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Revision: 1.103
Committed: Wed Mar 23 09:59:49 2022 UTC (2 years, 3 months ago) by root
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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 root 1.94 (C<ecb_rotl>). There are no restrictions on the value C<count>, i.e. both
737 root 1.95 zero and values equal or larger than the word width work correctly. Also,
738     notwithstanding C<count> being unsigned, negative numbers work and shift
739     to the opposite direction.
740 root 1.93
741 root 1.85 Current GCC/clang versions understand these functions and usually compile
742     them to "optimal" code (e.g. a single C<rol> or a combination of C<shld>
743     on x86).
744 root 1.20
745 root 1.77 =item T ecb_rotl (T x, unsigned int count) [C++]
746    
747     =item T ecb_rotr (T x, unsigned int count) [C++]
748    
749     Overloaded C++ rotl/rotr functions.
750    
751     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
752    
753 root 1.102 =item uint_fast8_t ecb_gray8_encode (uint_fast8_t b)
754    
755     =item uint_fast16_t ecb_gray16_encode (uint_fast16_t b)
756    
757     =item uint_fast32_t ecb_gray32_encode (uint_fast32_t b)
758    
759     =item uint_fast64_t ecb_gray64_encode (uint_fast64_t b)
760    
761     Encode an unsigned into its corresponding (reflective) gray code - the
762     kind of gray code meant when just talking about "gray code". These
763     functions are very fast and all have identical implementation, so there is
764 root 1.103 no need to use a smaller type, as long as your CPU can handle it natively.
765 root 1.102
766     =item T ecb_gray_encode (T b) [C++]
767    
768     Overloaded C++ version of the above, for C<uint{8,16,32,64}_t>.
769    
770     =item uint_fast8_t ecb_gray8_decode (uint_fast8_t b)
771    
772     =item uint_fast16_t ecb_gray16_decode (uint_fast16_t b)
773    
774     =item uint_fast32_t ecb_gray32_decode (uint_fast32_t b)
775    
776     =item uint_fast64_t ecb_gray64_decode (uint_fast64_t b)
777    
778 root 1.103 Decode a gray code back into linear index form (the reverse of
779     C<ecb_gray*_encode>. Unlike the encode functions, the decode functions
780     have higher time complexity for larger types, so it can pay off to use a
781 root 1.102 smaller type here.
782    
783     =item T ecb_gray_decode (T b) [C++]
784    
785     Overloaded C++ version of the above, for C<uint{8,16,32,64}_t>.
786    
787 root 1.3 =back
788 root 1.1
789 root 1.96 =head2 BIT MIXING, HASHING
790    
791     Sometimes you have an integer and want to distribute its bits well, for
792     example, to use it as a hash in a hashtable. A common example is pointer
793     values, which often only have a limited range (e.g. low and high bits are
794     often zero).
795    
796     The following functions try to mix the bits to get a good bias-free
797     distribution. They were mainly made for pointers, but the underlying
798     integer functions are exposed as well.
799    
800     As an added benefit, the functions are reversible, so if you find it
801     convenient to store only the hash value, you can recover the original
802     pointer from the hash ("unmix"), as long as your pinters are 32 or 64 bit
803     (if this isn't the case on your platform, drop us a note and we will add
804     functions for other bit widths).
805    
806     The unmix functions are very slightly slower than the mix functions, so
807     it is equally very slightly preferable to store the original values wehen
808     convenient.
809    
810     The underlying algorithm if subject to change, so currently these
811     functions are not suitable for persistent hash tables, as their result
812     value can change between diferent versions of libecb.
813    
814     =over
815    
816     =item uintptr_t ecb_ptrmix (void *ptr)
817    
818     Mixes the bits of a pointer so the result is suitable for hash table
819     lookups. In other words, this hashes the pointer value.
820    
821     =item uintptr_t ecb_ptrmix (T *ptr) [C++]
822    
823     Overload the C<ecb_ptrmix> function to work for any pointer in C++.
824    
825     =item void *ecb_ptrunmix (uintptr_t v)
826    
827     Unmix the hash value into the original pointer. This only works as long
828     as the hash value is not truncated, i.e. you used C<uintptr_t> (or
829     equivalent) throughout to store it.
830    
831     =item T *ecb_ptrunmix<T> (uintptr_t v) [C++]
832    
833     The somewhat less useful template version of C<ecb_ptrunmix> for
834     C++. Example:
835    
836     sometype *myptr;
837     uintptr_t hash = ecb_ptrmix (myptr);
838     sometype *orig = ecb_ptrunmix<sometype> (hash);
839    
840     =item uint32_t ecb_mix32 (uint32_t v)
841    
842     =item uint64_t ecb_mix64 (uint64_t v)
843    
844     Sometimes you don't have a pointer but an integer whose values are very
845     badly distributed. In this case you cna sue these integer versions of the
846     mixing function. No C++ template is provided currently.
847    
848     =item uint32_t ecb_unmix32 (uint32_t v)
849    
850     =item uint64_t ecb_unmix64 (uint64_t v)
851    
852     The reverse of the C<ecb_mix> functions - they take a mixed/hashed value
853     and recover the original value.
854    
855     =back
856    
857 root 1.76 =head2 HOST ENDIANNESS CONVERSION
858    
859 root 1.88 =over
860 root 1.76
861     =item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v)
862    
863     =item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v)
864    
865     =item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v)
866    
867     =item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v)
868    
869     =item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v)
870    
871     =item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v)
872    
873     Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order.
874    
875     The naming convention is C<ecb_>(C<be>|C<le>)C<_u>C<16|32|64>C<_to_host>,
876 root 1.79 where C<be> and C<le> stand for big endian and little endian, respectively.
877 root 1.76
878     =item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v)
879    
880     =item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v)
881    
882     =item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v)
883    
884     =item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v)
885    
886     =item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v)
887    
888     =item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v)
889    
890     Like above, but converts I<from> host byte order to the specified
891     endianness.
892    
893     =back
894    
895 root 1.77 In C++ the following additional template functions are supported:
896 root 1.76
897 root 1.88 =over
898 root 1.76
899     =item T ecb_be_to_host (T v)
900    
901     =item T ecb_le_to_host (T v)
902    
903     =item T ecb_host_to_be (T v)
904    
905     =item T ecb_host_to_le (T v)
906    
907 root 1.86 =back
908    
909 root 1.77 These functions work like their C counterparts, above, but use templates,
910     which make them useful in generic code.
911 root 1.76
912     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>
913     (so unlike their C counterparts, there is a version for C<uint8_t>, which
914     again can be useful in generic code).
915    
916     =head2 UNALIGNED LOAD/STORE
917    
918     These function load or store unaligned multi-byte values.
919    
920 root 1.88 =over
921 root 1.76
922     =item uint_fast16_t ecb_peek_u16_u (const void *ptr)
923    
924     =item uint_fast32_t ecb_peek_u32_u (const void *ptr)
925    
926     =item uint_fast64_t ecb_peek_u64_u (const void *ptr)
927    
928     These functions load an unaligned, unsigned 16, 32 or 64 bit value from
929     memory.
930    
931     =item uint_fast16_t ecb_peek_be_u16_u (const void *ptr)
932    
933     =item uint_fast32_t ecb_peek_be_u32_u (const void *ptr)
934    
935     =item uint_fast64_t ecb_peek_be_u64_u (const void *ptr)
936    
937     =item uint_fast16_t ecb_peek_le_u16_u (const void *ptr)
938    
939     =item uint_fast32_t ecb_peek_le_u32_u (const void *ptr)
940    
941     =item uint_fast64_t ecb_peek_le_u64_u (const void *ptr)
942    
943     Like above, but additionally convert from big endian (C<be>) or little
944     endian (C<le>) byte order to host byte order while doing so.
945    
946     =item ecb_poke_u16_u (void *ptr, uint16_t v)
947    
948     =item ecb_poke_u32_u (void *ptr, uint32_t v)
949    
950     =item ecb_poke_u64_u (void *ptr, uint64_t v)
951    
952     These functions store an unaligned, unsigned 16, 32 or 64 bit value to
953     memory.
954    
955     =item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v)
956    
957     =item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v)
958    
959     =item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v)
960    
961     =item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v)
962    
963     =item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v)
964    
965     =item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v)
966    
967     Like above, but additionally convert from host byte order to big endian
968     (C<be>) or little endian (C<le>) byte order while doing so.
969    
970     =back
971    
972 root 1.77 In C++ the following additional template functions are supported:
973 root 1.76
974 root 1.88 =over
975 root 1.76
976 root 1.80 =item T ecb_peek<T> (const void *ptr)
977 root 1.76
978 root 1.80 =item T ecb_peek_be<T> (const void *ptr)
979 root 1.76
980 root 1.80 =item T ecb_peek_le<T> (const void *ptr)
981 root 1.76
982 root 1.80 =item T ecb_peek_u<T> (const void *ptr)
983 root 1.76
984 root 1.80 =item T ecb_peek_be_u<T> (const void *ptr)
985 root 1.76
986 root 1.80 =item T ecb_peek_le_u<T> (const void *ptr)
987 root 1.76
988     Similarly to their C counterparts, these functions load an unsigned 8, 16,
989     32 or 64 bit value from memory, with optional conversion from big/little
990     endian.
991    
992 root 1.80 Since the type cannot be deduced, it has to be specified explicitly, e.g.
993 root 1.76
994     uint_fast16_t v = ecb_peek<uint16_t> (ptr);
995    
996     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
997    
998     Unlike their C counterparts, these functions support 8 bit quantities
999     (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
1000     all of which hopefully makes them more useful in generic code.
1001    
1002     =item ecb_poke (void *ptr, T v)
1003    
1004     =item ecb_poke_be (void *ptr, T v)
1005    
1006     =item ecb_poke_le (void *ptr, T v)
1007    
1008     =item ecb_poke_u (void *ptr, T v)
1009    
1010     =item ecb_poke_be_u (void *ptr, T v)
1011    
1012     =item ecb_poke_le_u (void *ptr, T v)
1013    
1014     Again, similarly to their C counterparts, these functions store an
1015     unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to
1016     big/little endian.
1017    
1018     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
1019    
1020     Unlike their C counterparts, these functions support 8 bit quantities
1021     (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
1022     all of which hopefully makes them more useful in generic code.
1023    
1024     =back
1025    
1026 root 1.89 =head2 FAST INTEGER TO STRING
1027    
1028     Libecb defines a set of very fast integer to decimal string (or integer
1029     to ascii, short C<i2a>) functions. These work by converting the integer
1030     to a fixed point representation and then successively multiplying out
1031     the topmost digits. Unlike some other, also very fast, libraries, ecb's
1032     algorithm should be completely branchless per digit, and does not rely on
1033     the presence of special cpu functions (such as clz).
1034    
1035     There is a high level API that takes an C<int32_t>, C<uint32_t>,
1036     C<int64_t> or C<uint64_t> as argument, and a low-level API, which is
1037     harder to use but supports slightly more formatting options.
1038    
1039     =head3 HIGH LEVEL API
1040    
1041     The high level API consists of four functions, one each for C<int32_t>,
1042     C<uint32_t>, C<int64_t> and C<uint64_t>:
1043    
1044 root 1.91 Example:
1045    
1046     char buf[ECB_I2A_MAX_DIGITS + 1];
1047     char *end = ecb_i2a_i32 (buf, 17262);
1048     *end = 0;
1049     // buf now contains "17262"
1050    
1051 root 1.89 =over
1052    
1053     =item ECB_I2A_I32_DIGITS (=11)
1054    
1055     =item char *ecb_i2a_u32 (char *ptr, uint32_t value)
1056    
1057     Takes an C<uint32_t> I<value> and formats it as a decimal number starting
1058     at I<ptr>, using at most C<ECB_I2A_I32_DIGITS> characters. Returns a
1059     pointer to just after the generated string, where you would normally put
1060 sf-exg 1.92 the terminating C<0> character. This function outputs the minimum number
1061 root 1.89 of digits.
1062    
1063     =item ECB_I2A_U32_DIGITS (=10)
1064    
1065     =item char *ecb_i2a_i32 (char *ptr, int32_t value)
1066    
1067     Same as C<ecb_i2a_u32>, but formats a C<int32_t> value, including a minus
1068     sign if needed.
1069    
1070     =item ECB_I2A_I64_DIGITS (=20)
1071    
1072     =item char *ecb_i2a_u64 (char *ptr, uint64_t value)
1073    
1074     =item ECB_I2A_U64_DIGITS (=21)
1075    
1076     =item char *ecb_i2a_i64 (char *ptr, int64_t value)
1077    
1078     Similar to their 32 bit counterparts, these take a 64 bit argument.
1079    
1080 root 1.90 =item ECB_I2A_MAX_DIGITS (=21)
1081 root 1.89
1082 root 1.97 Instead of using a type specific length macro, you can just use
1083 root 1.90 C<ECB_I2A_MAX_DIGITS>, which is good enough for any C<ecb_i2a> function.
1084 root 1.89
1085     =back
1086    
1087     =head3 LOW-LEVEL API
1088    
1089     The functions above use a number of low-level APIs which have some strict
1090 root 1.98 limitations, but can be used as building blocks (studying C<ecb_i2a_i32>
1091 sf-exg 1.92 and related functions is recommended).
1092 root 1.89
1093     There are three families of functions: functions that convert a number
1094     to a fixed number of digits with leading zeroes (C<ecb_i2a_0N>, C<0>
1095     for "leading zeroes"), functions that generate up to N digits, skipping
1096     leading zeroes (C<_N>), and functions that can generate more digits, but
1097     the leading digit has limited range (C<_xN>).
1098    
1099 sf-exg 1.92 None of the functions deal with negative numbers.
1100 root 1.89
1101 root 1.91 Example: convert an IP address in an u32 into dotted-quad:
1102    
1103     uint32_t ip = 0x0a000164; // 10.0.1.100
1104     char ips[3 * 4 + 3 + 1];
1105     char *ptr = ips;
1106     ptr = ecb_i2a_3 (ptr, ip >> 24 ); *ptr++ = '.';
1107     ptr = ecb_i2a_3 (ptr, (ip >> 16) & 0xff); *ptr++ = '.';
1108     ptr = ecb_i2a_3 (ptr, (ip >> 8) & 0xff); *ptr++ = '.';
1109     ptr = ecb_i2a_3 (ptr, ip & 0xff); *ptr++ = 0;
1110     printf ("ip: %s\n", ips); // prints "ip: 10.0.1.100"
1111    
1112 root 1.89 =over
1113    
1114     =item char *ecb_i2a_02 (char *ptr, uint32_t value) // 32 bit
1115    
1116     =item char *ecb_i2a_03 (char *ptr, uint32_t value) // 32 bit
1117    
1118     =item char *ecb_i2a_04 (char *ptr, uint32_t value) // 32 bit
1119    
1120     =item char *ecb_i2a_05 (char *ptr, uint32_t value) // 64 bit
1121    
1122     =item char *ecb_i2a_06 (char *ptr, uint32_t value) // 64 bit
1123    
1124     =item char *ecb_i2a_07 (char *ptr, uint32_t value) // 64 bit
1125    
1126     =item char *ecb_i2a_08 (char *ptr, uint32_t value) // 64 bit
1127    
1128     =item char *ecb_i2a_09 (char *ptr, uint32_t value) // 64 bit
1129    
1130 root 1.101 The C<< ecb_i2a_0I<N> >> functions take an unsigned I<value> and convert
1131 root 1.89 them to exactly I<N> digits, returning a pointer to the first character
1132     after the digits. The I<value> must be in range. The functions marked with
1133     I<32 bit> do their calculations internally in 32 bit, the ones marked with
1134     I<64 bit> internally use 64 bit integers, which might be slow on 32 bit
1135     architectures (the high level API decides on 32 vs. 64 bit versions using
1136     C<ECB_64BIT_NATIVE>).
1137    
1138     =item char *ecb_i2a_2 (char *ptr, uint32_t value) // 32 bit
1139    
1140     =item char *ecb_i2a_3 (char *ptr, uint32_t value) // 32 bit
1141    
1142     =item char *ecb_i2a_4 (char *ptr, uint32_t value) // 32 bit
1143    
1144     =item char *ecb_i2a_5 (char *ptr, uint32_t value) // 64 bit
1145    
1146     =item char *ecb_i2a_6 (char *ptr, uint32_t value) // 64 bit
1147    
1148     =item char *ecb_i2a_7 (char *ptr, uint32_t value) // 64 bit
1149    
1150     =item char *ecb_i2a_8 (char *ptr, uint32_t value) // 64 bit
1151    
1152     =item char *ecb_i2a_9 (char *ptr, uint32_t value) // 64 bit
1153    
1154 root 1.101 Similarly, the C<< ecb_i2a_I<N> >> functions take an unsigned I<value>
1155 root 1.89 and convert them to at most I<N> digits, suppressing leading zeroes, and
1156     returning a pointer to the first character after the digits.
1157    
1158     =item ECB_I2A_MAX_X5 (=59074)
1159    
1160     =item char *ecb_i2a_x5 (char *ptr, uint32_t value) // 32 bit
1161    
1162     =item ECB_I2A_MAX_X10 (=2932500665)
1163    
1164     =item char *ecb_i2a_x10 (char *ptr, uint32_t value) // 64 bit
1165    
1166 root 1.101 The C<< ecb_i2a_xI<N> >> functions are similar to the C<< ecb_i2a_I<N> >>
1167 root 1.89 functions, but they can generate one digit more, as long as the number
1168     is within range, which is given by the symbols C<ECB_I2A_MAX_X5> (almost
1169     16 bit range) and C<ECB_I2A_MAX_X10> (a bit more than 31 bit range),
1170     respectively.
1171    
1172 sf-exg 1.92 For example, the digit part of a 32 bit signed integer just fits into the
1173 root 1.89 C<ECB_I2A_MAX_X10> range, so while C<ecb_i2a_x10> cannot convert a 10
1174     digit number, it can convert all 32 bit signed numbers. Sadly, it's not
1175     good enough for 32 bit unsigned numbers.
1176    
1177     =back
1178    
1179 root 1.50 =head2 FLOATING POINT FIDDLING
1180    
1181 root 1.88 =over
1182 root 1.50
1183 root 1.71 =item ECB_INFINITY [-UECB_NO_LIBM]
1184 root 1.62
1185     Evaluates to positive infinity if supported by the platform, otherwise to
1186     a truly huge number.
1187    
1188 root 1.71 =item ECB_NAN [-UECB_NO_LIBM]
1189 root 1.62
1190     Evaluates to a quiet NAN if supported by the platform, otherwise to
1191     C<ECB_INFINITY>.
1192    
1193 root 1.71 =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
1194 root 1.62
1195     Same as C<ldexpf>, but always available.
1196    
1197 root 1.71 =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
1198    
1199 root 1.50 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
1200    
1201     =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
1202    
1203     These functions each take an argument in the native C<float> or C<double>
1204 root 1.71 type and return the IEEE 754 bit representation of it (binary16/half,
1205     binary32/single or binary64/double precision).
1206 root 1.50
1207     The bit representation is just as IEEE 754 defines it, i.e. the sign bit
1208     will be the most significant bit, followed by exponent and mantissa.
1209    
1210     This function should work even when the native floating point format isn't
1211     IEEE compliant, of course at a speed and code size penalty, and of course
1212     also within reasonable limits (it tries to convert NaNs, infinities and
1213     denormals, but will likely convert negative zero to positive zero).
1214    
1215     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1216 root 1.99 be able to completely optimise away the 32 and 64 bit functions.
1217 root 1.50
1218     These functions can be helpful when serialising floats to the network - you
1219 root 1.71 can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
1220 root 1.50
1221     Another use for these functions is to manipulate floating point values
1222     directly.
1223    
1224     Silly example: toggle the sign bit of a float.
1225    
1226     /* On gcc-4.7 on amd64, */
1227     /* this results in a single add instruction to toggle the bit, and 4 extra */
1228     /* instructions to move the float value to an integer register and back. */
1229    
1230     x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
1231    
1232 root 1.58 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
1233    
1234 root 1.50 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
1235    
1236 root 1.70 =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
1237 root 1.50
1238 sf-exg 1.59 The reverse operation of the previous function - takes the bit
1239 root 1.71 representation of an IEEE binary16, binary32 or binary64 number (half,
1240     single or double precision) and converts it to the native C<float> or
1241     C<double> format.
1242 root 1.50
1243     This function should work even when the native floating point format isn't
1244     IEEE compliant, of course at a speed and code size penalty, and of course
1245     also within reasonable limits (it tries to convert normals and denormals,
1246     and might be lucky for infinities, and with extraordinary luck, also for
1247     negative zero).
1248    
1249     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1250     be able to optimise away this function completely.
1251    
1252 root 1.71 =item uint16_t ecb_binary32_to_binary16 (uint32_t x)
1253    
1254     =item uint32_t ecb_binary16_to_binary32 (uint16_t x)
1255    
1256     Convert a IEEE binary32/single precision to binary16/half format, and vice
1257 root 1.72 versa, handling all details (round-to-nearest-even, subnormals, infinity
1258     and NaNs) correctly.
1259 root 1.71
1260     These are functions are available under C<-DECB_NO_LIBM>, since
1261     they do not rely on the platform floating point format. The
1262     C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
1263     usually what you want.
1264    
1265 root 1.50 =back
1266    
1267 root 1.1 =head2 ARITHMETIC
1268    
1269 root 1.88 =over
1270 root 1.3
1271 root 1.14 =item x = ecb_mod (m, n)
1272 root 1.3
1273 root 1.25 Returns C<m> modulo C<n>, which is the same as the positive remainder
1274     of the division operation between C<m> and C<n>, using floored
1275     division. Unlike the C remainder operator C<%>, this function ensures that
1276     the return value is always positive and that the two numbers I<m> and
1277     I<m' = m + i * n> result in the same value modulo I<n> - in other words,
1278     C<ecb_mod> implements the mathematical modulo operation, which is missing
1279     in the language.
1280 root 1.14
1281 sf-exg 1.23 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
1282 root 1.14 negatable, that is, both C<m> and C<-m> must be representable in its
1283 root 1.30 type (this typically excludes the minimum signed integer value, the same
1284 root 1.25 limitation as for C</> and C<%> in C).
1285 sf-exg 1.11
1286 root 1.85 Current GCC/clang versions compile this into an efficient branchless
1287     sequence on almost all CPUs.
1288 root 1.24
1289     For example, when you want to rotate forward through the members of an
1290     array for increasing C<m> (which might be negative), then you should use
1291     C<ecb_mod>, as the C<%> operator might give either negative results, or
1292     change direction for negative values:
1293    
1294     for (m = -100; m <= 100; ++m)
1295     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
1296    
1297 sf-exg 1.37 =item x = ecb_div_rd (val, div)
1298    
1299     =item x = ecb_div_ru (val, div)
1300    
1301     Returns C<val> divided by C<div> rounded down or up, respectively.
1302     C<val> and C<div> must have integer types and C<div> must be strictly
1303 sf-exg 1.38 positive. Note that these functions are implemented with macros in C
1304     and with function templates in C++.
1305 sf-exg 1.37
1306 root 1.3 =back
1307 root 1.1
1308     =head2 UTILITY
1309    
1310 root 1.88 =over
1311 root 1.3
1312 sf-exg 1.23 =item element_count = ecb_array_length (name)
1313 root 1.3
1314 sf-exg 1.13 Returns the number of elements in the array C<name>. For example:
1315    
1316     int primes[] = { 2, 3, 5, 7, 11 };
1317     int sum = 0;
1318    
1319     for (i = 0; i < ecb_array_length (primes); i++)
1320     sum += primes [i];
1321    
1322 root 1.3 =back
1323 root 1.1
1324 root 1.43 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
1325    
1326     These symbols need to be defined before including F<ecb.h> the first time.
1327    
1328 root 1.88 =over
1329 root 1.43
1330 root 1.51 =item ECB_NO_THREADS
1331 root 1.43
1332     If F<ecb.h> is never used from multiple threads, then this symbol can
1333     be defined, in which case memory fences (and similar constructs) are
1334     completely removed, leading to more efficient code and fewer dependencies.
1335    
1336     Setting this symbol to a true value implies C<ECB_NO_SMP>.
1337    
1338     =item ECB_NO_SMP
1339    
1340     The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
1341     multiple threads, but never concurrently (e.g. if the system the program
1342     runs on has only a single CPU with a single core, no hyperthreading and so
1343     on), then this symbol can be defined, leading to more efficient code and
1344     fewer dependencies.
1345    
1346 root 1.50 =item ECB_NO_LIBM
1347    
1348     When defined to C<1>, do not export any functions that might introduce
1349     dependencies on the math library (usually called F<-lm>) - these are
1350     marked with [-UECB_NO_LIBM].
1351    
1352 sf-exg 1.69 =back
1353    
1354 root 1.68 =head1 UNDOCUMENTED FUNCTIONALITY
1355    
1356     F<ecb.h> is full of undocumented functionality as well, some of which is
1357     intended to be internal-use only, some of which we forgot to document, and
1358     some of which we hide because we are not sure we will keep the interface
1359     stable.
1360    
1361     While you are welcome to rummage around and use whatever you find useful
1362 root 1.100 (we don't want to stop you), keep in mind that we will change undocumented
1363 root 1.68 functionality in incompatible ways without thinking twice, while we are
1364     considerably more conservative with documented things.
1365    
1366     =head1 AUTHORS
1367    
1368     C<libecb> is designed and maintained by:
1369    
1370     Emanuele Giaquinta <e.giaquinta@glauco.it>
1371     Marc Alexander Lehmann <schmorp@schmorp.de>