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

Comparing libecb/ecb.pod (file contents):
Revision 1.16 by sf-exg, Thu May 26 23:32:41 2011 UTC vs.
Revision 1.26 by sf-exg, Wed Jun 1 00:57:14 2011 UTC

…		…
14		14
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
		20	Or in other words, things that should be built into any standard C system,
		21	but aren't, implemented as efficient as possible with GCC, and still
		22	correct with other compilers.
19		23
20	More might come.	24	More might come.
21		25
22	=head2 ABOUT THE HEADER	26	=head2 ABOUT THE HEADER
23		27
…		…
52	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
53	refers to any kind of boolean value, not a specific type.	57	refers to any kind of boolean value, not a specific type.
54		58
55	=head2 GCC ATTRIBUTES	59	=head2 GCC ATTRIBUTES
56		60
57	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;
58		76
59	=over 4	77	=over 4
60		78
61	=item ecb_attribute ((attrs...))	79	=item ecb_attribute ((attrs...))
62		80
…		…
91	not inlined into other functions. This is useful if you know your function	109	not inlined into other functions. This is useful if you know your function
92	is rarely called and large enough for inlining not to be helpful.	110	is rarely called and large enough for inlining not to be helpful.
93		111
94	=item ecb_noreturn	112	=item ecb_noreturn
95		113
		114	Marks a function as "not returning, ever". Some typical functions that
		115	don't return are C<exit> or C<abort> (which really works hard to not
		116	return), and now you can make your own:
		117
		118	ecb_noreturn void
		119	my_abort (const char *errline)
		120	{
		121	puts (errline);
		122	abort ();
		123	}
		124
		125	In this case, the compiler would probably be smart enough to deduce it on
		126	its own, so this is mainly useful for declarations.
		127
96	=item ecb_const	128	=item ecb_const
97		129
		130	Declares that the function only depends on the values of its arguments,
		131	much like a mathematical function. It specifically does not read or write
		132	any memory any arguments might point to, global variables, or call any
		133	non-const functions. It also must not have any side effects.
		134
		135	Such a function can be optimised much more aggressively by the compiler -
		136	for example, multiple calls with the same arguments can be optimised into
		137	a single call, which wouldn't be possible if the compiler would have to
		138	expect any side effects.
		139
		140	It is best suited for functions in the sense of mathematical functions,
		141	such as a function returning the square root of its input argument.
		142
		143	Not suited would be a function that calculates the hash of some memory
		144	area you pass in, prints some messages or looks at a global variable to
		145	decide on rounding.
		146
		147	See C<ecb_pure> for a slightly less restrictive class of functions.
		148
98	=item ecb_pure	149	=item ecb_pure
99		150
		151	Similar to C<ecb_const>, declares a function that has no side
		152	effects. Unlike C<ecb_const>, the function is allowed to examine global
		153	variables and any other memory areas (such as the ones passed to it via
		154	pointers).
		155
		156	While these functions cannot be optimised as aggressively as C<ecb_const>
		157	functions, they can still be optimised away in many occasions, and the
		158	compiler has more freedom in moving calls to them around.
		159
		160	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		161	function that calculates the MD5 sum of some input and updates some MD5
		162	state passed as argument would I<NOT> be pure, however, as it would modify
		163	some memory area that is not the return value.
		164
100	=item ecb_hot	165	=item ecb_hot
101		166
		167	This declares a function as "hot" with regards to the cache - the function
		168	is used so often, that it is very beneficial to keep it in the cache if
		169	possible.
		170
		171	The compiler reacts by trying to place hot functions near to each other in
		172	memory.
		173
		174	Whether a function is hot or not often depends on the whole program,
		175	and less on the function itself. C<ecb_cold> is likely more useful in
		176	practise.
		177
102	=item ecb_cold	178	=item ecb_cold
103		179
		180	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		181	the cache, or in other words, this function is not called often, or not at
		182	speed-critical times, and keeping it in the cache might be a waste of said
		183	cache.
		184
		185	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
		187	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
		189	C<ecb_unlikely> had been used to reach them.
		190
		191	Good examples for such functions would be error reporting functions, or
		192	functions only called in exceptional or rare cases.
		193
104	=item ecb_artificial	194	=item ecb_artificial
		195
		196	Declares the function as "artificial", in this case meaning that this
		197	function is not really mean to be a function, but more like an accessor
		198	- many methods in C++ classes are mere accessor functions, and having a
		199	crash reported in such a method, or single-stepping through them, is not
		200	usually so helpful, especially when it's inlined to just a few instructions.
		201
		202	Marking them as artificial will instruct the debugger about just this,
		203	leading to happier debugging and thus happier lives.
		204
		205	Example: in some kind of smart-pointer class, mark the pointer accessor as
		206	artificial, so that the whole class acts more like a pointer and less like
		207	some C++ abstraction monster.
		208
		209	template<typename T>
		210	struct my_smart_ptr
		211	{
		212	T *value;
		213
		214	ecb_artificial
		215	operator T *()
		216	{
		217	return value;
		218	}
		219	};
105		220
106	=back	221	=back
107		222
108	=head2 OPTIMISATION HINTS	223	=head2 OPTIMISATION HINTS
109		224
…		…
272		387
273	These two functions return true if the byte order is big endian	388	These two functions return true if the byte order is big endian
274	(most-significant byte first) or little endian (least-significant byte	389	(most-significant byte first) or little endian (least-significant byte
275	first) respectively.	390	first) respectively.
276		391
		392	On systems that are neither, their return values are unspecified.
		393
277	=item int ecb_ctz32 (uint32_t x)	394	=item int ecb_ctz32 (uint32_t x)
278		395
279	Returns the index of the least significant bit set in C<x> (or	396	Returns the index of the least significant bit set in C<x> (or
280	equivalently the number of bits set to 0 before the least significant	397	equivalently the number of bits set to 0 before the least significant bit
281	bit set), starting from 0. If C<x> is 0 the result is undefined. A	398	set), starting from 0. If C<x> is 0 the result is undefined. A common use
282	common use case is to compute the integer binary logarithm, i.e.,	399	case is to compute the integer binary logarithm, i.e., C<floor (log2
283	floor(log2(n)). For example:	400	(n))>. For example:
284		401
285	ecb_ctz32 (3) = 0	402	ecb_ctz32 (3) = 0
286	ecb_ctz32 (6) = 1	403	ecb_ctz32 (6) = 1
287		404
288	=item int ecb_popcount32 (uint32_t x)	405	=item int ecb_popcount32 (uint32_t x)
…		…
294		411
295	=item uint32_t ecb_bswap16 (uint32_t x)	412	=item uint32_t ecb_bswap16 (uint32_t x)
296		413
297	=item uint32_t ecb_bswap32 (uint32_t x)	414	=item uint32_t ecb_bswap32 (uint32_t x)
298		415
299	These two functions return the value of the 16-bit (32-bit) variable	416	These two functions return the value of the 16-bit (32-bit) value C<x>
300	C<x> after reversing the order of bytes.	417	after reversing the order of bytes (0x11223344 becomes 0x44332211).
301		418
302	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	419	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
303		420
304	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	421	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
305		422
306	These two functions return the value of C<x> after shifting all the bits	423	These two functions return the value of C<x> after rotating all the bits
307	by C<count> positions to the right or left respectively.	424	by C<count> positions to the right or left respectively.
308		425
		426	Current GCC versions understand these functions and usually compile them
		427	to "optimal" code (e.g. a single C<roll> on x86).
		428
309	=back	429	=back
310		430
311	=head2 ARITHMETIC	431	=head2 ARITHMETIC
312		432
313	=over 4	433	=over 4
314		434
315	=item x = ecb_mod (m, n)	435	=item x = ecb_mod (m, n)
316		436
317	Returns the positive remainder of the modulo operation between C<m> and	437	Returns C<m> modulo C<n>, which is the same as the positive remainder
		438	of the division operation between C<m> and C<n>, using floored
318	C<n>. Unlike the C modulo operator C<%>, this function ensures that the	439	division. Unlike the C remainder operator C<%>, this function ensures that
319	return value is always positive).	440	the return value is always positive and that the two numbers I<m> and
		441	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		442	C<ecb_mod> implements the mathematical modulo operation, which is missing
		443	in the language.
320		444
321	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	445	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
322	negatable, that is, both C<m> and C<-m> must be representable in its	446	negatable, that is, both C<m> and C<-m> must be representable in its
323	type.	447	type (this typically includes the minimum signed integer value, the same
		448	limitation as for C</> and C<%> in C).
		449
		450	Current GCC versions compile this into an efficient branchless sequence on
		451	many systems.
		452
		453	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
		455	C<ecb_mod>, as the C<%> operator might give either negative results, or
		456	change direction for negative values:
		457
		458	for (m = -100; m <= 100; ++m)
		459	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
324		460
325	=back	461	=back
326		462
327	=head2 UTILITY	463	=head2 UTILITY
328		464
329	=over 4	465	=over 4
330		466
331	=item element_count = ecb_array_length (name) [MACRO]	467	=item element_count = ecb_array_length (name)
332		468
333	Returns the number of elements in the array C<name>. For example:	469	Returns the number of elements in the array C<name>. For example:
334		470
335	int primes[] = { 2, 3, 5, 7, 11 };	471	int primes[] = { 2, 3, 5, 7, 11 };
336	int sum = 0;	472	int sum = 0;

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Comparing libecb/ecb.pod (file contents): Revision 1.16 by sf-exg, Thu May 26 23:32:41 2011 UTC vs. Revision 1.26 by sf-exg, Wed Jun 1 00:57:14 2011 UTC

Diff Legend

Comparing libecb/ecb.pod (file contents):
Revision 1.16 by sf-exg, Thu May 26 23:32:41 2011 UTC vs.
Revision 1.26 by sf-exg, Wed Jun 1 00:57:14 2011 UTC