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_inline

This is not actually an attribute, but you use it like one. It expands
either to C<static inline> or to just C<static>, if inline isn't
supported. It should be used to declare functions that should be inlined,
for code size or speed reasons.

Example: inline this function, it surely will reduce codesize.

   ecb_inline int
   negmul (int a, int b)
   {
     return - (a * b);
   }

=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 / BIT WIZARDRY

=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)

=item int ecb_ctz64 (uint64_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.

For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.

For example:

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

=item int ecb_ld32 (uint32_t x)

=item int ecb_ld64 (uint64_t x)

Returns the index of the most significant bit set in C<x>, or the number
of digits the number requires in binary (so that C<< 2**ld <= x <
2**(ld+1) >>). 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 to see how many bits a certain number requires to be encoded.

This function is similar to the "count leading zero bits" function, except
that that one returns how many zero bits are "in front" of the number (in
the given data type), while C<ecb_ld> returns how many bits the number
itself requires.

For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.

=item int ecb_popcount32 (uint32_t x)

=item int ecb_popcount64 (uint64_t x)

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

For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.

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)

=item uint64_t ecb_bswap64 (uint64_t x)

These functions return the value of the 16-bit (32-bit, 64-bit) value
C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
C<ecb_bswap32>).

=item uint8_t  ecb_rotl8  (uint8_t  x, unsigned int count)

=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)

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

=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)

=item uint8_t  ecb_rotr8  (uint8_t  x, unsigned int count)

=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)

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

=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)

These two families of functions return the value of C<x> after rotating
all the bits by C<count> positions to the right (C<ecb_rotr>) or left
(C<ecb_rotl>).

Current GCC versions understand these functions and usually compile them
to "optimal" code (e.g. a single C<rol> or a combination of C<shld> 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 excludes 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))];

=item x = ecb_div_rd (val, div)

=item x = ecb_div_ru (val, div)

Returns C<val> divided by C<div> rounded down or up, respectively.
C<val> and C<div> must have integer types and C<div> must be strictly
positive.

=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:	1.37
Committed:	Wed Aug 24 23:28:47 2011 UTC (13 years ago) by sf-exg
Branch:	MAIN
Changes since 1.36:	+8 -0 lines
Log Message:	Port div_r{d,u} from deliantra.
#	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_inline
107
108	This is not actually an attribute, but you use it like one. It expands
109	either to C<static inline> or to just C<static>, if inline isn't
110	supported. It should be used to declare functions that should be inlined,
111	for code size or speed reasons.
112
113	Example: inline this function, it surely will reduce codesize.
114
115	ecb_inline int
116	negmul (int a, int b)
117	{
118	return - (a * b);
119	}
120
121	=item ecb_noinline
122
123	Prevent a function from being inlined - it might be optimised away, but
124	not inlined into other functions. This is useful if you know your function
125	is rarely called and large enough for inlining not to be helpful.
126
127	=item ecb_noreturn
128
129	Marks a function as "not returning, ever". Some typical functions that
130	don't return are C<exit> or C<abort> (which really works hard to not
131	return), and now you can make your own:
132
133	ecb_noreturn void
134	my_abort (const char *errline)
135	{
136	puts (errline);
137	abort ();
138	}
139
140	In this case, the compiler would probably be smart enough to deduce it on
141	its own, so this is mainly useful for declarations.
142
143	=item ecb_const
144
145	Declares that the function only depends on the values of its arguments,
146	much like a mathematical function. It specifically does not read or write
147	any memory any arguments might point to, global variables, or call any
148	non-const functions. It also must not have any side effects.
149
150	Such a function can be optimised much more aggressively by the compiler -
151	for example, multiple calls with the same arguments can be optimised into
152	a single call, which wouldn't be possible if the compiler would have to
153	expect any side effects.
154
155	It is best suited for functions in the sense of mathematical functions,
156	such as a function returning the square root of its input argument.
157
158	Not suited would be a function that calculates the hash of some memory
159	area you pass in, prints some messages or looks at a global variable to
160	decide on rounding.
161
162	See C<ecb_pure> for a slightly less restrictive class of functions.
163
164	=item ecb_pure
165
166	Similar to C<ecb_const>, declares a function that has no side
167	effects. Unlike C<ecb_const>, the function is allowed to examine global
168	variables and any other memory areas (such as the ones passed to it via
169	pointers).
170
171	While these functions cannot be optimised as aggressively as C<ecb_const>
172	functions, they can still be optimised away in many occasions, and the
173	compiler has more freedom in moving calls to them around.
174
175	Typical examples for such functions would be C<strlen> or C<memcmp>. A
176	function that calculates the MD5 sum of some input and updates some MD5
177	state passed as argument would I<NOT> be pure, however, as it would modify
178	some memory area that is not the return value.
179
180	=item ecb_hot
181
182	This declares a function as "hot" with regards to the cache - the function
183	is used so often, that it is very beneficial to keep it in the cache if
184	possible.
185
186	The compiler reacts by trying to place hot functions near to each other in
187	memory.
188
189	Whether a function is hot or not often depends on the whole program,
190	and less on the function itself. C<ecb_cold> is likely more useful in
191	practise.
192
193	=item ecb_cold
194
195	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
196	the cache, or in other words, this function is not called often, or not at
197	speed-critical times, and keeping it in the cache might be a waste of said
198	cache.
199
200	In addition to placing cold functions together (or at least away from hot
201	functions), this knowledge can be used in other ways, for example, the
202	function will be optimised for size, as opposed to speed, and codepaths
203	leading to calls to those functions can automatically be marked as if
204	C<ecb_expect_false> had been used to reach them.
205
206	Good examples for such functions would be error reporting functions, or
207	functions only called in exceptional or rare cases.
208
209	=item ecb_artificial
210
211	Declares the function as "artificial", in this case meaning that this
212	function is not really mean to be a function, but more like an accessor
213	- many methods in C++ classes are mere accessor functions, and having a
214	crash reported in such a method, or single-stepping through them, is not
215	usually so helpful, especially when it's inlined to just a few instructions.
216
217	Marking them as artificial will instruct the debugger about just this,
218	leading to happier debugging and thus happier lives.
219
220	Example: in some kind of smart-pointer class, mark the pointer accessor as
221	artificial, so that the whole class acts more like a pointer and less like
222	some C++ abstraction monster.
223
224	template<typename T>
225	struct my_smart_ptr
226	{
227	T *value;
228
229	ecb_artificial
230	operator T *()
231	{
232	return value;
233	}
234	};
235
236	=back
237
238	=head2 OPTIMISATION HINTS
239
240	=over 4
241
242	=item bool ecb_is_constant(expr)
243
244	Returns true iff the expression can be deduced to be a compile-time
245	constant, and false otherwise.
246
247	For example, when you have a C<rndm16> function that returns a 16 bit
248	random number, and you have a function that maps this to a range from
249	0..n-1, then you could use this inline function in a header file:
250
251	ecb_inline uint32_t
252	rndm (uint32_t n)
253	{
254	return (n * (uint32_t)rndm16 ()) >> 16;
255	}
256
257	However, for powers of two, you could use a normal mask, but that is only
258	worth it if, at compile time, you can detect this case. This is the case
259	when the passed number is a constant and also a power of two (C<n & (n -
260	1) == 0>):
261
262	ecb_inline uint32_t
263	rndm (uint32_t n)
264	{
265	return is_constant (n) && !(n & (n - 1))
266	? rndm16 () & (num - 1)
267	: (n * (uint32_t)rndm16 ()) >> 16;
268	}
269
270	=item bool ecb_expect (expr, value)
271
272	Evaluates C<expr> and returns it. In addition, it tells the compiler that
273	the C<expr> evaluates to C<value> a lot, which can be used for static
274	branch optimisations.
275
276	Usually, you want to use the more intuitive C<ecb_expect_true> and
277	C<ecb_expect_false> functions instead.
278
279	=item bool ecb_expect_true (cond)
280
281	=item bool ecb_expect_false (cond)
282
283	These two functions expect a expression that is true or false and return
284	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
285	other conditional statement, it will not change the program:
286
287	/* these two do the same thing */
288	if (some_condition) ...;
289	if (ecb_expect_true (some_condition)) ...;
290
291	However, by using C<ecb_expect_true>, you tell the compiler that the
292	condition is likely to be true (and for C<ecb_expect_false>, that it is
293	unlikely to be true).
294
295	For example, when you check for a null pointer and expect this to be a
296	rare, exceptional, case, then use C<ecb_expect_false>:
297
298	void my_free (void *ptr)
299	{
300	if (ecb_expect_false (ptr == 0))
301	return;
302	}
303
304	Consequent use of these functions to mark away exceptional cases or to
305	tell the compiler what the hot path through a function is can increase
306	performance considerably.
307
308	You might know these functions under the name C<likely> and C<unlikely>
309	- while these are common aliases, we find that the expect name is easier
310	to understand when quickly skimming code. If you wish, you can use
311	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
312	C<ecb_expect_false> - these are simply aliases.
313
314	A very good example is in a function that reserves more space for some
315	memory block (for example, inside an implementation of a string stream) -
316	each time something is added, you have to check for a buffer overrun, but
317	you expect that most checks will turn out to be false:
318
319	/* make sure we have "size" extra room in our buffer */
320	ecb_inline void
321	reserve (int size)
322	{
323	if (ecb_expect_false (current + size > end))
324	real_reserve_method (size); /* presumably noinline */
325	}
326
327	=item bool ecb_assume (cond)
328
329	Try to tell the compiler that some condition is true, even if it's not
330	obvious.
331
332	This can be used to teach the compiler about invariants or other
333	conditions that might improve code generation, but which are impossible to
334	deduce form the code itself.
335
336	For example, the example reservation function from the C<ecb_expect_false>
337	description could be written thus (only C<ecb_assume> was added):
338
339	ecb_inline void
340	reserve (int size)
341	{
342	if (ecb_expect_false (current + size > end))
343	real_reserve_method (size); /* presumably noinline */
344
345	ecb_assume (current + size <= end);
346	}
347
348	If you then call this function twice, like this:
349
350	reserve (10);
351	reserve (1);
352
353	Then the compiler I<might> be able to optimise out the second call
354	completely, as it knows that C<< current + 1 > end >> is false and the
355	call will never be executed.
356
357	=item bool ecb_unreachable ()
358
359	This function does nothing itself, except tell the compiler that it will
360	never be executed. Apart from suppressing a warning in some cases, this
361	function can be used to implement C<ecb_assume> or similar functions.
362
363	=item bool ecb_prefetch (addr, rw, locality)
364
365	Tells the compiler to try to prefetch memory at the given C<addr>ess
366	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
367	C<0> means that there will only be one access later, C<3> means that
368	the data will likely be accessed very often, and values in between mean
369	something... in between. The memory pointed to by the address does not
370	need to be accessible (it could be a null pointer for example), but C<rw>
371	and C<locality> must be compile-time constants.
372
373	An obvious way to use this is to prefetch some data far away, in a big
374	array you loop over. This prefetches memory some 128 array elements later,
375	in the hope that it will be ready when the CPU arrives at that location.
376
377	int sum = 0;
378
379	for (i = 0; i < N; ++i)
380	{
381	sum += arr [i]
382	ecb_prefetch (arr + i + 128, 0, 0);
383	}
384
385	It's hard to predict how far to prefetch, and most CPUs that can prefetch
386	are often good enough to predict this kind of behaviour themselves. It
387	gets more interesting with linked lists, especially when you do some fair
388	processing on each list element:
389
390	for (node *n = start; n; n = n->next)
391	{
392	ecb_prefetch (n->next, 0, 0);
393	... do medium amount of work with *n
394	}
395
396	After processing the node, (part of) the next node might already be in
397	cache.
398
399	=back
400
401	=head2 BIT FIDDLING / BIT WIZARDRY
402
403	=over 4
404
405	=item bool ecb_big_endian ()
406
407	=item bool ecb_little_endian ()
408
409	These two functions return true if the byte order is big endian
410	(most-significant byte first) or little endian (least-significant byte
411	first) respectively.
412
413	On systems that are neither, their return values are unspecified.
414
415	=item int ecb_ctz32 (uint32_t x)
416
417	=item int ecb_ctz64 (uint64_t x)
418
419	Returns the index of the least significant bit set in C<x> (or
420	equivalently the number of bits set to 0 before the least significant bit
421	set), starting from 0. If C<x> is 0 the result is undefined.
422
423	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
424
425	For example:
426
427	ecb_ctz32 (3) = 0
428	ecb_ctz32 (6) = 1
429
430	=item int ecb_ld32 (uint32_t x)
431
432	=item int ecb_ld64 (uint64_t x)
433
434	Returns the index of the most significant bit set in C<x>, or the number
435	of digits the number requires in binary (so that C<< 2**ld <= x <
436	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
437	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
438	example to see how many bits a certain number requires to be encoded.
439
440	This function is similar to the "count leading zero bits" function, except
441	that that one returns how many zero bits are "in front" of the number (in
442	the given data type), while C<ecb_ld> returns how many bits the number
443	itself requires.
444
445	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
446
447	=item int ecb_popcount32 (uint32_t x)
448
449	=item int ecb_popcount64 (uint64_t x)
450
451	Returns the number of bits set to 1 in C<x>.
452
453	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
454
455	For example:
456
457	ecb_popcount32 (7) = 3
458	ecb_popcount32 (255) = 8
459
460	=item uint32_t ecb_bswap16 (uint32_t x)
461
462	=item uint32_t ecb_bswap32 (uint32_t x)
463
464	=item uint64_t ecb_bswap64 (uint64_t x)
465
466	These functions return the value of the 16-bit (32-bit, 64-bit) value
467	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
468	C<ecb_bswap32>).
469
470	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
471
472	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
473
474	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
475
476	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
477
478	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
479
480	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
481
482	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
483
484	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
485
486	These two families of functions return the value of C<x> after rotating
487	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
488	(C<ecb_rotl>).
489
490	Current GCC versions understand these functions and usually compile them
491	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
492	x86).
493
494	=back
495
496	=head2 ARITHMETIC
497
498	=over 4
499
500	=item x = ecb_mod (m, n)
501
502	Returns C<m> modulo C<n>, which is the same as the positive remainder
503	of the division operation between C<m> and C<n>, using floored
504	division. Unlike the C remainder operator C<%>, this function ensures that
505	the return value is always positive and that the two numbers I<m> and
506	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
507	C<ecb_mod> implements the mathematical modulo operation, which is missing
508	in the language.
509
510	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
511	negatable, that is, both C<m> and C<-m> must be representable in its
512	type (this typically excludes the minimum signed integer value, the same
513	limitation as for C</> and C<%> in C).
514
515	Current GCC versions compile this into an efficient branchless sequence on
516	almost all CPUs.
517
518	For example, when you want to rotate forward through the members of an
519	array for increasing C<m> (which might be negative), then you should use
520	C<ecb_mod>, as the C<%> operator might give either negative results, or
521	change direction for negative values:
522
523	for (m = -100; m <= 100; ++m)
524	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
525
526	=item x = ecb_div_rd (val, div)
527
528	=item x = ecb_div_ru (val, div)
529
530	Returns C<val> divided by C<div> rounded down or up, respectively.
531	C<val> and C<div> must have integer types and C<div> must be strictly
532	positive.
533
534	=back
535
536	=head2 UTILITY
537
538	=over 4
539
540	=item element_count = ecb_array_length (name)
541
542	Returns the number of elements in the array C<name>. For example:
543
544	int primes[] = { 2, 3, 5, 7, 11 };
545	int sum = 0;
546
547	for (i = 0; i < ecb_array_length (primes); i++)
548	sum += primes [i];
549
550	=back
551
552