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

Comparing cvsroot/libecb/ecb.pod (file contents):
Revision 1.13 by sf-exg, Thu May 26 22:14:52 2011 UTC vs.
Revision 1.27 by root, Wed Jun 1 01:29:36 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 can 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 expected 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_expect_false> 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_expect_true> and
101	C<ecb_unlikely> functions instead.	262	C<ecb_expect_false> functions instead.
102		263
103	=item bool ecb_likely (bool) [MACRO]	264	=item bool ecb_expect_true (cond)
104		265
105	=item bool ecb_unlikely (bool) [MACRO]	266	=item bool ecb_expect_false (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
111	/* these two do the same thing */	272	/* these two do the same thing */
112	if (some_condition) ...;	273	if (some_condition) ...;
113	if (ecb_likely (some_condition)) ...;	274	if (ecb_expect_true (some_condition)) ...;
114		275
115	However, by using C<ecb_likely>, you tell the compiler that the condition	276	However, by using C<ecb_expect_true>, you tell the compiler that the
116	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be	277	condition is likely to be true (and for C<ecb_expect_false>, that it is
117	true).	278	unlikely to be true).
118		279
119	For example, when you check for a null pointer and expect this to be a	280	For example, when you check for a null pointer and expect this to be a
120	rare, exceptional, case, then use C<ecb_unlikely>:	281	rare, exceptional, case, then use C<ecb_expect_false>:
121		282
122	void my_free (void *ptr)	283	void my_free (void *ptr)
123	{	284	{
124	if (ecb_unlikely (ptr == 0))	285	if (ecb_expect_false (ptr == 0))
125	return;	286	return;
126	}	287	}
127		288
128	Consequent use of these functions to mark away exceptional cases or to	289	Consequent use of these functions to mark away exceptional cases or to
129	tell the compiler what the hot path through a function is can increase	290	tell the compiler what the hot path through a function is can increase
130	performance considerably.	291	performance considerably.
		292
		293	You might know these functions under the name C<likely> and C<unlikely>
		294	- while these are common aliases, we find that the expect name is easier
		295	to understand when quickly skimming code. If you wish, you can use
		296	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		297	C<ecb_expect_false> - these are simply aliases.
131		298
132	A very good example is in a function that reserves more space for some	299	A very good example is in a function that reserves more space for some
133	memory block (for example, inside an implementation of a string stream) -	300	memory block (for example, inside an implementation of a string stream) -
134	each time something is added, you have to check for a buffer overrun, but	301	each time something is added, you have to check for a buffer overrun, but
135	you expect that most checks will turn out to be false:	302	you expect that most checks will turn out to be false:
136		303
137	/* make sure we have "size" extra room in our buffer */	304	/* make sure we have "size" extra room in our buffer */
138	ecb_inline void	305	ecb_inline void
139	reserve (int size)	306	reserve (int size)
140	{	307	{
141	if (ecb_unlikely (current + size > end))	308	if (ecb_expect_false (current + size > end))
142	real_reserve_method (size); /* presumably noinline */	309	real_reserve_method (size); /* presumably noinline */
143	}	310	}
144		311
145	=item bool ecb_assume (cond) [MACRO]	312	=item bool ecb_assume (cond)
146		313
147	Try to tell the compiler that some condition is true, even if it's not	314	Try to tell the compiler that some condition is true, even if it's not
148	obvious.	315	obvious.
149		316
150	This can be used to teach the compiler about invariants or other	317	This can be used to teach the compiler about invariants or other
151	conditions that might improve code generation, but which are impossible to	318	conditions that might improve code generation, but which are impossible to
152	deduce form the code itself.	319	deduce form the code itself.
153		320
154	For example, the example reservation function from the C<ecb_unlikely>	321	For example, the example reservation function from the C<ecb_expect_false>
155	description could be written thus (only C<ecb_assume> was added):	322	description could be written thus (only C<ecb_assume> was added):
156		323
157	ecb_inline void	324	ecb_inline void
158	reserve (int size)	325	reserve (int size)
159	{	326	{
160	if (ecb_unlikely (current + size > end))	327	if (ecb_expect_false (current + size > end))
161	real_reserve_method (size); /* presumably noinline */	328	real_reserve_method (size); /* presumably noinline */
162		329
163	ecb_assume (current + size <= end);	330	ecb_assume (current + size <= end);
164	}	331	}
165		332
…		…
176		343
177	This function does nothing itself, except tell the compiler that it will	344	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	345	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.	346	function can be used to implement C<ecb_assume> or similar functions.
180		347
181	=item bool ecb_prefetch (addr, rw, locality) [MACRO]	348	=item bool ecb_prefetch (addr, rw, locality)
182		349
183	Tells the compiler to try to prefetch memory at the given C<addr>ess	350	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	351	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	352	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	353	the data will likely be accessed very often, and values in between mean
…		…
226		393
227	These two functions return true if the byte order is big endian	394	These two functions return true if the byte order is big endian
228	(most-significant byte first) or little endian (least-significant byte	395	(most-significant byte first) or little endian (least-significant byte
229	first) respectively.	396	first) respectively.
230		397
		398	On systems that are neither, their return values are unspecified.
		399
231	=item int ecb_ctz32 (uint32_t x)	400	=item int ecb_ctz32 (uint32_t x)
232		401
233	Returns the index of the least significant bit set in C<x> (or	402	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	403	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	404	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.,	405	case is to compute the integer binary logarithm, i.e., C<floor (log2
237	floor(log2(n)). For example:	406	(n))>. For example:
238		407
239	ecb_ctz32(3) = 0	408	ecb_ctz32 (3) = 0
240	ecb_ctz32(6) = 1	409	ecb_ctz32 (6) = 1
241		410
242	=item int ecb_popcount32 (uint32_t x)	411	=item int ecb_popcount32 (uint32_t x)
243		412
244	Returns the number of bits set to 1 in C<x>. For example:	413	Returns the number of bits set to 1 in C<x>. For example:
245		414
246	ecb_popcount32(7) = 3	415	ecb_popcount32 (7) = 3
247	ecb_popcount32(255) = 8	416	ecb_popcount32 (255) = 8
248		417
249	=item uint32_t ecb_bswap16 (uint32_t x)	418	=item uint32_t ecb_bswap16 (uint32_t x)
250		419
251	=item uint32_t ecb_bswap32 (uint32_t x)	420	=item uint32_t ecb_bswap32 (uint32_t x)
252		421
253	These two functions return the value of the 16-bit (32-bit) variable	422	These two functions return the value of the 16-bit (32-bit) value C<x>
254	C<x> after reversing the order of bytes.	423	after reversing the order of bytes (0x11223344 becomes 0x44332211).
255		424
256	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	425	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
257		426
258	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	427	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
259		428
260	These two functions return the value of C<x> after shifting all the bits	429	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.	430	by C<count> positions to the right or left respectively.
262		431
		432	Current GCC versions understand these functions and usually compile them
		433	to "optimal" code (e.g. a single C<roll> on x86).
		434
263	=back	435	=back
264		436
265	=head2 ARITHMETIC	437	=head2 ARITHMETIC
266		438
267	=over 4	439	=over 4
268		440
269	=item x = ecb_mod (m, n) [MACRO]	441	=item x = ecb_mod (m, n)
270		442
271	Returns the positive remainder of the modulo operation between C<m>	443	Returns C<m> modulo C<n>, which is the same as the positive remainder
272	and C<n>.	444	of the division operation between C<m> and C<n>, using floored
		445	division. Unlike the C remainder operator C<%>, this function ensures that
		446	the return value is always positive and that the two numbers I<m> and
		447	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		448	C<ecb_mod> implements the mathematical modulo operation, which is missing
		449	in the language.
		450
		451	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
		452	negatable, that is, both C<m> and C<-m> must be representable in its
		453	type (this typically includes the minimum signed integer value, the same
		454	limitation as for C</> and C<%> in C).
		455
		456	Current GCC versions compile this into an efficient branchless sequence on
		457	many systems.
		458
		459	For example, when you want to rotate forward through the members of an
		460	array for increasing C<m> (which might be negative), then you should use
		461	C<ecb_mod>, as the C<%> operator might give either negative results, or
		462	change direction for negative values:
		463
		464	for (m = -100; m <= 100; ++m)
		465	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
273		466
274	=back	467	=back
275		468
276	=head2 UTILITY	469	=head2 UTILITY
277		470
278	=over 4	471	=over 4
279		472
280	=item element_count = ecb_array_length (name) [MACRO]	473	=item element_count = ecb_array_length (name)
281		474
282	Returns the number of elements in the array C<name>. For example:	475	Returns the number of elements in the array C<name>. For example:
283		476
284	int primes[] = { 2, 3, 5, 7, 11 };	477	int primes[] = { 2, 3, 5, 7, 11 };
285	int sum = 0;	478	int sum = 0;

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Comparing cvsroot/libecb/ecb.pod (file contents): Revision 1.13 by sf-exg, Thu May 26 22:14:52 2011 UTC vs. Revision 1.27 by root, Wed Jun 1 01:29:36 2011 UTC

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Comparing cvsroot/libecb/ecb.pod (file contents):
Revision 1.13 by sf-exg, Thu May 26 22:14:52 2011 UTC vs.
Revision 1.27 by root, Wed Jun 1 01:29:36 2011 UTC