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		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 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.
		25
		26	=head2 ABOUT THE HEADER
		27
		28	At the moment, all you have to do is copy F<ecb.h> somewhere where your
		29	compiler can find it and include it:
		30
		31	#include <ecb.h>
		32
		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.
		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>).
1		69
2	=head2 GCC ATTRIBUTES	70	=head2 GCC ATTRIBUTES
3		71
		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;
		87
4	=over 4	88	=over 4
5		89
6	=item ecb_attribute(attrlist)	90	=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		91
		92	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
		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);
		99
		100	=item ecb_unused
		101
		102	Marks a function or a variable as "unused", which simply suppresses a
		103	warning by GCC when it detects it as unused. This is useful when you e.g.
		104	declare a variable but do not always use it:
		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)
		128	{
		129	return - (a * b);
		130	}
		131
		132	=item ecb_noinline
		133
		134	Prevent a function from being inlined - it might be optimised away, but
		135	not inlined into other functions. This is useful if you know your function
		136	is rarely called and large enough for inlining not to be helpful.
		137
		138	=item ecb_noreturn
		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
		154	=item ecb_const
		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
		175	=item ecb_pure
		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
		191	=item ecb_hot
		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
		204	=item ecb_cold
		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
		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	};
		246
15	=back	247	=back
16		248
17	=head2 OPTIMISATION HINTS	249	=head2 OPTIMISATION HINTS
18		250
19	=over 4	251	=over 4
20		252
21	=item bool ecb_is_constant(expr)	253	=item bool ecb_is_constant(expr)
22		254
		255	Returns true iff the expression can be deduced to be a compile-time
		256	constant, and false otherwise.
		257
		258	For example, when you have a C<rndm16> function that returns a 16 bit
		259	random number, and you have a function that maps this to a range from
		260	0..n-1, then you could use this inline function in a header file:
		261
		262	ecb_inline uint32_t
		263	rndm (uint32_t n)
		264	{
		265	return (n * (uint32_t)rndm16 ()) >> 16;
		266	}
		267
		268	However, for powers of two, you could use a normal mask, but that is only
		269	worth it if, at compile time, you can detect this case. This is the case
		270	when the passed number is a constant and also a power of two (C<n & (n -
		271	1) == 0>):
		272
		273	ecb_inline uint32_t
		274	rndm (uint32_t n)
		275	{
		276	return is_constant (n) && !(n & (n - 1))
		277	? rndm16 () & (num - 1)
		278	: (n * (uint32_t)rndm16 ()) >> 16;
		279	}
		280
23	=item bool ecb_expect(expr,value)	281	=item bool ecb_expect (expr, value)
24		282
25	=item bool ecb_unlikely(bool)	283	Evaluates C<expr> and returns it. In addition, it tells the compiler that
		284	the C<expr> evaluates to C<value> a lot, which can be used for static
		285	branch optimisations.
26		286
27	=item bool ecb_likely(bool)	287	Usually, you want to use the more intuitive C<ecb_expect_true> and
		288	C<ecb_expect_false> functions instead.
28		289
		290	=item bool ecb_expect_true (cond)
		291
		292	=item bool ecb_expect_false (cond)
		293
		294	These two functions expect a expression that is true or false and return
		295	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
		296	other conditional statement, it will not change the program:
		297
		298	/* these two do the same thing */
		299	if (some_condition) ...;
		300	if (ecb_expect_true (some_condition)) ...;
		301
		302	However, by using C<ecb_expect_true>, you tell the compiler that the
		303	condition is likely to be true (and for C<ecb_expect_false>, that it is
		304	unlikely to be true).
		305
		306	For example, when you check for a null pointer and expect this to be a
		307	rare, exceptional, case, then use C<ecb_expect_false>:
		308
		309	void my_free (void *ptr)
		310	{
		311	if (ecb_expect_false (ptr == 0))
		312	return;
		313	}
		314
		315	Consequent use of these functions to mark away exceptional cases or to
		316	tell the compiler what the hot path through a function is can increase
		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.
		324
		325	A very good example is in a function that reserves more space for some
		326	memory block (for example, inside an implementation of a string stream) -
		327	each time something is added, you have to check for a buffer overrun, but
		328	you expect that most checks will turn out to be false:
		329
		330	/* make sure we have "size" extra room in our buffer */
		331	ecb_inline void
		332	reserve (int size)
		333	{
		334	if (ecb_expect_false (current + size > end))
		335	real_reserve_method (size); /* presumably noinline */
		336	}
		337
29	=item bool ecb_assume(cond)	338	=item bool ecb_assume (cond)
30		339
		340	Try to tell the compiler that some condition is true, even if it's not
		341	obvious.
		342
		343	This can be used to teach the compiler about invariants or other
		344	conditions that might improve code generation, but which are impossible to
		345	deduce form the code itself.
		346
		347	For example, the example reservation function from the C<ecb_expect_false>
		348	description could be written thus (only C<ecb_assume> was added):
		349
		350	ecb_inline void
		351	reserve (int size)
		352	{
		353	if (ecb_expect_false (current + size > end))
		354	real_reserve_method (size); /* presumably noinline */
		355
		356	ecb_assume (current + size <= end);
		357	}
		358
		359	If you then call this function twice, like this:
		360
		361	reserve (10);
		362	reserve (1);
		363
		364	Then the compiler I<might> be able to optimise out the second call
		365	completely, as it knows that C<< current + 1 > end >> is false and the
		366	call will never be executed.
		367
31	=item bool ecb_unreachable()	368	=item bool ecb_unreachable ()
32		369
		370	This function does nothing itself, except tell the compiler that it will
		371	never be executed. Apart from suppressing a warning in some cases, this
		372	function can be used to implement C<ecb_assume> or similar functions.
		373
33	=item bool ecb_prefetch(addr,rw,locality)	374	=item bool ecb_prefetch (addr, rw, locality)
34		375
		376	Tells the compiler to try to prefetch memory at the given C<addr>ess
		377	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
		378	C<0> means that there will only be one access later, C<3> means that
		379	the data will likely be accessed very often, and values in between mean
		380	something... in between. The memory pointed to by the address does not
		381	need to be accessible (it could be a null pointer for example), but C<rw>
		382	and C<locality> must be compile-time constants.
		383
		384	An obvious way to use this is to prefetch some data far away, in a big
		385	array you loop over. This prefetches memory some 128 array elements later,
		386	in the hope that it will be ready when the CPU arrives at that location.
		387
		388	int sum = 0;
		389
		390	for (i = 0; i < N; ++i)
		391	{
		392	sum += arr [i]
		393	ecb_prefetch (arr + i + 128, 0, 0);
		394	}
		395
		396	It's hard to predict how far to prefetch, and most CPUs that can prefetch
		397	are often good enough to predict this kind of behaviour themselves. It
		398	gets more interesting with linked lists, especially when you do some fair
		399	processing on each list element:
		400
		401	for (node *n = start; n; n = n->next)
		402	{
		403	ecb_prefetch (n->next, 0, 0);
		404	... do medium amount of work with *n
		405	}
		406
		407	After processing the node, (part of) the next node might already be in
		408	cache.
		409
35	=back	410	=back
36		411
37	=head2 BIT FIDDLING / BITSTUFFS	412	=head2 BIT FIDDLING / BIT WIZARDRY
38		413
		414	=over 4
		415
39	bool ecb_big_endian ();	416	=item bool ecb_big_endian ()
		417
40	bool ecb_little_endian ();	418	=item bool ecb_little_endian ()
		419
		420	These two functions return true if the byte order is big endian
		421	(most-significant byte first) or little endian (least-significant byte
		422	first) respectively.
		423
		424	On systems that are neither, their return values are unspecified.
		425
41	int ecb_ctz32 (uint32_t x);	426	=item int ecb_ctz32 (uint32_t x)
		427
		428	=item int ecb_ctz64 (uint64_t x)
		429
		430	Returns the index of the least significant bit set in C<x> (or
		431	equivalently the number of bits set to 0 before the least significant bit
		432	set), starting from 0. If C<x> is 0 the result is undefined.
		433
		434	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		435
		436	For example:
		437
		438	ecb_ctz32 (3) = 0
		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>.
		465
42	int ecb_popcount32 (uint32_t x);	466	=item int ecb_popcount32 (uint32_t x)
		467
		468	=item int ecb_popcount64 (uint64_t x)
		469
		470	Returns the number of bits set to 1 in C<x>.
		471
		472	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		473
		474	For example:
		475
		476	ecb_popcount32 (7) = 3
		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
43	uint32_t ecb_bswap32 (uint32_t x);	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
		492
44	uint32_t ecb_bswap16 (uint32_t x);	493	=item uint32_t ecb_bswap16 (uint32_t x)
		494
		495	=item uint32_t ecb_bswap32 (uint32_t x)
		496
		497	=item uint64_t ecb_bswap64 (uint64_t x)
		498
		499	These functions return the value of the 16-bit (32-bit, 64-bit) value
		500	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		501	C<ecb_bswap32>).
		502
45	uint32_t ecb_rotr32 (uint32_t x, unsigned int count);	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
46	uint32_t ecb_rotl32 (uint32_t x, unsigned int count);	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)
		514
		515	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
		516
		517	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
		518
		519	These two families of functions return the value of C<x> after rotating
		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).
		526
		527	=back
47		528
48	=head2 ARITHMETIC	529	=head2 ARITHMETIC
49		530
		531	=over 4
		532
50	x = ecb_mod (m, n)	533	=item x = ecb_mod (m, n)
		534
		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
		537	division. Unlike the C remainder operator C<%>, this function ensures that
		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.
		542
		543	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
		544	negatable, that is, both C<m> and C<-m> must be representable in its
		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++.
		567
		568	=back
51		569
52	=head2 UTILITY	570	=head2 UTILITY
53		571
54	ecb_array_length (name)	572	=over 4
55		573
		574	=item element_count = ecb_array_length (name)
56		575
		576	Returns the number of elements in the array C<name>. For example:
		577
		578	int primes[] = { 2, 3, 5, 7, 11 };
		579	int sum = 0;
		580
		581	for (i = 0; i < ecb_array_length (primes); i++)
		582	sum += primes [i];
		583
		584	=back
		585
		586

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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 1.42 by root, Mon May 28 08:54:03 2012 UTC

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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 1.42 by root, Mon May 28 08:54:03 2012 UTC