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

A major part of libecb deals with GCC attributes. These are additional
attributes that you cna assign to functions, variables and sometimes even
types - much like C<const> or C<volatile> in C.

While GCC allows declarations to show up in many surprising places,
but not in many expeted places, the safest way is to put attribute
declarations before the whole declaration:

   ecb_const int mysqrt (int a);
   ecb_unused int i;

For variables, it is often nicer to put the attribute after the name, and
avoid multiple declarations using commas:

   int i ecb_unused;

=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 probably be smart enough to deduce it on
its own, so this is mainly useful for declarations.

=item ecb_const

Declares that the function only depends on the values of its 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 returning 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 depends on 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_unlikely> 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) value C<x>
after reversing the order of bytes (0x11223344 becomes 0x44332211).

=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.

Current GCC versions understand these functions and usually compile them
to "optimal" code (e.g. a single C<roll> on x86).

=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 - ISO C guarantees very little when
negative numbers are used with C<%>.

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:	1.21
Committed:	Fri May 27 00:14:10 2011 UTC (13 years, 1 month ago) by root
Branch:	MAIN
Changes since 1.20:	+2 -2 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	A major part of libecb deals with GCC attributes. These are additional
61	attributes that you cna assign to functions, variables and sometimes even
62	types - much like C<const> or C<volatile> in C.
63
64	While GCC allows declarations to show up in many surprising places,
65	but not in many expeted places, the safest way is to put attribute
66	declarations before the whole declaration:
67
68	ecb_const int mysqrt (int a);
69	ecb_unused int i;
70
71	For variables, it is often nicer to put the attribute after the name, and
72	avoid multiple declarations using commas:
73
74	int i ecb_unused;
75
76	=over 4
77
78	=item ecb_attribute ((attrs...))
79
80	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
81	nothing on other compilers, so the effect is that only GCC sees these.
82
83	Example: use the C<deprecated> attribute on a function.
84
85	ecb_attribute((__deprecated__)) void
86	do_not_use_me_anymore (void);
87
88	=item ecb_unused
89
90	Marks a function or a variable as "unused", which simply suppresses a
91	warning by GCC when it detects it as unused. This is useful when you e.g.
92	declare a variable but do not always use it:
93
94	{
95	int var ecb_unused;
96
97	#ifdef SOMECONDITION
98	var = ...;
99	return var;
100	#else
101	return 0;
102	#endif
103	}
104
105	=item ecb_noinline
106
107	Prevent a function from being inlined - it might be optimised away, but
108	not inlined into other functions. This is useful if you know your function
109	is rarely called and large enough for inlining not to be helpful.
110
111	=item ecb_noreturn
112
113	Marks a function as "not returning, ever". Some typical functions that
114	don't return are C<exit> or C<abort> (which really works hard to not
115	return), and now you can make your own:
116
117	ecb_noreturn void
118	my_abort (const char *errline)
119	{
120	puts (errline);
121	abort ();
122	}
123
124	In this case, the compiler would probably be smart enough to deduce it on
125	its own, so this is mainly useful for declarations.
126
127	=item ecb_const
128
129	Declares that the function only depends on the values of its arguments,
130	much like a mathematical function. It specifically does not read or write
131	any memory any arguments might point to, global variables, or call any
132	non-const functions. It also must not have any side effects.
133
134	Such a function can be optimised much more aggressively by the compiler -
135	for example, multiple calls with the same arguments can be optimised into
136	a single call, which wouldn't be possible if the compiler would have to
137	expect any side effects.
138
139	It is best suited for functions in the sense of mathematical functions,
140	such as a function returning the square root of its input argument.
141
142	Not suited would be a function that calculates the hash of some memory
143	area you pass in, prints some messages or looks at a global variable to
144	decide on rounding.
145
146	See C<ecb_pure> for a slightly less restrictive class of functions.
147
148	=item ecb_pure
149
150	Similar to C<ecb_const>, declares a function that has no side
151	effects. Unlike C<ecb_const>, the function is allowed to examine global
152	variables and any other memory areas (such as the ones passed to it via
153	pointers).
154
155	While these functions cannot be optimised as aggressively as C<ecb_const>
156	functions, they can still be optimised away in many occasions, and the
157	compiler has more freedom in moving calls to them around.
158
159	Typical examples for such functions would be C<strlen> or C<memcmp>. A
160	function that calculates the MD5 sum of some input and updates some MD5
161	state passed as argument would I<NOT> be pure, however, as it would modify
162	some memory area that is not the return value.
163
164	=item ecb_hot
165
166	This declares a function as "hot" with regards to the cache - the function
167	is used so often, that it is very beneficial to keep it in the cache if
168	possible.
169
170	The compiler reacts by trying to place hot functions near to each other in
171	memory.
172
173	Whether a function is hot or not often depends on the whole program,
174	and less on the function itself. C<ecb_cold> is likely more useful in
175	practise.
176
177	=item ecb_cold
178
179	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
180	the cache, or in other words, this function is not called often, or not at
181	speed-critical times, and keeping it in the cache might be a waste of said
182	cache.
183
184	In addition to placing cold functions together (or at least away from hot
185	functions), this knowledge can be used in other ways, for example, the
186	function will be optimised for size, as opposed to speed, and codepaths
187	leading to calls to those functions can automatically be marked as if
188	C<ecb_unlikely> had been used to reach them.
189
190	Good examples for such functions would be error reporting functions, or
191	functions only called in exceptional or rare cases.
192
193	=item ecb_artificial
194
195	Declares the function as "artificial", in this case meaning that this
196	function is not really mean to be a function, but more like an accessor
197	- many methods in C++ classes are mere accessor functions, and having a
198	crash reported in such a method, or single-stepping through them, is not
199	usually so helpful, especially when it's inlined to just a few instructions.
200
201	Marking them as artificial will instruct the debugger about just this,
202	leading to happier debugging and thus happier lives.
203
204	Example: in some kind of smart-pointer class, mark the pointer accessor as
205	artificial, so that the whole class acts more like a pointer and less like
206	some C++ abstraction monster.
207
208	template<typename T>
209	struct my_smart_ptr
210	{
211	T *value;
212
213	ecb_artificial
214	operator T *()
215	{
216	return value;
217	}
218	};
219
220	=back
221
222	=head2 OPTIMISATION HINTS
223
224	=over 4
225
226	=item bool ecb_is_constant(expr)
227
228	Returns true iff the expression can be deduced to be a compile-time
229	constant, and false otherwise.
230
231	For example, when you have a C<rndm16> function that returns a 16 bit
232	random number, and you have a function that maps this to a range from
233	0..n-1, then you could use this inline function in a header file:
234
235	ecb_inline uint32_t
236	rndm (uint32_t n)
237	{
238	return (n * (uint32_t)rndm16 ()) >> 16;
239	}
240
241	However, for powers of two, you could use a normal mask, but that is only
242	worth it if, at compile time, you can detect this case. This is the case
243	when the passed number is a constant and also a power of two (C<n & (n -
244	1) == 0>):
245
246	ecb_inline uint32_t
247	rndm (uint32_t n)
248	{
249	return is_constant (n) && !(n & (n - 1))
250	? rndm16 () & (num - 1)
251	: (n * (uint32_t)rndm16 ()) >> 16;
252	}
253
254	=item bool ecb_expect (expr, value)
255
256	Evaluates C<expr> and returns it. In addition, it tells the compiler that
257	the C<expr> evaluates to C<value> a lot, which can be used for static
258	branch optimisations.
259
260	Usually, you want to use the more intuitive C<ecb_likely> and
261	C<ecb_unlikely> functions instead.
262
263	=item bool ecb_likely (cond)
264
265	=item bool ecb_unlikely (cond)
266
267	These two functions expect a expression that is true or false and return
268	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
269	other conditional statement, it will not change the program:
270
271	/* these two do the same thing */
272	if (some_condition) ...;
273	if (ecb_likely (some_condition)) ...;
274
275	However, by using C<ecb_likely>, you tell the compiler that the condition
276	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
277	true).
278
279	For example, when you check for a null pointer and expect this to be a
280	rare, exceptional, case, then use C<ecb_unlikely>:
281
282	void my_free (void *ptr)
283	{
284	if (ecb_unlikely (ptr == 0))
285	return;
286	}
287
288	Consequent use of these functions to mark away exceptional cases or to
289	tell the compiler what the hot path through a function is can increase
290	performance considerably.
291
292	A very good example is in a function that reserves more space for some
293	memory block (for example, inside an implementation of a string stream) -
294	each time something is added, you have to check for a buffer overrun, but
295	you expect that most checks will turn out to be false:
296
297	/* make sure we have "size" extra room in our buffer */
298	ecb_inline void
299	reserve (int size)
300	{
301	if (ecb_unlikely (current + size > end))
302	real_reserve_method (size); /* presumably noinline */
303	}
304
305	=item bool ecb_assume (cond)
306
307	Try to tell the compiler that some condition is true, even if it's not
308	obvious.
309
310	This can be used to teach the compiler about invariants or other
311	conditions that might improve code generation, but which are impossible to
312	deduce form the code itself.
313
314	For example, the example reservation function from the C<ecb_unlikely>
315	description could be written thus (only C<ecb_assume> was added):
316
317	ecb_inline void
318	reserve (int size)
319	{
320	if (ecb_unlikely (current + size > end))
321	real_reserve_method (size); /* presumably noinline */
322
323	ecb_assume (current + size <= end);
324	}
325
326	If you then call this function twice, like this:
327
328	reserve (10);
329	reserve (1);
330
331	Then the compiler I<might> be able to optimise out the second call
332	completely, as it knows that C<< current + 1 > end >> is false and the
333	call will never be executed.
334
335	=item bool ecb_unreachable ()
336
337	This function does nothing itself, except tell the compiler that it will
338	never be executed. Apart from suppressing a warning in some cases, this
339	function can be used to implement C<ecb_assume> or similar functions.
340
341	=item bool ecb_prefetch (addr, rw, locality)
342
343	Tells the compiler to try to prefetch memory at the given C<addr>ess
344	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
345	C<0> means that there will only be one access later, C<3> means that
346	the data will likely be accessed very often, and values in between mean
347	something... in between. The memory pointed to by the address does not
348	need to be accessible (it could be a null pointer for example), but C<rw>
349	and C<locality> must be compile-time constants.
350
351	An obvious way to use this is to prefetch some data far away, in a big
352	array you loop over. This prefetches memory some 128 array elements later,
353	in the hope that it will be ready when the CPU arrives at that location.
354
355	int sum = 0;
356
357	for (i = 0; i < N; ++i)
358	{
359	sum += arr [i]
360	ecb_prefetch (arr + i + 128, 0, 0);
361	}
362
363	It's hard to predict how far to prefetch, and most CPUs that can prefetch
364	are often good enough to predict this kind of behaviour themselves. It
365	gets more interesting with linked lists, especially when you do some fair
366	processing on each list element:
367
368	for (node *n = start; n; n = n->next)
369	{
370	ecb_prefetch (n->next, 0, 0);
371	... do medium amount of work with *n
372	}
373
374	After processing the node, (part of) the next node might already be in
375	cache.
376
377	=back
378
379	=head2 BIT FIDDLING / BITSTUFFS
380
381	=over 4
382
383	=item bool ecb_big_endian ()
384
385	=item bool ecb_little_endian ()
386
387	These two functions return true if the byte order is big endian
388	(most-significant byte first) or little endian (least-significant byte
389	first) respectively.
390
391	=item int ecb_ctz32 (uint32_t x)
392
393	Returns the index of the least significant bit set in C<x> (or
394	equivalently the number of bits set to 0 before the least significant
395	bit set), starting from 0. If C<x> is 0 the result is undefined. A
396	common use case is to compute the integer binary logarithm, i.e.,
397	floor(log2(n)). For example:
398
399	ecb_ctz32 (3) = 0
400	ecb_ctz32 (6) = 1
401
402	=item int ecb_popcount32 (uint32_t x)
403
404	Returns the number of bits set to 1 in C<x>. For example:
405
406	ecb_popcount32 (7) = 3
407	ecb_popcount32 (255) = 8
408
409	=item uint32_t ecb_bswap16 (uint32_t x)
410
411	=item uint32_t ecb_bswap32 (uint32_t x)
412
413	These two functions return the value of the 16-bit (32-bit) value C<x>
414	after reversing the order of bytes (0x11223344 becomes 0x44332211).
415
416	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
417
418	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
419
420	These two functions return the value of C<x> after shifting all the bits
421	by C<count> positions to the right or left respectively.
422
423	Current GCC versions understand these functions and usually compile them
424	to "optimal" code (e.g. a single C<roll> on x86).
425
426	=back
427
428	=head2 ARITHMETIC
429
430	=over 4
431
432	=item x = ecb_mod (m, n)
433
434	Returns the positive remainder of the modulo operation between C<m> and
435	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
436	return value is always positive - ISO C guarantees very little when
437	negative numbers are used with C<%>.
438
439	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
440	negatable, that is, both C<m> and C<-m> must be representable in its
441	type.
442
443	=back
444
445	=head2 UTILITY
446
447	=over 4
448
449	=item element_count = ecb_array_length (name) [MACRO]
450
451	Returns the number of elements in the array C<name>. For example:
452
453	int primes[] = { 2, 3, 5, 7, 11 };
454	int sum = 0;
455
456	for (i = 0; i < ecb_array_length (primes); i++)
457	sum += primes [i];
458
459	=back
460
461