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 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) 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:	1.19
Committed:	Fri May 27 00:04:05 2011 UTC (13 years ago) by sf-exg
Branch:	MAIN
Changes since 1.18:	+6 -6 lines
Log Message:	Fix typos.
#	User	Rev	Content
1	root	1.14	=head1 LIBECB - e-C-Builtins
2	root	1.3
3	root	1.14	=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	sf-exg	1.16	It's part of the e-suite of libraries, other members of which include
9	root	1.14	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	sf-exg	1.16	it provides a number of other lowlevel C utilities, such as endianness
18	root	1.14	detection, byte swapping or bit rotations.
19
20	root	1.17	Or in other words, things that should be built-in into any standard C
21	root	1.18	system, but aren't.
22	root	1.17
23	root	1.14	More might come.
24	root	1.3
25			=head2 ABOUT THE HEADER
26
27	root	1.14	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	sf-exg	1.16	There are currently no object files to link to - future versions might
36	root	1.14	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	root	1.1
58			=head2 GCC ATTRIBUTES
59
60	root	1.3	blabla where to put, what others
61
62	root	1.1	=over 4
63
64	root	1.2	=item ecb_attribute ((attrs...))
65	root	1.1
66	root	1.15	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	root	1.2
74	root	1.3	=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	root	1.15	{
81			int var ecb_unused;
82	root	1.3
83	root	1.15	#ifdef SOMECONDITION
84			var = ...;
85			return var;
86			#else
87			return 0;
88			#endif
89			}
90	root	1.3
91	root	1.2	=item ecb_noinline
92
93	root	1.9	Prevent a function from being inlined - it might be optimised away, but
94	root	1.3	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	root	1.2	=item ecb_noreturn
98
99	root	1.17	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	sf-exg	1.19	In this case, the compiler would probably be smart enough to deduce it on
111			its own, so this is mainly useful for declarations.
112	root	1.17
113	root	1.2	=item ecb_const
114
115	sf-exg	1.19	Declares that the function only depends on the values of its arguments,
116	root	1.17	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	sf-exg	1.19	such as a function returning the square root of its input argument.
127	root	1.17
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	root	1.2	=item ecb_pure
135
136	root	1.17	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	root	1.2	=item ecb_hot
151
152	root	1.17	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	sf-exg	1.19	Whether a function is hot or not often depends on the whole program,
160	root	1.17	and less on the function itself. C<ecb_cold> is likely more useful in
161			practise.
162
163	root	1.2	=item ecb_cold
164
165	root	1.17	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	sf-exg	1.19	C<ecb_unlikely> had been used to reach them.
175	root	1.17
176			Good examples for such functions would be error reporting functions, or
177			functions only called in exceptional or rare cases.
178
179	root	1.2	=item ecb_artificial
180
181	root	1.17	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	root	1.2	=back
207	root	1.1
208			=head2 OPTIMISATION HINTS
209
210			=over 4
211
212	root	1.14	=item bool ecb_is_constant(expr)
213	root	1.1
214	root	1.3	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	root	1.5	0..n-1, then you could use this inline function in a header file:
220	root	1.3
221			ecb_inline uint32_t
222			rndm (uint32_t n)
223			{
224	root	1.6	return (n * (uint32_t)rndm16 ()) >> 16;
225	root	1.3	}
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	root	1.6	: (n * (uint32_t)rndm16 ()) >> 16;
238	root	1.3	}
239
240	root	1.14	=item bool ecb_expect (expr, value)
241	root	1.1
242	root	1.7	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	root	1.1
246	root	1.7	Usually, you want to use the more intuitive C<ecb_likely> and
247			C<ecb_unlikely> functions instead.
248	root	1.1
249	root	1.15	=item bool ecb_likely (cond)
250	root	1.1
251	root	1.15	=item bool ecb_unlikely (cond)
252	root	1.1
253	root	1.7	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	sf-exg	1.11	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
263	root	1.7	true).
264
265	root	1.9	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	root	1.7
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	root	1.9	each time something is added, you have to check for a buffer overrun, but
281	root	1.7	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	root	1.14	=item bool ecb_assume (cond)
292	root	1.7
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	root	1.9	never be executed. Apart from suppressing a warning in some cases, this
325	root	1.7	function can be used to implement C<ecb_assume> or similar functions.
326
327	root	1.14	=item bool ecb_prefetch (addr, rw, locality)
328	root	1.7
329			Tells the compiler to try to prefetch memory at the given C<addr>ess
330	root	1.10	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
331	root	1.7	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	root	1.9	array you loop over. This prefetches memory some 128 array elements later,
339	root	1.7	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	root	1.1
363	root	1.2	=back
364	root	1.1
365			=head2 BIT FIDDLING / BITSTUFFS
366
367	root	1.4	=over 4
368
369	root	1.3	=item bool ecb_big_endian ()
370
371			=item bool ecb_little_endian ()
372
373	sf-exg	1.11	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	root	1.3	=item int ecb_ctz32 (uint32_t x)
378
379	sf-exg	1.11	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	root	1.15	ecb_ctz32 (3) = 0
386			ecb_ctz32 (6) = 1
387	sf-exg	1.11
388	root	1.3	=item int ecb_popcount32 (uint32_t x)
389
390	sf-exg	1.11	Returns the number of bits set to 1 in C<x>. For example:
391
392	root	1.15	ecb_popcount32 (7) = 3
393			ecb_popcount32 (255) = 8
394	sf-exg	1.11
395	root	1.8	=item uint32_t ecb_bswap16 (uint32_t x)
396
397	root	1.3	=item uint32_t ecb_bswap32 (uint32_t x)
398
399	sf-exg	1.13	These two functions return the value of the 16-bit (32-bit) variable
400			C<x> after reversing the order of bytes.
401
402	root	1.3	=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	sf-exg	1.11	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	root	1.3	=back
410	root	1.1
411			=head2 ARITHMETIC
412
413	root	1.3	=over 4
414
415	root	1.14	=item x = ecb_mod (m, n)
416	root	1.3
417	root	1.14	Returns the positive remainder of the modulo operation between C<m> and
418	sf-exg	1.16	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
419	root	1.14	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	sf-exg	1.11
425	root	1.3	=back
426	root	1.1
427			=head2 UTILITY
428
429	root	1.3	=over 4
430
431	root	1.8	=item element_count = ecb_array_length (name) [MACRO]
432	root	1.3
433	sf-exg	1.13	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	root	1.3	=back
442	root	1.1
443