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1 =head1 LIBECB - e-C-Builtins
2
3 =head2 ABOUT LIBECB
4
5 Libecb is currently a simple header file that doesn't require any
6 configuration to use or include in your project.
7
8 It's part of the e-suite of libraries, other members of which include
9 libev and libeio.
10
11 Its homepage can be found here:
12
13 http://software.schmorp.de/pkg/libecb
14
15 It mainly provides a number of wrappers around GCC built-ins, together
16 with replacement functions for other compilers. In addition to this,
17 it provides a number of other lowlevel C utilities, such as endianness
18 detection, byte swapping or bit rotations.
19
20 Or in other words, things that should be built into any standard C system,
21 but aren't, implemented as efficient as possible with GCC, and still
22 correct with other compilers.
23
24 More might come.
25
26 =head2 ABOUT THE HEADER
27
28 At the moment, all you have to do is copy F<ecb.h> somewhere where your
29 compiler can find it and include it:
30
31 #include <ecb.h>
32
33 The header should work fine for both C and C++ compilation, and gives you
34 all of F<inttypes.h> in addition to the ECB symbols.
35
36 There are currently no object files to link to - future versions might
37 come with an (optional) object code library to link against, to reduce
38 code size or gain access to additional features.
39
40 It also currently includes everything from F<inttypes.h>.
41
42 =head2 ABOUT THIS MANUAL / CONVENTIONS
43
44 This manual mainly describes each (public) function available after
45 including the F<ecb.h> header. The header might define other symbols than
46 these, but these are not part of the public API, and not supported in any
47 way.
48
49 When the manual mentions a "function" then this could be defined either as
50 as inline function, a macro, or an external symbol.
51
52 When functions use a concrete standard type, such as C<int> or
53 C<uint32_t>, then the corresponding function works only with that type. If
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
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.
58
59 =head2 TYPES / TYPE SUPPORT
60
61 ecb.h makes sure that the following types are defined (in the expected way):
62
63 int8_t uint8_t int16_t uint16_t
64 int32_t uint32_t int64_t uint64_t
65 intptr_t uintptr_t
66
67 The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
68 platform (currently C<4> or C<8>) and can be used in preprocessor
69 expressions.
70
71 For C<ptrdiff_t> and C<size_t> use C<stddef.h>.
72
73 =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
74
75 All the following symbols expand to an expression that can be tested in
76 preprocessor instructions as well as treated as a boolean (use C<!!> to
77 ensure it's either C<0> or C<1> if you need that).
78
79 =over 4
80
81 =item ECB_C
82
83 True if the implementation defines the C<__STDC__> macro to a true value,
84 while not claiming to be C++.
85
86 =item ECB_C99
87
88 True if the implementation claims to be compliant to C99 (ISO/IEC
89 9899:1999) or any later version, while not claiming to be C++.
90
91 Note that later versions (ECB_C11) remove core features again (for
92 example, variable length arrays).
93
94 =item ECB_C11
95
96 True if the implementation claims to be compliant to C11 (ISO/IEC
97 9899:2011) or any later version, while not claiming to be C++.
98
99 =item ECB_CPP
100
101 True if the implementation defines the C<__cplusplus__> macro to a true
102 value, which is typically true for C++ compilers.
103
104 =item ECB_CPP11
105
106 True if the implementation claims to be compliant to ISO/IEC 14882:2011
107 (C++11) or any later version.
108
109 =item ECB_GCC_VERSION (major, minor)
110
111 Expands to a true value (suitable for testing in by the preprocessor)
112 if the compiler used is GNU C and the version is the given version, or
113 higher.
114
115 This macro tries to return false on compilers that claim to be GCC
116 compatible but aren't.
117
118 =item ECB_EXTERN_C
119
120 Expands to C<extern "C"> in C++, and a simple C<extern> in C.
121
122 This can be used to declare a single external C function:
123
124 ECB_EXTERN_C int printf (const char *format, ...);
125
126 =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
127
128 These two macros can be used to wrap multiple C<extern "C"> definitions -
129 they expand to nothing in C.
130
131 They are most useful in header files:
132
133 ECB_EXTERN_C_BEG
134
135 int mycfun1 (int x);
136 int mycfun2 (int x);
137
138 ECB_EXTERN_C_END
139
140 =item ECB_STDFP
141
142 If this evaluates to a true value (suitable for testing in by the
143 preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
144 and double/binary64 representations internally I<and> the endianness of
145 both types match the endianness of C<uint32_t> and C<uint64_t>.
146
147 This means you can just copy the bits of a C<float> (or C<double>) to an
148 C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
149 without having to think about format or endianness.
150
151 This is true for basically all modern platforms, although F<ecb.h> might
152 not be able to deduce this correctly everywhere and might err on the safe
153 side.
154
155 =item ECB_AMD64, ECB_AMD64_X32
156
157 These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
158 ABI, respectively, and undefined elsewhere.
159
160 The designers of the new X32 ABI for some inexplicable reason decided to
161 make it look exactly like amd64, even though it's completely incompatible
162 to that ABI, breaking about every piece of software that assumed that
163 C<__x86_64> stands for, well, the x86-64 ABI, making these macros
164 necessary.
165
166 =back
167
168 =head2 MACRO TRICKERY
169
170 =over 4
171
172 =item ECB_CONCAT (a, b)
173
174 Expands any macros in C<a> and C<b>, then concatenates the result to form
175 a single token. This is mainly useful to form identifiers from components,
176 e.g.:
177
178 #define S1 str
179 #define S2 cpy
180
181 ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
182
183 =item ECB_STRINGIFY (arg)
184
185 Expands any macros in C<arg> and returns the stringified version of
186 it. This is mainly useful to get the contents of a macro in string form,
187 e.g.:
188
189 #define SQL_LIMIT 100
190 sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
191
192 =back
193
194 =head2 ATTRIBUTES
195
196 A major part of libecb deals with additional attributes that can be
197 assigned to functions, variables and sometimes even types - much like
198 C<const> or C<volatile> in C. They are implemented using either GCC
199 attributes or other compiler/language specific features. Attributes
200 declarations must be put before the whole declaration:
201
202 ecb_const int mysqrt (int a);
203 ecb_unused int i;
204
205 =over 4
206
207 =item ecb_unused
208
209 Marks a function or a variable as "unused", which simply suppresses a
210 warning by GCC when it detects it as unused. This is useful when you e.g.
211 declare a variable but do not always use it:
212
213 {
214 ecb_unused int var;
215
216 #ifdef SOMECONDITION
217 var = ...;
218 return var;
219 #else
220 return 0;
221 #endif
222 }
223
224 =item ecb_deprecated
225
226 Similar to C<ecb_unused>, but marks a function, variable or type as
227 deprecated. This makes some compilers warn when the type is used.
228
229 =item ecb_deprecated_message (message)
230
231 Same as C<ecb_deprecated>, but if possible, supply a diagnostic that is
232 used instead of a generic depreciation message when the object is being
233 used.
234
235 =item ecb_inline
236
237 Expands either to C<static inline> or to just C<static>, if inline
238 isn't supported. It should be used to declare functions that should be
239 inlined, for code size or speed reasons.
240
241 Example: inline this function, it surely will reduce codesize.
242
243 ecb_inline int
244 negmul (int a, int b)
245 {
246 return - (a * b);
247 }
248
249 =item ecb_noinline
250
251 Prevent a function from being inlined - it might be optimised away, but
252 not inlined into other functions. This is useful if you know your function
253 is rarely called and large enough for inlining not to be helpful.
254
255 =item ecb_noreturn
256
257 Marks a function as "not returning, ever". Some typical functions that
258 don't return are C<exit> or C<abort> (which really works hard to not
259 return), and now you can make your own:
260
261 ecb_noreturn void
262 my_abort (const char *errline)
263 {
264 puts (errline);
265 abort ();
266 }
267
268 In this case, the compiler would probably be smart enough to deduce it on
269 its own, so this is mainly useful for declarations.
270
271 =item ecb_restrict
272
273 Expands to the C<restrict> keyword or equivalent on compilers that support
274 them, and to nothing on others. Must be specified on a pointer type or
275 an array index to indicate that the memory doesn't alias with any other
276 restricted pointer in the same scope.
277
278 Example: multiply a vector, and allow the compiler to parallelise the
279 loop, because it knows it doesn't overwrite input values.
280
281 void
282 multiply (ecb_restrict float *src,
283 ecb_restrict float *dst,
284 int len, float factor)
285 {
286 int i;
287
288 for (i = 0; i < len; ++i)
289 dst [i] = src [i] * factor;
290 }
291
292 =item ecb_const
293
294 Declares that the function only depends on the values of its arguments,
295 much like a mathematical function. It specifically does not read or write
296 any memory any arguments might point to, global variables, or call any
297 non-const functions. It also must not have any side effects.
298
299 Such a function can be optimised much more aggressively by the compiler -
300 for example, multiple calls with the same arguments can be optimised into
301 a single call, which wouldn't be possible if the compiler would have to
302 expect any side effects.
303
304 It is best suited for functions in the sense of mathematical functions,
305 such as a function returning the square root of its input argument.
306
307 Not suited would be a function that calculates the hash of some memory
308 area you pass in, prints some messages or looks at a global variable to
309 decide on rounding.
310
311 See C<ecb_pure> for a slightly less restrictive class of functions.
312
313 =item ecb_pure
314
315 Similar to C<ecb_const>, declares a function that has no side
316 effects. Unlike C<ecb_const>, the function is allowed to examine global
317 variables and any other memory areas (such as the ones passed to it via
318 pointers).
319
320 While these functions cannot be optimised as aggressively as C<ecb_const>
321 functions, they can still be optimised away in many occasions, and the
322 compiler has more freedom in moving calls to them around.
323
324 Typical examples for such functions would be C<strlen> or C<memcmp>. A
325 function that calculates the MD5 sum of some input and updates some MD5
326 state passed as argument would I<NOT> be pure, however, as it would modify
327 some memory area that is not the return value.
328
329 =item ecb_hot
330
331 This declares a function as "hot" with regards to the cache - the function
332 is used so often, that it is very beneficial to keep it in the cache if
333 possible.
334
335 The compiler reacts by trying to place hot functions near to each other in
336 memory.
337
338 Whether a function is hot or not often depends on the whole program,
339 and less on the function itself. C<ecb_cold> is likely more useful in
340 practise.
341
342 =item ecb_cold
343
344 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
345 the cache, or in other words, this function is not called often, or not at
346 speed-critical times, and keeping it in the cache might be a waste of said
347 cache.
348
349 In addition to placing cold functions together (or at least away from hot
350 functions), this knowledge can be used in other ways, for example, the
351 function will be optimised for size, as opposed to speed, and codepaths
352 leading to calls to those functions can automatically be marked as if
353 C<ecb_expect_false> had been used to reach them.
354
355 Good examples for such functions would be error reporting functions, or
356 functions only called in exceptional or rare cases.
357
358 =item ecb_artificial
359
360 Declares the function as "artificial", in this case meaning that this
361 function is not really meant to be a function, but more like an accessor
362 - many methods in C++ classes are mere accessor functions, and having a
363 crash reported in such a method, or single-stepping through them, is not
364 usually so helpful, especially when it's inlined to just a few instructions.
365
366 Marking them as artificial will instruct the debugger about just this,
367 leading to happier debugging and thus happier lives.
368
369 Example: in some kind of smart-pointer class, mark the pointer accessor as
370 artificial, so that the whole class acts more like a pointer and less like
371 some C++ abstraction monster.
372
373 template<typename T>
374 struct my_smart_ptr
375 {
376 T *value;
377
378 ecb_artificial
379 operator T *()
380 {
381 return value;
382 }
383 };
384
385 =back
386
387 =head2 OPTIMISATION HINTS
388
389 =over 4
390
391 =item bool ecb_is_constant (expr)
392
393 Returns true iff the expression can be deduced to be a compile-time
394 constant, and false otherwise.
395
396 For example, when you have a C<rndm16> function that returns a 16 bit
397 random number, and you have a function that maps this to a range from
398 0..n-1, then you could use this inline function in a header file:
399
400 ecb_inline uint32_t
401 rndm (uint32_t n)
402 {
403 return (n * (uint32_t)rndm16 ()) >> 16;
404 }
405
406 However, for powers of two, you could use a normal mask, but that is only
407 worth it if, at compile time, you can detect this case. This is the case
408 when the passed number is a constant and also a power of two (C<n & (n -
409 1) == 0>):
410
411 ecb_inline uint32_t
412 rndm (uint32_t n)
413 {
414 return is_constant (n) && !(n & (n - 1))
415 ? rndm16 () & (num - 1)
416 : (n * (uint32_t)rndm16 ()) >> 16;
417 }
418
419 =item ecb_expect (expr, value)
420
421 Evaluates C<expr> and returns it. In addition, it tells the compiler that
422 the C<expr> evaluates to C<value> a lot, which can be used for static
423 branch optimisations.
424
425 Usually, you want to use the more intuitive C<ecb_expect_true> and
426 C<ecb_expect_false> functions instead.
427
428 =item bool ecb_expect_true (cond)
429
430 =item bool ecb_expect_false (cond)
431
432 These two functions expect a expression that is true or false and return
433 C<1> or C<0>, respectively, so when used in the condition of an C<if> or
434 other conditional statement, it will not change the program:
435
436 /* these two do the same thing */
437 if (some_condition) ...;
438 if (ecb_expect_true (some_condition)) ...;
439
440 However, by using C<ecb_expect_true>, you tell the compiler that the
441 condition is likely to be true (and for C<ecb_expect_false>, that it is
442 unlikely to be true).
443
444 For example, when you check for a null pointer and expect this to be a
445 rare, exceptional, case, then use C<ecb_expect_false>:
446
447 void my_free (void *ptr)
448 {
449 if (ecb_expect_false (ptr == 0))
450 return;
451 }
452
453 Consequent use of these functions to mark away exceptional cases or to
454 tell the compiler what the hot path through a function is can increase
455 performance considerably.
456
457 You might know these functions under the name C<likely> and C<unlikely>
458 - while these are common aliases, we find that the expect name is easier
459 to understand when quickly skimming code. If you wish, you can use
460 C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
461 C<ecb_expect_false> - these are simply aliases.
462
463 A very good example is in a function that reserves more space for some
464 memory block (for example, inside an implementation of a string stream) -
465 each time something is added, you have to check for a buffer overrun, but
466 you expect that most checks will turn out to be false:
467
468 /* make sure we have "size" extra room in our buffer */
469 ecb_inline void
470 reserve (int size)
471 {
472 if (ecb_expect_false (current + size > end))
473 real_reserve_method (size); /* presumably noinline */
474 }
475
476 =item ecb_assume (cond)
477
478 Try to tell the compiler that some condition is true, even if it's not
479 obvious.
480
481 This can be used to teach the compiler about invariants or other
482 conditions that might improve code generation, but which are impossible to
483 deduce form the code itself.
484
485 For example, the example reservation function from the C<ecb_expect_false>
486 description could be written thus (only C<ecb_assume> was added):
487
488 ecb_inline void
489 reserve (int size)
490 {
491 if (ecb_expect_false (current + size > end))
492 real_reserve_method (size); /* presumably noinline */
493
494 ecb_assume (current + size <= end);
495 }
496
497 If you then call this function twice, like this:
498
499 reserve (10);
500 reserve (1);
501
502 Then the compiler I<might> be able to optimise out the second call
503 completely, as it knows that C<< current + 1 > end >> is false and the
504 call will never be executed.
505
506 =item ecb_unreachable ()
507
508 This function does nothing itself, except tell the compiler that it will
509 never be executed. Apart from suppressing a warning in some cases, this
510 function can be used to implement C<ecb_assume> or similar functions.
511
512 =item ecb_prefetch (addr, rw, locality)
513
514 Tells the compiler to try to prefetch memory at the given C<addr>ess
515 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
516 C<0> means that there will only be one access later, C<3> means that
517 the data will likely be accessed very often, and values in between mean
518 something... in between. The memory pointed to by the address does not
519 need to be accessible (it could be a null pointer for example), but C<rw>
520 and C<locality> must be compile-time constants.
521
522 An obvious way to use this is to prefetch some data far away, in a big
523 array you loop over. This prefetches memory some 128 array elements later,
524 in the hope that it will be ready when the CPU arrives at that location.
525
526 int sum = 0;
527
528 for (i = 0; i < N; ++i)
529 {
530 sum += arr [i]
531 ecb_prefetch (arr + i + 128, 0, 0);
532 }
533
534 It's hard to predict how far to prefetch, and most CPUs that can prefetch
535 are often good enough to predict this kind of behaviour themselves. It
536 gets more interesting with linked lists, especially when you do some fair
537 processing on each list element:
538
539 for (node *n = start; n; n = n->next)
540 {
541 ecb_prefetch (n->next, 0, 0);
542 ... do medium amount of work with *n
543 }
544
545 After processing the node, (part of) the next node might already be in
546 cache.
547
548 =back
549
550 =head2 BIT FIDDLING / BIT WIZARDRY
551
552 =over 4
553
554 =item bool ecb_big_endian ()
555
556 =item bool ecb_little_endian ()
557
558 These two functions return true if the byte order is big endian
559 (most-significant byte first) or little endian (least-significant byte
560 first) respectively.
561
562 On systems that are neither, their return values are unspecified.
563
564 =item int ecb_ctz32 (uint32_t x)
565
566 =item int ecb_ctz64 (uint64_t x)
567
568 Returns the index of the least significant bit set in C<x> (or
569 equivalently the number of bits set to 0 before the least significant bit
570 set), starting from 0. If C<x> is 0 the result is undefined.
571
572 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
573
574 For example:
575
576 ecb_ctz32 (3) = 0
577 ecb_ctz32 (6) = 1
578
579 =item bool ecb_is_pot32 (uint32_t x)
580
581 =item bool ecb_is_pot64 (uint32_t x)
582
583 Return true iff C<x> is a power of two or C<x == 0>.
584
585 For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
586
587 =item int ecb_ld32 (uint32_t x)
588
589 =item int ecb_ld64 (uint64_t x)
590
591 Returns the index of the most significant bit set in C<x>, or the number
592 of digits the number requires in binary (so that C<< 2**ld <= x <
593 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
594 to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
595 example to see how many bits a certain number requires to be encoded.
596
597 This function is similar to the "count leading zero bits" function, except
598 that that one returns how many zero bits are "in front" of the number (in
599 the given data type), while C<ecb_ld> returns how many bits the number
600 itself requires.
601
602 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
603
604 =item int ecb_popcount32 (uint32_t x)
605
606 =item int ecb_popcount64 (uint64_t x)
607
608 Returns the number of bits set to 1 in C<x>.
609
610 For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
611
612 For example:
613
614 ecb_popcount32 (7) = 3
615 ecb_popcount32 (255) = 8
616
617 =item uint8_t ecb_bitrev8 (uint8_t x)
618
619 =item uint16_t ecb_bitrev16 (uint16_t x)
620
621 =item uint32_t ecb_bitrev32 (uint32_t x)
622
623 Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
624 and so on.
625
626 Example:
627
628 ecb_bitrev8 (0xa7) = 0xea
629 ecb_bitrev32 (0xffcc4411) = 0x882233ff
630
631 =item uint32_t ecb_bswap16 (uint32_t x)
632
633 =item uint32_t ecb_bswap32 (uint32_t x)
634
635 =item uint64_t ecb_bswap64 (uint64_t x)
636
637 These functions return the value of the 16-bit (32-bit, 64-bit) value
638 C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
639 C<ecb_bswap32>).
640
641 =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
642
643 =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
644
645 =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
646
647 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
648
649 =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
650
651 =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
652
653 =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
654
655 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
656
657 These two families of functions return the value of C<x> after rotating
658 all the bits by C<count> positions to the right (C<ecb_rotr>) or left
659 (C<ecb_rotl>).
660
661 Current GCC versions understand these functions and usually compile them
662 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
663 x86).
664
665 =back
666
667 =head2 FLOATING POINT FIDDLING
668
669 =over 4
670
671 =item ECB_INFINITY
672
673 Evaluates to positive infinity if supported by the platform, otherwise to
674 a truly huge number.
675
676 =item ECB_NAN
677
678 Evaluates to a quiet NAN if supported by the platform, otherwise to
679 C<ECB_INFINITY>.
680
681 =item float ecb_ldexpf (float x, int exp)
682
683 Same as C<ldexpf>, but always available.
684
685 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
686
687 =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
688
689 These functions each take an argument in the native C<float> or C<double>
690 type and return the IEEE 754 bit representation of it.
691
692 The bit representation is just as IEEE 754 defines it, i.e. the sign bit
693 will be the most significant bit, followed by exponent and mantissa.
694
695 This function should work even when the native floating point format isn't
696 IEEE compliant, of course at a speed and code size penalty, and of course
697 also within reasonable limits (it tries to convert NaNs, infinities and
698 denormals, but will likely convert negative zero to positive zero).
699
700 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
701 be able to optimise away this function completely.
702
703 These functions can be helpful when serialising floats to the network - you
704 can serialise the return value like a normal uint32_t/uint64_t.
705
706 Another use for these functions is to manipulate floating point values
707 directly.
708
709 Silly example: toggle the sign bit of a float.
710
711 /* On gcc-4.7 on amd64, */
712 /* this results in a single add instruction to toggle the bit, and 4 extra */
713 /* instructions to move the float value to an integer register and back. */
714
715 x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
716
717 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
718
719 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
720
721 =item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
722
723 The reverse operation of the previous function - takes the bit
724 representation of an IEEE binary16, binary32 or binary64 number and
725 converts it to the native C<float> or C<double> format.
726
727 This function should work even when the native floating point format isn't
728 IEEE compliant, of course at a speed and code size penalty, and of course
729 also within reasonable limits (it tries to convert normals and denormals,
730 and might be lucky for infinities, and with extraordinary luck, also for
731 negative zero).
732
733 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
734 be able to optimise away this function completely.
735
736 =back
737
738 =head2 ARITHMETIC
739
740 =over 4
741
742 =item x = ecb_mod (m, n)
743
744 Returns C<m> modulo C<n>, which is the same as the positive remainder
745 of the division operation between C<m> and C<n>, using floored
746 division. Unlike the C remainder operator C<%>, this function ensures that
747 the return value is always positive and that the two numbers I<m> and
748 I<m' = m + i * n> result in the same value modulo I<n> - in other words,
749 C<ecb_mod> implements the mathematical modulo operation, which is missing
750 in the language.
751
752 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
753 negatable, that is, both C<m> and C<-m> must be representable in its
754 type (this typically excludes the minimum signed integer value, the same
755 limitation as for C</> and C<%> in C).
756
757 Current GCC versions compile this into an efficient branchless sequence on
758 almost all CPUs.
759
760 For example, when you want to rotate forward through the members of an
761 array for increasing C<m> (which might be negative), then you should use
762 C<ecb_mod>, as the C<%> operator might give either negative results, or
763 change direction for negative values:
764
765 for (m = -100; m <= 100; ++m)
766 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
767
768 =item x = ecb_div_rd (val, div)
769
770 =item x = ecb_div_ru (val, div)
771
772 Returns C<val> divided by C<div> rounded down or up, respectively.
773 C<val> and C<div> must have integer types and C<div> must be strictly
774 positive. Note that these functions are implemented with macros in C
775 and with function templates in C++.
776
777 =back
778
779 =head2 UTILITY
780
781 =over 4
782
783 =item element_count = ecb_array_length (name)
784
785 Returns the number of elements in the array C<name>. For example:
786
787 int primes[] = { 2, 3, 5, 7, 11 };
788 int sum = 0;
789
790 for (i = 0; i < ecb_array_length (primes); i++)
791 sum += primes [i];
792
793 =back
794
795 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
796
797 These symbols need to be defined before including F<ecb.h> the first time.
798
799 =over 4
800
801 =item ECB_NO_THREADS
802
803 If F<ecb.h> is never used from multiple threads, then this symbol can
804 be defined, in which case memory fences (and similar constructs) are
805 completely removed, leading to more efficient code and fewer dependencies.
806
807 Setting this symbol to a true value implies C<ECB_NO_SMP>.
808
809 =item ECB_NO_SMP
810
811 The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
812 multiple threads, but never concurrently (e.g. if the system the program
813 runs on has only a single CPU with a single core, no hyperthreading and so
814 on), then this symbol can be defined, leading to more efficient code and
815 fewer dependencies.
816
817 =item ECB_NO_LIBM
818
819 When defined to C<1>, do not export any functions that might introduce
820 dependencies on the math library (usually called F<-lm>) - these are
821 marked with [-UECB_NO_LIBM].
822
823 =back
824
825