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

Comparing cvsroot/libecb/ecb.pod (file contents):
Revision 1.6 by root, Thu May 26 20:06:43 2011 UTC vs.
Revision 1.26 by sf-exg, Wed Jun 1 00:57:14 2011 UTC

…		…
		1	=head1 LIBECB - e-C-Builtins
		2
1	=head1 LIBECB	3	=head2 ABOUT LIBECB
2		4
3	You suck, we don't(tm)	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.
4		25
5	=head2 ABOUT THE HEADER	26	=head2 ABOUT THE HEADER
6		27
7	- how to include it	28	At the moment, all you have to do is copy F<ecb.h> somewhere where your
8	- it includes inttypes.h	29	compiler can find it and include it:
9	- no .a	30
10	- whats a bool	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.
11		58
12	=head2 GCC ATTRIBUTES	59	=head2 GCC ATTRIBUTES
13		60
14	blabla where to put, what others	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;
15		76
16	=over 4	77	=over 4
17		78
18	=item ecb_attribute ((attrs...))	79	=item ecb_attribute ((attrs...))
19		80
20	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and	81	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
21	to nothing on other compilers, so the effect is that only GCC sees these.	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);
22		88
23	=item ecb_unused	89	=item ecb_unused
24		90
25	Marks a function or a variable as "unused", which simply suppresses a	91	Marks a function or a variable as "unused", which simply suppresses a
26	warning by GCC when it detects it as unused. This is useful when you e.g.	92	warning by GCC when it detects it as unused. This is useful when you e.g.
27	declare a variable but do not always use it:	93	declare a variable but do not always use it:
28		94
29	{	95	{
30	int var ecb_unused;	96	int var ecb_unused;
31		97
32	#ifdef SOMECONDITION	98	#ifdef SOMECONDITION
33	var = ...;	99	var = ...;
34	return var;	100	return var;
35	#else	101	#else
36	return 0;	102	return 0;
37	#endif	103	#endif
38	}	104	}
39		105
40	=item ecb_noinline	106	=item ecb_noinline
41		107
42	Prevent a function from being inlined - it might be optimsied away, but	108	Prevent a function from being inlined - it might be optimised away, but
43	not inlined into other functions. This is useful if you know your function	109	not inlined into other functions. This is useful if you know your function
44	is rarely called and large enough for inlining not to be helpful.	110	is rarely called and large enough for inlining not to be helpful.
45		111
46	=item ecb_noreturn	112	=item ecb_noreturn
47		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
48	=item ecb_const	128	=item ecb_const
49		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
50	=item ecb_pure	149	=item ecb_pure
51		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
52	=item ecb_hot	165	=item ecb_hot
53		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
54	=item ecb_cold	178	=item ecb_cold
55		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_unlikely> 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
56	=item ecb_artificial	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	};
57		220
58	=back	221	=back
59		222
60	=head2 OPTIMISATION HINTS	223	=head2 OPTIMISATION HINTS
61		224
87	return is_constant (n) && !(n & (n - 1))	250	return is_constant (n) && !(n & (n - 1))
88	? rndm16 () & (num - 1)	251	? rndm16 () & (num - 1)
89	: (n * (uint32_t)rndm16 ()) >> 16;	252	: (n * (uint32_t)rndm16 ()) >> 16;
90	}	253	}
91		254
92	=item bool ecb_expect(expr,value)	255	=item bool ecb_expect (expr, value)
93		256
		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.
		260
		261	Usually, you want to use the more intuitive C<ecb_likely> and
		262	C<ecb_unlikely> functions instead.
		263
		264	=item bool ecb_likely (cond)
		265
94	=item bool ecb_unlikely(bool)	266	=item bool ecb_unlikely (cond)
95		267
96	=item bool ecb_likely(bool)	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:
97		271
		272	/* these two do the same thing */
		273	if (some_condition) ...;
		274	if (ecb_likely (some_condition)) ...;
		275
		276	However, by using C<ecb_likely>, you tell the compiler that the condition
		277	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
		278	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_unlikely>:
		282
		283	void my_free (void *ptr)
		284	{
		285	if (ecb_unlikely (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	A very good example is in a function that reserves more space for some
		294	memory block (for example, inside an implementation of a string stream) -
		295	each time something is added, you have to check for a buffer overrun, but
		296	you expect that most checks will turn out to be false:
		297
		298	/* make sure we have "size" extra room in our buffer */
		299	ecb_inline void
		300	reserve (int size)
		301	{
		302	if (ecb_unlikely (current + size > end))
		303	real_reserve_method (size); /* presumably noinline */
		304	}
		305
98	=item bool ecb_assume(cond)	306	=item bool ecb_assume (cond)
99		307
		308	Try to tell the compiler that some condition is true, even if it's not
		309	obvious.
		310
		311	This can be used to teach the compiler about invariants or other
		312	conditions that might improve code generation, but which are impossible to
		313	deduce form the code itself.
		314
		315	For example, the example reservation function from the C<ecb_unlikely>
		316	description could be written thus (only C<ecb_assume> was added):
		317
		318	ecb_inline void
		319	reserve (int size)
		320	{
		321	if (ecb_unlikely (current + size > end))
		322	real_reserve_method (size); /* presumably noinline */
		323
		324	ecb_assume (current + size <= end);
		325	}
		326
		327	If you then call this function twice, like this:
		328
		329	reserve (10);
		330	reserve (1);
		331
		332	Then the compiler I<might> be able to optimise out the second call
		333	completely, as it knows that C<< current + 1 > end >> is false and the
		334	call will never be executed.
		335
100	=item bool ecb_unreachable()	336	=item bool ecb_unreachable ()
101		337
		338	This function does nothing itself, except tell the compiler that it will
		339	never be executed. Apart from suppressing a warning in some cases, this
		340	function can be used to implement C<ecb_assume> or similar functions.
		341
102	=item bool ecb_prefetch(addr,rw,locality)	342	=item bool ecb_prefetch (addr, rw, locality)
		343
		344	Tells the compiler to try to prefetch memory at the given C<addr>ess
		345	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
		346	C<0> means that there will only be one access later, C<3> means that
		347	the data will likely be accessed very often, and values in between mean
		348	something... in between. The memory pointed to by the address does not
		349	need to be accessible (it could be a null pointer for example), but C<rw>
		350	and C<locality> must be compile-time constants.
		351
		352	An obvious way to use this is to prefetch some data far away, in a big
		353	array you loop over. This prefetches memory some 128 array elements later,
		354	in the hope that it will be ready when the CPU arrives at that location.
		355
		356	int sum = 0;
		357
		358	for (i = 0; i < N; ++i)
		359	{
		360	sum += arr [i]
		361	ecb_prefetch (arr + i + 128, 0, 0);
		362	}
		363
		364	It's hard to predict how far to prefetch, and most CPUs that can prefetch
		365	are often good enough to predict this kind of behaviour themselves. It
		366	gets more interesting with linked lists, especially when you do some fair
		367	processing on each list element:
		368
		369	for (node *n = start; n; n = n->next)
		370	{
		371	ecb_prefetch (n->next, 0, 0);
		372	... do medium amount of work with *n
		373	}
		374
		375	After processing the node, (part of) the next node might already be in
		376	cache.
103		377
104	=back	378	=back
105		379
106	=head2 BIT FIDDLING / BITSTUFFS	380	=head2 BIT FIDDLING / BITSTUFFS
107		381
…		…
109		383
110	=item bool ecb_big_endian ()	384	=item bool ecb_big_endian ()
111		385
112	=item bool ecb_little_endian ()	386	=item bool ecb_little_endian ()
113		387
		388	These two functions return true if the byte order is big endian
		389	(most-significant byte first) or little endian (least-significant byte
		390	first) respectively.
		391
		392	On systems that are neither, their return values are unspecified.
		393
114	=item int ecb_ctz32 (uint32_t x)	394	=item int ecb_ctz32 (uint32_t x)
115		395
		396	Returns the index of the least significant bit set in C<x> (or
		397	equivalently the number of bits set to 0 before the least significant bit
		398	set), starting from 0. If C<x> is 0 the result is undefined. A common use
		399	case is to compute the integer binary logarithm, i.e., C<floor (log2
		400	(n))>. For example:
		401
		402	ecb_ctz32 (3) = 0
		403	ecb_ctz32 (6) = 1
		404
116	=item int ecb_popcount32 (uint32_t x)	405	=item int ecb_popcount32 (uint32_t x)
117		406
		407	Returns the number of bits set to 1 in C<x>. For example:
		408
		409	ecb_popcount32 (7) = 3
		410	ecb_popcount32 (255) = 8
		411
		412	=item uint32_t ecb_bswap16 (uint32_t x)
		413
118	=item uint32_t ecb_bswap32 (uint32_t x)	414	=item uint32_t ecb_bswap32 (uint32_t x)
119		415
120	=item uint32_t ecb_bswap16 (uint32_t x)	416	These two functions return the value of the 16-bit (32-bit) value C<x>
		417	after reversing the order of bytes (0x11223344 becomes 0x44332211).
121		418
122	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	419	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
123		420
124	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	421	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
125		422
		423	These two functions return the value of C<x> after rotating all the bits
		424	by C<count> positions to the right or left respectively.
		425
		426	Current GCC versions understand these functions and usually compile them
		427	to "optimal" code (e.g. a single C<roll> on x86).
		428
126	=back	429	=back
127		430
128	=head2 ARITHMETIC	431	=head2 ARITHMETIC
129		432
130	=over 4	433	=over 4
131		434
132	=item x = ecb_mod (m, n) [MACRO]	435	=item x = ecb_mod (m, n)
		436
		437	Returns C<m> modulo C<n>, which is the same as the positive remainder
		438	of the division operation between C<m> and C<n>, using floored
		439	division. Unlike the C remainder operator C<%>, this function ensures that
		440	the return value is always positive and that the two numbers I<m> and
		441	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		442	C<ecb_mod> implements the mathematical modulo operation, which is missing
		443	in the language.
		444
		445	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
		446	negatable, that is, both C<m> and C<-m> must be representable in its
		447	type (this typically includes the minimum signed integer value, the same
		448	limitation as for C</> and C<%> in C).
		449
		450	Current GCC versions compile this into an efficient branchless sequence on
		451	many systems.
		452
		453	For example, when you want to rotate forward through the members of an
		454	array for increasing C<m> (which might be negative), then you should use
		455	C<ecb_mod>, as the C<%> operator might give either negative results, or
		456	change direction for negative values:
		457
		458	for (m = -100; m <= 100; ++m)
		459	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
133		460
134	=back	461	=back
135		462
136	=head2 UTILITY	463	=head2 UTILITY
137		464
138	=over 4	465	=over 4
139		466
140	=item ecb_array_length (name) [MACRO]	467	=item element_count = ecb_array_length (name)
		468
		469	Returns the number of elements in the array C<name>. For example:
		470
		471	int primes[] = { 2, 3, 5, 7, 11 };
		472	int sum = 0;
		473
		474	for (i = 0; i < ecb_array_length (primes); i++)
		475	sum += primes [i];
141		476
142	=back	477	=back
143		478
144		479

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Comparing cvsroot/libecb/ecb.pod (file contents): Revision 1.6 by root, Thu May 26 20:06:43 2011 UTC vs. Revision 1.26 by sf-exg, Wed Jun 1 00:57:14 2011 UTC

Diff Legend

Comparing cvsroot/libecb/ecb.pod (file contents):
Revision 1.6 by root, Thu May 26 20:06:43 2011 UTC vs.
Revision 1.26 by sf-exg, Wed Jun 1 00:57:14 2011 UTC