[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 1.40 by root, Mon May 28 08:28:26 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
		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	A major part of libecb deals with GCC attributes. These are additional	72	A major part of libecb deals with GCC attributes. These are additional
61	attributes that you cna assign to functions, variables and sometimes even	73	attributes that you can assign to functions, variables and sometimes even
62	types - much like C<const> or C<volatile> in C.	74	types - much like C<const> or C<volatile> in C.
63		75
64	While GCC allows declarations to show up in many surprising places,	76	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	77	but not in many expected places, the safest way is to put attribute
66	declarations before the whole declaration:	78	declarations before the whole declaration:
67		79
68	ecb_const int mysqrt (int a);	80	ecb_const int mysqrt (int a);
69	ecb_unused int i;	81	ecb_unused int i;
70		82
…		…
100	#else	112	#else
101	return 0;	113	return 0;
102	#endif	114	#endif
103	}	115	}
104		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
105	=item ecb_noinline	132	=item ecb_noinline
106		133
107	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
108	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
109	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.
…		…
183		210
184	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
185	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
186	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
187	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
188	C<ecb_unlikely> had been used to reach them.	215	C<ecb_expect_false> had been used to reach them.
189		216
190	Good examples for such functions would be error reporting functions, or	217	Good examples for such functions would be error reporting functions, or
191	functions only called in exceptional or rare cases.	218	functions only called in exceptional or rare cases.
192		219
193	=item ecb_artificial	220	=item ecb_artificial
…		…
255		282
256	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
257	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
258	branch optimisations.	285	branch optimisations.
259		286
260	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
261	C<ecb_unlikely> functions instead.	288	C<ecb_expect_false> functions instead.
262		289
		290	=item bool ecb_expect_true (cond)
		291
263	=item bool ecb_likely (cond)	292	=item bool ecb_expect_false (cond)
264
265	=item bool ecb_unlikely (cond)
266		293
267	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
268	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
269	other conditional statement, it will not change the program:	296	other conditional statement, it will not change the program:
270		297
271	/* these two do the same thing */	298	/* these two do the same thing */
272	if (some_condition) ...;	299	if (some_condition) ...;
273	if (ecb_likely (some_condition)) ...;	300	if (ecb_expect_true (some_condition)) ...;
274		301
275	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
276	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
277	true).	304	unlikely to be true).
278		305
279	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
280	rare, exceptional, case, then use C<ecb_unlikely>:	307	rare, exceptional, case, then use C<ecb_expect_false>:
281		308
282	void my_free (void *ptr)	309	void my_free (void *ptr)
283	{	310	{
284	if (ecb_unlikely (ptr == 0))	311	if (ecb_expect_false (ptr == 0))
285	return;	312	return;
286	}	313	}
287		314
288	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
289	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
290	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.
291		324
292	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
293	memory block (for example, inside an implementation of a string stream) -	326	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	327	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:	328	you expect that most checks will turn out to be false:
296		329
297	/* make sure we have "size" extra room in our buffer */	330	/* make sure we have "size" extra room in our buffer */
298	ecb_inline void	331	ecb_inline void
299	reserve (int size)	332	reserve (int size)
300	{	333	{
301	if (ecb_unlikely (current + size > end))	334	if (ecb_expect_false (current + size > end))
302	real_reserve_method (size); /* presumably noinline */	335	real_reserve_method (size); /* presumably noinline */
303	}	336	}
304		337
305	=item bool ecb_assume (cond)	338	=item bool ecb_assume (cond)
306		339
…		…
309		342
310	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
311	conditions that might improve code generation, but which are impossible to	344	conditions that might improve code generation, but which are impossible to
312	deduce form the code itself.	345	deduce form the code itself.
313		346
314	For example, the example reservation function from the C<ecb_unlikely>	347	For example, the example reservation function from the C<ecb_expect_false>
315	description could be written thus (only C<ecb_assume> was added):	348	description could be written thus (only C<ecb_assume> was added):
316		349
317	ecb_inline void	350	ecb_inline void
318	reserve (int size)	351	reserve (int size)
319	{	352	{
320	if (ecb_unlikely (current + size > end))	353	if (ecb_expect_false (current + size > end))
321	real_reserve_method (size); /* presumably noinline */	354	real_reserve_method (size); /* presumably noinline */
322		355
323	ecb_assume (current + size <= end);	356	ecb_assume (current + size <= end);
324	}	357	}
325		358
…		…
374	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
375	cache.	408	cache.
376		409
377	=back	410	=back
378		411
379	=head2 BIT FIDDLING / BITSTUFFS	412	=head2 BIT FIDDLING / BIT WIZARDRY
380		413
381	=over 4	414	=over 4
382		415
383	=item bool ecb_big_endian ()	416	=item bool ecb_big_endian ()
384		417
…		…
386		419
387	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
388	(most-significant byte first) or little endian (least-significant byte	421	(most-significant byte first) or little endian (least-significant byte
389	first) respectively.	422	first) respectively.
390		423
		424	On systems that are neither, their return values are unspecified.
		425
391	=item int ecb_ctz32 (uint32_t x)	426	=item int ecb_ctz32 (uint32_t x)
392		427
		428	=item int ecb_ctz64 (uint64_t x)
		429
393	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
394	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
395	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.
396	common use case is to compute the integer binary logarithm, i.e.,	433
397	floor(log2(n)). For example:	434	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		435
		436	For example:
398		437
399	ecb_ctz32 (3) = 0	438	ecb_ctz32 (3) = 0
400	ecb_ctz32 (6) = 1	439	ecb_ctz32 (6) = 1
401		440
		441	=item int ecb_ld32 (uint32_t x)
		442
		443	=item int ecb_ld64 (uint64_t x)
		444
		445	Returns the index of the most significant bit set in C<x>, or the number
		446	of digits the number requires in binary (so that C<< 2**ld <= x <
		447	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		448	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		449	example to see how many bits a certain number requires to be encoded.
		450
		451	This function is similar to the "count leading zero bits" function, except
		452	that that one returns how many zero bits are "in front" of the number (in
		453	the given data type), while C<ecb_ld> returns how many bits the number
		454	itself requires.
		455
		456	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		457
402	=item int ecb_popcount32 (uint32_t x)	458	=item int ecb_popcount32 (uint32_t x)
403		459
		460	=item int ecb_popcount64 (uint64_t x)
		461
404	Returns the number of bits set to 1 in C<x>. For example:	462	Returns the number of bits set to 1 in C<x>.
		463
		464	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		465
		466	For example:
405		467
406	ecb_popcount32 (7) = 3	468	ecb_popcount32 (7) = 3
407	ecb_popcount32 (255) = 8	469	ecb_popcount32 (255) = 8
408		470
		471	=item uint8_t ecb_bitrev8 (uint8_t x)
		472
		473	=item uint16_t ecb_bitrev16 (uint16_t x)
		474
		475	=item uint32_t ecb_bitrev32 (uint32_t x)
		476
		477	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		478	and so on.
		479
		480	Example:
		481
		482	ecb_bitrev8 (0xa7) = 0xea
		483	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		484
409	=item uint32_t ecb_bswap16 (uint32_t x)	485	=item uint32_t ecb_bswap16 (uint32_t x)
410		486
411	=item uint32_t ecb_bswap32 (uint32_t x)	487	=item uint32_t ecb_bswap32 (uint32_t x)
412		488
		489	=item uint64_t ecb_bswap64 (uint64_t x)
		490
413	These two functions return the value of the 16-bit (32-bit) value C<x>	491	These functions return the value of the 16-bit (32-bit, 64-bit) value
414	after reversing the order of bytes (0x11223344 becomes 0x44332211).	492	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		493	C<ecb_bswap32>).
		494
		495	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		496
		497	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		498
		499	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		500
		501	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		502
		503	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		504
		505	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
415		506
416	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	507	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
417		508
418	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	509	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
419		510
420	These two functions return the value of C<x> after rotating all the bits	511	These two families of functions return the value of C<x> after rotating
421	by C<count> positions to the right or left respectively.	512	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		513	(C<ecb_rotl>).
422		514
423	Current GCC versions understand these functions and usually compile them	515	Current GCC versions understand these functions and usually compile them
424	to "optimal" code (e.g. a single C<roll> on x86).	516	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		517	x86).
425		518
426	=back	519	=back
427		520
428	=head2 ARITHMETIC	521	=head2 ARITHMETIC
429		522
430	=over 4	523	=over 4
431		524
432	=item x = ecb_mod (m, n)	525	=item x = ecb_mod (m, n)
433		526
434	Returns the positive remainder of the modulo operation between C<m> and	527	Returns C<m> modulo C<n>, which is the same as the positive remainder
		528	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	529	division. Unlike the C remainder operator C<%>, this function ensures that
436	return value is always positive - ISO C guarantees very little when	530	the return value is always positive and that the two numbers I<m> and
437	negative numbers are used with C<%>.	531	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		532	C<ecb_mod> implements the mathematical modulo operation, which is missing
		533	in the language.
438		534
439	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be	535	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	536	negatable, that is, both C<m> and C<-m> must be representable in its
441	type.	537	type (this typically excludes the minimum signed integer value, the same
		538	limitation as for C</> and C<%> in C).
		539
		540	Current GCC versions compile this into an efficient branchless sequence on
		541	almost all CPUs.
		542
		543	For example, when you want to rotate forward through the members of an
		544	array for increasing C<m> (which might be negative), then you should use
		545	C<ecb_mod>, as the C<%> operator might give either negative results, or
		546	change direction for negative values:
		547
		548	for (m = -100; m <= 100; ++m)
		549	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		550
		551	=item x = ecb_div_rd (val, div)
		552
		553	=item x = ecb_div_ru (val, div)
		554
		555	Returns C<val> divided by C<div> rounded down or up, respectively.
		556	C<val> and C<div> must have integer types and C<div> must be strictly
		557	positive. Note that these functions are implemented with macros in C
		558	and with function templates in C++.
442		559
443	=back	560	=back
444		561
445	=head2 UTILITY	562	=head2 UTILITY
446		563

Diff Legend

-–
+Removed lines
-+
+Added lines
-<
+Changed lines
->
+Changed lines

Comparing libecb/ecb.pod (file contents): Revision 1.23 by sf-exg, Fri May 27 01:35:46 2011 UTC vs. Revision 1.40 by root, Mon May 28 08:28:26 2012 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 1.40 by root, Mon May 28 08:28:26 2012 UTC