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# Content
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/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 GCC ATTRIBUTES
169
170 A major part of libecb deals with GCC attributes. These are additional
171 attributes that you can assign to functions, variables and sometimes even
172 types - much like C<const> or C<volatile> in C.
173
174 While GCC allows declarations to show up in many surprising places,
175 but not in many expected places, the safest way is to put attribute
176 declarations before the whole declaration:
177
178 ecb_const int mysqrt (int a);
179 ecb_unused int i;
180
181 For variables, it is often nicer to put the attribute after the name, and
182 avoid multiple declarations using commas:
183
184 int i ecb_unused;
185
186 =over 4
187
188 =item ecb_attribute ((attrs...))
189
190 A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
191 nothing on other compilers, so the effect is that only GCC sees these.
192
193 Example: use the C<deprecated> attribute on a function.
194
195 ecb_attribute((__deprecated__)) void
196 do_not_use_me_anymore (void);
197
198 =item ecb_unused
199
200 Marks a function or a variable as "unused", which simply suppresses a
201 warning by GCC when it detects it as unused. This is useful when you e.g.
202 declare a variable but do not always use it:
203
204 {
205 int var ecb_unused;
206
207 #ifdef SOMECONDITION
208 var = ...;
209 return var;
210 #else
211 return 0;
212 #endif
213 }
214
215 =item ecb_inline
216
217 This is not actually an attribute, but you use it like one. It expands
218 either to C<static inline> or to just C<static>, if inline isn't
219 supported. It should be used to declare functions that should be inlined,
220 for code size or speed reasons.
221
222 Example: inline this function, it surely will reduce codesize.
223
224 ecb_inline int
225 negmul (int a, int b)
226 {
227 return - (a * b);
228 }
229
230 =item ecb_noinline
231
232 Prevent a function from being inlined - it might be optimised away, but
233 not inlined into other functions. This is useful if you know your function
234 is rarely called and large enough for inlining not to be helpful.
235
236 =item ecb_noreturn
237
238 Marks a function as "not returning, ever". Some typical functions that
239 don't return are C<exit> or C<abort> (which really works hard to not
240 return), and now you can make your own:
241
242 ecb_noreturn void
243 my_abort (const char *errline)
244 {
245 puts (errline);
246 abort ();
247 }
248
249 In this case, the compiler would probably be smart enough to deduce it on
250 its own, so this is mainly useful for declarations.
251
252 =item ecb_restrict
253
254 Expands to the C<restrict> keyword or equivalent on compilers that support
255 them, and to nothing on others. Must be specified on a pointer type or
256 an array index to indicate that the memory doesn't alias with any other
257 restricted pointer in the same scope.
258
259 Example: multiply a vector, and allow the compiler to parallelise the
260 loop, because it knows it doesn't overwrite input values.
261
262 void
263 multiply (float *ecb_restrict src,
264 float *ecb_restrict dst,
265 int len, float factor)
266 {
267 int i;
268
269 for (i = 0; i < len; ++i)
270 dst [i] = src [i] * factor;
271 }
272
273 =item ecb_const
274
275 Declares that the function only depends on the values of its arguments,
276 much like a mathematical function. It specifically does not read or write
277 any memory any arguments might point to, global variables, or call any
278 non-const functions. It also must not have any side effects.
279
280 Such a function can be optimised much more aggressively by the compiler -
281 for example, multiple calls with the same arguments can be optimised into
282 a single call, which wouldn't be possible if the compiler would have to
283 expect any side effects.
284
285 It is best suited for functions in the sense of mathematical functions,
286 such as a function returning the square root of its input argument.
287
288 Not suited would be a function that calculates the hash of some memory
289 area you pass in, prints some messages or looks at a global variable to
290 decide on rounding.
291
292 See C<ecb_pure> for a slightly less restrictive class of functions.
293
294 =item ecb_pure
295
296 Similar to C<ecb_const>, declares a function that has no side
297 effects. Unlike C<ecb_const>, the function is allowed to examine global
298 variables and any other memory areas (such as the ones passed to it via
299 pointers).
300
301 While these functions cannot be optimised as aggressively as C<ecb_const>
302 functions, they can still be optimised away in many occasions, and the
303 compiler has more freedom in moving calls to them around.
304
305 Typical examples for such functions would be C<strlen> or C<memcmp>. A
306 function that calculates the MD5 sum of some input and updates some MD5
307 state passed as argument would I<NOT> be pure, however, as it would modify
308 some memory area that is not the return value.
309
310 =item ecb_hot
311
312 This declares a function as "hot" with regards to the cache - the function
313 is used so often, that it is very beneficial to keep it in the cache if
314 possible.
315
316 The compiler reacts by trying to place hot functions near to each other in
317 memory.
318
319 Whether a function is hot or not often depends on the whole program,
320 and less on the function itself. C<ecb_cold> is likely more useful in
321 practise.
322
323 =item ecb_cold
324
325 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
326 the cache, or in other words, this function is not called often, or not at
327 speed-critical times, and keeping it in the cache might be a waste of said
328 cache.
329
330 In addition to placing cold functions together (or at least away from hot
331 functions), this knowledge can be used in other ways, for example, the
332 function will be optimised for size, as opposed to speed, and codepaths
333 leading to calls to those functions can automatically be marked as if
334 C<ecb_expect_false> had been used to reach them.
335
336 Good examples for such functions would be error reporting functions, or
337 functions only called in exceptional or rare cases.
338
339 =item ecb_artificial
340
341 Declares the function as "artificial", in this case meaning that this
342 function is not really meant to be a function, but more like an accessor
343 - many methods in C++ classes are mere accessor functions, and having a
344 crash reported in such a method, or single-stepping through them, is not
345 usually so helpful, especially when it's inlined to just a few instructions.
346
347 Marking them as artificial will instruct the debugger about just this,
348 leading to happier debugging and thus happier lives.
349
350 Example: in some kind of smart-pointer class, mark the pointer accessor as
351 artificial, so that the whole class acts more like a pointer and less like
352 some C++ abstraction monster.
353
354 template<typename T>
355 struct my_smart_ptr
356 {
357 T *value;
358
359 ecb_artificial
360 operator T *()
361 {
362 return value;
363 }
364 };
365
366 =back
367
368 =head2 OPTIMISATION HINTS
369
370 =over 4
371
372 =item bool ecb_is_constant(expr)
373
374 Returns true iff the expression can be deduced to be a compile-time
375 constant, and false otherwise.
376
377 For example, when you have a C<rndm16> function that returns a 16 bit
378 random number, and you have a function that maps this to a range from
379 0..n-1, then you could use this inline function in a header file:
380
381 ecb_inline uint32_t
382 rndm (uint32_t n)
383 {
384 return (n * (uint32_t)rndm16 ()) >> 16;
385 }
386
387 However, for powers of two, you could use a normal mask, but that is only
388 worth it if, at compile time, you can detect this case. This is the case
389 when the passed number is a constant and also a power of two (C<n & (n -
390 1) == 0>):
391
392 ecb_inline uint32_t
393 rndm (uint32_t n)
394 {
395 return is_constant (n) && !(n & (n - 1))
396 ? rndm16 () & (num - 1)
397 : (n * (uint32_t)rndm16 ()) >> 16;
398 }
399
400 =item bool ecb_expect (expr, value)
401
402 Evaluates C<expr> and returns it. In addition, it tells the compiler that
403 the C<expr> evaluates to C<value> a lot, which can be used for static
404 branch optimisations.
405
406 Usually, you want to use the more intuitive C<ecb_expect_true> and
407 C<ecb_expect_false> functions instead.
408
409 =item bool ecb_expect_true (cond)
410
411 =item bool ecb_expect_false (cond)
412
413 These two functions expect a expression that is true or false and return
414 C<1> or C<0>, respectively, so when used in the condition of an C<if> or
415 other conditional statement, it will not change the program:
416
417 /* these two do the same thing */
418 if (some_condition) ...;
419 if (ecb_expect_true (some_condition)) ...;
420
421 However, by using C<ecb_expect_true>, you tell the compiler that the
422 condition is likely to be true (and for C<ecb_expect_false>, that it is
423 unlikely to be true).
424
425 For example, when you check for a null pointer and expect this to be a
426 rare, exceptional, case, then use C<ecb_expect_false>:
427
428 void my_free (void *ptr)
429 {
430 if (ecb_expect_false (ptr == 0))
431 return;
432 }
433
434 Consequent use of these functions to mark away exceptional cases or to
435 tell the compiler what the hot path through a function is can increase
436 performance considerably.
437
438 You might know these functions under the name C<likely> and C<unlikely>
439 - while these are common aliases, we find that the expect name is easier
440 to understand when quickly skimming code. If you wish, you can use
441 C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
442 C<ecb_expect_false> - these are simply aliases.
443
444 A very good example is in a function that reserves more space for some
445 memory block (for example, inside an implementation of a string stream) -
446 each time something is added, you have to check for a buffer overrun, but
447 you expect that most checks will turn out to be false:
448
449 /* make sure we have "size" extra room in our buffer */
450 ecb_inline void
451 reserve (int size)
452 {
453 if (ecb_expect_false (current + size > end))
454 real_reserve_method (size); /* presumably noinline */
455 }
456
457 =item bool ecb_assume (cond)
458
459 Try to tell the compiler that some condition is true, even if it's not
460 obvious.
461
462 This can be used to teach the compiler about invariants or other
463 conditions that might improve code generation, but which are impossible to
464 deduce form the code itself.
465
466 For example, the example reservation function from the C<ecb_expect_false>
467 description could be written thus (only C<ecb_assume> was added):
468
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 ecb_assume (current + size <= end);
476 }
477
478 If you then call this function twice, like this:
479
480 reserve (10);
481 reserve (1);
482
483 Then the compiler I<might> be able to optimise out the second call
484 completely, as it knows that C<< current + 1 > end >> is false and the
485 call will never be executed.
486
487 =item bool ecb_unreachable ()
488
489 This function does nothing itself, except tell the compiler that it will
490 never be executed. Apart from suppressing a warning in some cases, this
491 function can be used to implement C<ecb_assume> or similar functions.
492
493 =item bool ecb_prefetch (addr, rw, locality)
494
495 Tells the compiler to try to prefetch memory at the given C<addr>ess
496 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
497 C<0> means that there will only be one access later, C<3> means that
498 the data will likely be accessed very often, and values in between mean
499 something... in between. The memory pointed to by the address does not
500 need to be accessible (it could be a null pointer for example), but C<rw>
501 and C<locality> must be compile-time constants.
502
503 An obvious way to use this is to prefetch some data far away, in a big
504 array you loop over. This prefetches memory some 128 array elements later,
505 in the hope that it will be ready when the CPU arrives at that location.
506
507 int sum = 0;
508
509 for (i = 0; i < N; ++i)
510 {
511 sum += arr [i]
512 ecb_prefetch (arr + i + 128, 0, 0);
513 }
514
515 It's hard to predict how far to prefetch, and most CPUs that can prefetch
516 are often good enough to predict this kind of behaviour themselves. It
517 gets more interesting with linked lists, especially when you do some fair
518 processing on each list element:
519
520 for (node *n = start; n; n = n->next)
521 {
522 ecb_prefetch (n->next, 0, 0);
523 ... do medium amount of work with *n
524 }
525
526 After processing the node, (part of) the next node might already be in
527 cache.
528
529 =back
530
531 =head2 BIT FIDDLING / BIT WIZARDRY
532
533 =over 4
534
535 =item bool ecb_big_endian ()
536
537 =item bool ecb_little_endian ()
538
539 These two functions return true if the byte order is big endian
540 (most-significant byte first) or little endian (least-significant byte
541 first) respectively.
542
543 On systems that are neither, their return values are unspecified.
544
545 =item int ecb_ctz32 (uint32_t x)
546
547 =item int ecb_ctz64 (uint64_t x)
548
549 Returns the index of the least significant bit set in C<x> (or
550 equivalently the number of bits set to 0 before the least significant bit
551 set), starting from 0. If C<x> is 0 the result is undefined.
552
553 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
554
555 For example:
556
557 ecb_ctz32 (3) = 0
558 ecb_ctz32 (6) = 1
559
560 =item bool ecb_is_pot32 (uint32_t x)
561
562 =item bool ecb_is_pot64 (uint32_t x)
563
564 Return true iff C<x> is a power of two or C<x == 0>.
565
566 For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
567
568 =item int ecb_ld32 (uint32_t x)
569
570 =item int ecb_ld64 (uint64_t x)
571
572 Returns the index of the most significant bit set in C<x>, or the number
573 of digits the number requires in binary (so that C<< 2**ld <= x <
574 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
575 to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
576 example to see how many bits a certain number requires to be encoded.
577
578 This function is similar to the "count leading zero bits" function, except
579 that that one returns how many zero bits are "in front" of the number (in
580 the given data type), while C<ecb_ld> returns how many bits the number
581 itself requires.
582
583 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
584
585 =item int ecb_popcount32 (uint32_t x)
586
587 =item int ecb_popcount64 (uint64_t x)
588
589 Returns the number of bits set to 1 in C<x>.
590
591 For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
592
593 For example:
594
595 ecb_popcount32 (7) = 3
596 ecb_popcount32 (255) = 8
597
598 =item uint8_t ecb_bitrev8 (uint8_t x)
599
600 =item uint16_t ecb_bitrev16 (uint16_t x)
601
602 =item uint32_t ecb_bitrev32 (uint32_t x)
603
604 Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
605 and so on.
606
607 Example:
608
609 ecb_bitrev8 (0xa7) = 0xea
610 ecb_bitrev32 (0xffcc4411) = 0x882233ff
611
612 =item uint32_t ecb_bswap16 (uint32_t x)
613
614 =item uint32_t ecb_bswap32 (uint32_t x)
615
616 =item uint64_t ecb_bswap64 (uint64_t x)
617
618 These functions return the value of the 16-bit (32-bit, 64-bit) value
619 C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
620 C<ecb_bswap32>).
621
622 =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
623
624 =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
625
626 =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
627
628 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
629
630 =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
631
632 =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
633
634 =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
635
636 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
637
638 These two families of functions return the value of C<x> after rotating
639 all the bits by C<count> positions to the right (C<ecb_rotr>) or left
640 (C<ecb_rotl>).
641
642 Current GCC versions understand these functions and usually compile them
643 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
644 x86).
645
646 =back
647
648 =head2 FLOATING POINT FIDDLING
649
650 =over 4
651
652 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
653
654 =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
655
656 These functions each take an argument in the native C<float> or C<double>
657 type and return the IEEE 754 bit representation of it.
658
659 The bit representation is just as IEEE 754 defines it, i.e. the sign bit
660 will be the most significant bit, followed by exponent and mantissa.
661
662 This function should work even when the native floating point format isn't
663 IEEE compliant, of course at a speed and code size penalty, and of course
664 also within reasonable limits (it tries to convert NaNs, infinities and
665 denormals, but will likely convert negative zero to positive zero).
666
667 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
668 be able to optimise away this function completely.
669
670 These functions can be helpful when serialising floats to the network - you
671 can serialise the return value like a normal uint32_t/uint64_t.
672
673 Another use for these functions is to manipulate floating point values
674 directly.
675
676 Silly example: toggle the sign bit of a float.
677
678 /* On gcc-4.7 on amd64, */
679 /* this results in a single add instruction to toggle the bit, and 4 extra */
680 /* instructions to move the float value to an integer register and back. */
681
682 x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
683
684 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
685
686 =item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
687
688 The reverse operation of the previos function - takes the bit representation
689 of an IEEE binary32 or binary64 number and converts it to the native C<float>
690 or C<double> format.
691
692 This function should work even when the native floating point format isn't
693 IEEE compliant, of course at a speed and code size penalty, and of course
694 also within reasonable limits (it tries to convert normals and denormals,
695 and might be lucky for infinities, and with extraordinary luck, also for
696 negative zero).
697
698 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
699 be able to optimise away this function completely.
700
701 =back
702
703 =head2 ARITHMETIC
704
705 =over 4
706
707 =item x = ecb_mod (m, n)
708
709 Returns C<m> modulo C<n>, which is the same as the positive remainder
710 of the division operation between C<m> and C<n>, using floored
711 division. Unlike the C remainder operator C<%>, this function ensures that
712 the return value is always positive and that the two numbers I<m> and
713 I<m' = m + i * n> result in the same value modulo I<n> - in other words,
714 C<ecb_mod> implements the mathematical modulo operation, which is missing
715 in the language.
716
717 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
718 negatable, that is, both C<m> and C<-m> must be representable in its
719 type (this typically excludes the minimum signed integer value, the same
720 limitation as for C</> and C<%> in C).
721
722 Current GCC versions compile this into an efficient branchless sequence on
723 almost all CPUs.
724
725 For example, when you want to rotate forward through the members of an
726 array for increasing C<m> (which might be negative), then you should use
727 C<ecb_mod>, as the C<%> operator might give either negative results, or
728 change direction for negative values:
729
730 for (m = -100; m <= 100; ++m)
731 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
732
733 =item x = ecb_div_rd (val, div)
734
735 =item x = ecb_div_ru (val, div)
736
737 Returns C<val> divided by C<div> rounded down or up, respectively.
738 C<val> and C<div> must have integer types and C<div> must be strictly
739 positive. Note that these functions are implemented with macros in C
740 and with function templates in C++.
741
742 =back
743
744 =head2 UTILITY
745
746 =over 4
747
748 =item element_count = ecb_array_length (name)
749
750 Returns the number of elements in the array C<name>. For example:
751
752 int primes[] = { 2, 3, 5, 7, 11 };
753 int sum = 0;
754
755 for (i = 0; i < ecb_array_length (primes); i++)
756 sum += primes [i];
757
758 =back
759
760 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
761
762 These symbols need to be defined before including F<ecb.h> the first time.
763
764 =over 4
765
766 =item ECB_NO_THREADS
767
768 If F<ecb.h> is never used from multiple threads, then this symbol can
769 be defined, in which case memory fences (and similar constructs) are
770 completely removed, leading to more efficient code and fewer dependencies.
771
772 Setting this symbol to a true value implies C<ECB_NO_SMP>.
773
774 =item ECB_NO_SMP
775
776 The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
777 multiple threads, but never concurrently (e.g. if the system the program
778 runs on has only a single CPU with a single core, no hyperthreading and so
779 on), then this symbol can be defined, leading to more efficient code and
780 fewer dependencies.
781
782 =item ECB_NO_LIBM
783
784 When defined to C<1>, do not export any functions that might introduce
785 dependencies on the math library (usually called F<-lm>) - these are
786 marked with [-UECB_NO_LIBM].
787
788 =back
789
790