[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 1.42 by root, Mon May 28 08:54:03 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
58	=head2 GCC ATTRIBUTES	70	=head2 GCC ATTRIBUTES
59		71
60	blabla where to put, what others	72	A major part of libecb deals with GCC attributes. These are additional
		73	attributes that you can assign to functions, variables and sometimes even
		74	types - much like C<const> or C<volatile> in C.
		75
		76	While GCC allows declarations to show up in many surprising places,
		77	but not in many expected places, the safest way is to put attribute
		78	declarations before the whole declaration:
		79
		80	ecb_const int mysqrt (int a);
		81	ecb_unused int i;
		82
		83	For variables, it is often nicer to put the attribute after the name, and
		84	avoid multiple declarations using commas:
		85
		86	int i ecb_unused;
61		87
62	=over 4	88	=over 4
63		89
64	=item ecb_attribute ((attrs...))	90	=item ecb_attribute ((attrs...))
65		91
…		…
86	#else	112	#else
87	return 0;	113	return 0;
88	#endif	114	#endif
89	}	115	}
90		116
		117	=item ecb_inline
		118
		119	This is not actually an attribute, but you use it like one. It expands
		120	either to C<static inline> or to just C<static>, if inline isn't
		121	supported. It should be used to declare functions that should be inlined,
		122	for code size or speed reasons.
		123
		124	Example: inline this function, it surely will reduce codesize.
		125
		126	ecb_inline int
		127	negmul (int a, int b)
		128	{
		129	return - (a * b);
		130	}
		131
91	=item ecb_noinline	132	=item ecb_noinline
92		133
93	Prevent a function from being inlined - it might be optimised away, but	134	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	135	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.	136	is rarely called and large enough for inlining not to be helpful.
…		…
105	{	146	{
106	puts (errline);	147	puts (errline);
107	abort ();	148	abort ();
108	}	149	}
109		150
110	In this case, the compiler would probbaly be smart enough to decude it on	151	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.	152	its own, so this is mainly useful for declarations.
112		153
113	=item ecb_const	154	=item ecb_const
114		155
115	Declares that the function only depends on the values of it's arguments,	156	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	157	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	158	any memory any arguments might point to, global variables, or call any
118	non-const functions. It also must not have any side effects.	159	non-const functions. It also must not have any side effects.
119		160
120	Such a function can be optimised much more aggressively by the compiler -	161	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	162	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	163	a single call, which wouldn't be possible if the compiler would have to
123	expect any side effects.	164	expect any side effects.
124		165
125	It is best suited for functions in the sense of mathematical functions,	166	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.	167	such as a function returning the square root of its input argument.
127		168
128	Not suited would be a function that calculates the hash of some memory	169	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	170	area you pass in, prints some messages or looks at a global variable to
130	decide on rounding.	171	decide on rounding.
131		172
…		…
154	possible.	195	possible.
155		196
156	The compiler reacts by trying to place hot functions near to each other in	197	The compiler reacts by trying to place hot functions near to each other in
157	memory.	198	memory.
158		199
159	Whether a function is hot or not often depend son the whole program,	200	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	201	and less on the function itself. C<ecb_cold> is likely more useful in
161	practise.	202	practise.
162		203
163	=item ecb_cold	204	=item ecb_cold
164		205
…		…
169		210
170	In addition to placing cold functions together (or at least away from hot	211	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	212	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	213	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	214	leading to calls to those functions can automatically be marked as if
174	C<ecb_unlikel> had been used to reach them.	215	C<ecb_expect_false> had been used to reach them.
175		216
176	Good examples for such functions would be error reporting functions, or	217	Good examples for such functions would be error reporting functions, or
177	functions only called in exceptional or rare cases.	218	functions only called in exceptional or rare cases.
178		219
179	=item ecb_artificial	220	=item ecb_artificial
…		…
241		282
242	Evaluates C<expr> and returns it. In addition, it tells the compiler that	283	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	284	the C<expr> evaluates to C<value> a lot, which can be used for static
244	branch optimisations.	285	branch optimisations.
245		286
246	Usually, you want to use the more intuitive C<ecb_likely> and	287	Usually, you want to use the more intuitive C<ecb_expect_true> and
247	C<ecb_unlikely> functions instead.	288	C<ecb_expect_false> functions instead.
248		289
		290	=item bool ecb_expect_true (cond)
		291
249	=item bool ecb_likely (cond)	292	=item bool ecb_expect_false (cond)
250
251	=item bool ecb_unlikely (cond)
252		293
253	These two functions expect a expression that is true or false and return	294	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	295	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:	296	other conditional statement, it will not change the program:
256		297
257	/* these two do the same thing */	298	/* these two do the same thing */
258	if (some_condition) ...;	299	if (some_condition) ...;
259	if (ecb_likely (some_condition)) ...;	300	if (ecb_expect_true (some_condition)) ...;
260		301
261	However, by using C<ecb_likely>, you tell the compiler that the condition	302	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	303	condition is likely to be true (and for C<ecb_expect_false>, that it is
263	true).	304	unlikely to be true).
264		305
265	For example, when you check for a null pointer and expect this to be a	306	For example, when you check for a null pointer and expect this to be a
266	rare, exceptional, case, then use C<ecb_unlikely>:	307	rare, exceptional, case, then use C<ecb_expect_false>:
267		308
268	void my_free (void *ptr)	309	void my_free (void *ptr)
269	{	310	{
270	if (ecb_unlikely (ptr == 0))	311	if (ecb_expect_false (ptr == 0))
271	return;	312	return;
272	}	313	}
273		314
274	Consequent use of these functions to mark away exceptional cases or to	315	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	316	tell the compiler what the hot path through a function is can increase
276	performance considerably.	317	performance considerably.
		318
		319	You might know these functions under the name C<likely> and C<unlikely>
		320	- while these are common aliases, we find that the expect name is easier
		321	to understand when quickly skimming code. If you wish, you can use
		322	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		323	C<ecb_expect_false> - these are simply aliases.
277		324
278	A very good example is in a function that reserves more space for some	325	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) -	326	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	327	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:	328	you expect that most checks will turn out to be false:
282		329
283	/* make sure we have "size" extra room in our buffer */	330	/* make sure we have "size" extra room in our buffer */
284	ecb_inline void	331	ecb_inline void
285	reserve (int size)	332	reserve (int size)
286	{	333	{
287	if (ecb_unlikely (current + size > end))	334	if (ecb_expect_false (current + size > end))
288	real_reserve_method (size); /* presumably noinline */	335	real_reserve_method (size); /* presumably noinline */
289	}	336	}
290		337
291	=item bool ecb_assume (cond)	338	=item bool ecb_assume (cond)
292		339
…		…
295		342
296	This can be used to teach the compiler about invariants or other	343	This can be used to teach the compiler about invariants or other
297	conditions that might improve code generation, but which are impossible to	344	conditions that might improve code generation, but which are impossible to
298	deduce form the code itself.	345	deduce form the code itself.
299		346
300	For example, the example reservation function from the C<ecb_unlikely>	347	For example, the example reservation function from the C<ecb_expect_false>
301	description could be written thus (only C<ecb_assume> was added):	348	description could be written thus (only C<ecb_assume> was added):
302		349
303	ecb_inline void	350	ecb_inline void
304	reserve (int size)	351	reserve (int size)
305	{	352	{
306	if (ecb_unlikely (current + size > end))	353	if (ecb_expect_false (current + size > end))
307	real_reserve_method (size); /* presumably noinline */	354	real_reserve_method (size); /* presumably noinline */
308		355
309	ecb_assume (current + size <= end);	356	ecb_assume (current + size <= end);
310	}	357	}
311		358
…		…
360	After processing the node, (part of) the next node might already be in	407	After processing the node, (part of) the next node might already be in
361	cache.	408	cache.
362		409
363	=back	410	=back
364		411
365	=head2 BIT FIDDLING / BITSTUFFS	412	=head2 BIT FIDDLING / BIT WIZARDRY
366		413
367	=over 4	414	=over 4
368		415
369	=item bool ecb_big_endian ()	416	=item bool ecb_big_endian ()
370		417
…		…
372		419
373	These two functions return true if the byte order is big endian	420	These two functions return true if the byte order is big endian
374	(most-significant byte first) or little endian (least-significant byte	421	(most-significant byte first) or little endian (least-significant byte
375	first) respectively.	422	first) respectively.
376		423
		424	On systems that are neither, their return values are unspecified.
		425
377	=item int ecb_ctz32 (uint32_t x)	426	=item int ecb_ctz32 (uint32_t x)
378		427
		428	=item int ecb_ctz64 (uint64_t x)
		429
379	Returns the index of the least significant bit set in C<x> (or	430	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	431	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	432	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.,	433
383	floor(log2(n)). For example:	434	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		435
		436	For example:
384		437
385	ecb_ctz32 (3) = 0	438	ecb_ctz32 (3) = 0
386	ecb_ctz32 (6) = 1	439	ecb_ctz32 (6) = 1
387		440
		441	=item bool ecb_is_pot32 (uint32_t x)
		442
		443	=item bool ecb_is_pot64 (uint32_t x)
		444
		445	Return true iff C<x> is a power of two or C<x == 0>.
		446
		447	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		448
		449	=item int ecb_ld32 (uint32_t x)
		450
		451	=item int ecb_ld64 (uint64_t x)
		452
		453	Returns the index of the most significant bit set in C<x>, or the number
		454	of digits the number requires in binary (so that C<< 2**ld <= x <
		455	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		456	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		457	example to see how many bits a certain number requires to be encoded.
		458
		459	This function is similar to the "count leading zero bits" function, except
		460	that that one returns how many zero bits are "in front" of the number (in
		461	the given data type), while C<ecb_ld> returns how many bits the number
		462	itself requires.
		463
		464	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		465
388	=item int ecb_popcount32 (uint32_t x)	466	=item int ecb_popcount32 (uint32_t x)
389		467
		468	=item int ecb_popcount64 (uint64_t x)
		469
390	Returns the number of bits set to 1 in C<x>. For example:	470	Returns the number of bits set to 1 in C<x>.
		471
		472	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		473
		474	For example:
391		475
392	ecb_popcount32 (7) = 3	476	ecb_popcount32 (7) = 3
393	ecb_popcount32 (255) = 8	477	ecb_popcount32 (255) = 8
394		478
		479	=item uint8_t ecb_bitrev8 (uint8_t x)
		480
		481	=item uint16_t ecb_bitrev16 (uint16_t x)
		482
		483	=item uint32_t ecb_bitrev32 (uint32_t x)
		484
		485	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		486	and so on.
		487
		488	Example:
		489
		490	ecb_bitrev8 (0xa7) = 0xea
		491	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		492
395	=item uint32_t ecb_bswap16 (uint32_t x)	493	=item uint32_t ecb_bswap16 (uint32_t x)
396		494
397	=item uint32_t ecb_bswap32 (uint32_t x)	495	=item uint32_t ecb_bswap32 (uint32_t x)
398		496
		497	=item uint64_t ecb_bswap64 (uint64_t x)
		498
399	These two functions return the value of the 16-bit (32-bit) variable	499	These functions return the value of the 16-bit (32-bit, 64-bit) value
400	C<x> after reversing the order of bytes.	500	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		501	C<ecb_bswap32>).
		502
		503	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		504
		505	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		506
		507	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		508
		509	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		510
		511	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		512
		513	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
401		514
402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	515	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
403		516
404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	517	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
405		518
406	These two functions return the value of C<x> after shifting all the bits	519	These two families of functions return the value of C<x> after rotating
407	by C<count> positions to the right or left respectively.	520	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		521	(C<ecb_rotl>).
		522
		523	Current GCC versions understand these functions and usually compile them
		524	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		525	x86).
408		526
409	=back	527	=back
410		528
411	=head2 ARITHMETIC	529	=head2 ARITHMETIC
412		530
413	=over 4	531	=over 4
414		532
415	=item x = ecb_mod (m, n)	533	=item x = ecb_mod (m, n)
416		534
417	Returns the positive remainder of the modulo operation between C<m> and	535	Returns C<m> modulo C<n>, which is the same as the positive remainder
		536	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	537	division. Unlike the C remainder operator C<%>, this function ensures that
419	return value is always positive).	538	the return value is always positive and that the two numbers I<m> and
		539	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		540	C<ecb_mod> implements the mathematical modulo operation, which is missing
		541	in the language.
420		542
421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	543	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	544	negatable, that is, both C<m> and C<-m> must be representable in its
423	type.	545	type (this typically excludes the minimum signed integer value, the same
		546	limitation as for C</> and C<%> in C).
		547
		548	Current GCC versions compile this into an efficient branchless sequence on
		549	almost all CPUs.
		550
		551	For example, when you want to rotate forward through the members of an
		552	array for increasing C<m> (which might be negative), then you should use
		553	C<ecb_mod>, as the C<%> operator might give either negative results, or
		554	change direction for negative values:
		555
		556	for (m = -100; m <= 100; ++m)
		557	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		558
		559	=item x = ecb_div_rd (val, div)
		560
		561	=item x = ecb_div_ru (val, div)
		562
		563	Returns C<val> divided by C<div> rounded down or up, respectively.
		564	C<val> and C<div> must have integer types and C<div> must be strictly
		565	positive. Note that these functions are implemented with macros in C
		566	and with function templates in C++.
424		567
425	=back	568	=back
426		569
427	=head2 UTILITY	570	=head2 UTILITY
428		571
429	=over 4	572	=over 4
430		573
431	=item element_count = ecb_array_length (name) [MACRO]	574	=item element_count = ecb_array_length (name)
432		575
433	Returns the number of elements in the array C<name>. For example:	576	Returns the number of elements in the array C<name>. For example:
434		577
435	int primes[] = { 2, 3, 5, 7, 11 };	578	int primes[] = { 2, 3, 5, 7, 11 };
436	int sum = 0;	579	int sum = 0;

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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 1.42 by root, Mon May 28 08:54:03 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 1.42 by root, Mon May 28 08:54:03 2012 UTC