… | |
… | |
15 | It mainly provides a number of wrappers around GCC built-ins, together |
15 | It mainly provides a number of wrappers around GCC built-ins, together |
16 | with replacement functions for other compilers. In addition to this, |
16 | with replacement functions for other compilers. In addition to this, |
17 | it provides a number of other lowlevel C utilities, such as endianness |
17 | it provides a number of other lowlevel C utilities, such as endianness |
18 | detection, byte swapping or bit rotations. |
18 | detection, byte swapping or bit rotations. |
19 | |
19 | |
20 | Or in other words, things that should be built-in into any standard C |
20 | Or in other words, things that should be built into any standard C system, |
21 | system, but aren't. |
21 | but aren't, implemented as efficient as possible with GCC, and still |
|
|
22 | correct with other compilers. |
22 | |
23 | |
23 | More might come. |
24 | More might come. |
24 | |
25 | |
25 | =head2 ABOUT THE HEADER |
26 | =head2 ABOUT THE HEADER |
26 | |
27 | |
… | |
… | |
55 | 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 |
56 | refers to any kind of boolean value, not a specific type. |
57 | refers to any kind of boolean value, not a specific type. |
57 | |
58 | |
58 | =head2 GCC ATTRIBUTES |
59 | =head2 GCC ATTRIBUTES |
59 | |
60 | |
60 | blabla where to put, what others |
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; |
61 | |
76 | |
62 | =over 4 |
77 | =over 4 |
63 | |
78 | |
64 | =item ecb_attribute ((attrs...)) |
79 | =item ecb_attribute ((attrs...)) |
65 | |
80 | |
… | |
… | |
86 | #else |
101 | #else |
87 | return 0; |
102 | return 0; |
88 | #endif |
103 | #endif |
89 | } |
104 | } |
90 | |
105 | |
|
|
106 | =item ecb_inline |
|
|
107 | |
|
|
108 | This is not actually an attribute, but you use it like one. It expands |
|
|
109 | either to C<static inline> or to just C<static>, if inline isn't |
|
|
110 | supported. It should be used to declare functions that should be inlined, |
|
|
111 | for code size or speed reasons. |
|
|
112 | |
|
|
113 | Example: 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 | |
91 | =item ecb_noinline |
121 | =item ecb_noinline |
92 | |
122 | |
93 | Prevent a function from being inlined - it might be optimised away, but |
123 | Prevent a function from being inlined - it might be optimised away, but |
94 | not inlined into other functions. This is useful if you know your function |
124 | not inlined into other functions. This is useful if you know your function |
95 | is rarely called and large enough for inlining not to be helpful. |
125 | is rarely called and large enough for inlining not to be helpful. |
… | |
… | |
105 | { |
135 | { |
106 | puts (errline); |
136 | puts (errline); |
107 | abort (); |
137 | abort (); |
108 | } |
138 | } |
109 | |
139 | |
110 | In this case, the compiler would probbaly be smart enough to decude it on |
140 | In this case, the compiler would probably be smart enough to deduce it on |
111 | it's own, so this is mainly useful for declarations. |
141 | its own, so this is mainly useful for declarations. |
112 | |
142 | |
113 | =item ecb_const |
143 | =item ecb_const |
114 | |
144 | |
115 | Declares that the function only depends on the values of it's arguments, |
145 | Declares that the function only depends on the values of its arguments, |
116 | much like a mathematical function. It specifically does not read or write |
146 | much like a mathematical function. It specifically does not read or write |
117 | any memory any arguments might point to, global variables, or call any |
147 | any memory any arguments might point to, global variables, or call any |
118 | non-const functions. It also must not have any side effects. |
148 | non-const functions. It also must not have any side effects. |
119 | |
149 | |
120 | Such a function can be optimised much more aggressively by the compiler - |
150 | Such a function can be optimised much more aggressively by the compiler - |
121 | for example, multiple calls with the same arguments can be optimised into |
151 | for example, multiple calls with the same arguments can be optimised into |
122 | a single call, which wouldn't be possible if the compiler would have to |
152 | a single call, which wouldn't be possible if the compiler would have to |
123 | expect any side effects. |
153 | expect any side effects. |
124 | |
154 | |
125 | It is best suited for functions in the sense of mathematical functions, |
155 | It is best suited for functions in the sense of mathematical functions, |
126 | such as a function return the square root of its input argument. |
156 | such as a function returning the square root of its input argument. |
127 | |
157 | |
128 | Not suited would be a function that calculates the hash of some memory |
158 | Not suited would be a function that calculates the hash of some memory |
129 | area you pass in, prints some messages or looks at a global variable to |
159 | area you pass in, prints some messages or looks at a global variable to |
130 | decide on rounding. |
160 | decide on rounding. |
131 | |
161 | |
… | |
… | |
154 | possible. |
184 | possible. |
155 | |
185 | |
156 | The compiler reacts by trying to place hot functions near to each other in |
186 | The compiler reacts by trying to place hot functions near to each other in |
157 | memory. |
187 | memory. |
158 | |
188 | |
159 | Whether a function is hot or not often depend son the whole program, |
189 | Whether a function is hot or not often depends on the whole program, |
160 | and less on the function itself. C<ecb_cold> is likely more useful in |
190 | and less on the function itself. C<ecb_cold> is likely more useful in |
161 | practise. |
191 | practise. |
162 | |
192 | |
163 | =item ecb_cold |
193 | =item ecb_cold |
164 | |
194 | |
… | |
… | |
169 | |
199 | |
170 | In addition to placing cold functions together (or at least away from hot |
200 | In addition to placing cold functions together (or at least away from hot |
171 | functions), this knowledge can be used in other ways, for example, the |
201 | functions), this knowledge can be used in other ways, for example, the |
172 | function will be optimised for size, as opposed to speed, and codepaths |
202 | function will be optimised for size, as opposed to speed, and codepaths |
173 | leading to calls to those functions can automatically be marked as if |
203 | leading to calls to those functions can automatically be marked as if |
174 | C<ecb_unlikel> had been used to reach them. |
204 | C<ecb_expect_false> had been used to reach them. |
175 | |
205 | |
176 | Good examples for such functions would be error reporting functions, or |
206 | Good examples for such functions would be error reporting functions, or |
177 | functions only called in exceptional or rare cases. |
207 | functions only called in exceptional or rare cases. |
178 | |
208 | |
179 | =item ecb_artificial |
209 | =item ecb_artificial |
… | |
… | |
241 | |
271 | |
242 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
272 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
243 | the C<expr> evaluates to C<value> a lot, which can be used for static |
273 | the C<expr> evaluates to C<value> a lot, which can be used for static |
244 | branch optimisations. |
274 | branch optimisations. |
245 | |
275 | |
246 | Usually, you want to use the more intuitive C<ecb_likely> and |
276 | Usually, you want to use the more intuitive C<ecb_expect_true> and |
247 | C<ecb_unlikely> functions instead. |
277 | C<ecb_expect_false> functions instead. |
248 | |
278 | |
|
|
279 | =item bool ecb_expect_true (cond) |
|
|
280 | |
249 | =item bool ecb_likely (cond) |
281 | =item bool ecb_expect_false (cond) |
250 | |
|
|
251 | =item bool ecb_unlikely (cond) |
|
|
252 | |
282 | |
253 | These two functions expect a expression that is true or false and return |
283 | These two functions expect a expression that is true or false and return |
254 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
284 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
255 | other conditional statement, it will not change the program: |
285 | other conditional statement, it will not change the program: |
256 | |
286 | |
257 | /* these two do the same thing */ |
287 | /* these two do the same thing */ |
258 | if (some_condition) ...; |
288 | if (some_condition) ...; |
259 | if (ecb_likely (some_condition)) ...; |
289 | if (ecb_expect_true (some_condition)) ...; |
260 | |
290 | |
261 | However, by using C<ecb_likely>, you tell the compiler that the condition |
291 | However, by using C<ecb_expect_true>, you tell the compiler that the |
262 | is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
292 | condition is likely to be true (and for C<ecb_expect_false>, that it is |
263 | true). |
293 | unlikely to be true). |
264 | |
294 | |
265 | For example, when you check for a null pointer and expect this to be a |
295 | For example, when you check for a null pointer and expect this to be a |
266 | rare, exceptional, case, then use C<ecb_unlikely>: |
296 | rare, exceptional, case, then use C<ecb_expect_false>: |
267 | |
297 | |
268 | void my_free (void *ptr) |
298 | void my_free (void *ptr) |
269 | { |
299 | { |
270 | if (ecb_unlikely (ptr == 0)) |
300 | if (ecb_expect_false (ptr == 0)) |
271 | return; |
301 | return; |
272 | } |
302 | } |
273 | |
303 | |
274 | Consequent use of these functions to mark away exceptional cases or to |
304 | Consequent use of these functions to mark away exceptional cases or to |
275 | tell the compiler what the hot path through a function is can increase |
305 | tell the compiler what the hot path through a function is can increase |
276 | performance considerably. |
306 | performance considerably. |
|
|
307 | |
|
|
308 | You 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 |
|
|
310 | to understand when quickly skimming code. If you wish, you can use |
|
|
311 | C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
|
|
312 | C<ecb_expect_false> - these are simply aliases. |
277 | |
313 | |
278 | A very good example is in a function that reserves more space for some |
314 | A very good example is in a function that reserves more space for some |
279 | memory block (for example, inside an implementation of a string stream) - |
315 | memory block (for example, inside an implementation of a string stream) - |
280 | each time something is added, you have to check for a buffer overrun, but |
316 | each time something is added, you have to check for a buffer overrun, but |
281 | you expect that most checks will turn out to be false: |
317 | you expect that most checks will turn out to be false: |
282 | |
318 | |
283 | /* make sure we have "size" extra room in our buffer */ |
319 | /* make sure we have "size" extra room in our buffer */ |
284 | ecb_inline void |
320 | ecb_inline void |
285 | reserve (int size) |
321 | reserve (int size) |
286 | { |
322 | { |
287 | if (ecb_unlikely (current + size > end)) |
323 | if (ecb_expect_false (current + size > end)) |
288 | real_reserve_method (size); /* presumably noinline */ |
324 | real_reserve_method (size); /* presumably noinline */ |
289 | } |
325 | } |
290 | |
326 | |
291 | =item bool ecb_assume (cond) |
327 | =item bool ecb_assume (cond) |
292 | |
328 | |
… | |
… | |
295 | |
331 | |
296 | This can be used to teach the compiler about invariants or other |
332 | This can be used to teach the compiler about invariants or other |
297 | conditions that might improve code generation, but which are impossible to |
333 | conditions that might improve code generation, but which are impossible to |
298 | deduce form the code itself. |
334 | deduce form the code itself. |
299 | |
335 | |
300 | For example, the example reservation function from the C<ecb_unlikely> |
336 | For example, the example reservation function from the C<ecb_expect_false> |
301 | description could be written thus (only C<ecb_assume> was added): |
337 | description could be written thus (only C<ecb_assume> was added): |
302 | |
338 | |
303 | ecb_inline void |
339 | ecb_inline void |
304 | reserve (int size) |
340 | reserve (int size) |
305 | { |
341 | { |
306 | if (ecb_unlikely (current + size > end)) |
342 | if (ecb_expect_false (current + size > end)) |
307 | real_reserve_method (size); /* presumably noinline */ |
343 | real_reserve_method (size); /* presumably noinline */ |
308 | |
344 | |
309 | ecb_assume (current + size <= end); |
345 | ecb_assume (current + size <= end); |
310 | } |
346 | } |
311 | |
347 | |
… | |
… | |
360 | After processing the node, (part of) the next node might already be in |
396 | After processing the node, (part of) the next node might already be in |
361 | cache. |
397 | cache. |
362 | |
398 | |
363 | =back |
399 | =back |
364 | |
400 | |
365 | =head2 BIT FIDDLING / BITSTUFFS |
401 | =head2 BIT FIDDLING / BIT WIZARDRY |
366 | |
402 | |
367 | =over 4 |
403 | =over 4 |
368 | |
404 | |
369 | =item bool ecb_big_endian () |
405 | =item bool ecb_big_endian () |
370 | |
406 | |
… | |
… | |
372 | |
408 | |
373 | These two functions return true if the byte order is big endian |
409 | These two functions return true if the byte order is big endian |
374 | (most-significant byte first) or little endian (least-significant byte |
410 | (most-significant byte first) or little endian (least-significant byte |
375 | first) respectively. |
411 | first) respectively. |
376 | |
412 | |
|
|
413 | On systems that are neither, their return values are unspecified. |
|
|
414 | |
377 | =item int ecb_ctz32 (uint32_t x) |
415 | =item int ecb_ctz32 (uint32_t x) |
378 | |
416 | |
|
|
417 | =item int ecb_ctz64 (uint64_t x) |
|
|
418 | |
379 | Returns the index of the least significant bit set in C<x> (or |
419 | Returns the index of the least significant bit set in C<x> (or |
380 | equivalently the number of bits set to 0 before the least significant |
420 | equivalently the number of bits set to 0 before the least significant bit |
381 | bit set), starting from 0. If C<x> is 0 the result is undefined. A |
421 | set), starting from 0. If C<x> is 0 the result is undefined. |
382 | common use case is to compute the integer binary logarithm, i.e., |
422 | |
383 | floor(log2(n)). For example: |
423 | For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>. |
|
|
424 | |
|
|
425 | For example: |
384 | |
426 | |
385 | ecb_ctz32 (3) = 0 |
427 | ecb_ctz32 (3) = 0 |
386 | ecb_ctz32 (6) = 1 |
428 | ecb_ctz32 (6) = 1 |
387 | |
429 | |
|
|
430 | =item int ecb_ld32 (uint32_t x) |
|
|
431 | |
|
|
432 | =item int ecb_ld64 (uint64_t x) |
|
|
433 | |
|
|
434 | Returns the index of the most significant bit set in C<x>, or the number |
|
|
435 | of digits the number requires in binary (so that C<< 2**ld <= x < |
|
|
436 | 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is |
|
|
437 | to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for |
|
|
438 | example to see how many bits a certain number requires to be encoded. |
|
|
439 | |
|
|
440 | This function is similar to the "count leading zero bits" function, except |
|
|
441 | that that one returns how many zero bits are "in front" of the number (in |
|
|
442 | the given data type), while C<ecb_ld> returns how many bits the number |
|
|
443 | itself requires. |
|
|
444 | |
|
|
445 | For smaller types than C<uint32_t> you can safely use C<ecb_ld32>. |
|
|
446 | |
388 | =item int ecb_popcount32 (uint32_t x) |
447 | =item int ecb_popcount32 (uint32_t x) |
389 | |
448 | |
|
|
449 | =item int ecb_popcount64 (uint64_t x) |
|
|
450 | |
390 | Returns the number of bits set to 1 in C<x>. For example: |
451 | Returns the number of bits set to 1 in C<x>. |
|
|
452 | |
|
|
453 | For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>. |
|
|
454 | |
|
|
455 | For example: |
391 | |
456 | |
392 | ecb_popcount32 (7) = 3 |
457 | ecb_popcount32 (7) = 3 |
393 | ecb_popcount32 (255) = 8 |
458 | ecb_popcount32 (255) = 8 |
394 | |
459 | |
395 | =item uint32_t ecb_bswap16 (uint32_t x) |
460 | =item uint32_t ecb_bswap16 (uint32_t x) |
396 | |
461 | |
397 | =item uint32_t ecb_bswap32 (uint32_t x) |
462 | =item uint32_t ecb_bswap32 (uint32_t x) |
398 | |
463 | |
|
|
464 | =item uint64_t ecb_bswap64 (uint64_t x) |
|
|
465 | |
399 | These two functions return the value of the 16-bit (32-bit) variable |
466 | These functions return the value of the 16-bit (32-bit, 64-bit) value |
400 | C<x> after reversing the order of bytes. |
467 | C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in |
|
|
468 | C<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) |
401 | |
481 | |
402 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
482 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
403 | |
483 | |
404 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
484 | =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count) |
405 | |
485 | |
406 | These two functions return the value of C<x> after shifting all the bits |
486 | These two families of functions return the value of C<x> after rotating |
407 | by C<count> positions to the right or left respectively. |
487 | all the bits by C<count> positions to the right (C<ecb_rotr>) or left |
|
|
488 | (C<ecb_rotl>). |
|
|
489 | |
|
|
490 | Current GCC versions understand these functions and usually compile them |
|
|
491 | to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on |
|
|
492 | x86). |
408 | |
493 | |
409 | =back |
494 | =back |
410 | |
495 | |
411 | =head2 ARITHMETIC |
496 | =head2 ARITHMETIC |
412 | |
497 | |
413 | =over 4 |
498 | =over 4 |
414 | |
499 | |
415 | =item x = ecb_mod (m, n) |
500 | =item x = ecb_mod (m, n) |
416 | |
501 | |
417 | Returns the positive remainder of the modulo operation between C<m> and |
502 | Returns C<m> modulo C<n>, which is the same as the positive remainder |
|
|
503 | of the division operation between C<m> and C<n>, using floored |
418 | C<n>. Unlike the C modulo operator C<%>, this function ensures that the |
504 | division. Unlike the C remainder operator C<%>, this function ensures that |
419 | return value is always positive). |
505 | the return value is always positive and that the two numbers I<m> and |
|
|
506 | I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
|
|
507 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
|
|
508 | in the language. |
420 | |
509 | |
421 | C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be |
510 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
422 | negatable, that is, both C<m> and C<-m> must be representable in its |
511 | negatable, that is, both C<m> and C<-m> must be representable in its |
423 | type. |
512 | type (this typically excludes the minimum signed integer value, the same |
|
|
513 | limitation as for C</> and C<%> in C). |
|
|
514 | |
|
|
515 | Current GCC versions compile this into an efficient branchless sequence on |
|
|
516 | almost all CPUs. |
|
|
517 | |
|
|
518 | For example, when you want to rotate forward through the members of an |
|
|
519 | array for increasing C<m> (which might be negative), then you should use |
|
|
520 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
|
|
521 | change direction for negative values: |
|
|
522 | |
|
|
523 | for (m = -100; m <= 100; ++m) |
|
|
524 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
424 | |
525 | |
425 | =back |
526 | =back |
426 | |
527 | |
427 | =head2 UTILITY |
528 | =head2 UTILITY |
428 | |
529 | |
429 | =over 4 |
530 | =over 4 |
430 | |
531 | |
431 | =item element_count = ecb_array_length (name) [MACRO] |
532 | =item element_count = ecb_array_length (name) |
432 | |
533 | |
433 | Returns the number of elements in the array C<name>. For example: |
534 | Returns the number of elements in the array C<name>. For example: |
434 | |
535 | |
435 | int primes[] = { 2, 3, 5, 7, 11 }; |
536 | int primes[] = { 2, 3, 5, 7, 11 }; |
436 | int sum = 0; |
537 | int sum = 0; |