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 cna assign to functions, variables and sometimes even
types - much like C<const> or C<volatile> in C.

While GCC allows declarations to show up in many surprising places,
but not in many expeted places, the safest way is to put attribute
declarations before the whole declaration:

   ecb_const int mysqrt (int a);
   ecb_unused int i;

For variables, it is often nicer to put the attribute after the name, and
avoid multiple declarations using commas:

   int i ecb_unused;

=over 4

=item ecb_attribute ((attrs...))

A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
nothing on other compilers, so the effect is that only GCC sees these.

Example: use the C<deprecated> attribute on a function.

  ecb_attribute((__deprecated__)) void
  do_not_use_me_anymore (void);

=item ecb_unused

Marks a function or a variable as "unused", which simply suppresses a
warning by GCC when it detects it as unused. This is useful when you e.g.
declare a variable but do not always use it:

  {
    int var ecb_unused;

    #ifdef SOMECONDITION
       var = ...;
       return var;
    #else
       return 0;
    #endif
  }

=item ecb_noinline

Prevent a function from being inlined - it might be optimised away, but
not inlined into other functions. This is useful if you know your function
is rarely called and large enough for inlining not to be helpful.

=item ecb_noreturn

Marks a function as "not returning, ever". Some typical functions that
don't return are C<exit> or C<abort> (which really works hard to not
return), and now you can make your own:

   ecb_noreturn void
   my_abort (const char *errline)
   {
     puts (errline);
     abort ();
   }

In this case, the compiler would probably be smart enough to deduce it on
its own, so this is mainly useful for declarations.

=item ecb_const

Declares that the function only depends on the values of its arguments,
much like a mathematical function. It specifically does not read or write
any memory any arguments might point to, global variables, or call any
non-const functions. It also must not have any side effects.

Such a function can be optimised much more aggressively by the compiler -
for example, multiple calls with the same arguments can be optimised into
a single call, which wouldn't be possible if the compiler would have to
expect any side effects.

It is best suited for functions in the sense of mathematical functions,
such as a function returning the square root of its input argument.

Not suited would be a function that calculates the hash of some memory
area you pass in, prints some messages or looks at a global variable to
decide on rounding.

See C<ecb_pure> for a slightly less restrictive class of functions.

=item ecb_pure

Similar to C<ecb_const>, declares a function that has no side
effects. Unlike C<ecb_const>, the function is allowed to examine global
variables and any other memory areas (such as the ones passed to it via
pointers).

While these functions cannot be optimised as aggressively as C<ecb_const>
functions, they can still be optimised away in many occasions, and the
compiler has more freedom in moving calls to them around.

Typical examples for such functions would be C<strlen> or C<memcmp>. A
function that calculates the MD5 sum of some input and updates some MD5
state passed as argument would I<NOT> be pure, however, as it would modify
some memory area that is not the return value.

=item ecb_hot

This declares a function as "hot" with regards to the cache - the function
is used so often, that it is very beneficial to keep it in the cache if
possible.

The compiler reacts by trying to place hot functions near to each other in
memory.

Whether a function is hot or not often depends on the whole program,
and less on the function itself. C<ecb_cold> is likely more useful in
practise.

=item ecb_cold

The opposite of C<ecb_hot> - declares a function as "cold" with regards to
the cache, or in other words, this function is not called often, or not at
speed-critical times, and keeping it in the cache might be a waste of said
cache.

In addition to placing cold functions together (or at least away from hot
functions), this knowledge can be used in other ways, for example, the
function will be optimised for size, as opposed to speed, and codepaths
leading to calls to those functions can automatically be marked as if
C<ecb_unlikely> had been used to reach them.

Good examples for such functions would be error reporting functions, or
functions only called in exceptional or rare cases.

=item ecb_artificial

Declares the function as "artificial", in this case meaning that this
function is not really mean to be a function, but more like an accessor
- many methods in C++ classes are mere accessor functions, and having a
crash reported in such a method, or single-stepping through them, is not
usually so helpful, especially when it's inlined to just a few instructions.

Marking them as artificial will instruct the debugger about just this,
leading to happier debugging and thus happier lives.

Example: in some kind of smart-pointer class, mark the pointer accessor as
artificial, so that the whole class acts more like a pointer and less like
some C++ abstraction monster.

  template<typename T>
  struct my_smart_ptr
  {
    T *value;

    ecb_artificial
    operator T *()
    {
      return value;
    }
  };

=back

=head2 OPTIMISATION HINTS

=over 4

=item bool ecb_is_constant(expr)

Returns true iff the expression can be deduced to be a compile-time
constant, and false otherwise.

For example, when you have a C<rndm16> function that returns a 16 bit
random number, and you have a function that maps this to a range from
0..n-1, then you could use this inline function in a header file:

  ecb_inline uint32_t
  rndm (uint32_t n)
  {
    return (n * (uint32_t)rndm16 ()) >> 16;
  }

However, for powers of two, you could use a normal mask, but that is only
worth it if, at compile time, you can detect this case. This is the case
when the passed number is a constant and also a power of two (C<n & (n -
1) == 0>):

  ecb_inline uint32_t
  rndm (uint32_t n)
  {
    return is_constant (n) && !(n & (n - 1))
      ? rndm16 () & (num - 1)
      : (n * (uint32_t)rndm16 ()) >> 16;
  }

=item bool ecb_expect (expr, value)

Evaluates C<expr> and returns it. In addition, it tells the compiler that
the C<expr> evaluates to C<value> a lot, which can be used for static
branch optimisations.

Usually, you want to use the more intuitive C<ecb_likely> and
C<ecb_unlikely> functions instead.

=item bool ecb_likely (cond)

=item bool ecb_unlikely (cond)

These two functions expect a expression that is true or false and return
C<1> or C<0>, respectively, so when used in the condition of an C<if> or
other conditional statement, it will not change the program:

  /* these two do the same thing */
  if (some_condition) ...;
  if (ecb_likely (some_condition)) ...;

However, by using C<ecb_likely>, you tell the compiler that the condition
is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
true).

For example, when you check for a null pointer and expect this to be a
rare, exceptional, case, then use C<ecb_unlikely>:

  void my_free (void *ptr)
  {
    if (ecb_unlikely (ptr == 0))
      return;
  }

Consequent use of these functions to mark away exceptional cases or to
tell the compiler what the hot path through a function is can increase
performance considerably.

A very good example is in a function that reserves more space for some
memory block (for example, inside an implementation of a string stream) -
each time something is added, you have to check for a buffer overrun, but
you expect that most checks will turn out to be false:

  /* make sure we have "size" extra room in our buffer */
  ecb_inline void
  reserve (int size)
  {
    if (ecb_unlikely (current + size > end))
      real_reserve_method (size); /* presumably noinline */
  }

=item bool ecb_assume (cond)

Try to tell the compiler that some condition is true, even if it's not
obvious.

This can be used to teach the compiler about invariants or other
conditions that might improve code generation, but which are impossible to
deduce form the code itself.

For example, the example reservation function from the C<ecb_unlikely>
description could be written thus (only C<ecb_assume> was added):

  ecb_inline void
  reserve (int size)
  {
    if (ecb_unlikely (current + size > end))
      real_reserve_method (size); /* presumably noinline */

    ecb_assume (current + size <= end);
  }

If you then call this function twice, like this:

  reserve (10);
  reserve (1);

Then the compiler I<might> be able to optimise out the second call
completely, as it knows that C<< current + 1 > end >> is false and the
call will never be executed.

=item bool ecb_unreachable ()

This function does nothing itself, except tell the compiler that it will
never be executed. Apart from suppressing a warning in some cases, this
function can be used to implement C<ecb_assume> or similar functions.

=item bool ecb_prefetch (addr, rw, locality)

Tells the compiler to try to prefetch memory at the given C<addr>ess
for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
C<0> means that there will only be one access later, C<3> means that
the data will likely be accessed very often, and values in between mean
something... in between. The memory pointed to by the address does not
need to be accessible (it could be a null pointer for example), but C<rw>
and C<locality> must be compile-time constants.

An obvious way to use this is to prefetch some data far away, in a big
array you loop over. This prefetches memory some 128 array elements later,
in the hope that it will be ready when the CPU arrives at that location.

  int sum = 0;

  for (i = 0; i < N; ++i)
    {
      sum += arr [i]
      ecb_prefetch (arr + i + 128, 0, 0);
    }

It's hard to predict how far to prefetch, and most CPUs that can prefetch
are often good enough to predict this kind of behaviour themselves. It
gets more interesting with linked lists, especially when you do some fair
processing on each list element:

  for (node *n = start; n; n = n->next)
    {
      ecb_prefetch (n->next, 0, 0);
      ... do medium amount of work with *n
    }

After processing the node, (part of) the next node might already be in
cache.

=back

=head2 BIT FIDDLING / BITSTUFFS

=over 4

=item bool ecb_big_endian ()

=item bool ecb_little_endian ()

These two functions return true if the byte order is big endian
(most-significant byte first) or little endian (least-significant byte
first) respectively.

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 the positive remainder of the modulo operation between C<m> and
C<n>, using floored division. Unlike the C modulo 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> -
the C<%> operator usually has a behaviour change at C<m = 0>.

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.

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:	1.24
Committed:	Tue May 31 21:10:37 2011 UTC (14 years, 8 months ago) by root
Branch:	MAIN
Changes since 1.23:	+24 -9 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			attributes that you cna 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 expeted 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	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	sf-exg	1.19	C<ecb_unlikely> 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	1.7	Usually, you want to use the more intuitive C<ecb_likely> and
262			C<ecb_unlikely> functions instead.
263	root	1.1
264	root	1.15	=item bool ecb_likely (cond)
265	root	1.1
266	root	1.15	=item bool ecb_unlikely (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			if (ecb_likely (some_condition)) ...;
275
276			However, by using C<ecb_likely>, you tell the compiler that the condition
277	sf-exg	1.11	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
278	root	1.7	true).
279
280	root	1.9	For example, when you check for a null pointer and expect this to be a
281			rare, exceptional, case, then use C<ecb_unlikely>:
282	root	1.7
283			void my_free (void *ptr)
284			{
285			if (ecb_unlikely (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			A very good example is in a function that reserves more space for some
294			memory block (for example, inside an implementation of a string stream) -
295	root	1.9	each time something is added, you have to check for a buffer overrun, but
296	root	1.7	you expect that most checks will turn out to be false:
297
298			/* make sure we have "size" extra room in our buffer */
299			ecb_inline void
300			reserve (int size)
301			{
302			if (ecb_unlikely (current + size > end))
303			real_reserve_method (size); /* presumably noinline */
304			}
305
306	root	1.14	=item bool ecb_assume (cond)
307	root	1.7
308			Try to tell the compiler that some condition is true, even if it's not
309			obvious.
310
311			This can be used to teach the compiler about invariants or other
312			conditions that might improve code generation, but which are impossible to
313			deduce form the code itself.
314
315			For example, the example reservation function from the C<ecb_unlikely>
316			description could be written thus (only C<ecb_assume> was added):
317
318			ecb_inline void
319			reserve (int size)
320			{
321			if (ecb_unlikely (current + size > end))
322			real_reserve_method (size); /* presumably noinline */
323
324			ecb_assume (current + size <= end);
325			}
326
327			If you then call this function twice, like this:
328
329			reserve (10);
330			reserve (1);
331
332			Then the compiler I<might> be able to optimise out the second call
333			completely, as it knows that C<< current + 1 > end >> is false and the
334			call will never be executed.
335
336			=item bool ecb_unreachable ()
337
338			This function does nothing itself, except tell the compiler that it will
339	root	1.9	never be executed. Apart from suppressing a warning in some cases, this
340	root	1.7	function can be used to implement C<ecb_assume> or similar functions.
341
342	root	1.14	=item bool ecb_prefetch (addr, rw, locality)
343	root	1.7
344			Tells the compiler to try to prefetch memory at the given C<addr>ess
345	root	1.10	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
346	root	1.7	C<0> means that there will only be one access later, C<3> means that
347			the data will likely be accessed very often, and values in between mean
348			something... in between. The memory pointed to by the address does not
349			need to be accessible (it could be a null pointer for example), but C<rw>
350			and C<locality> must be compile-time constants.
351
352			An obvious way to use this is to prefetch some data far away, in a big
353	root	1.9	array you loop over. This prefetches memory some 128 array elements later,
354	root	1.7	in the hope that it will be ready when the CPU arrives at that location.
355
356			int sum = 0;
357
358			for (i = 0; i < N; ++i)
359			{
360			sum += arr [i]
361			ecb_prefetch (arr + i + 128, 0, 0);
362			}
363
364			It's hard to predict how far to prefetch, and most CPUs that can prefetch
365			are often good enough to predict this kind of behaviour themselves. It
366			gets more interesting with linked lists, especially when you do some fair
367			processing on each list element:
368
369			for (node *n = start; n; n = n->next)
370			{
371			ecb_prefetch (n->next, 0, 0);
372			... do medium amount of work with *n
373			}
374
375			After processing the node, (part of) the next node might already be in
376			cache.
377	root	1.1
378	root	1.2	=back
379	root	1.1
380			=head2 BIT FIDDLING / BITSTUFFS
381
382	root	1.4	=over 4
383
384	root	1.3	=item bool ecb_big_endian ()
385
386			=item bool ecb_little_endian ()
387
388	sf-exg	1.11	These two functions return true if the byte order is big endian
389			(most-significant byte first) or little endian (least-significant byte
390			first) respectively.
391
392	root	1.24	On systems that are neither, their return values are unspecified.
393
394	root	1.3	=item int ecb_ctz32 (uint32_t x)
395
396	sf-exg	1.11	Returns the index of the least significant bit set in C<x> (or
397	root	1.24	equivalently the number of bits set to 0 before the least significant bit
398			set), starting from 0. If C<x> is 0 the result is undefined. A common use
399			case is to compute the integer binary logarithm, i.e., C<floor (log2
400			(n))>. For example:
401	sf-exg	1.11
402	root	1.15	ecb_ctz32 (3) = 0
403			ecb_ctz32 (6) = 1
404	sf-exg	1.11
405	root	1.3	=item int ecb_popcount32 (uint32_t x)
406
407	sf-exg	1.11	Returns the number of bits set to 1 in C<x>. For example:
408
409	root	1.15	ecb_popcount32 (7) = 3
410			ecb_popcount32 (255) = 8
411	sf-exg	1.11
412	root	1.8	=item uint32_t ecb_bswap16 (uint32_t x)
413
414	root	1.3	=item uint32_t ecb_bswap32 (uint32_t x)
415
416	root	1.21	These two functions return the value of the 16-bit (32-bit) value C<x>
417			after reversing the order of bytes (0x11223344 becomes 0x44332211).
418	sf-exg	1.13
419	root	1.3	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
420
421			=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
422
423	root	1.22	These two functions return the value of C<x> after rotating all the bits
424	sf-exg	1.11	by C<count> positions to the right or left respectively.
425
426	root	1.20	Current GCC versions understand these functions and usually compile them
427			to "optimal" code (e.g. a single C<roll> on x86).
428
429	root	1.3	=back
430	root	1.1
431			=head2 ARITHMETIC
432
433	root	1.3	=over 4
434
435	root	1.14	=item x = ecb_mod (m, n)
436	root	1.3
437	root	1.14	Returns the positive remainder of the modulo operation between C<m> and
438	root	1.24	C<n>, using floored division. Unlike the C modulo operator C<%>, this
439			function ensures that the return value is always positive and that the two
440			numbers I<m> and I<m' = m + i * n> result in the same value modulo I<n> -
441			the C<%> operator usually has a behaviour change at C<m = 0>.
442	root	1.14
443	sf-exg	1.23	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
444	root	1.14	negatable, that is, both C<m> and C<-m> must be representable in its
445			type.
446	sf-exg	1.11
447	root	1.24	Current GCC versions compile this into an efficient branchless sequence on
448			many systems.
449
450			For example, when you want to rotate forward through the members of an
451			array for increasing C<m> (which might be negative), then you should use
452			C<ecb_mod>, as the C<%> operator might give either negative results, or
453			change direction for negative values:
454
455			for (m = -100; m <= 100; ++m)
456			int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
457
458	root	1.3	=back
459	root	1.1
460			=head2 UTILITY
461
462	root	1.3	=over 4
463
464	sf-exg	1.23	=item element_count = ecb_array_length (name)
465	root	1.3
466	sf-exg	1.13	Returns the number of elements in the array C<name>. For example:
467
468			int primes[] = { 2, 3, 5, 7, 11 };
469			int sum = 0;
470
471			for (i = 0; i < ecb_array_length (primes); i++)
472			sum += primes [i];
473
474	root	1.3	=back
475	root	1.1
476