libecb/ecb.pod

=head1 LIBECB - e-C-Builtins

=head2 ABOUT LIBECB

Libecb is currently a simple header file that doesn't require any
configuration to use or include in your project.

It's part of the e-suite of libraries, other members of which include
libev and libeio.

Its homepage can be found here:

    http://software.schmorp.de/pkg/libecb

It mainly provides a number of wrappers around GCC built-ins, together
with replacement functions for other compilers. In addition to this,
it provides a number of other lowlevel C utilities, such as endianness
detection, byte swapping or bit rotations.

Or in other words, things that should be built-in into any standard C
system, but aren't.

More might come.

=head2 ABOUT THE HEADER

At the moment, all you have to do is copy F<ecb.h> somewhere where your
compiler can find it and include it:

   #include <ecb.h>

The header should work fine for both C and C++ compilation, and gives you
all of F<inttypes.h> in addition to the ECB symbols.

There are currently no object files to link to - future versions might
come with an (optional) object code library to link against, to reduce
code size or gain access to additional features.

It also currently includes everything from F<inttypes.h>.

=head2 ABOUT THIS MANUAL / CONVENTIONS

This manual mainly describes each (public) function available after
including the F<ecb.h> header. The header might define other symbols than
these, but these are not part of the public API, and not supported in any
way.

When the manual mentions a "function" then this could be defined either as
as inline function, a macro, or an external symbol.

When functions use a concrete standard type, such as C<int> or
C<uint32_t>, then the corresponding function works only with that type. If
only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
the corresponding function relies on C to implement the correct types, and
is usually implemented as a macro. Specifically, a "bool" in this manual
refers to any kind of boolean value, not a specific type.

=head2 GCC ATTRIBUTES

blabla where to put, what others

=over 4

=item ecb_attribute ((attrs...))

A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
nothing on other compilers, so the effect is that only GCC sees these.

Example: use the C<deprecated> attribute on a function.

  ecb_attribute((__deprecated__)) void
  do_not_use_me_anymore (void);

=item ecb_unused

Marks a function or a variable as "unused", which simply suppresses a
warning by GCC when it detects it as unused. This is useful when you e.g.
declare a variable but do not always use it:

  {
    int var ecb_unused;

    #ifdef SOMECONDITION
       var = ...;
       return var;
    #else
       return 0;
    #endif
  }

=item ecb_noinline

Prevent a function from being inlined - it might be optimised away, but
not inlined into other functions. This is useful if you know your function
is rarely called and large enough for inlining not to be helpful.

=item ecb_noreturn

Marks a function as "not returning, ever". Some typical functions that
don't return are C<exit> or C<abort> (which really works hard to not
return), and now you can make your own:

   ecb_noreturn void
   my_abort (const char *errline)
   {
     puts (errline);
     abort ();
   }

In this case, the compiler would probbaly be smart enough to decude it on
it's own, so this is mainly useful for declarations.

=item ecb_const

Declares that the function only depends on the values of it's arguments,
much like a mathematical function. It specifically does not read or write
any memory any arguments might point to, global variables, or call any
non-const functions. It also must not have any side effects.

Such a function can be optimised much more aggressively by the compiler -
for example, multiple calls with the same arguments can be optimised into
a single call, which wouldn't be possible if the compiler would have to
expect any side effects.

It is best suited for functions in the sense of mathematical functions,
such as a function return the square root of its input argument.

Not suited would be a function that calculates the hash of some memory
area you pass in, prints some messages or looks at a global variable to
decide on rounding.

See C<ecb_pure> for a slightly less restrictive class of functions.

=item ecb_pure

Similar to C<ecb_const>, declares a function that has no side
effects. Unlike C<ecb_const>, the function is allowed to examine global
variables and any other memory areas (such as the ones passed to it via
pointers).

While these functions cannot be optimised as aggressively as C<ecb_const>
functions, they can still be optimised away in many occasions, and the
compiler has more freedom in moving calls to them around.

Typical examples for such functions would be C<strlen> or C<memcmp>. A
function that calculates the MD5 sum of some input and updates some MD5
state passed as argument would I<NOT> be pure, however, as it would modify
some memory area that is not the return value.

=item ecb_hot

This declares a function as "hot" with regards to the cache - the function
is used so often, that it is very beneficial to keep it in the cache if
possible.

The compiler reacts by trying to place hot functions near to each other in
memory.

Whether a function is hot or not often depend son the whole program,
and less on the function itself. C<ecb_cold> is likely more useful in
practise.

=item ecb_cold

The opposite of C<ecb_hot> - declares a function as "cold" with regards to
the cache, or in other words, this function is not called often, or not at
speed-critical times, and keeping it in the cache might be a waste of said
cache.

In addition to placing cold functions together (or at least away from hot
functions), this knowledge can be used in other ways, for example, the
function will be optimised for size, as opposed to speed, and codepaths
leading to calls to those functions can automatically be marked as if
C<ecb_unlikel> had been used to reach them.

Good examples for such functions would be error reporting functions, or
functions only called in exceptional or rare cases.

=item ecb_artificial

Declares the function as "artificial", in this case meaning that this
function is not really mean to be a function, but more like an accessor
- many methods in C++ classes are mere accessor functions, and having a
crash reported in such a method, or single-stepping through them, is not
usually so helpful, especially when it's inlined to just a few instructions.

Marking them as artificial will instruct the debugger about just this,
leading to happier debugging and thus happier lives.

Example: in some kind of smart-pointer class, mark the pointer accessor as
artificial, so that the whole class acts more like a pointer and less like
some C++ abstraction monster.

  template<typename T>
  struct my_smart_ptr
  {
    T *value;

    ecb_artificial
    operator T *()
    {
      return value;
    }
  };

=back

=head2 OPTIMISATION HINTS

=over 4

=item bool ecb_is_constant(expr)

Returns true iff the expression can be deduced to be a compile-time
constant, and false otherwise.

For example, when you have a C<rndm16> function that returns a 16 bit
random number, and you have a function that maps this to a range from
0..n-1, then you could use this inline function in a header file:

  ecb_inline uint32_t
  rndm (uint32_t n)
  {
    return (n * (uint32_t)rndm16 ()) >> 16;
  }

However, for powers of two, you could use a normal mask, but that is only
worth it if, at compile time, you can detect this case. This is the case
when the passed number is a constant and also a power of two (C<n & (n -
1) == 0>):

  ecb_inline uint32_t
  rndm (uint32_t n)
  {
    return is_constant (n) && !(n & (n - 1))
      ? rndm16 () & (num - 1)
      : (n * (uint32_t)rndm16 ()) >> 16;
  }

=item bool ecb_expect (expr, value)

Evaluates C<expr> and returns it. In addition, it tells the compiler that
the C<expr> evaluates to C<value> a lot, which can be used for static
branch optimisations.

Usually, you want to use the more intuitive C<ecb_likely> and
C<ecb_unlikely> functions instead.

=item bool ecb_likely (cond)

=item bool ecb_unlikely (cond)

These two functions expect a expression that is true or false and return
C<1> or C<0>, respectively, so when used in the condition of an C<if> or
other conditional statement, it will not change the program:

  /* these two do the same thing */
  if (some_condition) ...;
  if (ecb_likely (some_condition)) ...;

However, by using C<ecb_likely>, you tell the compiler that the condition
is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
true).

For example, when you check for a null pointer and expect this to be a
rare, exceptional, case, then use C<ecb_unlikely>:

  void my_free (void *ptr)
  {
    if (ecb_unlikely (ptr == 0))
      return;
  }

Consequent use of these functions to mark away exceptional cases or to
tell the compiler what the hot path through a function is can increase
performance considerably.

A very good example is in a function that reserves more space for some
memory block (for example, inside an implementation of a string stream) -
each time something is added, you have to check for a buffer overrun, but
you expect that most checks will turn out to be false:

  /* make sure we have "size" extra room in our buffer */
  ecb_inline void
  reserve (int size)
  {
    if (ecb_unlikely (current + size > end))
      real_reserve_method (size); /* presumably noinline */
  }

=item bool ecb_assume (cond)

Try to tell the compiler that some condition is true, even if it's not
obvious.

This can be used to teach the compiler about invariants or other
conditions that might improve code generation, but which are impossible to
deduce form the code itself.

For example, the example reservation function from the C<ecb_unlikely>
description could be written thus (only C<ecb_assume> was added):

  ecb_inline void
  reserve (int size)
  {
    if (ecb_unlikely (current + size > end))
      real_reserve_method (size); /* presumably noinline */

    ecb_assume (current + size <= end);
  }

If you then call this function twice, like this:

  reserve (10);
  reserve (1);

Then the compiler I<might> be able to optimise out the second call
completely, as it knows that C<< current + 1 > end >> is false and the
call will never be executed.

=item bool ecb_unreachable ()

This function does nothing itself, except tell the compiler that it will
never be executed. Apart from suppressing a warning in some cases, this
function can be used to implement C<ecb_assume> or similar functions.

=item bool ecb_prefetch (addr, rw, locality)

Tells the compiler to try to prefetch memory at the given C<addr>ess
for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
C<0> means that there will only be one access later, C<3> means that
the data will likely be accessed very often, and values in between mean
something... in between. The memory pointed to by the address does not
need to be accessible (it could be a null pointer for example), but C<rw>
and C<locality> must be compile-time constants.

An obvious way to use this is to prefetch some data far away, in a big
array you loop over. This prefetches memory some 128 array elements later,
in the hope that it will be ready when the CPU arrives at that location.

  int sum = 0;

  for (i = 0; i < N; ++i)
    {
      sum += arr [i]
      ecb_prefetch (arr + i + 128, 0, 0);
    }

It's hard to predict how far to prefetch, and most CPUs that can prefetch
are often good enough to predict this kind of behaviour themselves. It
gets more interesting with linked lists, especially when you do some fair
processing on each list element:

  for (node *n = start; n; n = n->next)
    {
      ecb_prefetch (n->next, 0, 0);
      ... do medium amount of work with *n
    }

After processing the node, (part of) the next node might already be in
cache.

=back

=head2 BIT FIDDLING / BITSTUFFS

=over 4

=item bool ecb_big_endian ()

=item bool ecb_little_endian ()

These two functions return true if the byte order is big endian
(most-significant byte first) or little endian (least-significant byte
first) respectively.

=item int ecb_ctz32 (uint32_t x)

Returns the index of the least significant bit set in C<x> (or
equivalently the number of bits set to 0 before the least significant
bit set), starting from 0. If C<x> is 0 the result is undefined. A
common use case is to compute the integer binary logarithm, i.e.,
floor(log2(n)). For example:

  ecb_ctz32 (3) = 0
  ecb_ctz32 (6) = 1

=item int ecb_popcount32 (uint32_t x)

Returns the number of bits set to 1 in C<x>. For example:

  ecb_popcount32 (7) = 3
  ecb_popcount32 (255) = 8

=item uint32_t ecb_bswap16 (uint32_t x)

=item uint32_t ecb_bswap32 (uint32_t x)

These two functions return the value of the 16-bit (32-bit) variable
C<x> after reversing the order of bytes.

=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)

=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)

These two functions return the value of C<x> after shifting all the bits
by C<count> positions to the right or left respectively.

=back

=head2 ARITHMETIC

=over 4

=item x = ecb_mod (m, n)

Returns the positive remainder of the modulo operation between C<m> and
C<n>. Unlike the C modulo operator C<%>, this function ensures that the
return value is always positive).

C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
negatable, that is, both C<m> and C<-m> must be representable in its
type.

=back

=head2 UTILITY

=over 4

=item element_count = ecb_array_length (name) [MACRO]

Returns the number of elements in the array C<name>. For example:

  int primes[] = { 2, 3, 5, 7, 11 };
  int sum = 0;

  for (i = 0; i < ecb_array_length (primes); i++)
    sum += primes [i];

=back


Revision:	1.18
Committed:	Fri May 27 00:01:28 2011 UTC (13 years, 1 month ago) by root
Branch:	MAIN
Changes since 1.17:	+1 -1 lines
Log Message:	* empty log message *
#	Content
1	=head1 LIBECB - e-C-Builtins
2
3	=head2 ABOUT LIBECB
4
5	Libecb is currently a simple header file that doesn't require any
6	configuration to use or include in your project.
7
8	It's part of the e-suite of libraries, other members of which include
9	libev and libeio.
10
11	Its homepage can be found here:
12
13	http://software.schmorp.de/pkg/libecb
14
15	It mainly provides a number of wrappers around GCC built-ins, together
16	with replacement functions for other compilers. In addition to this,
17	it provides a number of other lowlevel C utilities, such as endianness
18	detection, byte swapping or bit rotations.
19
20	Or in other words, things that should be built-in into any standard C
21	system, but aren't.
22
23	More might come.
24
25	=head2 ABOUT THE HEADER
26
27	At the moment, all you have to do is copy F<ecb.h> somewhere where your
28	compiler can find it and include it:
29
30	#include <ecb.h>
31
32	The header should work fine for both C and C++ compilation, and gives you
33	all of F<inttypes.h> in addition to the ECB symbols.
34
35	There are currently no object files to link to - future versions might
36	come with an (optional) object code library to link against, to reduce
37	code size or gain access to additional features.
38
39	It also currently includes everything from F<inttypes.h>.
40
41	=head2 ABOUT THIS MANUAL / CONVENTIONS
42
43	This manual mainly describes each (public) function available after
44	including the F<ecb.h> header. The header might define other symbols than
45	these, but these are not part of the public API, and not supported in any
46	way.
47
48	When the manual mentions a "function" then this could be defined either as
49	as inline function, a macro, or an external symbol.
50
51	When functions use a concrete standard type, such as C<int> or
52	C<uint32_t>, then the corresponding function works only with that type. If
53	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	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
58	=head2 GCC ATTRIBUTES
59
60	blabla where to put, what others
61
62	=over 4
63
64	=item ecb_attribute ((attrs...))
65
66	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
67	nothing on other compilers, so the effect is that only GCC sees these.
68
69	Example: use the C<deprecated> attribute on a function.
70
71	ecb_attribute((__deprecated__)) void
72	do_not_use_me_anymore (void);
73
74	=item ecb_unused
75
76	Marks a function or a variable as "unused", which simply suppresses a
77	warning by GCC when it detects it as unused. This is useful when you e.g.
78	declare a variable but do not always use it:
79
80	{
81	int var ecb_unused;
82
83	#ifdef SOMECONDITION
84	var = ...;
85	return var;
86	#else
87	return 0;
88	#endif
89	}
90
91	=item ecb_noinline
92
93	Prevent a function from being inlined - it might be optimised away, but
94	not inlined into other functions. This is useful if you know your function
95	is rarely called and large enough for inlining not to be helpful.
96
97	=item ecb_noreturn
98
99	Marks a function as "not returning, ever". Some typical functions that
100	don't return are C<exit> or C<abort> (which really works hard to not
101	return), and now you can make your own:
102
103	ecb_noreturn void
104	my_abort (const char *errline)
105	{
106	puts (errline);
107	abort ();
108	}
109
110	In this case, the compiler would probbaly be smart enough to decude it on
111	it's own, so this is mainly useful for declarations.
112
113	=item ecb_const
114
115	Declares that the function only depends on the values of it's arguments,
116	much like a mathematical function. It specifically does not read or write
117	any memory any arguments might point to, global variables, or call any
118	non-const functions. It also must not have any side effects.
119
120	Such a function can be optimised much more aggressively by the compiler -
121	for example, multiple calls with the same arguments can be optimised into
122	a single call, which wouldn't be possible if the compiler would have to
123	expect any side effects.
124
125	It is best suited for functions in the sense of mathematical functions,
126	such as a function return the square root of its input argument.
127
128	Not suited would be a function that calculates the hash of some memory
129	area you pass in, prints some messages or looks at a global variable to
130	decide on rounding.
131
132	See C<ecb_pure> for a slightly less restrictive class of functions.
133
134	=item ecb_pure
135
136	Similar to C<ecb_const>, declares a function that has no side
137	effects. Unlike C<ecb_const>, the function is allowed to examine global
138	variables and any other memory areas (such as the ones passed to it via
139	pointers).
140
141	While these functions cannot be optimised as aggressively as C<ecb_const>
142	functions, they can still be optimised away in many occasions, and the
143	compiler has more freedom in moving calls to them around.
144
145	Typical examples for such functions would be C<strlen> or C<memcmp>. A
146	function that calculates the MD5 sum of some input and updates some MD5
147	state passed as argument would I<NOT> be pure, however, as it would modify
148	some memory area that is not the return value.
149
150	=item ecb_hot
151
152	This declares a function as "hot" with regards to the cache - the function
153	is used so often, that it is very beneficial to keep it in the cache if
154	possible.
155
156	The compiler reacts by trying to place hot functions near to each other in
157	memory.
158
159	Whether a function is hot or not often depend son the whole program,
160	and less on the function itself. C<ecb_cold> is likely more useful in
161	practise.
162
163	=item ecb_cold
164
165	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
166	the cache, or in other words, this function is not called often, or not at
167	speed-critical times, and keeping it in the cache might be a waste of said
168	cache.
169
170	In addition to placing cold functions together (or at least away from hot
171	functions), this knowledge can be used in other ways, for example, the
172	function will be optimised for size, as opposed to speed, and codepaths
173	leading to calls to those functions can automatically be marked as if
174	C<ecb_unlikel> had been used to reach them.
175
176	Good examples for such functions would be error reporting functions, or
177	functions only called in exceptional or rare cases.
178
179	=item ecb_artificial
180
181	Declares the function as "artificial", in this case meaning that this
182	function is not really mean to be a function, but more like an accessor
183	- many methods in C++ classes are mere accessor functions, and having a
184	crash reported in such a method, or single-stepping through them, is not
185	usually so helpful, especially when it's inlined to just a few instructions.
186
187	Marking them as artificial will instruct the debugger about just this,
188	leading to happier debugging and thus happier lives.
189
190	Example: in some kind of smart-pointer class, mark the pointer accessor as
191	artificial, so that the whole class acts more like a pointer and less like
192	some C++ abstraction monster.
193
194	template<typename T>
195	struct my_smart_ptr
196	{
197	T *value;
198
199	ecb_artificial
200	operator T *()
201	{
202	return value;
203	}
204	};
205
206	=back
207
208	=head2 OPTIMISATION HINTS
209
210	=over 4
211
212	=item bool ecb_is_constant(expr)
213
214	Returns true iff the expression can be deduced to be a compile-time
215	constant, and false otherwise.
216
217	For example, when you have a C<rndm16> function that returns a 16 bit
218	random number, and you have a function that maps this to a range from
219	0..n-1, then you could use this inline function in a header file:
220
221	ecb_inline uint32_t
222	rndm (uint32_t n)
223	{
224	return (n * (uint32_t)rndm16 ()) >> 16;
225	}
226
227	However, for powers of two, you could use a normal mask, but that is only
228	worth it if, at compile time, you can detect this case. This is the case
229	when the passed number is a constant and also a power of two (C<n & (n -
230	1) == 0>):
231
232	ecb_inline uint32_t
233	rndm (uint32_t n)
234	{
235	return is_constant (n) && !(n & (n - 1))
236	? rndm16 () & (num - 1)
237	: (n * (uint32_t)rndm16 ()) >> 16;
238	}
239
240	=item bool ecb_expect (expr, value)
241
242	Evaluates C<expr> and returns it. In addition, it tells the compiler that
243	the C<expr> evaluates to C<value> a lot, which can be used for static
244	branch optimisations.
245
246	Usually, you want to use the more intuitive C<ecb_likely> and
247	C<ecb_unlikely> functions instead.
248
249	=item bool ecb_likely (cond)
250
251	=item bool ecb_unlikely (cond)
252
253	These two functions expect a expression that is true or false and return
254	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
255	other conditional statement, it will not change the program:
256
257	/* these two do the same thing */
258	if (some_condition) ...;
259	if (ecb_likely (some_condition)) ...;
260
261	However, by using C<ecb_likely>, you tell the compiler that the condition
262	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
263	true).
264
265	For example, when you check for a null pointer and expect this to be a
266	rare, exceptional, case, then use C<ecb_unlikely>:
267
268	void my_free (void *ptr)
269	{
270	if (ecb_unlikely (ptr == 0))
271	return;
272	}
273
274	Consequent use of these functions to mark away exceptional cases or to
275	tell the compiler what the hot path through a function is can increase
276	performance considerably.
277
278	A very good example is in a function that reserves more space for some
279	memory block (for example, inside an implementation of a string stream) -
280	each time something is added, you have to check for a buffer overrun, but
281	you expect that most checks will turn out to be false:
282
283	/* make sure we have "size" extra room in our buffer */
284	ecb_inline void
285	reserve (int size)
286	{
287	if (ecb_unlikely (current + size > end))
288	real_reserve_method (size); /* presumably noinline */
289	}
290
291	=item bool ecb_assume (cond)
292
293	Try to tell the compiler that some condition is true, even if it's not
294	obvious.
295
296	This can be used to teach the compiler about invariants or other
297	conditions that might improve code generation, but which are impossible to
298	deduce form the code itself.
299
300	For example, the example reservation function from the C<ecb_unlikely>
301	description could be written thus (only C<ecb_assume> was added):
302
303	ecb_inline void
304	reserve (int size)
305	{
306	if (ecb_unlikely (current + size > end))
307	real_reserve_method (size); /* presumably noinline */
308
309	ecb_assume (current + size <= end);
310	}
311
312	If you then call this function twice, like this:
313
314	reserve (10);
315	reserve (1);
316
317	Then the compiler I<might> be able to optimise out the second call
318	completely, as it knows that C<< current + 1 > end >> is false and the
319	call will never be executed.
320
321	=item bool ecb_unreachable ()
322
323	This function does nothing itself, except tell the compiler that it will
324	never be executed. Apart from suppressing a warning in some cases, this
325	function can be used to implement C<ecb_assume> or similar functions.
326
327	=item bool ecb_prefetch (addr, rw, locality)
328
329	Tells the compiler to try to prefetch memory at the given C<addr>ess
330	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
331	C<0> means that there will only be one access later, C<3> means that
332	the data will likely be accessed very often, and values in between mean
333	something... in between. The memory pointed to by the address does not
334	need to be accessible (it could be a null pointer for example), but C<rw>
335	and C<locality> must be compile-time constants.
336
337	An obvious way to use this is to prefetch some data far away, in a big
338	array you loop over. This prefetches memory some 128 array elements later,
339	in the hope that it will be ready when the CPU arrives at that location.
340
341	int sum = 0;
342
343	for (i = 0; i < N; ++i)
344	{
345	sum += arr [i]
346	ecb_prefetch (arr + i + 128, 0, 0);
347	}
348
349	It's hard to predict how far to prefetch, and most CPUs that can prefetch
350	are often good enough to predict this kind of behaviour themselves. It
351	gets more interesting with linked lists, especially when you do some fair
352	processing on each list element:
353
354	for (node *n = start; n; n = n->next)
355	{
356	ecb_prefetch (n->next, 0, 0);
357	... do medium amount of work with *n
358	}
359
360	After processing the node, (part of) the next node might already be in
361	cache.
362
363	=back
364
365	=head2 BIT FIDDLING / BITSTUFFS
366
367	=over 4
368
369	=item bool ecb_big_endian ()
370
371	=item bool ecb_little_endian ()
372
373	These two functions return true if the byte order is big endian
374	(most-significant byte first) or little endian (least-significant byte
375	first) respectively.
376
377	=item int ecb_ctz32 (uint32_t x)
378
379	Returns the index of the least significant bit set in C<x> (or
380	equivalently the number of bits set to 0 before the least significant
381	bit set), starting from 0. If C<x> is 0 the result is undefined. A
382	common use case is to compute the integer binary logarithm, i.e.,
383	floor(log2(n)). For example:
384
385	ecb_ctz32 (3) = 0
386	ecb_ctz32 (6) = 1
387
388	=item int ecb_popcount32 (uint32_t x)
389
390	Returns the number of bits set to 1 in C<x>. For example:
391
392	ecb_popcount32 (7) = 3
393	ecb_popcount32 (255) = 8
394
395	=item uint32_t ecb_bswap16 (uint32_t x)
396
397	=item uint32_t ecb_bswap32 (uint32_t x)
398
399	These two functions return the value of the 16-bit (32-bit) variable
400	C<x> after reversing the order of bytes.
401
402	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
403
404	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
405
406	These two functions return the value of C<x> after shifting all the bits
407	by C<count> positions to the right or left respectively.
408
409	=back
410
411	=head2 ARITHMETIC
412
413	=over 4
414
415	=item x = ecb_mod (m, n)
416
417	Returns the positive remainder of the modulo operation between C<m> and
418	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
419	return value is always positive).
420
421	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
422	negatable, that is, both C<m> and C<-m> must be representable in its
423	type.
424
425	=back
426
427	=head2 UTILITY
428
429	=over 4
430
431	=item element_count = ecb_array_length (name) [MACRO]
432
433	Returns the number of elements in the array C<name>. For example:
434
435	int primes[] = { 2, 3, 5, 7, 11 };
436	int sum = 0;
437
438	for (i = 0; i < ecb_array_length (primes); i++)
439	sum += primes [i];
440
441	=back
442
443