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

Comparing libecb/ecb.pod (file contents):
Revision 1.1 by root, Thu May 26 19:39:40 2011 UTC vs.
Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC

		1	=head1 LIBECB - e-C-Builtins
		2
		3	=head2 ABOUT LIBECB
		4
		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.
		24
		25	=head2 ABOUT THE HEADER
		26
		27	At the moment, all you have to do is copy F<ecb.h> somewhere where your
		28	compiler can find it and include it:
		29
		30	#include <ecb.h>
		31
		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.
1		57
2	=head2 GCC ATTRIBUTES	58	=head2 GCC ATTRIBUTES
3		59
		60	blabla where to put, what others
		61
4	=over 4	62	=over 4
5		63
6	=item ecb_attribute(attrlist)	64	=item ecb_attribute ((attrs...))
7	=item ecb_noinline ecb_attribute ((noinline))
8	=item ecb_noreturn ecb_attribute ((noreturn))
9	=item ecb_unused ecb_attribute ((unused))
10	=item ecb_const ecb_attribute ((const))
11	=item ecb_pure ecb_attribute ((pure))
12	=item ecb_hot ecb_attribute ((hot)) /* 4.3 */
13	=item ecb_cold ecb_attribute ((cold)) /* 4.3 */
14		65
		66	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
		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);
		73
		74	=item ecb_unused
		75
		76	Marks a function or a variable as "unused", which simply suppresses a
		77	warning by GCC when it detects it as unused. This is useful when you e.g.
		78	declare a variable but do not always use it:
		79
		80	{
		81	int var ecb_unused;
		82
		83	#ifdef SOMECONDITION
		84	var = ...;
		85	return var;
		86	#else
		87	return 0;
		88	#endif
		89	}
		90
		91	=item ecb_noinline
		92
		93	Prevent a function from being inlined - it might be optimised away, but
		94	not inlined into other functions. This is useful if you know your function
		95	is rarely called and large enough for inlining not to be helpful.
		96
		97	=item ecb_noreturn
		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
		113	=item ecb_const
		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
		134	=item ecb_pure
		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
		150	=item ecb_hot
		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
		163	=item ecb_cold
		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
		179	=item ecb_artificial
		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
15	=back	206	=back
16		207
17	=head2 OPTIMISATION HINTS	208	=head2 OPTIMISATION HINTS
18		209
19	=over 4	210	=over 4
20		211
21	=item bool ecb_is_constant(expr)	212	=item bool ecb_is_constant(expr)
22		213
		214	Returns true iff the expression can be deduced to be a compile-time
		215	constant, and false otherwise.
		216
		217	For example, when you have a C<rndm16> function that returns a 16 bit
		218	random number, and you have a function that maps this to a range from
		219	0..n-1, then you could use this inline function in a header file:
		220
		221	ecb_inline uint32_t
		222	rndm (uint32_t n)
		223	{
		224	return (n * (uint32_t)rndm16 ()) >> 16;
		225	}
		226
		227	However, for powers of two, you could use a normal mask, but that is only
		228	worth it if, at compile time, you can detect this case. This is the case
		229	when the passed number is a constant and also a power of two (C<n & (n -
		230	1) == 0>):
		231
		232	ecb_inline uint32_t
		233	rndm (uint32_t n)
		234	{
		235	return is_constant (n) && !(n & (n - 1))
		236	? rndm16 () & (num - 1)
		237	: (n * (uint32_t)rndm16 ()) >> 16;
		238	}
		239
23	=item bool ecb_expect(expr,value)	240	=item bool ecb_expect (expr, value)
24		241
		242	Evaluates C<expr> and returns it. In addition, it tells the compiler that
		243	the C<expr> evaluates to C<value> a lot, which can be used for static
		244	branch optimisations.
		245
		246	Usually, you want to use the more intuitive C<ecb_likely> and
		247	C<ecb_unlikely> functions instead.
		248
		249	=item bool ecb_likely (cond)
		250
25	=item bool ecb_unlikely(bool)	251	=item bool ecb_unlikely (cond)
26		252
27	=item bool ecb_likely(bool)	253	These two functions expect a expression that is true or false and return
		254	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
		255	other conditional statement, it will not change the program:
28		256
		257	/* these two do the same thing */
		258	if (some_condition) ...;
		259	if (ecb_likely (some_condition)) ...;
		260
		261	However, by using C<ecb_likely>, you tell the compiler that the condition
		262	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
		263	true).
		264
		265	For example, when you check for a null pointer and expect this to be a
		266	rare, exceptional, case, then use C<ecb_unlikely>:
		267
		268	void my_free (void *ptr)
		269	{
		270	if (ecb_unlikely (ptr == 0))
		271	return;
		272	}
		273
		274	Consequent use of these functions to mark away exceptional cases or to
		275	tell the compiler what the hot path through a function is can increase
		276	performance considerably.
		277
		278	A very good example is in a function that reserves more space for some
		279	memory block (for example, inside an implementation of a string stream) -
		280	each time something is added, you have to check for a buffer overrun, but
		281	you expect that most checks will turn out to be false:
		282
		283	/* make sure we have "size" extra room in our buffer */
		284	ecb_inline void
		285	reserve (int size)
		286	{
		287	if (ecb_unlikely (current + size > end))
		288	real_reserve_method (size); /* presumably noinline */
		289	}
		290
29	=item bool ecb_assume(cond)	291	=item bool ecb_assume (cond)
30		292
		293	Try to tell the compiler that some condition is true, even if it's not
		294	obvious.
		295
		296	This can be used to teach the compiler about invariants or other
		297	conditions that might improve code generation, but which are impossible to
		298	deduce form the code itself.
		299
		300	For example, the example reservation function from the C<ecb_unlikely>
		301	description could be written thus (only C<ecb_assume> was added):
		302
		303	ecb_inline void
		304	reserve (int size)
		305	{
		306	if (ecb_unlikely (current + size > end))
		307	real_reserve_method (size); /* presumably noinline */
		308
		309	ecb_assume (current + size <= end);
		310	}
		311
		312	If you then call this function twice, like this:
		313
		314	reserve (10);
		315	reserve (1);
		316
		317	Then the compiler I<might> be able to optimise out the second call
		318	completely, as it knows that C<< current + 1 > end >> is false and the
		319	call will never be executed.
		320
31	=item bool ecb_unreachable()	321	=item bool ecb_unreachable ()
32		322
		323	This function does nothing itself, except tell the compiler that it will
		324	never be executed. Apart from suppressing a warning in some cases, this
		325	function can be used to implement C<ecb_assume> or similar functions.
		326
33	=item bool ecb_prefetch(addr,rw,locality)	327	=item bool ecb_prefetch (addr, rw, locality)
34		328
		329	Tells the compiler to try to prefetch memory at the given C<addr>ess
		330	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
		331	C<0> means that there will only be one access later, C<3> means that
		332	the data will likely be accessed very often, and values in between mean
		333	something... in between. The memory pointed to by the address does not
		334	need to be accessible (it could be a null pointer for example), but C<rw>
		335	and C<locality> must be compile-time constants.
		336
		337	An obvious way to use this is to prefetch some data far away, in a big
		338	array you loop over. This prefetches memory some 128 array elements later,
		339	in the hope that it will be ready when the CPU arrives at that location.
		340
		341	int sum = 0;
		342
		343	for (i = 0; i < N; ++i)
		344	{
		345	sum += arr [i]
		346	ecb_prefetch (arr + i + 128, 0, 0);
		347	}
		348
		349	It's hard to predict how far to prefetch, and most CPUs that can prefetch
		350	are often good enough to predict this kind of behaviour themselves. It
		351	gets more interesting with linked lists, especially when you do some fair
		352	processing on each list element:
		353
		354	for (node *n = start; n; n = n->next)
		355	{
		356	ecb_prefetch (n->next, 0, 0);
		357	... do medium amount of work with *n
		358	}
		359
		360	After processing the node, (part of) the next node might already be in
		361	cache.
		362
35	=back	363	=back
36		364
37	=head2 BIT FIDDLING / BITSTUFFS	365	=head2 BIT FIDDLING / BITSTUFFS
38		366
		367	=over 4
		368
39	bool ecb_big_endian ();	369	=item bool ecb_big_endian ()
		370
40	bool ecb_little_endian ();	371	=item bool ecb_little_endian ()
		372
		373	These two functions return true if the byte order is big endian
		374	(most-significant byte first) or little endian (least-significant byte
		375	first) respectively.
		376
41	int ecb_ctz32 (uint32_t x);	377	=item int ecb_ctz32 (uint32_t x)
		378
		379	Returns the index of the least significant bit set in C<x> (or
		380	equivalently the number of bits set to 0 before the least significant
		381	bit set), starting from 0. If C<x> is 0 the result is undefined. A
		382	common use case is to compute the integer binary logarithm, i.e.,
		383	floor(log2(n)). For example:
		384
		385	ecb_ctz32 (3) = 0
		386	ecb_ctz32 (6) = 1
		387
42	int ecb_popcount32 (uint32_t x);	388	=item int ecb_popcount32 (uint32_t x)
43	uint32_t ecb_bswap32 (uint32_t x);	389
		390	Returns the number of bits set to 1 in C<x>. For example:
		391
		392	ecb_popcount32 (7) = 3
		393	ecb_popcount32 (255) = 8
		394
44	uint32_t ecb_bswap16 (uint32_t x);	395	=item uint32_t ecb_bswap16 (uint32_t x)
		396
		397	=item uint32_t ecb_bswap32 (uint32_t x)
		398
		399	These two functions return the value of the 16-bit (32-bit) variable
		400	C<x> after reversing the order of bytes.
		401
45	uint32_t ecb_rotr32 (uint32_t x, unsigned int count);	402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
		403
46	uint32_t ecb_rotl32 (uint32_t x, unsigned int count);	404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		405
		406	These two functions return the value of C<x> after shifting all the bits
		407	by C<count> positions to the right or left respectively.
		408
		409	=back
47		410
48	=head2 ARITHMETIC	411	=head2 ARITHMETIC
49		412
		413	=over 4
		414
50	x = ecb_mod (m, n)	415	=item x = ecb_mod (m, n)
		416
		417	Returns the positive remainder of the modulo operation between C<m> and
		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.
		424
		425	=back
51		426
52	=head2 UTILITY	427	=head2 UTILITY
53		428
54	ecb_array_length (name)	429	=over 4
55		430
		431	=item element_count = ecb_array_length (name) [MACRO]
56		432
		433	Returns the number of elements in the array C<name>. For example:
		434
		435	int primes[] = { 2, 3, 5, 7, 11 };
		436	int sum = 0;
		437
		438	for (i = 0; i < ecb_array_length (primes); i++)
		439	sum += primes [i];
		440
		441	=back
		442
		443

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Comparing libecb/ecb.pod (file contents): Revision 1.1 by root, Thu May 26 19:39:40 2011 UTC vs. Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.1 by root, Thu May 26 19:39:40 2011 UTC vs.
Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC