[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 1.39 by root, Sat Dec 10 11:58:38 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
…		…
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	A major part of libecb deals with GCC attributes. These are additional	61	A major part of libecb deals with GCC attributes. These are additional
61	attributes that you cna assign to functions, variables and sometimes even	62	attributes that you can assign to functions, variables and sometimes even
62	types - much like C<const> or C<volatile> in C.	63	types - much like C<const> or C<volatile> in C.
63		64
64	While GCC allows declarations to show up in many surprising places,	65	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	66	but not in many expected places, the safest way is to put attribute
66	declarations before the whole declaration:	67	declarations before the whole declaration:
67		68
68	ecb_const int mysqrt (int a);	69	ecb_const int mysqrt (int a);
69	ecb_unused int i;	70	ecb_unused int i;
70		71
…		…
100	#else	101	#else
101	return 0;	102	return 0;
102	#endif	103	#endif
103	}	104	}
104		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
105	=item ecb_noinline	121	=item ecb_noinline
106		122
107	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
108	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
109	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.
…		…
183		199
184	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
185	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
186	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
187	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
188	C<ecb_unlikely> had been used to reach them.	204	C<ecb_expect_false> had been used to reach them.
189		205
190	Good examples for such functions would be error reporting functions, or	206	Good examples for such functions would be error reporting functions, or
191	functions only called in exceptional or rare cases.	207	functions only called in exceptional or rare cases.
192		208
193	=item ecb_artificial	209	=item ecb_artificial
…		…
255		271
256	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
257	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
258	branch optimisations.	274	branch optimisations.
259		275
260	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
261	C<ecb_unlikely> functions instead.	277	C<ecb_expect_false> functions instead.
262		278
		279	=item bool ecb_expect_true (cond)
		280
263	=item bool ecb_likely (cond)	281	=item bool ecb_expect_false (cond)
264
265	=item bool ecb_unlikely (cond)
266		282
267	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
268	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
269	other conditional statement, it will not change the program:	285	other conditional statement, it will not change the program:
270		286
271	/* these two do the same thing */	287	/* these two do the same thing */
272	if (some_condition) ...;	288	if (some_condition) ...;
273	if (ecb_likely (some_condition)) ...;	289	if (ecb_expect_true (some_condition)) ...;
274		290
275	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
276	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
277	true).	293	unlikely to be true).
278		294
279	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
280	rare, exceptional, case, then use C<ecb_unlikely>:	296	rare, exceptional, case, then use C<ecb_expect_false>:
281		297
282	void my_free (void *ptr)	298	void my_free (void *ptr)
283	{	299	{
284	if (ecb_unlikely (ptr == 0))	300	if (ecb_expect_false (ptr == 0))
285	return;	301	return;
286	}	302	}
287		303
288	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
289	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
290	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.
291		313
292	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
293	memory block (for example, inside an implementation of a string stream) -	315	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	316	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:	317	you expect that most checks will turn out to be false:
296		318
297	/* make sure we have "size" extra room in our buffer */	319	/* make sure we have "size" extra room in our buffer */
298	ecb_inline void	320	ecb_inline void
299	reserve (int size)	321	reserve (int size)
300	{	322	{
301	if (ecb_unlikely (current + size > end))	323	if (ecb_expect_false (current + size > end))
302	real_reserve_method (size); /* presumably noinline */	324	real_reserve_method (size); /* presumably noinline */
303	}	325	}
304		326
305	=item bool ecb_assume (cond)	327	=item bool ecb_assume (cond)
306		328
…		…
309		331
310	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
311	conditions that might improve code generation, but which are impossible to	333	conditions that might improve code generation, but which are impossible to
312	deduce form the code itself.	334	deduce form the code itself.
313		335
314	For example, the example reservation function from the C<ecb_unlikely>	336	For example, the example reservation function from the C<ecb_expect_false>
315	description could be written thus (only C<ecb_assume> was added):	337	description could be written thus (only C<ecb_assume> was added):
316		338
317	ecb_inline void	339	ecb_inline void
318	reserve (int size)	340	reserve (int size)
319	{	341	{
320	if (ecb_unlikely (current + size > end))	342	if (ecb_expect_false (current + size > end))
321	real_reserve_method (size); /* presumably noinline */	343	real_reserve_method (size); /* presumably noinline */
322		344
323	ecb_assume (current + size <= end);	345	ecb_assume (current + size <= end);
324	}	346	}
325		347
…		…
374	After processing the node, (part of) the next node might already be in	396	After processing the node, (part of) the next node might already be in
375	cache.	397	cache.
376		398
377	=back	399	=back
378		400
379	=head2 BIT FIDDLING / BITSTUFFS	401	=head2 BIT FIDDLING / BIT WIZARDRY
380		402
381	=over 4	403	=over 4
382		404
383	=item bool ecb_big_endian ()	405	=item bool ecb_big_endian ()
384		406
…		…
386		408
387	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
388	(most-significant byte first) or little endian (least-significant byte	410	(most-significant byte first) or little endian (least-significant byte
389	first) respectively.	411	first) respectively.
390		412
		413	On systems that are neither, their return values are unspecified.
		414
391	=item int ecb_ctz32 (uint32_t x)	415	=item int ecb_ctz32 (uint32_t x)
392		416
		417	=item int ecb_ctz64 (uint64_t x)
		418
393	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
394	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
395	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.
396	common use case is to compute the integer binary logarithm, i.e.,	422
397	floor(log2(n)). For example:	423	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		424
		425	For example:
398		426
399	ecb_ctz32 (3) = 0	427	ecb_ctz32 (3) = 0
400	ecb_ctz32 (6) = 1	428	ecb_ctz32 (6) = 1
401		429
		430	=item int ecb_ld32 (uint32_t x)
		431
		432	=item int ecb_ld64 (uint64_t x)
		433
		434	Returns the index of the most significant bit set in C<x>, or the number
		435	of digits the number requires in binary (so that C<< 2**ld <= x <
		436	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		437	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		438	example to see how many bits a certain number requires to be encoded.
		439
		440	This function is similar to the "count leading zero bits" function, except
		441	that that one returns how many zero bits are "in front" of the number (in
		442	the given data type), while C<ecb_ld> returns how many bits the number
		443	itself requires.
		444
		445	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		446
402	=item int ecb_popcount32 (uint32_t x)	447	=item int ecb_popcount32 (uint32_t x)
403		448
		449	=item int ecb_popcount64 (uint64_t x)
		450
404	Returns the number of bits set to 1 in C<x>. For example:	451	Returns the number of bits set to 1 in C<x>.
		452
		453	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		454
		455	For example:
405		456
406	ecb_popcount32 (7) = 3	457	ecb_popcount32 (7) = 3
407	ecb_popcount32 (255) = 8	458	ecb_popcount32 (255) = 8
408		459
		460	=item uint8_t ecb_bitrev8 (uint8_t x)
		461
		462	=item uint16_t ecb_bitrev16 (uint16_t x)
		463
		464	=item uint32_t ecb_bitrev32 (uint32_t x)
		465
		466	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		467	and so on.
		468
		469	Example:
		470
		471	ecb_bitrev8 (0xa7) = 0xea
		472	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		473
409	=item uint32_t ecb_bswap16 (uint32_t x)	474	=item uint32_t ecb_bswap16 (uint32_t x)
410		475
411	=item uint32_t ecb_bswap32 (uint32_t x)	476	=item uint32_t ecb_bswap32 (uint32_t x)
412		477
		478	=item uint64_t ecb_bswap64 (uint64_t x)
		479
413	These two functions return the value of the 16-bit (32-bit) value C<x>	480	These functions return the value of the 16-bit (32-bit, 64-bit) value
414	after reversing the order of bytes (0x11223344 becomes 0x44332211).	481	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		482	C<ecb_bswap32>).
		483
		484	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		485
		486	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		487
		488	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		489
		490	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		491
		492	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		493
		494	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
415		495
416	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	496	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
417		497
418	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	498	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
419		499
420	These two functions return the value of C<x> after rotating all the bits	500	These two families of functions return the value of C<x> after rotating
421	by C<count> positions to the right or left respectively.	501	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		502	(C<ecb_rotl>).
422		503
423	Current GCC versions understand these functions and usually compile them	504	Current GCC versions understand these functions and usually compile them
424	to "optimal" code (e.g. a single C<roll> on x86).	505	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		506	x86).
425		507
426	=back	508	=back
427		509
428	=head2 ARITHMETIC	510	=head2 ARITHMETIC
429		511
430	=over 4	512	=over 4
431		513
432	=item x = ecb_mod (m, n)	514	=item x = ecb_mod (m, n)
433		515
434	Returns the positive remainder of the modulo operation between C<m> and	516	Returns C<m> modulo C<n>, which is the same as the positive remainder
		517	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	518	division. Unlike the C remainder operator C<%>, this function ensures that
436	return value is always positive - ISO C guarantees very little when	519	the return value is always positive and that the two numbers I<m> and
437	negative numbers are used with C<%>.	520	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		521	C<ecb_mod> implements the mathematical modulo operation, which is missing
		522	in the language.
438		523
439	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be	524	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	525	negatable, that is, both C<m> and C<-m> must be representable in its
441	type.	526	type (this typically excludes the minimum signed integer value, the same
		527	limitation as for C</> and C<%> in C).
		528
		529	Current GCC versions compile this into an efficient branchless sequence on
		530	almost all CPUs.
		531
		532	For example, when you want to rotate forward through the members of an
		533	array for increasing C<m> (which might be negative), then you should use
		534	C<ecb_mod>, as the C<%> operator might give either negative results, or
		535	change direction for negative values:
		536
		537	for (m = -100; m <= 100; ++m)
		538	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		539
		540	=item x = ecb_div_rd (val, div)
		541
		542	=item x = ecb_div_ru (val, div)
		543
		544	Returns C<val> divided by C<div> rounded down or up, respectively.
		545	C<val> and C<div> must have integer types and C<div> must be strictly
		546	positive. Note that these functions are implemented with macros in C
		547	and with function templates in C++.
442		548
443	=back	549	=back
444		550
445	=head2 UTILITY	551	=head2 UTILITY
446		552

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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 1.39 by root, Sat Dec 10 11:58:38 2011 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.23 by sf-exg, Fri May 27 01:35:46 2011 UTC vs.
Revision 1.39 by root, Sat Dec 10 11:58:38 2011 UTC