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

Comparing cvsroot/libecb/ecb.pod (file contents):
Revision 1.17 by root, Thu May 26 23:55:58 2011 UTC vs.
Revision 1.43 by root, Tue May 29 14:09:49 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 isn'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	=over 4
		73
		74	=item ECB_C99
		75
		76	Expands to a true value (suitable for testing in by the preprocessor)
		77	if the environment claims to be C99 compliant.
		78
		79	=item ECB_C11
		80
		81	Expands to a true value (suitable for testing in by the preprocessor)
		82	if the environment claims to be C11 compliant.
		83
		84	=item ECB_GCC_VERSION(major,minor)
		85
		86	Expands to a true value (suitable for testing in by the preprocessor)
		87	if the compiler used is GNU C and the version is the givne version, or
		88	higher.
		89
		90	This macro tries to return false on compilers that claim to be GCC
		91	compatible but aren't.
		92
		93	=back
		94
58	=head2 GCC ATTRIBUTES	95	=head2 GCC ATTRIBUTES
59		96
60	blabla where to put, what others	97	A major part of libecb deals with GCC attributes. These are additional
		98	attributes that you can assign to functions, variables and sometimes even
		99	types - much like C<const> or C<volatile> in C.
		100
		101	While GCC allows declarations to show up in many surprising places,
		102	but not in many expected places, the safest way is to put attribute
		103	declarations before the whole declaration:
		104
		105	ecb_const int mysqrt (int a);
		106	ecb_unused int i;
		107
		108	For variables, it is often nicer to put the attribute after the name, and
		109	avoid multiple declarations using commas:
		110
		111	int i ecb_unused;
61		112
62	=over 4	113	=over 4
63		114
64	=item ecb_attribute ((attrs...))	115	=item ecb_attribute ((attrs...))
65		116
…		…
86	#else	137	#else
87	return 0;	138	return 0;
88	#endif	139	#endif
89	}	140	}
90		141
		142	=item ecb_inline
		143
		144	This is not actually an attribute, but you use it like one. It expands
		145	either to C<static inline> or to just C<static>, if inline isn't
		146	supported. It should be used to declare functions that should be inlined,
		147	for code size or speed reasons.
		148
		149	Example: inline this function, it surely will reduce codesize.
		150
		151	ecb_inline int
		152	negmul (int a, int b)
		153	{
		154	return - (a * b);
		155	}
		156
91	=item ecb_noinline	157	=item ecb_noinline
92		158
93	Prevent a function from being inlined - it might be optimised away, but	159	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	160	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.	161	is rarely called and large enough for inlining not to be helpful.
…		…
105	{	171	{
106	puts (errline);	172	puts (errline);
107	abort ();	173	abort ();
108	}	174	}
109		175
110	In this case, the compiler would probbaly be smart enough to decude it on	176	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.	177	its own, so this is mainly useful for declarations.
112		178
113	=item ecb_const	179	=item ecb_const
114		180
115	Declares that the function only depends on the values of it's arguments,	181	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	182	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	183	any memory any arguments might point to, global variables, or call any
118	non-const functions. It also must not have any side effects.	184	non-const functions. It also must not have any side effects.
119		185
120	Such a function can be optimised much more aggressively by the compiler -	186	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	187	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	188	a single call, which wouldn't be possible if the compiler would have to
123	expect any side effects.	189	expect any side effects.
124		190
125	It is best suited for functions in the sense of mathematical functions,	191	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.	192	such as a function returning the square root of its input argument.
127		193
128	Not suited would be a function that calculates the hash of some memory	194	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	195	area you pass in, prints some messages or looks at a global variable to
130	decide on rounding.	196	decide on rounding.
131		197
…		…
154	possible.	220	possible.
155		221
156	The compiler reacts by trying to place hot functions near to each other in	222	The compiler reacts by trying to place hot functions near to each other in
157	memory.	223	memory.
158		224
159	Whether a function is hot or not often depend son the whole program,	225	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	226	and less on the function itself. C<ecb_cold> is likely more useful in
161	practise.	227	practise.
162		228
163	=item ecb_cold	229	=item ecb_cold
164		230
…		…
169		235
170	In addition to placing cold functions together (or at least away from hot	236	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	237	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	238	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	239	leading to calls to those functions can automatically be marked as if
174	C<ecb_unlikel> had been used to reach them.	240	C<ecb_expect_false> had been used to reach them.
175		241
176	Good examples for such functions would be error reporting functions, or	242	Good examples for such functions would be error reporting functions, or
177	functions only called in exceptional or rare cases.	243	functions only called in exceptional or rare cases.
178		244
179	=item ecb_artificial	245	=item ecb_artificial
…		…
241		307
242	Evaluates C<expr> and returns it. In addition, it tells the compiler that	308	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	309	the C<expr> evaluates to C<value> a lot, which can be used for static
244	branch optimisations.	310	branch optimisations.
245		311
246	Usually, you want to use the more intuitive C<ecb_likely> and	312	Usually, you want to use the more intuitive C<ecb_expect_true> and
247	C<ecb_unlikely> functions instead.	313	C<ecb_expect_false> functions instead.
248		314
		315	=item bool ecb_expect_true (cond)
		316
249	=item bool ecb_likely (cond)	317	=item bool ecb_expect_false (cond)
250
251	=item bool ecb_unlikely (cond)
252		318
253	These two functions expect a expression that is true or false and return	319	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	320	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:	321	other conditional statement, it will not change the program:
256		322
257	/* these two do the same thing */	323	/* these two do the same thing */
258	if (some_condition) ...;	324	if (some_condition) ...;
259	if (ecb_likely (some_condition)) ...;	325	if (ecb_expect_true (some_condition)) ...;
260		326
261	However, by using C<ecb_likely>, you tell the compiler that the condition	327	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	328	condition is likely to be true (and for C<ecb_expect_false>, that it is
263	true).	329	unlikely to be true).
264		330
265	For example, when you check for a null pointer and expect this to be a	331	For example, when you check for a null pointer and expect this to be a
266	rare, exceptional, case, then use C<ecb_unlikely>:	332	rare, exceptional, case, then use C<ecb_expect_false>:
267		333
268	void my_free (void *ptr)	334	void my_free (void *ptr)
269	{	335	{
270	if (ecb_unlikely (ptr == 0))	336	if (ecb_expect_false (ptr == 0))
271	return;	337	return;
272	}	338	}
273		339
274	Consequent use of these functions to mark away exceptional cases or to	340	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	341	tell the compiler what the hot path through a function is can increase
276	performance considerably.	342	performance considerably.
		343
		344	You might know these functions under the name C<likely> and C<unlikely>
		345	- while these are common aliases, we find that the expect name is easier
		346	to understand when quickly skimming code. If you wish, you can use
		347	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		348	C<ecb_expect_false> - these are simply aliases.
277		349
278	A very good example is in a function that reserves more space for some	350	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) -	351	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	352	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:	353	you expect that most checks will turn out to be false:
282		354
283	/* make sure we have "size" extra room in our buffer */	355	/* make sure we have "size" extra room in our buffer */
284	ecb_inline void	356	ecb_inline void
285	reserve (int size)	357	reserve (int size)
286	{	358	{
287	if (ecb_unlikely (current + size > end))	359	if (ecb_expect_false (current + size > end))
288	real_reserve_method (size); /* presumably noinline */	360	real_reserve_method (size); /* presumably noinline */
289	}	361	}
290		362
291	=item bool ecb_assume (cond)	363	=item bool ecb_assume (cond)
292		364
…		…
295		367
296	This can be used to teach the compiler about invariants or other	368	This can be used to teach the compiler about invariants or other
297	conditions that might improve code generation, but which are impossible to	369	conditions that might improve code generation, but which are impossible to
298	deduce form the code itself.	370	deduce form the code itself.
299		371
300	For example, the example reservation function from the C<ecb_unlikely>	372	For example, the example reservation function from the C<ecb_expect_false>
301	description could be written thus (only C<ecb_assume> was added):	373	description could be written thus (only C<ecb_assume> was added):
302		374
303	ecb_inline void	375	ecb_inline void
304	reserve (int size)	376	reserve (int size)
305	{	377	{
306	if (ecb_unlikely (current + size > end))	378	if (ecb_expect_false (current + size > end))
307	real_reserve_method (size); /* presumably noinline */	379	real_reserve_method (size); /* presumably noinline */
308		380
309	ecb_assume (current + size <= end);	381	ecb_assume (current + size <= end);
310	}	382	}
311		383
…		…
360	After processing the node, (part of) the next node might already be in	432	After processing the node, (part of) the next node might already be in
361	cache.	433	cache.
362		434
363	=back	435	=back
364		436
365	=head2 BIT FIDDLING / BITSTUFFS	437	=head2 BIT FIDDLING / BIT WIZARDRY
366		438
367	=over 4	439	=over 4
368		440
369	=item bool ecb_big_endian ()	441	=item bool ecb_big_endian ()
370		442
…		…
372		444
373	These two functions return true if the byte order is big endian	445	These two functions return true if the byte order is big endian
374	(most-significant byte first) or little endian (least-significant byte	446	(most-significant byte first) or little endian (least-significant byte
375	first) respectively.	447	first) respectively.
376		448
		449	On systems that are neither, their return values are unspecified.
		450
377	=item int ecb_ctz32 (uint32_t x)	451	=item int ecb_ctz32 (uint32_t x)
378		452
		453	=item int ecb_ctz64 (uint64_t x)
		454
379	Returns the index of the least significant bit set in C<x> (or	455	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	456	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	457	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.,	458
383	floor(log2(n)). For example:	459	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		460
		461	For example:
384		462
385	ecb_ctz32 (3) = 0	463	ecb_ctz32 (3) = 0
386	ecb_ctz32 (6) = 1	464	ecb_ctz32 (6) = 1
387		465
		466	=item bool ecb_is_pot32 (uint32_t x)
		467
		468	=item bool ecb_is_pot64 (uint32_t x)
		469
		470	Return true iff C<x> is a power of two or C<x == 0>.
		471
		472	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		473
		474	=item int ecb_ld32 (uint32_t x)
		475
		476	=item int ecb_ld64 (uint64_t x)
		477
		478	Returns the index of the most significant bit set in C<x>, or the number
		479	of digits the number requires in binary (so that C<< 2**ld <= x <
		480	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		481	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		482	example to see how many bits a certain number requires to be encoded.
		483
		484	This function is similar to the "count leading zero bits" function, except
		485	that that one returns how many zero bits are "in front" of the number (in
		486	the given data type), while C<ecb_ld> returns how many bits the number
		487	itself requires.
		488
		489	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		490
388	=item int ecb_popcount32 (uint32_t x)	491	=item int ecb_popcount32 (uint32_t x)
389		492
		493	=item int ecb_popcount64 (uint64_t x)
		494
390	Returns the number of bits set to 1 in C<x>. For example:	495	Returns the number of bits set to 1 in C<x>.
		496
		497	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		498
		499	For example:
391		500
392	ecb_popcount32 (7) = 3	501	ecb_popcount32 (7) = 3
393	ecb_popcount32 (255) = 8	502	ecb_popcount32 (255) = 8
394		503
		504	=item uint8_t ecb_bitrev8 (uint8_t x)
		505
		506	=item uint16_t ecb_bitrev16 (uint16_t x)
		507
		508	=item uint32_t ecb_bitrev32 (uint32_t x)
		509
		510	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		511	and so on.
		512
		513	Example:
		514
		515	ecb_bitrev8 (0xa7) = 0xea
		516	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		517
395	=item uint32_t ecb_bswap16 (uint32_t x)	518	=item uint32_t ecb_bswap16 (uint32_t x)
396		519
397	=item uint32_t ecb_bswap32 (uint32_t x)	520	=item uint32_t ecb_bswap32 (uint32_t x)
398		521
		522	=item uint64_t ecb_bswap64 (uint64_t x)
		523
399	These two functions return the value of the 16-bit (32-bit) variable	524	These functions return the value of the 16-bit (32-bit, 64-bit) value
400	C<x> after reversing the order of bytes.	525	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		526	C<ecb_bswap32>).
		527
		528	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		529
		530	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		531
		532	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		533
		534	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		535
		536	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		537
		538	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
401		539
402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	540	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
403		541
404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	542	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
405		543
406	These two functions return the value of C<x> after shifting all the bits	544	These two families of functions return the value of C<x> after rotating
407	by C<count> positions to the right or left respectively.	545	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		546	(C<ecb_rotl>).
		547
		548	Current GCC versions understand these functions and usually compile them
		549	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		550	x86).
408		551
409	=back	552	=back
410		553
411	=head2 ARITHMETIC	554	=head2 ARITHMETIC
412		555
413	=over 4	556	=over 4
414		557
415	=item x = ecb_mod (m, n)	558	=item x = ecb_mod (m, n)
416		559
417	Returns the positive remainder of the modulo operation between C<m> and	560	Returns C<m> modulo C<n>, which is the same as the positive remainder
		561	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	562	division. Unlike the C remainder operator C<%>, this function ensures that
419	return value is always positive).	563	the return value is always positive and that the two numbers I<m> and
		564	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		565	C<ecb_mod> implements the mathematical modulo operation, which is missing
		566	in the language.
420		567
421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	568	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	569	negatable, that is, both C<m> and C<-m> must be representable in its
423	type.	570	type (this typically excludes the minimum signed integer value, the same
		571	limitation as for C</> and C<%> in C).
		572
		573	Current GCC versions compile this into an efficient branchless sequence on
		574	almost all CPUs.
		575
		576	For example, when you want to rotate forward through the members of an
		577	array for increasing C<m> (which might be negative), then you should use
		578	C<ecb_mod>, as the C<%> operator might give either negative results, or
		579	change direction for negative values:
		580
		581	for (m = -100; m <= 100; ++m)
		582	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		583
		584	=item x = ecb_div_rd (val, div)
		585
		586	=item x = ecb_div_ru (val, div)
		587
		588	Returns C<val> divided by C<div> rounded down or up, respectively.
		589	C<val> and C<div> must have integer types and C<div> must be strictly
		590	positive. Note that these functions are implemented with macros in C
		591	and with function templates in C++.
424		592
425	=back	593	=back
426		594
427	=head2 UTILITY	595	=head2 UTILITY
428		596
429	=over 4	597	=over 4
430		598
431	=item element_count = ecb_array_length (name) [MACRO]	599	=item element_count = ecb_array_length (name)
432		600
433	Returns the number of elements in the array C<name>. For example:	601	Returns the number of elements in the array C<name>. For example:
434		602
435	int primes[] = { 2, 3, 5, 7, 11 };	603	int primes[] = { 2, 3, 5, 7, 11 };
436	int sum = 0;	604	int sum = 0;
…		…
438	for (i = 0; i < ecb_array_length (primes); i++)	606	for (i = 0; i < ecb_array_length (primes); i++)
439	sum += primes [i];	607	sum += primes [i];
440		608
441	=back	609	=back
442		610
		611	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
443		612
		613	These symbols need to be defined before including F<ecb.h> the first time.
		614
		615	=over 4
		616
		617	=item ECB_NO_THRADS
		618
		619	If F<ecb.h> is never used from multiple threads, then this symbol can
		620	be defined, in which case memory fences (and similar constructs) are
		621	completely removed, leading to more efficient code and fewer dependencies.
		622
		623	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		624
		625	=item ECB_NO_SMP
		626
		627	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		628	multiple threads, but never concurrently (e.g. if the system the program
		629	runs on has only a single CPU with a single core, no hyperthreading and so
		630	on), then this symbol can be defined, leading to more efficient code and
		631	fewer dependencies.
		632
		633	=back
		634
		635

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Comparing cvsroot/libecb/ecb.pod (file contents): Revision 1.17 by root, Thu May 26 23:55:58 2011 UTC vs. Revision 1.43 by root, Tue May 29 14:09:49 2012 UTC

Diff Legend

Comparing cvsroot/libecb/ecb.pod (file contents):
Revision 1.17 by root, Thu May 26 23:55:58 2011 UTC vs.
Revision 1.43 by root, Tue May 29 14:09:49 2012 UTC