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Revision: 1.92
Committed: Tue Jun 22 05:28:44 2021 UTC (2 years, 11 months ago) by sf-exg
Branch: MAIN
Changes since 1.91: +7 -7 lines
Log Message:
Fix typos

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# User Rev Content
1 root 1.14 =head1 LIBECB - e-C-Builtins
2 root 1.3
3 root 1.14 =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 sf-exg 1.16 It's part of the e-suite of libraries, other members of which include
9 root 1.14 libev and libeio.
10    
11     Its homepage can be found here:
12    
13     http://software.schmorp.de/pkg/libecb
14    
15 root 1.85 It mainly provides a number of wrappers around many compiler built-ins,
16     together with replacement functions for other compilers. In addition
17     to this, it provides a number of other lowlevel C utilities, such as
18     endianness detection, byte swapping or bit rotations.
19    
20     Or in other words, things that should be built into any standard C
21     system, but aren't, implemented as efficient as possible with GCC (clang,
22     msvc...), and still correct with other compilers.
23 root 1.17
24 root 1.14 More might come.
25 root 1.3
26     =head2 ABOUT THE HEADER
27    
28 root 1.14 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 sf-exg 1.16 There are currently no object files to link to - future versions might
37 root 1.14 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 root 1.1
59 root 1.40 =head2 TYPES / TYPE SUPPORT
60    
61     ecb.h makes sure that the following types are defined (in the expected way):
62    
63 root 1.76 int8_t uint8_
64     int16_t uint16_t
65     int32_t uint32_
66     int64_t uint64_t
67     int_fast8_t uint_fast8_t
68     int_fast16_t uint_fast16_t
69     int_fast32_t uint_fast32_t
70     int_fast64_t uint_fast64_t
71     intptr_t uintptr_t
72 root 1.40
73     The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
74 root 1.45 platform (currently C<4> or C<8>) and can be used in preprocessor
75     expressions.
76 root 1.40
77 root 1.74 For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>.
78 root 1.49
79 root 1.62 =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
80 root 1.43
81 sf-exg 1.46 All the following symbols expand to an expression that can be tested in
82 root 1.44 preprocessor instructions as well as treated as a boolean (use C<!!> to
83     ensure it's either C<0> or C<1> if you need that).
84    
85 root 1.88 =over
86 root 1.43
87 root 1.44 =item ECB_C
88    
89     True if the implementation defines the C<__STDC__> macro to a true value,
90 root 1.82 while not claiming to be C++, i..e C, but not C++.
91 root 1.44
92 root 1.43 =item ECB_C99
93    
94 root 1.47 True if the implementation claims to be compliant to C99 (ISO/IEC
95 root 1.55 9899:1999) or any later version, while not claiming to be C++.
96 root 1.47
97     Note that later versions (ECB_C11) remove core features again (for
98     example, variable length arrays).
99 root 1.43
100 root 1.74 =item ECB_C11, ECB_C17
101 root 1.43
102 root 1.74 True if the implementation claims to be compliant to C11/C17 (ISO/IEC
103     9899:2011, :20187) or any later version, while not claiming to be C++.
104 root 1.44
105     =item ECB_CPP
106    
107     True if the implementation defines the C<__cplusplus__> macro to a true
108     value, which is typically true for C++ compilers.
109    
110 root 1.74 =item ECB_CPP11, ECB_CPP14, ECB_CPP17
111 root 1.44
112 root 1.74 True if the implementation claims to be compliant to C++11/C++14/C++17
113     (ISO/IEC 14882:2011, :2014, :2017) or any later version.
114 root 1.43
115 root 1.83 Note that many C++20 features will likely have their own feature test
116     macros (see e.g. L<http://eel.is/c++draft/cpp.predefined#1.8>).
117    
118 root 1.81 =item ECB_OPTIMIZE_SIZE
119    
120     Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
121     can also be defined before including F<ecb.h>, in which case it will be
122     unchanged.
123    
124 root 1.57 =item ECB_GCC_VERSION (major, minor)
125 root 1.43
126 root 1.84 Expands to a true value (suitable for testing by the preprocessor) if the
127     compiler used is GNU C and the version is the given version, or higher.
128 root 1.43
129     This macro tries to return false on compilers that claim to be GCC
130     compatible but aren't.
131    
132 root 1.50 =item ECB_EXTERN_C
133    
134     Expands to C<extern "C"> in C++, and a simple C<extern> in C.
135    
136     This can be used to declare a single external C function:
137    
138     ECB_EXTERN_C int printf (const char *format, ...);
139    
140     =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
141    
142     These two macros can be used to wrap multiple C<extern "C"> definitions -
143     they expand to nothing in C.
144    
145     They are most useful in header files:
146    
147     ECB_EXTERN_C_BEG
148    
149     int mycfun1 (int x);
150     int mycfun2 (int x);
151    
152     ECB_EXTERN_C_END
153    
154     =item ECB_STDFP
155    
156 root 1.84 If this evaluates to a true value (suitable for testing by the
157 root 1.50 preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
158     and double/binary64 representations internally I<and> the endianness of
159     both types match the endianness of C<uint32_t> and C<uint64_t>.
160    
161     This means you can just copy the bits of a C<float> (or C<double>) to an
162     C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
163     without having to think about format or endianness.
164    
165     This is true for basically all modern platforms, although F<ecb.h> might
166     not be able to deduce this correctly everywhere and might err on the safe
167     side.
168    
169 root 1.87 =item ECB_64BIT_NATIVE
170    
171     Evaluates to a true value (suitable for both preprocessor and C code
172     testing) if 64 bit integer types on this architecture are evaluated
173 sf-exg 1.92 "natively", that is, with similar speeds as 32 bit integers. While 64 bit
174     integer support is very common (and in fact required by libecb), 32 bit
175 root 1.87 cpus have to emulate operations on them, so you might want to avoid them.
176    
177 root 1.54 =item ECB_AMD64, ECB_AMD64_X32
178    
179     These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
180     ABI, respectively, and undefined elsewhere.
181    
182     The designers of the new X32 ABI for some inexplicable reason decided to
183     make it look exactly like amd64, even though it's completely incompatible
184     to that ABI, breaking about every piece of software that assumed that
185     C<__x86_64> stands for, well, the x86-64 ABI, making these macros
186     necessary.
187    
188 root 1.43 =back
189    
190 root 1.62 =head2 MACRO TRICKERY
191    
192 root 1.88 =over
193 root 1.62
194     =item ECB_CONCAT (a, b)
195    
196     Expands any macros in C<a> and C<b>, then concatenates the result to form
197     a single token. This is mainly useful to form identifiers from components,
198     e.g.:
199    
200     #define S1 str
201     #define S2 cpy
202    
203     ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
204    
205     =item ECB_STRINGIFY (arg)
206    
207     Expands any macros in C<arg> and returns the stringified version of
208     it. This is mainly useful to get the contents of a macro in string form,
209     e.g.:
210    
211     #define SQL_LIMIT 100
212     sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
213    
214 root 1.64 =item ECB_STRINGIFY_EXPR (expr)
215    
216     Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
217     is a valid expression. This is useful to catch typos or cases where the
218     macro isn't available:
219    
220     #include <errno.h>
221    
222     ECB_STRINGIFY (EDOM); // "33" (on my system at least)
223     ECB_STRINGIFY_EXPR (EDOM); // "33"
224    
225     // now imagine we had a typo:
226    
227     ECB_STRINGIFY (EDAM); // "EDAM"
228     ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
229    
230 root 1.62 =back
231    
232 sf-exg 1.60 =head2 ATTRIBUTES
233 root 1.1
234 sf-exg 1.60 A major part of libecb deals with additional attributes that can be
235     assigned to functions, variables and sometimes even types - much like
236     C<const> or C<volatile> in C. They are implemented using either GCC
237     attributes or other compiler/language specific features. Attributes
238     declarations must be put before the whole declaration:
239 root 1.20
240     ecb_const int mysqrt (int a);
241     ecb_unused int i;
242    
243 root 1.88 =over
244 root 1.1
245 root 1.3 =item ecb_unused
246    
247     Marks a function or a variable as "unused", which simply suppresses a
248 root 1.85 warning by the compiler when it detects it as unused. This is useful when
249     you e.g. declare a variable but do not always use it:
250 root 1.3
251 root 1.15 {
252 sf-exg 1.61 ecb_unused int var;
253 root 1.3
254 root 1.15 #ifdef SOMECONDITION
255     var = ...;
256     return var;
257     #else
258     return 0;
259     #endif
260     }
261 root 1.3
262 root 1.56 =item ecb_deprecated
263    
264     Similar to C<ecb_unused>, but marks a function, variable or type as
265     deprecated. This makes some compilers warn when the type is used.
266    
267 root 1.62 =item ecb_deprecated_message (message)
268    
269 root 1.67 Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
270 root 1.62 used instead of a generic depreciation message when the object is being
271     used.
272    
273 root 1.31 =item ecb_inline
274 root 1.29
275 root 1.73 Expands either to (a compiler-specific equivalent of) C<static inline> or
276     to just C<static>, if inline isn't supported. It should be used to declare
277     functions that should be inlined, for code size or speed reasons.
278 root 1.29
279     Example: inline this function, it surely will reduce codesize.
280    
281 root 1.31 ecb_inline int
282 root 1.29 negmul (int a, int b)
283     {
284     return - (a * b);
285     }
286    
287 root 1.2 =item ecb_noinline
288    
289 sf-exg 1.66 Prevents a function from being inlined - it might be optimised away, but
290 root 1.3 not inlined into other functions. This is useful if you know your function
291     is rarely called and large enough for inlining not to be helpful.
292    
293 root 1.2 =item ecb_noreturn
294    
295 root 1.17 Marks a function as "not returning, ever". Some typical functions that
296     don't return are C<exit> or C<abort> (which really works hard to not
297     return), and now you can make your own:
298    
299     ecb_noreturn void
300     my_abort (const char *errline)
301     {
302     puts (errline);
303     abort ();
304     }
305    
306 sf-exg 1.19 In this case, the compiler would probably be smart enough to deduce it on
307     its own, so this is mainly useful for declarations.
308 root 1.17
309 root 1.53 =item ecb_restrict
310    
311     Expands to the C<restrict> keyword or equivalent on compilers that support
312     them, and to nothing on others. Must be specified on a pointer type or
313     an array index to indicate that the memory doesn't alias with any other
314     restricted pointer in the same scope.
315    
316     Example: multiply a vector, and allow the compiler to parallelise the
317     loop, because it knows it doesn't overwrite input values.
318    
319     void
320 sf-exg 1.61 multiply (ecb_restrict float *src,
321     ecb_restrict float *dst,
322 root 1.53 int len, float factor)
323     {
324     int i;
325    
326     for (i = 0; i < len; ++i)
327     dst [i] = src [i] * factor;
328     }
329    
330 root 1.2 =item ecb_const
331    
332 sf-exg 1.19 Declares that the function only depends on the values of its arguments,
333 root 1.17 much like a mathematical function. It specifically does not read or write
334     any memory any arguments might point to, global variables, or call any
335     non-const functions. It also must not have any side effects.
336    
337     Such a function can be optimised much more aggressively by the compiler -
338     for example, multiple calls with the same arguments can be optimised into
339     a single call, which wouldn't be possible if the compiler would have to
340     expect any side effects.
341    
342     It is best suited for functions in the sense of mathematical functions,
343 sf-exg 1.19 such as a function returning the square root of its input argument.
344 root 1.17
345     Not suited would be a function that calculates the hash of some memory
346     area you pass in, prints some messages or looks at a global variable to
347     decide on rounding.
348    
349     See C<ecb_pure> for a slightly less restrictive class of functions.
350    
351 root 1.2 =item ecb_pure
352    
353 root 1.17 Similar to C<ecb_const>, declares a function that has no side
354     effects. Unlike C<ecb_const>, the function is allowed to examine global
355     variables and any other memory areas (such as the ones passed to it via
356     pointers).
357    
358     While these functions cannot be optimised as aggressively as C<ecb_const>
359     functions, they can still be optimised away in many occasions, and the
360     compiler has more freedom in moving calls to them around.
361    
362     Typical examples for such functions would be C<strlen> or C<memcmp>. A
363     function that calculates the MD5 sum of some input and updates some MD5
364     state passed as argument would I<NOT> be pure, however, as it would modify
365     some memory area that is not the return value.
366    
367 root 1.2 =item ecb_hot
368    
369 root 1.17 This declares a function as "hot" with regards to the cache - the function
370     is used so often, that it is very beneficial to keep it in the cache if
371     possible.
372    
373     The compiler reacts by trying to place hot functions near to each other in
374     memory.
375    
376 sf-exg 1.19 Whether a function is hot or not often depends on the whole program,
377 root 1.17 and less on the function itself. C<ecb_cold> is likely more useful in
378     practise.
379    
380 root 1.2 =item ecb_cold
381    
382 root 1.17 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
383     the cache, or in other words, this function is not called often, or not at
384     speed-critical times, and keeping it in the cache might be a waste of said
385     cache.
386    
387     In addition to placing cold functions together (or at least away from hot
388     functions), this knowledge can be used in other ways, for example, the
389     function will be optimised for size, as opposed to speed, and codepaths
390     leading to calls to those functions can automatically be marked as if
391 root 1.27 C<ecb_expect_false> had been used to reach them.
392 root 1.17
393     Good examples for such functions would be error reporting functions, or
394     functions only called in exceptional or rare cases.
395    
396 root 1.2 =item ecb_artificial
397    
398 root 1.17 Declares the function as "artificial", in this case meaning that this
399 root 1.52 function is not really meant to be a function, but more like an accessor
400 root 1.17 - many methods in C++ classes are mere accessor functions, and having a
401     crash reported in such a method, or single-stepping through them, is not
402     usually so helpful, especially when it's inlined to just a few instructions.
403    
404     Marking them as artificial will instruct the debugger about just this,
405     leading to happier debugging and thus happier lives.
406    
407     Example: in some kind of smart-pointer class, mark the pointer accessor as
408     artificial, so that the whole class acts more like a pointer and less like
409     some C++ abstraction monster.
410    
411     template<typename T>
412     struct my_smart_ptr
413     {
414     T *value;
415    
416     ecb_artificial
417     operator T *()
418     {
419     return value;
420     }
421     };
422    
423 root 1.2 =back
424 root 1.1
425     =head2 OPTIMISATION HINTS
426    
427 root 1.88 =over
428 root 1.1
429 root 1.58 =item bool ecb_is_constant (expr)
430 root 1.1
431 root 1.3 Returns true iff the expression can be deduced to be a compile-time
432     constant, and false otherwise.
433    
434     For example, when you have a C<rndm16> function that returns a 16 bit
435     random number, and you have a function that maps this to a range from
436 root 1.5 0..n-1, then you could use this inline function in a header file:
437 root 1.3
438     ecb_inline uint32_t
439     rndm (uint32_t n)
440     {
441 root 1.6 return (n * (uint32_t)rndm16 ()) >> 16;
442 root 1.3 }
443    
444     However, for powers of two, you could use a normal mask, but that is only
445     worth it if, at compile time, you can detect this case. This is the case
446     when the passed number is a constant and also a power of two (C<n & (n -
447     1) == 0>):
448    
449     ecb_inline uint32_t
450     rndm (uint32_t n)
451     {
452     return is_constant (n) && !(n & (n - 1))
453     ? rndm16 () & (num - 1)
454 root 1.6 : (n * (uint32_t)rndm16 ()) >> 16;
455 root 1.3 }
456    
457 root 1.62 =item ecb_expect (expr, value)
458 root 1.1
459 root 1.7 Evaluates C<expr> and returns it. In addition, it tells the compiler that
460     the C<expr> evaluates to C<value> a lot, which can be used for static
461     branch optimisations.
462 root 1.1
463 root 1.27 Usually, you want to use the more intuitive C<ecb_expect_true> and
464     C<ecb_expect_false> functions instead.
465 root 1.1
466 root 1.27 =item bool ecb_expect_true (cond)
467 root 1.1
468 root 1.27 =item bool ecb_expect_false (cond)
469 root 1.1
470 root 1.7 These two functions expect a expression that is true or false and return
471     C<1> or C<0>, respectively, so when used in the condition of an C<if> or
472     other conditional statement, it will not change the program:
473    
474     /* these two do the same thing */
475     if (some_condition) ...;
476 root 1.27 if (ecb_expect_true (some_condition)) ...;
477 root 1.7
478 root 1.27 However, by using C<ecb_expect_true>, you tell the compiler that the
479     condition is likely to be true (and for C<ecb_expect_false>, that it is
480     unlikely to be true).
481 root 1.7
482 root 1.9 For example, when you check for a null pointer and expect this to be a
483 root 1.27 rare, exceptional, case, then use C<ecb_expect_false>:
484 root 1.7
485     void my_free (void *ptr)
486     {
487 root 1.27 if (ecb_expect_false (ptr == 0))
488 root 1.7 return;
489     }
490    
491     Consequent use of these functions to mark away exceptional cases or to
492     tell the compiler what the hot path through a function is can increase
493     performance considerably.
494    
495 root 1.27 You might know these functions under the name C<likely> and C<unlikely>
496     - while these are common aliases, we find that the expect name is easier
497     to understand when quickly skimming code. If you wish, you can use
498     C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
499     C<ecb_expect_false> - these are simply aliases.
500    
501 root 1.7 A very good example is in a function that reserves more space for some
502     memory block (for example, inside an implementation of a string stream) -
503 root 1.9 each time something is added, you have to check for a buffer overrun, but
504 root 1.7 you expect that most checks will turn out to be false:
505    
506     /* make sure we have "size" extra room in our buffer */
507     ecb_inline void
508     reserve (int size)
509     {
510 root 1.27 if (ecb_expect_false (current + size > end))
511 root 1.7 real_reserve_method (size); /* presumably noinline */
512     }
513    
514 root 1.62 =item ecb_assume (cond)
515 root 1.7
516 sf-exg 1.66 Tries to tell the compiler that some condition is true, even if it's not
517 root 1.65 obvious. This is not a function, but a statement: it cannot be used in
518     another expression.
519 root 1.7
520     This can be used to teach the compiler about invariants or other
521     conditions that might improve code generation, but which are impossible to
522     deduce form the code itself.
523    
524 root 1.27 For example, the example reservation function from the C<ecb_expect_false>
525 root 1.7 description could be written thus (only C<ecb_assume> was added):
526    
527     ecb_inline void
528     reserve (int size)
529     {
530 root 1.27 if (ecb_expect_false (current + size > end))
531 root 1.7 real_reserve_method (size); /* presumably noinline */
532    
533     ecb_assume (current + size <= end);
534     }
535    
536     If you then call this function twice, like this:
537    
538     reserve (10);
539     reserve (1);
540    
541     Then the compiler I<might> be able to optimise out the second call
542     completely, as it knows that C<< current + 1 > end >> is false and the
543     call will never be executed.
544    
545 root 1.62 =item ecb_unreachable ()
546 root 1.7
547     This function does nothing itself, except tell the compiler that it will
548 root 1.9 never be executed. Apart from suppressing a warning in some cases, this
549 root 1.65 function can be used to implement C<ecb_assume> or similar functionality.
550 root 1.7
551 root 1.62 =item ecb_prefetch (addr, rw, locality)
552 root 1.7
553     Tells the compiler to try to prefetch memory at the given C<addr>ess
554 root 1.10 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
555 root 1.7 C<0> means that there will only be one access later, C<3> means that
556     the data will likely be accessed very often, and values in between mean
557     something... in between. The memory pointed to by the address does not
558     need to be accessible (it could be a null pointer for example), but C<rw>
559     and C<locality> must be compile-time constants.
560    
561 root 1.65 This is a statement, not a function: you cannot use it as part of an
562     expression.
563    
564 root 1.7 An obvious way to use this is to prefetch some data far away, in a big
565 root 1.9 array you loop over. This prefetches memory some 128 array elements later,
566 root 1.7 in the hope that it will be ready when the CPU arrives at that location.
567    
568     int sum = 0;
569    
570     for (i = 0; i < N; ++i)
571     {
572     sum += arr [i]
573     ecb_prefetch (arr + i + 128, 0, 0);
574     }
575    
576     It's hard to predict how far to prefetch, and most CPUs that can prefetch
577     are often good enough to predict this kind of behaviour themselves. It
578     gets more interesting with linked lists, especially when you do some fair
579     processing on each list element:
580    
581     for (node *n = start; n; n = n->next)
582     {
583     ecb_prefetch (n->next, 0, 0);
584     ... do medium amount of work with *n
585     }
586    
587     After processing the node, (part of) the next node might already be in
588     cache.
589 root 1.1
590 root 1.2 =back
591 root 1.1
592 root 1.36 =head2 BIT FIDDLING / BIT WIZARDRY
593 root 1.1
594 root 1.88 =over
595 root 1.4
596 root 1.3 =item bool ecb_big_endian ()
597    
598     =item bool ecb_little_endian ()
599    
600 sf-exg 1.11 These two functions return true if the byte order is big endian
601     (most-significant byte first) or little endian (least-significant byte
602     first) respectively.
603    
604 root 1.24 On systems that are neither, their return values are unspecified.
605    
606 root 1.3 =item int ecb_ctz32 (uint32_t x)
607    
608 root 1.35 =item int ecb_ctz64 (uint64_t x)
609    
610 root 1.77 =item int ecb_ctz (T x) [C++]
611    
612 sf-exg 1.11 Returns the index of the least significant bit set in C<x> (or
613 root 1.24 equivalently the number of bits set to 0 before the least significant bit
614 root 1.35 set), starting from 0. If C<x> is 0 the result is undefined.
615    
616 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
617    
618 root 1.77 The overloaded C++ C<ecb_ctz> function supports C<uint8_t>, C<uint16_t>,
619     C<uint32_t> and C<uint64_t> types.
620    
621 root 1.35 For example:
622 sf-exg 1.11
623 root 1.15 ecb_ctz32 (3) = 0
624     ecb_ctz32 (6) = 1
625 sf-exg 1.11
626 root 1.41 =item bool ecb_is_pot32 (uint32_t x)
627    
628     =item bool ecb_is_pot64 (uint32_t x)
629    
630 root 1.77 =item bool ecb_is_pot (T x) [C++]
631    
632 sf-exg 1.66 Returns true iff C<x> is a power of two or C<x == 0>.
633 root 1.41
634 sf-exg 1.66 For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
635 root 1.41
636 root 1.77 The overloaded C++ C<ecb_is_pot> function supports C<uint8_t>, C<uint16_t>,
637     C<uint32_t> and C<uint64_t> types.
638    
639 root 1.35 =item int ecb_ld32 (uint32_t x)
640    
641     =item int ecb_ld64 (uint64_t x)
642    
643 root 1.77 =item int ecb_ld64 (T x) [C++]
644    
645 root 1.35 Returns the index of the most significant bit set in C<x>, or the number
646     of digits the number requires in binary (so that C<< 2**ld <= x <
647     2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
648     to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
649     example to see how many bits a certain number requires to be encoded.
650    
651     This function is similar to the "count leading zero bits" function, except
652     that that one returns how many zero bits are "in front" of the number (in
653     the given data type), while C<ecb_ld> returns how many bits the number
654     itself requires.
655    
656 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
657    
658 root 1.77 The overloaded C++ C<ecb_ld> function supports C<uint8_t>, C<uint16_t>,
659     C<uint32_t> and C<uint64_t> types.
660    
661 root 1.3 =item int ecb_popcount32 (uint32_t x)
662    
663 root 1.35 =item int ecb_popcount64 (uint64_t x)
664    
665 root 1.77 =item int ecb_popcount (T x) [C++]
666    
667 root 1.36 Returns the number of bits set to 1 in C<x>.
668    
669     For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
670    
671 root 1.77 The overloaded C++ C<ecb_popcount> function supports C<uint8_t>, C<uint16_t>,
672     C<uint32_t> and C<uint64_t> types.
673    
674 root 1.36 For example:
675 sf-exg 1.11
676 root 1.15 ecb_popcount32 (7) = 3
677     ecb_popcount32 (255) = 8
678 sf-exg 1.11
679 root 1.39 =item uint8_t ecb_bitrev8 (uint8_t x)
680    
681     =item uint16_t ecb_bitrev16 (uint16_t x)
682    
683     =item uint32_t ecb_bitrev32 (uint32_t x)
684    
685 root 1.77 =item T ecb_bitrev (T x) [C++]
686    
687 root 1.39 Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
688     and so on.
689    
690 root 1.77 The overloaded C++ C<ecb_bitrev> function supports C<uint8_t>, C<uint16_t> and C<uint32_t> types.
691    
692 root 1.39 Example:
693    
694     ecb_bitrev8 (0xa7) = 0xea
695     ecb_bitrev32 (0xffcc4411) = 0x882233ff
696    
697 root 1.77 =item T ecb_bitrev (T x) [C++]
698    
699     Overloaded C++ bitrev function.
700    
701     C<T> must be one of C<uint8_t>, C<uint16_t> or C<uint32_t>.
702    
703 root 1.8 =item uint32_t ecb_bswap16 (uint32_t x)
704    
705 root 1.3 =item uint32_t ecb_bswap32 (uint32_t x)
706    
707 root 1.34 =item uint64_t ecb_bswap64 (uint64_t x)
708 sf-exg 1.13
709 root 1.78 =item T ecb_bswap (T x)
710    
711 root 1.34 These functions return the value of the 16-bit (32-bit, 64-bit) value
712     C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
713     C<ecb_bswap32>).
714    
715 root 1.77 The overloaded C++ C<ecb_bswap> function supports C<uint8_t>, C<uint16_t>,
716     C<uint32_t> and C<uint64_t> types.
717 root 1.76
718 root 1.34 =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
719    
720     =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
721 root 1.3
722     =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
723    
724 root 1.34 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
725    
726     =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
727    
728     =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
729    
730     =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
731    
732 root 1.33 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
733    
734 root 1.34 These two families of functions return the value of C<x> after rotating
735     all the bits by C<count> positions to the right (C<ecb_rotr>) or left
736     (C<ecb_rotl>).
737 sf-exg 1.11
738 root 1.85 Current GCC/clang versions understand these functions and usually compile
739     them to "optimal" code (e.g. a single C<rol> or a combination of C<shld>
740     on x86).
741 root 1.20
742 root 1.77 =item T ecb_rotl (T x, unsigned int count) [C++]
743    
744     =item T ecb_rotr (T x, unsigned int count) [C++]
745    
746     Overloaded C++ rotl/rotr functions.
747    
748     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
749    
750 root 1.3 =back
751 root 1.1
752 root 1.76 =head2 HOST ENDIANNESS CONVERSION
753    
754 root 1.88 =over
755 root 1.76
756     =item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v)
757    
758     =item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v)
759    
760     =item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v)
761    
762     =item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v)
763    
764     =item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v)
765    
766     =item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v)
767    
768     Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order.
769    
770     The naming convention is C<ecb_>(C<be>|C<le>)C<_u>C<16|32|64>C<_to_host>,
771 root 1.79 where C<be> and C<le> stand for big endian and little endian, respectively.
772 root 1.76
773     =item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v)
774    
775     =item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v)
776    
777     =item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v)
778    
779     =item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v)
780    
781     =item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v)
782    
783     =item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v)
784    
785     Like above, but converts I<from> host byte order to the specified
786     endianness.
787    
788     =back
789    
790 root 1.77 In C++ the following additional template functions are supported:
791 root 1.76
792 root 1.88 =over
793 root 1.76
794     =item T ecb_be_to_host (T v)
795    
796     =item T ecb_le_to_host (T v)
797    
798     =item T ecb_host_to_be (T v)
799    
800     =item T ecb_host_to_le (T v)
801    
802 root 1.86 =back
803    
804 root 1.77 These functions work like their C counterparts, above, but use templates,
805     which make them useful in generic code.
806 root 1.76
807     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>
808     (so unlike their C counterparts, there is a version for C<uint8_t>, which
809     again can be useful in generic code).
810    
811     =head2 UNALIGNED LOAD/STORE
812    
813     These function load or store unaligned multi-byte values.
814    
815 root 1.88 =over
816 root 1.76
817     =item uint_fast16_t ecb_peek_u16_u (const void *ptr)
818    
819     =item uint_fast32_t ecb_peek_u32_u (const void *ptr)
820    
821     =item uint_fast64_t ecb_peek_u64_u (const void *ptr)
822    
823     These functions load an unaligned, unsigned 16, 32 or 64 bit value from
824     memory.
825    
826     =item uint_fast16_t ecb_peek_be_u16_u (const void *ptr)
827    
828     =item uint_fast32_t ecb_peek_be_u32_u (const void *ptr)
829    
830     =item uint_fast64_t ecb_peek_be_u64_u (const void *ptr)
831    
832     =item uint_fast16_t ecb_peek_le_u16_u (const void *ptr)
833    
834     =item uint_fast32_t ecb_peek_le_u32_u (const void *ptr)
835    
836     =item uint_fast64_t ecb_peek_le_u64_u (const void *ptr)
837    
838     Like above, but additionally convert from big endian (C<be>) or little
839     endian (C<le>) byte order to host byte order while doing so.
840    
841     =item ecb_poke_u16_u (void *ptr, uint16_t v)
842    
843     =item ecb_poke_u32_u (void *ptr, uint32_t v)
844    
845     =item ecb_poke_u64_u (void *ptr, uint64_t v)
846    
847     These functions store an unaligned, unsigned 16, 32 or 64 bit value to
848     memory.
849    
850     =item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v)
851    
852     =item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v)
853    
854     =item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v)
855    
856     =item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v)
857    
858     =item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v)
859    
860     =item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v)
861    
862     Like above, but additionally convert from host byte order to big endian
863     (C<be>) or little endian (C<le>) byte order while doing so.
864    
865     =back
866    
867 root 1.77 In C++ the following additional template functions are supported:
868 root 1.76
869 root 1.88 =over
870 root 1.76
871 root 1.80 =item T ecb_peek<T> (const void *ptr)
872 root 1.76
873 root 1.80 =item T ecb_peek_be<T> (const void *ptr)
874 root 1.76
875 root 1.80 =item T ecb_peek_le<T> (const void *ptr)
876 root 1.76
877 root 1.80 =item T ecb_peek_u<T> (const void *ptr)
878 root 1.76
879 root 1.80 =item T ecb_peek_be_u<T> (const void *ptr)
880 root 1.76
881 root 1.80 =item T ecb_peek_le_u<T> (const void *ptr)
882 root 1.76
883     Similarly to their C counterparts, these functions load an unsigned 8, 16,
884     32 or 64 bit value from memory, with optional conversion from big/little
885     endian.
886    
887 root 1.80 Since the type cannot be deduced, it has to be specified explicitly, e.g.
888 root 1.76
889     uint_fast16_t v = ecb_peek<uint16_t> (ptr);
890    
891     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
892    
893     Unlike their C counterparts, these functions support 8 bit quantities
894     (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
895     all of which hopefully makes them more useful in generic code.
896    
897     =item ecb_poke (void *ptr, T v)
898    
899     =item ecb_poke_be (void *ptr, T v)
900    
901     =item ecb_poke_le (void *ptr, T v)
902    
903     =item ecb_poke_u (void *ptr, T v)
904    
905     =item ecb_poke_be_u (void *ptr, T v)
906    
907     =item ecb_poke_le_u (void *ptr, T v)
908    
909     Again, similarly to their C counterparts, these functions store an
910     unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to
911     big/little endian.
912    
913     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
914    
915     Unlike their C counterparts, these functions support 8 bit quantities
916     (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
917     all of which hopefully makes them more useful in generic code.
918    
919     =back
920    
921 root 1.89 =head2 FAST INTEGER TO STRING
922    
923     Libecb defines a set of very fast integer to decimal string (or integer
924     to ascii, short C<i2a>) functions. These work by converting the integer
925     to a fixed point representation and then successively multiplying out
926     the topmost digits. Unlike some other, also very fast, libraries, ecb's
927     algorithm should be completely branchless per digit, and does not rely on
928     the presence of special cpu functions (such as clz).
929    
930     There is a high level API that takes an C<int32_t>, C<uint32_t>,
931     C<int64_t> or C<uint64_t> as argument, and a low-level API, which is
932     harder to use but supports slightly more formatting options.
933    
934     =head3 HIGH LEVEL API
935    
936     The high level API consists of four functions, one each for C<int32_t>,
937     C<uint32_t>, C<int64_t> and C<uint64_t>:
938    
939 root 1.91 Example:
940    
941     char buf[ECB_I2A_MAX_DIGITS + 1];
942     char *end = ecb_i2a_i32 (buf, 17262);
943     *end = 0;
944     // buf now contains "17262"
945    
946 root 1.89 =over
947    
948     =item ECB_I2A_I32_DIGITS (=11)
949    
950     =item char *ecb_i2a_u32 (char *ptr, uint32_t value)
951    
952     Takes an C<uint32_t> I<value> and formats it as a decimal number starting
953     at I<ptr>, using at most C<ECB_I2A_I32_DIGITS> characters. Returns a
954     pointer to just after the generated string, where you would normally put
955 sf-exg 1.92 the terminating C<0> character. This function outputs the minimum number
956 root 1.89 of digits.
957    
958     =item ECB_I2A_U32_DIGITS (=10)
959    
960     =item char *ecb_i2a_i32 (char *ptr, int32_t value)
961    
962     Same as C<ecb_i2a_u32>, but formats a C<int32_t> value, including a minus
963     sign if needed.
964    
965     =item ECB_I2A_I64_DIGITS (=20)
966    
967     =item char *ecb_i2a_u64 (char *ptr, uint64_t value)
968    
969     =item ECB_I2A_U64_DIGITS (=21)
970    
971     =item char *ecb_i2a_i64 (char *ptr, int64_t value)
972    
973     Similar to their 32 bit counterparts, these take a 64 bit argument.
974    
975 root 1.90 =item ECB_I2A_MAX_DIGITS (=21)
976 root 1.89
977     Instead of using a type specific length macro, youi can just use
978 root 1.90 C<ECB_I2A_MAX_DIGITS>, which is good enough for any C<ecb_i2a> function.
979 root 1.89
980     =back
981    
982     =head3 LOW-LEVEL API
983    
984     The functions above use a number of low-level APIs which have some strict
985 sf-exg 1.92 limitations, but can be used as building blocks (study of C<ecb_i2a_i32>
986     and related functions is recommended).
987 root 1.89
988     There are three families of functions: functions that convert a number
989     to a fixed number of digits with leading zeroes (C<ecb_i2a_0N>, C<0>
990     for "leading zeroes"), functions that generate up to N digits, skipping
991     leading zeroes (C<_N>), and functions that can generate more digits, but
992     the leading digit has limited range (C<_xN>).
993    
994 sf-exg 1.92 None of the functions deal with negative numbers.
995 root 1.89
996 root 1.91 Example: convert an IP address in an u32 into dotted-quad:
997    
998     uint32_t ip = 0x0a000164; // 10.0.1.100
999     char ips[3 * 4 + 3 + 1];
1000     char *ptr = ips;
1001     ptr = ecb_i2a_3 (ptr, ip >> 24 ); *ptr++ = '.';
1002     ptr = ecb_i2a_3 (ptr, (ip >> 16) & 0xff); *ptr++ = '.';
1003     ptr = ecb_i2a_3 (ptr, (ip >> 8) & 0xff); *ptr++ = '.';
1004     ptr = ecb_i2a_3 (ptr, ip & 0xff); *ptr++ = 0;
1005     printf ("ip: %s\n", ips); // prints "ip: 10.0.1.100"
1006    
1007 root 1.89 =over
1008    
1009     =item char *ecb_i2a_02 (char *ptr, uint32_t value) // 32 bit
1010    
1011     =item char *ecb_i2a_03 (char *ptr, uint32_t value) // 32 bit
1012    
1013     =item char *ecb_i2a_04 (char *ptr, uint32_t value) // 32 bit
1014    
1015     =item char *ecb_i2a_05 (char *ptr, uint32_t value) // 64 bit
1016    
1017     =item char *ecb_i2a_06 (char *ptr, uint32_t value) // 64 bit
1018    
1019     =item char *ecb_i2a_07 (char *ptr, uint32_t value) // 64 bit
1020    
1021     =item char *ecb_i2a_08 (char *ptr, uint32_t value) // 64 bit
1022    
1023     =item char *ecb_i2a_09 (char *ptr, uint32_t value) // 64 bit
1024    
1025     The C<< ecb_i2a_0I<N> > functions take an unsigned I<value> and convert
1026     them to exactly I<N> digits, returning a pointer to the first character
1027     after the digits. The I<value> must be in range. The functions marked with
1028     I<32 bit> do their calculations internally in 32 bit, the ones marked with
1029     I<64 bit> internally use 64 bit integers, which might be slow on 32 bit
1030     architectures (the high level API decides on 32 vs. 64 bit versions using
1031     C<ECB_64BIT_NATIVE>).
1032    
1033     =item char *ecb_i2a_2 (char *ptr, uint32_t value) // 32 bit
1034    
1035     =item char *ecb_i2a_3 (char *ptr, uint32_t value) // 32 bit
1036    
1037     =item char *ecb_i2a_4 (char *ptr, uint32_t value) // 32 bit
1038    
1039     =item char *ecb_i2a_5 (char *ptr, uint32_t value) // 64 bit
1040    
1041     =item char *ecb_i2a_6 (char *ptr, uint32_t value) // 64 bit
1042    
1043     =item char *ecb_i2a_7 (char *ptr, uint32_t value) // 64 bit
1044    
1045     =item char *ecb_i2a_8 (char *ptr, uint32_t value) // 64 bit
1046    
1047     =item char *ecb_i2a_9 (char *ptr, uint32_t value) // 64 bit
1048    
1049     Similarly, the C<< ecb_i2a_I<N> > functions take an unsigned I<value>
1050     and convert them to at most I<N> digits, suppressing leading zeroes, and
1051     returning a pointer to the first character after the digits.
1052    
1053     =item ECB_I2A_MAX_X5 (=59074)
1054    
1055     =item char *ecb_i2a_x5 (char *ptr, uint32_t value) // 32 bit
1056    
1057     =item ECB_I2A_MAX_X10 (=2932500665)
1058    
1059     =item char *ecb_i2a_x10 (char *ptr, uint32_t value) // 64 bit
1060    
1061     The C<< ecb_i2a_xI<N> >> functions are similar to the C<< ecb_i2a_I<N> >
1062     functions, but they can generate one digit more, as long as the number
1063     is within range, which is given by the symbols C<ECB_I2A_MAX_X5> (almost
1064     16 bit range) and C<ECB_I2A_MAX_X10> (a bit more than 31 bit range),
1065     respectively.
1066    
1067 sf-exg 1.92 For example, the digit part of a 32 bit signed integer just fits into the
1068 root 1.89 C<ECB_I2A_MAX_X10> range, so while C<ecb_i2a_x10> cannot convert a 10
1069     digit number, it can convert all 32 bit signed numbers. Sadly, it's not
1070     good enough for 32 bit unsigned numbers.
1071    
1072     =back
1073    
1074 root 1.50 =head2 FLOATING POINT FIDDLING
1075    
1076 root 1.88 =over
1077 root 1.50
1078 root 1.71 =item ECB_INFINITY [-UECB_NO_LIBM]
1079 root 1.62
1080     Evaluates to positive infinity if supported by the platform, otherwise to
1081     a truly huge number.
1082    
1083 root 1.71 =item ECB_NAN [-UECB_NO_LIBM]
1084 root 1.62
1085     Evaluates to a quiet NAN if supported by the platform, otherwise to
1086     C<ECB_INFINITY>.
1087    
1088 root 1.71 =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
1089 root 1.62
1090     Same as C<ldexpf>, but always available.
1091    
1092 root 1.71 =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
1093    
1094 root 1.50 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
1095    
1096     =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
1097    
1098     These functions each take an argument in the native C<float> or C<double>
1099 root 1.71 type and return the IEEE 754 bit representation of it (binary16/half,
1100     binary32/single or binary64/double precision).
1101 root 1.50
1102     The bit representation is just as IEEE 754 defines it, i.e. the sign bit
1103     will be the most significant bit, followed by exponent and mantissa.
1104    
1105     This function should work even when the native floating point format isn't
1106     IEEE compliant, of course at a speed and code size penalty, and of course
1107     also within reasonable limits (it tries to convert NaNs, infinities and
1108     denormals, but will likely convert negative zero to positive zero).
1109    
1110     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1111     be able to optimise away this function completely.
1112    
1113     These functions can be helpful when serialising floats to the network - you
1114 root 1.71 can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
1115 root 1.50
1116     Another use for these functions is to manipulate floating point values
1117     directly.
1118    
1119     Silly example: toggle the sign bit of a float.
1120    
1121     /* On gcc-4.7 on amd64, */
1122     /* this results in a single add instruction to toggle the bit, and 4 extra */
1123     /* instructions to move the float value to an integer register and back. */
1124    
1125     x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
1126    
1127 root 1.58 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
1128    
1129 root 1.50 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
1130    
1131 root 1.70 =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
1132 root 1.50
1133 sf-exg 1.59 The reverse operation of the previous function - takes the bit
1134 root 1.71 representation of an IEEE binary16, binary32 or binary64 number (half,
1135     single or double precision) and converts it to the native C<float> or
1136     C<double> format.
1137 root 1.50
1138     This function should work even when the native floating point format isn't
1139     IEEE compliant, of course at a speed and code size penalty, and of course
1140     also within reasonable limits (it tries to convert normals and denormals,
1141     and might be lucky for infinities, and with extraordinary luck, also for
1142     negative zero).
1143    
1144     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1145     be able to optimise away this function completely.
1146    
1147 root 1.71 =item uint16_t ecb_binary32_to_binary16 (uint32_t x)
1148    
1149     =item uint32_t ecb_binary16_to_binary32 (uint16_t x)
1150    
1151     Convert a IEEE binary32/single precision to binary16/half format, and vice
1152 root 1.72 versa, handling all details (round-to-nearest-even, subnormals, infinity
1153     and NaNs) correctly.
1154 root 1.71
1155     These are functions are available under C<-DECB_NO_LIBM>, since
1156     they do not rely on the platform floating point format. The
1157     C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
1158     usually what you want.
1159    
1160 root 1.50 =back
1161    
1162 root 1.1 =head2 ARITHMETIC
1163    
1164 root 1.88 =over
1165 root 1.3
1166 root 1.14 =item x = ecb_mod (m, n)
1167 root 1.3
1168 root 1.25 Returns C<m> modulo C<n>, which is the same as the positive remainder
1169     of the division operation between C<m> and C<n>, using floored
1170     division. Unlike the C remainder operator C<%>, this function ensures that
1171     the return value is always positive and that the two numbers I<m> and
1172     I<m' = m + i * n> result in the same value modulo I<n> - in other words,
1173     C<ecb_mod> implements the mathematical modulo operation, which is missing
1174     in the language.
1175 root 1.14
1176 sf-exg 1.23 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
1177 root 1.14 negatable, that is, both C<m> and C<-m> must be representable in its
1178 root 1.30 type (this typically excludes the minimum signed integer value, the same
1179 root 1.25 limitation as for C</> and C<%> in C).
1180 sf-exg 1.11
1181 root 1.85 Current GCC/clang versions compile this into an efficient branchless
1182     sequence on almost all CPUs.
1183 root 1.24
1184     For example, when you want to rotate forward through the members of an
1185     array for increasing C<m> (which might be negative), then you should use
1186     C<ecb_mod>, as the C<%> operator might give either negative results, or
1187     change direction for negative values:
1188    
1189     for (m = -100; m <= 100; ++m)
1190     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
1191    
1192 sf-exg 1.37 =item x = ecb_div_rd (val, div)
1193    
1194     =item x = ecb_div_ru (val, div)
1195    
1196     Returns C<val> divided by C<div> rounded down or up, respectively.
1197     C<val> and C<div> must have integer types and C<div> must be strictly
1198 sf-exg 1.38 positive. Note that these functions are implemented with macros in C
1199     and with function templates in C++.
1200 sf-exg 1.37
1201 root 1.3 =back
1202 root 1.1
1203     =head2 UTILITY
1204    
1205 root 1.88 =over
1206 root 1.3
1207 sf-exg 1.23 =item element_count = ecb_array_length (name)
1208 root 1.3
1209 sf-exg 1.13 Returns the number of elements in the array C<name>. For example:
1210    
1211     int primes[] = { 2, 3, 5, 7, 11 };
1212     int sum = 0;
1213    
1214     for (i = 0; i < ecb_array_length (primes); i++)
1215     sum += primes [i];
1216    
1217 root 1.3 =back
1218 root 1.1
1219 root 1.43 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
1220    
1221     These symbols need to be defined before including F<ecb.h> the first time.
1222    
1223 root 1.88 =over
1224 root 1.43
1225 root 1.51 =item ECB_NO_THREADS
1226 root 1.43
1227     If F<ecb.h> is never used from multiple threads, then this symbol can
1228     be defined, in which case memory fences (and similar constructs) are
1229     completely removed, leading to more efficient code and fewer dependencies.
1230    
1231     Setting this symbol to a true value implies C<ECB_NO_SMP>.
1232    
1233     =item ECB_NO_SMP
1234    
1235     The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
1236     multiple threads, but never concurrently (e.g. if the system the program
1237     runs on has only a single CPU with a single core, no hyperthreading and so
1238     on), then this symbol can be defined, leading to more efficient code and
1239     fewer dependencies.
1240    
1241 root 1.50 =item ECB_NO_LIBM
1242    
1243     When defined to C<1>, do not export any functions that might introduce
1244     dependencies on the math library (usually called F<-lm>) - these are
1245     marked with [-UECB_NO_LIBM].
1246    
1247 sf-exg 1.69 =back
1248    
1249 root 1.68 =head1 UNDOCUMENTED FUNCTIONALITY
1250    
1251     F<ecb.h> is full of undocumented functionality as well, some of which is
1252     intended to be internal-use only, some of which we forgot to document, and
1253     some of which we hide because we are not sure we will keep the interface
1254     stable.
1255    
1256     While you are welcome to rummage around and use whatever you find useful
1257     (we can't stop you), keep in mind that we will change undocumented
1258     functionality in incompatible ways without thinking twice, while we are
1259     considerably more conservative with documented things.
1260    
1261     =head1 AUTHORS
1262    
1263     C<libecb> is designed and maintained by:
1264    
1265     Emanuele Giaquinta <e.giaquinta@glauco.it>
1266     Marc Alexander Lehmann <schmorp@schmorp.de>
1267    
1268 root 1.1