… | |
… | |
54 | only a generic name is used (C<expr>, C<cond>, C<value> and so on), then |
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 |
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 |
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. |
57 | refers to any kind of boolean value, not a specific type. |
58 | |
58 | |
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59 | =head2 TYPES / TYPE SUPPORT |
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60 | |
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61 | ecb.h makes sure that the following types are defined (in the expected way): |
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62 | |
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63 | int8_t uint8_t int16_t uint16_t |
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64 | int32_t uint32_t int64_t uint64_t |
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65 | intptr_t uintptr_t ptrdiff_t |
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66 | |
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67 | The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this |
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68 | platform (currently C<4> or C<8>) and can be used in preprocessor |
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69 | expressions. |
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70 | |
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71 | =head2 LANGUAGE/COMPILER VERSIONS |
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72 | |
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73 | All the following symbols expand to an expression that can be tested in |
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74 | preprocessor instructions as well as treated as a boolean (use C<!!> to |
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75 | ensure it's either C<0> or C<1> if you need that). |
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76 | |
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77 | =over 4 |
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78 | |
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79 | =item ECB_C |
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80 | |
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81 | True if the implementation defines the C<__STDC__> macro to a true value, |
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82 | which is typically true for both C and C++ compilers. |
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83 | |
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84 | =item ECB_C99 |
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85 | |
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86 | True if the implementation claims to be C99 compliant. |
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87 | |
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88 | =item ECB_C11 |
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89 | |
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90 | True if the implementation claims to be C11 compliant. |
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91 | |
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92 | =item ECB_CPP |
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93 | |
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94 | True if the implementation defines the C<__cplusplus__> macro to a true |
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95 | value, which is typically true for C++ compilers. |
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96 | |
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97 | =item ECB_CPP98 |
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98 | |
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99 | True if the implementation claims to be compliant to ISO/IEC 14882:1998 |
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100 | (the first C++ ISO standard) or any later version. Typically true for all |
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101 | C++ compilers. |
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102 | |
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103 | =item ECB_CPP11 |
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104 | |
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105 | True if the implementation claims to be compliant to ISO/IEC 14882:2011 |
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106 | (C++11) or any later version. |
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107 | |
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108 | =item ECB_GCC_VERSION(major,minor) |
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109 | |
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110 | Expands to a true value (suitable for testing in by the preprocessor) |
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111 | if the compiler used is GNU C and the version is the given version, or |
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112 | higher. |
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113 | |
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114 | This macro tries to return false on compilers that claim to be GCC |
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115 | compatible but aren't. |
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116 | |
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117 | =back |
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118 | |
59 | =head2 GCC ATTRIBUTES |
119 | =head2 GCC ATTRIBUTES |
60 | |
120 | |
61 | A major part of libecb deals with GCC attributes. These are additional |
121 | A major part of libecb deals with GCC attributes. These are additional |
62 | attributes that you cna assign to functions, variables and sometimes even |
122 | attributes that you can assign to functions, variables and sometimes even |
63 | types - much like C<const> or C<volatile> in C. |
123 | types - much like C<const> or C<volatile> in C. |
64 | |
124 | |
65 | While GCC allows declarations to show up in many surprising places, |
125 | While GCC allows declarations to show up in many surprising places, |
66 | but not in many expeted places, the safest way is to put attribute |
126 | but not in many expected places, the safest way is to put attribute |
67 | declarations before the whole declaration: |
127 | declarations before the whole declaration: |
68 | |
128 | |
69 | ecb_const int mysqrt (int a); |
129 | ecb_const int mysqrt (int a); |
70 | ecb_unused int i; |
130 | ecb_unused int i; |
71 | |
131 | |
… | |
… | |
101 | #else |
161 | #else |
102 | return 0; |
162 | return 0; |
103 | #endif |
163 | #endif |
104 | } |
164 | } |
105 | |
165 | |
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166 | =item ecb_inline |
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167 | |
|
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168 | This is not actually an attribute, but you use it like one. It expands |
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169 | either to C<static inline> or to just C<static>, if inline isn't |
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170 | supported. It should be used to declare functions that should be inlined, |
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171 | for code size or speed reasons. |
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172 | |
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173 | Example: inline this function, it surely will reduce codesize. |
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174 | |
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175 | ecb_inline int |
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176 | negmul (int a, int b) |
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177 | { |
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178 | return - (a * b); |
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179 | } |
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180 | |
106 | =item ecb_noinline |
181 | =item ecb_noinline |
107 | |
182 | |
108 | Prevent a function from being inlined - it might be optimised away, but |
183 | 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 |
184 | 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. |
185 | is rarely called and large enough for inlining not to be helpful. |
… | |
… | |
184 | |
259 | |
185 | In addition to placing cold functions together (or at least away from hot |
260 | 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 |
261 | 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 |
262 | 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 |
263 | leading to calls to those functions can automatically be marked as if |
189 | C<ecb_unlikely> had been used to reach them. |
264 | C<ecb_expect_false> had been used to reach them. |
190 | |
265 | |
191 | Good examples for such functions would be error reporting functions, or |
266 | Good examples for such functions would be error reporting functions, or |
192 | functions only called in exceptional or rare cases. |
267 | functions only called in exceptional or rare cases. |
193 | |
268 | |
194 | =item ecb_artificial |
269 | =item ecb_artificial |
… | |
… | |
256 | |
331 | |
257 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
332 | 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 |
333 | the C<expr> evaluates to C<value> a lot, which can be used for static |
259 | branch optimisations. |
334 | branch optimisations. |
260 | |
335 | |
261 | Usually, you want to use the more intuitive C<ecb_likely> and |
336 | Usually, you want to use the more intuitive C<ecb_expect_true> and |
262 | C<ecb_unlikely> functions instead. |
337 | C<ecb_expect_false> functions instead. |
263 | |
338 | |
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339 | =item bool ecb_expect_true (cond) |
|
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340 | |
264 | =item bool ecb_likely (cond) |
341 | =item bool ecb_expect_false (cond) |
265 | |
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266 | =item bool ecb_unlikely (cond) |
|
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267 | |
342 | |
268 | These two functions expect a expression that is true or false and return |
343 | 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 |
344 | 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: |
345 | other conditional statement, it will not change the program: |
271 | |
346 | |
272 | /* these two do the same thing */ |
347 | /* these two do the same thing */ |
273 | if (some_condition) ...; |
348 | if (some_condition) ...; |
274 | if (ecb_likely (some_condition)) ...; |
349 | if (ecb_expect_true (some_condition)) ...; |
275 | |
350 | |
276 | However, by using C<ecb_likely>, you tell the compiler that the condition |
351 | However, by using C<ecb_expect_true>, you tell the compiler that the |
277 | is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
352 | condition is likely to be true (and for C<ecb_expect_false>, that it is |
278 | true). |
353 | unlikely to be true). |
279 | |
354 | |
280 | For example, when you check for a null pointer and expect this to be a |
355 | For example, when you check for a null pointer and expect this to be a |
281 | rare, exceptional, case, then use C<ecb_unlikely>: |
356 | rare, exceptional, case, then use C<ecb_expect_false>: |
282 | |
357 | |
283 | void my_free (void *ptr) |
358 | void my_free (void *ptr) |
284 | { |
359 | { |
285 | if (ecb_unlikely (ptr == 0)) |
360 | if (ecb_expect_false (ptr == 0)) |
286 | return; |
361 | return; |
287 | } |
362 | } |
288 | |
363 | |
289 | Consequent use of these functions to mark away exceptional cases or to |
364 | 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 |
365 | tell the compiler what the hot path through a function is can increase |
291 | performance considerably. |
366 | performance considerably. |
|
|
367 | |
|
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368 | You might know these functions under the name C<likely> and C<unlikely> |
|
|
369 | - while these are common aliases, we find that the expect name is easier |
|
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370 | to understand when quickly skimming code. If you wish, you can use |
|
|
371 | C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
|
|
372 | C<ecb_expect_false> - these are simply aliases. |
292 | |
373 | |
293 | A very good example is in a function that reserves more space for some |
374 | A very good example is in a function that reserves more space for some |
294 | memory block (for example, inside an implementation of a string stream) - |
375 | memory block (for example, inside an implementation of a string stream) - |
295 | each time something is added, you have to check for a buffer overrun, but |
376 | each time something is added, you have to check for a buffer overrun, but |
296 | you expect that most checks will turn out to be false: |
377 | you expect that most checks will turn out to be false: |
297 | |
378 | |
298 | /* make sure we have "size" extra room in our buffer */ |
379 | /* make sure we have "size" extra room in our buffer */ |
299 | ecb_inline void |
380 | ecb_inline void |
300 | reserve (int size) |
381 | reserve (int size) |
301 | { |
382 | { |
302 | if (ecb_unlikely (current + size > end)) |
383 | if (ecb_expect_false (current + size > end)) |
303 | real_reserve_method (size); /* presumably noinline */ |
384 | real_reserve_method (size); /* presumably noinline */ |
304 | } |
385 | } |
305 | |
386 | |
306 | =item bool ecb_assume (cond) |
387 | =item bool ecb_assume (cond) |
307 | |
388 | |
… | |
… | |
310 | |
391 | |
311 | This can be used to teach the compiler about invariants or other |
392 | This can be used to teach the compiler about invariants or other |
312 | conditions that might improve code generation, but which are impossible to |
393 | conditions that might improve code generation, but which are impossible to |
313 | deduce form the code itself. |
394 | deduce form the code itself. |
314 | |
395 | |
315 | For example, the example reservation function from the C<ecb_unlikely> |
396 | For example, the example reservation function from the C<ecb_expect_false> |
316 | description could be written thus (only C<ecb_assume> was added): |
397 | description could be written thus (only C<ecb_assume> was added): |
317 | |
398 | |
318 | ecb_inline void |
399 | ecb_inline void |
319 | reserve (int size) |
400 | reserve (int size) |
320 | { |
401 | { |
321 | if (ecb_unlikely (current + size > end)) |
402 | if (ecb_expect_false (current + size > end)) |
322 | real_reserve_method (size); /* presumably noinline */ |
403 | real_reserve_method (size); /* presumably noinline */ |
323 | |
404 | |
324 | ecb_assume (current + size <= end); |
405 | ecb_assume (current + size <= end); |
325 | } |
406 | } |
326 | |
407 | |
… | |
… | |
375 | After processing the node, (part of) the next node might already be in |
456 | After processing the node, (part of) the next node might already be in |
376 | cache. |
457 | cache. |
377 | |
458 | |
378 | =back |
459 | =back |
379 | |
460 | |
380 | =head2 BIT FIDDLING / BITSTUFFS |
461 | =head2 BIT FIDDLING / BIT WIZARDRY |
381 | |
462 | |
382 | =over 4 |
463 | =over 4 |
383 | |
464 | |
384 | =item bool ecb_big_endian () |
465 | =item bool ecb_big_endian () |
385 | |
466 | |
… | |
… | |
391 | |
472 | |
392 | On systems that are neither, their return values are unspecified. |
473 | On systems that are neither, their return values are unspecified. |
393 | |
474 | |
394 | =item int ecb_ctz32 (uint32_t x) |
475 | =item int ecb_ctz32 (uint32_t x) |
395 | |
476 | |
|
|
477 | =item int ecb_ctz64 (uint64_t x) |
|
|
478 | |
396 | Returns the index of the least significant bit set in C<x> (or |
479 | Returns the index of the least significant bit set in C<x> (or |
397 | equivalently the number of bits set to 0 before the least significant bit |
480 | equivalently the number of bits set to 0 before the least significant bit |
398 | set), starting from 0. If C<x> is 0 the result is undefined. A common use |
481 | set), starting from 0. If C<x> is 0 the result is undefined. |
399 | case is to compute the integer binary logarithm, i.e., C<floor (log2 |
482 | |
|
|
483 | For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>. |
|
|
484 | |
400 | (n))>. For example: |
485 | For example: |
401 | |
486 | |
402 | ecb_ctz32 (3) = 0 |
487 | ecb_ctz32 (3) = 0 |
403 | ecb_ctz32 (6) = 1 |
488 | ecb_ctz32 (6) = 1 |
404 | |
489 | |
|
|
490 | =item bool ecb_is_pot32 (uint32_t x) |
|
|
491 | |
|
|
492 | =item bool ecb_is_pot64 (uint32_t x) |
|
|
493 | |
|
|
494 | Return true iff C<x> is a power of two or C<x == 0>. |
|
|
495 | |
|
|
496 | For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>. |
|
|
497 | |
|
|
498 | =item int ecb_ld32 (uint32_t x) |
|
|
499 | |
|
|
500 | =item int ecb_ld64 (uint64_t x) |
|
|
501 | |
|
|
502 | Returns the index of the most significant bit set in C<x>, or the number |
|
|
503 | of digits the number requires in binary (so that C<< 2**ld <= x < |
|
|
504 | 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is |
|
|
505 | to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for |
|
|
506 | example to see how many bits a certain number requires to be encoded. |
|
|
507 | |
|
|
508 | This function is similar to the "count leading zero bits" function, except |
|
|
509 | that that one returns how many zero bits are "in front" of the number (in |
|
|
510 | the given data type), while C<ecb_ld> returns how many bits the number |
|
|
511 | itself requires. |
|
|
512 | |
|
|
513 | For smaller types than C<uint32_t> you can safely use C<ecb_ld32>. |
|
|
514 | |
405 | =item int ecb_popcount32 (uint32_t x) |
515 | =item int ecb_popcount32 (uint32_t x) |
406 | |
516 | |
|
|
517 | =item int ecb_popcount64 (uint64_t x) |
|
|
518 | |
407 | Returns the number of bits set to 1 in C<x>. For example: |
519 | Returns the number of bits set to 1 in C<x>. |
|
|
520 | |
|
|
521 | For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>. |
|
|
522 | |
|
|
523 | For example: |
408 | |
524 | |
409 | ecb_popcount32 (7) = 3 |
525 | ecb_popcount32 (7) = 3 |
410 | ecb_popcount32 (255) = 8 |
526 | ecb_popcount32 (255) = 8 |
411 | |
527 | |
|
|
528 | =item uint8_t ecb_bitrev8 (uint8_t x) |
|
|
529 | |
|
|
530 | =item uint16_t ecb_bitrev16 (uint16_t x) |
|
|
531 | |
|
|
532 | =item uint32_t ecb_bitrev32 (uint32_t x) |
|
|
533 | |
|
|
534 | Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1 |
|
|
535 | and so on. |
|
|
536 | |
|
|
537 | Example: |
|
|
538 | |
|
|
539 | ecb_bitrev8 (0xa7) = 0xea |
|
|
540 | ecb_bitrev32 (0xffcc4411) = 0x882233ff |
|
|
541 | |
412 | =item uint32_t ecb_bswap16 (uint32_t x) |
542 | =item uint32_t ecb_bswap16 (uint32_t x) |
413 | |
543 | |
414 | =item uint32_t ecb_bswap32 (uint32_t x) |
544 | =item uint32_t ecb_bswap32 (uint32_t x) |
415 | |
545 | |
|
|
546 | =item uint64_t ecb_bswap64 (uint64_t x) |
|
|
547 | |
416 | These two functions return the value of the 16-bit (32-bit) value C<x> |
548 | These functions return the value of the 16-bit (32-bit, 64-bit) value |
417 | after reversing the order of bytes (0x11223344 becomes 0x44332211). |
549 | C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in |
|
|
550 | C<ecb_bswap32>). |
|
|
551 | |
|
|
552 | =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count) |
|
|
553 | |
|
|
554 | =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count) |
|
|
555 | |
|
|
556 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
|
|
557 | |
|
|
558 | =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count) |
|
|
559 | |
|
|
560 | =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count) |
|
|
561 | |
|
|
562 | =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count) |
418 | |
563 | |
419 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
564 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
420 | |
565 | |
421 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
566 | =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count) |
422 | |
567 | |
423 | These two functions return the value of C<x> after rotating all the bits |
568 | These two families of functions return the value of C<x> after rotating |
424 | by C<count> positions to the right or left respectively. |
569 | all the bits by C<count> positions to the right (C<ecb_rotr>) or left |
|
|
570 | (C<ecb_rotl>). |
425 | |
571 | |
426 | Current GCC versions understand these functions and usually compile them |
572 | Current GCC versions understand these functions and usually compile them |
427 | to "optimal" code (e.g. a single C<roll> on x86). |
573 | to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on |
|
|
574 | x86). |
428 | |
575 | |
429 | =back |
576 | =back |
430 | |
577 | |
431 | =head2 ARITHMETIC |
578 | =head2 ARITHMETIC |
432 | |
579 | |
433 | =over 4 |
580 | =over 4 |
434 | |
581 | |
435 | =item x = ecb_mod (m, n) |
582 | =item x = ecb_mod (m, n) |
436 | |
583 | |
437 | Returns the positive remainder of the modulo operation between C<m> and |
584 | Returns C<m> modulo C<n>, which is the same as the positive remainder |
438 | C<n>, using floored division. Unlike the C modulo operator C<%>, this |
585 | of the division operation between C<m> and C<n>, using floored |
439 | function ensures that the return value is always positive and that the two |
586 | division. Unlike the C remainder operator C<%>, this function ensures that |
|
|
587 | the return value is always positive and that the two numbers I<m> and |
440 | numbers I<m> and I<m' = m + i * n> result in the same value modulo I<n> - |
588 | I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
441 | the C<%> operator usually has a behaviour change at C<m = 0>. |
589 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
|
|
590 | in the language. |
442 | |
591 | |
443 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
592 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
444 | negatable, that is, both C<m> and C<-m> must be representable in its |
593 | negatable, that is, both C<m> and C<-m> must be representable in its |
445 | type. |
594 | type (this typically excludes the minimum signed integer value, the same |
|
|
595 | limitation as for C</> and C<%> in C). |
446 | |
596 | |
447 | Current GCC versions compile this into an efficient branchless sequence on |
597 | Current GCC versions compile this into an efficient branchless sequence on |
448 | many systems. |
598 | almost all CPUs. |
449 | |
599 | |
450 | For example, when you want to rotate forward through the members of an |
600 | For example, when you want to rotate forward through the members of an |
451 | array for increasing C<m> (which might be negative), then you should use |
601 | array for increasing C<m> (which might be negative), then you should use |
452 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
602 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
453 | change direction for negative values: |
603 | change direction for negative values: |
454 | |
604 | |
455 | for (m = -100; m <= 100; ++m) |
605 | for (m = -100; m <= 100; ++m) |
456 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
606 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
457 | |
607 | |
|
|
608 | =item x = ecb_div_rd (val, div) |
|
|
609 | |
|
|
610 | =item x = ecb_div_ru (val, div) |
|
|
611 | |
|
|
612 | Returns C<val> divided by C<div> rounded down or up, respectively. |
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|
613 | C<val> and C<div> must have integer types and C<div> must be strictly |
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614 | positive. Note that these functions are implemented with macros in C |
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615 | and with function templates in C++. |
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616 | |
458 | =back |
617 | =back |
459 | |
618 | |
460 | =head2 UTILITY |
619 | =head2 UTILITY |
461 | |
620 | |
462 | =over 4 |
621 | =over 4 |
… | |
… | |
471 | for (i = 0; i < ecb_array_length (primes); i++) |
630 | for (i = 0; i < ecb_array_length (primes); i++) |
472 | sum += primes [i]; |
631 | sum += primes [i]; |
473 | |
632 | |
474 | =back |
633 | =back |
475 | |
634 | |
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635 | =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF |
476 | |
636 | |
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637 | These symbols need to be defined before including F<ecb.h> the first time. |
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638 | |
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639 | =over 4 |
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640 | |
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641 | =item ECB_NO_THRADS |
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642 | |
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643 | If F<ecb.h> is never used from multiple threads, then this symbol can |
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644 | be defined, in which case memory fences (and similar constructs) are |
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645 | completely removed, leading to more efficient code and fewer dependencies. |
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646 | |
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647 | Setting this symbol to a true value implies C<ECB_NO_SMP>. |
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648 | |
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649 | =item ECB_NO_SMP |
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650 | |
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651 | The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from |
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652 | multiple threads, but never concurrently (e.g. if the system the program |
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653 | runs on has only a single CPU with a single core, no hyperthreading and so |
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654 | on), then this symbol can be defined, leading to more efficient code and |
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655 | fewer dependencies. |
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656 | |
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657 | =back |
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658 | |
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659 | |