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

Comparing libecb/ecb.pod (file contents):
Revision 1.13 by sf-exg, Thu May 26 22:14:52 2011 UTC vs.
Revision 1.18 by root, Fri May 27 00:01:28 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	blabla where to put, what others
17		61
18	=over 4	62	=over 4
19		63
20	=item ecb_attribute ((attrs...))	64	=item ecb_attribute ((attrs...))
21		65
22	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and	66	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.	67	nothing on other compilers, so the effect is that only GCC sees these.
		68
		69	Example: use the C<deprecated> attribute on a function.
		70
		71	ecb_attribute((__deprecated__)) void
		72	do_not_use_me_anymore (void);
24		73
25	=item ecb_unused	74	=item ecb_unused
26		75
27	Marks a function or a variable as "unused", which simply suppresses a	76	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.	77	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:	78	declare a variable but do not always use it:
30		79
31	{	80	{
32	int var ecb_unused;	81	int var ecb_unused;
33		82
34	#ifdef SOMECONDITION	83	#ifdef SOMECONDITION
35	var = ...;	84	var = ...;
36	return var;	85	return var;
37	#else	86	#else
38	return 0;	87	return 0;
39	#endif	88	#endif
40	}	89	}
41		90
42	=item ecb_noinline	91	=item ecb_noinline
43		92
44	Prevent a function from being inlined - it might be optimised away, but	93	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	94	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.	95	is rarely called and large enough for inlining not to be helpful.
47		96
48	=item ecb_noreturn	97	=item ecb_noreturn
49		98
		99	Marks a function as "not returning, ever". Some typical functions that
		100	don't return are C<exit> or C<abort> (which really works hard to not
		101	return), and now you can make your own:
		102
		103	ecb_noreturn void
		104	my_abort (const char *errline)
		105	{
		106	puts (errline);
		107	abort ();
		108	}
		109
		110	In this case, the compiler would probbaly be smart enough to decude it on
		111	it's own, so this is mainly useful for declarations.
		112
50	=item ecb_const	113	=item ecb_const
51		114
		115	Declares that the function only depends on the values of it's arguments,
		116	much like a mathematical function. It specifically does not read or write
		117	any memory any arguments might point to, global variables, or call any
		118	non-const functions. It also must not have any side effects.
		119
		120	Such a function can be optimised much more aggressively by the compiler -
		121	for example, multiple calls with the same arguments can be optimised into
		122	a single call, which wouldn't be possible if the compiler would have to
		123	expect any side effects.
		124
		125	It is best suited for functions in the sense of mathematical functions,
		126	such as a function return the square root of its input argument.
		127
		128	Not suited would be a function that calculates the hash of some memory
		129	area you pass in, prints some messages or looks at a global variable to
		130	decide on rounding.
		131
		132	See C<ecb_pure> for a slightly less restrictive class of functions.
		133
52	=item ecb_pure	134	=item ecb_pure
53		135
		136	Similar to C<ecb_const>, declares a function that has no side
		137	effects. Unlike C<ecb_const>, the function is allowed to examine global
		138	variables and any other memory areas (such as the ones passed to it via
		139	pointers).
		140
		141	While these functions cannot be optimised as aggressively as C<ecb_const>
		142	functions, they can still be optimised away in many occasions, and the
		143	compiler has more freedom in moving calls to them around.
		144
		145	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		146	function that calculates the MD5 sum of some input and updates some MD5
		147	state passed as argument would I<NOT> be pure, however, as it would modify
		148	some memory area that is not the return value.
		149
54	=item ecb_hot	150	=item ecb_hot
55		151
		152	This declares a function as "hot" with regards to the cache - the function
		153	is used so often, that it is very beneficial to keep it in the cache if
		154	possible.
		155
		156	The compiler reacts by trying to place hot functions near to each other in
		157	memory.
		158
		159	Whether a function is hot or not often depend son the whole program,
		160	and less on the function itself. C<ecb_cold> is likely more useful in
		161	practise.
		162
56	=item ecb_cold	163	=item ecb_cold
57		164
		165	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		166	the cache, or in other words, this function is not called often, or not at
		167	speed-critical times, and keeping it in the cache might be a waste of said
		168	cache.
		169
		170	In addition to placing cold functions together (or at least away from hot
		171	functions), this knowledge can be used in other ways, for example, the
		172	function will be optimised for size, as opposed to speed, and codepaths
		173	leading to calls to those functions can automatically be marked as if
		174	C<ecb_unlikel> had been used to reach them.
		175
		176	Good examples for such functions would be error reporting functions, or
		177	functions only called in exceptional or rare cases.
		178
58	=item ecb_artificial	179	=item ecb_artificial
59		180
		181	Declares the function as "artificial", in this case meaning that this
		182	function is not really mean to be a function, but more like an accessor
		183	- many methods in C++ classes are mere accessor functions, and having a
		184	crash reported in such a method, or single-stepping through them, is not
		185	usually so helpful, especially when it's inlined to just a few instructions.
		186
		187	Marking them as artificial will instruct the debugger about just this,
		188	leading to happier debugging and thus happier lives.
		189
		190	Example: in some kind of smart-pointer class, mark the pointer accessor as
		191	artificial, so that the whole class acts more like a pointer and less like
		192	some C++ abstraction monster.
		193
		194	template<typename T>
		195	struct my_smart_ptr
		196	{
		197	T *value;
		198
		199	ecb_artificial
		200	operator T *()
		201	{
		202	return value;
		203	}
		204	};
		205
60	=back	206	=back
61		207
62	=head2 OPTIMISATION HINTS	208	=head2 OPTIMISATION HINTS
63		209
64	=over 4	210	=over 4
65		211
66	=item bool ecb_is_constant(expr) [MACRO]	212	=item bool ecb_is_constant(expr)
67		213
68	Returns true iff the expression can be deduced to be a compile-time	214	Returns true iff the expression can be deduced to be a compile-time
69	constant, and false otherwise.	215	constant, and false otherwise.
70		216
71	For example, when you have a C<rndm16> function that returns a 16 bit	217	For example, when you have a C<rndm16> function that returns a 16 bit
89	return is_constant (n) && !(n & (n - 1))	235	return is_constant (n) && !(n & (n - 1))
90	? rndm16 () & (num - 1)	236	? rndm16 () & (num - 1)
91	: (n * (uint32_t)rndm16 ()) >> 16;	237	: (n * (uint32_t)rndm16 ()) >> 16;
92	}	238	}
93		239
94	=item bool ecb_expect (expr, value) [MACRO]	240	=item bool ecb_expect (expr, value)
95		241
96	Evaluates C<expr> and returns it. In addition, it tells the compiler that	242	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	243	the C<expr> evaluates to C<value> a lot, which can be used for static
98	branch optimisations.	244	branch optimisations.
99		245
100	Usually, you want to use the more intuitive C<ecb_likely> and	246	Usually, you want to use the more intuitive C<ecb_likely> and
101	C<ecb_unlikely> functions instead.	247	C<ecb_unlikely> functions instead.
102		248
103	=item bool ecb_likely (bool) [MACRO]	249	=item bool ecb_likely (cond)
104		250
105	=item bool ecb_unlikely (bool) [MACRO]	251	=item bool ecb_unlikely (cond)
106		252
107	These two functions expect a expression that is true or false and return	253	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	254	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:	255	other conditional statement, it will not change the program:
110		256
…		…
140	{	286	{
141	if (ecb_unlikely (current + size > end))	287	if (ecb_unlikely (current + size > end))
142	real_reserve_method (size); /* presumably noinline */	288	real_reserve_method (size); /* presumably noinline */
143	}	289	}
144		290
145	=item bool ecb_assume (cond) [MACRO]	291	=item bool ecb_assume (cond)
146		292
147	Try to tell the compiler that some condition is true, even if it's not	293	Try to tell the compiler that some condition is true, even if it's not
148	obvious.	294	obvious.
149		295
150	This can be used to teach the compiler about invariants or other	296	This can be used to teach the compiler about invariants or other
…		…
176		322
177	This function does nothing itself, except tell the compiler that it will	323	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	324	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.	325	function can be used to implement C<ecb_assume> or similar functions.
180		326
181	=item bool ecb_prefetch (addr, rw, locality) [MACRO]	327	=item bool ecb_prefetch (addr, rw, locality)
182		328
183	Tells the compiler to try to prefetch memory at the given C<addr>ess	329	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	330	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	331	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	332	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	380	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	381	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.,	382	common use case is to compute the integer binary logarithm, i.e.,
237	floor(log2(n)). For example:	383	floor(log2(n)). For example:
238		384
239	ecb_ctz32(3) = 0	385	ecb_ctz32 (3) = 0
240	ecb_ctz32(6) = 1	386	ecb_ctz32 (6) = 1
241		387
242	=item int ecb_popcount32 (uint32_t x)	388	=item int ecb_popcount32 (uint32_t x)
243		389
244	Returns the number of bits set to 1 in C<x>. For example:	390	Returns the number of bits set to 1 in C<x>. For example:
245		391
246	ecb_popcount32(7) = 3	392	ecb_popcount32 (7) = 3
247	ecb_popcount32(255) = 8	393	ecb_popcount32 (255) = 8
248		394
249	=item uint32_t ecb_bswap16 (uint32_t x)	395	=item uint32_t ecb_bswap16 (uint32_t x)
250		396
251	=item uint32_t ecb_bswap32 (uint32_t x)	397	=item uint32_t ecb_bswap32 (uint32_t x)
252		398
…		…
264		410
265	=head2 ARITHMETIC	411	=head2 ARITHMETIC
266		412
267	=over 4	413	=over 4
268		414
269	=item x = ecb_mod (m, n) [MACRO]	415	=item x = ecb_mod (m, n)
270		416
271	Returns the positive remainder of the modulo operation between C<m>	417	Returns the positive remainder of the modulo operation between C<m> and
272	and C<n>.	418	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
		419	return value is always positive).
		420
		421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
		422	negatable, that is, both C<m> and C<-m> must be representable in its
		423	type.
273		424
274	=back	425	=back
275		426
276	=head2 UTILITY	427	=head2 UTILITY
277		428

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Comparing libecb/ecb.pod (file contents): Revision 1.13 by sf-exg, Thu May 26 22:14:52 2011 UTC vs. Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC

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Comparing libecb/ecb.pod (file contents):
Revision 1.13 by sf-exg, Thu May 26 22:14:52 2011 UTC vs.
Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC