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

Comparing libecb/ecb.pod (file contents):
Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC vs.
Revision 1.44 by root, Tue May 29 14:30:44 2012 UTC

…		…
15	It mainly provides a number of wrappers around GCC built-ins, together	15	It mainly provides a number of wrappers around GCC built-ins, together
16	with replacement functions for other compilers. In addition to this,	16	with replacement functions for other compilers. In addition to this,
17	it provides a number of other lowlevel C utilities, such as endianness	17	it provides a number of other lowlevel C utilities, such as endianness
18	detection, byte swapping or bit rotations.	18	detection, byte swapping or bit rotations.
19		19
20	Or in other words, things that should be built-in into any standard C	20	Or in other words, things that should be built into any standard C system,
21	system, but aren't.	21	but aren't, implemented as efficient as possible with GCC, and still
		22	correct with other compilers.
22		23
23	More might come.	24	More might come.
24		25
25	=head2 ABOUT THE HEADER	26	=head2 ABOUT THE HEADER
26		27
…		…
53	only a generic name is used (C<expr>, C<cond>, C<value> and so on), then	54	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	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	is usually implemented as a macro. Specifically, a "bool" in this manual
56	refers to any kind of boolean value, not a specific type.	57	refers to any kind of boolean value, not a specific type.
57		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>).
		69
		70	=head2 LANGUAGE/COMPILER VERSIONS
		71
		72	All the following symbols expand to an expressionb that cna be tested in
		73	preprocessor instructions as well as treated as a boolean (use C<!!> to
		74	ensure it's either C<0> or C<1> if you need that).
		75
		76	=over 4
		77
		78	=item ECB_C
		79
		80	True if the implementation defines the C<__STDC__> macro to a true value,
		81	which is typically true for both C and C++ compilers.
		82
		83	=item ECB_C99
		84
		85	True if the implementation claims to be C99 compliant.
		86
		87	=item ECB_C11
		88
		89	True if the implementation claims to be C11 compliant.
		90
		91	=item ECB_CPP
		92
		93	True if the implementation defines the C<__cplusplus__> macro to a true
		94	value, which is typically true for C++ compilers.
		95
		96	=item ECB_CPP98
		97
		98	True if the implementation claims to be compliant to ISO/IEC 14882:1998
		99	(the first C++ ISO standard) or any later vwersion. Typically true for all
		100	C++ compilers.
		101
		102	=item ECB_CPP11
		103
		104	True if the implementation claims to be compliant to ISO/IEC 14882:2011
		105	(C++11) or any later vwersion.
		106
		107	=item ECB_GCC_VERSION(major,minor)
		108
		109	Expands to a true value (suitable for testing in by the preprocessor)
		110	if the compiler used is GNU C and the version is the givne version, or
		111	higher.
		112
		113	This macro tries to return false on compilers that claim to be GCC
		114	compatible but aren't.
		115
		116	=back
		117
58	=head2 GCC ATTRIBUTES	118	=head2 GCC ATTRIBUTES
59		119
60	blabla where to put, what others	120	A major part of libecb deals with GCC attributes. These are additional
		121	attributes that you can assign to functions, variables and sometimes even
		122	types - much like C<const> or C<volatile> in C.
		123
		124	While GCC allows declarations to show up in many surprising places,
		125	but not in many expected places, the safest way is to put attribute
		126	declarations before the whole declaration:
		127
		128	ecb_const int mysqrt (int a);
		129	ecb_unused int i;
		130
		131	For variables, it is often nicer to put the attribute after the name, and
		132	avoid multiple declarations using commas:
		133
		134	int i ecb_unused;
61		135
62	=over 4	136	=over 4
63		137
64	=item ecb_attribute ((attrs...))	138	=item ecb_attribute ((attrs...))
65		139
…		…
86	#else	160	#else
87	return 0;	161	return 0;
88	#endif	162	#endif
89	}	163	}
90		164
		165	=item ecb_inline
		166
		167	This is not actually an attribute, but you use it like one. It expands
		168	either to C<static inline> or to just C<static>, if inline isn't
		169	supported. It should be used to declare functions that should be inlined,
		170	for code size or speed reasons.
		171
		172	Example: inline this function, it surely will reduce codesize.
		173
		174	ecb_inline int
		175	negmul (int a, int b)
		176	{
		177	return - (a * b);
		178	}
		179
91	=item ecb_noinline	180	=item ecb_noinline
92		181
93	Prevent a function from being inlined - it might be optimised away, but	182	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	183	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.	184	is rarely called and large enough for inlining not to be helpful.
…		…
105	{	194	{
106	puts (errline);	195	puts (errline);
107	abort ();	196	abort ();
108	}	197	}
109		198
110	In this case, the compiler would probbaly be smart enough to decude it on	199	In this case, the compiler would probably be smart enough to deduce it on
111	it's own, so this is mainly useful for declarations.	200	its own, so this is mainly useful for declarations.
112		201
113	=item ecb_const	202	=item ecb_const
114		203
115	Declares that the function only depends on the values of it's arguments,	204	Declares that the function only depends on the values of its arguments,
116	much like a mathematical function. It specifically does not read or write	205	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	206	any memory any arguments might point to, global variables, or call any
118	non-const functions. It also must not have any side effects.	207	non-const functions. It also must not have any side effects.
119		208
120	Such a function can be optimised much more aggressively by the compiler -	209	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	210	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	211	a single call, which wouldn't be possible if the compiler would have to
123	expect any side effects.	212	expect any side effects.
124		213
125	It is best suited for functions in the sense of mathematical functions,	214	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.	215	such as a function returning the square root of its input argument.
127		216
128	Not suited would be a function that calculates the hash of some memory	217	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	218	area you pass in, prints some messages or looks at a global variable to
130	decide on rounding.	219	decide on rounding.
131		220
…		…
154	possible.	243	possible.
155		244
156	The compiler reacts by trying to place hot functions near to each other in	245	The compiler reacts by trying to place hot functions near to each other in
157	memory.	246	memory.
158		247
159	Whether a function is hot or not often depend son the whole program,	248	Whether a function is hot or not often depends on the whole program,
160	and less on the function itself. C<ecb_cold> is likely more useful in	249	and less on the function itself. C<ecb_cold> is likely more useful in
161	practise.	250	practise.
162		251
163	=item ecb_cold	252	=item ecb_cold
164		253
…		…
169		258
170	In addition to placing cold functions together (or at least away from hot	259	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	260	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	261	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	262	leading to calls to those functions can automatically be marked as if
174	C<ecb_unlikel> had been used to reach them.	263	C<ecb_expect_false> had been used to reach them.
175		264
176	Good examples for such functions would be error reporting functions, or	265	Good examples for such functions would be error reporting functions, or
177	functions only called in exceptional or rare cases.	266	functions only called in exceptional or rare cases.
178		267
179	=item ecb_artificial	268	=item ecb_artificial
…		…
241		330
242	Evaluates C<expr> and returns it. In addition, it tells the compiler that	331	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	332	the C<expr> evaluates to C<value> a lot, which can be used for static
244	branch optimisations.	333	branch optimisations.
245		334
246	Usually, you want to use the more intuitive C<ecb_likely> and	335	Usually, you want to use the more intuitive C<ecb_expect_true> and
247	C<ecb_unlikely> functions instead.	336	C<ecb_expect_false> functions instead.
248		337
		338	=item bool ecb_expect_true (cond)
		339
249	=item bool ecb_likely (cond)	340	=item bool ecb_expect_false (cond)
250
251	=item bool ecb_unlikely (cond)
252		341
253	These two functions expect a expression that is true or false and return	342	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	343	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:	344	other conditional statement, it will not change the program:
256		345
257	/* these two do the same thing */	346	/* these two do the same thing */
258	if (some_condition) ...;	347	if (some_condition) ...;
259	if (ecb_likely (some_condition)) ...;	348	if (ecb_expect_true (some_condition)) ...;
260		349
261	However, by using C<ecb_likely>, you tell the compiler that the condition	350	However, by using C<ecb_expect_true>, you tell the compiler that the
262	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be	351	condition is likely to be true (and for C<ecb_expect_false>, that it is
263	true).	352	unlikely to be true).
264		353
265	For example, when you check for a null pointer and expect this to be a	354	For example, when you check for a null pointer and expect this to be a
266	rare, exceptional, case, then use C<ecb_unlikely>:	355	rare, exceptional, case, then use C<ecb_expect_false>:
267		356
268	void my_free (void *ptr)	357	void my_free (void *ptr)
269	{	358	{
270	if (ecb_unlikely (ptr == 0))	359	if (ecb_expect_false (ptr == 0))
271	return;	360	return;
272	}	361	}
273		362
274	Consequent use of these functions to mark away exceptional cases or to	363	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	364	tell the compiler what the hot path through a function is can increase
276	performance considerably.	365	performance considerably.
		366
		367	You might know these functions under the name C<likely> and C<unlikely>
		368	- while these are common aliases, we find that the expect name is easier
		369	to understand when quickly skimming code. If you wish, you can use
		370	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		371	C<ecb_expect_false> - these are simply aliases.
277		372
278	A very good example is in a function that reserves more space for some	373	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) -	374	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	375	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:	376	you expect that most checks will turn out to be false:
282		377
283	/* make sure we have "size" extra room in our buffer */	378	/* make sure we have "size" extra room in our buffer */
284	ecb_inline void	379	ecb_inline void
285	reserve (int size)	380	reserve (int size)
286	{	381	{
287	if (ecb_unlikely (current + size > end))	382	if (ecb_expect_false (current + size > end))
288	real_reserve_method (size); /* presumably noinline */	383	real_reserve_method (size); /* presumably noinline */
289	}	384	}
290		385
291	=item bool ecb_assume (cond)	386	=item bool ecb_assume (cond)
292		387
…		…
295		390
296	This can be used to teach the compiler about invariants or other	391	This can be used to teach the compiler about invariants or other
297	conditions that might improve code generation, but which are impossible to	392	conditions that might improve code generation, but which are impossible to
298	deduce form the code itself.	393	deduce form the code itself.
299		394
300	For example, the example reservation function from the C<ecb_unlikely>	395	For example, the example reservation function from the C<ecb_expect_false>
301	description could be written thus (only C<ecb_assume> was added):	396	description could be written thus (only C<ecb_assume> was added):
302		397
303	ecb_inline void	398	ecb_inline void
304	reserve (int size)	399	reserve (int size)
305	{	400	{
306	if (ecb_unlikely (current + size > end))	401	if (ecb_expect_false (current + size > end))
307	real_reserve_method (size); /* presumably noinline */	402	real_reserve_method (size); /* presumably noinline */
308		403
309	ecb_assume (current + size <= end);	404	ecb_assume (current + size <= end);
310	}	405	}
311		406
…		…
360	After processing the node, (part of) the next node might already be in	455	After processing the node, (part of) the next node might already be in
361	cache.	456	cache.
362		457
363	=back	458	=back
364		459
365	=head2 BIT FIDDLING / BITSTUFFS	460	=head2 BIT FIDDLING / BIT WIZARDRY
366		461
367	=over 4	462	=over 4
368		463
369	=item bool ecb_big_endian ()	464	=item bool ecb_big_endian ()
370		465
…		…
372		467
373	These two functions return true if the byte order is big endian	468	These two functions return true if the byte order is big endian
374	(most-significant byte first) or little endian (least-significant byte	469	(most-significant byte first) or little endian (least-significant byte
375	first) respectively.	470	first) respectively.
376		471
		472	On systems that are neither, their return values are unspecified.
		473
377	=item int ecb_ctz32 (uint32_t x)	474	=item int ecb_ctz32 (uint32_t x)
378		475
		476	=item int ecb_ctz64 (uint64_t x)
		477
379	Returns the index of the least significant bit set in C<x> (or	478	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	479	equivalently the number of bits set to 0 before the least significant bit
381	bit set), starting from 0. If C<x> is 0 the result is undefined. A	480	set), starting from 0. If C<x> is 0 the result is undefined.
382	common use case is to compute the integer binary logarithm, i.e.,	481
383	floor(log2(n)). For example:	482	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		483
		484	For example:
384		485
385	ecb_ctz32 (3) = 0	486	ecb_ctz32 (3) = 0
386	ecb_ctz32 (6) = 1	487	ecb_ctz32 (6) = 1
387		488
		489	=item bool ecb_is_pot32 (uint32_t x)
		490
		491	=item bool ecb_is_pot64 (uint32_t x)
		492
		493	Return true iff C<x> is a power of two or C<x == 0>.
		494
		495	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		496
		497	=item int ecb_ld32 (uint32_t x)
		498
		499	=item int ecb_ld64 (uint64_t x)
		500
		501	Returns the index of the most significant bit set in C<x>, or the number
		502	of digits the number requires in binary (so that C<< 2**ld <= x <
		503	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		504	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		505	example to see how many bits a certain number requires to be encoded.
		506
		507	This function is similar to the "count leading zero bits" function, except
		508	that that one returns how many zero bits are "in front" of the number (in
		509	the given data type), while C<ecb_ld> returns how many bits the number
		510	itself requires.
		511
		512	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		513
388	=item int ecb_popcount32 (uint32_t x)	514	=item int ecb_popcount32 (uint32_t x)
389		515
		516	=item int ecb_popcount64 (uint64_t x)
		517
390	Returns the number of bits set to 1 in C<x>. For example:	518	Returns the number of bits set to 1 in C<x>.
		519
		520	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		521
		522	For example:
391		523
392	ecb_popcount32 (7) = 3	524	ecb_popcount32 (7) = 3
393	ecb_popcount32 (255) = 8	525	ecb_popcount32 (255) = 8
394		526
		527	=item uint8_t ecb_bitrev8 (uint8_t x)
		528
		529	=item uint16_t ecb_bitrev16 (uint16_t x)
		530
		531	=item uint32_t ecb_bitrev32 (uint32_t x)
		532
		533	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		534	and so on.
		535
		536	Example:
		537
		538	ecb_bitrev8 (0xa7) = 0xea
		539	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		540
395	=item uint32_t ecb_bswap16 (uint32_t x)	541	=item uint32_t ecb_bswap16 (uint32_t x)
396		542
397	=item uint32_t ecb_bswap32 (uint32_t x)	543	=item uint32_t ecb_bswap32 (uint32_t x)
398		544
		545	=item uint64_t ecb_bswap64 (uint64_t x)
		546
399	These two functions return the value of the 16-bit (32-bit) variable	547	These functions return the value of the 16-bit (32-bit, 64-bit) value
400	C<x> after reversing the order of bytes.	548	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		549	C<ecb_bswap32>).
		550
		551	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		552
		553	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		554
		555	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		556
		557	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		558
		559	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		560
		561	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
401		562
402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	563	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
403		564
404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	565	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
405		566
406	These two functions return the value of C<x> after shifting all the bits	567	These two families of functions return the value of C<x> after rotating
407	by C<count> positions to the right or left respectively.	568	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		569	(C<ecb_rotl>).
		570
		571	Current GCC versions understand these functions and usually compile them
		572	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		573	x86).
408		574
409	=back	575	=back
410		576
411	=head2 ARITHMETIC	577	=head2 ARITHMETIC
412		578
413	=over 4	579	=over 4
414		580
415	=item x = ecb_mod (m, n)	581	=item x = ecb_mod (m, n)
416		582
417	Returns the positive remainder of the modulo operation between C<m> and	583	Returns C<m> modulo C<n>, which is the same as the positive remainder
		584	of the division operation between C<m> and C<n>, using floored
418	C<n>. Unlike the C modulo operator C<%>, this function ensures that the	585	division. Unlike the C remainder operator C<%>, this function ensures that
419	return value is always positive).	586	the return value is always positive and that the two numbers I<m> and
		587	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		588	C<ecb_mod> implements the mathematical modulo operation, which is missing
		589	in the language.
420		590
421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	591	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	592	negatable, that is, both C<m> and C<-m> must be representable in its
423	type.	593	type (this typically excludes the minimum signed integer value, the same
		594	limitation as for C</> and C<%> in C).
		595
		596	Current GCC versions compile this into an efficient branchless sequence on
		597	almost all CPUs.
		598
		599	For example, when you want to rotate forward through the members of an
		600	array for increasing C<m> (which might be negative), then you should use
		601	C<ecb_mod>, as the C<%> operator might give either negative results, or
		602	change direction for negative values:
		603
		604	for (m = -100; m <= 100; ++m)
		605	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		606
		607	=item x = ecb_div_rd (val, div)
		608
		609	=item x = ecb_div_ru (val, div)
		610
		611	Returns C<val> divided by C<div> rounded down or up, respectively.
		612	C<val> and C<div> must have integer types and C<div> must be strictly
		613	positive. Note that these functions are implemented with macros in C
		614	and with function templates in C++.
424		615
425	=back	616	=back
426		617
427	=head2 UTILITY	618	=head2 UTILITY
428		619
429	=over 4	620	=over 4
430		621
431	=item element_count = ecb_array_length (name) [MACRO]	622	=item element_count = ecb_array_length (name)
432		623
433	Returns the number of elements in the array C<name>. For example:	624	Returns the number of elements in the array C<name>. For example:
434		625
435	int primes[] = { 2, 3, 5, 7, 11 };	626	int primes[] = { 2, 3, 5, 7, 11 };
436	int sum = 0;	627	int sum = 0;
…		…
438	for (i = 0; i < ecb_array_length (primes); i++)	629	for (i = 0; i < ecb_array_length (primes); i++)
439	sum += primes [i];	630	sum += primes [i];
440		631
441	=back	632	=back
442		633
		634	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
443		635
		636	These symbols need to be defined before including F<ecb.h> the first time.
		637
		638	=over 4
		639
		640	=item ECB_NO_THRADS
		641
		642	If F<ecb.h> is never used from multiple threads, then this symbol can
		643	be defined, in which case memory fences (and similar constructs) are
		644	completely removed, leading to more efficient code and fewer dependencies.
		645
		646	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		647
		648	=item ECB_NO_SMP
		649
		650	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		651	multiple threads, but never concurrently (e.g. if the system the program
		652	runs on has only a single CPU with a single core, no hyperthreading and so
		653	on), then this symbol can be defined, leading to more efficient code and
		654	fewer dependencies.
		655
		656	=back
		657
		658

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Comparing libecb/ecb.pod (file contents): Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC vs. Revision 1.44 by root, Tue May 29 14:30:44 2012 UTC

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Comparing libecb/ecb.pod (file contents):
Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC vs.
Revision 1.44 by root, Tue May 29 14:30:44 2012 UTC