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 into any standard C system,
but aren't, implemented as efficient as possible with GCC, and still
correct with other compilers.

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 can 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 expected 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_expect_false> 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_expect_true> and
C<ecb_expect_false> functions instead.

=item bool ecb_expect_true (cond)

=item bool ecb_expect_false (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_expect_true (some_condition)) ...;

However, by using C<ecb_expect_true>, you tell the compiler that the
condition is likely to be true (and for C<ecb_expect_false>, 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_expect_false>:

  void my_free (void *ptr)
  {
    if (ecb_expect_false (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.

You might know these functions under the name C<likely> and C<unlikely>
- while these are common aliases, we find that the expect name is easier
to understand when quickly skimming code. If you wish, you can use
C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
C<ecb_expect_false> - these are simply aliases.

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_expect_false (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_expect_false>
description could be written thus (only C<ecb_assume> was added):

  ecb_inline void
  reserve (int size)
  {
    if (ecb_expect_false (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.

On systems that are neither, their return values are unspecified.

=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.,  C<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 C<m> modulo C<n>, which is the same as the positive remainder
of the division operation between C<m> and C<n>, using floored
division. Unlike the C remainder operator C<%>, this function ensures that
the return value is always positive and that the two numbers I<m> and
I<m' = m + i * n> result in the same value modulo I<n> - in other words,
C<ecb_mod> implements the mathematical modulo operation, which is missing
in the language.

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 (this typically includes the minimum signed integer value, the same
limitation as for C</> and C<%> in C).

Current GCC versions compile this into an efficient branchless sequence on
almost all CPUs.

For example, when you want to rotate forward through the members of an
array for increasing C<m> (which might be negative), then you should use
C<ecb_mod>, as the C<%> operator might give either negative results, or
change direction for negative values:

   for (m = -100; m <= 100; ++m)
     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];

=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:	1.28
Committed:	Fri Jun 10 11:51:03 2011 UTC (13 years ago) by root
Branch:	MAIN
Changes since 1.27:	+1 -1 lines
Log Message:	* empty log message *
#	Content
1	=head1 LIBECB - e-C-Builtins
2
3	=head2 ABOUT LIBECB
4
5	Libecb is currently a simple header file that doesn't require any
6	configuration to use or include in your project.
7
8	It's part of the e-suite of libraries, other members of which include
9	libev and libeio.
10
11	Its homepage can be found here:
12
13	http://software.schmorp.de/pkg/libecb
14
15	It mainly provides a number of wrappers around GCC built-ins, together
16	with replacement functions for other compilers. In addition to this,
17	it provides a number of other lowlevel C utilities, such as endianness
18	detection, byte swapping or bit rotations.
19
20	Or in other words, things that should be built into any standard C system,
21	but aren't, implemented as efficient as possible with GCC, and still
22	correct with other compilers.
23
24	More might come.
25
26	=head2 ABOUT THE HEADER
27
28	At the moment, all you have to do is copy F<ecb.h> somewhere where your
29	compiler can find it and include it:
30
31	#include <ecb.h>
32
33	The header should work fine for both C and C++ compilation, and gives you
34	all of F<inttypes.h> in addition to the ECB symbols.
35
36	There are currently no object files to link to - future versions might
37	come with an (optional) object code library to link against, to reduce
38	code size or gain access to additional features.
39
40	It also currently includes everything from F<inttypes.h>.
41
42	=head2 ABOUT THIS MANUAL / CONVENTIONS
43
44	This manual mainly describes each (public) function available after
45	including the F<ecb.h> header. The header might define other symbols than
46	these, but these are not part of the public API, and not supported in any
47	way.
48
49	When the manual mentions a "function" then this could be defined either as
50	as inline function, a macro, or an external symbol.
51
52	When functions use a concrete standard type, such as C<int> or
53	C<uint32_t>, then the corresponding function works only with that type. If
54	only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
55	the corresponding function relies on C to implement the correct types, and
56	is usually implemented as a macro. Specifically, a "bool" in this manual
57	refers to any kind of boolean value, not a specific type.
58
59	=head2 GCC ATTRIBUTES
60
61	A major part of libecb deals with GCC attributes. These are additional
62	attributes that you can assign to functions, variables and sometimes even
63	types - much like C<const> or C<volatile> in C.
64
65	While GCC allows declarations to show up in many surprising places,
66	but not in many expected places, the safest way is to put attribute
67	declarations before the whole declaration:
68
69	ecb_const int mysqrt (int a);
70	ecb_unused int i;
71
72	For variables, it is often nicer to put the attribute after the name, and
73	avoid multiple declarations using commas:
74
75	int i ecb_unused;
76
77	=over 4
78
79	=item ecb_attribute ((attrs...))
80
81	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
82	nothing on other compilers, so the effect is that only GCC sees these.
83
84	Example: use the C<deprecated> attribute on a function.
85
86	ecb_attribute((__deprecated__)) void
87	do_not_use_me_anymore (void);
88
89	=item ecb_unused
90
91	Marks a function or a variable as "unused", which simply suppresses a
92	warning by GCC when it detects it as unused. This is useful when you e.g.
93	declare a variable but do not always use it:
94
95	{
96	int var ecb_unused;
97
98	#ifdef SOMECONDITION
99	var = ...;
100	return var;
101	#else
102	return 0;
103	#endif
104	}
105
106	=item ecb_noinline
107
108	Prevent a function from being inlined - it might be optimised away, but
109	not inlined into other functions. This is useful if you know your function
110	is rarely called and large enough for inlining not to be helpful.
111
112	=item ecb_noreturn
113
114	Marks a function as "not returning, ever". Some typical functions that
115	don't return are C<exit> or C<abort> (which really works hard to not
116	return), and now you can make your own:
117
118	ecb_noreturn void
119	my_abort (const char *errline)
120	{
121	puts (errline);
122	abort ();
123	}
124
125	In this case, the compiler would probably be smart enough to deduce it on
126	its own, so this is mainly useful for declarations.
127
128	=item ecb_const
129
130	Declares that the function only depends on the values of its arguments,
131	much like a mathematical function. It specifically does not read or write
132	any memory any arguments might point to, global variables, or call any
133	non-const functions. It also must not have any side effects.
134
135	Such a function can be optimised much more aggressively by the compiler -
136	for example, multiple calls with the same arguments can be optimised into
137	a single call, which wouldn't be possible if the compiler would have to
138	expect any side effects.
139
140	It is best suited for functions in the sense of mathematical functions,
141	such as a function returning the square root of its input argument.
142
143	Not suited would be a function that calculates the hash of some memory
144	area you pass in, prints some messages or looks at a global variable to
145	decide on rounding.
146
147	See C<ecb_pure> for a slightly less restrictive class of functions.
148
149	=item ecb_pure
150
151	Similar to C<ecb_const>, declares a function that has no side
152	effects. Unlike C<ecb_const>, the function is allowed to examine global
153	variables and any other memory areas (such as the ones passed to it via
154	pointers).
155
156	While these functions cannot be optimised as aggressively as C<ecb_const>
157	functions, they can still be optimised away in many occasions, and the
158	compiler has more freedom in moving calls to them around.
159
160	Typical examples for such functions would be C<strlen> or C<memcmp>. A
161	function that calculates the MD5 sum of some input and updates some MD5
162	state passed as argument would I<NOT> be pure, however, as it would modify
163	some memory area that is not the return value.
164
165	=item ecb_hot
166
167	This declares a function as "hot" with regards to the cache - the function
168	is used so often, that it is very beneficial to keep it in the cache if
169	possible.
170
171	The compiler reacts by trying to place hot functions near to each other in
172	memory.
173
174	Whether a function is hot or not often depends on the whole program,
175	and less on the function itself. C<ecb_cold> is likely more useful in
176	practise.
177
178	=item ecb_cold
179
180	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
181	the cache, or in other words, this function is not called often, or not at
182	speed-critical times, and keeping it in the cache might be a waste of said
183	cache.
184
185	In addition to placing cold functions together (or at least away from hot
186	functions), this knowledge can be used in other ways, for example, the
187	function will be optimised for size, as opposed to speed, and codepaths
188	leading to calls to those functions can automatically be marked as if
189	C<ecb_expect_false> had been used to reach them.
190
191	Good examples for such functions would be error reporting functions, or
192	functions only called in exceptional or rare cases.
193
194	=item ecb_artificial
195
196	Declares the function as "artificial", in this case meaning that this
197	function is not really mean to be a function, but more like an accessor
198	- many methods in C++ classes are mere accessor functions, and having a
199	crash reported in such a method, or single-stepping through them, is not
200	usually so helpful, especially when it's inlined to just a few instructions.
201
202	Marking them as artificial will instruct the debugger about just this,
203	leading to happier debugging and thus happier lives.
204
205	Example: in some kind of smart-pointer class, mark the pointer accessor as
206	artificial, so that the whole class acts more like a pointer and less like
207	some C++ abstraction monster.
208
209	template<typename T>
210	struct my_smart_ptr
211	{
212	T *value;
213
214	ecb_artificial
215	operator T *()
216	{
217	return value;
218	}
219	};
220
221	=back
222
223	=head2 OPTIMISATION HINTS
224
225	=over 4
226
227	=item bool ecb_is_constant(expr)
228
229	Returns true iff the expression can be deduced to be a compile-time
230	constant, and false otherwise.
231
232	For example, when you have a C<rndm16> function that returns a 16 bit
233	random number, and you have a function that maps this to a range from
234	0..n-1, then you could use this inline function in a header file:
235
236	ecb_inline uint32_t
237	rndm (uint32_t n)
238	{
239	return (n * (uint32_t)rndm16 ()) >> 16;
240	}
241
242	However, for powers of two, you could use a normal mask, but that is only
243	worth it if, at compile time, you can detect this case. This is the case
244	when the passed number is a constant and also a power of two (C<n & (n -
245	1) == 0>):
246
247	ecb_inline uint32_t
248	rndm (uint32_t n)
249	{
250	return is_constant (n) && !(n & (n - 1))
251	? rndm16 () & (num - 1)
252	: (n * (uint32_t)rndm16 ()) >> 16;
253	}
254
255	=item bool ecb_expect (expr, value)
256
257	Evaluates C<expr> and returns it. In addition, it tells the compiler that
258	the C<expr> evaluates to C<value> a lot, which can be used for static
259	branch optimisations.
260
261	Usually, you want to use the more intuitive C<ecb_expect_true> and
262	C<ecb_expect_false> functions instead.
263
264	=item bool ecb_expect_true (cond)
265
266	=item bool ecb_expect_false (cond)
267
268	These two functions expect a expression that is true or false and return
269	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
270	other conditional statement, it will not change the program:
271
272	/* these two do the same thing */
273	if (some_condition) ...;
274	if (ecb_expect_true (some_condition)) ...;
275
276	However, by using C<ecb_expect_true>, you tell the compiler that the
277	condition is likely to be true (and for C<ecb_expect_false>, that it is
278	unlikely to be true).
279
280	For example, when you check for a null pointer and expect this to be a
281	rare, exceptional, case, then use C<ecb_expect_false>:
282
283	void my_free (void *ptr)
284	{
285	if (ecb_expect_false (ptr == 0))
286	return;
287	}
288
289	Consequent use of these functions to mark away exceptional cases or to
290	tell the compiler what the hot path through a function is can increase
291	performance considerably.
292
293	You might know these functions under the name C<likely> and C<unlikely>
294	- while these are common aliases, we find that the expect name is easier
295	to understand when quickly skimming code. If you wish, you can use
296	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
297	C<ecb_expect_false> - these are simply aliases.
298
299	A very good example is in a function that reserves more space for some
300	memory block (for example, inside an implementation of a string stream) -
301	each time something is added, you have to check for a buffer overrun, but
302	you expect that most checks will turn out to be false:
303
304	/* make sure we have "size" extra room in our buffer */
305	ecb_inline void
306	reserve (int size)
307	{
308	if (ecb_expect_false (current + size > end))
309	real_reserve_method (size); /* presumably noinline */
310	}
311
312	=item bool ecb_assume (cond)
313
314	Try to tell the compiler that some condition is true, even if it's not
315	obvious.
316
317	This can be used to teach the compiler about invariants or other
318	conditions that might improve code generation, but which are impossible to
319	deduce form the code itself.
320
321	For example, the example reservation function from the C<ecb_expect_false>
322	description could be written thus (only C<ecb_assume> was added):
323
324	ecb_inline void
325	reserve (int size)
326	{
327	if (ecb_expect_false (current + size > end))
328	real_reserve_method (size); /* presumably noinline */
329
330	ecb_assume (current + size <= end);
331	}
332
333	If you then call this function twice, like this:
334
335	reserve (10);
336	reserve (1);
337
338	Then the compiler I<might> be able to optimise out the second call
339	completely, as it knows that C<< current + 1 > end >> is false and the
340	call will never be executed.
341
342	=item bool ecb_unreachable ()
343
344	This function does nothing itself, except tell the compiler that it will
345	never be executed. Apart from suppressing a warning in some cases, this
346	function can be used to implement C<ecb_assume> or similar functions.
347
348	=item bool ecb_prefetch (addr, rw, locality)
349
350	Tells the compiler to try to prefetch memory at the given C<addr>ess
351	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
352	C<0> means that there will only be one access later, C<3> means that
353	the data will likely be accessed very often, and values in between mean
354	something... in between. The memory pointed to by the address does not
355	need to be accessible (it could be a null pointer for example), but C<rw>
356	and C<locality> must be compile-time constants.
357
358	An obvious way to use this is to prefetch some data far away, in a big
359	array you loop over. This prefetches memory some 128 array elements later,
360	in the hope that it will be ready when the CPU arrives at that location.
361
362	int sum = 0;
363
364	for (i = 0; i < N; ++i)
365	{
366	sum += arr [i]
367	ecb_prefetch (arr + i + 128, 0, 0);
368	}
369
370	It's hard to predict how far to prefetch, and most CPUs that can prefetch
371	are often good enough to predict this kind of behaviour themselves. It
372	gets more interesting with linked lists, especially when you do some fair
373	processing on each list element:
374
375	for (node *n = start; n; n = n->next)
376	{
377	ecb_prefetch (n->next, 0, 0);
378	... do medium amount of work with *n
379	}
380
381	After processing the node, (part of) the next node might already be in
382	cache.
383
384	=back
385
386	=head2 BIT FIDDLING / BITSTUFFS
387
388	=over 4
389
390	=item bool ecb_big_endian ()
391
392	=item bool ecb_little_endian ()
393
394	These two functions return true if the byte order is big endian
395	(most-significant byte first) or little endian (least-significant byte
396	first) respectively.
397
398	On systems that are neither, their return values are unspecified.
399
400	=item int ecb_ctz32 (uint32_t x)
401
402	Returns the index of the least significant bit set in C<x> (or
403	equivalently the number of bits set to 0 before the least significant bit
404	set), starting from 0. If C<x> is 0 the result is undefined. A common use
405	case is to compute the integer binary logarithm, i.e., C<floor (log2
406	(n))>. For example:
407
408	ecb_ctz32 (3) = 0
409	ecb_ctz32 (6) = 1
410
411	=item int ecb_popcount32 (uint32_t x)
412
413	Returns the number of bits set to 1 in C<x>. For example:
414
415	ecb_popcount32 (7) = 3
416	ecb_popcount32 (255) = 8
417
418	=item uint32_t ecb_bswap16 (uint32_t x)
419
420	=item uint32_t ecb_bswap32 (uint32_t x)
421
422	These two functions return the value of the 16-bit (32-bit) value C<x>
423	after reversing the order of bytes (0x11223344 becomes 0x44332211).
424
425	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
426
427	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
428
429	These two functions return the value of C<x> after rotating all the bits
430	by C<count> positions to the right or left respectively.
431
432	Current GCC versions understand these functions and usually compile them
433	to "optimal" code (e.g. a single C<roll> on x86).
434
435	=back
436
437	=head2 ARITHMETIC
438
439	=over 4
440
441	=item x = ecb_mod (m, n)
442
443	Returns C<m> modulo C<n>, which is the same as the positive remainder
444	of the division operation between C<m> and C<n>, using floored
445	division. Unlike the C remainder operator C<%>, this function ensures that
446	the return value is always positive and that the two numbers I<m> and
447	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
448	C<ecb_mod> implements the mathematical modulo operation, which is missing
449	in the language.
450
451	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
452	negatable, that is, both C<m> and C<-m> must be representable in its
453	type (this typically includes the minimum signed integer value, the same
454	limitation as for C</> and C<%> in C).
455
456	Current GCC versions compile this into an efficient branchless sequence on
457	almost all CPUs.
458
459	For example, when you want to rotate forward through the members of an
460	array for increasing C<m> (which might be negative), then you should use
461	C<ecb_mod>, as the C<%> operator might give either negative results, or
462	change direction for negative values:
463
464	for (m = -100; m <= 100; ++m)
465	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
466
467	=back
468
469	=head2 UTILITY
470
471	=over 4
472
473	=item element_count = ecb_array_length (name)
474
475	Returns the number of elements in the array C<name>. For example:
476
477	int primes[] = { 2, 3, 5, 7, 11 };
478	int sum = 0;
479
480	for (i = 0; i < ecb_array_length (primes); i++)
481	sum += primes [i];
482
483	=back
484
485