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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 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     =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 1.54 =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 1.1 =head2 GCC ATTRIBUTES
169    
170 root 1.20 A major part of libecb deals with GCC attributes. These are additional
171 sf-exg 1.26 attributes that you can assign to functions, variables and sometimes even
172 root 1.20 types - much like C<const> or C<volatile> in C.
173    
174     While GCC allows declarations to show up in many surprising places,
175 sf-exg 1.26 but not in many expected places, the safest way is to put attribute
176 root 1.20 declarations before the whole declaration:
177    
178     ecb_const int mysqrt (int a);
179     ecb_unused int i;
180    
181     For variables, it is often nicer to put the attribute after the name, and
182     avoid multiple declarations using commas:
183    
184     int i ecb_unused;
185 root 1.3
186 root 1.1 =over 4
187    
188 root 1.2 =item ecb_attribute ((attrs...))
189 root 1.1
190 root 1.15 A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
191     nothing on other compilers, so the effect is that only GCC sees these.
192    
193     Example: use the C<deprecated> attribute on a function.
194    
195     ecb_attribute((__deprecated__)) void
196     do_not_use_me_anymore (void);
197 root 1.2
198 root 1.3 =item ecb_unused
199    
200     Marks a function or a variable as "unused", which simply suppresses a
201     warning by GCC when it detects it as unused. This is useful when you e.g.
202     declare a variable but do not always use it:
203    
204 root 1.15 {
205     int var ecb_unused;
206 root 1.3
207 root 1.15 #ifdef SOMECONDITION
208     var = ...;
209     return var;
210     #else
211     return 0;
212     #endif
213     }
214 root 1.3
215 root 1.56 =item ecb_deprecated
216    
217     Similar to C<ecb_unused>, but marks a function, variable or type as
218     deprecated. This makes some compilers warn when the type is used.
219    
220 root 1.31 =item ecb_inline
221 root 1.29
222     This is not actually an attribute, but you use it like one. It expands
223     either to C<static inline> or to just C<static>, if inline isn't
224     supported. It should be used to declare functions that should be inlined,
225     for code size or speed reasons.
226    
227     Example: inline this function, it surely will reduce codesize.
228    
229 root 1.31 ecb_inline int
230 root 1.29 negmul (int a, int b)
231     {
232     return - (a * b);
233     }
234    
235 root 1.2 =item ecb_noinline
236    
237 root 1.9 Prevent a function from being inlined - it might be optimised away, but
238 root 1.3 not inlined into other functions. This is useful if you know your function
239     is rarely called and large enough for inlining not to be helpful.
240    
241 root 1.2 =item ecb_noreturn
242    
243 root 1.17 Marks a function as "not returning, ever". Some typical functions that
244     don't return are C<exit> or C<abort> (which really works hard to not
245     return), and now you can make your own:
246    
247     ecb_noreturn void
248     my_abort (const char *errline)
249     {
250     puts (errline);
251     abort ();
252     }
253    
254 sf-exg 1.19 In this case, the compiler would probably be smart enough to deduce it on
255     its own, so this is mainly useful for declarations.
256 root 1.17
257 root 1.53 =item ecb_restrict
258    
259     Expands to the C<restrict> keyword or equivalent on compilers that support
260     them, and to nothing on others. Must be specified on a pointer type or
261     an array index to indicate that the memory doesn't alias with any other
262     restricted pointer in the same scope.
263    
264     Example: multiply a vector, and allow the compiler to parallelise the
265     loop, because it knows it doesn't overwrite input values.
266    
267     void
268     multiply (float *ecb_restrict src,
269     float *ecb_restrict dst,
270     int len, float factor)
271     {
272     int i;
273    
274     for (i = 0; i < len; ++i)
275     dst [i] = src [i] * factor;
276     }
277    
278 root 1.2 =item ecb_const
279    
280 sf-exg 1.19 Declares that the function only depends on the values of its arguments,
281 root 1.17 much like a mathematical function. It specifically does not read or write
282     any memory any arguments might point to, global variables, or call any
283     non-const functions. It also must not have any side effects.
284    
285     Such a function can be optimised much more aggressively by the compiler -
286     for example, multiple calls with the same arguments can be optimised into
287     a single call, which wouldn't be possible if the compiler would have to
288     expect any side effects.
289    
290     It is best suited for functions in the sense of mathematical functions,
291 sf-exg 1.19 such as a function returning the square root of its input argument.
292 root 1.17
293     Not suited would be a function that calculates the hash of some memory
294     area you pass in, prints some messages or looks at a global variable to
295     decide on rounding.
296    
297     See C<ecb_pure> for a slightly less restrictive class of functions.
298    
299 root 1.2 =item ecb_pure
300    
301 root 1.17 Similar to C<ecb_const>, declares a function that has no side
302     effects. Unlike C<ecb_const>, the function is allowed to examine global
303     variables and any other memory areas (such as the ones passed to it via
304     pointers).
305    
306     While these functions cannot be optimised as aggressively as C<ecb_const>
307     functions, they can still be optimised away in many occasions, and the
308     compiler has more freedom in moving calls to them around.
309    
310     Typical examples for such functions would be C<strlen> or C<memcmp>. A
311     function that calculates the MD5 sum of some input and updates some MD5
312     state passed as argument would I<NOT> be pure, however, as it would modify
313     some memory area that is not the return value.
314    
315 root 1.2 =item ecb_hot
316    
317 root 1.17 This declares a function as "hot" with regards to the cache - the function
318     is used so often, that it is very beneficial to keep it in the cache if
319     possible.
320    
321     The compiler reacts by trying to place hot functions near to each other in
322     memory.
323    
324 sf-exg 1.19 Whether a function is hot or not often depends on the whole program,
325 root 1.17 and less on the function itself. C<ecb_cold> is likely more useful in
326     practise.
327    
328 root 1.2 =item ecb_cold
329    
330 root 1.17 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
331     the cache, or in other words, this function is not called often, or not at
332     speed-critical times, and keeping it in the cache might be a waste of said
333     cache.
334    
335     In addition to placing cold functions together (or at least away from hot
336     functions), this knowledge can be used in other ways, for example, the
337     function will be optimised for size, as opposed to speed, and codepaths
338     leading to calls to those functions can automatically be marked as if
339 root 1.27 C<ecb_expect_false> had been used to reach them.
340 root 1.17
341     Good examples for such functions would be error reporting functions, or
342     functions only called in exceptional or rare cases.
343    
344 root 1.2 =item ecb_artificial
345    
346 root 1.17 Declares the function as "artificial", in this case meaning that this
347 root 1.52 function is not really meant to be a function, but more like an accessor
348 root 1.17 - many methods in C++ classes are mere accessor functions, and having a
349     crash reported in such a method, or single-stepping through them, is not
350     usually so helpful, especially when it's inlined to just a few instructions.
351    
352     Marking them as artificial will instruct the debugger about just this,
353     leading to happier debugging and thus happier lives.
354    
355     Example: in some kind of smart-pointer class, mark the pointer accessor as
356     artificial, so that the whole class acts more like a pointer and less like
357     some C++ abstraction monster.
358    
359     template<typename T>
360     struct my_smart_ptr
361     {
362     T *value;
363    
364     ecb_artificial
365     operator T *()
366     {
367     return value;
368     }
369     };
370    
371 root 1.2 =back
372 root 1.1
373     =head2 OPTIMISATION HINTS
374    
375     =over 4
376    
377 root 1.14 =item bool ecb_is_constant(expr)
378 root 1.1
379 root 1.3 Returns true iff the expression can be deduced to be a compile-time
380     constant, and false otherwise.
381    
382     For example, when you have a C<rndm16> function that returns a 16 bit
383     random number, and you have a function that maps this to a range from
384 root 1.5 0..n-1, then you could use this inline function in a header file:
385 root 1.3
386     ecb_inline uint32_t
387     rndm (uint32_t n)
388     {
389 root 1.6 return (n * (uint32_t)rndm16 ()) >> 16;
390 root 1.3 }
391    
392     However, for powers of two, you could use a normal mask, but that is only
393     worth it if, at compile time, you can detect this case. This is the case
394     when the passed number is a constant and also a power of two (C<n & (n -
395     1) == 0>):
396    
397     ecb_inline uint32_t
398     rndm (uint32_t n)
399     {
400     return is_constant (n) && !(n & (n - 1))
401     ? rndm16 () & (num - 1)
402 root 1.6 : (n * (uint32_t)rndm16 ()) >> 16;
403 root 1.3 }
404    
405 root 1.14 =item bool ecb_expect (expr, value)
406 root 1.1
407 root 1.7 Evaluates C<expr> and returns it. In addition, it tells the compiler that
408     the C<expr> evaluates to C<value> a lot, which can be used for static
409     branch optimisations.
410 root 1.1
411 root 1.27 Usually, you want to use the more intuitive C<ecb_expect_true> and
412     C<ecb_expect_false> functions instead.
413 root 1.1
414 root 1.27 =item bool ecb_expect_true (cond)
415 root 1.1
416 root 1.27 =item bool ecb_expect_false (cond)
417 root 1.1
418 root 1.7 These two functions expect a expression that is true or false and return
419     C<1> or C<0>, respectively, so when used in the condition of an C<if> or
420     other conditional statement, it will not change the program:
421    
422     /* these two do the same thing */
423     if (some_condition) ...;
424 root 1.27 if (ecb_expect_true (some_condition)) ...;
425 root 1.7
426 root 1.27 However, by using C<ecb_expect_true>, you tell the compiler that the
427     condition is likely to be true (and for C<ecb_expect_false>, that it is
428     unlikely to be true).
429 root 1.7
430 root 1.9 For example, when you check for a null pointer and expect this to be a
431 root 1.27 rare, exceptional, case, then use C<ecb_expect_false>:
432 root 1.7
433     void my_free (void *ptr)
434     {
435 root 1.27 if (ecb_expect_false (ptr == 0))
436 root 1.7 return;
437     }
438    
439     Consequent use of these functions to mark away exceptional cases or to
440     tell the compiler what the hot path through a function is can increase
441     performance considerably.
442    
443 root 1.27 You might know these functions under the name C<likely> and C<unlikely>
444     - while these are common aliases, we find that the expect name is easier
445     to understand when quickly skimming code. If you wish, you can use
446     C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
447     C<ecb_expect_false> - these are simply aliases.
448    
449 root 1.7 A very good example is in a function that reserves more space for some
450     memory block (for example, inside an implementation of a string stream) -
451 root 1.9 each time something is added, you have to check for a buffer overrun, but
452 root 1.7 you expect that most checks will turn out to be false:
453    
454     /* make sure we have "size" extra room in our buffer */
455     ecb_inline void
456     reserve (int size)
457     {
458 root 1.27 if (ecb_expect_false (current + size > end))
459 root 1.7 real_reserve_method (size); /* presumably noinline */
460     }
461    
462 root 1.14 =item bool ecb_assume (cond)
463 root 1.7
464     Try to tell the compiler that some condition is true, even if it's not
465     obvious.
466    
467     This can be used to teach the compiler about invariants or other
468     conditions that might improve code generation, but which are impossible to
469     deduce form the code itself.
470    
471 root 1.27 For example, the example reservation function from the C<ecb_expect_false>
472 root 1.7 description could be written thus (only C<ecb_assume> was added):
473    
474     ecb_inline void
475     reserve (int size)
476     {
477 root 1.27 if (ecb_expect_false (current + size > end))
478 root 1.7 real_reserve_method (size); /* presumably noinline */
479    
480     ecb_assume (current + size <= end);
481     }
482    
483     If you then call this function twice, like this:
484    
485     reserve (10);
486     reserve (1);
487    
488     Then the compiler I<might> be able to optimise out the second call
489     completely, as it knows that C<< current + 1 > end >> is false and the
490     call will never be executed.
491    
492     =item bool ecb_unreachable ()
493    
494     This function does nothing itself, except tell the compiler that it will
495 root 1.9 never be executed. Apart from suppressing a warning in some cases, this
496 root 1.7 function can be used to implement C<ecb_assume> or similar functions.
497    
498 root 1.14 =item bool ecb_prefetch (addr, rw, locality)
499 root 1.7
500     Tells the compiler to try to prefetch memory at the given C<addr>ess
501 root 1.10 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
502 root 1.7 C<0> means that there will only be one access later, C<3> means that
503     the data will likely be accessed very often, and values in between mean
504     something... in between. The memory pointed to by the address does not
505     need to be accessible (it could be a null pointer for example), but C<rw>
506     and C<locality> must be compile-time constants.
507    
508     An obvious way to use this is to prefetch some data far away, in a big
509 root 1.9 array you loop over. This prefetches memory some 128 array elements later,
510 root 1.7 in the hope that it will be ready when the CPU arrives at that location.
511    
512     int sum = 0;
513    
514     for (i = 0; i < N; ++i)
515     {
516     sum += arr [i]
517     ecb_prefetch (arr + i + 128, 0, 0);
518     }
519    
520     It's hard to predict how far to prefetch, and most CPUs that can prefetch
521     are often good enough to predict this kind of behaviour themselves. It
522     gets more interesting with linked lists, especially when you do some fair
523     processing on each list element:
524    
525     for (node *n = start; n; n = n->next)
526     {
527     ecb_prefetch (n->next, 0, 0);
528     ... do medium amount of work with *n
529     }
530    
531     After processing the node, (part of) the next node might already be in
532     cache.
533 root 1.1
534 root 1.2 =back
535 root 1.1
536 root 1.36 =head2 BIT FIDDLING / BIT WIZARDRY
537 root 1.1
538 root 1.4 =over 4
539    
540 root 1.3 =item bool ecb_big_endian ()
541    
542     =item bool ecb_little_endian ()
543    
544 sf-exg 1.11 These two functions return true if the byte order is big endian
545     (most-significant byte first) or little endian (least-significant byte
546     first) respectively.
547    
548 root 1.24 On systems that are neither, their return values are unspecified.
549    
550 root 1.3 =item int ecb_ctz32 (uint32_t x)
551    
552 root 1.35 =item int ecb_ctz64 (uint64_t x)
553    
554 sf-exg 1.11 Returns the index of the least significant bit set in C<x> (or
555 root 1.24 equivalently the number of bits set to 0 before the least significant bit
556 root 1.35 set), starting from 0. If C<x> is 0 the result is undefined.
557    
558 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
559    
560 root 1.35 For example:
561 sf-exg 1.11
562 root 1.15 ecb_ctz32 (3) = 0
563     ecb_ctz32 (6) = 1
564 sf-exg 1.11
565 root 1.41 =item bool ecb_is_pot32 (uint32_t x)
566    
567     =item bool ecb_is_pot64 (uint32_t x)
568    
569     Return true iff C<x> is a power of two or C<x == 0>.
570    
571     For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
572    
573 root 1.35 =item int ecb_ld32 (uint32_t x)
574    
575     =item int ecb_ld64 (uint64_t x)
576    
577     Returns the index of the most significant bit set in C<x>, or the number
578     of digits the number requires in binary (so that C<< 2**ld <= x <
579     2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
580     to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
581     example to see how many bits a certain number requires to be encoded.
582    
583     This function is similar to the "count leading zero bits" function, except
584     that that one returns how many zero bits are "in front" of the number (in
585     the given data type), while C<ecb_ld> returns how many bits the number
586     itself requires.
587    
588 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
589    
590 root 1.3 =item int ecb_popcount32 (uint32_t x)
591    
592 root 1.35 =item int ecb_popcount64 (uint64_t x)
593    
594 root 1.36 Returns the number of bits set to 1 in C<x>.
595    
596     For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
597    
598     For example:
599 sf-exg 1.11
600 root 1.15 ecb_popcount32 (7) = 3
601     ecb_popcount32 (255) = 8
602 sf-exg 1.11
603 root 1.39 =item uint8_t ecb_bitrev8 (uint8_t x)
604    
605     =item uint16_t ecb_bitrev16 (uint16_t x)
606    
607     =item uint32_t ecb_bitrev32 (uint32_t x)
608    
609     Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
610     and so on.
611    
612     Example:
613    
614     ecb_bitrev8 (0xa7) = 0xea
615     ecb_bitrev32 (0xffcc4411) = 0x882233ff
616    
617 root 1.8 =item uint32_t ecb_bswap16 (uint32_t x)
618    
619 root 1.3 =item uint32_t ecb_bswap32 (uint32_t x)
620    
621 root 1.34 =item uint64_t ecb_bswap64 (uint64_t x)
622 sf-exg 1.13
623 root 1.34 These functions return the value of the 16-bit (32-bit, 64-bit) value
624     C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
625     C<ecb_bswap32>).
626    
627     =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
628    
629     =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
630 root 1.3
631     =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
632    
633 root 1.34 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
634    
635     =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
636    
637     =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
638    
639     =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
640    
641 root 1.33 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
642    
643 root 1.34 These two families of functions return the value of C<x> after rotating
644     all the bits by C<count> positions to the right (C<ecb_rotr>) or left
645     (C<ecb_rotl>).
646 sf-exg 1.11
647 root 1.20 Current GCC versions understand these functions and usually compile them
648 root 1.34 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
649     x86).
650 root 1.20
651 root 1.3 =back
652 root 1.1
653 root 1.50 =head2 FLOATING POINT FIDDLING
654    
655     =over 4
656    
657     =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
658    
659     =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
660    
661     These functions each take an argument in the native C<float> or C<double>
662     type and return the IEEE 754 bit representation of it.
663    
664     The bit representation is just as IEEE 754 defines it, i.e. the sign bit
665     will be the most significant bit, followed by exponent and mantissa.
666    
667     This function should work even when the native floating point format isn't
668     IEEE compliant, of course at a speed and code size penalty, and of course
669     also within reasonable limits (it tries to convert NaNs, infinities and
670     denormals, but will likely convert negative zero to positive zero).
671    
672     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
673     be able to optimise away this function completely.
674    
675     These functions can be helpful when serialising floats to the network - you
676     can serialise the return value like a normal uint32_t/uint64_t.
677    
678     Another use for these functions is to manipulate floating point values
679     directly.
680    
681     Silly example: toggle the sign bit of a float.
682    
683     /* On gcc-4.7 on amd64, */
684     /* this results in a single add instruction to toggle the bit, and 4 extra */
685     /* instructions to move the float value to an integer register and back. */
686    
687     x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
688    
689     =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
690    
691     =item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
692    
693     The reverse operation of the previos function - takes the bit representation
694     of an IEEE binary32 or binary64 number and converts it to the native C<float>
695     or C<double> format.
696    
697     This function should work even when the native floating point format isn't
698     IEEE compliant, of course at a speed and code size penalty, and of course
699     also within reasonable limits (it tries to convert normals and denormals,
700     and might be lucky for infinities, and with extraordinary luck, also for
701     negative zero).
702    
703     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
704     be able to optimise away this function completely.
705    
706     =back
707    
708 root 1.1 =head2 ARITHMETIC
709    
710 root 1.3 =over 4
711    
712 root 1.14 =item x = ecb_mod (m, n)
713 root 1.3
714 root 1.25 Returns C<m> modulo C<n>, which is the same as the positive remainder
715     of the division operation between C<m> and C<n>, using floored
716     division. Unlike the C remainder operator C<%>, this function ensures that
717     the return value is always positive and that the two numbers I<m> and
718     I<m' = m + i * n> result in the same value modulo I<n> - in other words,
719     C<ecb_mod> implements the mathematical modulo operation, which is missing
720     in the language.
721 root 1.14
722 sf-exg 1.23 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
723 root 1.14 negatable, that is, both C<m> and C<-m> must be representable in its
724 root 1.30 type (this typically excludes the minimum signed integer value, the same
725 root 1.25 limitation as for C</> and C<%> in C).
726 sf-exg 1.11
727 root 1.24 Current GCC versions compile this into an efficient branchless sequence on
728 root 1.28 almost all CPUs.
729 root 1.24
730     For example, when you want to rotate forward through the members of an
731     array for increasing C<m> (which might be negative), then you should use
732     C<ecb_mod>, as the C<%> operator might give either negative results, or
733     change direction for negative values:
734    
735     for (m = -100; m <= 100; ++m)
736     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
737    
738 sf-exg 1.37 =item x = ecb_div_rd (val, div)
739    
740     =item x = ecb_div_ru (val, div)
741    
742     Returns C<val> divided by C<div> rounded down or up, respectively.
743     C<val> and C<div> must have integer types and C<div> must be strictly
744 sf-exg 1.38 positive. Note that these functions are implemented with macros in C
745     and with function templates in C++.
746 sf-exg 1.37
747 root 1.3 =back
748 root 1.1
749     =head2 UTILITY
750    
751 root 1.3 =over 4
752    
753 sf-exg 1.23 =item element_count = ecb_array_length (name)
754 root 1.3
755 sf-exg 1.13 Returns the number of elements in the array C<name>. For example:
756    
757     int primes[] = { 2, 3, 5, 7, 11 };
758     int sum = 0;
759    
760     for (i = 0; i < ecb_array_length (primes); i++)
761     sum += primes [i];
762    
763 root 1.3 =back
764 root 1.1
765 root 1.43 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
766    
767     These symbols need to be defined before including F<ecb.h> the first time.
768    
769     =over 4
770    
771 root 1.51 =item ECB_NO_THREADS
772 root 1.43
773     If F<ecb.h> is never used from multiple threads, then this symbol can
774     be defined, in which case memory fences (and similar constructs) are
775     completely removed, leading to more efficient code and fewer dependencies.
776    
777     Setting this symbol to a true value implies C<ECB_NO_SMP>.
778    
779     =item ECB_NO_SMP
780    
781     The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
782     multiple threads, but never concurrently (e.g. if the system the program
783     runs on has only a single CPU with a single core, no hyperthreading and so
784     on), then this symbol can be defined, leading to more efficient code and
785     fewer dependencies.
786    
787 root 1.50 =item ECB_NO_LIBM
788    
789     When defined to C<1>, do not export any functions that might introduce
790     dependencies on the math library (usually called F<-lm>) - these are
791     marked with [-UECB_NO_LIBM].
792    
793 root 1.43 =back
794    
795 root 1.1