[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 1.21 by root, Fri May 27 00:14:10 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	A major part of libecb deals with GCC attributes. These are additional
		61	attributes that you cna assign to functions, variables and sometimes even
		62	types - much like C<const> or C<volatile> in C.
		63
		64	While GCC allows declarations to show up in many surprising places,
		65	but not in many expeted places, the safest way is to put attribute
		66	declarations before the whole declaration:
		67
		68	ecb_const int mysqrt (int a);
		69	ecb_unused int i;
		70
		71	For variables, it is often nicer to put the attribute after the name, and
		72	avoid multiple declarations using commas:
		73
		74	int i ecb_unused;
		75
4	=over 4	76	=over 4
5		77
6	=item ecb_attribute(attrlist)	78	=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		79
		80	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
		81	nothing on other compilers, so the effect is that only GCC sees these.
		82
		83	Example: use the C<deprecated> attribute on a function.
		84
		85	ecb_attribute((__deprecated__)) void
		86	do_not_use_me_anymore (void);
		87
		88	=item ecb_unused
		89
		90	Marks a function or a variable as "unused", which simply suppresses a
		91	warning by GCC when it detects it as unused. This is useful when you e.g.
		92	declare a variable but do not always use it:
		93
		94	{
		95	int var ecb_unused;
		96
		97	#ifdef SOMECONDITION
		98	var = ...;
		99	return var;
		100	#else
		101	return 0;
		102	#endif
		103	}
		104
		105	=item ecb_noinline
		106
		107	Prevent a function from being inlined - it might be optimised away, but
		108	not inlined into other functions. This is useful if you know your function
		109	is rarely called and large enough for inlining not to be helpful.
		110
		111	=item ecb_noreturn
		112
		113	Marks a function as "not returning, ever". Some typical functions that
		114	don't return are C<exit> or C<abort> (which really works hard to not
		115	return), and now you can make your own:
		116
		117	ecb_noreturn void
		118	my_abort (const char *errline)
		119	{
		120	puts (errline);
		121	abort ();
		122	}
		123
		124	In this case, the compiler would probably be smart enough to deduce it on
		125	its own, so this is mainly useful for declarations.
		126
		127	=item ecb_const
		128
		129	Declares that the function only depends on the values of its arguments,
		130	much like a mathematical function. It specifically does not read or write
		131	any memory any arguments might point to, global variables, or call any
		132	non-const functions. It also must not have any side effects.
		133
		134	Such a function can be optimised much more aggressively by the compiler -
		135	for example, multiple calls with the same arguments can be optimised into
		136	a single call, which wouldn't be possible if the compiler would have to
		137	expect any side effects.
		138
		139	It is best suited for functions in the sense of mathematical functions,
		140	such as a function returning the square root of its input argument.
		141
		142	Not suited would be a function that calculates the hash of some memory
		143	area you pass in, prints some messages or looks at a global variable to
		144	decide on rounding.
		145
		146	See C<ecb_pure> for a slightly less restrictive class of functions.
		147
		148	=item ecb_pure
		149
		150	Similar to C<ecb_const>, declares a function that has no side
		151	effects. Unlike C<ecb_const>, the function is allowed to examine global
		152	variables and any other memory areas (such as the ones passed to it via
		153	pointers).
		154
		155	While these functions cannot be optimised as aggressively as C<ecb_const>
		156	functions, they can still be optimised away in many occasions, and the
		157	compiler has more freedom in moving calls to them around.
		158
		159	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		160	function that calculates the MD5 sum of some input and updates some MD5
		161	state passed as argument would I<NOT> be pure, however, as it would modify
		162	some memory area that is not the return value.
		163
		164	=item ecb_hot
		165
		166	This declares a function as "hot" with regards to the cache - the function
		167	is used so often, that it is very beneficial to keep it in the cache if
		168	possible.
		169
		170	The compiler reacts by trying to place hot functions near to each other in
		171	memory.
		172
		173	Whether a function is hot or not often depends on the whole program,
		174	and less on the function itself. C<ecb_cold> is likely more useful in
		175	practise.
		176
		177	=item ecb_cold
		178
		179	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		180	the cache, or in other words, this function is not called often, or not at
		181	speed-critical times, and keeping it in the cache might be a waste of said
		182	cache.
		183
		184	In addition to placing cold functions together (or at least away from hot
		185	functions), this knowledge can be used in other ways, for example, the
		186	function will be optimised for size, as opposed to speed, and codepaths
		187	leading to calls to those functions can automatically be marked as if
		188	C<ecb_unlikely> had been used to reach them.
		189
		190	Good examples for such functions would be error reporting functions, or
		191	functions only called in exceptional or rare cases.
		192
		193	=item ecb_artificial
		194
		195	Declares the function as "artificial", in this case meaning that this
		196	function is not really mean to be a function, but more like an accessor
		197	- many methods in C++ classes are mere accessor functions, and having a
		198	crash reported in such a method, or single-stepping through them, is not
		199	usually so helpful, especially when it's inlined to just a few instructions.
		200
		201	Marking them as artificial will instruct the debugger about just this,
		202	leading to happier debugging and thus happier lives.
		203
		204	Example: in some kind of smart-pointer class, mark the pointer accessor as
		205	artificial, so that the whole class acts more like a pointer and less like
		206	some C++ abstraction monster.
		207
		208	template<typename T>
		209	struct my_smart_ptr
		210	{
		211	T *value;
		212
		213	ecb_artificial
		214	operator T *()
		215	{
		216	return value;
		217	}
		218	};
		219
15	=back	220	=back
16		221
17	=head2 OPTIMISATION HINTS	222	=head2 OPTIMISATION HINTS
18		223
19	=over 4	224	=over 4
20		225
21	=item bool ecb_is_constant(expr)	226	=item bool ecb_is_constant(expr)
22		227
		228	Returns true iff the expression can be deduced to be a compile-time
		229	constant, and false otherwise.
		230
		231	For example, when you have a C<rndm16> function that returns a 16 bit
		232	random number, and you have a function that maps this to a range from
		233	0..n-1, then you could use this inline function in a header file:
		234
		235	ecb_inline uint32_t
		236	rndm (uint32_t n)
		237	{
		238	return (n * (uint32_t)rndm16 ()) >> 16;
		239	}
		240
		241	However, for powers of two, you could use a normal mask, but that is only
		242	worth it if, at compile time, you can detect this case. This is the case
		243	when the passed number is a constant and also a power of two (C<n & (n -
		244	1) == 0>):
		245
		246	ecb_inline uint32_t
		247	rndm (uint32_t n)
		248	{
		249	return is_constant (n) && !(n & (n - 1))
		250	? rndm16 () & (num - 1)
		251	: (n * (uint32_t)rndm16 ()) >> 16;
		252	}
		253
23	=item bool ecb_expect(expr,value)	254	=item bool ecb_expect (expr, value)
24		255
		256	Evaluates C<expr> and returns it. In addition, it tells the compiler that
		257	the C<expr> evaluates to C<value> a lot, which can be used for static
		258	branch optimisations.
		259
		260	Usually, you want to use the more intuitive C<ecb_likely> and
		261	C<ecb_unlikely> functions instead.
		262
		263	=item bool ecb_likely (cond)
		264
25	=item bool ecb_unlikely(bool)	265	=item bool ecb_unlikely (cond)
26		266
27	=item bool ecb_likely(bool)	267	These two functions expect a expression that is true or false and return
		268	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
		269	other conditional statement, it will not change the program:
28		270
		271	/* these two do the same thing */
		272	if (some_condition) ...;
		273	if (ecb_likely (some_condition)) ...;
		274
		275	However, by using C<ecb_likely>, you tell the compiler that the condition
		276	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
		277	true).
		278
		279	For example, when you check for a null pointer and expect this to be a
		280	rare, exceptional, case, then use C<ecb_unlikely>:
		281
		282	void my_free (void *ptr)
		283	{
		284	if (ecb_unlikely (ptr == 0))
		285	return;
		286	}
		287
		288	Consequent use of these functions to mark away exceptional cases or to
		289	tell the compiler what the hot path through a function is can increase
		290	performance considerably.
		291
		292	A very good example is in a function that reserves more space for some
		293	memory block (for example, inside an implementation of a string stream) -
		294	each time something is added, you have to check for a buffer overrun, but
		295	you expect that most checks will turn out to be false:
		296
		297	/* make sure we have "size" extra room in our buffer */
		298	ecb_inline void
		299	reserve (int size)
		300	{
		301	if (ecb_unlikely (current + size > end))
		302	real_reserve_method (size); /* presumably noinline */
		303	}
		304
29	=item bool ecb_assume(cond)	305	=item bool ecb_assume (cond)
30		306
		307	Try to tell the compiler that some condition is true, even if it's not
		308	obvious.
		309
		310	This can be used to teach the compiler about invariants or other
		311	conditions that might improve code generation, but which are impossible to
		312	deduce form the code itself.
		313
		314	For example, the example reservation function from the C<ecb_unlikely>
		315	description could be written thus (only C<ecb_assume> was added):
		316
		317	ecb_inline void
		318	reserve (int size)
		319	{
		320	if (ecb_unlikely (current + size > end))
		321	real_reserve_method (size); /* presumably noinline */
		322
		323	ecb_assume (current + size <= end);
		324	}
		325
		326	If you then call this function twice, like this:
		327
		328	reserve (10);
		329	reserve (1);
		330
		331	Then the compiler I<might> be able to optimise out the second call
		332	completely, as it knows that C<< current + 1 > end >> is false and the
		333	call will never be executed.
		334
31	=item bool ecb_unreachable()	335	=item bool ecb_unreachable ()
32		336
		337	This function does nothing itself, except tell the compiler that it will
		338	never be executed. Apart from suppressing a warning in some cases, this
		339	function can be used to implement C<ecb_assume> or similar functions.
		340
33	=item bool ecb_prefetch(addr,rw,locality)	341	=item bool ecb_prefetch (addr, rw, locality)
34		342
		343	Tells the compiler to try to prefetch memory at the given C<addr>ess
		344	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
		345	C<0> means that there will only be one access later, C<3> means that
		346	the data will likely be accessed very often, and values in between mean
		347	something... in between. The memory pointed to by the address does not
		348	need to be accessible (it could be a null pointer for example), but C<rw>
		349	and C<locality> must be compile-time constants.
		350
		351	An obvious way to use this is to prefetch some data far away, in a big
		352	array you loop over. This prefetches memory some 128 array elements later,
		353	in the hope that it will be ready when the CPU arrives at that location.
		354
		355	int sum = 0;
		356
		357	for (i = 0; i < N; ++i)
		358	{
		359	sum += arr [i]
		360	ecb_prefetch (arr + i + 128, 0, 0);
		361	}
		362
		363	It's hard to predict how far to prefetch, and most CPUs that can prefetch
		364	are often good enough to predict this kind of behaviour themselves. It
		365	gets more interesting with linked lists, especially when you do some fair
		366	processing on each list element:
		367
		368	for (node *n = start; n; n = n->next)
		369	{
		370	ecb_prefetch (n->next, 0, 0);
		371	... do medium amount of work with *n
		372	}
		373
		374	After processing the node, (part of) the next node might already be in
		375	cache.
		376
35	=back	377	=back
36		378
37	=head2 BIT FIDDLING / BITSTUFFS	379	=head2 BIT FIDDLING / BITSTUFFS
38		380
		381	=over 4
		382
39	bool ecb_big_endian ();	383	=item bool ecb_big_endian ()
		384
40	bool ecb_little_endian ();	385	=item bool ecb_little_endian ()
		386
		387	These two functions return true if the byte order is big endian
		388	(most-significant byte first) or little endian (least-significant byte
		389	first) respectively.
		390
41	int ecb_ctz32 (uint32_t x);	391	=item int ecb_ctz32 (uint32_t x)
		392
		393	Returns the index of the least significant bit set in C<x> (or
		394	equivalently the number of bits set to 0 before the least significant
		395	bit set), starting from 0. If C<x> is 0 the result is undefined. A
		396	common use case is to compute the integer binary logarithm, i.e.,
		397	floor(log2(n)). For example:
		398
		399	ecb_ctz32 (3) = 0
		400	ecb_ctz32 (6) = 1
		401
42	int ecb_popcount32 (uint32_t x);	402	=item int ecb_popcount32 (uint32_t x)
43	uint32_t ecb_bswap32 (uint32_t x);	403
		404	Returns the number of bits set to 1 in C<x>. For example:
		405
		406	ecb_popcount32 (7) = 3
		407	ecb_popcount32 (255) = 8
		408
44	uint32_t ecb_bswap16 (uint32_t x);	409	=item uint32_t ecb_bswap16 (uint32_t x)
		410
		411	=item uint32_t ecb_bswap32 (uint32_t x)
		412
		413	These two functions return the value of the 16-bit (32-bit) value C<x>
		414	after reversing the order of bytes (0x11223344 becomes 0x44332211).
		415
45	uint32_t ecb_rotr32 (uint32_t x, unsigned int count);	416	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
		417
46	uint32_t ecb_rotl32 (uint32_t x, unsigned int count);	418	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		419
		420	These two functions return the value of C<x> after shifting all the bits
		421	by C<count> positions to the right or left respectively.
		422
		423	Current GCC versions understand these functions and usually compile them
		424	to "optimal" code (e.g. a single C<roll> on x86).
		425
		426	=back
47		427
48	=head2 ARITHMETIC	428	=head2 ARITHMETIC
49		429
		430	=over 4
		431
50	x = ecb_mod (m, n)	432	=item x = ecb_mod (m, n)
		433
		434	Returns the positive remainder of the modulo operation between C<m> and
		435	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
		436	return value is always positive - ISO C guarantees very little when
		437	negative numbers are used with C<%>.
		438
		439	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
		440	negatable, that is, both C<m> and C<-m> must be representable in its
		441	type.
		442
		443	=back
51		444
52	=head2 UTILITY	445	=head2 UTILITY
53		446
54	ecb_array_length (name)	447	=over 4
55		448
		449	=item element_count = ecb_array_length (name) [MACRO]
56		450
		451	Returns the number of elements in the array C<name>. For example:
		452
		453	int primes[] = { 2, 3, 5, 7, 11 };
		454	int sum = 0;
		455
		456	for (i = 0; i < ecb_array_length (primes); i++)
		457	sum += primes [i];
		458
		459	=back
		460
		461

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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 1.21 by root, Fri May 27 00:14:10 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 1.21 by root, Fri May 27 00:14:10 2011 UTC