[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 1.35 by root, Fri Jun 17 21:38:31 2011 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
…		…
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
58	=head2 GCC ATTRIBUTES	59	=head2 GCC ATTRIBUTES
59		60
60	blabla where to put, what others	61	A major part of libecb deals with GCC attributes. These are additional
		62	attributes that you can assign to functions, variables and sometimes even
		63	types - much like C<const> or C<volatile> in C.
		64
		65	While GCC allows declarations to show up in many surprising places,
		66	but not in many expected places, the safest way is to put attribute
		67	declarations before the whole declaration:
		68
		69	ecb_const int mysqrt (int a);
		70	ecb_unused int i;
		71
		72	For variables, it is often nicer to put the attribute after the name, and
		73	avoid multiple declarations using commas:
		74
		75	int i ecb_unused;
61		76
62	=over 4	77	=over 4
63		78
64	=item ecb_attribute ((attrs...))	79	=item ecb_attribute ((attrs...))
65		80
…		…
86	#else	101	#else
87	return 0;	102	return 0;
88	#endif	103	#endif
89	}	104	}
90		105
		106	=item ecb_inline
		107
		108	This is not actually an attribute, but you use it like one. It expands
		109	either to C<static inline> or to just C<static>, if inline isn't
		110	supported. It should be used to declare functions that should be inlined,
		111	for code size or speed reasons.
		112
		113	Example: inline this function, it surely will reduce codesize.
		114
		115	ecb_inline int
		116	negmul (int a, int b)
		117	{
		118	return - (a * b);
		119	}
		120
91	=item ecb_noinline	121	=item ecb_noinline
92		122
93	Prevent a function from being inlined - it might be optimised away, but	123	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	124	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.	125	is rarely called and large enough for inlining not to be helpful.
…		…
105	{	135	{
106	puts (errline);	136	puts (errline);
107	abort ();	137	abort ();
108	}	138	}
109		139
110	In this case, the compiler would probbaly be smart enough to decude it on	140	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.	141	its own, so this is mainly useful for declarations.
112		142
113	=item ecb_const	143	=item ecb_const
114		144
115	Declares that the function only depends on the values of it's arguments,	145	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	146	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	147	any memory any arguments might point to, global variables, or call any
118	non-const functions. It also must not have any side effects.	148	non-const functions. It also must not have any side effects.
119		149
120	Such a function can be optimised much more aggressively by the compiler -	150	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	151	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	152	a single call, which wouldn't be possible if the compiler would have to
123	expect any side effects.	153	expect any side effects.
124		154
125	It is best suited for functions in the sense of mathematical functions,	155	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.	156	such as a function returning the square root of its input argument.
127		157
128	Not suited would be a function that calculates the hash of some memory	158	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	159	area you pass in, prints some messages or looks at a global variable to
130	decide on rounding.	160	decide on rounding.
131		161
…		…
154	possible.	184	possible.
155		185
156	The compiler reacts by trying to place hot functions near to each other in	186	The compiler reacts by trying to place hot functions near to each other in
157	memory.	187	memory.
158		188
159	Whether a function is hot or not often depend son the whole program,	189	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	190	and less on the function itself. C<ecb_cold> is likely more useful in
161	practise.	191	practise.
162		192
163	=item ecb_cold	193	=item ecb_cold
164		194
…		…
169		199
170	In addition to placing cold functions together (or at least away from hot	200	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	201	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	202	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	203	leading to calls to those functions can automatically be marked as if
174	C<ecb_unlikel> had been used to reach them.	204	C<ecb_expect_false> had been used to reach them.
175		205
176	Good examples for such functions would be error reporting functions, or	206	Good examples for such functions would be error reporting functions, or
177	functions only called in exceptional or rare cases.	207	functions only called in exceptional or rare cases.
178		208
179	=item ecb_artificial	209	=item ecb_artificial
…		…
241		271
242	Evaluates C<expr> and returns it. In addition, it tells the compiler that	272	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	273	the C<expr> evaluates to C<value> a lot, which can be used for static
244	branch optimisations.	274	branch optimisations.
245		275
246	Usually, you want to use the more intuitive C<ecb_likely> and	276	Usually, you want to use the more intuitive C<ecb_expect_true> and
247	C<ecb_unlikely> functions instead.	277	C<ecb_expect_false> functions instead.
248		278
		279	=item bool ecb_expect_true (cond)
		280
249	=item bool ecb_likely (cond)	281	=item bool ecb_expect_false (cond)
250
251	=item bool ecb_unlikely (cond)
252		282
253	These two functions expect a expression that is true or false and return	283	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	284	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:	285	other conditional statement, it will not change the program:
256		286
257	/* these two do the same thing */	287	/* these two do the same thing */
258	if (some_condition) ...;	288	if (some_condition) ...;
259	if (ecb_likely (some_condition)) ...;	289	if (ecb_expect_true (some_condition)) ...;
260		290
261	However, by using C<ecb_likely>, you tell the compiler that the condition	291	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	292	condition is likely to be true (and for C<ecb_expect_false>, that it is
263	true).	293	unlikely to be true).
264		294
265	For example, when you check for a null pointer and expect this to be a	295	For example, when you check for a null pointer and expect this to be a
266	rare, exceptional, case, then use C<ecb_unlikely>:	296	rare, exceptional, case, then use C<ecb_expect_false>:
267		297
268	void my_free (void *ptr)	298	void my_free (void *ptr)
269	{	299	{
270	if (ecb_unlikely (ptr == 0))	300	if (ecb_expect_false (ptr == 0))
271	return;	301	return;
272	}	302	}
273		303
274	Consequent use of these functions to mark away exceptional cases or to	304	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	305	tell the compiler what the hot path through a function is can increase
276	performance considerably.	306	performance considerably.
		307
		308	You might know these functions under the name C<likely> and C<unlikely>
		309	- while these are common aliases, we find that the expect name is easier
		310	to understand when quickly skimming code. If you wish, you can use
		311	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		312	C<ecb_expect_false> - these are simply aliases.
277		313
278	A very good example is in a function that reserves more space for some	314	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) -	315	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	316	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:	317	you expect that most checks will turn out to be false:
282		318
283	/* make sure we have "size" extra room in our buffer */	319	/* make sure we have "size" extra room in our buffer */
284	ecb_inline void	320	ecb_inline void
285	reserve (int size)	321	reserve (int size)
286	{	322	{
287	if (ecb_unlikely (current + size > end))	323	if (ecb_expect_false (current + size > end))
288	real_reserve_method (size); /* presumably noinline */	324	real_reserve_method (size); /* presumably noinline */
289	}	325	}
290		326
291	=item bool ecb_assume (cond)	327	=item bool ecb_assume (cond)
292		328
…		…
295		331
296	This can be used to teach the compiler about invariants or other	332	This can be used to teach the compiler about invariants or other
297	conditions that might improve code generation, but which are impossible to	333	conditions that might improve code generation, but which are impossible to
298	deduce form the code itself.	334	deduce form the code itself.
299		335
300	For example, the example reservation function from the C<ecb_unlikely>	336	For example, the example reservation function from the C<ecb_expect_false>
301	description could be written thus (only C<ecb_assume> was added):	337	description could be written thus (only C<ecb_assume> was added):
302		338
303	ecb_inline void	339	ecb_inline void
304	reserve (int size)	340	reserve (int size)
305	{	341	{
306	if (ecb_unlikely (current + size > end))	342	if (ecb_expect_false (current + size > end))
307	real_reserve_method (size); /* presumably noinline */	343	real_reserve_method (size); /* presumably noinline */
308		344
309	ecb_assume (current + size <= end);	345	ecb_assume (current + size <= end);
310	}	346	}
311		347
…		…
372		408
373	These two functions return true if the byte order is big endian	409	These two functions return true if the byte order is big endian
374	(most-significant byte first) or little endian (least-significant byte	410	(most-significant byte first) or little endian (least-significant byte
375	first) respectively.	411	first) respectively.
376		412
		413	On systems that are neither, their return values are unspecified.
		414
377	=item int ecb_ctz32 (uint32_t x)	415	=item int ecb_ctz32 (uint32_t x)
378		416
		417	=item int ecb_ctz64 (uint64_t x)
		418
379	Returns the index of the least significant bit set in C<x> (or	419	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	420	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	421	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.,	422
383	floor(log2(n)). For example:	423	For example:
384		424
385	ecb_ctz32 (3) = 0	425	ecb_ctz32 (3) = 0
386	ecb_ctz32 (6) = 1	426	ecb_ctz32 (6) = 1
387		427
		428	=item int ecb_ld32 (uint32_t x)
		429
		430	=item int ecb_ld64 (uint64_t x)
		431
		432	Returns the index of the most significant bit set in C<x>, or the number
		433	of digits the number requires in binary (so that C<< 2**ld <= x <
		434	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		435	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		436	example to see how many bits a certain number requires to be encoded.
		437
		438	This function is similar to the "count leading zero bits" function, except
		439	that that one returns how many zero bits are "in front" of the number (in
		440	the given data type), while C<ecb_ld> returns how many bits the number
		441	itself requires.
		442
388	=item int ecb_popcount32 (uint32_t x)	443	=item int ecb_popcount32 (uint32_t x)
		444
		445	=item int ecb_popcount64 (uint64_t x)
389		446
390	Returns the number of bits set to 1 in C<x>. For example:	447	Returns the number of bits set to 1 in C<x>. For example:
391		448
392	ecb_popcount32 (7) = 3	449	ecb_popcount32 (7) = 3
393	ecb_popcount32 (255) = 8	450	ecb_popcount32 (255) = 8
394		451
395	=item uint32_t ecb_bswap16 (uint32_t x)	452	=item uint32_t ecb_bswap16 (uint32_t x)
396		453
397	=item uint32_t ecb_bswap32 (uint32_t x)	454	=item uint32_t ecb_bswap32 (uint32_t x)
398		455
		456	=item uint64_t ecb_bswap64 (uint64_t x)
		457
399	These two functions return the value of the 16-bit (32-bit) variable	458	These functions return the value of the 16-bit (32-bit, 64-bit) value
400	C<x> after reversing the order of bytes.	459	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		460	C<ecb_bswap32>).
		461
		462	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		463
		464	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		465
		466	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		467
		468	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		469
		470	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		471
		472	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
401		473
402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	474	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
403		475
404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	476	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
405		477
406	These two functions return the value of C<x> after shifting all the bits	478	These two families of functions return the value of C<x> after rotating
407	by C<count> positions to the right or left respectively.	479	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		480	(C<ecb_rotl>).
		481
		482	Current GCC versions understand these functions and usually compile them
		483	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		484	x86).
408		485
409	=back	486	=back
410		487
411	=head2 ARITHMETIC	488	=head2 ARITHMETIC
412		489
413	=over 4	490	=over 4
414		491
415	=item x = ecb_mod (m, n)	492	=item x = ecb_mod (m, n)
416		493
417	Returns the positive remainder of the modulo operation between C<m> and	494	Returns C<m> modulo C<n>, which is the same as the positive remainder
		495	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	496	division. Unlike the C remainder operator C<%>, this function ensures that
419	return value is always positive).	497	the return value is always positive and that the two numbers I<m> and
		498	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		499	C<ecb_mod> implements the mathematical modulo operation, which is missing
		500	in the language.
420		501
421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	502	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	503	negatable, that is, both C<m> and C<-m> must be representable in its
423	type.	504	type (this typically excludes the minimum signed integer value, the same
		505	limitation as for C</> and C<%> in C).
		506
		507	Current GCC versions compile this into an efficient branchless sequence on
		508	almost all CPUs.
		509
		510	For example, when you want to rotate forward through the members of an
		511	array for increasing C<m> (which might be negative), then you should use
		512	C<ecb_mod>, as the C<%> operator might give either negative results, or
		513	change direction for negative values:
		514
		515	for (m = -100; m <= 100; ++m)
		516	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
424		517
425	=back	518	=back
426		519
427	=head2 UTILITY	520	=head2 UTILITY
428		521
429	=over 4	522	=over 4
430		523
431	=item element_count = ecb_array_length (name) [MACRO]	524	=item element_count = ecb_array_length (name)
432		525
433	Returns the number of elements in the array C<name>. For example:	526	Returns the number of elements in the array C<name>. For example:
434		527
435	int primes[] = { 2, 3, 5, 7, 11 };	528	int primes[] = { 2, 3, 5, 7, 11 };
436	int sum = 0;	529	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 1.35 by root, Fri Jun 17 21:38:31 2011 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.18 by root, Fri May 27 00:01:28 2011 UTC vs.
Revision 1.35 by root, Fri Jun 17 21:38:31 2011 UTC