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Revision 1.39 by root, Sat Dec 10 11:58:38 2011 UTC

… …
57refers to any kind of boolean value, not a specific type. 57refers to any kind of boolean value, not a specific type.
58 58
59=head2 GCC ATTRIBUTES 59=head2 GCC ATTRIBUTES
60 60
61A major part of libecb deals with GCC attributes. These are additional 61A major part of libecb deals with GCC attributes. These are additional
62attributes that you cna assign to functions, variables and sometimes even 62attributes that you can assign to functions, variables and sometimes even
63types - much like C<const> or C<volatile> in C. 63types - much like C<const> or C<volatile> in C.
64 64
65While GCC allows declarations to show up in many surprising places, 65While GCC allows declarations to show up in many surprising places,
66but not in many expeted places, the safest way is to put attribute 66but not in many expected places, the safest way is to put attribute
67declarations before the whole declaration: 67declarations before the whole declaration:
68 68
69 ecb_const int mysqrt (int a); 69 ecb_const int mysqrt (int a);
70 ecb_unused int i; 70 ecb_unused int i;
71 71
… …
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
460=item uint8_t ecb_bitrev8 (uint8_t x)
461
462=item uint16_t ecb_bitrev16 (uint16_t x)
463
464=item uint32_t ecb_bitrev32 (uint32_t x)
465
466Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
467and so on.
468
469Example:
470
471 ecb_bitrev8 (0xa7) = 0xea
472 ecb_bitrev32 (0xffcc4411) = 0x882233ff
473
412=item uint32_t ecb_bswap16 (uint32_t x) 474=item uint32_t ecb_bswap16 (uint32_t x)
413 475
414=item uint32_t ecb_bswap32 (uint32_t x) 476=item uint32_t ecb_bswap32 (uint32_t x)
415 477
478=item uint64_t ecb_bswap64 (uint64_t x)
479
416These two functions return the value of the 16-bit (32-bit) value C<x> 480These functions return the value of the 16-bit (32-bit, 64-bit) value
417after reversing the order of bytes (0x11223344 becomes 0x44332211). 481C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
482C<ecb_bswap32>).
483
484=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
485
486=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
487
488=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
489
490=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
491
492=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
493
494=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
418 495
419=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) 496=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
420 497
421=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) 498=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
422 499
423These two functions return the value of C<x> after rotating all the bits 500These two families of functions return the value of C<x> after rotating
424by C<count> positions to the right or left respectively. 501all the bits by C<count> positions to the right (C<ecb_rotr>) or left
502(C<ecb_rotl>).
425 503
426Current GCC versions understand these functions and usually compile them 504Current GCC versions understand these functions and usually compile them
427to "optimal" code (e.g. a single C<roll> on x86). 505to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
506x86).
428 507
429=back 508=back
430 509
431=head2 ARITHMETIC 510=head2 ARITHMETIC
432 511
433=over 4 512=over 4
434 513
435=item x = ecb_mod (m, n) 514=item x = ecb_mod (m, n)
436 515
437Returns the positive remainder of the modulo operation between C<m> and 516Returns C<m> modulo C<n>, which is the same as the positive remainder
438C<n>, using floored division. Unlike the C modulo operator C<%>, this 517of the division operation between C<m> and C<n>, using floored
439function ensures that the return value is always positive and that the two 518division. Unlike the C remainder operator C<%>, this function ensures that
519the return value is always positive and that the two numbers I<m> and
440numbers I<m> and I<m' = m + i * n> result in the same value modulo I<n> - 520I<m' = m + i * n> result in the same value modulo I<n> - in other words,
441the C<%> operator usually has a behaviour change at C<m = 0>. 521C<ecb_mod> implements the mathematical modulo operation, which is missing
522in the language.
442 523
443C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be 524C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
444negatable, that is, both C<m> and C<-m> must be representable in its 525negatable, that is, both C<m> and C<-m> must be representable in its
445type. 526type (this typically excludes the minimum signed integer value, the same
527limitation as for C</> and C<%> in C).
446 528
447Current GCC versions compile this into an efficient branchless sequence on 529Current GCC versions compile this into an efficient branchless sequence on
448many systems. 530almost all CPUs.
449 531
450For example, when you want to rotate forward through the members of an 532For example, when you want to rotate forward through the members of an
451array for increasing C<m> (which might be negative), then you should use 533array for increasing C<m> (which might be negative), then you should use
452C<ecb_mod>, as the C<%> operator might give either negative results, or 534C<ecb_mod>, as the C<%> operator might give either negative results, or
453change direction for negative values: 535change direction for negative values:
454 536
455 for (m = -100; m <= 100; ++m) 537 for (m = -100; m <= 100; ++m)
456 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; 538 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
457 539
540=item x = ecb_div_rd (val, div)
541
542=item x = ecb_div_ru (val, div)
543
544Returns C<val> divided by C<div> rounded down or up, respectively.
545C<val> and C<div> must have integer types and C<div> must be strictly
546positive. Note that these functions are implemented with macros in C
547and with function templates in C++.
548
458=back 549=back
459 550
460=head2 UTILITY 551=head2 UTILITY
461 552
462=over 4 553=over 4

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