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

Comparing libecb/ecb.pod (file contents):
Revision 1.14 by root, Thu May 26 23:23:08 2011 UTC vs.
Revision 1.42 by root, Mon May 28 08:54:03 2012 UTC

…		…
3	=head2 ABOUT LIBECB	3	=head2 ABOUT LIBECB
4		4
5	Libecb is currently a simple header file that doesn't require any	5	Libecb is currently a simple header file that doesn't require any
6	configuration to use or include in your project.	6	configuration to use or include in your project.
7		7
8	It's part of the e-suite of libraries, other memembers of which include	8	It's part of the e-suite of libraries, other members of which include
9	libev and libeio.	9	libev and libeio.
10		10
11	Its homepage can be found here:	11	Its homepage can be found here:
12		12
13	http://software.schmorp.de/pkg/libecb	13	http://software.schmorp.de/pkg/libecb
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 endienness	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
…		…
27	#include <ecb.h>	31	#include <ecb.h>
28		32
29	The header should work fine for both C and C++ compilation, and gives you	33	The header should work fine for both C and C++ compilation, and gives you
30	all of F<inttypes.h> in addition to the ECB symbols.	34	all of F<inttypes.h> in addition to the ECB symbols.
31		35
32	There are currently no objetc files to link to - future versions might	36	There are currently no object files to link to - future versions might
33	come with an (optional) object code library to link against, to reduce	37	come with an (optional) object code library to link against, to reduce
34	code size or gain access to additional features.	38	code size or gain access to additional features.
35		39
36	It also currently includes everything from F<inttypes.h>.	40	It also currently includes everything from F<inttypes.h>.
37		41
…		…
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
		59	=head2 TYPES / TYPE SUPPORT
		60
		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 ptrdiff_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>).
		69
55	=head2 GCC ATTRIBUTES	70	=head2 GCC ATTRIBUTES
56		71
57	blabla where to put, what others	72	A major part of libecb deals with GCC attributes. These are additional
		73	attributes that you can assign to functions, variables and sometimes even
		74	types - much like C<const> or C<volatile> in C.
		75
		76	While GCC allows declarations to show up in many surprising places,
		77	but not in many expected places, the safest way is to put attribute
		78	declarations before the whole declaration:
		79
		80	ecb_const int mysqrt (int a);
		81	ecb_unused int i;
		82
		83	For variables, it is often nicer to put the attribute after the name, and
		84	avoid multiple declarations using commas:
		85
		86	int i ecb_unused;
58		87
59	=over 4	88	=over 4
60		89
61	=item ecb_attribute ((attrs...))	90	=item ecb_attribute ((attrs...))
62		91
63	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and	92	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
64	to nothing on other compilers, so the effect is that only GCC sees these.	93	nothing on other compilers, so the effect is that only GCC sees these.
		94
		95	Example: use the C<deprecated> attribute on a function.
		96
		97	ecb_attribute((__deprecated__)) void
		98	do_not_use_me_anymore (void);
65		99
66	=item ecb_unused	100	=item ecb_unused
67		101
68	Marks a function or a variable as "unused", which simply suppresses a	102	Marks a function or a variable as "unused", which simply suppresses a
69	warning by GCC when it detects it as unused. This is useful when you e.g.	103	warning by GCC when it detects it as unused. This is useful when you e.g.
70	declare a variable but do not always use it:	104	declare a variable but do not always use it:
71		105
		106	{
		107	int var ecb_unused;
		108
		109	#ifdef SOMECONDITION
		110	var = ...;
		111	return var;
		112	#else
		113	return 0;
		114	#endif
		115	}
		116
		117	=item ecb_inline
		118
		119	This is not actually an attribute, but you use it like one. It expands
		120	either to C<static inline> or to just C<static>, if inline isn't
		121	supported. It should be used to declare functions that should be inlined,
		122	for code size or speed reasons.
		123
		124	Example: inline this function, it surely will reduce codesize.
		125
		126	ecb_inline int
		127	negmul (int a, int b)
72	{	128	{
73	int var ecb_unused;	129	return - (a * b);
74
75	#ifdef SOMECONDITION
76	var = ...;
77	return var;
78	#else
79	return 0;
80	#endif
81	}	130	}
82		131
83	=item ecb_noinline	132	=item ecb_noinline
84		133
85	Prevent a function from being inlined - it might be optimised away, but	134	Prevent a function from being inlined - it might be optimised away, but
86	not inlined into other functions. This is useful if you know your function	135	not inlined into other functions. This is useful if you know your function
87	is rarely called and large enough for inlining not to be helpful.	136	is rarely called and large enough for inlining not to be helpful.
88		137
89	=item ecb_noreturn	138	=item ecb_noreturn
90		139
		140	Marks a function as "not returning, ever". Some typical functions that
		141	don't return are C<exit> or C<abort> (which really works hard to not
		142	return), and now you can make your own:
		143
		144	ecb_noreturn void
		145	my_abort (const char *errline)
		146	{
		147	puts (errline);
		148	abort ();
		149	}
		150
		151	In this case, the compiler would probably be smart enough to deduce it on
		152	its own, so this is mainly useful for declarations.
		153
91	=item ecb_const	154	=item ecb_const
92		155
		156	Declares that the function only depends on the values of its arguments,
		157	much like a mathematical function. It specifically does not read or write
		158	any memory any arguments might point to, global variables, or call any
		159	non-const functions. It also must not have any side effects.
		160
		161	Such a function can be optimised much more aggressively by the compiler -
		162	for example, multiple calls with the same arguments can be optimised into
		163	a single call, which wouldn't be possible if the compiler would have to
		164	expect any side effects.
		165
		166	It is best suited for functions in the sense of mathematical functions,
		167	such as a function returning the square root of its input argument.
		168
		169	Not suited would be a function that calculates the hash of some memory
		170	area you pass in, prints some messages or looks at a global variable to
		171	decide on rounding.
		172
		173	See C<ecb_pure> for a slightly less restrictive class of functions.
		174
93	=item ecb_pure	175	=item ecb_pure
94		176
		177	Similar to C<ecb_const>, declares a function that has no side
		178	effects. Unlike C<ecb_const>, the function is allowed to examine global
		179	variables and any other memory areas (such as the ones passed to it via
		180	pointers).
		181
		182	While these functions cannot be optimised as aggressively as C<ecb_const>
		183	functions, they can still be optimised away in many occasions, and the
		184	compiler has more freedom in moving calls to them around.
		185
		186	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		187	function that calculates the MD5 sum of some input and updates some MD5
		188	state passed as argument would I<NOT> be pure, however, as it would modify
		189	some memory area that is not the return value.
		190
95	=item ecb_hot	191	=item ecb_hot
96		192
		193	This declares a function as "hot" with regards to the cache - the function
		194	is used so often, that it is very beneficial to keep it in the cache if
		195	possible.
		196
		197	The compiler reacts by trying to place hot functions near to each other in
		198	memory.
		199
		200	Whether a function is hot or not often depends on the whole program,
		201	and less on the function itself. C<ecb_cold> is likely more useful in
		202	practise.
		203
97	=item ecb_cold	204	=item ecb_cold
98		205
		206	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		207	the cache, or in other words, this function is not called often, or not at
		208	speed-critical times, and keeping it in the cache might be a waste of said
		209	cache.
		210
		211	In addition to placing cold functions together (or at least away from hot
		212	functions), this knowledge can be used in other ways, for example, the
		213	function will be optimised for size, as opposed to speed, and codepaths
		214	leading to calls to those functions can automatically be marked as if
		215	C<ecb_expect_false> had been used to reach them.
		216
		217	Good examples for such functions would be error reporting functions, or
		218	functions only called in exceptional or rare cases.
		219
99	=item ecb_artificial	220	=item ecb_artificial
		221
		222	Declares the function as "artificial", in this case meaning that this
		223	function is not really mean to be a function, but more like an accessor
		224	- many methods in C++ classes are mere accessor functions, and having a
		225	crash reported in such a method, or single-stepping through them, is not
		226	usually so helpful, especially when it's inlined to just a few instructions.
		227
		228	Marking them as artificial will instruct the debugger about just this,
		229	leading to happier debugging and thus happier lives.
		230
		231	Example: in some kind of smart-pointer class, mark the pointer accessor as
		232	artificial, so that the whole class acts more like a pointer and less like
		233	some C++ abstraction monster.
		234
		235	template<typename T>
		236	struct my_smart_ptr
		237	{
		238	T *value;
		239
		240	ecb_artificial
		241	operator T *()
		242	{
		243	return value;
		244	}
		245	};
100		246
101	=back	247	=back
102		248
103	=head2 OPTIMISATION HINTS	249	=head2 OPTIMISATION HINTS
104		250
…		…
136		282
137	Evaluates C<expr> and returns it. In addition, it tells the compiler that	283	Evaluates C<expr> and returns it. In addition, it tells the compiler that
138	the C<expr> evaluates to C<value> a lot, which can be used for static	284	the C<expr> evaluates to C<value> a lot, which can be used for static
139	branch optimisations.	285	branch optimisations.
140		286
141	Usually, you want to use the more intuitive C<ecb_likely> and	287	Usually, you want to use the more intuitive C<ecb_expect_true> and
142	C<ecb_unlikely> functions instead.	288	C<ecb_expect_false> functions instead.
143		289
144	=item bool ecb_likely (bool)	290	=item bool ecb_expect_true (cond)
145		291
146	=item bool ecb_unlikely (bool)	292	=item bool ecb_expect_false (cond)
147		293
148	These two functions expect a expression that is true or false and return	294	These two functions expect a expression that is true or false and return
149	C<1> or C<0>, respectively, so when used in the condition of an C<if> or	295	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
150	other conditional statement, it will not change the program:	296	other conditional statement, it will not change the program:
151		297
152	/* these two do the same thing */	298	/* these two do the same thing */
153	if (some_condition) ...;	299	if (some_condition) ...;
154	if (ecb_likely (some_condition)) ...;	300	if (ecb_expect_true (some_condition)) ...;
155		301
156	However, by using C<ecb_likely>, you tell the compiler that the condition	302	However, by using C<ecb_expect_true>, you tell the compiler that the
157	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be	303	condition is likely to be true (and for C<ecb_expect_false>, that it is
158	true).	304	unlikely to be true).
159		305
160	For example, when you check for a null pointer and expect this to be a	306	For example, when you check for a null pointer and expect this to be a
161	rare, exceptional, case, then use C<ecb_unlikely>:	307	rare, exceptional, case, then use C<ecb_expect_false>:
162		308
163	void my_free (void *ptr)	309	void my_free (void *ptr)
164	{	310	{
165	if (ecb_unlikely (ptr == 0))	311	if (ecb_expect_false (ptr == 0))
166	return;	312	return;
167	}	313	}
168		314
169	Consequent use of these functions to mark away exceptional cases or to	315	Consequent use of these functions to mark away exceptional cases or to
170	tell the compiler what the hot path through a function is can increase	316	tell the compiler what the hot path through a function is can increase
171	performance considerably.	317	performance considerably.
		318
		319	You might know these functions under the name C<likely> and C<unlikely>
		320	- while these are common aliases, we find that the expect name is easier
		321	to understand when quickly skimming code. If you wish, you can use
		322	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		323	C<ecb_expect_false> - these are simply aliases.
172		324
173	A very good example is in a function that reserves more space for some	325	A very good example is in a function that reserves more space for some
174	memory block (for example, inside an implementation of a string stream) -	326	memory block (for example, inside an implementation of a string stream) -
175	each time something is added, you have to check for a buffer overrun, but	327	each time something is added, you have to check for a buffer overrun, but
176	you expect that most checks will turn out to be false:	328	you expect that most checks will turn out to be false:
177		329
178	/* make sure we have "size" extra room in our buffer */	330	/* make sure we have "size" extra room in our buffer */
179	ecb_inline void	331	ecb_inline void
180	reserve (int size)	332	reserve (int size)
181	{	333	{
182	if (ecb_unlikely (current + size > end))	334	if (ecb_expect_false (current + size > end))
183	real_reserve_method (size); /* presumably noinline */	335	real_reserve_method (size); /* presumably noinline */
184	}	336	}
185		337
186	=item bool ecb_assume (cond)	338	=item bool ecb_assume (cond)
187		339
…		…
190		342
191	This can be used to teach the compiler about invariants or other	343	This can be used to teach the compiler about invariants or other
192	conditions that might improve code generation, but which are impossible to	344	conditions that might improve code generation, but which are impossible to
193	deduce form the code itself.	345	deduce form the code itself.
194		346
195	For example, the example reservation function from the C<ecb_unlikely>	347	For example, the example reservation function from the C<ecb_expect_false>
196	description could be written thus (only C<ecb_assume> was added):	348	description could be written thus (only C<ecb_assume> was added):
197		349
198	ecb_inline void	350	ecb_inline void
199	reserve (int size)	351	reserve (int size)
200	{	352	{
201	if (ecb_unlikely (current + size > end))	353	if (ecb_expect_false (current + size > end))
202	real_reserve_method (size); /* presumably noinline */	354	real_reserve_method (size); /* presumably noinline */
203		355
204	ecb_assume (current + size <= end);	356	ecb_assume (current + size <= end);
205	}	357	}
206		358
…		…
255	After processing the node, (part of) the next node might already be in	407	After processing the node, (part of) the next node might already be in
256	cache.	408	cache.
257		409
258	=back	410	=back
259		411
260	=head2 BIT FIDDLING / BITSTUFFS	412	=head2 BIT FIDDLING / BIT WIZARDRY
261		413
262	=over 4	414	=over 4
263		415
264	=item bool ecb_big_endian ()	416	=item bool ecb_big_endian ()
265		417
…		…
267		419
268	These two functions return true if the byte order is big endian	420	These two functions return true if the byte order is big endian
269	(most-significant byte first) or little endian (least-significant byte	421	(most-significant byte first) or little endian (least-significant byte
270	first) respectively.	422	first) respectively.
271		423
		424	On systems that are neither, their return values are unspecified.
		425
272	=item int ecb_ctz32 (uint32_t x)	426	=item int ecb_ctz32 (uint32_t x)
273		427
		428	=item int ecb_ctz64 (uint64_t x)
		429
274	Returns the index of the least significant bit set in C<x> (or	430	Returns the index of the least significant bit set in C<x> (or
275	equivalently the number of bits set to 0 before the least significant	431	equivalently the number of bits set to 0 before the least significant bit
276	bit set), starting from 0. If C<x> is 0 the result is undefined. A	432	set), starting from 0. If C<x> is 0 the result is undefined.
277	common use case is to compute the integer binary logarithm, i.e.,
278	floor(log2(n)). For example:
279		433
		434	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		435
		436	For example:
		437
280	ecb_ctz32(3) = 0	438	ecb_ctz32 (3) = 0
281	ecb_ctz32(6) = 1	439	ecb_ctz32 (6) = 1
		440
		441	=item bool ecb_is_pot32 (uint32_t x)
		442
		443	=item bool ecb_is_pot64 (uint32_t x)
		444
		445	Return true iff C<x> is a power of two or C<x == 0>.
		446
		447	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		448
		449	=item int ecb_ld32 (uint32_t x)
		450
		451	=item int ecb_ld64 (uint64_t x)
		452
		453	Returns the index of the most significant bit set in C<x>, or the number
		454	of digits the number requires in binary (so that C<< 2**ld <= x <
		455	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		456	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		457	example to see how many bits a certain number requires to be encoded.
		458
		459	This function is similar to the "count leading zero bits" function, except
		460	that that one returns how many zero bits are "in front" of the number (in
		461	the given data type), while C<ecb_ld> returns how many bits the number
		462	itself requires.
		463
		464	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
282		465
283	=item int ecb_popcount32 (uint32_t x)	466	=item int ecb_popcount32 (uint32_t x)
284		467
		468	=item int ecb_popcount64 (uint64_t x)
		469
285	Returns the number of bits set to 1 in C<x>. For example:	470	Returns the number of bits set to 1 in C<x>.
286		471
		472	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		473
		474	For example:
		475
287	ecb_popcount32(7) = 3	476	ecb_popcount32 (7) = 3
288	ecb_popcount32(255) = 8	477	ecb_popcount32 (255) = 8
		478
		479	=item uint8_t ecb_bitrev8 (uint8_t x)
		480
		481	=item uint16_t ecb_bitrev16 (uint16_t x)
		482
		483	=item uint32_t ecb_bitrev32 (uint32_t x)
		484
		485	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		486	and so on.
		487
		488	Example:
		489
		490	ecb_bitrev8 (0xa7) = 0xea
		491	ecb_bitrev32 (0xffcc4411) = 0x882233ff
289		492
290	=item uint32_t ecb_bswap16 (uint32_t x)	493	=item uint32_t ecb_bswap16 (uint32_t x)
291		494
292	=item uint32_t ecb_bswap32 (uint32_t x)	495	=item uint32_t ecb_bswap32 (uint32_t x)
293		496
		497	=item uint64_t ecb_bswap64 (uint64_t x)
		498
294	These two functions return the value of the 16-bit (32-bit) variable	499	These functions return the value of the 16-bit (32-bit, 64-bit) value
295	C<x> after reversing the order of bytes.	500	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		501	C<ecb_bswap32>).
		502
		503	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		504
		505	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		506
		507	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		508
		509	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		510
		511	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		512
		513	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
296		514
297	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	515	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
298		516
299	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	517	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
300		518
301	These two functions return the value of C<x> after shifting all the bits	519	These two families of functions return the value of C<x> after rotating
302	by C<count> positions to the right or left respectively.	520	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		521	(C<ecb_rotl>).
		522
		523	Current GCC versions understand these functions and usually compile them
		524	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		525	x86).
303		526
304	=back	527	=back
305		528
306	=head2 ARITHMETIC	529	=head2 ARITHMETIC
307		530
308	=over 4	531	=over 4
309		532
310	=item x = ecb_mod (m, n)	533	=item x = ecb_mod (m, n)
311		534
312	Returns the positive remainder of the modulo operation between C<m> and	535	Returns C<m> modulo C<n>, which is the same as the positive remainder
		536	of the division operation between C<m> and C<n>, using floored
313	C<n>. Unlike the C moduloe operator C<%>, this function ensures that the	537	division. Unlike the C remainder operator C<%>, this function ensures that
314	return value is always positive).	538	the return value is always positive and that the two numbers I<m> and
		539	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		540	C<ecb_mod> implements the mathematical modulo operation, which is missing
		541	in the language.
315		542
316	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	543	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
317	negatable, that is, both C<m> and C<-m> must be representable in its	544	negatable, that is, both C<m> and C<-m> must be representable in its
318	type.	545	type (this typically excludes the minimum signed integer value, the same
		546	limitation as for C</> and C<%> in C).
		547
		548	Current GCC versions compile this into an efficient branchless sequence on
		549	almost all CPUs.
		550
		551	For example, when you want to rotate forward through the members of an
		552	array for increasing C<m> (which might be negative), then you should use
		553	C<ecb_mod>, as the C<%> operator might give either negative results, or
		554	change direction for negative values:
		555
		556	for (m = -100; m <= 100; ++m)
		557	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		558
		559	=item x = ecb_div_rd (val, div)
		560
		561	=item x = ecb_div_ru (val, div)
		562
		563	Returns C<val> divided by C<div> rounded down or up, respectively.
		564	C<val> and C<div> must have integer types and C<div> must be strictly
		565	positive. Note that these functions are implemented with macros in C
		566	and with function templates in C++.
319		567
320	=back	568	=back
321		569
322	=head2 UTILITY	570	=head2 UTILITY
323		571
324	=over 4	572	=over 4
325		573
326	=item element_count = ecb_array_length (name) [MACRO]	574	=item element_count = ecb_array_length (name)
327		575
328	Returns the number of elements in the array C<name>. For example:	576	Returns the number of elements in the array C<name>. For example:
329		577
330	int primes[] = { 2, 3, 5, 7, 11 };	578	int primes[] = { 2, 3, 5, 7, 11 };
331	int sum = 0;	579	int sum = 0;

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Comparing libecb/ecb.pod (file contents): Revision 1.14 by root, Thu May 26 23:23:08 2011 UTC vs. Revision 1.42 by root, Mon May 28 08:54:03 2012 UTC

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Comparing libecb/ecb.pod (file contents):
Revision 1.14 by root, Thu May 26 23:23:08 2011 UTC vs.
Revision 1.42 by root, Mon May 28 08:54:03 2012 UTC