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Revision: 1.61
Committed: Thu Feb 12 12:37:33 2015 UTC (9 years, 4 months ago) by sf-exg
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# User Rev Content
1 root 1.14 =head1 LIBECB - e-C-Builtins
2 root 1.3
3 root 1.14 =head2 ABOUT LIBECB
4    
5     Libecb is currently a simple header file that doesn't require any
6     configuration to use or include in your project.
7    
8 sf-exg 1.16 It's part of the e-suite of libraries, other members of which include
9 root 1.14 libev and libeio.
10    
11     Its homepage can be found here:
12    
13     http://software.schmorp.de/pkg/libecb
14    
15     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 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     =item ECB_C11
95    
96 root 1.47 True if the implementation claims to be compliant to C11 (ISO/IEC
97 root 1.55 9899:2011) 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     =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 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 sf-exg 1.60 =head2 ATTRIBUTES
169 root 1.1
170 sf-exg 1.60 A major part of libecb deals with additional attributes that can be
171     assigned to functions, variables and sometimes even types - much like
172     C<const> or C<volatile> in C. They are implemented using either GCC
173     attributes or other compiler/language specific features. Attributes
174     declarations must be put before the whole declaration:
175 root 1.20
176     ecb_const int mysqrt (int a);
177     ecb_unused int i;
178    
179 root 1.1 =over 4
180    
181 root 1.3 =item ecb_unused
182    
183     Marks a function or a variable as "unused", which simply suppresses a
184     warning by GCC when it detects it as unused. This is useful when you e.g.
185     declare a variable but do not always use it:
186    
187 root 1.15 {
188 sf-exg 1.61 ecb_unused int var;
189 root 1.3
190 root 1.15 #ifdef SOMECONDITION
191     var = ...;
192     return var;
193     #else
194     return 0;
195     #endif
196     }
197 root 1.3
198 root 1.56 =item ecb_deprecated
199    
200     Similar to C<ecb_unused>, but marks a function, variable or type as
201     deprecated. This makes some compilers warn when the type is used.
202    
203 root 1.31 =item ecb_inline
204 root 1.29
205 sf-exg 1.60 Expands either to C<static inline> or to just C<static>, if inline
206     isn't supported. It should be used to declare functions that should be
207     inlined, for code size or speed reasons.
208 root 1.29
209     Example: inline this function, it surely will reduce codesize.
210    
211 root 1.31 ecb_inline int
212 root 1.29 negmul (int a, int b)
213     {
214     return - (a * b);
215     }
216    
217 root 1.2 =item ecb_noinline
218    
219 root 1.9 Prevent a function from being inlined - it might be optimised away, but
220 root 1.3 not inlined into other functions. This is useful if you know your function
221     is rarely called and large enough for inlining not to be helpful.
222    
223 root 1.2 =item ecb_noreturn
224    
225 root 1.17 Marks a function as "not returning, ever". Some typical functions that
226     don't return are C<exit> or C<abort> (which really works hard to not
227     return), and now you can make your own:
228    
229     ecb_noreturn void
230     my_abort (const char *errline)
231     {
232     puts (errline);
233     abort ();
234     }
235    
236 sf-exg 1.19 In this case, the compiler would probably be smart enough to deduce it on
237     its own, so this is mainly useful for declarations.
238 root 1.17
239 root 1.53 =item ecb_restrict
240    
241     Expands to the C<restrict> keyword or equivalent on compilers that support
242     them, and to nothing on others. Must be specified on a pointer type or
243     an array index to indicate that the memory doesn't alias with any other
244     restricted pointer in the same scope.
245    
246     Example: multiply a vector, and allow the compiler to parallelise the
247     loop, because it knows it doesn't overwrite input values.
248    
249     void
250 sf-exg 1.61 multiply (ecb_restrict float *src,
251     ecb_restrict float *dst,
252 root 1.53 int len, float factor)
253     {
254     int i;
255    
256     for (i = 0; i < len; ++i)
257     dst [i] = src [i] * factor;
258     }
259    
260 root 1.2 =item ecb_const
261    
262 sf-exg 1.19 Declares that the function only depends on the values of its arguments,
263 root 1.17 much like a mathematical function. It specifically does not read or write
264     any memory any arguments might point to, global variables, or call any
265     non-const functions. It also must not have any side effects.
266    
267     Such a function can be optimised much more aggressively by the compiler -
268     for example, multiple calls with the same arguments can be optimised into
269     a single call, which wouldn't be possible if the compiler would have to
270     expect any side effects.
271    
272     It is best suited for functions in the sense of mathematical functions,
273 sf-exg 1.19 such as a function returning the square root of its input argument.
274 root 1.17
275     Not suited would be a function that calculates the hash of some memory
276     area you pass in, prints some messages or looks at a global variable to
277     decide on rounding.
278    
279     See C<ecb_pure> for a slightly less restrictive class of functions.
280    
281 root 1.2 =item ecb_pure
282    
283 root 1.17 Similar to C<ecb_const>, declares a function that has no side
284     effects. Unlike C<ecb_const>, the function is allowed to examine global
285     variables and any other memory areas (such as the ones passed to it via
286     pointers).
287    
288     While these functions cannot be optimised as aggressively as C<ecb_const>
289     functions, they can still be optimised away in many occasions, and the
290     compiler has more freedom in moving calls to them around.
291    
292     Typical examples for such functions would be C<strlen> or C<memcmp>. A
293     function that calculates the MD5 sum of some input and updates some MD5
294     state passed as argument would I<NOT> be pure, however, as it would modify
295     some memory area that is not the return value.
296    
297 root 1.2 =item ecb_hot
298    
299 root 1.17 This declares a function as "hot" with regards to the cache - the function
300     is used so often, that it is very beneficial to keep it in the cache if
301     possible.
302    
303     The compiler reacts by trying to place hot functions near to each other in
304     memory.
305    
306 sf-exg 1.19 Whether a function is hot or not often depends on the whole program,
307 root 1.17 and less on the function itself. C<ecb_cold> is likely more useful in
308     practise.
309    
310 root 1.2 =item ecb_cold
311    
312 root 1.17 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
313     the cache, or in other words, this function is not called often, or not at
314     speed-critical times, and keeping it in the cache might be a waste of said
315     cache.
316    
317     In addition to placing cold functions together (or at least away from hot
318     functions), this knowledge can be used in other ways, for example, the
319     function will be optimised for size, as opposed to speed, and codepaths
320     leading to calls to those functions can automatically be marked as if
321 root 1.27 C<ecb_expect_false> had been used to reach them.
322 root 1.17
323     Good examples for such functions would be error reporting functions, or
324     functions only called in exceptional or rare cases.
325    
326 root 1.2 =item ecb_artificial
327    
328 root 1.17 Declares the function as "artificial", in this case meaning that this
329 root 1.52 function is not really meant to be a function, but more like an accessor
330 root 1.17 - many methods in C++ classes are mere accessor functions, and having a
331     crash reported in such a method, or single-stepping through them, is not
332     usually so helpful, especially when it's inlined to just a few instructions.
333    
334     Marking them as artificial will instruct the debugger about just this,
335     leading to happier debugging and thus happier lives.
336    
337     Example: in some kind of smart-pointer class, mark the pointer accessor as
338     artificial, so that the whole class acts more like a pointer and less like
339     some C++ abstraction monster.
340    
341     template<typename T>
342     struct my_smart_ptr
343     {
344     T *value;
345    
346     ecb_artificial
347     operator T *()
348     {
349     return value;
350     }
351     };
352    
353 root 1.2 =back
354 root 1.1
355     =head2 OPTIMISATION HINTS
356    
357     =over 4
358    
359 root 1.58 =item bool ecb_is_constant (expr)
360 root 1.1
361 root 1.3 Returns true iff the expression can be deduced to be a compile-time
362     constant, and false otherwise.
363    
364     For example, when you have a C<rndm16> function that returns a 16 bit
365     random number, and you have a function that maps this to a range from
366 root 1.5 0..n-1, then you could use this inline function in a header file:
367 root 1.3
368     ecb_inline uint32_t
369     rndm (uint32_t n)
370     {
371 root 1.6 return (n * (uint32_t)rndm16 ()) >> 16;
372 root 1.3 }
373    
374     However, for powers of two, you could use a normal mask, but that is only
375     worth it if, at compile time, you can detect this case. This is the case
376     when the passed number is a constant and also a power of two (C<n & (n -
377     1) == 0>):
378    
379     ecb_inline uint32_t
380     rndm (uint32_t n)
381     {
382     return is_constant (n) && !(n & (n - 1))
383     ? rndm16 () & (num - 1)
384 root 1.6 : (n * (uint32_t)rndm16 ()) >> 16;
385 root 1.3 }
386    
387 root 1.14 =item bool ecb_expect (expr, value)
388 root 1.1
389 root 1.7 Evaluates C<expr> and returns it. In addition, it tells the compiler that
390     the C<expr> evaluates to C<value> a lot, which can be used for static
391     branch optimisations.
392 root 1.1
393 root 1.27 Usually, you want to use the more intuitive C<ecb_expect_true> and
394     C<ecb_expect_false> functions instead.
395 root 1.1
396 root 1.27 =item bool ecb_expect_true (cond)
397 root 1.1
398 root 1.27 =item bool ecb_expect_false (cond)
399 root 1.1
400 root 1.7 These two functions expect a expression that is true or false and return
401     C<1> or C<0>, respectively, so when used in the condition of an C<if> or
402     other conditional statement, it will not change the program:
403    
404     /* these two do the same thing */
405     if (some_condition) ...;
406 root 1.27 if (ecb_expect_true (some_condition)) ...;
407 root 1.7
408 root 1.27 However, by using C<ecb_expect_true>, you tell the compiler that the
409     condition is likely to be true (and for C<ecb_expect_false>, that it is
410     unlikely to be true).
411 root 1.7
412 root 1.9 For example, when you check for a null pointer and expect this to be a
413 root 1.27 rare, exceptional, case, then use C<ecb_expect_false>:
414 root 1.7
415     void my_free (void *ptr)
416     {
417 root 1.27 if (ecb_expect_false (ptr == 0))
418 root 1.7 return;
419     }
420    
421     Consequent use of these functions to mark away exceptional cases or to
422     tell the compiler what the hot path through a function is can increase
423     performance considerably.
424    
425 root 1.27 You might know these functions under the name C<likely> and C<unlikely>
426     - while these are common aliases, we find that the expect name is easier
427     to understand when quickly skimming code. If you wish, you can use
428     C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
429     C<ecb_expect_false> - these are simply aliases.
430    
431 root 1.7 A very good example is in a function that reserves more space for some
432     memory block (for example, inside an implementation of a string stream) -
433 root 1.9 each time something is added, you have to check for a buffer overrun, but
434 root 1.7 you expect that most checks will turn out to be false:
435    
436     /* make sure we have "size" extra room in our buffer */
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    
444 root 1.14 =item bool ecb_assume (cond)
445 root 1.7
446     Try to tell the compiler that some condition is true, even if it's not
447     obvious.
448    
449     This can be used to teach the compiler about invariants or other
450     conditions that might improve code generation, but which are impossible to
451     deduce form the code itself.
452    
453 root 1.27 For example, the example reservation function from the C<ecb_expect_false>
454 root 1.7 description could be written thus (only C<ecb_assume> was added):
455    
456     ecb_inline void
457     reserve (int size)
458     {
459 root 1.27 if (ecb_expect_false (current + size > end))
460 root 1.7 real_reserve_method (size); /* presumably noinline */
461    
462     ecb_assume (current + size <= end);
463     }
464    
465     If you then call this function twice, like this:
466    
467     reserve (10);
468     reserve (1);
469    
470     Then the compiler I<might> be able to optimise out the second call
471     completely, as it knows that C<< current + 1 > end >> is false and the
472     call will never be executed.
473    
474     =item bool ecb_unreachable ()
475    
476     This function does nothing itself, except tell the compiler that it will
477 root 1.9 never be executed. Apart from suppressing a warning in some cases, this
478 root 1.7 function can be used to implement C<ecb_assume> or similar functions.
479    
480 root 1.14 =item bool ecb_prefetch (addr, rw, locality)
481 root 1.7
482     Tells the compiler to try to prefetch memory at the given C<addr>ess
483 root 1.10 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
484 root 1.7 C<0> means that there will only be one access later, C<3> means that
485     the data will likely be accessed very often, and values in between mean
486     something... in between. The memory pointed to by the address does not
487     need to be accessible (it could be a null pointer for example), but C<rw>
488     and C<locality> must be compile-time constants.
489    
490     An obvious way to use this is to prefetch some data far away, in a big
491 root 1.9 array you loop over. This prefetches memory some 128 array elements later,
492 root 1.7 in the hope that it will be ready when the CPU arrives at that location.
493    
494     int sum = 0;
495    
496     for (i = 0; i < N; ++i)
497     {
498     sum += arr [i]
499     ecb_prefetch (arr + i + 128, 0, 0);
500     }
501    
502     It's hard to predict how far to prefetch, and most CPUs that can prefetch
503     are often good enough to predict this kind of behaviour themselves. It
504     gets more interesting with linked lists, especially when you do some fair
505     processing on each list element:
506    
507     for (node *n = start; n; n = n->next)
508     {
509     ecb_prefetch (n->next, 0, 0);
510     ... do medium amount of work with *n
511     }
512    
513     After processing the node, (part of) the next node might already be in
514     cache.
515 root 1.1
516 root 1.2 =back
517 root 1.1
518 root 1.36 =head2 BIT FIDDLING / BIT WIZARDRY
519 root 1.1
520 root 1.4 =over 4
521    
522 root 1.3 =item bool ecb_big_endian ()
523    
524     =item bool ecb_little_endian ()
525    
526 sf-exg 1.11 These two functions return true if the byte order is big endian
527     (most-significant byte first) or little endian (least-significant byte
528     first) respectively.
529    
530 root 1.24 On systems that are neither, their return values are unspecified.
531    
532 root 1.3 =item int ecb_ctz32 (uint32_t x)
533    
534 root 1.35 =item int ecb_ctz64 (uint64_t x)
535    
536 sf-exg 1.11 Returns the index of the least significant bit set in C<x> (or
537 root 1.24 equivalently the number of bits set to 0 before the least significant bit
538 root 1.35 set), starting from 0. If C<x> is 0 the result is undefined.
539    
540 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
541    
542 root 1.35 For example:
543 sf-exg 1.11
544 root 1.15 ecb_ctz32 (3) = 0
545     ecb_ctz32 (6) = 1
546 sf-exg 1.11
547 root 1.41 =item bool ecb_is_pot32 (uint32_t x)
548    
549     =item bool ecb_is_pot64 (uint32_t x)
550    
551     Return true iff C<x> is a power of two or C<x == 0>.
552    
553     For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
554    
555 root 1.35 =item int ecb_ld32 (uint32_t x)
556    
557     =item int ecb_ld64 (uint64_t x)
558    
559     Returns the index of the most significant bit set in C<x>, or the number
560     of digits the number requires in binary (so that C<< 2**ld <= x <
561     2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
562     to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
563     example to see how many bits a certain number requires to be encoded.
564    
565     This function is similar to the "count leading zero bits" function, except
566     that that one returns how many zero bits are "in front" of the number (in
567     the given data type), while C<ecb_ld> returns how many bits the number
568     itself requires.
569    
570 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
571    
572 root 1.3 =item int ecb_popcount32 (uint32_t x)
573    
574 root 1.35 =item int ecb_popcount64 (uint64_t x)
575    
576 root 1.36 Returns the number of bits set to 1 in C<x>.
577    
578     For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
579    
580     For example:
581 sf-exg 1.11
582 root 1.15 ecb_popcount32 (7) = 3
583     ecb_popcount32 (255) = 8
584 sf-exg 1.11
585 root 1.39 =item uint8_t ecb_bitrev8 (uint8_t x)
586    
587     =item uint16_t ecb_bitrev16 (uint16_t x)
588    
589     =item uint32_t ecb_bitrev32 (uint32_t x)
590    
591     Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
592     and so on.
593    
594     Example:
595    
596     ecb_bitrev8 (0xa7) = 0xea
597     ecb_bitrev32 (0xffcc4411) = 0x882233ff
598    
599 root 1.8 =item uint32_t ecb_bswap16 (uint32_t x)
600    
601 root 1.3 =item uint32_t ecb_bswap32 (uint32_t x)
602    
603 root 1.34 =item uint64_t ecb_bswap64 (uint64_t x)
604 sf-exg 1.13
605 root 1.34 These functions return the value of the 16-bit (32-bit, 64-bit) value
606     C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
607     C<ecb_bswap32>).
608    
609     =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
610    
611     =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
612 root 1.3
613     =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
614    
615 root 1.34 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
616    
617     =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
618    
619     =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
620    
621     =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
622    
623 root 1.33 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
624    
625 root 1.34 These two families of functions return the value of C<x> after rotating
626     all the bits by C<count> positions to the right (C<ecb_rotr>) or left
627     (C<ecb_rotl>).
628 sf-exg 1.11
629 root 1.20 Current GCC versions understand these functions and usually compile them
630 root 1.34 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
631     x86).
632 root 1.20
633 root 1.3 =back
634 root 1.1
635 root 1.50 =head2 FLOATING POINT FIDDLING
636    
637     =over 4
638    
639     =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
640    
641     =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
642    
643     These functions each take an argument in the native C<float> or C<double>
644     type and return the IEEE 754 bit representation of it.
645    
646     The bit representation is just as IEEE 754 defines it, i.e. the sign bit
647     will be the most significant bit, followed by exponent and mantissa.
648    
649     This function should work even when the native floating point format isn't
650     IEEE compliant, of course at a speed and code size penalty, and of course
651     also within reasonable limits (it tries to convert NaNs, infinities and
652     denormals, but will likely convert negative zero to positive zero).
653    
654     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
655     be able to optimise away this function completely.
656    
657     These functions can be helpful when serialising floats to the network - you
658     can serialise the return value like a normal uint32_t/uint64_t.
659    
660     Another use for these functions is to manipulate floating point values
661     directly.
662    
663     Silly example: toggle the sign bit of a float.
664    
665     /* On gcc-4.7 on amd64, */
666     /* this results in a single add instruction to toggle the bit, and 4 extra */
667     /* instructions to move the float value to an integer register and back. */
668    
669     x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
670    
671 root 1.58 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
672    
673 root 1.50 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
674    
675     =item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
676    
677 sf-exg 1.59 The reverse operation of the previous function - takes the bit
678 root 1.58 representation of an IEEE binary16, binary32 or binary64 number and
679     converts it to the native C<float> or C<double> format.
680 root 1.50
681     This function should work even when the native floating point format isn't
682     IEEE compliant, of course at a speed and code size penalty, and of course
683     also within reasonable limits (it tries to convert normals and denormals,
684     and might be lucky for infinities, and with extraordinary luck, also for
685     negative zero).
686    
687     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
688     be able to optimise away this function completely.
689    
690     =back
691    
692 root 1.1 =head2 ARITHMETIC
693    
694 root 1.3 =over 4
695    
696 root 1.14 =item x = ecb_mod (m, n)
697 root 1.3
698 root 1.25 Returns C<m> modulo C<n>, which is the same as the positive remainder
699     of the division operation between C<m> and C<n>, using floored
700     division. Unlike the C remainder operator C<%>, this function ensures that
701     the return value is always positive and that the two numbers I<m> and
702     I<m' = m + i * n> result in the same value modulo I<n> - in other words,
703     C<ecb_mod> implements the mathematical modulo operation, which is missing
704     in the language.
705 root 1.14
706 sf-exg 1.23 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
707 root 1.14 negatable, that is, both C<m> and C<-m> must be representable in its
708 root 1.30 type (this typically excludes the minimum signed integer value, the same
709 root 1.25 limitation as for C</> and C<%> in C).
710 sf-exg 1.11
711 root 1.24 Current GCC versions compile this into an efficient branchless sequence on
712 root 1.28 almost all CPUs.
713 root 1.24
714     For example, when you want to rotate forward through the members of an
715     array for increasing C<m> (which might be negative), then you should use
716     C<ecb_mod>, as the C<%> operator might give either negative results, or
717     change direction for negative values:
718    
719     for (m = -100; m <= 100; ++m)
720     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
721    
722 sf-exg 1.37 =item x = ecb_div_rd (val, div)
723    
724     =item x = ecb_div_ru (val, div)
725    
726     Returns C<val> divided by C<div> rounded down or up, respectively.
727     C<val> and C<div> must have integer types and C<div> must be strictly
728 sf-exg 1.38 positive. Note that these functions are implemented with macros in C
729     and with function templates in C++.
730 sf-exg 1.37
731 root 1.3 =back
732 root 1.1
733     =head2 UTILITY
734    
735 root 1.3 =over 4
736    
737 sf-exg 1.23 =item element_count = ecb_array_length (name)
738 root 1.3
739 sf-exg 1.13 Returns the number of elements in the array C<name>. For example:
740    
741     int primes[] = { 2, 3, 5, 7, 11 };
742     int sum = 0;
743    
744     for (i = 0; i < ecb_array_length (primes); i++)
745     sum += primes [i];
746    
747 root 1.3 =back
748 root 1.1
749 root 1.43 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
750    
751     These symbols need to be defined before including F<ecb.h> the first time.
752    
753     =over 4
754    
755 root 1.51 =item ECB_NO_THREADS
756 root 1.43
757     If F<ecb.h> is never used from multiple threads, then this symbol can
758     be defined, in which case memory fences (and similar constructs) are
759     completely removed, leading to more efficient code and fewer dependencies.
760    
761     Setting this symbol to a true value implies C<ECB_NO_SMP>.
762    
763     =item ECB_NO_SMP
764    
765     The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
766     multiple threads, but never concurrently (e.g. if the system the program
767     runs on has only a single CPU with a single core, no hyperthreading and so
768     on), then this symbol can be defined, leading to more efficient code and
769     fewer dependencies.
770    
771 root 1.50 =item ECB_NO_LIBM
772    
773     When defined to C<1>, do not export any functions that might introduce
774     dependencies on the math library (usually called F<-lm>) - these are
775     marked with [-UECB_NO_LIBM].
776    
777 root 1.43 =back
778    
779 root 1.1