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

Comparing libecb/ecb.pod (file contents):
Revision 1.11 by sf-exg, Thu May 26 21:18:52 2011 UTC vs.
Revision 1.20 by root, Fri May 27 00:12:00 2011 UTC

…		…
		1	=head1 LIBECB - e-C-Builtins
		2
1	=head1 LIBECB	3	=head2 ABOUT LIBECB
2		4
3	You suck, we don't(tm)	5	Libecb is currently a simple header file that doesn't require any
		6	configuration to use or include in your project.
		7
		8	It's part of the e-suite of libraries, other members of which include
		9	libev and libeio.
		10
		11	Its homepage can be found here:
		12
		13	http://software.schmorp.de/pkg/libecb
		14
		15	It mainly provides a number of wrappers around GCC built-ins, together
		16	with replacement functions for other compilers. In addition to this,
		17	it provides a number of other lowlevel C utilities, such as endianness
		18	detection, byte swapping or bit rotations.
		19
		20	Or in other words, things that should be built-in into any standard C
		21	system, but aren't.
		22
		23	More might come.
4		24
5	=head2 ABOUT THE HEADER	25	=head2 ABOUT THE HEADER
6		26
7	- how to include it	27	At the moment, all you have to do is copy F<ecb.h> somewhere where your
8	- it includes inttypes.h	28	compiler can find it and include it:
9	- no .a	29
10	- whats a bool	30	#include <ecb.h>
11	- function mean macro or function	31
12	- macro means untyped	32	The header should work fine for both C and C++ compilation, and gives you
		33	all of F<inttypes.h> in addition to the ECB symbols.
		34
		35	There are currently no object files to link to - future versions might
		36	come with an (optional) object code library to link against, to reduce
		37	code size or gain access to additional features.
		38
		39	It also currently includes everything from F<inttypes.h>.
		40
		41	=head2 ABOUT THIS MANUAL / CONVENTIONS
		42
		43	This manual mainly describes each (public) function available after
		44	including the F<ecb.h> header. The header might define other symbols than
		45	these, but these are not part of the public API, and not supported in any
		46	way.
		47
		48	When the manual mentions a "function" then this could be defined either as
		49	as inline function, a macro, or an external symbol.
		50
		51	When functions use a concrete standard type, such as C<int> or
		52	C<uint32_t>, then the corresponding function works only with that type. If
		53	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	is usually implemented as a macro. Specifically, a "bool" in this manual
		56	refers to any kind of boolean value, not a specific type.
13		57
14	=head2 GCC ATTRIBUTES	58	=head2 GCC ATTRIBUTES
15		59
16	blabla where to put, what others	60	A major part of libecb deals with GCC attributes. These are additional
		61	attributes that you cna assign to functions, variables and sometimes even
		62	types - much like C<const> or C<volatile> in C.
		63
		64	While GCC allows declarations to show up in many surprising places,
		65	but not in many expeted places, the safest way is to put attribute
		66	declarations before the whole declaration:
		67
		68	ecb_const int mysqrt (int a);
		69	ecb_unused int i;
		70
		71	For variables, it is often nicer to put the attribute after the name, and
		72	avoid multiple declarations using commas:
		73
		74	int i ecb_unused;
17		75
18	=over 4	76	=over 4
19		77
20	=item ecb_attribute ((attrs...))	78	=item ecb_attribute ((attrs...))
21		79
22	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and	80	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
23	to nothing on other compilers, so the effect is that only GCC sees these.	81	nothing on other compilers, so the effect is that only GCC sees these.
		82
		83	Example: use the C<deprecated> attribute on a function.
		84
		85	ecb_attribute((__deprecated__)) void
		86	do_not_use_me_anymore (void);
24		87
25	=item ecb_unused	88	=item ecb_unused
26		89
27	Marks a function or a variable as "unused", which simply suppresses a	90	Marks a function or a variable as "unused", which simply suppresses a
28	warning by GCC when it detects it as unused. This is useful when you e.g.	91	warning by GCC when it detects it as unused. This is useful when you e.g.
29	declare a variable but do not always use it:	92	declare a variable but do not always use it:
30		93
31	{	94	{
32	int var ecb_unused;	95	int var ecb_unused;
33		96
34	#ifdef SOMECONDITION	97	#ifdef SOMECONDITION
35	var = ...;	98	var = ...;
36	return var;	99	return var;
37	#else	100	#else
38	return 0;	101	return 0;
39	#endif	102	#endif
40	}	103	}
41		104
42	=item ecb_noinline	105	=item ecb_noinline
43		106
44	Prevent a function from being inlined - it might be optimised away, but	107	Prevent a function from being inlined - it might be optimised away, but
45	not inlined into other functions. This is useful if you know your function	108	not inlined into other functions. This is useful if you know your function
46	is rarely called and large enough for inlining not to be helpful.	109	is rarely called and large enough for inlining not to be helpful.
47		110
48	=item ecb_noreturn	111	=item ecb_noreturn
49		112
		113	Marks a function as "not returning, ever". Some typical functions that
		114	don't return are C<exit> or C<abort> (which really works hard to not
		115	return), and now you can make your own:
		116
		117	ecb_noreturn void
		118	my_abort (const char *errline)
		119	{
		120	puts (errline);
		121	abort ();
		122	}
		123
		124	In this case, the compiler would probably be smart enough to deduce it on
		125	its own, so this is mainly useful for declarations.
		126
50	=item ecb_const	127	=item ecb_const
51		128
		129	Declares that the function only depends on the values of its arguments,
		130	much like a mathematical function. It specifically does not read or write
		131	any memory any arguments might point to, global variables, or call any
		132	non-const functions. It also must not have any side effects.
		133
		134	Such a function can be optimised much more aggressively by the compiler -
		135	for example, multiple calls with the same arguments can be optimised into
		136	a single call, which wouldn't be possible if the compiler would have to
		137	expect any side effects.
		138
		139	It is best suited for functions in the sense of mathematical functions,
		140	such as a function returning the square root of its input argument.
		141
		142	Not suited would be a function that calculates the hash of some memory
		143	area you pass in, prints some messages or looks at a global variable to
		144	decide on rounding.
		145
		146	See C<ecb_pure> for a slightly less restrictive class of functions.
		147
52	=item ecb_pure	148	=item ecb_pure
53		149
		150	Similar to C<ecb_const>, declares a function that has no side
		151	effects. Unlike C<ecb_const>, the function is allowed to examine global
		152	variables and any other memory areas (such as the ones passed to it via
		153	pointers).
		154
		155	While these functions cannot be optimised as aggressively as C<ecb_const>
		156	functions, they can still be optimised away in many occasions, and the
		157	compiler has more freedom in moving calls to them around.
		158
		159	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		160	function that calculates the MD5 sum of some input and updates some MD5
		161	state passed as argument would I<NOT> be pure, however, as it would modify
		162	some memory area that is not the return value.
		163
54	=item ecb_hot	164	=item ecb_hot
55		165
		166	This declares a function as "hot" with regards to the cache - the function
		167	is used so often, that it is very beneficial to keep it in the cache if
		168	possible.
		169
		170	The compiler reacts by trying to place hot functions near to each other in
		171	memory.
		172
		173	Whether a function is hot or not often depends on the whole program,
		174	and less on the function itself. C<ecb_cold> is likely more useful in
		175	practise.
		176
56	=item ecb_cold	177	=item ecb_cold
57		178
		179	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		180	the cache, or in other words, this function is not called often, or not at
		181	speed-critical times, and keeping it in the cache might be a waste of said
		182	cache.
		183
		184	In addition to placing cold functions together (or at least away from hot
		185	functions), this knowledge can be used in other ways, for example, the
		186	function will be optimised for size, as opposed to speed, and codepaths
		187	leading to calls to those functions can automatically be marked as if
		188	C<ecb_unlikely> had been used to reach them.
		189
		190	Good examples for such functions would be error reporting functions, or
		191	functions only called in exceptional or rare cases.
		192
58	=item ecb_artificial	193	=item ecb_artificial
59		194
		195	Declares the function as "artificial", in this case meaning that this
		196	function is not really mean to be a function, but more like an accessor
		197	- many methods in C++ classes are mere accessor functions, and having a
		198	crash reported in such a method, or single-stepping through them, is not
		199	usually so helpful, especially when it's inlined to just a few instructions.
		200
		201	Marking them as artificial will instruct the debugger about just this,
		202	leading to happier debugging and thus happier lives.
		203
		204	Example: in some kind of smart-pointer class, mark the pointer accessor as
		205	artificial, so that the whole class acts more like a pointer and less like
		206	some C++ abstraction monster.
		207
		208	template<typename T>
		209	struct my_smart_ptr
		210	{
		211	T *value;
		212
		213	ecb_artificial
		214	operator T *()
		215	{
		216	return value;
		217	}
		218	};
		219
60	=back	220	=back
61		221
62	=head2 OPTIMISATION HINTS	222	=head2 OPTIMISATION HINTS
63		223
64	=over 4	224	=over 4
65		225
66	=item bool ecb_is_constant(expr) [MACRO]	226	=item bool ecb_is_constant(expr)
67		227
68	Returns true iff the expression can be deduced to be a compile-time	228	Returns true iff the expression can be deduced to be a compile-time
69	constant, and false otherwise.	229	constant, and false otherwise.
70		230
71	For example, when you have a C<rndm16> function that returns a 16 bit	231	For example, when you have a C<rndm16> function that returns a 16 bit
89	return is_constant (n) && !(n & (n - 1))	249	return is_constant (n) && !(n & (n - 1))
90	? rndm16 () & (num - 1)	250	? rndm16 () & (num - 1)
91	: (n * (uint32_t)rndm16 ()) >> 16;	251	: (n * (uint32_t)rndm16 ()) >> 16;
92	}	252	}
93		253
94	=item bool ecb_expect (expr, value) [MACRO]	254	=item bool ecb_expect (expr, value)
95		255
96	Evaluates C<expr> and returns it. In addition, it tells the compiler that	256	Evaluates C<expr> and returns it. In addition, it tells the compiler that
97	the C<expr> evaluates to C<value> a lot, which can be used for static	257	the C<expr> evaluates to C<value> a lot, which can be used for static
98	branch optimisations.	258	branch optimisations.
99		259
100	Usually, you want to use the more intuitive C<ecb_likely> and	260	Usually, you want to use the more intuitive C<ecb_likely> and
101	C<ecb_unlikely> functions instead.	261	C<ecb_unlikely> functions instead.
102		262
103	=item bool ecb_likely (bool) [MACRO]	263	=item bool ecb_likely (cond)
104		264
105	=item bool ecb_unlikely (bool) [MACRO]	265	=item bool ecb_unlikely (cond)
106		266
107	These two functions expect a expression that is true or false and return	267	These two functions expect a expression that is true or false and return
108	C<1> or C<0>, respectively, so when used in the condition of an C<if> or	268	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
109	other conditional statement, it will not change the program:	269	other conditional statement, it will not change the program:
110		270
…		…
140	{	300	{
141	if (ecb_unlikely (current + size > end))	301	if (ecb_unlikely (current + size > end))
142	real_reserve_method (size); /* presumably noinline */	302	real_reserve_method (size); /* presumably noinline */
143	}	303	}
144		304
145	=item bool ecb_assume (cond) [MACRO]	305	=item bool ecb_assume (cond)
146		306
147	Try to tell the compiler that some condition is true, even if it's not	307	Try to tell the compiler that some condition is true, even if it's not
148	obvious.	308	obvious.
149		309
150	This can be used to teach the compiler about invariants or other	310	This can be used to teach the compiler about invariants or other
…		…
176		336
177	This function does nothing itself, except tell the compiler that it will	337	This function does nothing itself, except tell the compiler that it will
178	never be executed. Apart from suppressing a warning in some cases, this	338	never be executed. Apart from suppressing a warning in some cases, this
179	function can be used to implement C<ecb_assume> or similar functions.	339	function can be used to implement C<ecb_assume> or similar functions.
180		340
181	=item bool ecb_prefetch (addr, rw, locality) [MACRO]	341	=item bool ecb_prefetch (addr, rw, locality)
182		342
183	Tells the compiler to try to prefetch memory at the given C<addr>ess	343	Tells the compiler to try to prefetch memory at the given C<addr>ess
184	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of	344	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
185	C<0> means that there will only be one access later, C<3> means that	345	C<0> means that there will only be one access later, C<3> means that
186	the data will likely be accessed very often, and values in between mean	346	the data will likely be accessed very often, and values in between mean
…		…
234	equivalently the number of bits set to 0 before the least significant	394	equivalently the number of bits set to 0 before the least significant
235	bit set), starting from 0. If C<x> is 0 the result is undefined. A	395	bit set), starting from 0. If C<x> is 0 the result is undefined. A
236	common use case is to compute the integer binary logarithm, i.e.,	396	common use case is to compute the integer binary logarithm, i.e.,
237	floor(log2(n)). For example:	397	floor(log2(n)). For example:
238		398
239	ecb_ctz32(3) = 1	399	ecb_ctz32 (3) = 0
240	ecb_ctz32(6) = 2	400	ecb_ctz32 (6) = 1
241		401
242	=item int ecb_popcount32 (uint32_t x)	402	=item int ecb_popcount32 (uint32_t x)
243		403
244	Returns the number of bits set to 1 in C<x>. For example:	404	Returns the number of bits set to 1 in C<x>. For example:
245		405
246	ecb_popcount32(7) = 3	406	ecb_popcount32 (7) = 3
247	ecb_popcount32(255) = 8	407	ecb_popcount32 (255) = 8
248		408
249	=item uint32_t ecb_bswap16 (uint32_t x)	409	=item uint32_t ecb_bswap16 (uint32_t x)
250		410
251	=item uint32_t ecb_bswap32 (uint32_t x)	411	=item uint32_t ecb_bswap32 (uint32_t x)
		412
		413	These two functions return the value of the 16-bit (32-bit) variable
		414	C<x> after reversing the order of bytes.
252		415
253	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	416	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
254		417
255	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	418	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
256		419
257	These two functions return the value of C<x> after shifting all the bits	420	These two functions return the value of C<x> after shifting all the bits
258	by C<count> positions to the right or left respectively.	421	by C<count> positions to the right or left respectively.
259		422
		423	Current GCC versions understand these functions and usually compile them
		424	to "optimal" code (e.g. a single C<roll> on x86).
		425
260	=back	426	=back
261		427
262	=head2 ARITHMETIC	428	=head2 ARITHMETIC
263		429
264	=over 4	430	=over 4
265		431
266	=item x = ecb_mod (m, n) [MACRO]	432	=item x = ecb_mod (m, n)
267		433
268	Returns the positive remainder of the modulo operation between C<m>	434	Returns the positive remainder of the modulo operation between C<m> and
269	and C<n>.	435	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
		436	return value is always positive - ISO C guarantees very little when
		437	negative numbers are used with C<%>.
		438
		439	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
		440	negatable, that is, both C<m> and C<-m> must be representable in its
		441	type.
270		442
271	=back	443	=back
272		444
273	=head2 UTILITY	445	=head2 UTILITY
274		446
275	=over 4	447	=over 4
276		448
277	=item element_count = ecb_array_length (name) [MACRO]	449	=item element_count = ecb_array_length (name) [MACRO]
278		450
		451	Returns the number of elements in the array C<name>. For example:
		452
		453	int primes[] = { 2, 3, 5, 7, 11 };
		454	int sum = 0;
		455
		456	for (i = 0; i < ecb_array_length (primes); i++)
		457	sum += primes [i];
		458
279	=back	459	=back
280		460
281		461

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Comparing libecb/ecb.pod (file contents): Revision 1.11 by sf-exg, Thu May 26 21:18:52 2011 UTC vs. Revision 1.20 by root, Fri May 27 00:12:00 2011 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.11 by sf-exg, Thu May 26 21:18:52 2011 UTC vs.
Revision 1.20 by root, Fri May 27 00:12:00 2011 UTC