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Revision 1.25 by root, Tue May 31 21:52:31 2011 UTC vs.
Revision 1.32 by sf-exg, Fri Jun 17 15:55:41 2011 UTC

… …
57refers to any kind of boolean value, not a specific type. 57refers to any kind of boolean value, not a specific type.
58 58
59=head2 GCC ATTRIBUTES 59=head2 GCC ATTRIBUTES
60 60
61A major part of libecb deals with GCC attributes. These are additional 61A major part of libecb deals with GCC attributes. These are additional
62attributes that you cna assign to functions, variables and sometimes even 62attributes that you can assign to functions, variables and sometimes even
63types - much like C<const> or C<volatile> in C. 63types - much like C<const> or C<volatile> in C.
64 64
65While GCC allows declarations to show up in many surprising places, 65While GCC allows declarations to show up in many surprising places,
66but not in many expeted places, the safest way is to put attribute 66but not in many expected places, the safest way is to put attribute
67declarations before the whole declaration: 67declarations before the whole declaration:
68 68
69 ecb_const int mysqrt (int a); 69 ecb_const int mysqrt (int a);
70 ecb_unused int i; 70 ecb_unused int i;
71 71
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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.
… …
184 199
185In addition to placing cold functions together (or at least away from hot 200In addition to placing cold functions together (or at least away from hot
186functions), this knowledge can be used in other ways, for example, the 201functions), this knowledge can be used in other ways, for example, the
187function will be optimised for size, as opposed to speed, and codepaths 202function will be optimised for size, as opposed to speed, and codepaths
188leading to calls to those functions can automatically be marked as if 203leading to calls to those functions can automatically be marked as if
189C<ecb_unlikely> had been used to reach them. 204C<ecb_expect_false> had been used to reach them.
190 205
191Good examples for such functions would be error reporting functions, or 206Good examples for such functions would be error reporting functions, or
192functions only called in exceptional or rare cases. 207functions only called in exceptional or rare cases.
193 208
194=item ecb_artificial 209=item ecb_artificial
… …
256 271
257Evaluates C<expr> and returns it. In addition, it tells the compiler that 272Evaluates C<expr> and returns it. In addition, it tells the compiler that
258the C<expr> evaluates to C<value> a lot, which can be used for static 273the C<expr> evaluates to C<value> a lot, which can be used for static
259branch optimisations. 274branch optimisations.
260 275
261Usually, you want to use the more intuitive C<ecb_likely> and 276Usually, you want to use the more intuitive C<ecb_expect_true> and
262C<ecb_unlikely> functions instead. 277C<ecb_expect_false> functions instead.
263 278
279=item bool ecb_expect_true (cond)
280
264=item bool ecb_likely (cond) 281=item bool ecb_expect_false (cond)
265
266=item bool ecb_unlikely (cond)
267 282
268These two functions expect a expression that is true or false and return 283These two functions expect a expression that is true or false and return
269C<1> or C<0>, respectively, so when used in the condition of an C<if> or 284C<1> or C<0>, respectively, so when used in the condition of an C<if> or
270other conditional statement, it will not change the program: 285other conditional statement, it will not change the program:
271 286
272 /* these two do the same thing */ 287 /* these two do the same thing */
273 if (some_condition) ...; 288 if (some_condition) ...;
274 if (ecb_likely (some_condition)) ...; 289 if (ecb_expect_true (some_condition)) ...;
275 290
276However, by using C<ecb_likely>, you tell the compiler that the condition 291However, by using C<ecb_expect_true>, you tell the compiler that the
277is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be 292condition is likely to be true (and for C<ecb_expect_false>, that it is
278true). 293unlikely to be true).
279 294
280For example, when you check for a null pointer and expect this to be a 295For example, when you check for a null pointer and expect this to be a
281rare, exceptional, case, then use C<ecb_unlikely>: 296rare, exceptional, case, then use C<ecb_expect_false>:
282 297
283 void my_free (void *ptr) 298 void my_free (void *ptr)
284 { 299 {
285 if (ecb_unlikely (ptr == 0)) 300 if (ecb_expect_false (ptr == 0))
286 return; 301 return;
287 } 302 }
288 303
289Consequent use of these functions to mark away exceptional cases or to 304Consequent use of these functions to mark away exceptional cases or to
290tell the compiler what the hot path through a function is can increase 305tell the compiler what the hot path through a function is can increase
291performance considerably. 306performance considerably.
307
308You 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
310to understand when quickly skimming code. If you wish, you can use
311C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
312C<ecb_expect_false> - these are simply aliases.
292 313
293A very good example is in a function that reserves more space for some 314A very good example is in a function that reserves more space for some
294memory block (for example, inside an implementation of a string stream) - 315memory block (for example, inside an implementation of a string stream) -
295each time something is added, you have to check for a buffer overrun, but 316each time something is added, you have to check for a buffer overrun, but
296you expect that most checks will turn out to be false: 317you expect that most checks will turn out to be false:
297 318
298 /* make sure we have "size" extra room in our buffer */ 319 /* make sure we have "size" extra room in our buffer */
299 ecb_inline void 320 ecb_inline void
300 reserve (int size) 321 reserve (int size)
301 { 322 {
302 if (ecb_unlikely (current + size > end)) 323 if (ecb_expect_false (current + size > end))
303 real_reserve_method (size); /* presumably noinline */ 324 real_reserve_method (size); /* presumably noinline */
304 } 325 }
305 326
306=item bool ecb_assume (cond) 327=item bool ecb_assume (cond)
307 328
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310 331
311This can be used to teach the compiler about invariants or other 332This can be used to teach the compiler about invariants or other
312conditions that might improve code generation, but which are impossible to 333conditions that might improve code generation, but which are impossible to
313deduce form the code itself. 334deduce form the code itself.
314 335
315For example, the example reservation function from the C<ecb_unlikely> 336For example, the example reservation function from the C<ecb_expect_false>
316description could be written thus (only C<ecb_assume> was added): 337description could be written thus (only C<ecb_assume> was added):
317 338
318 ecb_inline void 339 ecb_inline void
319 reserve (int size) 340 reserve (int size)
320 { 341 {
321 if (ecb_unlikely (current + size > end)) 342 if (ecb_expect_false (current + size > end))
322 real_reserve_method (size); /* presumably noinline */ 343 real_reserve_method (size); /* presumably noinline */
323 344
324 ecb_assume (current + size <= end); 345 ecb_assume (current + size <= end);
325 } 346 }
326 347
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393 414
394=item int ecb_ctz32 (uint32_t x) 415=item int ecb_ctz32 (uint32_t x)
395 416
396Returns the index of the least significant bit set in C<x> (or 417Returns the index of the least significant bit set in C<x> (or
397equivalently the number of bits set to 0 before the least significant bit 418equivalently the number of bits set to 0 before the least significant bit
398set), starting from 0. If C<x> is 0 the result is undefined. A common use 419set), starting from 0. If C<x> is 0 the result is undefined. For example:
399case is to compute the integer binary logarithm, i.e., C<floor (log2
400(n))>. For example:
401 420
402 ecb_ctz32 (3) = 0 421 ecb_ctz32 (3) = 0
403 ecb_ctz32 (6) = 1 422 ecb_ctz32 (6) = 1
404 423
405=item int ecb_popcount32 (uint32_t x) 424=item int ecb_popcount32 (uint32_t x)
… …
442C<ecb_mod> implements the mathematical modulo operation, which is missing 461C<ecb_mod> implements the mathematical modulo operation, which is missing
443in the language. 462in the language.
444 463
445C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be 464C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
446negatable, that is, both C<m> and C<-m> must be representable in its 465negatable, that is, both C<m> and C<-m> must be representable in its
447type (this typically includes the minimum signed integer value, the same 466type (this typically excludes the minimum signed integer value, the same
448limitation as for C</> and C<%> in C). 467limitation as for C</> and C<%> in C).
449 468
450Current GCC versions compile this into an efficient branchless sequence on 469Current GCC versions compile this into an efficient branchless sequence on
451many systems. 470almost all CPUs.
452 471
453For example, when you want to rotate forward through the members of an 472For example, when you want to rotate forward through the members of an
454array for increasing C<m> (which might be negative), then you should use 473array for increasing C<m> (which might be negative), then you should use
455C<ecb_mod>, as the C<%> operator might give either negative results, or 474C<ecb_mod>, as the C<%> operator might give either negative results, or
456change direction for negative values: 475change direction for negative values:

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