[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 1.45 by root, Tue May 29 14:35:43 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>) and can be used in preprocessor
		69	expressions.
		70
		71	=head2 LANGUAGE/COMPILER VERSIONS
		72
		73	All the following symbols expand to an expressionb that cna be tested in
		74	preprocessor instructions as well as treated as a boolean (use C<!!> to
		75	ensure it's either C<0> or C<1> if you need that).
		76
		77	=over 4
		78
		79	=item ECB_C
		80
		81	True if the implementation defines the C<__STDC__> macro to a true value,
		82	which is typically true for both C and C++ compilers.
		83
		84	=item ECB_C99
		85
		86	True if the implementation claims to be C99 compliant.
		87
		88	=item ECB_C11
		89
		90	True if the implementation claims to be C11 compliant.
		91
		92	=item ECB_CPP
		93
		94	True if the implementation defines the C<__cplusplus__> macro to a true
		95	value, which is typically true for C++ compilers.
		96
		97	=item ECB_CPP98
		98
		99	True if the implementation claims to be compliant to ISO/IEC 14882:1998
		100	(the first C++ ISO standard) or any later vwersion. Typically true for all
		101	C++ compilers.
		102
		103	=item ECB_CPP11
		104
		105	True if the implementation claims to be compliant to ISO/IEC 14882:2011
		106	(C++11) or any later vwersion.
		107
		108	=item ECB_GCC_VERSION(major,minor)
		109
		110	Expands to a true value (suitable for testing in by the preprocessor)
		111	if the compiler used is GNU C and the version is the givne version, or
		112	higher.
		113
		114	This macro tries to return false on compilers that claim to be GCC
		115	compatible but aren't.
		116
		117	=back
		118
58	=head2 GCC ATTRIBUTES	119	=head2 GCC ATTRIBUTES
59		120
60	blabla where to put, what others	121	A major part of libecb deals with GCC attributes. These are additional
		122	attributes that you can assign to functions, variables and sometimes even
		123	types - much like C<const> or C<volatile> in C.
		124
		125	While GCC allows declarations to show up in many surprising places,
		126	but not in many expected places, the safest way is to put attribute
		127	declarations before the whole declaration:
		128
		129	ecb_const int mysqrt (int a);
		130	ecb_unused int i;
		131
		132	For variables, it is often nicer to put the attribute after the name, and
		133	avoid multiple declarations using commas:
		134
		135	int i ecb_unused;
61		136
62	=over 4	137	=over 4
63		138
64	=item ecb_attribute ((attrs...))	139	=item ecb_attribute ((attrs...))
65		140
…		…
86	#else	161	#else
87	return 0;	162	return 0;
88	#endif	163	#endif
89	}	164	}
90		165
		166	=item ecb_inline
		167
		168	This is not actually an attribute, but you use it like one. It expands
		169	either to C<static inline> or to just C<static>, if inline isn't
		170	supported. It should be used to declare functions that should be inlined,
		171	for code size or speed reasons.
		172
		173	Example: inline this function, it surely will reduce codesize.
		174
		175	ecb_inline int
		176	negmul (int a, int b)
		177	{
		178	return - (a * b);
		179	}
		180
91	=item ecb_noinline	181	=item ecb_noinline
92		182
93	Prevent a function from being inlined - it might be optimised away, but	183	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	184	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.	185	is rarely called and large enough for inlining not to be helpful.
…		…
105	{	195	{
106	puts (errline);	196	puts (errline);
107	abort ();	197	abort ();
108	}	198	}
109		199
110	In this case, the compiler would probbaly be smart enough to decude it on	200	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.	201	its own, so this is mainly useful for declarations.
112		202
113	=item ecb_const	203	=item ecb_const
114		204
115	Declares that the function only depends on the values of it's arguments,	205	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	206	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	207	any memory any arguments might point to, global variables, or call any
118	non-const functions. It also must not have any side effects.	208	non-const functions. It also must not have any side effects.
119		209
120	Such a function can be optimised much more aggressively by the compiler -	210	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	211	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	212	a single call, which wouldn't be possible if the compiler would have to
123	expect any side effects.	213	expect any side effects.
124		214
125	It is best suited for functions in the sense of mathematical functions,	215	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.	216	such as a function returning the square root of its input argument.
127		217
128	Not suited would be a function that calculates the hash of some memory	218	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	219	area you pass in, prints some messages or looks at a global variable to
130	decide on rounding.	220	decide on rounding.
131		221
…		…
154	possible.	244	possible.
155		245
156	The compiler reacts by trying to place hot functions near to each other in	246	The compiler reacts by trying to place hot functions near to each other in
157	memory.	247	memory.
158		248
159	Whether a function is hot or not often depend son the whole program,	249	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	250	and less on the function itself. C<ecb_cold> is likely more useful in
161	practise.	251	practise.
162		252
163	=item ecb_cold	253	=item ecb_cold
164		254
…		…
169		259
170	In addition to placing cold functions together (or at least away from hot	260	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	261	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	262	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	263	leading to calls to those functions can automatically be marked as if
174	C<ecb_unlikel> had been used to reach them.	264	C<ecb_expect_false> had been used to reach them.
175		265
176	Good examples for such functions would be error reporting functions, or	266	Good examples for such functions would be error reporting functions, or
177	functions only called in exceptional or rare cases.	267	functions only called in exceptional or rare cases.
178		268
179	=item ecb_artificial	269	=item ecb_artificial
…		…
241		331
242	Evaluates C<expr> and returns it. In addition, it tells the compiler that	332	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	333	the C<expr> evaluates to C<value> a lot, which can be used for static
244	branch optimisations.	334	branch optimisations.
245		335
246	Usually, you want to use the more intuitive C<ecb_likely> and	336	Usually, you want to use the more intuitive C<ecb_expect_true> and
247	C<ecb_unlikely> functions instead.	337	C<ecb_expect_false> functions instead.
248		338
		339	=item bool ecb_expect_true (cond)
		340
249	=item bool ecb_likely (cond)	341	=item bool ecb_expect_false (cond)
250
251	=item bool ecb_unlikely (cond)
252		342
253	These two functions expect a expression that is true or false and return	343	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	344	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:	345	other conditional statement, it will not change the program:
256		346
257	/* these two do the same thing */	347	/* these two do the same thing */
258	if (some_condition) ...;	348	if (some_condition) ...;
259	if (ecb_likely (some_condition)) ...;	349	if (ecb_expect_true (some_condition)) ...;
260		350
261	However, by using C<ecb_likely>, you tell the compiler that the condition	351	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	352	condition is likely to be true (and for C<ecb_expect_false>, that it is
263	true).	353	unlikely to be true).
264		354
265	For example, when you check for a null pointer and expect this to be a	355	For example, when you check for a null pointer and expect this to be a
266	rare, exceptional, case, then use C<ecb_unlikely>:	356	rare, exceptional, case, then use C<ecb_expect_false>:
267		357
268	void my_free (void *ptr)	358	void my_free (void *ptr)
269	{	359	{
270	if (ecb_unlikely (ptr == 0))	360	if (ecb_expect_false (ptr == 0))
271	return;	361	return;
272	}	362	}
273		363
274	Consequent use of these functions to mark away exceptional cases or to	364	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	365	tell the compiler what the hot path through a function is can increase
276	performance considerably.	366	performance considerably.
		367
		368	You might know these functions under the name C<likely> and C<unlikely>
		369	- while these are common aliases, we find that the expect name is easier
		370	to understand when quickly skimming code. If you wish, you can use
		371	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		372	C<ecb_expect_false> - these are simply aliases.
277		373
278	A very good example is in a function that reserves more space for some	374	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) -	375	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	376	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:	377	you expect that most checks will turn out to be false:
282		378
283	/* make sure we have "size" extra room in our buffer */	379	/* make sure we have "size" extra room in our buffer */
284	ecb_inline void	380	ecb_inline void
285	reserve (int size)	381	reserve (int size)
286	{	382	{
287	if (ecb_unlikely (current + size > end))	383	if (ecb_expect_false (current + size > end))
288	real_reserve_method (size); /* presumably noinline */	384	real_reserve_method (size); /* presumably noinline */
289	}	385	}
290		386
291	=item bool ecb_assume (cond)	387	=item bool ecb_assume (cond)
292		388
…		…
295		391
296	This can be used to teach the compiler about invariants or other	392	This can be used to teach the compiler about invariants or other
297	conditions that might improve code generation, but which are impossible to	393	conditions that might improve code generation, but which are impossible to
298	deduce form the code itself.	394	deduce form the code itself.
299		395
300	For example, the example reservation function from the C<ecb_unlikely>	396	For example, the example reservation function from the C<ecb_expect_false>
301	description could be written thus (only C<ecb_assume> was added):	397	description could be written thus (only C<ecb_assume> was added):
302		398
303	ecb_inline void	399	ecb_inline void
304	reserve (int size)	400	reserve (int size)
305	{	401	{
306	if (ecb_unlikely (current + size > end))	402	if (ecb_expect_false (current + size > end))
307	real_reserve_method (size); /* presumably noinline */	403	real_reserve_method (size); /* presumably noinline */
308		404
309	ecb_assume (current + size <= end);	405	ecb_assume (current + size <= end);
310	}	406	}
311		407
…		…
360	After processing the node, (part of) the next node might already be in	456	After processing the node, (part of) the next node might already be in
361	cache.	457	cache.
362		458
363	=back	459	=back
364		460
365	=head2 BIT FIDDLING / BITSTUFFS	461	=head2 BIT FIDDLING / BIT WIZARDRY
366		462
367	=over 4	463	=over 4
368		464
369	=item bool ecb_big_endian ()	465	=item bool ecb_big_endian ()
370		466
…		…
372		468
373	These two functions return true if the byte order is big endian	469	These two functions return true if the byte order is big endian
374	(most-significant byte first) or little endian (least-significant byte	470	(most-significant byte first) or little endian (least-significant byte
375	first) respectively.	471	first) respectively.
376		472
		473	On systems that are neither, their return values are unspecified.
		474
377	=item int ecb_ctz32 (uint32_t x)	475	=item int ecb_ctz32 (uint32_t x)
378		476
		477	=item int ecb_ctz64 (uint64_t x)
		478
379	Returns the index of the least significant bit set in C<x> (or	479	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	480	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	481	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.,	482
383	floor(log2(n)). For example:	483	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		484
		485	For example:
384		486
385	ecb_ctz32 (3) = 0	487	ecb_ctz32 (3) = 0
386	ecb_ctz32 (6) = 1	488	ecb_ctz32 (6) = 1
387		489
		490	=item bool ecb_is_pot32 (uint32_t x)
		491
		492	=item bool ecb_is_pot64 (uint32_t x)
		493
		494	Return true iff C<x> is a power of two or C<x == 0>.
		495
		496	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		497
		498	=item int ecb_ld32 (uint32_t x)
		499
		500	=item int ecb_ld64 (uint64_t x)
		501
		502	Returns the index of the most significant bit set in C<x>, or the number
		503	of digits the number requires in binary (so that C<< 2**ld <= x <
		504	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		505	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		506	example to see how many bits a certain number requires to be encoded.
		507
		508	This function is similar to the "count leading zero bits" function, except
		509	that that one returns how many zero bits are "in front" of the number (in
		510	the given data type), while C<ecb_ld> returns how many bits the number
		511	itself requires.
		512
		513	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		514
388	=item int ecb_popcount32 (uint32_t x)	515	=item int ecb_popcount32 (uint32_t x)
389		516
		517	=item int ecb_popcount64 (uint64_t x)
		518
390	Returns the number of bits set to 1 in C<x>. For example:	519	Returns the number of bits set to 1 in C<x>.
		520
		521	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		522
		523	For example:
391		524
392	ecb_popcount32 (7) = 3	525	ecb_popcount32 (7) = 3
393	ecb_popcount32 (255) = 8	526	ecb_popcount32 (255) = 8
394		527
		528	=item uint8_t ecb_bitrev8 (uint8_t x)
		529
		530	=item uint16_t ecb_bitrev16 (uint16_t x)
		531
		532	=item uint32_t ecb_bitrev32 (uint32_t x)
		533
		534	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		535	and so on.
		536
		537	Example:
		538
		539	ecb_bitrev8 (0xa7) = 0xea
		540	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		541
395	=item uint32_t ecb_bswap16 (uint32_t x)	542	=item uint32_t ecb_bswap16 (uint32_t x)
396		543
397	=item uint32_t ecb_bswap32 (uint32_t x)	544	=item uint32_t ecb_bswap32 (uint32_t x)
398		545
		546	=item uint64_t ecb_bswap64 (uint64_t x)
		547
399	These two functions return the value of the 16-bit (32-bit) variable	548	These functions return the value of the 16-bit (32-bit, 64-bit) value
400	C<x> after reversing the order of bytes.	549	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		550	C<ecb_bswap32>).
		551
		552	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		553
		554	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		555
		556	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		557
		558	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		559
		560	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		561
		562	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
401		563
402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	564	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
403		565
404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	566	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
405		567
406	These two functions return the value of C<x> after shifting all the bits	568	These two families of functions return the value of C<x> after rotating
407	by C<count> positions to the right or left respectively.	569	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		570	(C<ecb_rotl>).
		571
		572	Current GCC versions understand these functions and usually compile them
		573	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		574	x86).
408		575
409	=back	576	=back
410		577
411	=head2 ARITHMETIC	578	=head2 ARITHMETIC
412		579
413	=over 4	580	=over 4
414		581
415	=item x = ecb_mod (m, n)	582	=item x = ecb_mod (m, n)
416		583
417	Returns the positive remainder of the modulo operation between C<m> and	584	Returns C<m> modulo C<n>, which is the same as the positive remainder
		585	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	586	division. Unlike the C remainder operator C<%>, this function ensures that
419	return value is always positive).	587	the return value is always positive and that the two numbers I<m> and
		588	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		589	C<ecb_mod> implements the mathematical modulo operation, which is missing
		590	in the language.
420		591
421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	592	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	593	negatable, that is, both C<m> and C<-m> must be representable in its
423	type.	594	type (this typically excludes the minimum signed integer value, the same
		595	limitation as for C</> and C<%> in C).
		596
		597	Current GCC versions compile this into an efficient branchless sequence on
		598	almost all CPUs.
		599
		600	For example, when you want to rotate forward through the members of an
		601	array for increasing C<m> (which might be negative), then you should use
		602	C<ecb_mod>, as the C<%> operator might give either negative results, or
		603	change direction for negative values:
		604
		605	for (m = -100; m <= 100; ++m)
		606	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		607
		608	=item x = ecb_div_rd (val, div)
		609
		610	=item x = ecb_div_ru (val, div)
		611
		612	Returns C<val> divided by C<div> rounded down or up, respectively.
		613	C<val> and C<div> must have integer types and C<div> must be strictly
		614	positive. Note that these functions are implemented with macros in C
		615	and with function templates in C++.
424		616
425	=back	617	=back
426		618
427	=head2 UTILITY	619	=head2 UTILITY
428		620
429	=over 4	621	=over 4
430		622
431	=item element_count = ecb_array_length (name) [MACRO]	623	=item element_count = ecb_array_length (name)
432		624
433	Returns the number of elements in the array C<name>. For example:	625	Returns the number of elements in the array C<name>. For example:
434		626
435	int primes[] = { 2, 3, 5, 7, 11 };	627	int primes[] = { 2, 3, 5, 7, 11 };
436	int sum = 0;	628	int sum = 0;
…		…
438	for (i = 0; i < ecb_array_length (primes); i++)	630	for (i = 0; i < ecb_array_length (primes); i++)
439	sum += primes [i];	631	sum += primes [i];
440		632
441	=back	633	=back
442		634
		635	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
443		636
		637	These symbols need to be defined before including F<ecb.h> the first time.
		638
		639	=over 4
		640
		641	=item ECB_NO_THRADS
		642
		643	If F<ecb.h> is never used from multiple threads, then this symbol can
		644	be defined, in which case memory fences (and similar constructs) are
		645	completely removed, leading to more efficient code and fewer dependencies.
		646
		647	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		648
		649	=item ECB_NO_SMP
		650
		651	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		652	multiple threads, but never concurrently (e.g. if the system the program
		653	runs on has only a single CPU with a single core, no hyperthreading and so
		654	on), then this symbol can be defined, leading to more efficient code and
		655	fewer dependencies.
		656
		657	=back
		658
		659

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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 1.45 by root, Tue May 29 14:35:43 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 1.45 by root, Tue May 29 14:35:43 2012 UTC