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Revision: 1.75
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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     It mainly provides a number of wrappers around GCC built-ins, together
16     with replacement functions for other compilers. In addition to this,
17 sf-exg 1.16 it provides a number of other lowlevel C utilities, such as endianness
18 root 1.14 detection, byte swapping or bit rotations.
19    
20 root 1.24 Or in other words, things that should be built into any standard C system,
21     but aren't, implemented as efficient as possible with GCC, and still
22     correct with other compilers.
23 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 1.42 int8_t uint8_t int16_t uint16_t
64     int32_t uint32_t int64_t uint64_t
65 root 1.49 intptr_t uintptr_t
66 root 1.40
67     The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
68 root 1.45 platform (currently C<4> or C<8>) and can be used in preprocessor
69     expressions.
70 root 1.40
71 root 1.74 For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>.
72 root 1.49
73 root 1.62 =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
74 root 1.43
75 sf-exg 1.46 All the following symbols expand to an expression that can be tested in
76 root 1.44 preprocessor instructions as well as treated as a boolean (use C<!!> to
77     ensure it's either C<0> or C<1> if you need that).
78    
79 root 1.43 =over 4
80    
81 root 1.44 =item ECB_C
82    
83     True if the implementation defines the C<__STDC__> macro to a true value,
84 root 1.55 while not claiming to be C++.
85 root 1.44
86 root 1.43 =item ECB_C99
87    
88 root 1.47 True if the implementation claims to be compliant to C99 (ISO/IEC
89 root 1.55 9899:1999) or any later version, while not claiming to be C++.
90 root 1.47
91     Note that later versions (ECB_C11) remove core features again (for
92     example, variable length arrays).
93 root 1.43
94 root 1.74 =item ECB_C11, ECB_C17
95 root 1.43
96 root 1.74 True if the implementation claims to be compliant to C11/C17 (ISO/IEC
97     9899:2011, :20187) or any later version, while not claiming to be C++.
98 root 1.44
99     =item ECB_CPP
100    
101     True if the implementation defines the C<__cplusplus__> macro to a true
102     value, which is typically true for C++ compilers.
103    
104 root 1.74 =item ECB_CPP11, ECB_CPP14, ECB_CPP17
105 root 1.44
106 root 1.74 True if the implementation claims to be compliant to C++11/C++14/C++17
107     (ISO/IEC 14882:2011, :2014, :2017) or any later version.
108 root 1.43
109 root 1.57 =item ECB_GCC_VERSION (major, minor)
110 root 1.43
111     Expands to a true value (suitable for testing in by the preprocessor)
112 sf-exg 1.46 if the compiler used is GNU C and the version is the given version, or
113 root 1.43 higher.
114    
115     This macro tries to return false on compilers that claim to be GCC
116     compatible but aren't.
117    
118 root 1.50 =item ECB_EXTERN_C
119    
120     Expands to C<extern "C"> in C++, and a simple C<extern> in C.
121    
122     This can be used to declare a single external C function:
123    
124     ECB_EXTERN_C int printf (const char *format, ...);
125    
126     =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
127    
128     These two macros can be used to wrap multiple C<extern "C"> definitions -
129     they expand to nothing in C.
130    
131     They are most useful in header files:
132    
133     ECB_EXTERN_C_BEG
134    
135     int mycfun1 (int x);
136     int mycfun2 (int x);
137    
138     ECB_EXTERN_C_END
139    
140     =item ECB_STDFP
141    
142     If this evaluates to a true value (suitable for testing in by the
143     preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
144     and double/binary64 representations internally I<and> the endianness of
145     both types match the endianness of C<uint32_t> and C<uint64_t>.
146    
147     This means you can just copy the bits of a C<float> (or C<double>) to an
148     C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
149     without having to think about format or endianness.
150    
151     This is true for basically all modern platforms, although F<ecb.h> might
152     not be able to deduce this correctly everywhere and might err on the safe
153     side.
154    
155 root 1.54 =item ECB_AMD64, ECB_AMD64_X32
156    
157     These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
158     ABI, respectively, and undefined elsewhere.
159    
160     The designers of the new X32 ABI for some inexplicable reason decided to
161     make it look exactly like amd64, even though it's completely incompatible
162     to that ABI, breaking about every piece of software that assumed that
163     C<__x86_64> stands for, well, the x86-64 ABI, making these macros
164     necessary.
165    
166 root 1.43 =back
167    
168 root 1.62 =head2 MACRO TRICKERY
169    
170     =over 4
171    
172     =item ECB_CONCAT (a, b)
173    
174     Expands any macros in C<a> and C<b>, then concatenates the result to form
175     a single token. This is mainly useful to form identifiers from components,
176     e.g.:
177    
178     #define S1 str
179     #define S2 cpy
180    
181     ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
182    
183     =item ECB_STRINGIFY (arg)
184    
185     Expands any macros in C<arg> and returns the stringified version of
186     it. This is mainly useful to get the contents of a macro in string form,
187     e.g.:
188    
189     #define SQL_LIMIT 100
190     sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
191    
192 root 1.64 =item ECB_STRINGIFY_EXPR (expr)
193    
194     Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
195     is a valid expression. This is useful to catch typos or cases where the
196     macro isn't available:
197    
198     #include <errno.h>
199    
200     ECB_STRINGIFY (EDOM); // "33" (on my system at least)
201     ECB_STRINGIFY_EXPR (EDOM); // "33"
202    
203     // now imagine we had a typo:
204    
205     ECB_STRINGIFY (EDAM); // "EDAM"
206     ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
207    
208 root 1.62 =back
209    
210 sf-exg 1.60 =head2 ATTRIBUTES
211 root 1.1
212 sf-exg 1.60 A major part of libecb deals with additional attributes that can be
213     assigned to functions, variables and sometimes even types - much like
214     C<const> or C<volatile> in C. They are implemented using either GCC
215     attributes or other compiler/language specific features. Attributes
216     declarations must be put before the whole declaration:
217 root 1.20
218     ecb_const int mysqrt (int a);
219     ecb_unused int i;
220    
221 root 1.1 =over 4
222    
223 root 1.3 =item ecb_unused
224    
225     Marks a function or a variable as "unused", which simply suppresses a
226     warning by GCC when it detects it as unused. This is useful when you e.g.
227     declare a variable but do not always use it:
228    
229 root 1.15 {
230 sf-exg 1.61 ecb_unused int var;
231 root 1.3
232 root 1.15 #ifdef SOMECONDITION
233     var = ...;
234     return var;
235     #else
236     return 0;
237     #endif
238     }
239 root 1.3
240 root 1.56 =item ecb_deprecated
241    
242     Similar to C<ecb_unused>, but marks a function, variable or type as
243     deprecated. This makes some compilers warn when the type is used.
244    
245 root 1.62 =item ecb_deprecated_message (message)
246    
247 root 1.67 Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
248 root 1.62 used instead of a generic depreciation message when the object is being
249     used.
250    
251 root 1.31 =item ecb_inline
252 root 1.29
253 root 1.73 Expands either to (a compiler-specific equivalent of) C<static inline> or
254     to just C<static>, if inline isn't supported. It should be used to declare
255     functions that should be inlined, for code size or speed reasons.
256 root 1.29
257     Example: inline this function, it surely will reduce codesize.
258    
259 root 1.31 ecb_inline int
260 root 1.29 negmul (int a, int b)
261     {
262     return - (a * b);
263     }
264    
265 root 1.2 =item ecb_noinline
266    
267 sf-exg 1.66 Prevents a function from being inlined - it might be optimised away, but
268 root 1.3 not inlined into other functions. This is useful if you know your function
269     is rarely called and large enough for inlining not to be helpful.
270    
271 root 1.2 =item ecb_noreturn
272    
273 root 1.17 Marks a function as "not returning, ever". Some typical functions that
274     don't return are C<exit> or C<abort> (which really works hard to not
275     return), and now you can make your own:
276    
277     ecb_noreturn void
278     my_abort (const char *errline)
279     {
280     puts (errline);
281     abort ();
282     }
283    
284 sf-exg 1.19 In this case, the compiler would probably be smart enough to deduce it on
285     its own, so this is mainly useful for declarations.
286 root 1.17
287 root 1.53 =item ecb_restrict
288    
289     Expands to the C<restrict> keyword or equivalent on compilers that support
290     them, and to nothing on others. Must be specified on a pointer type or
291     an array index to indicate that the memory doesn't alias with any other
292     restricted pointer in the same scope.
293    
294     Example: multiply a vector, and allow the compiler to parallelise the
295     loop, because it knows it doesn't overwrite input values.
296    
297     void
298 sf-exg 1.61 multiply (ecb_restrict float *src,
299     ecb_restrict float *dst,
300 root 1.53 int len, float factor)
301     {
302     int i;
303    
304     for (i = 0; i < len; ++i)
305     dst [i] = src [i] * factor;
306     }
307    
308 root 1.2 =item ecb_const
309    
310 sf-exg 1.19 Declares that the function only depends on the values of its arguments,
311 root 1.17 much like a mathematical function. It specifically does not read or write
312     any memory any arguments might point to, global variables, or call any
313     non-const functions. It also must not have any side effects.
314    
315     Such a function can be optimised much more aggressively by the compiler -
316     for example, multiple calls with the same arguments can be optimised into
317     a single call, which wouldn't be possible if the compiler would have to
318     expect any side effects.
319    
320     It is best suited for functions in the sense of mathematical functions,
321 sf-exg 1.19 such as a function returning the square root of its input argument.
322 root 1.17
323     Not suited would be a function that calculates the hash of some memory
324     area you pass in, prints some messages or looks at a global variable to
325     decide on rounding.
326    
327     See C<ecb_pure> for a slightly less restrictive class of functions.
328    
329 root 1.2 =item ecb_pure
330    
331 root 1.17 Similar to C<ecb_const>, declares a function that has no side
332     effects. Unlike C<ecb_const>, the function is allowed to examine global
333     variables and any other memory areas (such as the ones passed to it via
334     pointers).
335    
336     While these functions cannot be optimised as aggressively as C<ecb_const>
337     functions, they can still be optimised away in many occasions, and the
338     compiler has more freedom in moving calls to them around.
339    
340     Typical examples for such functions would be C<strlen> or C<memcmp>. A
341     function that calculates the MD5 sum of some input and updates some MD5
342     state passed as argument would I<NOT> be pure, however, as it would modify
343     some memory area that is not the return value.
344    
345 root 1.2 =item ecb_hot
346    
347 root 1.17 This declares a function as "hot" with regards to the cache - the function
348     is used so often, that it is very beneficial to keep it in the cache if
349     possible.
350    
351     The compiler reacts by trying to place hot functions near to each other in
352     memory.
353    
354 sf-exg 1.19 Whether a function is hot or not often depends on the whole program,
355 root 1.17 and less on the function itself. C<ecb_cold> is likely more useful in
356     practise.
357    
358 root 1.2 =item ecb_cold
359    
360 root 1.17 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
361     the cache, or in other words, this function is not called often, or not at
362     speed-critical times, and keeping it in the cache might be a waste of said
363     cache.
364    
365     In addition to placing cold functions together (or at least away from hot
366     functions), this knowledge can be used in other ways, for example, the
367     function will be optimised for size, as opposed to speed, and codepaths
368     leading to calls to those functions can automatically be marked as if
369 root 1.27 C<ecb_expect_false> had been used to reach them.
370 root 1.17
371     Good examples for such functions would be error reporting functions, or
372     functions only called in exceptional or rare cases.
373    
374 root 1.2 =item ecb_artificial
375    
376 root 1.17 Declares the function as "artificial", in this case meaning that this
377 root 1.52 function is not really meant to be a function, but more like an accessor
378 root 1.17 - many methods in C++ classes are mere accessor functions, and having a
379     crash reported in such a method, or single-stepping through them, is not
380     usually so helpful, especially when it's inlined to just a few instructions.
381    
382     Marking them as artificial will instruct the debugger about just this,
383     leading to happier debugging and thus happier lives.
384    
385     Example: in some kind of smart-pointer class, mark the pointer accessor as
386     artificial, so that the whole class acts more like a pointer and less like
387     some C++ abstraction monster.
388    
389     template<typename T>
390     struct my_smart_ptr
391     {
392     T *value;
393    
394     ecb_artificial
395     operator T *()
396     {
397     return value;
398     }
399     };
400    
401 root 1.2 =back
402 root 1.1
403     =head2 OPTIMISATION HINTS
404    
405     =over 4
406    
407 root 1.75 =item ECB_OPTIMIZE_SIZE
408    
409     Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
410     can also be defined before including F<ecb.h>, in which case it will be
411     unchanged.
412    
413 root 1.58 =item bool ecb_is_constant (expr)
414 root 1.1
415 root 1.3 Returns true iff the expression can be deduced to be a compile-time
416     constant, and false otherwise.
417    
418     For example, when you have a C<rndm16> function that returns a 16 bit
419     random number, and you have a function that maps this to a range from
420 root 1.5 0..n-1, then you could use this inline function in a header file:
421 root 1.3
422     ecb_inline uint32_t
423     rndm (uint32_t n)
424     {
425 root 1.6 return (n * (uint32_t)rndm16 ()) >> 16;
426 root 1.3 }
427    
428     However, for powers of two, you could use a normal mask, but that is only
429     worth it if, at compile time, you can detect this case. This is the case
430     when the passed number is a constant and also a power of two (C<n & (n -
431     1) == 0>):
432    
433     ecb_inline uint32_t
434     rndm (uint32_t n)
435     {
436     return is_constant (n) && !(n & (n - 1))
437     ? rndm16 () & (num - 1)
438 root 1.6 : (n * (uint32_t)rndm16 ()) >> 16;
439 root 1.3 }
440    
441 root 1.62 =item ecb_expect (expr, value)
442 root 1.1
443 root 1.7 Evaluates C<expr> and returns it. In addition, it tells the compiler that
444     the C<expr> evaluates to C<value> a lot, which can be used for static
445     branch optimisations.
446 root 1.1
447 root 1.27 Usually, you want to use the more intuitive C<ecb_expect_true> and
448     C<ecb_expect_false> functions instead.
449 root 1.1
450 root 1.27 =item bool ecb_expect_true (cond)
451 root 1.1
452 root 1.27 =item bool ecb_expect_false (cond)
453 root 1.1
454 root 1.7 These two functions expect a expression that is true or false and return
455     C<1> or C<0>, respectively, so when used in the condition of an C<if> or
456     other conditional statement, it will not change the program:
457    
458     /* these two do the same thing */
459     if (some_condition) ...;
460 root 1.27 if (ecb_expect_true (some_condition)) ...;
461 root 1.7
462 root 1.27 However, by using C<ecb_expect_true>, you tell the compiler that the
463     condition is likely to be true (and for C<ecb_expect_false>, that it is
464     unlikely to be true).
465 root 1.7
466 root 1.9 For example, when you check for a null pointer and expect this to be a
467 root 1.27 rare, exceptional, case, then use C<ecb_expect_false>:
468 root 1.7
469     void my_free (void *ptr)
470     {
471 root 1.27 if (ecb_expect_false (ptr == 0))
472 root 1.7 return;
473     }
474    
475     Consequent use of these functions to mark away exceptional cases or to
476     tell the compiler what the hot path through a function is can increase
477     performance considerably.
478    
479 root 1.27 You might know these functions under the name C<likely> and C<unlikely>
480     - while these are common aliases, we find that the expect name is easier
481     to understand when quickly skimming code. If you wish, you can use
482     C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
483     C<ecb_expect_false> - these are simply aliases.
484    
485 root 1.7 A very good example is in a function that reserves more space for some
486     memory block (for example, inside an implementation of a string stream) -
487 root 1.9 each time something is added, you have to check for a buffer overrun, but
488 root 1.7 you expect that most checks will turn out to be false:
489    
490     /* make sure we have "size" extra room in our buffer */
491     ecb_inline void
492     reserve (int size)
493     {
494 root 1.27 if (ecb_expect_false (current + size > end))
495 root 1.7 real_reserve_method (size); /* presumably noinline */
496     }
497    
498 root 1.62 =item ecb_assume (cond)
499 root 1.7
500 sf-exg 1.66 Tries to tell the compiler that some condition is true, even if it's not
501 root 1.65 obvious. This is not a function, but a statement: it cannot be used in
502     another expression.
503 root 1.7
504     This can be used to teach the compiler about invariants or other
505     conditions that might improve code generation, but which are impossible to
506     deduce form the code itself.
507    
508 root 1.27 For example, the example reservation function from the C<ecb_expect_false>
509 root 1.7 description could be written thus (only C<ecb_assume> was added):
510    
511     ecb_inline void
512     reserve (int size)
513     {
514 root 1.27 if (ecb_expect_false (current + size > end))
515 root 1.7 real_reserve_method (size); /* presumably noinline */
516    
517     ecb_assume (current + size <= end);
518     }
519    
520     If you then call this function twice, like this:
521    
522     reserve (10);
523     reserve (1);
524    
525     Then the compiler I<might> be able to optimise out the second call
526     completely, as it knows that C<< current + 1 > end >> is false and the
527     call will never be executed.
528    
529 root 1.62 =item ecb_unreachable ()
530 root 1.7
531     This function does nothing itself, except tell the compiler that it will
532 root 1.9 never be executed. Apart from suppressing a warning in some cases, this
533 root 1.65 function can be used to implement C<ecb_assume> or similar functionality.
534 root 1.7
535 root 1.62 =item ecb_prefetch (addr, rw, locality)
536 root 1.7
537     Tells the compiler to try to prefetch memory at the given C<addr>ess
538 root 1.10 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
539 root 1.7 C<0> means that there will only be one access later, C<3> means that
540     the data will likely be accessed very often, and values in between mean
541     something... in between. The memory pointed to by the address does not
542     need to be accessible (it could be a null pointer for example), but C<rw>
543     and C<locality> must be compile-time constants.
544    
545 root 1.65 This is a statement, not a function: you cannot use it as part of an
546     expression.
547    
548 root 1.7 An obvious way to use this is to prefetch some data far away, in a big
549 root 1.9 array you loop over. This prefetches memory some 128 array elements later,
550 root 1.7 in the hope that it will be ready when the CPU arrives at that location.
551    
552     int sum = 0;
553    
554     for (i = 0; i < N; ++i)
555     {
556     sum += arr [i]
557     ecb_prefetch (arr + i + 128, 0, 0);
558     }
559    
560     It's hard to predict how far to prefetch, and most CPUs that can prefetch
561     are often good enough to predict this kind of behaviour themselves. It
562     gets more interesting with linked lists, especially when you do some fair
563     processing on each list element:
564    
565     for (node *n = start; n; n = n->next)
566     {
567     ecb_prefetch (n->next, 0, 0);
568     ... do medium amount of work with *n
569     }
570    
571     After processing the node, (part of) the next node might already be in
572     cache.
573 root 1.1
574 root 1.2 =back
575 root 1.1
576 root 1.36 =head2 BIT FIDDLING / BIT WIZARDRY
577 root 1.1
578 root 1.4 =over 4
579    
580 root 1.3 =item bool ecb_big_endian ()
581    
582     =item bool ecb_little_endian ()
583    
584 sf-exg 1.11 These two functions return true if the byte order is big endian
585     (most-significant byte first) or little endian (least-significant byte
586     first) respectively.
587    
588 root 1.24 On systems that are neither, their return values are unspecified.
589    
590 root 1.3 =item int ecb_ctz32 (uint32_t x)
591    
592 root 1.35 =item int ecb_ctz64 (uint64_t x)
593    
594 sf-exg 1.11 Returns the index of the least significant bit set in C<x> (or
595 root 1.24 equivalently the number of bits set to 0 before the least significant bit
596 root 1.35 set), starting from 0. If C<x> is 0 the result is undefined.
597    
598 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
599    
600 root 1.35 For example:
601 sf-exg 1.11
602 root 1.15 ecb_ctz32 (3) = 0
603     ecb_ctz32 (6) = 1
604 sf-exg 1.11
605 root 1.41 =item bool ecb_is_pot32 (uint32_t x)
606    
607     =item bool ecb_is_pot64 (uint32_t x)
608    
609 sf-exg 1.66 Returns true iff C<x> is a power of two or C<x == 0>.
610 root 1.41
611 sf-exg 1.66 For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
612 root 1.41
613 root 1.35 =item int ecb_ld32 (uint32_t x)
614    
615     =item int ecb_ld64 (uint64_t x)
616    
617     Returns the index of the most significant bit set in C<x>, or the number
618     of digits the number requires in binary (so that C<< 2**ld <= x <
619     2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
620     to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
621     example to see how many bits a certain number requires to be encoded.
622    
623     This function is similar to the "count leading zero bits" function, except
624     that that one returns how many zero bits are "in front" of the number (in
625     the given data type), while C<ecb_ld> returns how many bits the number
626     itself requires.
627    
628 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
629    
630 root 1.3 =item int ecb_popcount32 (uint32_t x)
631    
632 root 1.35 =item int ecb_popcount64 (uint64_t x)
633    
634 root 1.36 Returns the number of bits set to 1 in C<x>.
635    
636     For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
637    
638     For example:
639 sf-exg 1.11
640 root 1.15 ecb_popcount32 (7) = 3
641     ecb_popcount32 (255) = 8
642 sf-exg 1.11
643 root 1.39 =item uint8_t ecb_bitrev8 (uint8_t x)
644    
645     =item uint16_t ecb_bitrev16 (uint16_t x)
646    
647     =item uint32_t ecb_bitrev32 (uint32_t x)
648    
649     Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
650     and so on.
651    
652     Example:
653    
654     ecb_bitrev8 (0xa7) = 0xea
655     ecb_bitrev32 (0xffcc4411) = 0x882233ff
656    
657 root 1.8 =item uint32_t ecb_bswap16 (uint32_t x)
658    
659 root 1.3 =item uint32_t ecb_bswap32 (uint32_t x)
660    
661 root 1.34 =item uint64_t ecb_bswap64 (uint64_t x)
662 sf-exg 1.13
663 root 1.34 These functions return the value of the 16-bit (32-bit, 64-bit) value
664     C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
665     C<ecb_bswap32>).
666    
667     =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
668    
669     =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
670 root 1.3
671     =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
672    
673 root 1.34 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
674    
675     =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
676    
677     =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
678    
679     =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
680    
681 root 1.33 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
682    
683 root 1.34 These two families of functions return the value of C<x> after rotating
684     all the bits by C<count> positions to the right (C<ecb_rotr>) or left
685     (C<ecb_rotl>).
686 sf-exg 1.11
687 root 1.20 Current GCC versions understand these functions and usually compile them
688 root 1.34 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
689     x86).
690 root 1.20
691 root 1.3 =back
692 root 1.1
693 root 1.50 =head2 FLOATING POINT FIDDLING
694    
695     =over 4
696    
697 root 1.71 =item ECB_INFINITY [-UECB_NO_LIBM]
698 root 1.62
699     Evaluates to positive infinity if supported by the platform, otherwise to
700     a truly huge number.
701    
702 root 1.71 =item ECB_NAN [-UECB_NO_LIBM]
703 root 1.62
704     Evaluates to a quiet NAN if supported by the platform, otherwise to
705     C<ECB_INFINITY>.
706    
707 root 1.71 =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
708 root 1.62
709     Same as C<ldexpf>, but always available.
710    
711 root 1.71 =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
712    
713 root 1.50 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
714    
715     =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
716    
717     These functions each take an argument in the native C<float> or C<double>
718 root 1.71 type and return the IEEE 754 bit representation of it (binary16/half,
719     binary32/single or binary64/double precision).
720 root 1.50
721     The bit representation is just as IEEE 754 defines it, i.e. the sign bit
722     will be the most significant bit, followed by exponent and mantissa.
723    
724     This function should work even when the native floating point format isn't
725     IEEE compliant, of course at a speed and code size penalty, and of course
726     also within reasonable limits (it tries to convert NaNs, infinities and
727     denormals, but will likely convert negative zero to positive zero).
728    
729     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
730     be able to optimise away this function completely.
731    
732     These functions can be helpful when serialising floats to the network - you
733 root 1.71 can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
734 root 1.50
735     Another use for these functions is to manipulate floating point values
736     directly.
737    
738     Silly example: toggle the sign bit of a float.
739    
740     /* On gcc-4.7 on amd64, */
741     /* this results in a single add instruction to toggle the bit, and 4 extra */
742     /* instructions to move the float value to an integer register and back. */
743    
744     x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
745    
746 root 1.58 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
747    
748 root 1.50 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
749    
750 root 1.70 =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
751 root 1.50
752 sf-exg 1.59 The reverse operation of the previous function - takes the bit
753 root 1.71 representation of an IEEE binary16, binary32 or binary64 number (half,
754     single or double precision) and converts it to the native C<float> or
755     C<double> format.
756 root 1.50
757     This function should work even when the native floating point format isn't
758     IEEE compliant, of course at a speed and code size penalty, and of course
759     also within reasonable limits (it tries to convert normals and denormals,
760     and might be lucky for infinities, and with extraordinary luck, also for
761     negative zero).
762    
763     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
764     be able to optimise away this function completely.
765    
766 root 1.71 =item uint16_t ecb_binary32_to_binary16 (uint32_t x)
767    
768     =item uint32_t ecb_binary16_to_binary32 (uint16_t x)
769    
770     Convert a IEEE binary32/single precision to binary16/half format, and vice
771 root 1.72 versa, handling all details (round-to-nearest-even, subnormals, infinity
772     and NaNs) correctly.
773 root 1.71
774     These are functions are available under C<-DECB_NO_LIBM>, since
775     they do not rely on the platform floating point format. The
776     C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
777     usually what you want.
778    
779 root 1.50 =back
780    
781 root 1.1 =head2 ARITHMETIC
782    
783 root 1.3 =over 4
784    
785 root 1.14 =item x = ecb_mod (m, n)
786 root 1.3
787 root 1.25 Returns C<m> modulo C<n>, which is the same as the positive remainder
788     of the division operation between C<m> and C<n>, using floored
789     division. Unlike the C remainder operator C<%>, this function ensures that
790     the return value is always positive and that the two numbers I<m> and
791     I<m' = m + i * n> result in the same value modulo I<n> - in other words,
792     C<ecb_mod> implements the mathematical modulo operation, which is missing
793     in the language.
794 root 1.14
795 sf-exg 1.23 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
796 root 1.14 negatable, that is, both C<m> and C<-m> must be representable in its
797 root 1.30 type (this typically excludes the minimum signed integer value, the same
798 root 1.25 limitation as for C</> and C<%> in C).
799 sf-exg 1.11
800 root 1.24 Current GCC versions compile this into an efficient branchless sequence on
801 root 1.28 almost all CPUs.
802 root 1.24
803     For example, when you want to rotate forward through the members of an
804     array for increasing C<m> (which might be negative), then you should use
805     C<ecb_mod>, as the C<%> operator might give either negative results, or
806     change direction for negative values:
807    
808     for (m = -100; m <= 100; ++m)
809     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
810    
811 sf-exg 1.37 =item x = ecb_div_rd (val, div)
812    
813     =item x = ecb_div_ru (val, div)
814    
815     Returns C<val> divided by C<div> rounded down or up, respectively.
816     C<val> and C<div> must have integer types and C<div> must be strictly
817 sf-exg 1.38 positive. Note that these functions are implemented with macros in C
818     and with function templates in C++.
819 sf-exg 1.37
820 root 1.3 =back
821 root 1.1
822     =head2 UTILITY
823    
824 root 1.3 =over 4
825    
826 sf-exg 1.23 =item element_count = ecb_array_length (name)
827 root 1.3
828 sf-exg 1.13 Returns the number of elements in the array C<name>. For example:
829    
830     int primes[] = { 2, 3, 5, 7, 11 };
831     int sum = 0;
832    
833     for (i = 0; i < ecb_array_length (primes); i++)
834     sum += primes [i];
835    
836 root 1.3 =back
837 root 1.1
838 root 1.43 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
839    
840     These symbols need to be defined before including F<ecb.h> the first time.
841    
842     =over 4
843    
844 root 1.51 =item ECB_NO_THREADS
845 root 1.43
846     If F<ecb.h> is never used from multiple threads, then this symbol can
847     be defined, in which case memory fences (and similar constructs) are
848     completely removed, leading to more efficient code and fewer dependencies.
849    
850     Setting this symbol to a true value implies C<ECB_NO_SMP>.
851    
852     =item ECB_NO_SMP
853    
854     The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
855     multiple threads, but never concurrently (e.g. if the system the program
856     runs on has only a single CPU with a single core, no hyperthreading and so
857     on), then this symbol can be defined, leading to more efficient code and
858     fewer dependencies.
859    
860 root 1.50 =item ECB_NO_LIBM
861    
862     When defined to C<1>, do not export any functions that might introduce
863     dependencies on the math library (usually called F<-lm>) - these are
864     marked with [-UECB_NO_LIBM].
865    
866 sf-exg 1.69 =back
867    
868 root 1.68 =head1 UNDOCUMENTED FUNCTIONALITY
869    
870     F<ecb.h> is full of undocumented functionality as well, some of which is
871     intended to be internal-use only, some of which we forgot to document, and
872     some of which we hide because we are not sure we will keep the interface
873     stable.
874    
875     While you are welcome to rummage around and use whatever you find useful
876     (we can't stop you), keep in mind that we will change undocumented
877     functionality in incompatible ways without thinking twice, while we are
878     considerably more conservative with documented things.
879    
880     =head1 AUTHORS
881    
882     C<libecb> is designed and maintained by:
883    
884     Emanuele Giaquinta <e.giaquinta@glauco.it>
885     Marc Alexander Lehmann <schmorp@schmorp.de>
886    
887 root 1.1