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 rotating 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)

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:	1.23
Committed:	Fri May 27 01:35:46 2011 UTC (12 years, 11 months ago) by sf-exg
Branch:	MAIN
Changes since 1.22:	+2 -2 lines
Log Message:	Doc fixes.
#	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	1.20	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	root	1.3
76	root	1.1	=over 4
77
78	root	1.2	=item ecb_attribute ((attrs...))
79	root	1.1
80	root	1.15	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	root	1.2
88	root	1.3	=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	root	1.15	{
95			int var ecb_unused;
96	root	1.3
97	root	1.15	#ifdef SOMECONDITION
98			var = ...;
99			return var;
100			#else
101			return 0;
102			#endif
103			}
104	root	1.3
105	root	1.2	=item ecb_noinline
106
107	root	1.9	Prevent a function from being inlined - it might be optimised away, but
108	root	1.3	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	root	1.2	=item ecb_noreturn
112
113	root	1.17	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	sf-exg	1.19	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	root	1.17
127	root	1.2	=item ecb_const
128
129	sf-exg	1.19	Declares that the function only depends on the values of its arguments,
130	root	1.17	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	sf-exg	1.19	such as a function returning the square root of its input argument.
141	root	1.17
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	root	1.2	=item ecb_pure
149
150	root	1.17	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	root	1.2	=item ecb_hot
165
166	root	1.17	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	sf-exg	1.19	Whether a function is hot or not often depends on the whole program,
174	root	1.17	and less on the function itself. C<ecb_cold> is likely more useful in
175			practise.
176
177	root	1.2	=item ecb_cold
178
179	root	1.17	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	sf-exg	1.19	C<ecb_unlikely> had been used to reach them.
189	root	1.17
190			Good examples for such functions would be error reporting functions, or
191			functions only called in exceptional or rare cases.
192
193	root	1.2	=item ecb_artificial
194
195	root	1.17	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	root	1.2	=back
221	root	1.1
222			=head2 OPTIMISATION HINTS
223
224			=over 4
225
226	root	1.14	=item bool ecb_is_constant(expr)
227	root	1.1
228	root	1.3	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	root	1.5	0..n-1, then you could use this inline function in a header file:
234	root	1.3
235			ecb_inline uint32_t
236			rndm (uint32_t n)
237			{
238	root	1.6	return (n * (uint32_t)rndm16 ()) >> 16;
239	root	1.3	}
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	root	1.6	: (n * (uint32_t)rndm16 ()) >> 16;
252	root	1.3	}
253
254	root	1.14	=item bool ecb_expect (expr, value)
255	root	1.1
256	root	1.7	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	root	1.1
260	root	1.7	Usually, you want to use the more intuitive C<ecb_likely> and
261			C<ecb_unlikely> functions instead.
262	root	1.1
263	root	1.15	=item bool ecb_likely (cond)
264	root	1.1
265	root	1.15	=item bool ecb_unlikely (cond)
266	root	1.1
267	root	1.7	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	sf-exg	1.11	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
277	root	1.7	true).
278
279	root	1.9	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	root	1.7
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	root	1.9	each time something is added, you have to check for a buffer overrun, but
295	root	1.7	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	root	1.14	=item bool ecb_assume (cond)
306	root	1.7
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	root	1.9	never be executed. Apart from suppressing a warning in some cases, this
339	root	1.7	function can be used to implement C<ecb_assume> or similar functions.
340
341	root	1.14	=item bool ecb_prefetch (addr, rw, locality)
342	root	1.7
343			Tells the compiler to try to prefetch memory at the given C<addr>ess
344	root	1.10	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
345	root	1.7	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	root	1.9	array you loop over. This prefetches memory some 128 array elements later,
353	root	1.7	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	root	1.1
377	root	1.2	=back
378	root	1.1
379			=head2 BIT FIDDLING / BITSTUFFS
380
381	root	1.4	=over 4
382
383	root	1.3	=item bool ecb_big_endian ()
384
385			=item bool ecb_little_endian ()
386
387	sf-exg	1.11	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	root	1.3	=item int ecb_ctz32 (uint32_t x)
392
393	sf-exg	1.11	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	root	1.15	ecb_ctz32 (3) = 0
400			ecb_ctz32 (6) = 1
401	sf-exg	1.11
402	root	1.3	=item int ecb_popcount32 (uint32_t x)
403
404	sf-exg	1.11	Returns the number of bits set to 1 in C<x>. For example:
405
406	root	1.15	ecb_popcount32 (7) = 3
407			ecb_popcount32 (255) = 8
408	sf-exg	1.11
409	root	1.8	=item uint32_t ecb_bswap16 (uint32_t x)
410
411	root	1.3	=item uint32_t ecb_bswap32 (uint32_t x)
412
413	root	1.21	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	sf-exg	1.13
416	root	1.3	=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	root	1.22	These two functions return the value of C<x> after rotating all the bits
421	sf-exg	1.11	by C<count> positions to the right or left respectively.
422
423	root	1.20	Current GCC versions understand these functions and usually compile them
424			to "optimal" code (e.g. a single C<roll> on x86).
425
426	root	1.3	=back
427	root	1.1
428			=head2 ARITHMETIC
429
430	root	1.3	=over 4
431
432	root	1.14	=item x = ecb_mod (m, n)
433	root	1.3
434	root	1.14	Returns the positive remainder of the modulo operation between C<m> and
435	sf-exg	1.16	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
436	root	1.20	return value is always positive - ISO C guarantees very little when
437			negative numbers are used with C<%>.
438	root	1.14
439	sf-exg	1.23	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
440	root	1.14	negatable, that is, both C<m> and C<-m> must be representable in its
441			type.
442	sf-exg	1.11
443	root	1.3	=back
444	root	1.1
445			=head2 UTILITY
446
447	root	1.3	=over 4
448
449	sf-exg	1.23	=item element_count = ecb_array_length (name)
450	root	1.3
451	sf-exg	1.13	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	root	1.3	=back
460	root	1.1
461