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

Comparing libecb/ecb.pod (file contents):
Revision 1.23 by sf-exg, Fri May 27 01:35:46 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	A major part of libecb deals with GCC attributes. These are additional	120	A major part of libecb deals with GCC attributes. These are additional
61	attributes that you cna assign to functions, variables and sometimes even	121	attributes that you can assign to functions, variables and sometimes even
62	types - much like C<const> or C<volatile> in C.	122	types - much like C<const> or C<volatile> in C.
63		123
64	While GCC allows declarations to show up in many surprising places,	124	While GCC allows declarations to show up in many surprising places,
65	but not in many expeted places, the safest way is to put attribute	125	but not in many expected places, the safest way is to put attribute
66	declarations before the whole declaration:	126	declarations before the whole declaration:
67		127
68	ecb_const int mysqrt (int a);	128	ecb_const int mysqrt (int a);
69	ecb_unused int i;	129	ecb_unused int i;
70		130
…		…
100	#else	160	#else
101	return 0;	161	return 0;
102	#endif	162	#endif
103	}	163	}
104		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
105	=item ecb_noinline	180	=item ecb_noinline
106		181
107	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
108	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
109	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.
…		…
183		258
184	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
185	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
186	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
187	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
188	C<ecb_unlikely> had been used to reach them.	263	C<ecb_expect_false> had been used to reach them.
189		264
190	Good examples for such functions would be error reporting functions, or	265	Good examples for such functions would be error reporting functions, or
191	functions only called in exceptional or rare cases.	266	functions only called in exceptional or rare cases.
192		267
193	=item ecb_artificial	268	=item ecb_artificial
…		…
255		330
256	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
257	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
258	branch optimisations.	333	branch optimisations.
259		334
260	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
261	C<ecb_unlikely> functions instead.	336	C<ecb_expect_false> functions instead.
262		337
		338	=item bool ecb_expect_true (cond)
		339
263	=item bool ecb_likely (cond)	340	=item bool ecb_expect_false (cond)
264
265	=item bool ecb_unlikely (cond)
266		341
267	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
268	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
269	other conditional statement, it will not change the program:	344	other conditional statement, it will not change the program:
270		345
271	/* these two do the same thing */	346	/* these two do the same thing */
272	if (some_condition) ...;	347	if (some_condition) ...;
273	if (ecb_likely (some_condition)) ...;	348	if (ecb_expect_true (some_condition)) ...;
274		349
275	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
276	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
277	true).	352	unlikely to be true).
278		353
279	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
280	rare, exceptional, case, then use C<ecb_unlikely>:	355	rare, exceptional, case, then use C<ecb_expect_false>:
281		356
282	void my_free (void *ptr)	357	void my_free (void *ptr)
283	{	358	{
284	if (ecb_unlikely (ptr == 0))	359	if (ecb_expect_false (ptr == 0))
285	return;	360	return;
286	}	361	}
287		362
288	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
289	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
290	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.
291		372
292	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
293	memory block (for example, inside an implementation of a string stream) -	374	memory block (for example, inside an implementation of a string stream) -
294	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
295	you expect that most checks will turn out to be false:	376	you expect that most checks will turn out to be false:
296		377
297	/* make sure we have "size" extra room in our buffer */	378	/* make sure we have "size" extra room in our buffer */
298	ecb_inline void	379	ecb_inline void
299	reserve (int size)	380	reserve (int size)
300	{	381	{
301	if (ecb_unlikely (current + size > end))	382	if (ecb_expect_false (current + size > end))
302	real_reserve_method (size); /* presumably noinline */	383	real_reserve_method (size); /* presumably noinline */
303	}	384	}
304		385
305	=item bool ecb_assume (cond)	386	=item bool ecb_assume (cond)
306		387
…		…
309		390
310	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
311	conditions that might improve code generation, but which are impossible to	392	conditions that might improve code generation, but which are impossible to
312	deduce form the code itself.	393	deduce form the code itself.
313		394
314	For example, the example reservation function from the C<ecb_unlikely>	395	For example, the example reservation function from the C<ecb_expect_false>
315	description could be written thus (only C<ecb_assume> was added):	396	description could be written thus (only C<ecb_assume> was added):
316		397
317	ecb_inline void	398	ecb_inline void
318	reserve (int size)	399	reserve (int size)
319	{	400	{
320	if (ecb_unlikely (current + size > end))	401	if (ecb_expect_false (current + size > end))
321	real_reserve_method (size); /* presumably noinline */	402	real_reserve_method (size); /* presumably noinline */
322		403
323	ecb_assume (current + size <= end);	404	ecb_assume (current + size <= end);
324	}	405	}
325		406
…		…
374	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
375	cache.	456	cache.
376		457
377	=back	458	=back
378		459
379	=head2 BIT FIDDLING / BITSTUFFS	460	=head2 BIT FIDDLING / BIT WIZARDRY
380		461
381	=over 4	462	=over 4
382		463
383	=item bool ecb_big_endian ()	464	=item bool ecb_big_endian ()
384		465
…		…
386		467
387	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
388	(most-significant byte first) or little endian (least-significant byte	469	(most-significant byte first) or little endian (least-significant byte
389	first) respectively.	470	first) respectively.
390		471
		472	On systems that are neither, their return values are unspecified.
		473
391	=item int ecb_ctz32 (uint32_t x)	474	=item int ecb_ctz32 (uint32_t x)
392		475
		476	=item int ecb_ctz64 (uint64_t x)
		477
393	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
394	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
395	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.
396	common use case is to compute the integer binary logarithm, i.e.,	481
397	floor(log2(n)). For example:	482	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		483
		484	For example:
398		485
399	ecb_ctz32 (3) = 0	486	ecb_ctz32 (3) = 0
400	ecb_ctz32 (6) = 1	487	ecb_ctz32 (6) = 1
401		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
402	=item int ecb_popcount32 (uint32_t x)	514	=item int ecb_popcount32 (uint32_t x)
403		515
		516	=item int ecb_popcount64 (uint64_t x)
		517
404	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:
405		523
406	ecb_popcount32 (7) = 3	524	ecb_popcount32 (7) = 3
407	ecb_popcount32 (255) = 8	525	ecb_popcount32 (255) = 8
408		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
409	=item uint32_t ecb_bswap16 (uint32_t x)	541	=item uint32_t ecb_bswap16 (uint32_t x)
410		542
411	=item uint32_t ecb_bswap32 (uint32_t x)	543	=item uint32_t ecb_bswap32 (uint32_t x)
412		544
		545	=item uint64_t ecb_bswap64 (uint64_t x)
		546
413	These two functions return the value of the 16-bit (32-bit) value C<x>	547	These functions return the value of the 16-bit (32-bit, 64-bit) value
414	after reversing the order of bytes (0x11223344 becomes 0x44332211).	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)
415		562
416	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	563	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
417		564
418	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	565	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
419		566
420	These two functions return the value of C<x> after rotating all the bits	567	These two families of functions return the value of C<x> after rotating
421	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>).
422		570
423	Current GCC versions understand these functions and usually compile them	571	Current GCC versions understand these functions and usually compile them
424	to "optimal" code (e.g. a single C<roll> on x86).	572	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		573	x86).
425		574
426	=back	575	=back
427		576
428	=head2 ARITHMETIC	577	=head2 ARITHMETIC
429		578
430	=over 4	579	=over 4
431		580
432	=item x = ecb_mod (m, n)	581	=item x = ecb_mod (m, n)
433		582
434	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
435	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
436	return value is always positive - ISO C guarantees very little when	586	the return value is always positive and that the two numbers I<m> and
437	negative numbers are used with C<%>.	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.
438		590
439	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
440	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
441	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++.
442		615
443	=back	616	=back
444		617
445	=head2 UTILITY	618	=head2 UTILITY
446		619
…		…
456	for (i = 0; i < ecb_array_length (primes); i++)	629	for (i = 0; i < ecb_array_length (primes); i++)
457	sum += primes [i];	630	sum += primes [i];
458		631
459	=back	632	=back
460		633
		634	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
461		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 1.23 by sf-exg, Fri May 27 01:35:46 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 1.23 by sf-exg, Fri May 27 01:35:46 2011 UTC vs.
Revision 1.44 by root, Tue May 29 14:30:44 2012 UTC