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
many systems.

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:	1.27
Committed:	Wed Jun 1 01:29:36 2011 UTC (13 years, 1 month ago) by root
Branch:	MAIN
Changes since 1.26:	+20 -14 lines
Log Message:	* empty log message *
#	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	1.24	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	root	1.17
24	root	1.14	More might come.
25	root	1.3
26			=head2 ABOUT THE HEADER
27
28	root	1.14	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	sf-exg	1.16	There are currently no object files to link to - future versions might
37	root	1.14	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	root	1.1
59			=head2 GCC ATTRIBUTES
60
61	root	1.20	A major part of libecb deals with GCC attributes. These are additional
62	sf-exg	1.26	attributes that you can assign to functions, variables and sometimes even
63	root	1.20	types - much like C<const> or C<volatile> in C.
64
65			While GCC allows declarations to show up in many surprising places,
66	sf-exg	1.26	but not in many expected places, the safest way is to put attribute
67	root	1.20	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	root	1.3
77	root	1.1	=over 4
78
79	root	1.2	=item ecb_attribute ((attrs...))
80	root	1.1
81	root	1.15	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	root	1.2
89	root	1.3	=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	root	1.15	{
96			int var ecb_unused;
97	root	1.3
98	root	1.15	#ifdef SOMECONDITION
99			var = ...;
100			return var;
101			#else
102			return 0;
103			#endif
104			}
105	root	1.3
106	root	1.2	=item ecb_noinline
107
108	root	1.9	Prevent a function from being inlined - it might be optimised away, but
109	root	1.3	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	root	1.2	=item ecb_noreturn
113
114	root	1.17	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	sf-exg	1.19	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	root	1.17
128	root	1.2	=item ecb_const
129
130	sf-exg	1.19	Declares that the function only depends on the values of its arguments,
131	root	1.17	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	sf-exg	1.19	such as a function returning the square root of its input argument.
142	root	1.17
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	root	1.2	=item ecb_pure
150
151	root	1.17	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	root	1.2	=item ecb_hot
166
167	root	1.17	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	sf-exg	1.19	Whether a function is hot or not often depends on the whole program,
175	root	1.17	and less on the function itself. C<ecb_cold> is likely more useful in
176			practise.
177
178	root	1.2	=item ecb_cold
179
180	root	1.17	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	root	1.27	C<ecb_expect_false> had been used to reach them.
190	root	1.17
191			Good examples for such functions would be error reporting functions, or
192			functions only called in exceptional or rare cases.
193
194	root	1.2	=item ecb_artificial
195
196	root	1.17	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	root	1.2	=back
222	root	1.1
223			=head2 OPTIMISATION HINTS
224
225			=over 4
226
227	root	1.14	=item bool ecb_is_constant(expr)
228	root	1.1
229	root	1.3	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	root	1.5	0..n-1, then you could use this inline function in a header file:
235	root	1.3
236			ecb_inline uint32_t
237			rndm (uint32_t n)
238			{
239	root	1.6	return (n * (uint32_t)rndm16 ()) >> 16;
240	root	1.3	}
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	root	1.6	: (n * (uint32_t)rndm16 ()) >> 16;
253	root	1.3	}
254
255	root	1.14	=item bool ecb_expect (expr, value)
256	root	1.1
257	root	1.7	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	root	1.1
261	root	1.27	Usually, you want to use the more intuitive C<ecb_expect_true> and
262			C<ecb_expect_false> functions instead.
263	root	1.1
264	root	1.27	=item bool ecb_expect_true (cond)
265	root	1.1
266	root	1.27	=item bool ecb_expect_false (cond)
267	root	1.1
268	root	1.7	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	root	1.27	if (ecb_expect_true (some_condition)) ...;
275	root	1.7
276	root	1.27	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	root	1.7
280	root	1.9	For example, when you check for a null pointer and expect this to be a
281	root	1.27	rare, exceptional, case, then use C<ecb_expect_false>:
282	root	1.7
283			void my_free (void *ptr)
284			{
285	root	1.27	if (ecb_expect_false (ptr == 0))
286	root	1.7	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	root	1.27	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	root	1.7	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	root	1.9	each time something is added, you have to check for a buffer overrun, but
302	root	1.7	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	root	1.27	if (ecb_expect_false (current + size > end))
309	root	1.7	real_reserve_method (size); /* presumably noinline */
310			}
311
312	root	1.14	=item bool ecb_assume (cond)
313	root	1.7
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	root	1.27	For example, the example reservation function from the C<ecb_expect_false>
322	root	1.7	description could be written thus (only C<ecb_assume> was added):
323
324			ecb_inline void
325			reserve (int size)
326			{
327	root	1.27	if (ecb_expect_false (current + size > end))
328	root	1.7	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	root	1.9	never be executed. Apart from suppressing a warning in some cases, this
346	root	1.7	function can be used to implement C<ecb_assume> or similar functions.
347
348	root	1.14	=item bool ecb_prefetch (addr, rw, locality)
349	root	1.7
350			Tells the compiler to try to prefetch memory at the given C<addr>ess
351	root	1.10	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
352	root	1.7	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	root	1.9	array you loop over. This prefetches memory some 128 array elements later,
360	root	1.7	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	root	1.1
384	root	1.2	=back
385	root	1.1
386			=head2 BIT FIDDLING / BITSTUFFS
387
388	root	1.4	=over 4
389
390	root	1.3	=item bool ecb_big_endian ()
391
392			=item bool ecb_little_endian ()
393
394	sf-exg	1.11	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	root	1.24	On systems that are neither, their return values are unspecified.
399
400	root	1.3	=item int ecb_ctz32 (uint32_t x)
401
402	sf-exg	1.11	Returns the index of the least significant bit set in C<x> (or
403	root	1.24	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	sf-exg	1.11
408	root	1.15	ecb_ctz32 (3) = 0
409			ecb_ctz32 (6) = 1
410	sf-exg	1.11
411	root	1.3	=item int ecb_popcount32 (uint32_t x)
412
413	sf-exg	1.11	Returns the number of bits set to 1 in C<x>. For example:
414
415	root	1.15	ecb_popcount32 (7) = 3
416			ecb_popcount32 (255) = 8
417	sf-exg	1.11
418	root	1.8	=item uint32_t ecb_bswap16 (uint32_t x)
419
420	root	1.3	=item uint32_t ecb_bswap32 (uint32_t x)
421
422	root	1.21	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	sf-exg	1.13
425	root	1.3	=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	root	1.22	These two functions return the value of C<x> after rotating all the bits
430	sf-exg	1.11	by C<count> positions to the right or left respectively.
431
432	root	1.20	Current GCC versions understand these functions and usually compile them
433			to "optimal" code (e.g. a single C<roll> on x86).
434
435	root	1.3	=back
436	root	1.1
437			=head2 ARITHMETIC
438
439	root	1.3	=over 4
440
441	root	1.14	=item x = ecb_mod (m, n)
442	root	1.3
443	root	1.25	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	root	1.14
451	sf-exg	1.23	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
452	root	1.14	negatable, that is, both C<m> and C<-m> must be representable in its
453	root	1.25	type (this typically includes the minimum signed integer value, the same
454			limitation as for C</> and C<%> in C).
455	sf-exg	1.11
456	root	1.24	Current GCC versions compile this into an efficient branchless sequence on
457			many systems.
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	root	1.3	=back
468	root	1.1
469			=head2 UTILITY
470
471	root	1.3	=over 4
472
473	sf-exg	1.23	=item element_count = ecb_array_length (name)
474	root	1.3
475	sf-exg	1.13	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	root	1.3	=back
484	root	1.1
485