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

Comparing libecb/ecb.pod (file contents):
Revision 1.24 by root, Tue May 31 21:10:37 2011 UTC vs.
Revision 1.45 by root, Tue May 29 14:35:43 2012 UTC

…		…
54	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
55	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
56	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
57	refers to any kind of boolean value, not a specific type.	57	refers to any kind of boolean value, not a specific type.
58		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
59	=head2 GCC ATTRIBUTES	119	=head2 GCC ATTRIBUTES
60		120
61	A major part of libecb deals with GCC attributes. These are additional	121	A major part of libecb deals with GCC attributes. These are additional
62	attributes that you cna assign to functions, variables and sometimes even	122	attributes that you can assign to functions, variables and sometimes even
63	types - much like C<const> or C<volatile> in C.	123	types - much like C<const> or C<volatile> in C.
64		124
65	While GCC allows declarations to show up in many surprising places,	125	While GCC allows declarations to show up in many surprising places,
66	but not in many expeted places, the safest way is to put attribute	126	but not in many expected places, the safest way is to put attribute
67	declarations before the whole declaration:	127	declarations before the whole declaration:
68		128
69	ecb_const int mysqrt (int a);	129	ecb_const int mysqrt (int a);
70	ecb_unused int i;	130	ecb_unused int i;
71		131
…		…
101	#else	161	#else
102	return 0;	162	return 0;
103	#endif	163	#endif
104	}	164	}
105		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
106	=item ecb_noinline	181	=item ecb_noinline
107		182
108	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
109	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
110	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.
…		…
184		259
185	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
186	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
187	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
188	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
189	C<ecb_unlikely> had been used to reach them.	264	C<ecb_expect_false> had been used to reach them.
190		265
191	Good examples for such functions would be error reporting functions, or	266	Good examples for such functions would be error reporting functions, or
192	functions only called in exceptional or rare cases.	267	functions only called in exceptional or rare cases.
193		268
194	=item ecb_artificial	269	=item ecb_artificial
…		…
256		331
257	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
258	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
259	branch optimisations.	334	branch optimisations.
260		335
261	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
262	C<ecb_unlikely> functions instead.	337	C<ecb_expect_false> functions instead.
263		338
		339	=item bool ecb_expect_true (cond)
		340
264	=item bool ecb_likely (cond)	341	=item bool ecb_expect_false (cond)
265
266	=item bool ecb_unlikely (cond)
267		342
268	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
269	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
270	other conditional statement, it will not change the program:	345	other conditional statement, it will not change the program:
271		346
272	/* these two do the same thing */	347	/* these two do the same thing */
273	if (some_condition) ...;	348	if (some_condition) ...;
274	if (ecb_likely (some_condition)) ...;	349	if (ecb_expect_true (some_condition)) ...;
275		350
276	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
277	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
278	true).	353	unlikely to be true).
279		354
280	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
281	rare, exceptional, case, then use C<ecb_unlikely>:	356	rare, exceptional, case, then use C<ecb_expect_false>:
282		357
283	void my_free (void *ptr)	358	void my_free (void *ptr)
284	{	359	{
285	if (ecb_unlikely (ptr == 0))	360	if (ecb_expect_false (ptr == 0))
286	return;	361	return;
287	}	362	}
288		363
289	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
290	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
291	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.
292		373
293	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
294	memory block (for example, inside an implementation of a string stream) -	375	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	376	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:	377	you expect that most checks will turn out to be false:
297		378
298	/* make sure we have "size" extra room in our buffer */	379	/* make sure we have "size" extra room in our buffer */
299	ecb_inline void	380	ecb_inline void
300	reserve (int size)	381	reserve (int size)
301	{	382	{
302	if (ecb_unlikely (current + size > end))	383	if (ecb_expect_false (current + size > end))
303	real_reserve_method (size); /* presumably noinline */	384	real_reserve_method (size); /* presumably noinline */
304	}	385	}
305		386
306	=item bool ecb_assume (cond)	387	=item bool ecb_assume (cond)
307		388
…		…
310		391
311	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
312	conditions that might improve code generation, but which are impossible to	393	conditions that might improve code generation, but which are impossible to
313	deduce form the code itself.	394	deduce form the code itself.
314		395
315	For example, the example reservation function from the C<ecb_unlikely>	396	For example, the example reservation function from the C<ecb_expect_false>
316	description could be written thus (only C<ecb_assume> was added):	397	description could be written thus (only C<ecb_assume> was added):
317		398
318	ecb_inline void	399	ecb_inline void
319	reserve (int size)	400	reserve (int size)
320	{	401	{
321	if (ecb_unlikely (current + size > end))	402	if (ecb_expect_false (current + size > end))
322	real_reserve_method (size); /* presumably noinline */	403	real_reserve_method (size); /* presumably noinline */
323		404
324	ecb_assume (current + size <= end);	405	ecb_assume (current + size <= end);
325	}	406	}
326		407
…		…
375	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
376	cache.	457	cache.
377		458
378	=back	459	=back
379		460
380	=head2 BIT FIDDLING / BITSTUFFS	461	=head2 BIT FIDDLING / BIT WIZARDRY
381		462
382	=over 4	463	=over 4
383		464
384	=item bool ecb_big_endian ()	465	=item bool ecb_big_endian ()
385		466
…		…
391		472
392	On systems that are neither, their return values are unspecified.	473	On systems that are neither, their return values are unspecified.
393		474
394	=item int ecb_ctz32 (uint32_t x)	475	=item int ecb_ctz32 (uint32_t x)
395		476
		477	=item int ecb_ctz64 (uint64_t x)
		478
396	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
397	equivalently the number of bits set to 0 before the least significant bit	480	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	481	set), starting from 0. If C<x> is 0 the result is undefined.
399	case is to compute the integer binary logarithm, i.e., C<floor (log2	482
		483	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		484
400	(n))>. For example:	485	For example:
401		486
402	ecb_ctz32 (3) = 0	487	ecb_ctz32 (3) = 0
403	ecb_ctz32 (6) = 1	488	ecb_ctz32 (6) = 1
404		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
405	=item int ecb_popcount32 (uint32_t x)	515	=item int ecb_popcount32 (uint32_t x)
406		516
		517	=item int ecb_popcount64 (uint64_t x)
		518
407	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:
408		524
409	ecb_popcount32 (7) = 3	525	ecb_popcount32 (7) = 3
410	ecb_popcount32 (255) = 8	526	ecb_popcount32 (255) = 8
411		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
412	=item uint32_t ecb_bswap16 (uint32_t x)	542	=item uint32_t ecb_bswap16 (uint32_t x)
413		543
414	=item uint32_t ecb_bswap32 (uint32_t x)	544	=item uint32_t ecb_bswap32 (uint32_t x)
415		545
		546	=item uint64_t ecb_bswap64 (uint64_t x)
		547
416	These two functions return the value of the 16-bit (32-bit) value C<x>	548	These functions return the value of the 16-bit (32-bit, 64-bit) value
417	after reversing the order of bytes (0x11223344 becomes 0x44332211).	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)
418		563
419	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	564	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
420		565
421	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	566	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
422		567
423	These two functions return the value of C<x> after rotating all the bits	568	These two families of functions return the value of C<x> after rotating
424	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>).
425		571
426	Current GCC versions understand these functions and usually compile them	572	Current GCC versions understand these functions and usually compile them
427	to "optimal" code (e.g. a single C<roll> on x86).	573	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		574	x86).
428		575
429	=back	576	=back
430		577
431	=head2 ARITHMETIC	578	=head2 ARITHMETIC
432		579
433	=over 4	580	=over 4
434		581
435	=item x = ecb_mod (m, n)	582	=item x = ecb_mod (m, n)
436		583
437	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
438	C<n>, using floored division. Unlike the C modulo operator C<%>, this	585	of the division operation between C<m> and C<n>, using floored
439	function ensures that the return value is always positive and that the two	586	division. Unlike the C remainder operator C<%>, this function ensures that
		587	the return value is always positive and that the two numbers I<m> and
440	numbers I<m> and I<m' = m + i * n> result in the same value modulo I<n> -	588	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
441	the C<%> operator usually has a behaviour change at C<m = 0>.	589	C<ecb_mod> implements the mathematical modulo operation, which is missing
		590	in the language.
442		591
443	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
444	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
445	type.	594	type (this typically excludes the minimum signed integer value, the same
		595	limitation as for C</> and C<%> in C).
446		596
447	Current GCC versions compile this into an efficient branchless sequence on	597	Current GCC versions compile this into an efficient branchless sequence on
448	many systems.	598	almost all CPUs.
449		599
450	For example, when you want to rotate forward through the members of an	600	For example, when you want to rotate forward through the members of an
451	array for increasing C<m> (which might be negative), then you should use	601	array for increasing C<m> (which might be negative), then you should use
452	C<ecb_mod>, as the C<%> operator might give either negative results, or	602	C<ecb_mod>, as the C<%> operator might give either negative results, or
453	change direction for negative values:	603	change direction for negative values:
454		604
455	for (m = -100; m <= 100; ++m)	605	for (m = -100; m <= 100; ++m)
456	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];	606	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
457		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++.
		616
458	=back	617	=back
459		618
460	=head2 UTILITY	619	=head2 UTILITY
461		620
462	=over 4	621	=over 4
…		…
471	for (i = 0; i < ecb_array_length (primes); i++)	630	for (i = 0; i < ecb_array_length (primes); i++)
472	sum += primes [i];	631	sum += primes [i];
473		632
474	=back	633	=back
475		634
		635	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
476		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 1.24 by root, Tue May 31 21:10:37 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 1.24 by root, Tue May 31 21:10:37 2011 UTC vs.
Revision 1.45 by root, Tue May 29 14:35:43 2012 UTC