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 ago) by sf-exg
Branch:	MAIN
Changes since 1.15:	+4 -4 lines
Log Message:	Fix typos.
#	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			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	sf-exg	1.16	There are currently no object files to link to - future versions might
33	root	1.14	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	1.15	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	root	1.2
71	root	1.3	=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	root	1.15	{
78			int var ecb_unused;
79	root	1.3
80	root	1.15	#ifdef SOMECONDITION
81			var = ...;
82			return var;
83			#else
84			return 0;
85			#endif
86			}
87	root	1.3
88	root	1.2	=item ecb_noinline
89
90	root	1.9	Prevent a function from being inlined - it might be optimised away, but
91	root	1.3	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	root	1.2	=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	root	1.1
108			=head2 OPTIMISATION HINTS
109
110			=over 4
111
112	root	1.14	=item bool ecb_is_constant(expr)
113	root	1.1
114	root	1.3	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	root	1.5	0..n-1, then you could use this inline function in a header file:
120	root	1.3
121			ecb_inline uint32_t
122			rndm (uint32_t n)
123			{
124	root	1.6	return (n * (uint32_t)rndm16 ()) >> 16;
125	root	1.3	}
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	root	1.6	: (n * (uint32_t)rndm16 ()) >> 16;
138	root	1.3	}
139
140	root	1.14	=item bool ecb_expect (expr, value)
141	root	1.1
142	root	1.7	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	root	1.1
146	root	1.7	Usually, you want to use the more intuitive C<ecb_likely> and
147			C<ecb_unlikely> functions instead.
148	root	1.1
149	root	1.15	=item bool ecb_likely (cond)
150	root	1.1
151	root	1.15	=item bool ecb_unlikely (cond)
152	root	1.1
153	root	1.7	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	sf-exg	1.11	is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be
163	root	1.7	true).
164
165	root	1.9	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	root	1.7
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	root	1.9	each time something is added, you have to check for a buffer overrun, but
181	root	1.7	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	root	1.14	=item bool ecb_assume (cond)
192	root	1.7
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	root	1.9	never be executed. Apart from suppressing a warning in some cases, this
225	root	1.7	function can be used to implement C<ecb_assume> or similar functions.
226
227	root	1.14	=item bool ecb_prefetch (addr, rw, locality)
228	root	1.7
229			Tells the compiler to try to prefetch memory at the given C<addr>ess
230	root	1.10	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
231	root	1.7	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	root	1.9	array you loop over. This prefetches memory some 128 array elements later,
239	root	1.7	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	root	1.1
263	root	1.2	=back
264	root	1.1
265			=head2 BIT FIDDLING / BITSTUFFS
266
267	root	1.4	=over 4
268
269	root	1.3	=item bool ecb_big_endian ()
270
271			=item bool ecb_little_endian ()
272
273	sf-exg	1.11	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	root	1.3	=item int ecb_ctz32 (uint32_t x)
278
279	sf-exg	1.11	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	root	1.15	ecb_ctz32 (3) = 0
286			ecb_ctz32 (6) = 1
287	sf-exg	1.11
288	root	1.3	=item int ecb_popcount32 (uint32_t x)
289
290	sf-exg	1.11	Returns the number of bits set to 1 in C<x>. For example:
291
292	root	1.15	ecb_popcount32 (7) = 3
293			ecb_popcount32 (255) = 8
294	sf-exg	1.11
295	root	1.8	=item uint32_t ecb_bswap16 (uint32_t x)
296
297	root	1.3	=item uint32_t ecb_bswap32 (uint32_t x)
298
299	sf-exg	1.13	These two functions return the value of the 16-bit (32-bit) variable
300			C<x> after reversing the order of bytes.
301
302	root	1.3	=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	sf-exg	1.11	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	root	1.3	=back
310	root	1.1
311			=head2 ARITHMETIC
312
313	root	1.3	=over 4
314
315	root	1.14	=item x = ecb_mod (m, n)
316	root	1.3
317	root	1.14	Returns the positive remainder of the modulo operation between C<m> and
318	sf-exg	1.16	C<n>. Unlike the C modulo operator C<%>, this function ensures that the
319	root	1.14	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	sf-exg	1.11
325	root	1.3	=back
326	root	1.1
327			=head2 UTILITY
328
329	root	1.3	=over 4
330
331	root	1.8	=item element_count = ecb_array_length (name) [MACRO]
332	root	1.3
333	sf-exg	1.13	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	root	1.3	=back
342	root	1.1
343