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

Comparing libecb/ecb.pod (file contents):
Revision 1.25 by root, Tue May 31 21:52:31 2011 UTC vs.
Revision 1.42 by root, Mon May 28 08:54:03 2012 UTC

…		…
54	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
55	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
56	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
57	refers to any kind of boolean value, not a specific type.	57	refers to any kind of boolean value, not a specific type.
58		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
59	=head2 GCC ATTRIBUTES	70	=head2 GCC ATTRIBUTES
60		71
61	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
62	attributes that you cna assign to functions, variables and sometimes even	73	attributes that you can assign to functions, variables and sometimes even
63	types - much like C<const> or C<volatile> in C.	74	types - much like C<const> or C<volatile> in C.
64		75
65	While GCC allows declarations to show up in many surprising places,	76	While GCC allows declarations to show up in many surprising places,
66	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
67	declarations before the whole declaration:	78	declarations before the whole declaration:
68		79
69	ecb_const int mysqrt (int a);	80	ecb_const int mysqrt (int a);
70	ecb_unused int i;	81	ecb_unused int i;
71		82
…		…
101	#else	112	#else
102	return 0;	113	return 0;
103	#endif	114	#endif
104	}	115	}
105		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
106	=item ecb_noinline	132	=item ecb_noinline
107		133
108	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
109	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
110	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.
…		…
184		210
185	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
186	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
187	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
188	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
189	C<ecb_unlikely> had been used to reach them.	215	C<ecb_expect_false> had been used to reach them.
190		216
191	Good examples for such functions would be error reporting functions, or	217	Good examples for such functions would be error reporting functions, or
192	functions only called in exceptional or rare cases.	218	functions only called in exceptional or rare cases.
193		219
194	=item ecb_artificial	220	=item ecb_artificial
…		…
256		282
257	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
258	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
259	branch optimisations.	285	branch optimisations.
260		286
261	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
262	C<ecb_unlikely> functions instead.	288	C<ecb_expect_false> functions instead.
263		289
		290	=item bool ecb_expect_true (cond)
		291
264	=item bool ecb_likely (cond)	292	=item bool ecb_expect_false (cond)
265
266	=item bool ecb_unlikely (cond)
267		293
268	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
269	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
270	other conditional statement, it will not change the program:	296	other conditional statement, it will not change the program:
271		297
272	/* these two do the same thing */	298	/* these two do the same thing */
273	if (some_condition) ...;	299	if (some_condition) ...;
274	if (ecb_likely (some_condition)) ...;	300	if (ecb_expect_true (some_condition)) ...;
275		301
276	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
277	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
278	true).	304	unlikely to be true).
279		305
280	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
281	rare, exceptional, case, then use C<ecb_unlikely>:	307	rare, exceptional, case, then use C<ecb_expect_false>:
282		308
283	void my_free (void *ptr)	309	void my_free (void *ptr)
284	{	310	{
285	if (ecb_unlikely (ptr == 0))	311	if (ecb_expect_false (ptr == 0))
286	return;	312	return;
287	}	313	}
288		314
289	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
290	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
291	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.
292		324
293	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
294	memory block (for example, inside an implementation of a string stream) -	326	memory block (for example, inside an implementation of a string stream) -
295	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
296	you expect that most checks will turn out to be false:	328	you expect that most checks will turn out to be false:
297		329
298	/* make sure we have "size" extra room in our buffer */	330	/* make sure we have "size" extra room in our buffer */
299	ecb_inline void	331	ecb_inline void
300	reserve (int size)	332	reserve (int size)
301	{	333	{
302	if (ecb_unlikely (current + size > end))	334	if (ecb_expect_false (current + size > end))
303	real_reserve_method (size); /* presumably noinline */	335	real_reserve_method (size); /* presumably noinline */
304	}	336	}
305		337
306	=item bool ecb_assume (cond)	338	=item bool ecb_assume (cond)
307		339
…		…
310		342
311	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
312	conditions that might improve code generation, but which are impossible to	344	conditions that might improve code generation, but which are impossible to
313	deduce form the code itself.	345	deduce form the code itself.
314		346
315	For example, the example reservation function from the C<ecb_unlikely>	347	For example, the example reservation function from the C<ecb_expect_false>
316	description could be written thus (only C<ecb_assume> was added):	348	description could be written thus (only C<ecb_assume> was added):
317		349
318	ecb_inline void	350	ecb_inline void
319	reserve (int size)	351	reserve (int size)
320	{	352	{
321	if (ecb_unlikely (current + size > end))	353	if (ecb_expect_false (current + size > end))
322	real_reserve_method (size); /* presumably noinline */	354	real_reserve_method (size); /* presumably noinline */
323		355
324	ecb_assume (current + size <= end);	356	ecb_assume (current + size <= end);
325	}	357	}
326		358
…		…
375	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
376	cache.	408	cache.
377		409
378	=back	410	=back
379		411
380	=head2 BIT FIDDLING / BITSTUFFS	412	=head2 BIT FIDDLING / BIT WIZARDRY
381		413
382	=over 4	414	=over 4
383		415
384	=item bool ecb_big_endian ()	416	=item bool ecb_big_endian ()
385		417
…		…
391		423
392	On systems that are neither, their return values are unspecified.	424	On systems that are neither, their return values are unspecified.
393		425
394	=item int ecb_ctz32 (uint32_t x)	426	=item int ecb_ctz32 (uint32_t x)
395		427
		428	=item int ecb_ctz64 (uint64_t x)
		429
396	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
397	equivalently the number of bits set to 0 before the least significant bit	431	equivalently the number of bits set to 0 before the least significant bit
398	set), starting from 0. If C<x> is 0 the result is undefined. A common use	432	set), starting from 0. If C<x> is 0 the result is undefined.
399	case is to compute the integer binary logarithm, i.e., C<floor (log2	433
		434	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		435
400	(n))>. For example:	436	For example:
401		437
402	ecb_ctz32 (3) = 0	438	ecb_ctz32 (3) = 0
403	ecb_ctz32 (6) = 1	439	ecb_ctz32 (6) = 1
404		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
405	=item int ecb_popcount32 (uint32_t x)	466	=item int ecb_popcount32 (uint32_t x)
406		467
		468	=item int ecb_popcount64 (uint64_t x)
		469
407	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:
408		475
409	ecb_popcount32 (7) = 3	476	ecb_popcount32 (7) = 3
410	ecb_popcount32 (255) = 8	477	ecb_popcount32 (255) = 8
411		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
412	=item uint32_t ecb_bswap16 (uint32_t x)	493	=item uint32_t ecb_bswap16 (uint32_t x)
413		494
414	=item uint32_t ecb_bswap32 (uint32_t x)	495	=item uint32_t ecb_bswap32 (uint32_t x)
415		496
		497	=item uint64_t ecb_bswap64 (uint64_t x)
		498
416	These two functions return the value of the 16-bit (32-bit) value C<x>	499	These functions return the value of the 16-bit (32-bit, 64-bit) value
417	after reversing the order of bytes (0x11223344 becomes 0x44332211).	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)
418		514
419	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	515	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
420		516
421	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	517	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
422		518
423	These two functions return the value of C<x> after rotating all the bits	519	These two families of functions return the value of C<x> after rotating
424	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>).
425		522
426	Current GCC versions understand these functions and usually compile them	523	Current GCC versions understand these functions and usually compile them
427	to "optimal" code (e.g. a single C<roll> on x86).	524	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		525	x86).
428		526
429	=back	527	=back
430		528
431	=head2 ARITHMETIC	529	=head2 ARITHMETIC
432		530
…		…
442	C<ecb_mod> implements the mathematical modulo operation, which is missing	540	C<ecb_mod> implements the mathematical modulo operation, which is missing
443	in the language.	541	in the language.
444		542
445	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
446	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
447	type (this typically includes the minimum signed integer value, the same	545	type (this typically excludes the minimum signed integer value, the same
448	limitation as for C</> and C<%> in C).	546	limitation as for C</> and C<%> in C).
449		547
450	Current GCC versions compile this into an efficient branchless sequence on	548	Current GCC versions compile this into an efficient branchless sequence on
451	many systems.	549	almost all CPUs.
452		550
453	For example, when you want to rotate forward through the members of an	551	For example, when you want to rotate forward through the members of an
454	array for increasing C<m> (which might be negative), then you should use	552	array for increasing C<m> (which might be negative), then you should use
455	C<ecb_mod>, as the C<%> operator might give either negative results, or	553	C<ecb_mod>, as the C<%> operator might give either negative results, or
456	change direction for negative values:	554	change direction for negative values:
457		555
458	for (m = -100; m <= 100; ++m)	556	for (m = -100; m <= 100; ++m)
459	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];	557	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
460		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++.
		567
461	=back	568	=back
462		569
463	=head2 UTILITY	570	=head2 UTILITY
464		571
465	=over 4	572	=over 4

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Comparing libecb/ecb.pod (file contents): Revision 1.25 by root, Tue May 31 21:52:31 2011 UTC vs. Revision 1.42 by root, Mon May 28 08:54:03 2012 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.25 by root, Tue May 31 21:52:31 2011 UTC vs.
Revision 1.42 by root, Mon May 28 08:54:03 2012 UTC