ViewVC Help
View File | Revision Log | Show Annotations | Download File
/cvs/libecb/ecb.pod
Revision: 1.27
Committed: Wed Jun 1 01:29:36 2011 UTC (13 years ago) by root
Branch: MAIN
Changes since 1.26: +20 -14 lines
Log Message:
*** empty log message ***

File Contents

# 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 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
24 More might come.
25
26 =head2 ABOUT THE HEADER
27
28 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 There are currently no object files to link to - future versions might
37 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
59 =head2 GCC ATTRIBUTES
60
61 A major part of libecb deals with GCC attributes. These are additional
62 attributes that you can 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 expected 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
77 =over 4
78
79 =item ecb_attribute ((attrs...))
80
81 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
89 =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 {
96 int var ecb_unused;
97
98 #ifdef SOMECONDITION
99 var = ...;
100 return var;
101 #else
102 return 0;
103 #endif
104 }
105
106 =item ecb_noinline
107
108 Prevent a function from being inlined - it might be optimised away, but
109 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 =item ecb_noreturn
113
114 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 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
128 =item ecb_const
129
130 Declares that the function only depends on the values of its arguments,
131 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 such as a function returning the square root of its input argument.
142
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 =item ecb_pure
150
151 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 =item ecb_hot
166
167 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 Whether a function is hot or not often depends on the whole program,
175 and less on the function itself. C<ecb_cold> is likely more useful in
176 practise.
177
178 =item ecb_cold
179
180 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 C<ecb_expect_false> had been used to reach them.
190
191 Good examples for such functions would be error reporting functions, or
192 functions only called in exceptional or rare cases.
193
194 =item ecb_artificial
195
196 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 =back
222
223 =head2 OPTIMISATION HINTS
224
225 =over 4
226
227 =item bool ecb_is_constant(expr)
228
229 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 0..n-1, then you could use this inline function in a header file:
235
236 ecb_inline uint32_t
237 rndm (uint32_t n)
238 {
239 return (n * (uint32_t)rndm16 ()) >> 16;
240 }
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 : (n * (uint32_t)rndm16 ()) >> 16;
253 }
254
255 =item bool ecb_expect (expr, value)
256
257 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
261 Usually, you want to use the more intuitive C<ecb_expect_true> and
262 C<ecb_expect_false> functions instead.
263
264 =item bool ecb_expect_true (cond)
265
266 =item bool ecb_expect_false (cond)
267
268 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_expect_true (some_condition)) ...;
275
276 However, by using C<ecb_expect_true>, you tell the compiler that the
277 condition is likely to be true (and for C<ecb_expect_false>, that it is
278 unlikely to be true).
279
280 For example, when you check for a null pointer and expect this to be a
281 rare, exceptional, case, then use C<ecb_expect_false>:
282
283 void my_free (void *ptr)
284 {
285 if (ecb_expect_false (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 You might know these functions under the name C<likely> and C<unlikely>
294 - while these are common aliases, we find that the expect name is easier
295 to understand when quickly skimming code. If you wish, you can use
296 C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
297 C<ecb_expect_false> - these are simply aliases.
298
299 A very good example is in a function that reserves more space for some
300 memory block (for example, inside an implementation of a string stream) -
301 each time something is added, you have to check for a buffer overrun, but
302 you expect that most checks will turn out to be false:
303
304 /* make sure we have "size" extra room in our buffer */
305 ecb_inline void
306 reserve (int size)
307 {
308 if (ecb_expect_false (current + size > end))
309 real_reserve_method (size); /* presumably noinline */
310 }
311
312 =item bool ecb_assume (cond)
313
314 Try to tell the compiler that some condition is true, even if it's not
315 obvious.
316
317 This can be used to teach the compiler about invariants or other
318 conditions that might improve code generation, but which are impossible to
319 deduce form the code itself.
320
321 For example, the example reservation function from the C<ecb_expect_false>
322 description could be written thus (only C<ecb_assume> was added):
323
324 ecb_inline void
325 reserve (int size)
326 {
327 if (ecb_expect_false (current + size > end))
328 real_reserve_method (size); /* presumably noinline */
329
330 ecb_assume (current + size <= end);
331 }
332
333 If you then call this function twice, like this:
334
335 reserve (10);
336 reserve (1);
337
338 Then the compiler I<might> be able to optimise out the second call
339 completely, as it knows that C<< current + 1 > end >> is false and the
340 call will never be executed.
341
342 =item bool ecb_unreachable ()
343
344 This function does nothing itself, except tell the compiler that it will
345 never be executed. Apart from suppressing a warning in some cases, this
346 function can be used to implement C<ecb_assume> or similar functions.
347
348 =item bool ecb_prefetch (addr, rw, locality)
349
350 Tells the compiler to try to prefetch memory at the given C<addr>ess
351 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
352 C<0> means that there will only be one access later, C<3> means that
353 the data will likely be accessed very often, and values in between mean
354 something... in between. The memory pointed to by the address does not
355 need to be accessible (it could be a null pointer for example), but C<rw>
356 and C<locality> must be compile-time constants.
357
358 An obvious way to use this is to prefetch some data far away, in a big
359 array you loop over. This prefetches memory some 128 array elements later,
360 in the hope that it will be ready when the CPU arrives at that location.
361
362 int sum = 0;
363
364 for (i = 0; i < N; ++i)
365 {
366 sum += arr [i]
367 ecb_prefetch (arr + i + 128, 0, 0);
368 }
369
370 It's hard to predict how far to prefetch, and most CPUs that can prefetch
371 are often good enough to predict this kind of behaviour themselves. It
372 gets more interesting with linked lists, especially when you do some fair
373 processing on each list element:
374
375 for (node *n = start; n; n = n->next)
376 {
377 ecb_prefetch (n->next, 0, 0);
378 ... do medium amount of work with *n
379 }
380
381 After processing the node, (part of) the next node might already be in
382 cache.
383
384 =back
385
386 =head2 BIT FIDDLING / BITSTUFFS
387
388 =over 4
389
390 =item bool ecb_big_endian ()
391
392 =item bool ecb_little_endian ()
393
394 These two functions return true if the byte order is big endian
395 (most-significant byte first) or little endian (least-significant byte
396 first) respectively.
397
398 On systems that are neither, their return values are unspecified.
399
400 =item int ecb_ctz32 (uint32_t x)
401
402 Returns the index of the least significant bit set in C<x> (or
403 equivalently the number of bits set to 0 before the least significant bit
404 set), starting from 0. If C<x> is 0 the result is undefined. A common use
405 case is to compute the integer binary logarithm, i.e., C<floor (log2
406 (n))>. For example:
407
408 ecb_ctz32 (3) = 0
409 ecb_ctz32 (6) = 1
410
411 =item int ecb_popcount32 (uint32_t x)
412
413 Returns the number of bits set to 1 in C<x>. For example:
414
415 ecb_popcount32 (7) = 3
416 ecb_popcount32 (255) = 8
417
418 =item uint32_t ecb_bswap16 (uint32_t x)
419
420 =item uint32_t ecb_bswap32 (uint32_t x)
421
422 These two functions return the value of the 16-bit (32-bit) value C<x>
423 after reversing the order of bytes (0x11223344 becomes 0x44332211).
424
425 =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
426
427 =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
428
429 These two functions return the value of C<x> after rotating all the bits
430 by C<count> positions to the right or left respectively.
431
432 Current GCC versions understand these functions and usually compile them
433 to "optimal" code (e.g. a single C<roll> on x86).
434
435 =back
436
437 =head2 ARITHMETIC
438
439 =over 4
440
441 =item x = ecb_mod (m, n)
442
443 Returns C<m> modulo C<n>, which is the same as the positive remainder
444 of the division operation between C<m> and C<n>, using floored
445 division. Unlike the C remainder operator C<%>, this function ensures that
446 the return value is always positive and that the two numbers I<m> and
447 I<m' = m + i * n> result in the same value modulo I<n> - in other words,
448 C<ecb_mod> implements the mathematical modulo operation, which is missing
449 in the language.
450
451 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
452 negatable, that is, both C<m> and C<-m> must be representable in its
453 type (this typically includes the minimum signed integer value, the same
454 limitation as for C</> and C<%> in C).
455
456 Current GCC versions compile this into an efficient branchless sequence on
457 many systems.
458
459 For example, when you want to rotate forward through the members of an
460 array for increasing C<m> (which might be negative), then you should use
461 C<ecb_mod>, as the C<%> operator might give either negative results, or
462 change direction for negative values:
463
464 for (m = -100; m <= 100; ++m)
465 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
466
467 =back
468
469 =head2 UTILITY
470
471 =over 4
472
473 =item element_count = ecb_array_length (name)
474
475 Returns the number of elements in the array C<name>. For example:
476
477 int primes[] = { 2, 3, 5, 7, 11 };
478 int sum = 0;
479
480 for (i = 0; i < ecb_array_length (primes); i++)
481 sum += primes [i];
482
483 =back
484
485