[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 1.24 by root, Tue May 31 21:10:37 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 into any standard C system,
		21	but aren't, implemented as efficient as possible with GCC, and still
		22	correct with other compilers.
		23
		24	More might come.
4		25
5	=head2 ABOUT THE HEADER	26	=head2 ABOUT THE HEADER
6		27
7	- how to include it	28	At the moment, all you have to do is copy F<ecb.h> somewhere where your
8	- it includes inttypes.h	29	compiler can find it and include it:
9	- no .a	30
10	- whats a bool	31	#include <ecb.h>
11	- function mean macro or function	32
12	- macro means untyped	33	The header should work fine for both C and C++ compilation, and gives you
		34	all of F<inttypes.h> in addition to the ECB symbols.
		35
		36	There are currently no object files to link to - future versions might
		37	come with an (optional) object code library to link against, to reduce
		38	code size or gain access to additional features.
		39
		40	It also currently includes everything from F<inttypes.h>.
		41
		42	=head2 ABOUT THIS MANUAL / CONVENTIONS
		43
		44	This manual mainly describes each (public) function available after
		45	including the F<ecb.h> header. The header might define other symbols than
		46	these, but these are not part of the public API, and not supported in any
		47	way.
		48
		49	When the manual mentions a "function" then this could be defined either as
		50	as inline function, a macro, or an external symbol.
		51
		52	When functions use a concrete standard type, such as C<int> or
		53	C<uint32_t>, then the corresponding function works only with that type. If
		54	only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
		55	the corresponding function relies on C to implement the correct types, and
		56	is usually implemented as a macro. Specifically, a "bool" in this manual
		57	refers to any kind of boolean value, not a specific type.
13		58
14	=head2 GCC ATTRIBUTES	59	=head2 GCC ATTRIBUTES
15		60
16	blabla where to put, what others	61	A major part of libecb deals with GCC attributes. These are additional
		62	attributes that you cna assign to functions, variables and sometimes even
		63	types - much like C<const> or C<volatile> in C.
		64
		65	While GCC allows declarations to show up in many surprising places,
		66	but not in many expeted places, the safest way is to put attribute
		67	declarations before the whole declaration:
		68
		69	ecb_const int mysqrt (int a);
		70	ecb_unused int i;
		71
		72	For variables, it is often nicer to put the attribute after the name, and
		73	avoid multiple declarations using commas:
		74
		75	int i ecb_unused;
17		76
18	=over 4	77	=over 4
19		78
20	=item ecb_attribute ((attrs...))	79	=item ecb_attribute ((attrs...))
21		80
22	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and	81	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.	82	nothing on other compilers, so the effect is that only GCC sees these.
		83
		84	Example: use the C<deprecated> attribute on a function.
		85
		86	ecb_attribute((__deprecated__)) void
		87	do_not_use_me_anymore (void);
24		88
25	=item ecb_unused	89	=item ecb_unused
26		90
27	Marks a function or a variable as "unused", which simply suppresses a	91	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.	92	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:	93	declare a variable but do not always use it:
30		94
31	{	95	{
32	int var ecb_unused;	96	int var ecb_unused;
33		97
34	#ifdef SOMECONDITION	98	#ifdef SOMECONDITION
35	var = ...;	99	var = ...;
36	return var;	100	return var;
37	#else	101	#else
38	return 0;	102	return 0;
39	#endif	103	#endif
40	}	104	}
41		105
42	=item ecb_noinline	106	=item ecb_noinline
43		107
44	Prevent a function from being inlined - it might be optimised away, but	108	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	109	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.	110	is rarely called and large enough for inlining not to be helpful.
47		111
48	=item ecb_noreturn	112	=item ecb_noreturn
49		113
		114	Marks a function as "not returning, ever". Some typical functions that
		115	don't return are C<exit> or C<abort> (which really works hard to not
		116	return), and now you can make your own:
		117
		118	ecb_noreturn void
		119	my_abort (const char *errline)
		120	{
		121	puts (errline);
		122	abort ();
		123	}
		124
		125	In this case, the compiler would probably be smart enough to deduce it on
		126	its own, so this is mainly useful for declarations.
		127
50	=item ecb_const	128	=item ecb_const
51		129
		130	Declares that the function only depends on the values of its arguments,
		131	much like a mathematical function. It specifically does not read or write
		132	any memory any arguments might point to, global variables, or call any
		133	non-const functions. It also must not have any side effects.
		134
		135	Such a function can be optimised much more aggressively by the compiler -
		136	for example, multiple calls with the same arguments can be optimised into
		137	a single call, which wouldn't be possible if the compiler would have to
		138	expect any side effects.
		139
		140	It is best suited for functions in the sense of mathematical functions,
		141	such as a function returning the square root of its input argument.
		142
		143	Not suited would be a function that calculates the hash of some memory
		144	area you pass in, prints some messages or looks at a global variable to
		145	decide on rounding.
		146
		147	See C<ecb_pure> for a slightly less restrictive class of functions.
		148
52	=item ecb_pure	149	=item ecb_pure
53		150
		151	Similar to C<ecb_const>, declares a function that has no side
		152	effects. Unlike C<ecb_const>, the function is allowed to examine global
		153	variables and any other memory areas (such as the ones passed to it via
		154	pointers).
		155
		156	While these functions cannot be optimised as aggressively as C<ecb_const>
		157	functions, they can still be optimised away in many occasions, and the
		158	compiler has more freedom in moving calls to them around.
		159
		160	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		161	function that calculates the MD5 sum of some input and updates some MD5
		162	state passed as argument would I<NOT> be pure, however, as it would modify
		163	some memory area that is not the return value.
		164
54	=item ecb_hot	165	=item ecb_hot
55		166
		167	This declares a function as "hot" with regards to the cache - the function
		168	is used so often, that it is very beneficial to keep it in the cache if
		169	possible.
		170
		171	The compiler reacts by trying to place hot functions near to each other in
		172	memory.
		173
		174	Whether a function is hot or not often depends on the whole program,
		175	and less on the function itself. C<ecb_cold> is likely more useful in
		176	practise.
		177
56	=item ecb_cold	178	=item ecb_cold
57		179
		180	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		181	the cache, or in other words, this function is not called often, or not at
		182	speed-critical times, and keeping it in the cache might be a waste of said
		183	cache.
		184
		185	In addition to placing cold functions together (or at least away from hot
		186	functions), this knowledge can be used in other ways, for example, the
		187	function will be optimised for size, as opposed to speed, and codepaths
		188	leading to calls to those functions can automatically be marked as if
		189	C<ecb_unlikely> had been used to reach them.
		190
		191	Good examples for such functions would be error reporting functions, or
		192	functions only called in exceptional or rare cases.
		193
58	=item ecb_artificial	194	=item ecb_artificial
59		195
		196	Declares the function as "artificial", in this case meaning that this
		197	function is not really mean to be a function, but more like an accessor
		198	- many methods in C++ classes are mere accessor functions, and having a
		199	crash reported in such a method, or single-stepping through them, is not
		200	usually so helpful, especially when it's inlined to just a few instructions.
		201
		202	Marking them as artificial will instruct the debugger about just this,
		203	leading to happier debugging and thus happier lives.
		204
		205	Example: in some kind of smart-pointer class, mark the pointer accessor as
		206	artificial, so that the whole class acts more like a pointer and less like
		207	some C++ abstraction monster.
		208
		209	template<typename T>
		210	struct my_smart_ptr
		211	{
		212	T *value;
		213
		214	ecb_artificial
		215	operator T *()
		216	{
		217	return value;
		218	}
		219	};
		220
60	=back	221	=back
61		222
62	=head2 OPTIMISATION HINTS	223	=head2 OPTIMISATION HINTS
63		224
64	=over 4	225	=over 4
65		226
66	=item bool ecb_is_constant(expr) [MACRO]	227	=item bool ecb_is_constant(expr)
67		228
68	Returns true iff the expression can be deduced to be a compile-time	229	Returns true iff the expression can be deduced to be a compile-time
69	constant, and false otherwise.	230	constant, and false otherwise.
70		231
71	For example, when you have a C<rndm16> function that returns a 16 bit	232	For example, when you have a C<rndm16> function that returns a 16 bit
89	return is_constant (n) && !(n & (n - 1))	250	return is_constant (n) && !(n & (n - 1))
90	? rndm16 () & (num - 1)	251	? rndm16 () & (num - 1)
91	: (n * (uint32_t)rndm16 ()) >> 16;	252	: (n * (uint32_t)rndm16 ()) >> 16;
92	}	253	}
93		254
94	=item bool ecb_expect (expr, value) [MACRO]	255	=item bool ecb_expect (expr, value)
95		256
96	Evaluates C<expr> and returns it. In addition, it tells the compiler that	257	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	258	the C<expr> evaluates to C<value> a lot, which can be used for static
98	branch optimisations.	259	branch optimisations.
99		260
100	Usually, you want to use the more intuitive C<ecb_likely> and	261	Usually, you want to use the more intuitive C<ecb_likely> and
101	C<ecb_unlikely> functions instead.	262	C<ecb_unlikely> functions instead.
102		263
103	=item bool ecb_likely (bool) [MACRO]	264	=item bool ecb_likely (cond)
104		265
105	=item bool ecb_unlikely (bool) [MACRO]	266	=item bool ecb_unlikely (cond)
106		267
107	These two functions expect a expression that is true or false and return	268	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	269	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:	270	other conditional statement, it will not change the program:
110		271
…		…
140	{	301	{
141	if (ecb_unlikely (current + size > end))	302	if (ecb_unlikely (current + size > end))
142	real_reserve_method (size); /* presumably noinline */	303	real_reserve_method (size); /* presumably noinline */
143	}	304	}
144		305
145	=item bool ecb_assume (cond) [MACRO]	306	=item bool ecb_assume (cond)
146		307
147	Try to tell the compiler that some condition is true, even if it's not	308	Try to tell the compiler that some condition is true, even if it's not
148	obvious.	309	obvious.
149		310
150	This can be used to teach the compiler about invariants or other	311	This can be used to teach the compiler about invariants or other
…		…
176		337
177	This function does nothing itself, except tell the compiler that it will	338	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	339	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.	340	function can be used to implement C<ecb_assume> or similar functions.
180		341
181	=item bool ecb_prefetch (addr, rw, locality) [MACRO]	342	=item bool ecb_prefetch (addr, rw, locality)
182		343
183	Tells the compiler to try to prefetch memory at the given C<addr>ess	344	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	345	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	346	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	347	the data will likely be accessed very often, and values in between mean
…		…
226		387
227	These two functions return true if the byte order is big endian	388	These two functions return true if the byte order is big endian
228	(most-significant byte first) or little endian (least-significant byte	389	(most-significant byte first) or little endian (least-significant byte
229	first) respectively.	390	first) respectively.
230		391
		392	On systems that are neither, their return values are unspecified.
		393
231	=item int ecb_ctz32 (uint32_t x)	394	=item int ecb_ctz32 (uint32_t x)
232		395
233	Returns the index of the least significant bit set in C<x> (or	396	Returns the index of the least significant bit set in C<x> (or
234	equivalently the number of bits set to 0 before the least significant	397	equivalently the number of bits set to 0 before the least significant bit
235	bit set), starting from 0. If C<x> is 0 the result is undefined. A	398	set), starting from 0. If C<x> is 0 the result is undefined. A common use
236	common use case is to compute the integer binary logarithm, i.e.,	399	case is to compute the integer binary logarithm, i.e., C<floor (log2
237	floor(log2(n)). For example:	400	(n))>. For example:
238		401
239	ecb_ctz32(3) = 0	402	ecb_ctz32 (3) = 0
240	ecb_ctz32(6) = 1	403	ecb_ctz32 (6) = 1
241		404
242	=item int ecb_popcount32 (uint32_t x)	405	=item int ecb_popcount32 (uint32_t x)
243		406
244	Returns the number of bits set to 1 in C<x>. For example:	407	Returns the number of bits set to 1 in C<x>. For example:
245		408
246	ecb_popcount32(7) = 3	409	ecb_popcount32 (7) = 3
247	ecb_popcount32(255) = 8	410	ecb_popcount32 (255) = 8
248		411
249	=item uint32_t ecb_bswap16 (uint32_t x)	412	=item uint32_t ecb_bswap16 (uint32_t x)
250		413
251	=item uint32_t ecb_bswap32 (uint32_t x)	414	=item uint32_t ecb_bswap32 (uint32_t x)
252		415
253	These two functions return the value of the 16-bit (32-bit) variable	416	These two functions return the value of the 16-bit (32-bit) value C<x>
254	C<x> after reversing the order of bytes.	417	after reversing the order of bytes (0x11223344 becomes 0x44332211).
255		418
256	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	419	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
257		420
258	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	421	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
259		422
260	These two functions return the value of C<x> after shifting all the bits	423	These two functions return the value of C<x> after rotating all the bits
261	by C<count> positions to the right or left respectively.	424	by C<count> positions to the right or left respectively.
262		425
		426	Current GCC versions understand these functions and usually compile them
		427	to "optimal" code (e.g. a single C<roll> on x86).
		428
263	=back	429	=back
264		430
265	=head2 ARITHMETIC	431	=head2 ARITHMETIC
266		432
267	=over 4	433	=over 4
268		434
269	=item x = ecb_mod (m, n) [MACRO]	435	=item x = ecb_mod (m, n)
270		436
271	Returns the positive remainder of the modulo operation between C<m>	437	Returns the positive remainder of the modulo operation between C<m> and
272	and C<n>.	438	C<n>, using floored division. Unlike the C modulo operator C<%>, this
		439	function ensures that the return value is always positive and that the two
		440	numbers I<m> and I<m' = m + i * n> result in the same value modulo I<n> -
		441	the C<%> operator usually has a behaviour change at C<m = 0>.
		442
		443	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
		444	negatable, that is, both C<m> and C<-m> must be representable in its
		445	type.
		446
		447	Current GCC versions compile this into an efficient branchless sequence on
		448	many systems.
		449
		450	For example, when you want to rotate forward through the members of an
		451	array for increasing C<m> (which might be negative), then you should use
		452	C<ecb_mod>, as the C<%> operator might give either negative results, or
		453	change direction for negative values:
		454
		455	for (m = -100; m <= 100; ++m)
		456	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
273		457
274	=back	458	=back
275		459
276	=head2 UTILITY	460	=head2 UTILITY
277		461
278	=over 4	462	=over 4
279		463
280	=item element_count = ecb_array_length (name) [MACRO]	464	=item element_count = ecb_array_length (name)
281		465
282	Returns the number of elements in the array C<name>. For example:	466	Returns the number of elements in the array C<name>. For example:
283		467
284	int primes[] = { 2, 3, 5, 7, 11 };	468	int primes[] = { 2, 3, 5, 7, 11 };
285	int sum = 0;	469	int sum = 0;

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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 1.24 by root, Tue May 31 21:10:37 2011 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.13 by sf-exg, Thu May 26 22:14:52 2011 UTC vs.
Revision 1.24 by root, Tue May 31 21:10:37 2011 UTC