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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 |
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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 | |
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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 | A major part of libecb deals with GCC attributes. These are additional |
61 | A major part of libecb deals with GCC attributes. These are additional |
61 | attributes that you cna assign to functions, variables and sometimes even |
62 | attributes that you can assign to functions, variables and sometimes even |
62 | types - much like C<const> or C<volatile> in C. |
63 | types - much like C<const> or C<volatile> in C. |
63 | |
64 | |
64 | While GCC allows declarations to show up in many surprising places, |
65 | While GCC allows declarations to show up in many surprising places, |
65 | but not in many expeted places, the safest way is to put attribute |
66 | but not in many expected places, the safest way is to put attribute |
66 | declarations before the whole declaration: |
67 | declarations before the whole declaration: |
67 | |
68 | |
68 | ecb_const int mysqrt (int a); |
69 | ecb_const int mysqrt (int a); |
69 | ecb_unused int i; |
70 | ecb_unused int i; |
70 | |
71 | |
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183 | |
184 | |
184 | In addition to placing cold functions together (or at least away from hot |
185 | In addition to placing cold functions together (or at least away from hot |
185 | functions), this knowledge can be used in other ways, for example, the |
186 | functions), this knowledge can be used in other ways, for example, the |
186 | function will be optimised for size, as opposed to speed, and codepaths |
187 | function will be optimised for size, as opposed to speed, and codepaths |
187 | leading to calls to those functions can automatically be marked as if |
188 | leading to calls to those functions can automatically be marked as if |
188 | C<ecb_unlikely> had been used to reach them. |
189 | C<ecb_expect_false> had been used to reach them. |
189 | |
190 | |
190 | Good examples for such functions would be error reporting functions, or |
191 | Good examples for such functions would be error reporting functions, or |
191 | functions only called in exceptional or rare cases. |
192 | functions only called in exceptional or rare cases. |
192 | |
193 | |
193 | =item ecb_artificial |
194 | =item ecb_artificial |
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255 | |
256 | |
256 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
257 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
257 | the C<expr> evaluates to C<value> a lot, which can be used for static |
258 | the C<expr> evaluates to C<value> a lot, which can be used for static |
258 | branch optimisations. |
259 | branch optimisations. |
259 | |
260 | |
260 | Usually, you want to use the more intuitive C<ecb_likely> and |
261 | Usually, you want to use the more intuitive C<ecb_expect_true> and |
261 | C<ecb_unlikely> functions instead. |
262 | C<ecb_expect_false> functions instead. |
262 | |
263 | |
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264 | =item bool ecb_expect_true (cond) |
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265 | |
263 | =item bool ecb_likely (cond) |
266 | =item bool ecb_expect_false (cond) |
264 | |
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265 | =item bool ecb_unlikely (cond) |
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266 | |
267 | |
267 | These two functions expect a expression that is true or false and return |
268 | These two functions expect a expression that is true or false and return |
268 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
269 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
269 | other conditional statement, it will not change the program: |
270 | other conditional statement, it will not change the program: |
270 | |
271 | |
271 | /* these two do the same thing */ |
272 | /* these two do the same thing */ |
272 | if (some_condition) ...; |
273 | if (some_condition) ...; |
273 | if (ecb_likely (some_condition)) ...; |
274 | if (ecb_expect_true (some_condition)) ...; |
274 | |
275 | |
275 | However, by using C<ecb_likely>, you tell the compiler that the condition |
276 | However, by using C<ecb_expect_true>, you tell the compiler that the |
276 | is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
277 | condition is likely to be true (and for C<ecb_expect_false>, that it is |
277 | true). |
278 | unlikely to be true). |
278 | |
279 | |
279 | For example, when you check for a null pointer and expect this to be a |
280 | For example, when you check for a null pointer and expect this to be a |
280 | rare, exceptional, case, then use C<ecb_unlikely>: |
281 | rare, exceptional, case, then use C<ecb_expect_false>: |
281 | |
282 | |
282 | void my_free (void *ptr) |
283 | void my_free (void *ptr) |
283 | { |
284 | { |
284 | if (ecb_unlikely (ptr == 0)) |
285 | if (ecb_expect_false (ptr == 0)) |
285 | return; |
286 | return; |
286 | } |
287 | } |
287 | |
288 | |
288 | Consequent use of these functions to mark away exceptional cases or to |
289 | Consequent use of these functions to mark away exceptional cases or to |
289 | tell the compiler what the hot path through a function is can increase |
290 | tell the compiler what the hot path through a function is can increase |
290 | performance considerably. |
291 | performance considerably. |
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292 | |
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293 | You might know these functions under the name C<likely> and C<unlikely> |
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294 | - while these are common aliases, we find that the expect name is easier |
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295 | to understand when quickly skimming code. If you wish, you can use |
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296 | C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
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297 | C<ecb_expect_false> - these are simply aliases. |
291 | |
298 | |
292 | A very good example is in a function that reserves more space for some |
299 | A very good example is in a function that reserves more space for some |
293 | memory block (for example, inside an implementation of a string stream) - |
300 | memory block (for example, inside an implementation of a string stream) - |
294 | each time something is added, you have to check for a buffer overrun, but |
301 | each time something is added, you have to check for a buffer overrun, but |
295 | you expect that most checks will turn out to be false: |
302 | you expect that most checks will turn out to be false: |
296 | |
303 | |
297 | /* make sure we have "size" extra room in our buffer */ |
304 | /* make sure we have "size" extra room in our buffer */ |
298 | ecb_inline void |
305 | ecb_inline void |
299 | reserve (int size) |
306 | reserve (int size) |
300 | { |
307 | { |
301 | if (ecb_unlikely (current + size > end)) |
308 | if (ecb_expect_false (current + size > end)) |
302 | real_reserve_method (size); /* presumably noinline */ |
309 | real_reserve_method (size); /* presumably noinline */ |
303 | } |
310 | } |
304 | |
311 | |
305 | =item bool ecb_assume (cond) |
312 | =item bool ecb_assume (cond) |
306 | |
313 | |
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309 | |
316 | |
310 | This can be used to teach the compiler about invariants or other |
317 | This can be used to teach the compiler about invariants or other |
311 | conditions that might improve code generation, but which are impossible to |
318 | conditions that might improve code generation, but which are impossible to |
312 | deduce form the code itself. |
319 | deduce form the code itself. |
313 | |
320 | |
314 | For example, the example reservation function from the C<ecb_unlikely> |
321 | For example, the example reservation function from the C<ecb_expect_false> |
315 | description could be written thus (only C<ecb_assume> was added): |
322 | description could be written thus (only C<ecb_assume> was added): |
316 | |
323 | |
317 | ecb_inline void |
324 | ecb_inline void |
318 | reserve (int size) |
325 | reserve (int size) |
319 | { |
326 | { |
320 | if (ecb_unlikely (current + size > end)) |
327 | if (ecb_expect_false (current + size > end)) |
321 | real_reserve_method (size); /* presumably noinline */ |
328 | real_reserve_method (size); /* presumably noinline */ |
322 | |
329 | |
323 | ecb_assume (current + size <= end); |
330 | ecb_assume (current + size <= end); |
324 | } |
331 | } |
325 | |
332 | |
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386 | |
393 | |
387 | These two functions return true if the byte order is big endian |
394 | These two functions return true if the byte order is big endian |
388 | (most-significant byte first) or little endian (least-significant byte |
395 | (most-significant byte first) or little endian (least-significant byte |
389 | first) respectively. |
396 | first) respectively. |
390 | |
397 | |
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398 | On systems that are neither, their return values are unspecified. |
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399 | |
391 | =item int ecb_ctz32 (uint32_t x) |
400 | =item int ecb_ctz32 (uint32_t x) |
392 | |
401 | |
393 | Returns the index of the least significant bit set in C<x> (or |
402 | Returns the index of the least significant bit set in C<x> (or |
394 | equivalently the number of bits set to 0 before the least significant |
403 | equivalently the number of bits set to 0 before the least significant bit |
395 | bit set), starting from 0. If C<x> is 0 the result is undefined. A |
404 | set), starting from 0. If C<x> is 0 the result is undefined. A common use |
396 | common use case is to compute the integer binary logarithm, i.e., |
405 | case is to compute the integer binary logarithm, i.e., C<floor (log2 |
397 | floor(log2(n)). For example: |
406 | (n))>. For example: |
398 | |
407 | |
399 | ecb_ctz32 (3) = 0 |
408 | ecb_ctz32 (3) = 0 |
400 | ecb_ctz32 (6) = 1 |
409 | ecb_ctz32 (6) = 1 |
401 | |
410 | |
402 | =item int ecb_popcount32 (uint32_t x) |
411 | =item int ecb_popcount32 (uint32_t x) |
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429 | |
438 | |
430 | =over 4 |
439 | =over 4 |
431 | |
440 | |
432 | =item x = ecb_mod (m, n) |
441 | =item x = ecb_mod (m, n) |
433 | |
442 | |
434 | Returns the positive remainder of the modulo operation between C<m> and |
443 | Returns C<m> modulo C<n>, which is the same as the positive remainder |
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444 | of the division operation between C<m> and C<n>, using floored |
435 | C<n>. Unlike the C modulo operator C<%>, this function ensures that the |
445 | division. Unlike the C remainder operator C<%>, this function ensures that |
436 | return value is always positive - ISO C guarantees very little when |
446 | the return value is always positive and that the two numbers I<m> and |
437 | negative numbers are used with C<%>. |
447 | I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
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448 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
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449 | in the language. |
438 | |
450 | |
439 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
451 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
440 | negatable, that is, both C<m> and C<-m> must be representable in its |
452 | negatable, that is, both C<m> and C<-m> must be representable in its |
441 | type. |
453 | type (this typically includes the minimum signed integer value, the same |
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454 | limitation as for C</> and C<%> in C). |
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455 | |
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456 | Current GCC versions compile this into an efficient branchless sequence on |
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457 | many systems. |
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458 | |
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459 | For example, when you want to rotate forward through the members of an |
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460 | array for increasing C<m> (which might be negative), then you should use |
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461 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
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462 | change direction for negative values: |
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463 | |
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464 | for (m = -100; m <= 100; ++m) |
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465 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
442 | |
466 | |
443 | =back |
467 | =back |
444 | |
468 | |
445 | =head2 UTILITY |
469 | =head2 UTILITY |
446 | |
470 | |