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

Comparing libecb/ecb.pod (file contents):
Revision 1.24 by root, Tue May 31 21:10:37 2011 UTC vs.
Revision 1.43 by root, Tue May 29 14:09:49 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
		70	=head2 LANGUAGE/COMPILER VERSIONS
		71
		72	=over 4
		73
		74	=item ECB_C99
		75
		76	Expands to a true value (suitable for testing in by the preprocessor)
		77	if the environment claims to be C99 compliant.
		78
		79	=item ECB_C11
		80
		81	Expands to a true value (suitable for testing in by the preprocessor)
		82	if the environment claims to be C11 compliant.
		83
		84	=item ECB_GCC_VERSION(major,minor)
		85
		86	Expands to a true value (suitable for testing in by the preprocessor)
		87	if the compiler used is GNU C and the version is the givne version, or
		88	higher.
		89
		90	This macro tries to return false on compilers that claim to be GCC
		91	compatible but aren't.
		92
		93	=back
		94
59	=head2 GCC ATTRIBUTES	95	=head2 GCC ATTRIBUTES
60		96
61	A major part of libecb deals with GCC attributes. These are additional	97	A major part of libecb deals with GCC attributes. These are additional
62	attributes that you cna assign to functions, variables and sometimes even	98	attributes that you can assign to functions, variables and sometimes even
63	types - much like C<const> or C<volatile> in C.	99	types - much like C<const> or C<volatile> in C.
64		100
65	While GCC allows declarations to show up in many surprising places,	101	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	102	but not in many expected places, the safest way is to put attribute
67	declarations before the whole declaration:	103	declarations before the whole declaration:
68		104
69	ecb_const int mysqrt (int a);	105	ecb_const int mysqrt (int a);
70	ecb_unused int i;	106	ecb_unused int i;
71		107
…		…
101	#else	137	#else
102	return 0;	138	return 0;
103	#endif	139	#endif
104	}	140	}
105		141
		142	=item ecb_inline
		143
		144	This is not actually an attribute, but you use it like one. It expands
		145	either to C<static inline> or to just C<static>, if inline isn't
		146	supported. It should be used to declare functions that should be inlined,
		147	for code size or speed reasons.
		148
		149	Example: inline this function, it surely will reduce codesize.
		150
		151	ecb_inline int
		152	negmul (int a, int b)
		153	{
		154	return - (a * b);
		155	}
		156
106	=item ecb_noinline	157	=item ecb_noinline
107		158
108	Prevent a function from being inlined - it might be optimised away, but	159	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	160	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.	161	is rarely called and large enough for inlining not to be helpful.
…		…
184		235
185	In addition to placing cold functions together (or at least away from hot	236	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	237	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	238	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	239	leading to calls to those functions can automatically be marked as if
189	C<ecb_unlikely> had been used to reach them.	240	C<ecb_expect_false> had been used to reach them.
190		241
191	Good examples for such functions would be error reporting functions, or	242	Good examples for such functions would be error reporting functions, or
192	functions only called in exceptional or rare cases.	243	functions only called in exceptional or rare cases.
193		244
194	=item ecb_artificial	245	=item ecb_artificial
…		…
256		307
257	Evaluates C<expr> and returns it. In addition, it tells the compiler that	308	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	309	the C<expr> evaluates to C<value> a lot, which can be used for static
259	branch optimisations.	310	branch optimisations.
260		311
261	Usually, you want to use the more intuitive C<ecb_likely> and	312	Usually, you want to use the more intuitive C<ecb_expect_true> and
262	C<ecb_unlikely> functions instead.	313	C<ecb_expect_false> functions instead.
263		314
		315	=item bool ecb_expect_true (cond)
		316
264	=item bool ecb_likely (cond)	317	=item bool ecb_expect_false (cond)
265
266	=item bool ecb_unlikely (cond)
267		318
268	These two functions expect a expression that is true or false and return	319	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	320	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:	321	other conditional statement, it will not change the program:
271		322
272	/* these two do the same thing */	323	/* these two do the same thing */
273	if (some_condition) ...;	324	if (some_condition) ...;
274	if (ecb_likely (some_condition)) ...;	325	if (ecb_expect_true (some_condition)) ...;
275		326
276	However, by using C<ecb_likely>, you tell the compiler that the condition	327	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	328	condition is likely to be true (and for C<ecb_expect_false>, that it is
278	true).	329	unlikely to be true).
279		330
280	For example, when you check for a null pointer and expect this to be a	331	For example, when you check for a null pointer and expect this to be a
281	rare, exceptional, case, then use C<ecb_unlikely>:	332	rare, exceptional, case, then use C<ecb_expect_false>:
282		333
283	void my_free (void *ptr)	334	void my_free (void *ptr)
284	{	335	{
285	if (ecb_unlikely (ptr == 0))	336	if (ecb_expect_false (ptr == 0))
286	return;	337	return;
287	}	338	}
288		339
289	Consequent use of these functions to mark away exceptional cases or to	340	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	341	tell the compiler what the hot path through a function is can increase
291	performance considerably.	342	performance considerably.
		343
		344	You might know these functions under the name C<likely> and C<unlikely>
		345	- while these are common aliases, we find that the expect name is easier
		346	to understand when quickly skimming code. If you wish, you can use
		347	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		348	C<ecb_expect_false> - these are simply aliases.
292		349
293	A very good example is in a function that reserves more space for some	350	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) -	351	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	352	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:	353	you expect that most checks will turn out to be false:
297		354
298	/* make sure we have "size" extra room in our buffer */	355	/* make sure we have "size" extra room in our buffer */
299	ecb_inline void	356	ecb_inline void
300	reserve (int size)	357	reserve (int size)
301	{	358	{
302	if (ecb_unlikely (current + size > end))	359	if (ecb_expect_false (current + size > end))
303	real_reserve_method (size); /* presumably noinline */	360	real_reserve_method (size); /* presumably noinline */
304	}	361	}
305		362
306	=item bool ecb_assume (cond)	363	=item bool ecb_assume (cond)
307		364
…		…
310		367
311	This can be used to teach the compiler about invariants or other	368	This can be used to teach the compiler about invariants or other
312	conditions that might improve code generation, but which are impossible to	369	conditions that might improve code generation, but which are impossible to
313	deduce form the code itself.	370	deduce form the code itself.
314		371
315	For example, the example reservation function from the C<ecb_unlikely>	372	For example, the example reservation function from the C<ecb_expect_false>
316	description could be written thus (only C<ecb_assume> was added):	373	description could be written thus (only C<ecb_assume> was added):
317		374
318	ecb_inline void	375	ecb_inline void
319	reserve (int size)	376	reserve (int size)
320	{	377	{
321	if (ecb_unlikely (current + size > end))	378	if (ecb_expect_false (current + size > end))
322	real_reserve_method (size); /* presumably noinline */	379	real_reserve_method (size); /* presumably noinline */
323		380
324	ecb_assume (current + size <= end);	381	ecb_assume (current + size <= end);
325	}	382	}
326		383
…		…
375	After processing the node, (part of) the next node might already be in	432	After processing the node, (part of) the next node might already be in
376	cache.	433	cache.
377		434
378	=back	435	=back
379		436
380	=head2 BIT FIDDLING / BITSTUFFS	437	=head2 BIT FIDDLING / BIT WIZARDRY
381		438
382	=over 4	439	=over 4
383		440
384	=item bool ecb_big_endian ()	441	=item bool ecb_big_endian ()
385		442
…		…
391		448
392	On systems that are neither, their return values are unspecified.	449	On systems that are neither, their return values are unspecified.
393		450
394	=item int ecb_ctz32 (uint32_t x)	451	=item int ecb_ctz32 (uint32_t x)
395		452
		453	=item int ecb_ctz64 (uint64_t x)
		454
396	Returns the index of the least significant bit set in C<x> (or	455	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	456	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	457	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	458
		459	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		460
400	(n))>. For example:	461	For example:
401		462
402	ecb_ctz32 (3) = 0	463	ecb_ctz32 (3) = 0
403	ecb_ctz32 (6) = 1	464	ecb_ctz32 (6) = 1
404		465
		466	=item bool ecb_is_pot32 (uint32_t x)
		467
		468	=item bool ecb_is_pot64 (uint32_t x)
		469
		470	Return true iff C<x> is a power of two or C<x == 0>.
		471
		472	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		473
		474	=item int ecb_ld32 (uint32_t x)
		475
		476	=item int ecb_ld64 (uint64_t x)
		477
		478	Returns the index of the most significant bit set in C<x>, or the number
		479	of digits the number requires in binary (so that C<< 2**ld <= x <
		480	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		481	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		482	example to see how many bits a certain number requires to be encoded.
		483
		484	This function is similar to the "count leading zero bits" function, except
		485	that that one returns how many zero bits are "in front" of the number (in
		486	the given data type), while C<ecb_ld> returns how many bits the number
		487	itself requires.
		488
		489	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		490
405	=item int ecb_popcount32 (uint32_t x)	491	=item int ecb_popcount32 (uint32_t x)
406		492
		493	=item int ecb_popcount64 (uint64_t x)
		494
407	Returns the number of bits set to 1 in C<x>. For example:	495	Returns the number of bits set to 1 in C<x>.
		496
		497	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		498
		499	For example:
408		500
409	ecb_popcount32 (7) = 3	501	ecb_popcount32 (7) = 3
410	ecb_popcount32 (255) = 8	502	ecb_popcount32 (255) = 8
411		503
		504	=item uint8_t ecb_bitrev8 (uint8_t x)
		505
		506	=item uint16_t ecb_bitrev16 (uint16_t x)
		507
		508	=item uint32_t ecb_bitrev32 (uint32_t x)
		509
		510	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		511	and so on.
		512
		513	Example:
		514
		515	ecb_bitrev8 (0xa7) = 0xea
		516	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		517
412	=item uint32_t ecb_bswap16 (uint32_t x)	518	=item uint32_t ecb_bswap16 (uint32_t x)
413		519
414	=item uint32_t ecb_bswap32 (uint32_t x)	520	=item uint32_t ecb_bswap32 (uint32_t x)
415		521
		522	=item uint64_t ecb_bswap64 (uint64_t x)
		523
416	These two functions return the value of the 16-bit (32-bit) value C<x>	524	These functions return the value of the 16-bit (32-bit, 64-bit) value
417	after reversing the order of bytes (0x11223344 becomes 0x44332211).	525	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		526	C<ecb_bswap32>).
		527
		528	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		529
		530	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		531
		532	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		533
		534	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		535
		536	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		537
		538	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
418		539
419	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	540	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
420		541
421	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	542	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
422		543
423	These two functions return the value of C<x> after rotating all the bits	544	These two families of functions return the value of C<x> after rotating
424	by C<count> positions to the right or left respectively.	545	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		546	(C<ecb_rotl>).
425		547
426	Current GCC versions understand these functions and usually compile them	548	Current GCC versions understand these functions and usually compile them
427	to "optimal" code (e.g. a single C<roll> on x86).	549	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		550	x86).
428		551
429	=back	552	=back
430		553
431	=head2 ARITHMETIC	554	=head2 ARITHMETIC
432		555
433	=over 4	556	=over 4
434		557
435	=item x = ecb_mod (m, n)	558	=item x = ecb_mod (m, n)
436		559
437	Returns the positive remainder of the modulo operation between C<m> and	560	Returns C<m> modulo C<n>, which is the same as the positive remainder
438	C<n>, using floored division. Unlike the C modulo operator C<%>, this	561	of the division operation between C<m> and C<n>, using floored
439	function ensures that the return value is always positive and that the two	562	division. Unlike the C remainder operator C<%>, this function ensures that
		563	the return value is always positive and that the two numbers I<m> and
440	numbers I<m> and I<m' = m + i * n> result in the same value modulo I<n> -	564	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
441	the C<%> operator usually has a behaviour change at C<m = 0>.	565	C<ecb_mod> implements the mathematical modulo operation, which is missing
		566	in the language.
442		567
443	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be	568	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
444	negatable, that is, both C<m> and C<-m> must be representable in its	569	negatable, that is, both C<m> and C<-m> must be representable in its
445	type.	570	type (this typically excludes the minimum signed integer value, the same
		571	limitation as for C</> and C<%> in C).
446		572
447	Current GCC versions compile this into an efficient branchless sequence on	573	Current GCC versions compile this into an efficient branchless sequence on
448	many systems.	574	almost all CPUs.
449		575
450	For example, when you want to rotate forward through the members of an	576	For example, when you want to rotate forward through the members of an
451	array for increasing C<m> (which might be negative), then you should use	577	array for increasing C<m> (which might be negative), then you should use
452	C<ecb_mod>, as the C<%> operator might give either negative results, or	578	C<ecb_mod>, as the C<%> operator might give either negative results, or
453	change direction for negative values:	579	change direction for negative values:
454		580
455	for (m = -100; m <= 100; ++m)	581	for (m = -100; m <= 100; ++m)
456	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];	582	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
457		583
		584	=item x = ecb_div_rd (val, div)
		585
		586	=item x = ecb_div_ru (val, div)
		587
		588	Returns C<val> divided by C<div> rounded down or up, respectively.
		589	C<val> and C<div> must have integer types and C<div> must be strictly
		590	positive. Note that these functions are implemented with macros in C
		591	and with function templates in C++.
		592
458	=back	593	=back
459		594
460	=head2 UTILITY	595	=head2 UTILITY
461		596
462	=over 4	597	=over 4
…		…
471	for (i = 0; i < ecb_array_length (primes); i++)	606	for (i = 0; i < ecb_array_length (primes); i++)
472	sum += primes [i];	607	sum += primes [i];
473		608
474	=back	609	=back
475		610
		611	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
476		612
		613	These symbols need to be defined before including F<ecb.h> the first time.
		614
		615	=over 4
		616
		617	=item ECB_NO_THRADS
		618
		619	If F<ecb.h> is never used from multiple threads, then this symbol can
		620	be defined, in which case memory fences (and similar constructs) are
		621	completely removed, leading to more efficient code and fewer dependencies.
		622
		623	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		624
		625	=item ECB_NO_SMP
		626
		627	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		628	multiple threads, but never concurrently (e.g. if the system the program
		629	runs on has only a single CPU with a single core, no hyperthreading and so
		630	on), then this symbol can be defined, leading to more efficient code and
		631	fewer dependencies.
		632
		633	=back
		634
		635

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Comparing libecb/ecb.pod (file contents): Revision 1.24 by root, Tue May 31 21:10:37 2011 UTC vs. Revision 1.43 by root, Tue May 29 14:09:49 2012 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.24 by root, Tue May 31 21:10:37 2011 UTC vs.
Revision 1.43 by root, Tue May 29 14:09:49 2012 UTC