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.

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

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.

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

=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 (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.

=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 modulo 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:	1.16
Committed:	Thu May 26 23:32:41 2011 UTC (13 years, 1 month ago) by sf-exg
Branch:	MAIN
Changes since 1.15:	+4 -4 lines
Log Message:	Fix typos.
#	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	More might come.
21
22	=head2 ABOUT THE HEADER
23
24	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 object 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
55	=head2 GCC ATTRIBUTES
56
57	blabla where to put, what others
58
59	=over 4
60
61	=item ecb_attribute ((attrs...))
62
63	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
64	nothing on other compilers, so the effect is that only GCC sees these.
65
66	Example: use the C<deprecated> attribute on a function.
67
68	ecb_attribute((__deprecated__)) void
69	do_not_use_me_anymore (void);
70
71	=item ecb_unused
72
73	Marks a function or a variable as "unused", which simply suppresses a
74	warning by GCC when it detects it as unused. This is useful when you e.g.
75	declare a variable but do not always use it:
76
77	{
78	int var ecb_unused;
79
80	#ifdef SOMECONDITION
81	var = ...;
82	return var;
83	#else
84	return 0;
85	#endif
86	}
87
88	=item ecb_noinline
89
90	Prevent a function from being inlined - it might be optimised away, but
91	not inlined into other functions. This is useful if you know your function
92	is rarely called and large enough for inlining not to be helpful.
93
94	=item ecb_noreturn
95
96	=item ecb_const
97
98	=item ecb_pure
99
100	=item ecb_hot
101
102	=item ecb_cold
103
104	=item ecb_artificial
105
106	=back
107
108	=head2 OPTIMISATION HINTS
109
110	=over 4
111
112	=item bool ecb_is_constant(expr)
113
114	Returns true iff the expression can be deduced to be a compile-time
115	constant, and false otherwise.
116
117	For example, when you have a C<rndm16> function that returns a 16 bit
118	random number, and you have a function that maps this to a range from
119	0..n-1, then you could use this inline function in a header file:
120
121	ecb_inline uint32_t
122	rndm (uint32_t n)
123	{
124	return (n * (uint32_t)rndm16 ()) >> 16;
125	}
126
127	However, for powers of two, you could use a normal mask, but that is only
128	worth it if, at compile time, you can detect this case. This is the case
129	when the passed number is a constant and also a power of two (C<n & (n -
130	1) == 0>):
131
132	ecb_inline uint32_t
133	rndm (uint32_t n)
134	{
135	return is_constant (n) && !(n & (n - 1))
136	? rndm16 () & (num - 1)
137	: (n * (uint32_t)rndm16 ()) >> 16;
138	}
139
140	=item bool ecb_expect (expr, value)
141
142	Evaluates C<expr> and returns it. In addition, it tells the compiler that
143	the C<expr> evaluates to C<value> a lot, which can be used for static
144	branch optimisations.
145
146	Usually, you want to use the more intuitive C<ecb_likely> and
147	C<ecb_unlikely> functions instead.
148
149	=item bool ecb_likely (cond)
150
151	=item bool ecb_unlikely (cond)
152
153	These two functions expect a expression that is true or false and return
154	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
155	other conditional statement, it will not change the program:
156
157	/* these two do the same thing */
158	if (some_condition) ...;
159	if (ecb_likely (some_condition)) ...;
160
161	However, by using C<ecb_likely>, you tell the compiler that the condition
162	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
163	true).
164
165	For example, when you check for a null pointer and expect this to be a
166	rare, exceptional, case, then use C<ecb_unlikely>:
167
168	void my_free (void *ptr)
169	{
170	if (ecb_unlikely (ptr == 0))
171	return;
172	}
173
174	Consequent use of these functions to mark away exceptional cases or to
175	tell the compiler what the hot path through a function is can increase
176	performance considerably.
177
178	A very good example is in a function that reserves more space for some
179	memory block (for example, inside an implementation of a string stream) -
180	each time something is added, you have to check for a buffer overrun, but
181	you expect that most checks will turn out to be false:
182
183	/* make sure we have "size" extra room in our buffer */
184	ecb_inline void
185	reserve (int size)
186	{
187	if (ecb_unlikely (current + size > end))
188	real_reserve_method (size); /* presumably noinline */
189	}
190
191	=item bool ecb_assume (cond)
192
193	Try to tell the compiler that some condition is true, even if it's not
194	obvious.
195
196	This can be used to teach the compiler about invariants or other
197	conditions that might improve code generation, but which are impossible to
198	deduce form the code itself.
199
200	For example, the example reservation function from the C<ecb_unlikely>
201	description could be written thus (only C<ecb_assume> was added):
202
203	ecb_inline void
204	reserve (int size)
205	{
206	if (ecb_unlikely (current + size > end))
207	real_reserve_method (size); /* presumably noinline */
208
209	ecb_assume (current + size <= end);
210	}
211
212	If you then call this function twice, like this:
213
214	reserve (10);
215	reserve (1);
216
217	Then the compiler I<might> be able to optimise out the second call
218	completely, as it knows that C<< current + 1 > end >> is false and the
219	call will never be executed.
220
221	=item bool ecb_unreachable ()
222
223	This function does nothing itself, except tell the compiler that it will
224	never be executed. Apart from suppressing a warning in some cases, this
225	function can be used to implement C<ecb_assume> or similar functions.
226
227	=item bool ecb_prefetch (addr, rw, locality)
228
229	Tells the compiler to try to prefetch memory at the given C<addr>ess
230	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
231	C<0> means that there will only be one access later, C<3> means that
232	the data will likely be accessed very often, and values in between mean
233	something... in between. The memory pointed to by the address does not
234	need to be accessible (it could be a null pointer for example), but C<rw>
235	and C<locality> must be compile-time constants.
236
237	An obvious way to use this is to prefetch some data far away, in a big
238	array you loop over. This prefetches memory some 128 array elements later,
239	in the hope that it will be ready when the CPU arrives at that location.
240
241	int sum = 0;
242
243	for (i = 0; i < N; ++i)
244	{
245	sum += arr [i]
246	ecb_prefetch (arr + i + 128, 0, 0);
247	}
248
249	It's hard to predict how far to prefetch, and most CPUs that can prefetch
250	are often good enough to predict this kind of behaviour themselves. It
251	gets more interesting with linked lists, especially when you do some fair
252	processing on each list element:
253
254	for (node *n = start; n; n = n->next)
255	{
256	ecb_prefetch (n->next, 0, 0);
257	... do medium amount of work with *n
258	}
259
260	After processing the node, (part of) the next node might already be in
261	cache.
262
263	=back
264
265	=head2 BIT FIDDLING / BITSTUFFS
266
267	=over 4
268
269	=item bool ecb_big_endian ()
270
271	=item bool ecb_little_endian ()
272
273	These two functions return true if the byte order is big endian
274	(most-significant byte first) or little endian (least-significant byte
275	first) respectively.
276
277	=item int ecb_ctz32 (uint32_t x)
278
279	Returns the index of the least significant bit set in C<x> (or
280	equivalently the number of bits set to 0 before the least significant
281	bit set), starting from 0. If C<x> is 0 the result is undefined. A
282	common use case is to compute the integer binary logarithm, i.e.,
283	floor(log2(n)). For example:
284
285	ecb_ctz32 (3) = 0
286	ecb_ctz32 (6) = 1
287
288	=item int ecb_popcount32 (uint32_t x)
289
290	Returns the number of bits set to 1 in C<x>. For example:
291
292	ecb_popcount32 (7) = 3
293	ecb_popcount32 (255) = 8
294
295	=item uint32_t ecb_bswap16 (uint32_t x)
296
297	=item uint32_t ecb_bswap32 (uint32_t x)
298
299	These two functions return the value of the 16-bit (32-bit) variable
300	C<x> after reversing the order of bytes.
301
302	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
303
304	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
305
306	These two functions return the value of C<x> after shifting all the bits
307	by C<count> positions to the right or left respectively.
308
309	=back
310
311	=head2 ARITHMETIC
312
313	=over 4
314
315	=item x = ecb_mod (m, n)
316
317	Returns the positive remainder of the modulo operation between C<m> and
318	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
319	return value is always positive).
320
321	C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be
322	negatable, that is, both C<m> and C<-m> must be representable in its
323	type.
324
325	=back
326
327	=head2 UTILITY
328
329	=over 4
330
331	=item element_count = ecb_array_length (name) [MACRO]
332
333	Returns the number of elements in the array C<name>. For example:
334
335	int primes[] = { 2, 3, 5, 7, 11 };
336	int sum = 0;
337
338	for (i = 0; i < ecb_array_length (primes); i++)
339	sum += primes [i];
340
341	=back
342
343