ViewVC Help
View File | Revision Log | Show Annotations | Download File
/cvs/cvsroot/libecb/ecb.pod
(Generate patch)

Comparing cvsroot/libecb/ecb.pod (file contents):
Revision 1.26 by sf-exg, Wed Jun 1 00:57:14 2011 UTC vs.
Revision 1.37 by sf-exg, Wed Aug 24 23:28:47 2011 UTC

… …
101 #else 101 #else
102 return 0; 102 return 0;
103 #endif 103 #endif
104 } 104 }
105 105
106=item ecb_inline
107
108This is not actually an attribute, but you use it like one. It expands
109either to C<static inline> or to just C<static>, if inline isn't
110supported. It should be used to declare functions that should be inlined,
111for code size or speed reasons.
112
113Example: inline this function, it surely will reduce codesize.
114
115 ecb_inline int
116 negmul (int a, int b)
117 {
118 return - (a * b);
119 }
120
106=item ecb_noinline 121=item ecb_noinline
107 122
108Prevent a function from being inlined - it might be optimised away, but 123Prevent a function from being inlined - it might be optimised away, but
109not inlined into other functions. This is useful if you know your function 124not inlined into other functions. This is useful if you know your function
110is rarely called and large enough for inlining not to be helpful. 125is rarely called and large enough for inlining not to be helpful.
… …
184 199
185In addition to placing cold functions together (or at least away from hot 200In addition to placing cold functions together (or at least away from hot
186functions), this knowledge can be used in other ways, for example, the 201functions), this knowledge can be used in other ways, for example, the
187function will be optimised for size, as opposed to speed, and codepaths 202function will be optimised for size, as opposed to speed, and codepaths
188leading to calls to those functions can automatically be marked as if 203leading to calls to those functions can automatically be marked as if
189C<ecb_unlikely> had been used to reach them. 204C<ecb_expect_false> had been used to reach them.
190 205
191Good examples for such functions would be error reporting functions, or 206Good examples for such functions would be error reporting functions, or
192functions only called in exceptional or rare cases. 207functions only called in exceptional or rare cases.
193 208
194=item ecb_artificial 209=item ecb_artificial
… …
256 271
257Evaluates C<expr> and returns it. In addition, it tells the compiler that 272Evaluates C<expr> and returns it. In addition, it tells the compiler that
258the C<expr> evaluates to C<value> a lot, which can be used for static 273the C<expr> evaluates to C<value> a lot, which can be used for static
259branch optimisations. 274branch optimisations.
260 275
261Usually, you want to use the more intuitive C<ecb_likely> and 276Usually, you want to use the more intuitive C<ecb_expect_true> and
262C<ecb_unlikely> functions instead. 277C<ecb_expect_false> functions instead.
263 278
279=item bool ecb_expect_true (cond)
280
264=item bool ecb_likely (cond) 281=item bool ecb_expect_false (cond)
265
266=item bool ecb_unlikely (cond)
267 282
268These two functions expect a expression that is true or false and return 283These two functions expect a expression that is true or false and return
269C<1> or C<0>, respectively, so when used in the condition of an C<if> or 284C<1> or C<0>, respectively, so when used in the condition of an C<if> or
270other conditional statement, it will not change the program: 285other conditional statement, it will not change the program:
271 286
272 /* these two do the same thing */ 287 /* these two do the same thing */
273 if (some_condition) ...; 288 if (some_condition) ...;
274 if (ecb_likely (some_condition)) ...; 289 if (ecb_expect_true (some_condition)) ...;
275 290
276However, by using C<ecb_likely>, you tell the compiler that the condition 291However, by using C<ecb_expect_true>, you tell the compiler that the
277is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be 292condition is likely to be true (and for C<ecb_expect_false>, that it is
278true). 293unlikely to be true).
279 294
280For example, when you check for a null pointer and expect this to be a 295For example, when you check for a null pointer and expect this to be a
281rare, exceptional, case, then use C<ecb_unlikely>: 296rare, exceptional, case, then use C<ecb_expect_false>:
282 297
283 void my_free (void *ptr) 298 void my_free (void *ptr)
284 { 299 {
285 if (ecb_unlikely (ptr == 0)) 300 if (ecb_expect_false (ptr == 0))
286 return; 301 return;
287 } 302 }
288 303
289Consequent use of these functions to mark away exceptional cases or to 304Consequent use of these functions to mark away exceptional cases or to
290tell the compiler what the hot path through a function is can increase 305tell the compiler what the hot path through a function is can increase
291performance considerably. 306performance considerably.
307
308You might know these functions under the name C<likely> and C<unlikely>
309- while these are common aliases, we find that the expect name is easier
310to understand when quickly skimming code. If you wish, you can use
311C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
312C<ecb_expect_false> - these are simply aliases.
292 313
293A very good example is in a function that reserves more space for some 314A very good example is in a function that reserves more space for some
294memory block (for example, inside an implementation of a string stream) - 315memory block (for example, inside an implementation of a string stream) -
295each time something is added, you have to check for a buffer overrun, but 316each time something is added, you have to check for a buffer overrun, but
296you expect that most checks will turn out to be false: 317you expect that most checks will turn out to be false:
297 318
298 /* make sure we have "size" extra room in our buffer */ 319 /* make sure we have "size" extra room in our buffer */
299 ecb_inline void 320 ecb_inline void
300 reserve (int size) 321 reserve (int size)
301 { 322 {
302 if (ecb_unlikely (current + size > end)) 323 if (ecb_expect_false (current + size > end))
303 real_reserve_method (size); /* presumably noinline */ 324 real_reserve_method (size); /* presumably noinline */
304 } 325 }
305 326
306=item bool ecb_assume (cond) 327=item bool ecb_assume (cond)
307 328
… …
310 331
311This can be used to teach the compiler about invariants or other 332This can be used to teach the compiler about invariants or other
312conditions that might improve code generation, but which are impossible to 333conditions that might improve code generation, but which are impossible to
313deduce form the code itself. 334deduce form the code itself.
314 335
315For example, the example reservation function from the C<ecb_unlikely> 336For example, the example reservation function from the C<ecb_expect_false>
316description could be written thus (only C<ecb_assume> was added): 337description could be written thus (only C<ecb_assume> was added):
317 338
318 ecb_inline void 339 ecb_inline void
319 reserve (int size) 340 reserve (int size)
320 { 341 {
321 if (ecb_unlikely (current + size > end)) 342 if (ecb_expect_false (current + size > end))
322 real_reserve_method (size); /* presumably noinline */ 343 real_reserve_method (size); /* presumably noinline */
323 344
324 ecb_assume (current + size <= end); 345 ecb_assume (current + size <= end);
325 } 346 }
326 347
… …
375After processing the node, (part of) the next node might already be in 396After processing the node, (part of) the next node might already be in
376cache. 397cache.
377 398
378=back 399=back
379 400
380=head2 BIT FIDDLING / BITSTUFFS 401=head2 BIT FIDDLING / BIT WIZARDRY
381 402
382=over 4 403=over 4
383 404
384=item bool ecb_big_endian () 405=item bool ecb_big_endian ()
385 406
… …
391 412
392On systems that are neither, their return values are unspecified. 413On systems that are neither, their return values are unspecified.
393 414
394=item int ecb_ctz32 (uint32_t x) 415=item int ecb_ctz32 (uint32_t x)
395 416
417=item int ecb_ctz64 (uint64_t x)
418
396Returns the index of the least significant bit set in C<x> (or 419Returns the index of the least significant bit set in C<x> (or
397equivalently the number of bits set to 0 before the least significant bit 420equivalently the number of bits set to 0 before the least significant bit
398set), starting from 0. If C<x> is 0 the result is undefined. A common use 421set), starting from 0. If C<x> is 0 the result is undefined.
399case is to compute the integer binary logarithm, i.e., C<floor (log2 422
423For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
424
400(n))>. For example: 425For example:
401 426
402 ecb_ctz32 (3) = 0 427 ecb_ctz32 (3) = 0
403 ecb_ctz32 (6) = 1 428 ecb_ctz32 (6) = 1
404 429
430=item int ecb_ld32 (uint32_t x)
431
432=item int ecb_ld64 (uint64_t x)
433
434Returns the index of the most significant bit set in C<x>, or the number
435of digits the number requires in binary (so that C<< 2**ld <= x <
4362**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
437to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
438example to see how many bits a certain number requires to be encoded.
439
440This function is similar to the "count leading zero bits" function, except
441that that one returns how many zero bits are "in front" of the number (in
442the given data type), while C<ecb_ld> returns how many bits the number
443itself requires.
444
445For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
446
405=item int ecb_popcount32 (uint32_t x) 447=item int ecb_popcount32 (uint32_t x)
406 448
449=item int ecb_popcount64 (uint64_t x)
450
407Returns the number of bits set to 1 in C<x>. For example: 451Returns the number of bits set to 1 in C<x>.
452
453For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
454
455For example:
408 456
409 ecb_popcount32 (7) = 3 457 ecb_popcount32 (7) = 3
410 ecb_popcount32 (255) = 8 458 ecb_popcount32 (255) = 8
411 459
412=item uint32_t ecb_bswap16 (uint32_t x) 460=item uint32_t ecb_bswap16 (uint32_t x)
413 461
414=item uint32_t ecb_bswap32 (uint32_t x) 462=item uint32_t ecb_bswap32 (uint32_t x)
415 463
464=item uint64_t ecb_bswap64 (uint64_t x)
465
416These two functions return the value of the 16-bit (32-bit) value C<x> 466These functions return the value of the 16-bit (32-bit, 64-bit) value
417after reversing the order of bytes (0x11223344 becomes 0x44332211). 467C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
468C<ecb_bswap32>).
469
470=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
471
472=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
473
474=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
475
476=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
477
478=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
479
480=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
418 481
419=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) 482=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
420 483
421=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) 484=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
422 485
423These two functions return the value of C<x> after rotating all the bits 486These two families of functions return the value of C<x> after rotating
424by C<count> positions to the right or left respectively. 487all the bits by C<count> positions to the right (C<ecb_rotr>) or left
488(C<ecb_rotl>).
425 489
426Current GCC versions understand these functions and usually compile them 490Current GCC versions understand these functions and usually compile them
427to "optimal" code (e.g. a single C<roll> on x86). 491to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
492x86).
428 493
429=back 494=back
430 495
431=head2 ARITHMETIC 496=head2 ARITHMETIC
432 497
… …
442C<ecb_mod> implements the mathematical modulo operation, which is missing 507C<ecb_mod> implements the mathematical modulo operation, which is missing
443in the language. 508in the language.
444 509
445C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be 510C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
446negatable, that is, both C<m> and C<-m> must be representable in its 511negatable, that is, both C<m> and C<-m> must be representable in its
447type (this typically includes the minimum signed integer value, the same 512type (this typically excludes the minimum signed integer value, the same
448limitation as for C</> and C<%> in C). 513limitation as for C</> and C<%> in C).
449 514
450Current GCC versions compile this into an efficient branchless sequence on 515Current GCC versions compile this into an efficient branchless sequence on
451many systems. 516almost all CPUs.
452 517
453For example, when you want to rotate forward through the members of an 518For example, when you want to rotate forward through the members of an
454array for increasing C<m> (which might be negative), then you should use 519array for increasing C<m> (which might be negative), then you should use
455C<ecb_mod>, as the C<%> operator might give either negative results, or 520C<ecb_mod>, as the C<%> operator might give either negative results, or
456change direction for negative values: 521change direction for negative values:
457 522
458 for (m = -100; m <= 100; ++m) 523 for (m = -100; m <= 100; ++m)
459 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; 524 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
460 525
526=item x = ecb_div_rd (val, div)
527
528=item x = ecb_div_ru (val, div)
529
530Returns C<val> divided by C<div> rounded down or up, respectively.
531C<val> and C<div> must have integer types and C<div> must be strictly
532positive.
533
461=back 534=back
462 535
463=head2 UTILITY 536=head2 UTILITY
464 537
465=over 4 538=over 4

Diff Legend

– Removed lines
+ Added lines
< Changed lines
> Changed lines