	change direction for negative values:
		801
		802	for (m = -100; m <= 100; ++m)
		803	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		804
		805	=item x = ecb_div_rd (val, div)
		806
		807	=item x = ecb_div_ru (val, div)
		808
		809	Returns C<val> divided by C<div> rounded down or up, respectively.
		810	C<val> and C<div> must have integer types and C<div> must be strictly
		811	positive. Note that these functions are implemented with macros in C
		812	and with function templates in C++.
324		813
325	=back	814	=back
326		815
327	=head2 UTILITY	816	=head2 UTILITY
328		817
329	=over 4	818	=over 4
330		819
331	=item element_count = ecb_array_length (name) [MACRO]	820	=item element_count = ecb_array_length (name)
332		821
333	Returns the number of elements in the array C<name>. For example:	822	Returns the number of elements in the array C<name>. For example:
334		823
335	int primes[] = { 2, 3, 5, 7, 11 };	824	int primes[] = { 2, 3, 5, 7, 11 };
336	int sum = 0;	825	int sum = 0;
…		…
338	for (i = 0; i < ecb_array_length (primes); i++)	827	for (i = 0; i < ecb_array_length (primes); i++)
339	sum += primes [i];	828	sum += primes [i];
340		829
341	=back	830	=back
342		831
		832	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
343		833
		834	These symbols need to be defined before including F<ecb.h> the first time.
		835
		836	=over 4
		837
		838	=item ECB_NO_THREADS
		839
		840	If F<ecb.h> is never used from multiple threads, then this symbol can
		841	be defined, in which case memory fences (and similar constructs) are
		842	completely removed, leading to more efficient code and fewer dependencies.
		843
		844	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		845
		846	=item ECB_NO_SMP
		847
		848	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		849	multiple threads, but never concurrently (e.g. if the system the program
		850	runs on has only a single CPU with a single core, no hyperthreading and so
		851	on), then this symbol can be defined, leading to more efficient code and
		852	fewer dependencies.
		853
		854	=item ECB_NO_LIBM
		855
		856	When defined to C<1>, do not export any functions that might introduce
		857	dependencies on the math library (usually called F<-lm>) - these are
		858	marked with [-UECB_NO_LIBM].
		859
		860	=back
		861
		862	=head1 UNDOCUMENTED FUNCTIONALITY
		863
		864	F<ecb.h> is full of undocumented functionality as well, some of which is
		865	intended to be internal-use only, some of which we forgot to document, and
		866	some of which we hide because we are not sure we will keep the interface
		867	stable.
		868
		869	While you are welcome to rummage around and use whatever you find useful
		870	(we can't stop you), keep in mind that we will change undocumented
		871	functionality in incompatible ways without thinking twice, while we are
		872	considerably more conservative with documented things.
		873
		874	=head1 AUTHORS
		875
		876	C<libecb> is designed and maintained by:
		877
		878	Emanuele Giaquinta <e.giaquinta@glauco.it>
		879	Marc Alexander Lehmann <schmorp@schmorp.de>
		880
		881

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 1.74 by root, Mon Nov 19 00:27:38 2018 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 1.74 by root, Mon Nov 19 00:27:38 2018 UTC