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Revision 1.15 by root, Thu May 26 23:26:29 2011 UTC vs.
Revision 1.58 by root, Thu Oct 16 15:01:54 2014 UTC

…		…
3	=head2 ABOUT LIBECB	3	=head2 ABOUT LIBECB
4		4
5	Libecb is currently a simple header file that doesn't require any	5	Libecb is currently a simple header file that doesn't require any
6	configuration to use or include in your project.	6	configuration to use or include in your project.
7		7
8	It's part of the e-suite of libraries, other memembers of which include	8	It's part of the e-suite of libraries, other members of which include
9	libev and libeio.	9	libev and libeio.
10		10
11	Its homepage can be found here:	11	Its homepage can be found here:
12		12
13	http://software.schmorp.de/pkg/libecb	13	http://software.schmorp.de/pkg/libecb
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 endienness	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
…		…
27	#include <ecb.h>	31	#include <ecb.h>
28		32
29	The header should work fine for both C and C++ compilation, and gives you	33	The header should work fine for both C and C++ compilation, and gives you
30	all of F<inttypes.h> in addition to the ECB symbols.	34	all of F<inttypes.h> in addition to the ECB symbols.
31		35
32	There are currently no objetc files to link to - future versions might	36	There are currently no object files to link to - future versions might
33	come with an (optional) object code library to link against, to reduce	37	come with an (optional) object code library to link against, to reduce
34	code size or gain access to additional features.	38	code size or gain access to additional features.
35		39
36	It also currently includes everything from F<inttypes.h>.	40	It also currently includes everything from F<inttypes.h>.
37		41
…		…
50	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
51	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
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
		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>) and can be used in preprocessor
		69	expressions.
		70
		71	For C<ptrdiff_t> and C<size_t> use C<stddef.h>.
		72
		73	=head2 LANGUAGE/COMPILER VERSIONS
		74
		75	All the following symbols expand to an expression that can be tested in
		76	preprocessor instructions as well as treated as a boolean (use C<!!> to
		77	ensure it's either C<0> or C<1> if you need that).
		78
		79	=over 4
		80
		81	=item ECB_C
		82
		83	True if the implementation defines the C<__STDC__> macro to a true value,
		84	while not claiming to be C++.
		85
		86	=item ECB_C99
		87
		88	True if the implementation claims to be compliant to C99 (ISO/IEC
		89	9899:1999) or any later version, while not claiming to be C++.
		90
		91	Note that later versions (ECB_C11) remove core features again (for
		92	example, variable length arrays).
		93
		94	=item ECB_C11
		95
		96	True if the implementation claims to be compliant to C11 (ISO/IEC
		97	9899:2011) or any later version, while not claiming to be C++.
		98
		99	=item ECB_CPP
		100
		101	True if the implementation defines the C<__cplusplus__> macro to a true
		102	value, which is typically true for C++ compilers.
		103
		104	=item ECB_CPP11
		105
		106	True if the implementation claims to be compliant to ISO/IEC 14882:2011
		107	(C++11) or any later version.
		108
		109	=item ECB_GCC_VERSION (major, minor)
		110
		111	Expands to a true value (suitable for testing in by the preprocessor)
		112	if the compiler used is GNU C and the version is the given version, or
		113	higher.
		114
		115	This macro tries to return false on compilers that claim to be GCC
		116	compatible but aren't.
		117
		118	=item ECB_EXTERN_C
		119
		120	Expands to C<extern "C"> in C++, and a simple C<extern> in C.
		121
		122	This can be used to declare a single external C function:
		123
		124	ECB_EXTERN_C int printf (const char *format, ...);
		125
		126	=item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
		127
		128	These two macros can be used to wrap multiple C<extern "C"> definitions -
		129	they expand to nothing in C.
		130
		131	They are most useful in header files:
		132
		133	ECB_EXTERN_C_BEG
		134
		135	int mycfun1 (int x);
		136	int mycfun2 (int x);
		137
		138	ECB_EXTERN_C_END
		139
		140	=item ECB_STDFP
		141
		142	If this evaluates to a true value (suitable for testing in by the
		143	preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
		144	and double/binary64 representations internally I<and> the endianness of
		145	both types match the endianness of C<uint32_t> and C<uint64_t>.
		146
		147	This means you can just copy the bits of a C<float> (or C<double>) to an
		148	C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
		149	without having to think about format or endianness.
		150
		151	This is true for basically all modern platforms, although F<ecb.h> might
		152	not be able to deduce this correctly everywhere and might err on the safe
		153	side.
		154
		155	=item ECB_AMD64, ECB_AMD64_X32
		156
		157	These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
		158	ABI, respectively, and undefined elsewhere.
		159
		160	The designers of the new X32 ABI for some inexplicable reason decided to
		161	make it look exactly like amd64, even though it's completely incompatible
		162	to that ABI, breaking about every piece of software that assumed that
		163	C<__x86_64> stands for, well, the x86-64 ABI, making these macros
		164	necessary.
		165
		166	=back
		167
55	=head2 GCC ATTRIBUTES	168	=head2 GCC ATTRIBUTES
56		169
57	blabla where to put, what others	170	A major part of libecb deals with GCC attributes. These are additional
		171	attributes that you can assign to functions, variables and sometimes even
		172	types - much like C<const> or C<volatile> in C.
		173
		174	While GCC allows declarations to show up in many surprising places,
		175	but not in many expected places, the safest way is to put attribute
		176	declarations before the whole declaration:
		177
		178	ecb_const int mysqrt (int a);
		179	ecb_unused int i;
		180
		181	For variables, it is often nicer to put the attribute after the name, and
		182	avoid multiple declarations using commas:
		183
		184	int i ecb_unused;
58		185
59	=over 4	186	=over 4
60		187
61	=item ecb_attribute ((attrs...))	188	=item ecb_attribute ((attrs...))
62		189
63	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to	190	A simple wrapper that expands to C<__attribute__((attrs))> on GCC 3.1+ and
64	nothing on other compilers, so the effect is that only GCC sees these.	191	Clang 2.8+, and to nothing on other compilers, so the effect is that only
		192	GCC and Clang see these.
65		193
66	Example: use the C<deprecated> attribute on a function.	194	Example: use the C<deprecated> attribute on a function.
67		195
68	ecb_attribute((__deprecated__)) void	196	ecb_attribute((__deprecated__)) void
69	do_not_use_me_anymore (void);	197	do_not_use_me_anymore (void);
…		…
83	#else	211	#else
84	return 0;	212	return 0;
85	#endif	213	#endif
86	}	214	}
87		215
		216	=item ecb_deprecated
		217
		218	Similar to C<ecb_unused>, but marks a function, variable or type as
		219	deprecated. This makes some compilers warn when the type is used.
		220
		221	=item ecb_inline
		222
		223	This is not actually an attribute, but you use it like one. It expands
		224	either to C<static inline> or to just C<static>, if inline isn't
		225	supported. It should be used to declare functions that should be inlined,
		226	for code size or speed reasons.
		227
		228	Example: inline this function, it surely will reduce codesize.
		229
		230	ecb_inline int
		231	negmul (int a, int b)
		232	{
		233	return - (a * b);
		234	}
		235
88	=item ecb_noinline	236	=item ecb_noinline
89		237
90	Prevent a function from being inlined - it might be optimised away, but	238	Prevent a function from being inlined - it might be optimised away, but
91	not inlined into other functions. This is useful if you know your function	239	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.	240	is rarely called and large enough for inlining not to be helpful.
93		241
94	=item ecb_noreturn	242	=item ecb_noreturn
95		243
		244	Marks a function as "not returning, ever". Some typical functions that
		245	don't return are C<exit> or C<abort> (which really works hard to not
		246	return), and now you can make your own:
		247
		248	ecb_noreturn void
		249	my_abort (const char *errline)
		250	{
		251	puts (errline);
		252	abort ();
		253	}
		254
		255	In this case, the compiler would probably be smart enough to deduce it on
		256	its own, so this is mainly useful for declarations.
		257
		258	=item ecb_restrict
		259
		260	Expands to the C<restrict> keyword or equivalent on compilers that support
		261	them, and to nothing on others. Must be specified on a pointer type or
		262	an array index to indicate that the memory doesn't alias with any other
		263	restricted pointer in the same scope.
		264
		265	Example: multiply a vector, and allow the compiler to parallelise the
		266	loop, because it knows it doesn't overwrite input values.
		267
		268	void
		269	multiply (float *ecb_restrict src,
		270	float *ecb_restrict dst,
		271	int len, float factor)
		272	{
		273	int i;
		274
		275	for (i = 0; i < len; ++i)
		276	dst [i] = src [i] * factor;
		277	}
		278
96	=item ecb_const	279	=item ecb_const
97		280
		281	Declares that the function only depends on the values of its arguments,
		282	much like a mathematical function. It specifically does not read or write
		283	any memory any arguments might point to, global variables, or call any
		284	non-const functions. It also must not have any side effects.
		285
		286	Such a function can be optimised much more aggressively by the compiler -
		287	for example, multiple calls with the same arguments can be optimised into
		288	a single call, which wouldn't be possible if the compiler would have to
		289	expect any side effects.
		290
		291	It is best suited for functions in the sense of mathematical functions,
		292	such as a function returning the square root of its input argument.
		293
		294	Not suited would be a function that calculates the hash of some memory
		295	area you pass in, prints some messages or looks at a global variable to
		296	decide on rounding.
		297
		298	See C<ecb_pure> for a slightly less restrictive class of functions.
		299
98	=item ecb_pure	300	=item ecb_pure
99		301
		302	Similar to C<ecb_const>, declares a function that has no side
		303	effects. Unlike C<ecb_const>, the function is allowed to examine global
		304	variables and any other memory areas (such as the ones passed to it via
		305	pointers).
		306
		307	While these functions cannot be optimised as aggressively as C<ecb_const>
		308	functions, they can still be optimised away in many occasions, and the
		309	compiler has more freedom in moving calls to them around.
		310
		311	Typical examples for such functions would be C<strlen> or C<memcmp>. A
		312	function that calculates the MD5 sum of some input and updates some MD5
		313	state passed as argument would I<NOT> be pure, however, as it would modify
		314	some memory area that is not the return value.
		315
100	=item ecb_hot	316	=item ecb_hot
101		317
		318	This declares a function as "hot" with regards to the cache - the function
		319	is used so often, that it is very beneficial to keep it in the cache if
		320	possible.
		321
		322	The compiler reacts by trying to place hot functions near to each other in
		323	memory.
		324
		325	Whether a function is hot or not often depends on the whole program,
		326	and less on the function itself. C<ecb_cold> is likely more useful in
		327	practise.
		328
102	=item ecb_cold	329	=item ecb_cold
103		330
		331	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
		332	the cache, or in other words, this function is not called often, or not at
		333	speed-critical times, and keeping it in the cache might be a waste of said
		334	cache.
		335
		336	In addition to placing cold functions together (or at least away from hot
		337	functions), this knowledge can be used in other ways, for example, the
		338	function will be optimised for size, as opposed to speed, and codepaths
		339	leading to calls to those functions can automatically be marked as if
		340	C<ecb_expect_false> had been used to reach them.
		341
		342	Good examples for such functions would be error reporting functions, or
		343	functions only called in exceptional or rare cases.
		344
104	=item ecb_artificial	345	=item ecb_artificial
105		346
		347	Declares the function as "artificial", in this case meaning that this
		348	function is not really meant to be a function, but more like an accessor
		349	- many methods in C++ classes are mere accessor functions, and having a
		350	crash reported in such a method, or single-stepping through them, is not
		351	usually so helpful, especially when it's inlined to just a few instructions.
		352
		353	Marking them as artificial will instruct the debugger about just this,
		354	leading to happier debugging and thus happier lives.
		355
		356	Example: in some kind of smart-pointer class, mark the pointer accessor as
		357	artificial, so that the whole class acts more like a pointer and less like
		358	some C++ abstraction monster.
		359
		360	template<typename T>
		361	struct my_smart_ptr
		362	{
		363	T *value;
		364
		365	ecb_artificial
		366	operator T *()
		367	{
		368	return value;
		369	}
		370	};
		371
106	=back	372	=back
107		373
108	=head2 OPTIMISATION HINTS	374	=head2 OPTIMISATION HINTS
109		375
110	=over 4	376	=over 4
111		377
112	=item bool ecb_is_constant(expr)	378	=item bool ecb_is_constant (expr)
113		379
114	Returns true iff the expression can be deduced to be a compile-time	380	Returns true iff the expression can be deduced to be a compile-time
115	constant, and false otherwise.	381	constant, and false otherwise.
116		382
117	For example, when you have a C<rndm16> function that returns a 16 bit	383	For example, when you have a C<rndm16> function that returns a 16 bit
…		…
141		407
142	Evaluates C<expr> and returns it. In addition, it tells the compiler that	408	Evaluates C<expr> and returns it. In addition, it tells the compiler that
143	the C<expr> evaluates to C<value> a lot, which can be used for static	409	the C<expr> evaluates to C<value> a lot, which can be used for static
144	branch optimisations.	410	branch optimisations.
145		411
146	Usually, you want to use the more intuitive C<ecb_likely> and	412	Usually, you want to use the more intuitive C<ecb_expect_true> and
147	C<ecb_unlikely> functions instead.	413	C<ecb_expect_false> functions instead.
148		414
		415	=item bool ecb_expect_true (cond)
		416
149	=item bool ecb_likely (cond)	417	=item bool ecb_expect_false (cond)
150
151	=item bool ecb_unlikely (cond)
152		418
153	These two functions expect a expression that is true or false and return	419	These two functions expect a expression that is true or false and return
154	C<1> or C<0>, respectively, so when used in the condition of an C<if> or	420	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
155	other conditional statement, it will not change the program:	421	other conditional statement, it will not change the program:
156		422
157	/* these two do the same thing */	423	/* these two do the same thing */
158	if (some_condition) ...;	424	if (some_condition) ...;
159	if (ecb_likely (some_condition)) ...;	425	if (ecb_expect_true (some_condition)) ...;
160		426
161	However, by using C<ecb_likely>, you tell the compiler that the condition	427	However, by using C<ecb_expect_true>, you tell the compiler that the
162	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be	428	condition is likely to be true (and for C<ecb_expect_false>, that it is
163	true).	429	unlikely to be true).
164		430
165	For example, when you check for a null pointer and expect this to be a	431	For example, when you check for a null pointer and expect this to be a
166	rare, exceptional, case, then use C<ecb_unlikely>:	432	rare, exceptional, case, then use C<ecb_expect_false>:
167		433
168	void my_free (void *ptr)	434	void my_free (void *ptr)
169	{	435	{
170	if (ecb_unlikely (ptr == 0))	436	if (ecb_expect_false (ptr == 0))
171	return;	437	return;
172	}	438	}
173		439
174	Consequent use of these functions to mark away exceptional cases or to	440	Consequent use of these functions to mark away exceptional cases or to
175	tell the compiler what the hot path through a function is can increase	441	tell the compiler what the hot path through a function is can increase
176	performance considerably.	442	performance considerably.
		443
		444	You might know these functions under the name C<likely> and C<unlikely>
		445	- while these are common aliases, we find that the expect name is easier
		446	to understand when quickly skimming code. If you wish, you can use
		447	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
		448	C<ecb_expect_false> - these are simply aliases.
177		449
178	A very good example is in a function that reserves more space for some	450	A very good example is in a function that reserves more space for some
179	memory block (for example, inside an implementation of a string stream) -	451	memory block (for example, inside an implementation of a string stream) -
180	each time something is added, you have to check for a buffer overrun, but	452	each time something is added, you have to check for a buffer overrun, but
181	you expect that most checks will turn out to be false:	453	you expect that most checks will turn out to be false:
182		454
183	/* make sure we have "size" extra room in our buffer */	455	/* make sure we have "size" extra room in our buffer */
184	ecb_inline void	456	ecb_inline void
185	reserve (int size)	457	reserve (int size)
186	{	458	{
187	if (ecb_unlikely (current + size > end))	459	if (ecb_expect_false (current + size > end))
188	real_reserve_method (size); /* presumably noinline */	460	real_reserve_method (size); /* presumably noinline */
189	}	461	}
190		462
191	=item bool ecb_assume (cond)	463	=item bool ecb_assume (cond)
192		464
…		…
195		467
196	This can be used to teach the compiler about invariants or other	468	This can be used to teach the compiler about invariants or other
197	conditions that might improve code generation, but which are impossible to	469	conditions that might improve code generation, but which are impossible to
198	deduce form the code itself.	470	deduce form the code itself.
199		471
200	For example, the example reservation function from the C<ecb_unlikely>	472	For example, the example reservation function from the C<ecb_expect_false>
201	description could be written thus (only C<ecb_assume> was added):	473	description could be written thus (only C<ecb_assume> was added):
202		474
203	ecb_inline void	475	ecb_inline void
204	reserve (int size)	476	reserve (int size)
205	{	477	{
206	if (ecb_unlikely (current + size > end))	478	if (ecb_expect_false (current + size > end))
207	real_reserve_method (size); /* presumably noinline */	479	real_reserve_method (size); /* presumably noinline */
208		480
209	ecb_assume (current + size <= end);	481	ecb_assume (current + size <= end);
210	}	482	}
211		483
…		…
260	After processing the node, (part of) the next node might already be in	532	After processing the node, (part of) the next node might already be in
261	cache.	533	cache.
262		534
263	=back	535	=back
264		536
265	=head2 BIT FIDDLING / BITSTUFFS	537	=head2 BIT FIDDLING / BIT WIZARDRY
266		538
267	=over 4	539	=over 4
268		540
269	=item bool ecb_big_endian ()	541	=item bool ecb_big_endian ()
270		542
…		…
272		544
273	These two functions return true if the byte order is big endian	545	These two functions return true if the byte order is big endian
274	(most-significant byte first) or little endian (least-significant byte	546	(most-significant byte first) or little endian (least-significant byte
275	first) respectively.	547	first) respectively.
276		548
		549	On systems that are neither, their return values are unspecified.
		550
277	=item int ecb_ctz32 (uint32_t x)	551	=item int ecb_ctz32 (uint32_t x)
278		552
		553	=item int ecb_ctz64 (uint64_t x)
		554
279	Returns the index of the least significant bit set in C<x> (or	555	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	556	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	557	set), starting from 0. If C<x> is 0 the result is undefined.
282	common use case is to compute the integer binary logarithm, i.e.,	558
283	floor(log2(n)). For example:	559	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
		560
		561	For example:
284		562
285	ecb_ctz32 (3) = 0	563	ecb_ctz32 (3) = 0
286	ecb_ctz32 (6) = 1	564	ecb_ctz32 (6) = 1
287		565
		566	=item bool ecb_is_pot32 (uint32_t x)
		567
		568	=item bool ecb_is_pot64 (uint32_t x)
		569
		570	Return true iff C<x> is a power of two or C<x == 0>.
		571
		572	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
		573
		574	=item int ecb_ld32 (uint32_t x)
		575
		576	=item int ecb_ld64 (uint64_t x)
		577
		578	Returns the index of the most significant bit set in C<x>, or the number
		579	of digits the number requires in binary (so that C<< 2**ld <= x <
		580	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
		581	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
		582	example to see how many bits a certain number requires to be encoded.
		583
		584	This function is similar to the "count leading zero bits" function, except
		585	that that one returns how many zero bits are "in front" of the number (in
		586	the given data type), while C<ecb_ld> returns how many bits the number
		587	itself requires.
		588
		589	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
		590
288	=item int ecb_popcount32 (uint32_t x)	591	=item int ecb_popcount32 (uint32_t x)
289		592
		593	=item int ecb_popcount64 (uint64_t x)
		594
290	Returns the number of bits set to 1 in C<x>. For example:	595	Returns the number of bits set to 1 in C<x>.
		596
		597	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
		598
		599	For example:
291		600
292	ecb_popcount32 (7) = 3	601	ecb_popcount32 (7) = 3
293	ecb_popcount32 (255) = 8	602	ecb_popcount32 (255) = 8
294		603
		604	=item uint8_t ecb_bitrev8 (uint8_t x)
		605
		606	=item uint16_t ecb_bitrev16 (uint16_t x)
		607
		608	=item uint32_t ecb_bitrev32 (uint32_t x)
		609
		610	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
		611	and so on.
		612
		613	Example:
		614
		615	ecb_bitrev8 (0xa7) = 0xea
		616	ecb_bitrev32 (0xffcc4411) = 0x882233ff
		617
295	=item uint32_t ecb_bswap16 (uint32_t x)	618	=item uint32_t ecb_bswap16 (uint32_t x)
296		619
297	=item uint32_t ecb_bswap32 (uint32_t x)	620	=item uint32_t ecb_bswap32 (uint32_t x)
298		621
		622	=item uint64_t ecb_bswap64 (uint64_t x)
		623
299	These two functions return the value of the 16-bit (32-bit) variable	624	These functions return the value of the 16-bit (32-bit, 64-bit) value
300	C<x> after reversing the order of bytes.	625	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
		626	C<ecb_bswap32>).
		627
		628	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
		629
		630	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
		631
		632	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
		633
		634	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
		635
		636	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
		637
		638	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
301		639
302	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)	640	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
303		641
304	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)	642	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
305		643
306	These two functions return the value of C<x> after shifting all the bits	644	These two families of functions return the value of C<x> after rotating
307	by C<count> positions to the right or left respectively.	645	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
		646	(C<ecb_rotl>).
		647
		648	Current GCC versions understand these functions and usually compile them
		649	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
		650	x86).
308		651
309	=back	652	=back
310		653
		654	=head2 FLOATING POINT FIDDLING
		655
		656	=over 4
		657
		658	=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
		659
		660	=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
		661
		662	These functions each take an argument in the native C<float> or C<double>
		663	type and return the IEEE 754 bit representation of it.
		664
		665	The bit representation is just as IEEE 754 defines it, i.e. the sign bit
		666	will be the most significant bit, followed by exponent and mantissa.
		667
		668	This function should work even when the native floating point format isn't
		669	IEEE compliant, of course at a speed and code size penalty, and of course
		670	also within reasonable limits (it tries to convert NaNs, infinities and
		671	denormals, but will likely convert negative zero to positive zero).
		672
		673	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		674	be able to optimise away this function completely.
		675
		676	These functions can be helpful when serialising floats to the network - you
		677	can serialise the return value like a normal uint32_t/uint64_t.
		678
		679	Another use for these functions is to manipulate floating point values
		680	directly.
		681
		682	Silly example: toggle the sign bit of a float.
		683
		684	/* On gcc-4.7 on amd64, */
		685	/* this results in a single add instruction to toggle the bit, and 4 extra */
		686	/* instructions to move the float value to an integer register and back. */
		687
		688	x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
		689
		690	=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
		691
		692	=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
		693
		694	=item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
		695
		696	The reverse operation of the previos function - takes the bit
		697	representation of an IEEE binary16, binary32 or binary64 number and
		698	converts it to the native C<float> or C<double> format.
		699
		700	This function should work even when the native floating point format isn't
		701	IEEE compliant, of course at a speed and code size penalty, and of course
		702	also within reasonable limits (it tries to convert normals and denormals,
		703	and might be lucky for infinities, and with extraordinary luck, also for
		704	negative zero).
		705
		706	On all modern platforms (where C<ECB_STDFP> is true), the compiler should
		707	be able to optimise away this function completely.
		708
		709	=back
		710
311	=head2 ARITHMETIC	711	=head2 ARITHMETIC
312		712
313	=over 4	713	=over 4
314		714
315	=item x = ecb_mod (m, n)	715	=item x = ecb_mod (m, n)
316		716
317	Returns the positive remainder of the modulo operation between C<m> and	717	Returns C<m> modulo C<n>, which is the same as the positive remainder
		718	of the division operation between C<m> and C<n>, using floored
318	C<n>. Unlike the C moduloe operator C<%>, this function ensures that the	719	division. Unlike the C remainder operator C<%>, this function ensures that
319	return value is always positive).	720	the return value is always positive and that the two numbers I<m> and
		721	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
		722	C<ecb_mod> implements the mathematical modulo operation, which is missing
		723	in the language.
320		724
321	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be	725	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	726	negatable, that is, both C<m> and C<-m> must be representable in its
323	type.	727	type (this typically excludes the minimum signed integer value, the same
		728	limitation as for C</> and C<%> in C).
		729
		730	Current GCC versions compile this into an efficient branchless sequence on
		731	almost all CPUs.
		732
		733	For example, when you want to rotate forward through the members of an
		734	array for increasing C<m> (which might be negative), then you should use
		735	C<ecb_mod>, as the C<%> operator might give either negative results, or
		736	change direction for negative values:
		737
		738	for (m = -100; m <= 100; ++m)
		739	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
		740
		741	=item x = ecb_div_rd (val, div)
		742
		743	=item x = ecb_div_ru (val, div)
		744
		745	Returns C<val> divided by C<div> rounded down or up, respectively.
		746	C<val> and C<div> must have integer types and C<div> must be strictly
		747	positive. Note that these functions are implemented with macros in C
		748	and with function templates in C++.
324		749
325	=back	750	=back
326		751
327	=head2 UTILITY	752	=head2 UTILITY
328		753
329	=over 4	754	=over 4
330		755
331	=item element_count = ecb_array_length (name) [MACRO]	756	=item element_count = ecb_array_length (name)
332		757
333	Returns the number of elements in the array C<name>. For example:	758	Returns the number of elements in the array C<name>. For example:
334		759
335	int primes[] = { 2, 3, 5, 7, 11 };	760	int primes[] = { 2, 3, 5, 7, 11 };
336	int sum = 0;	761	int sum = 0;
…		…
338	for (i = 0; i < ecb_array_length (primes); i++)	763	for (i = 0; i < ecb_array_length (primes); i++)
339	sum += primes [i];	764	sum += primes [i];
340		765
341	=back	766	=back
342		767
		768	=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
343		769
		770	These symbols need to be defined before including F<ecb.h> the first time.
		771
		772	=over 4
		773
		774	=item ECB_NO_THREADS
		775
		776	If F<ecb.h> is never used from multiple threads, then this symbol can
		777	be defined, in which case memory fences (and similar constructs) are
		778	completely removed, leading to more efficient code and fewer dependencies.
		779
		780	Setting this symbol to a true value implies C<ECB_NO_SMP>.
		781
		782	=item ECB_NO_SMP
		783
		784	The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
		785	multiple threads, but never concurrently (e.g. if the system the program
		786	runs on has only a single CPU with a single core, no hyperthreading and so
		787	on), then this symbol can be defined, leading to more efficient code and
		788	fewer dependencies.
		789
		790	=item ECB_NO_LIBM
		791
		792	When defined to C<1>, do not export any functions that might introduce
		793	dependencies on the math library (usually called F<-lm>) - these are
		794	marked with [-UECB_NO_LIBM].
		795
		796	=back
		797
		798

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Revision 1.15 by root, Thu May 26 23:26:29 2011 UTC vs.
Revision 1.58 by root, Thu Oct 16 15:01:54 2014 UTC