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Revision: 1.50
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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 1.49 For C<ptrdiff_t> and C<size_t> use C<stddef.h>.
72    
73 root 1.43 =head2 LANGUAGE/COMPILER VERSIONS
74    
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     which is typically true for both C and C++ compilers.
85    
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     9899:1999) or any later version.
90    
91     Note that later versions (ECB_C11) remove core features again (for
92     example, variable length arrays).
93 root 1.43
94     =item ECB_C11
95    
96 root 1.47 True if the implementation claims to be compliant to C11 (ISO/IEC
97     9899:2011) or any later version.
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     =item ECB_CPP11
105    
106     True if the implementation claims to be compliant to ISO/IEC 14882:2011
107 sf-exg 1.46 (C++11) or any later version.
108 root 1.43
109     =item ECB_GCC_VERSION(major,minor)
110    
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 1.43 =back
156    
157 root 1.1 =head2 GCC ATTRIBUTES
158    
159 root 1.20 A major part of libecb deals with GCC attributes. These are additional
160 sf-exg 1.26 attributes that you can assign to functions, variables and sometimes even
161 root 1.20 types - much like C<const> or C<volatile> in C.
162    
163     While GCC allows declarations to show up in many surprising places,
164 sf-exg 1.26 but not in many expected places, the safest way is to put attribute
165 root 1.20 declarations before the whole declaration:
166    
167     ecb_const int mysqrt (int a);
168     ecb_unused int i;
169    
170     For variables, it is often nicer to put the attribute after the name, and
171     avoid multiple declarations using commas:
172    
173     int i ecb_unused;
174 root 1.3
175 root 1.1 =over 4
176    
177 root 1.2 =item ecb_attribute ((attrs...))
178 root 1.1
179 root 1.15 A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
180     nothing on other compilers, so the effect is that only GCC sees these.
181    
182     Example: use the C<deprecated> attribute on a function.
183    
184     ecb_attribute((__deprecated__)) void
185     do_not_use_me_anymore (void);
186 root 1.2
187 root 1.3 =item ecb_unused
188    
189     Marks a function or a variable as "unused", which simply suppresses a
190     warning by GCC when it detects it as unused. This is useful when you e.g.
191     declare a variable but do not always use it:
192    
193 root 1.15 {
194     int var ecb_unused;
195 root 1.3
196 root 1.15 #ifdef SOMECONDITION
197     var = ...;
198     return var;
199     #else
200     return 0;
201     #endif
202     }
203 root 1.3
204 root 1.31 =item ecb_inline
205 root 1.29
206     This is not actually an attribute, but you use it like one. It expands
207     either to C<static inline> or to just C<static>, if inline isn't
208     supported. It should be used to declare functions that should be inlined,
209     for code size or speed reasons.
210    
211     Example: inline this function, it surely will reduce codesize.
212    
213 root 1.31 ecb_inline int
214 root 1.29 negmul (int a, int b)
215     {
216     return - (a * b);
217     }
218    
219 root 1.2 =item ecb_noinline
220    
221 root 1.9 Prevent a function from being inlined - it might be optimised away, but
222 root 1.3 not inlined into other functions. This is useful if you know your function
223     is rarely called and large enough for inlining not to be helpful.
224    
225 root 1.2 =item ecb_noreturn
226    
227 root 1.17 Marks a function as "not returning, ever". Some typical functions that
228     don't return are C<exit> or C<abort> (which really works hard to not
229     return), and now you can make your own:
230    
231     ecb_noreturn void
232     my_abort (const char *errline)
233     {
234     puts (errline);
235     abort ();
236     }
237    
238 sf-exg 1.19 In this case, the compiler would probably be smart enough to deduce it on
239     its own, so this is mainly useful for declarations.
240 root 1.17
241 root 1.2 =item ecb_const
242    
243 sf-exg 1.19 Declares that the function only depends on the values of its arguments,
244 root 1.17 much like a mathematical function. It specifically does not read or write
245     any memory any arguments might point to, global variables, or call any
246     non-const functions. It also must not have any side effects.
247    
248     Such a function can be optimised much more aggressively by the compiler -
249     for example, multiple calls with the same arguments can be optimised into
250     a single call, which wouldn't be possible if the compiler would have to
251     expect any side effects.
252    
253     It is best suited for functions in the sense of mathematical functions,
254 sf-exg 1.19 such as a function returning the square root of its input argument.
255 root 1.17
256     Not suited would be a function that calculates the hash of some memory
257     area you pass in, prints some messages or looks at a global variable to
258     decide on rounding.
259    
260     See C<ecb_pure> for a slightly less restrictive class of functions.
261    
262 root 1.2 =item ecb_pure
263    
264 root 1.17 Similar to C<ecb_const>, declares a function that has no side
265     effects. Unlike C<ecb_const>, the function is allowed to examine global
266     variables and any other memory areas (such as the ones passed to it via
267     pointers).
268    
269     While these functions cannot be optimised as aggressively as C<ecb_const>
270     functions, they can still be optimised away in many occasions, and the
271     compiler has more freedom in moving calls to them around.
272    
273     Typical examples for such functions would be C<strlen> or C<memcmp>. A
274     function that calculates the MD5 sum of some input and updates some MD5
275     state passed as argument would I<NOT> be pure, however, as it would modify
276     some memory area that is not the return value.
277    
278 root 1.2 =item ecb_hot
279    
280 root 1.17 This declares a function as "hot" with regards to the cache - the function
281     is used so often, that it is very beneficial to keep it in the cache if
282     possible.
283    
284     The compiler reacts by trying to place hot functions near to each other in
285     memory.
286    
287 sf-exg 1.19 Whether a function is hot or not often depends on the whole program,
288 root 1.17 and less on the function itself. C<ecb_cold> is likely more useful in
289     practise.
290    
291 root 1.2 =item ecb_cold
292    
293 root 1.17 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
294     the cache, or in other words, this function is not called often, or not at
295     speed-critical times, and keeping it in the cache might be a waste of said
296     cache.
297    
298     In addition to placing cold functions together (or at least away from hot
299     functions), this knowledge can be used in other ways, for example, the
300     function will be optimised for size, as opposed to speed, and codepaths
301     leading to calls to those functions can automatically be marked as if
302 root 1.27 C<ecb_expect_false> had been used to reach them.
303 root 1.17
304     Good examples for such functions would be error reporting functions, or
305     functions only called in exceptional or rare cases.
306    
307 root 1.2 =item ecb_artificial
308    
309 root 1.17 Declares the function as "artificial", in this case meaning that this
310     function is not really mean to be a function, but more like an accessor
311     - many methods in C++ classes are mere accessor functions, and having a
312     crash reported in such a method, or single-stepping through them, is not
313     usually so helpful, especially when it's inlined to just a few instructions.
314    
315     Marking them as artificial will instruct the debugger about just this,
316     leading to happier debugging and thus happier lives.
317    
318     Example: in some kind of smart-pointer class, mark the pointer accessor as
319     artificial, so that the whole class acts more like a pointer and less like
320     some C++ abstraction monster.
321    
322     template<typename T>
323     struct my_smart_ptr
324     {
325     T *value;
326    
327     ecb_artificial
328     operator T *()
329     {
330     return value;
331     }
332     };
333    
334 root 1.2 =back
335 root 1.1
336     =head2 OPTIMISATION HINTS
337    
338     =over 4
339    
340 root 1.14 =item bool ecb_is_constant(expr)
341 root 1.1
342 root 1.3 Returns true iff the expression can be deduced to be a compile-time
343     constant, and false otherwise.
344    
345     For example, when you have a C<rndm16> function that returns a 16 bit
346     random number, and you have a function that maps this to a range from
347 root 1.5 0..n-1, then you could use this inline function in a header file:
348 root 1.3
349     ecb_inline uint32_t
350     rndm (uint32_t n)
351     {
352 root 1.6 return (n * (uint32_t)rndm16 ()) >> 16;
353 root 1.3 }
354    
355     However, for powers of two, you could use a normal mask, but that is only
356     worth it if, at compile time, you can detect this case. This is the case
357     when the passed number is a constant and also a power of two (C<n & (n -
358     1) == 0>):
359    
360     ecb_inline uint32_t
361     rndm (uint32_t n)
362     {
363     return is_constant (n) && !(n & (n - 1))
364     ? rndm16 () & (num - 1)
365 root 1.6 : (n * (uint32_t)rndm16 ()) >> 16;
366 root 1.3 }
367    
368 root 1.14 =item bool ecb_expect (expr, value)
369 root 1.1
370 root 1.7 Evaluates C<expr> and returns it. In addition, it tells the compiler that
371     the C<expr> evaluates to C<value> a lot, which can be used for static
372     branch optimisations.
373 root 1.1
374 root 1.27 Usually, you want to use the more intuitive C<ecb_expect_true> and
375     C<ecb_expect_false> functions instead.
376 root 1.1
377 root 1.27 =item bool ecb_expect_true (cond)
378 root 1.1
379 root 1.27 =item bool ecb_expect_false (cond)
380 root 1.1
381 root 1.7 These two functions expect a expression that is true or false and return
382     C<1> or C<0>, respectively, so when used in the condition of an C<if> or
383     other conditional statement, it will not change the program:
384    
385     /* these two do the same thing */
386     if (some_condition) ...;
387 root 1.27 if (ecb_expect_true (some_condition)) ...;
388 root 1.7
389 root 1.27 However, by using C<ecb_expect_true>, you tell the compiler that the
390     condition is likely to be true (and for C<ecb_expect_false>, that it is
391     unlikely to be true).
392 root 1.7
393 root 1.9 For example, when you check for a null pointer and expect this to be a
394 root 1.27 rare, exceptional, case, then use C<ecb_expect_false>:
395 root 1.7
396     void my_free (void *ptr)
397     {
398 root 1.27 if (ecb_expect_false (ptr == 0))
399 root 1.7 return;
400     }
401    
402     Consequent use of these functions to mark away exceptional cases or to
403     tell the compiler what the hot path through a function is can increase
404     performance considerably.
405    
406 root 1.27 You might know these functions under the name C<likely> and C<unlikely>
407     - while these are common aliases, we find that the expect name is easier
408     to understand when quickly skimming code. If you wish, you can use
409     C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
410     C<ecb_expect_false> - these are simply aliases.
411    
412 root 1.7 A very good example is in a function that reserves more space for some
413     memory block (for example, inside an implementation of a string stream) -
414 root 1.9 each time something is added, you have to check for a buffer overrun, but
415 root 1.7 you expect that most checks will turn out to be false:
416    
417     /* make sure we have "size" extra room in our buffer */
418     ecb_inline void
419     reserve (int size)
420     {
421 root 1.27 if (ecb_expect_false (current + size > end))
422 root 1.7 real_reserve_method (size); /* presumably noinline */
423     }
424    
425 root 1.14 =item bool ecb_assume (cond)
426 root 1.7
427     Try to tell the compiler that some condition is true, even if it's not
428     obvious.
429    
430     This can be used to teach the compiler about invariants or other
431     conditions that might improve code generation, but which are impossible to
432     deduce form the code itself.
433    
434 root 1.27 For example, the example reservation function from the C<ecb_expect_false>
435 root 1.7 description could be written thus (only C<ecb_assume> was added):
436    
437     ecb_inline void
438     reserve (int size)
439     {
440 root 1.27 if (ecb_expect_false (current + size > end))
441 root 1.7 real_reserve_method (size); /* presumably noinline */
442    
443     ecb_assume (current + size <= end);
444     }
445    
446     If you then call this function twice, like this:
447    
448     reserve (10);
449     reserve (1);
450    
451     Then the compiler I<might> be able to optimise out the second call
452     completely, as it knows that C<< current + 1 > end >> is false and the
453     call will never be executed.
454    
455     =item bool ecb_unreachable ()
456    
457     This function does nothing itself, except tell the compiler that it will
458 root 1.9 never be executed. Apart from suppressing a warning in some cases, this
459 root 1.7 function can be used to implement C<ecb_assume> or similar functions.
460    
461 root 1.14 =item bool ecb_prefetch (addr, rw, locality)
462 root 1.7
463     Tells the compiler to try to prefetch memory at the given C<addr>ess
464 root 1.10 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
465 root 1.7 C<0> means that there will only be one access later, C<3> means that
466     the data will likely be accessed very often, and values in between mean
467     something... in between. The memory pointed to by the address does not
468     need to be accessible (it could be a null pointer for example), but C<rw>
469     and C<locality> must be compile-time constants.
470    
471     An obvious way to use this is to prefetch some data far away, in a big
472 root 1.9 array you loop over. This prefetches memory some 128 array elements later,
473 root 1.7 in the hope that it will be ready when the CPU arrives at that location.
474    
475     int sum = 0;
476    
477     for (i = 0; i < N; ++i)
478     {
479     sum += arr [i]
480     ecb_prefetch (arr + i + 128, 0, 0);
481     }
482    
483     It's hard to predict how far to prefetch, and most CPUs that can prefetch
484     are often good enough to predict this kind of behaviour themselves. It
485     gets more interesting with linked lists, especially when you do some fair
486     processing on each list element:
487    
488     for (node *n = start; n; n = n->next)
489     {
490     ecb_prefetch (n->next, 0, 0);
491     ... do medium amount of work with *n
492     }
493    
494     After processing the node, (part of) the next node might already be in
495     cache.
496 root 1.1
497 root 1.2 =back
498 root 1.1
499 root 1.36 =head2 BIT FIDDLING / BIT WIZARDRY
500 root 1.1
501 root 1.4 =over 4
502    
503 root 1.3 =item bool ecb_big_endian ()
504    
505     =item bool ecb_little_endian ()
506    
507 sf-exg 1.11 These two functions return true if the byte order is big endian
508     (most-significant byte first) or little endian (least-significant byte
509     first) respectively.
510    
511 root 1.24 On systems that are neither, their return values are unspecified.
512    
513 root 1.3 =item int ecb_ctz32 (uint32_t x)
514    
515 root 1.35 =item int ecb_ctz64 (uint64_t x)
516    
517 sf-exg 1.11 Returns the index of the least significant bit set in C<x> (or
518 root 1.24 equivalently the number of bits set to 0 before the least significant bit
519 root 1.35 set), starting from 0. If C<x> is 0 the result is undefined.
520    
521 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
522    
523 root 1.35 For example:
524 sf-exg 1.11
525 root 1.15 ecb_ctz32 (3) = 0
526     ecb_ctz32 (6) = 1
527 sf-exg 1.11
528 root 1.41 =item bool ecb_is_pot32 (uint32_t x)
529    
530     =item bool ecb_is_pot64 (uint32_t x)
531    
532     Return true iff C<x> is a power of two or C<x == 0>.
533    
534     For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
535    
536 root 1.35 =item int ecb_ld32 (uint32_t x)
537    
538     =item int ecb_ld64 (uint64_t x)
539    
540     Returns the index of the most significant bit set in C<x>, or the number
541     of digits the number requires in binary (so that C<< 2**ld <= x <
542     2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
543     to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
544     example to see how many bits a certain number requires to be encoded.
545    
546     This function is similar to the "count leading zero bits" function, except
547     that that one returns how many zero bits are "in front" of the number (in
548     the given data type), while C<ecb_ld> returns how many bits the number
549     itself requires.
550    
551 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
552    
553 root 1.3 =item int ecb_popcount32 (uint32_t x)
554    
555 root 1.35 =item int ecb_popcount64 (uint64_t x)
556    
557 root 1.36 Returns the number of bits set to 1 in C<x>.
558    
559     For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
560    
561     For example:
562 sf-exg 1.11
563 root 1.15 ecb_popcount32 (7) = 3
564     ecb_popcount32 (255) = 8
565 sf-exg 1.11
566 root 1.39 =item uint8_t ecb_bitrev8 (uint8_t x)
567    
568     =item uint16_t ecb_bitrev16 (uint16_t x)
569    
570     =item uint32_t ecb_bitrev32 (uint32_t x)
571    
572     Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
573     and so on.
574    
575     Example:
576    
577     ecb_bitrev8 (0xa7) = 0xea
578     ecb_bitrev32 (0xffcc4411) = 0x882233ff
579    
580 root 1.8 =item uint32_t ecb_bswap16 (uint32_t x)
581    
582 root 1.3 =item uint32_t ecb_bswap32 (uint32_t x)
583    
584 root 1.34 =item uint64_t ecb_bswap64 (uint64_t x)
585 sf-exg 1.13
586 root 1.34 These functions return the value of the 16-bit (32-bit, 64-bit) value
587     C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
588     C<ecb_bswap32>).
589    
590     =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
591    
592     =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
593 root 1.3
594     =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
595    
596 root 1.34 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
597    
598     =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
599    
600     =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
601    
602     =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
603    
604 root 1.33 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
605    
606 root 1.34 These two families of functions return the value of C<x> after rotating
607     all the bits by C<count> positions to the right (C<ecb_rotr>) or left
608     (C<ecb_rotl>).
609 sf-exg 1.11
610 root 1.20 Current GCC versions understand these functions and usually compile them
611 root 1.34 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
612     x86).
613 root 1.20
614 root 1.3 =back
615 root 1.1
616 root 1.50 =head2 FLOATING POINT FIDDLING
617    
618     =over 4
619    
620     =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
621    
622     =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
623    
624     These functions each take an argument in the native C<float> or C<double>
625     type and return the IEEE 754 bit representation of it.
626    
627     The bit representation is just as IEEE 754 defines it, i.e. the sign bit
628     will be the most significant bit, followed by exponent and mantissa.
629    
630     This function should work even when the native floating point format isn't
631     IEEE compliant, of course at a speed and code size penalty, and of course
632     also within reasonable limits (it tries to convert NaNs, infinities and
633     denormals, but will likely convert negative zero to positive zero).
634    
635     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
636     be able to optimise away this function completely.
637    
638     These functions can be helpful when serialising floats to the network - you
639     can serialise the return value like a normal uint32_t/uint64_t.
640    
641     Another use for these functions is to manipulate floating point values
642     directly.
643    
644     Silly example: toggle the sign bit of a float.
645    
646     /* On gcc-4.7 on amd64, */
647     /* this results in a single add instruction to toggle the bit, and 4 extra */
648     /* instructions to move the float value to an integer register and back. */
649    
650     x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
651    
652     =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
653    
654     =item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
655    
656     The reverse operation of the previos function - takes the bit representation
657     of an IEEE binary32 or binary64 number and converts it to the native C<float>
658     or C<double> format.
659    
660     This function should work even when the native floating point format isn't
661     IEEE compliant, of course at a speed and code size penalty, and of course
662     also within reasonable limits (it tries to convert normals and denormals,
663     and might be lucky for infinities, and with extraordinary luck, also for
664     negative zero).
665    
666     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
667     be able to optimise away this function completely.
668    
669     =back
670    
671 root 1.1 =head2 ARITHMETIC
672    
673 root 1.3 =over 4
674    
675 root 1.14 =item x = ecb_mod (m, n)
676 root 1.3
677 root 1.25 Returns C<m> modulo C<n>, which is the same as the positive remainder
678     of the division operation between C<m> and C<n>, using floored
679     division. Unlike the C remainder operator C<%>, this function ensures that
680     the return value is always positive and that the two numbers I<m> and
681     I<m' = m + i * n> result in the same value modulo I<n> - in other words,
682     C<ecb_mod> implements the mathematical modulo operation, which is missing
683     in the language.
684 root 1.14
685 sf-exg 1.23 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
686 root 1.14 negatable, that is, both C<m> and C<-m> must be representable in its
687 root 1.30 type (this typically excludes the minimum signed integer value, the same
688 root 1.25 limitation as for C</> and C<%> in C).
689 sf-exg 1.11
690 root 1.24 Current GCC versions compile this into an efficient branchless sequence on
691 root 1.28 almost all CPUs.
692 root 1.24
693     For example, when you want to rotate forward through the members of an
694     array for increasing C<m> (which might be negative), then you should use
695     C<ecb_mod>, as the C<%> operator might give either negative results, or
696     change direction for negative values:
697    
698     for (m = -100; m <= 100; ++m)
699     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
700    
701 sf-exg 1.37 =item x = ecb_div_rd (val, div)
702    
703     =item x = ecb_div_ru (val, div)
704    
705     Returns C<val> divided by C<div> rounded down or up, respectively.
706     C<val> and C<div> must have integer types and C<div> must be strictly
707 sf-exg 1.38 positive. Note that these functions are implemented with macros in C
708     and with function templates in C++.
709 sf-exg 1.37
710 root 1.3 =back
711 root 1.1
712     =head2 UTILITY
713    
714 root 1.3 =over 4
715    
716 sf-exg 1.23 =item element_count = ecb_array_length (name)
717 root 1.3
718 sf-exg 1.13 Returns the number of elements in the array C<name>. For example:
719    
720     int primes[] = { 2, 3, 5, 7, 11 };
721     int sum = 0;
722    
723     for (i = 0; i < ecb_array_length (primes); i++)
724     sum += primes [i];
725    
726 root 1.3 =back
727 root 1.1
728 root 1.43 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
729    
730     These symbols need to be defined before including F<ecb.h> the first time.
731    
732     =over 4
733    
734     =item ECB_NO_THRADS
735    
736     If F<ecb.h> is never used from multiple threads, then this symbol can
737     be defined, in which case memory fences (and similar constructs) are
738     completely removed, leading to more efficient code and fewer dependencies.
739    
740     Setting this symbol to a true value implies C<ECB_NO_SMP>.
741    
742     =item ECB_NO_SMP
743    
744     The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
745     multiple threads, but never concurrently (e.g. if the system the program
746     runs on has only a single CPU with a single core, no hyperthreading and so
747     on), then this symbol can be defined, leading to more efficient code and
748     fewer dependencies.
749    
750 root 1.50 =item ECB_NO_LIBM
751    
752     When defined to C<1>, do not export any functions that might introduce
753     dependencies on the math library (usually called F<-lm>) - these are
754     marked with [-UECB_NO_LIBM].
755    
756 root 1.43 =back
757    
758 root 1.1