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Revision 1.27 by root, Wed Jun 1 01:29:36 2011 UTC vs.
Revision 1.39 by root, Sat Dec 10 11:58:38 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.
… …
381After 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
382cache. 397cache.
383 398
384=back 399=back
385 400
386=head2 BIT FIDDLING / BITSTUFFS 401=head2 BIT FIDDLING / BIT WIZARDRY
387 402
388=over 4 403=over 4
389 404
390=item bool ecb_big_endian () 405=item bool ecb_big_endian ()
391 406
… …
397 412
398On systems that are neither, their return values are unspecified. 413On systems that are neither, their return values are unspecified.
399 414
400=item int ecb_ctz32 (uint32_t x) 415=item int ecb_ctz32 (uint32_t x)
401 416
417=item int ecb_ctz64 (uint64_t x)
418
402Returns 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
403equivalently 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
404set), 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.
405case 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
406(n))>. For example: 425For example:
407 426
408 ecb_ctz32 (3) = 0 427 ecb_ctz32 (3) = 0
409 ecb_ctz32 (6) = 1 428 ecb_ctz32 (6) = 1
410 429
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
411=item int ecb_popcount32 (uint32_t x) 447=item int ecb_popcount32 (uint32_t x)
412 448
449=item int ecb_popcount64 (uint64_t x)
450
413Returns 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:
414 456
415 ecb_popcount32 (7) = 3 457 ecb_popcount32 (7) = 3
416 ecb_popcount32 (255) = 8 458 ecb_popcount32 (255) = 8
417 459
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
418=item uint32_t ecb_bswap16 (uint32_t x) 474=item uint32_t ecb_bswap16 (uint32_t x)
419 475
420=item uint32_t ecb_bswap32 (uint32_t x) 476=item uint32_t ecb_bswap32 (uint32_t x)
421 477
478=item uint64_t ecb_bswap64 (uint64_t x)
479
422These 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
423after 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)
424 495
425=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) 496=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
426 497
427=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) 498=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
428 499
429These 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
430by 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>).
431 503
432Current GCC versions understand these functions and usually compile them 504Current GCC versions understand these functions and usually compile them
433to "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).
434 507
435=back 508=back
436 509
437=head2 ARITHMETIC 510=head2 ARITHMETIC
438 511
… …
448C<ecb_mod> implements the mathematical modulo operation, which is missing 521C<ecb_mod> implements the mathematical modulo operation, which is missing
449in the language. 522in the language.
450 523
451C<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
452negatable, 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
453type (this typically includes the minimum signed integer value, the same 526type (this typically excludes the minimum signed integer value, the same
454limitation as for C</> and C<%> in C). 527limitation as for C</> and C<%> in C).
455 528
456Current GCC versions compile this into an efficient branchless sequence on 529Current GCC versions compile this into an efficient branchless sequence on
457many systems. 530almost all CPUs.
458 531
459For 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
460array 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
461C<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
462change direction for negative values: 535change direction for negative values:
463 536
464 for (m = -100; m <= 100; ++m) 537 for (m = -100; m <= 100; ++m)
465 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; 538 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
466 539
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
467=back 549=back
468 550
469=head2 UTILITY 551=head2 UTILITY
470 552
471=over 4 553=over 4

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