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 *
#	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 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
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 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
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
64	to nothing on other compilers, so the effect is that only GCC sees these.
65
66	=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	=item ecb_noinline
84
85	Prevent a function from being inlined - it might be optimised away, but
86	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	=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
103	=head2 OPTIMISATION HINTS
104
105	=over 4
106
107	=item bool ecb_is_constant(expr)
108
109	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	0..n-1, then you could use this inline function in a header file:
115
116	ecb_inline uint32_t
117	rndm (uint32_t n)
118	{
119	return (n * (uint32_t)rndm16 ()) >> 16;
120	}
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	: (n * (uint32_t)rndm16 ()) >> 16;
133	}
134
135	=item bool ecb_expect (expr, value)
136
137	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
141	Usually, you want to use the more intuitive C<ecb_likely> and
142	C<ecb_unlikely> functions instead.
143
144	=item bool ecb_likely (bool)
145
146	=item bool ecb_unlikely (bool)
147
148	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	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
158	true).
159
160	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
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	each time something is added, you have to check for a buffer overrun, but
176	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	=item bool ecb_assume (cond)
187
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	never be executed. Apart from suppressing a warning in some cases, this
220	function can be used to implement C<ecb_assume> or similar functions.
221
222	=item bool ecb_prefetch (addr, rw, locality)
223
224	Tells the compiler to try to prefetch memory at the given C<addr>ess
225	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
226	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	array you loop over. This prefetches memory some 128 array elements later,
234	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
258	=back
259
260	=head2 BIT FIDDLING / BITSTUFFS
261
262	=over 4
263
264	=item bool ecb_big_endian ()
265
266	=item bool ecb_little_endian ()
267
268	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	=item int ecb_ctz32 (uint32_t x)
273
274	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	ecb_ctz32(3) = 0
281	ecb_ctz32(6) = 1
282
283	=item int ecb_popcount32 (uint32_t x)
284
285	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	=item uint32_t ecb_bswap16 (uint32_t x)
291
292	=item uint32_t ecb_bswap32 (uint32_t x)
293
294	These two functions return the value of the 16-bit (32-bit) variable
295	C<x> after reversing the order of bytes.
296
297	=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	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	=back
305
306	=head2 ARITHMETIC
307
308	=over 4
309
310	=item x = ecb_mod (m, n)
311
312	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
320	=back
321
322	=head2 UTILITY
323
324	=over 4
325
326	=item element_count = ecb_array_length (name) [MACRO]
327
328	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	=back
337
338