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Revision 1.1 by root, Thu May 26 19:39:40 2011 UTC vs.
Revision 1.27 by root, Wed Jun 1 01:29:36 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 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.
1		58
2	=head2 GCC ATTRIBUTES	59	=head2 GCC ATTRIBUTES
3		60
		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;
		76
4	=over 4	77	=over 4
5		78
6	=item ecb_attribute(attrlist)	79	=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		80
		81	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
		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);
		88
		89	=item ecb_unused
		90
		91	Marks a function or a variable as "unused", which simply suppresses a
		92	warning by GCC when it detects it as unused. This is useful when you e.g.
		93	declare a variable but do not always use it:
		94
		95	{
		96	int var ecb_unused;
		97
		98	#ifdef SOMECONDITION
		99	var = ...;
		100	return var;
		101	#else
		102	return 0;
		103	#endif
		104	}
		105
		106	=item ecb_noinline
		107
		108	Prevent a function from being inlined - it might be optimised away, but
		109	not inlined into other functions. This is useful if you know your function
		110	is rarely called and large enough for inlining not to be helpful.
		111
		112	=item ecb_noreturn
		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
		128	=item ecb_const
		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
		149	=item ecb_pure
		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
		165	=item ecb_hot
		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
		178	=item ecb_cold
		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
		194	=item ecb_artificial
		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
15	=back	221	=back
16		222
17	=head2 OPTIMISATION HINTS	223	=head2 OPTIMISATION HINTS
18		224
19	=over 4	225	=over 4
20		226
21	=item bool ecb_is_constant(expr)	227	=item bool ecb_is_constant(expr)
22		228
		229	Returns true iff the expression can be deduced to be a compile-time
		230	constant, and false otherwise.
		231
		232	For example, when you have a C<rndm16> function that returns a 16 bit
		233	random number, and you have a function that maps this to a range from
		234	0..n-1, then you could use this inline function in a header file:
		235
		236	ecb_inline uint32_t
		237	rndm (uint32_t n)
		238	{
		239	return (n * (uint32_t)rndm16 ()) >> 16;
		240	}
		241
		242	However, for powers of two, you could use a normal mask, but that is only
		243	worth it if, at compile time, you can detect this case. This is the case
		244	when the passed number is a constant and also a power of two (C<n & (n -
		245	1) == 0>):
		246
		247	ecb_inline uint32_t
		248	rndm (uint32_t n)
		249	{
		250	return is_constant (n) && !(n & (n - 1))
		251	? rndm16 () & (num - 1)
		252	: (n * (uint32_t)rndm16 ()) >> 16;
		253	}
		254
23	=item bool ecb_expect(expr,value)	255	=item bool ecb_expect (expr, value)
24		256
25	=item bool ecb_unlikely(bool)	257	Evaluates C<expr> and returns it. In addition, it tells the compiler that
		258	the C<expr> evaluates to C<value> a lot, which can be used for static
		259	branch optimisations.
26		260
27	=item bool ecb_likely(bool)	261	Usually, you want to use the more intuitive C<ecb_expect_true> and
		262	C<ecb_expect_false> functions instead.
28		263
		264	=item bool ecb_expect_true (cond)
		265
		266	=item bool ecb_expect_false (cond)
		267
		268	These two functions expect a expression that is true or false and return
		269	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
		270	other conditional statement, it will not change the program:
		271
		272	/* these two do the same thing */
		273	if (some_condition) ...;
		274	if (ecb_expect_true (some_condition)) ...;
		275
		276	However, by using C<ecb_expect_true>, you tell the compiler that the
		277	condition is likely to be true (and for C<ecb_expect_false>, that it is
		278	unlikely to be true).
		279
		280	For example, when you check for a null pointer and expect this to be a
		281	rare, exceptional, case, then use C<ecb_expect_false>:
		282
		283	void my_free (void *ptr)
		284	{
		285	if (ecb_expect_false (ptr == 0))
		286	return;
		287	}
		288
		289	Consequent use of these functions to mark away exceptional cases or to
		290	tell the compiler what the hot path through a function is can increase
		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.
		298
		299	A very good example is in a function that reserves more space for some
		300	memory block (for example, inside an implementation of a string stream) -
		301	each time something is added, you have to check for a buffer overrun, but
		302	you expect that most checks will turn out to be false:
		303
		304	/* make sure we have "size" extra room in our buffer */
		305	ecb_inline void
		306	reserve (int size)
		307	{
		308	if (ecb_expect_false (current + size > end))
		309	real_reserve_method (size); /* presumably noinline */
		310	}
		311
29	=item bool ecb_assume(cond)	312	=item bool ecb_assume (cond)
30		313
		314	Try to tell the compiler that some condition is true, even if it's not
		315	obvious.
		316
		317	This can be used to teach the compiler about invariants or other
		318	conditions that might improve code generation, but which are impossible to
		319	deduce form the code itself.
		320
		321	For example, the example reservation function from the C<ecb_expect_false>
		322	description could be written thus (only C<ecb_assume> was added):
		323
		324	ecb_inline void
		325	reserve (int size)
		326	{
		327	if (ecb_expect_false (current + size > end))
		328	real_reserve_method (size); /* presumably noinline */
		329
		330	ecb_assume (current + size <= end);
		331	}
		332
		333	If you then call this function twice, like this:
		334
		335	reserve (10);
		336	reserve (1);
		337
		338	Then the compiler I<might> be able to optimise out the second call
		339	completely, as it knows that C<< current + 1 > end >> is false and the
		340	call will never be executed.
		341
31	=item bool ecb_unreachable()	342	=item bool ecb_unreachable ()
32		343
		344	This function does nothing itself, except tell the compiler that it will
		345	never be executed. Apart from suppressing a warning in some cases, this
		346	function can be used to implement C<ecb_assume> or similar functions.
		347
33	=item bool ecb_prefetch(addr,rw,locality)	348	=item bool ecb_prefetch (addr, rw, locality)
34		349
		350	Tells the compiler to try to prefetch memory at the given C<addr>ess
		351	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
		352	C<0> means that there will only be one access later, C<3> means that
		353	the data will likely be accessed very often, and values in between mean
		354	something... in between. The memory pointed to by the address does not
		355	need to be accessible (it could be a null pointer for example), but C<rw>
		356	and C<locality> must be compile-time constants.
		357
		358	An obvious way to use this is to prefetch some data far away, in a big
		359	array you loop over. This prefetches memory some 128 array elements later,
		360	in the hope that it will be ready when the CPU arrives at that location.
		361
		362	int sum = 0;
		363
		364	for (i = 0; i < N; ++i)
		365	{
		366	sum += arr [i]
		367	ecb_prefetch (arr + i + 128, 0, 0);
		368	}
		369
		370	It's hard to predict how far to prefetch, and most CPUs that can prefetch
		371	are often good enough to predict this kind of behaviour themselves. It
		372	gets more interesting with linked lists, especially when you do some fair
		373	processing on each list element:
		374
		375	for (node *n = start; n; n = n->next)
		376	{
		377	ecb_prefetch (n->next, 0, 0);
		378	... do medium amount of work with *n
		379	}
		380
		381	After processing the node, (part of) the next node might already be in
		382	cache.
		383
35	=back	384	=back
36		385
37	=head2 BIT FIDDLING / BITSTUFFS	386	=head2 BIT FIDDLING / BITSTUFFS
38		387
		388	=over 4
		389
39	bool ecb_big_endian ();	390	=item bool ecb_big_endian ()
		391
40	bool ecb_little_endian ();	392	=item bool ecb_little_endian ()
		393
		394	These two functions return true if the byte order is big endian
		395	(most-significant byte first) or little endian (least-significant byte
		396	first) respectively.
		397
		398	On systems that are neither, their return values are unspecified.
		399
41	int ecb_ctz32 (uint32_t x);	400	=item int ecb_ctz32 (uint32_t x)
		401
		402	Returns the index of the least significant bit set in C<x> (or
		403	equivalently the number of bits set to 0 before the least significant bit
		404	set), starting from 0. If C<x> is 0 the result is undefined. A common use
		405	case is to compute the integer binary logarithm, i.e., C<floor (log2
		406	(n))>. For example:
		407
		408	ecb_ctz32 (3) = 0
		409	ecb_ctz32 (6) = 1
		410
42	int ecb_popcount32 (uint32_t x);	411	=item int ecb_popcount32 (uint32_t x)
43	uint32_t ecb_bswap32 (uint32_t x);	412
		413	Returns the number of bits set to 1 in C<x>. For example:
		414
		415	ecb_popcount32 (7) = 3
		416	ecb_popcount32 (255) = 8
		417
44	uint32_t ecb_bswap16 (uint32_t x);	418	=item uint32_t ecb_bswap16 (uint32_t x)
		419
		420	=item uint32_t ecb_bswap32 (uint32_t x)
		421
		422	These two functions return the value of the 16-bit (32-bit) value C<x>
		423	after reversing the order of bytes (0x11223344 becomes 0x44332211).
		424
45	uint32_t ecb_rotr32 (uint32_t x, unsigned int count);	425	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
		426
46	uint32_t ecb_rotl32 (uint32_t x, unsigned int count);	427	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		428
		429	These two functions return the value of C<x> after rotating all the bits
		430	by C<count> positions to the right or left respectively.
		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
		435	=back
47		436
48	=head2 ARITHMETIC	437	=head2 ARITHMETIC
49		438
		439	=over 4
		440
50	x = ecb_mod (m, n)	441	=item x = ecb_mod (m, n)
		442
		443	Returns C<m> modulo C<n>, which is the same as the positive remainder
		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))];
		466
		467	=back
51		468
52	=head2 UTILITY	469	=head2 UTILITY
53		470
54	ecb_array_length (name)	471	=over 4
55		472
		473	=item element_count = ecb_array_length (name)
56		474
		475	Returns the number of elements in the array C<name>. For example:
		476
		477	int primes[] = { 2, 3, 5, 7, 11 };
		478	int sum = 0;
		479
		480	for (i = 0; i < ecb_array_length (primes); i++)
		481	sum += primes [i];
		482
		483	=back
		484
		485

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Comparing cvsroot/libecb/ecb.pod (file contents): Revision 1.1 by root, Thu May 26 19:39:40 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 1.1 by root, Thu May 26 19:39:40 2011 UTC vs.
Revision 1.27 by root, Wed Jun 1 01:29:36 2011 UTC