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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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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 |
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62 | attributes that you cna assign to functions, variables and sometimes even |
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63 | types - much like C<const> or C<volatile> in C. |
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64 | |
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65 | While GCC allows declarations to show up in many surprising places, |
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66 | but not in many expeted places, the safest way is to put attribute |
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67 | declarations before the whole declaration: |
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68 | |
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69 | ecb_const int mysqrt (int a); |
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70 | ecb_unused int i; |
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71 | |
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72 | For variables, it is often nicer to put the attribute after the name, and |
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73 | avoid multiple declarations using commas: |
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74 | |
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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 | |
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105 | { |
120 | { |
106 | puts (errline); |
121 | puts (errline); |
107 | abort (); |
122 | abort (); |
108 | } |
123 | } |
109 | |
124 | |
110 | In this case, the compiler would probbaly be smart enough to decude it on |
125 | 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. |
126 | its own, so this is mainly useful for declarations. |
112 | |
127 | |
113 | =item ecb_const |
128 | =item ecb_const |
114 | |
129 | |
115 | Declares that the function only depends on the values of it's arguments, |
130 | 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 |
131 | 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 |
132 | any memory any arguments might point to, global variables, or call any |
118 | non-const functions. It also must not have any side effects. |
133 | non-const functions. It also must not have any side effects. |
119 | |
134 | |
120 | Such a function can be optimised much more aggressively by the compiler - |
135 | 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 |
136 | 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 |
137 | a single call, which wouldn't be possible if the compiler would have to |
123 | expect any side effects. |
138 | expect any side effects. |
124 | |
139 | |
125 | It is best suited for functions in the sense of mathematical functions, |
140 | 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. |
141 | such as a function returning the square root of its input argument. |
127 | |
142 | |
128 | Not suited would be a function that calculates the hash of some memory |
143 | 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 |
144 | area you pass in, prints some messages or looks at a global variable to |
130 | decide on rounding. |
145 | decide on rounding. |
131 | |
146 | |
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154 | possible. |
169 | possible. |
155 | |
170 | |
156 | The compiler reacts by trying to place hot functions near to each other in |
171 | The compiler reacts by trying to place hot functions near to each other in |
157 | memory. |
172 | memory. |
158 | |
173 | |
159 | Whether a function is hot or not often depend son the whole program, |
174 | 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 |
175 | and less on the function itself. C<ecb_cold> is likely more useful in |
161 | practise. |
176 | practise. |
162 | |
177 | |
163 | =item ecb_cold |
178 | =item ecb_cold |
164 | |
179 | |
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169 | |
184 | |
170 | 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 |
171 | 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 |
172 | 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 |
173 | 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 |
174 | C<ecb_unlikel> had been used to reach them. |
189 | C<ecb_unlikely> had been used to reach them. |
175 | |
190 | |
176 | Good examples for such functions would be error reporting functions, or |
191 | Good examples for such functions would be error reporting functions, or |
177 | functions only called in exceptional or rare cases. |
192 | functions only called in exceptional or rare cases. |
178 | |
193 | |
179 | =item ecb_artificial |
194 | =item ecb_artificial |
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372 | |
387 | |
373 | These two functions return true if the byte order is big endian |
388 | These two functions return true if the byte order is big endian |
374 | (most-significant byte first) or little endian (least-significant byte |
389 | (most-significant byte first) or little endian (least-significant byte |
375 | first) respectively. |
390 | first) respectively. |
376 | |
391 | |
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392 | On systems that are neither, their return values are unspecified. |
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393 | |
377 | =item int ecb_ctz32 (uint32_t x) |
394 | =item int ecb_ctz32 (uint32_t x) |
378 | |
395 | |
379 | Returns the index of the least significant bit set in C<x> (or |
396 | 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 |
397 | 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 |
398 | set), starting from 0. If C<x> is 0 the result is undefined. A common use |
382 | common use case is to compute the integer binary logarithm, i.e., |
399 | case is to compute the integer binary logarithm, i.e., C<floor (log2 |
383 | floor(log2(n)). For example: |
400 | (n))>. For example: |
384 | |
401 | |
385 | ecb_ctz32 (3) = 0 |
402 | ecb_ctz32 (3) = 0 |
386 | ecb_ctz32 (6) = 1 |
403 | ecb_ctz32 (6) = 1 |
387 | |
404 | |
388 | =item int ecb_popcount32 (uint32_t x) |
405 | =item int ecb_popcount32 (uint32_t x) |
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394 | |
411 | |
395 | =item uint32_t ecb_bswap16 (uint32_t x) |
412 | =item uint32_t ecb_bswap16 (uint32_t x) |
396 | |
413 | |
397 | =item uint32_t ecb_bswap32 (uint32_t x) |
414 | =item uint32_t ecb_bswap32 (uint32_t x) |
398 | |
415 | |
399 | These two functions return the value of the 16-bit (32-bit) variable |
416 | These two functions return the value of the 16-bit (32-bit) value C<x> |
400 | C<x> after reversing the order of bytes. |
417 | after reversing the order of bytes (0x11223344 becomes 0x44332211). |
401 | |
418 | |
402 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
419 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
403 | |
420 | |
404 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
421 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
405 | |
422 | |
406 | These two functions return the value of C<x> after shifting all the bits |
423 | These two functions return the value of C<x> after rotating all the bits |
407 | by C<count> positions to the right or left respectively. |
424 | by C<count> positions to the right or left respectively. |
408 | |
425 | |
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426 | Current GCC versions understand these functions and usually compile them |
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427 | to "optimal" code (e.g. a single C<roll> on x86). |
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428 | |
409 | =back |
429 | =back |
410 | |
430 | |
411 | =head2 ARITHMETIC |
431 | =head2 ARITHMETIC |
412 | |
432 | |
413 | =over 4 |
433 | =over 4 |
414 | |
434 | |
415 | =item x = ecb_mod (m, n) |
435 | =item x = ecb_mod (m, n) |
416 | |
436 | |
417 | Returns the positive remainder of the modulo operation between C<m> and |
437 | Returns C<m> modulo C<n>, which is the same as the positive remainder |
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438 | 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 |
439 | division. Unlike the C remainder operator C<%>, this function ensures that |
419 | return value is always positive). |
440 | the return value is always positive and that the two numbers I<m> and |
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441 | I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
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442 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
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443 | in the language. |
420 | |
444 | |
421 | C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be |
445 | 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 |
446 | negatable, that is, both C<m> and C<-m> must be representable in its |
423 | type. |
447 | type (this typically includes the minimum signed integer value, the same |
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448 | limitation as for C</> and C<%> in C). |
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449 | |
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450 | Current GCC versions compile this into an efficient branchless sequence on |
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451 | many systems. |
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452 | |
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453 | For example, when you want to rotate forward through the members of an |
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454 | array for increasing C<m> (which might be negative), then you should use |
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455 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
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456 | change direction for negative values: |
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457 | |
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458 | for (m = -100; m <= 100; ++m) |
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459 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
424 | |
460 | |
425 | =back |
461 | =back |
426 | |
462 | |
427 | =head2 UTILITY |
463 | =head2 UTILITY |
428 | |
464 | |
429 | =over 4 |
465 | =over 4 |
430 | |
466 | |
431 | =item element_count = ecb_array_length (name) [MACRO] |
467 | =item element_count = ecb_array_length (name) |
432 | |
468 | |
433 | Returns the number of elements in the array C<name>. For example: |
469 | Returns the number of elements in the array C<name>. For example: |
434 | |
470 | |
435 | int primes[] = { 2, 3, 5, 7, 11 }; |
471 | int primes[] = { 2, 3, 5, 7, 11 }; |
436 | int sum = 0; |
472 | int sum = 0; |