cvsroot/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 memembers 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 endienness
detection, byte swapping or bit rotations.

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 objetc 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

blabla where to put, what others

=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.

=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

=item ecb_const

=item ecb_pure

=item ecb_hot

=item ecb_cold

=item ecb_artificial

=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 (bool)

=item bool ecb_unlikely (bool)

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

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

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

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

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

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

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

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

=item bool ecb_assume (cond)

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

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

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

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

    ecb_assume (current + size <= end);
  }

If you then call this function twice, like this:

  reserve (10);
  reserve (1);

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

=item bool ecb_unreachable ()

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

=item bool ecb_prefetch (addr, rw, locality)

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

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

  int sum = 0;

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

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

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

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

=back

=head2 BIT FIDDLING / BITSTUFFS

=over 4

=item bool ecb_big_endian ()

=item bool ecb_little_endian ()

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

=item int ecb_ctz32 (uint32_t x)

Returns the index of the least significant bit set in C<x> (or
equivalently the number of bits set to 0 before the least significant
bit set), starting from 0. If C<x> is 0 the result is undefined. A
common use case is to compute the integer binary logarithm, i.e.,
floor(log2(n)). For example:

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

=item int ecb_popcount32 (uint32_t x)

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

  ecb_popcount32(7) = 3
  ecb_popcount32(255) = 8

=item uint32_t ecb_bswap16 (uint32_t x)

=item uint32_t ecb_bswap32 (uint32_t x)

These two functions return the value of the 16-bit (32-bit) variable
C<x> after reversing the order of bytes.

=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 shifting all the bits
by C<count> positions to the right or left respectively.

=back

=head2 ARITHMETIC

=over 4

=item x = ecb_mod (m, n)

Returns the positive remainder of the modulo operation between C<m> and
C<n>. Unlike the C moduloe operator C<%>, this function ensures that the
return value is always positive).

C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
negatable, that is, both C<m> and C<-m> must be representable in its
type.

=back

=head2 UTILITY

=over 4

=item element_count = ecb_array_length (name) [MACRO]

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:	1.14
Committed:	Thu May 26 23:23:08 2011 UTC (13 years, 1 month ago) by root
Branch:	MAIN
Changes since 1.13:	+63 -17 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			It's part of the e-suite of libraries, other memembers 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 endienness
18			detection, byte swapping or bit rotations.
19
20			More might come.
21	root	1.3
22			=head2 ABOUT THE HEADER
23
24	root	1.14	At the moment, all you have to do is copy F<ecb.h> somewhere where your
25			compiler can find it and include it:
26
27			#include <ecb.h>
28
29			The header should work fine for both C and C++ compilation, and gives you
30			all of F<inttypes.h> in addition to the ECB symbols.
31
32			There are currently no objetc files to link to - future versions might
33			come with an (optional) object code library to link against, to reduce
34			code size or gain access to additional features.
35
36			It also currently includes everything from F<inttypes.h>.
37
38			=head2 ABOUT THIS MANUAL / CONVENTIONS
39
40			This manual mainly describes each (public) function available after
41			including the F<ecb.h> header. The header might define other symbols than
42			these, but these are not part of the public API, and not supported in any
43			way.
44
45			When the manual mentions a "function" then this could be defined either as
46			as inline function, a macro, or an external symbol.
47
48			When functions use a concrete standard type, such as C<int> or
49			C<uint32_t>, then the corresponding function works only with that type. If
50			only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
51			the corresponding function relies on C to implement the correct types, and
52			is usually implemented as a macro. Specifically, a "bool" in this manual
53			refers to any kind of boolean value, not a specific type.
54	root	1.1
55			=head2 GCC ATTRIBUTES
56
57	root	1.3	blabla where to put, what others
58
59	root	1.1	=over 4
60
61	root	1.2	=item ecb_attribute ((attrs...))
62	root	1.1
63	root	1.2	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and
64			to nothing on other compilers, so the effect is that only GCC sees these.
65
66	root	1.3	=item ecb_unused
67
68			Marks a function or a variable as "unused", which simply suppresses a
69			warning by GCC when it detects it as unused. This is useful when you e.g.
70			declare a variable but do not always use it:
71
72			{
73			int var ecb_unused;
74
75			#ifdef SOMECONDITION
76			var = ...;
77			return var;
78			#else
79			return 0;
80			#endif
81			}
82
83	root	1.2	=item ecb_noinline
84
85	root	1.9	Prevent a function from being inlined - it might be optimised away, but
86	root	1.3	not inlined into other functions. This is useful if you know your function
87			is rarely called and large enough for inlining not to be helpful.
88
89	root	1.2	=item ecb_noreturn
90
91			=item ecb_const
92
93			=item ecb_pure
94
95			=item ecb_hot
96
97			=item ecb_cold
98
99			=item ecb_artificial
100
101			=back
102	root	1.1
103			=head2 OPTIMISATION HINTS
104
105			=over 4
106
107	root	1.14	=item bool ecb_is_constant(expr)
108	root	1.1
109	root	1.3	Returns true iff the expression can be deduced to be a compile-time
110			constant, and false otherwise.
111
112			For example, when you have a C<rndm16> function that returns a 16 bit
113			random number, and you have a function that maps this to a range from
114	root	1.5	0..n-1, then you could use this inline function in a header file:
115	root	1.3
116			ecb_inline uint32_t
117			rndm (uint32_t n)
118			{
119	root	1.6	return (n * (uint32_t)rndm16 ()) >> 16;
120	root	1.3	}
121
122			However, for powers of two, you could use a normal mask, but that is only
123			worth it if, at compile time, you can detect this case. This is the case
124			when the passed number is a constant and also a power of two (C<n & (n -
125			1) == 0>):
126
127			ecb_inline uint32_t
128			rndm (uint32_t n)
129			{
130			return is_constant (n) && !(n & (n - 1))
131			? rndm16 () & (num - 1)
132	root	1.6	: (n * (uint32_t)rndm16 ()) >> 16;
133	root	1.3	}
134
135	root	1.14	=item bool ecb_expect (expr, value)
136	root	1.1
137	root	1.7	Evaluates C<expr> and returns it. In addition, it tells the compiler that
138			the C<expr> evaluates to C<value> a lot, which can be used for static
139			branch optimisations.
140	root	1.1
141	root	1.7	Usually, you want to use the more intuitive C<ecb_likely> and
142			C<ecb_unlikely> functions instead.
143	root	1.1
144	root	1.14	=item bool ecb_likely (bool)
145	root	1.1
146	root	1.14	=item bool ecb_unlikely (bool)
147	root	1.1
148	root	1.7	These two functions expect a expression that is true or false and return
149			C<1> or C<0>, respectively, so when used in the condition of an C<if> or
150			other conditional statement, it will not change the program:
151
152			/* these two do the same thing */
153			if (some_condition) ...;
154			if (ecb_likely (some_condition)) ...;
155
156			However, by using C<ecb_likely>, you tell the compiler that the condition
157	sf-exg	1.11	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
158	root	1.7	true).
159
160	root	1.9	For example, when you check for a null pointer and expect this to be a
161			rare, exceptional, case, then use C<ecb_unlikely>:
162	root	1.7
163			void my_free (void *ptr)
164			{
165			if (ecb_unlikely (ptr == 0))
166			return;
167			}
168
169			Consequent use of these functions to mark away exceptional cases or to
170			tell the compiler what the hot path through a function is can increase
171			performance considerably.
172
173			A very good example is in a function that reserves more space for some
174			memory block (for example, inside an implementation of a string stream) -
175	root	1.9	each time something is added, you have to check for a buffer overrun, but
176	root	1.7	you expect that most checks will turn out to be false:
177
178			/* make sure we have "size" extra room in our buffer */
179			ecb_inline void
180			reserve (int size)
181			{
182			if (ecb_unlikely (current + size > end))
183			real_reserve_method (size); /* presumably noinline */
184			}
185
186	root	1.14	=item bool ecb_assume (cond)
187	root	1.7
188			Try to tell the compiler that some condition is true, even if it's not
189			obvious.
190
191			This can be used to teach the compiler about invariants or other
192			conditions that might improve code generation, but which are impossible to
193			deduce form the code itself.
194
195			For example, the example reservation function from the C<ecb_unlikely>
196			description could be written thus (only C<ecb_assume> was added):
197
198			ecb_inline void
199			reserve (int size)
200			{
201			if (ecb_unlikely (current + size > end))
202			real_reserve_method (size); /* presumably noinline */
203
204			ecb_assume (current + size <= end);
205			}
206
207			If you then call this function twice, like this:
208
209			reserve (10);
210			reserve (1);
211
212			Then the compiler I<might> be able to optimise out the second call
213			completely, as it knows that C<< current + 1 > end >> is false and the
214			call will never be executed.
215
216			=item bool ecb_unreachable ()
217
218			This function does nothing itself, except tell the compiler that it will
219	root	1.9	never be executed. Apart from suppressing a warning in some cases, this
220	root	1.7	function can be used to implement C<ecb_assume> or similar functions.
221
222	root	1.14	=item bool ecb_prefetch (addr, rw, locality)
223	root	1.7
224			Tells the compiler to try to prefetch memory at the given C<addr>ess
225	root	1.10	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
226	root	1.7	C<0> means that there will only be one access later, C<3> means that
227			the data will likely be accessed very often, and values in between mean
228			something... in between. The memory pointed to by the address does not
229			need to be accessible (it could be a null pointer for example), but C<rw>
230			and C<locality> must be compile-time constants.
231
232			An obvious way to use this is to prefetch some data far away, in a big
233	root	1.9	array you loop over. This prefetches memory some 128 array elements later,
234	root	1.7	in the hope that it will be ready when the CPU arrives at that location.
235
236			int sum = 0;
237
238			for (i = 0; i < N; ++i)
239			{
240			sum += arr [i]
241			ecb_prefetch (arr + i + 128, 0, 0);
242			}
243
244			It's hard to predict how far to prefetch, and most CPUs that can prefetch
245			are often good enough to predict this kind of behaviour themselves. It
246			gets more interesting with linked lists, especially when you do some fair
247			processing on each list element:
248
249			for (node *n = start; n; n = n->next)
250			{
251			ecb_prefetch (n->next, 0, 0);
252			... do medium amount of work with *n
253			}
254
255			After processing the node, (part of) the next node might already be in
256			cache.
257	root	1.1
258	root	1.2	=back
259	root	1.1
260			=head2 BIT FIDDLING / BITSTUFFS
261
262	root	1.4	=over 4
263
264	root	1.3	=item bool ecb_big_endian ()
265
266			=item bool ecb_little_endian ()
267
268	sf-exg	1.11	These two functions return true if the byte order is big endian
269			(most-significant byte first) or little endian (least-significant byte
270			first) respectively.
271
272	root	1.3	=item int ecb_ctz32 (uint32_t x)
273
274	sf-exg	1.11	Returns the index of the least significant bit set in C<x> (or
275			equivalently the number of bits set to 0 before the least significant
276			bit set), starting from 0. If C<x> is 0 the result is undefined. A
277			common use case is to compute the integer binary logarithm, i.e.,
278			floor(log2(n)). For example:
279
280	sf-exg	1.12	ecb_ctz32(3) = 0
281			ecb_ctz32(6) = 1
282	sf-exg	1.11
283	root	1.3	=item int ecb_popcount32 (uint32_t x)
284
285	sf-exg	1.11	Returns the number of bits set to 1 in C<x>. For example:
286
287			ecb_popcount32(7) = 3
288			ecb_popcount32(255) = 8
289
290	root	1.8	=item uint32_t ecb_bswap16 (uint32_t x)
291
292	root	1.3	=item uint32_t ecb_bswap32 (uint32_t x)
293
294	sf-exg	1.13	These two functions return the value of the 16-bit (32-bit) variable
295			C<x> after reversing the order of bytes.
296
297	root	1.3	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
298
299			=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
300
301	sf-exg	1.11	These two functions return the value of C<x> after shifting all the bits
302			by C<count> positions to the right or left respectively.
303
304	root	1.3	=back
305	root	1.1
306			=head2 ARITHMETIC
307
308	root	1.3	=over 4
309
310	root	1.14	=item x = ecb_mod (m, n)
311	root	1.3
312	root	1.14	Returns the positive remainder of the modulo operation between C<m> and
313			C<n>. Unlike the C moduloe operator C<%>, this function ensures that the
314			return value is always positive).
315
316			C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
317			negatable, that is, both C<m> and C<-m> must be representable in its
318			type.
319	sf-exg	1.11
320	root	1.3	=back
321	root	1.1
322			=head2 UTILITY
323
324	root	1.3	=over 4
325
326	root	1.8	=item element_count = ecb_array_length (name) [MACRO]
327	root	1.3
328	sf-exg	1.13	Returns the number of elements in the array C<name>. For example:
329
330			int primes[] = { 2, 3, 5, 7, 11 };
331			int sum = 0;
332
333			for (i = 0; i < ecb_array_length (primes); i++)
334			sum += primes [i];
335
336	root	1.3	=back
337	root	1.1
338