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1 =head1 LIBECB - e-C-Builtins
2
3 =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 It's part of the e-suite of libraries, other members of which include
9 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 it provides a number of other lowlevel C utilities, such as endianness
18 detection, byte swapping or bit rotations.
19
20 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
24 More might come.
25
26 =head2 ABOUT THE HEADER
27
28 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 There are currently no object files to link to - future versions might
37 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
59 =head2 TYPES / TYPE SUPPORT
60
61 ecb.h makes sure that the following types are defined (in the expected way):
62
63 int8_t uint8_
64 int16_t uint16_t
65 int32_t uint32_
66 int64_t uint64_t
67 int_fast8_t uint_fast8_t
68 int_fast16_t uint_fast16_t
69 int_fast32_t uint_fast32_t
70 int_fast64_t uint_fast64_t
71 intptr_t uintptr_t
72
73 The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
74 platform (currently C<4> or C<8>) and can be used in preprocessor
75 expressions.
76
77 For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>.
78
79 =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
80
81 All the following symbols expand to an expression that can be tested in
82 preprocessor instructions as well as treated as a boolean (use C<!!> to
83 ensure it's either C<0> or C<1> if you need that).
84
85 =over 4
86
87 =item ECB_C
88
89 True if the implementation defines the C<__STDC__> macro to a true value,
90 while not claiming to be C++.
91
92 =item ECB_C99
93
94 True if the implementation claims to be compliant to C99 (ISO/IEC
95 9899:1999) or any later version, while not claiming to be C++.
96
97 Note that later versions (ECB_C11) remove core features again (for
98 example, variable length arrays).
99
100 =item ECB_C11, ECB_C17
101
102 True if the implementation claims to be compliant to C11/C17 (ISO/IEC
103 9899:2011, :20187) or any later version, while not claiming to be C++.
104
105 =item ECB_CPP
106
107 True if the implementation defines the C<__cplusplus__> macro to a true
108 value, which is typically true for C++ compilers.
109
110 =item ECB_CPP11, ECB_CPP14, ECB_CPP17
111
112 True if the implementation claims to be compliant to C++11/C++14/C++17
113 (ISO/IEC 14882:2011, :2014, :2017) or any later version.
114
115 =item ECB_OPTIMIZE_SIZE
116
117 Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
118 can also be defined before including F<ecb.h>, in which case it will be
119 unchanged.
120
121 =item ECB_GCC_VERSION (major, minor)
122
123 Expands to a true value (suitable for testing in by the preprocessor)
124 if the compiler used is GNU C and the version is the given version, or
125 higher.
126
127 This macro tries to return false on compilers that claim to be GCC
128 compatible but aren't.
129
130 =item ECB_EXTERN_C
131
132 Expands to C<extern "C"> in C++, and a simple C<extern> in C.
133
134 This can be used to declare a single external C function:
135
136 ECB_EXTERN_C int printf (const char *format, ...);
137
138 =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
139
140 These two macros can be used to wrap multiple C<extern "C"> definitions -
141 they expand to nothing in C.
142
143 They are most useful in header files:
144
145 ECB_EXTERN_C_BEG
146
147 int mycfun1 (int x);
148 int mycfun2 (int x);
149
150 ECB_EXTERN_C_END
151
152 =item ECB_STDFP
153
154 If this evaluates to a true value (suitable for testing in by the
155 preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
156 and double/binary64 representations internally I<and> the endianness of
157 both types match the endianness of C<uint32_t> and C<uint64_t>.
158
159 This means you can just copy the bits of a C<float> (or C<double>) to an
160 C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
161 without having to think about format or endianness.
162
163 This is true for basically all modern platforms, although F<ecb.h> might
164 not be able to deduce this correctly everywhere and might err on the safe
165 side.
166
167 =item ECB_AMD64, ECB_AMD64_X32
168
169 These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
170 ABI, respectively, and undefined elsewhere.
171
172 The designers of the new X32 ABI for some inexplicable reason decided to
173 make it look exactly like amd64, even though it's completely incompatible
174 to that ABI, breaking about every piece of software that assumed that
175 C<__x86_64> stands for, well, the x86-64 ABI, making these macros
176 necessary.
177
178 =back
179
180 =head2 MACRO TRICKERY
181
182 =over 4
183
184 =item ECB_CONCAT (a, b)
185
186 Expands any macros in C<a> and C<b>, then concatenates the result to form
187 a single token. This is mainly useful to form identifiers from components,
188 e.g.:
189
190 #define S1 str
191 #define S2 cpy
192
193 ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
194
195 =item ECB_STRINGIFY (arg)
196
197 Expands any macros in C<arg> and returns the stringified version of
198 it. This is mainly useful to get the contents of a macro in string form,
199 e.g.:
200
201 #define SQL_LIMIT 100
202 sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
203
204 =item ECB_STRINGIFY_EXPR (expr)
205
206 Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
207 is a valid expression. This is useful to catch typos or cases where the
208 macro isn't available:
209
210 #include <errno.h>
211
212 ECB_STRINGIFY (EDOM); // "33" (on my system at least)
213 ECB_STRINGIFY_EXPR (EDOM); // "33"
214
215 // now imagine we had a typo:
216
217 ECB_STRINGIFY (EDAM); // "EDAM"
218 ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
219
220 =back
221
222 =head2 ATTRIBUTES
223
224 A major part of libecb deals with additional attributes that can be
225 assigned to functions, variables and sometimes even types - much like
226 C<const> or C<volatile> in C. They are implemented using either GCC
227 attributes or other compiler/language specific features. Attributes
228 declarations must be put before the whole declaration:
229
230 ecb_const int mysqrt (int a);
231 ecb_unused int i;
232
233 =over 4
234
235 =item ecb_unused
236
237 Marks a function or a variable as "unused", which simply suppresses a
238 warning by GCC when it detects it as unused. This is useful when you e.g.
239 declare a variable but do not always use it:
240
241 {
242 ecb_unused int var;
243
244 #ifdef SOMECONDITION
245 var = ...;
246 return var;
247 #else
248 return 0;
249 #endif
250 }
251
252 =item ecb_deprecated
253
254 Similar to C<ecb_unused>, but marks a function, variable or type as
255 deprecated. This makes some compilers warn when the type is used.
256
257 =item ecb_deprecated_message (message)
258
259 Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
260 used instead of a generic depreciation message when the object is being
261 used.
262
263 =item ecb_inline
264
265 Expands either to (a compiler-specific equivalent of) C<static inline> or
266 to just C<static>, if inline isn't supported. It should be used to declare
267 functions that should be inlined, for code size or speed reasons.
268
269 Example: inline this function, it surely will reduce codesize.
270
271 ecb_inline int
272 negmul (int a, int b)
273 {
274 return - (a * b);
275 }
276
277 =item ecb_noinline
278
279 Prevents a function from being inlined - it might be optimised away, but
280 not inlined into other functions. This is useful if you know your function
281 is rarely called and large enough for inlining not to be helpful.
282
283 =item ecb_noreturn
284
285 Marks a function as "not returning, ever". Some typical functions that
286 don't return are C<exit> or C<abort> (which really works hard to not
287 return), and now you can make your own:
288
289 ecb_noreturn void
290 my_abort (const char *errline)
291 {
292 puts (errline);
293 abort ();
294 }
295
296 In this case, the compiler would probably be smart enough to deduce it on
297 its own, so this is mainly useful for declarations.
298
299 =item ecb_restrict
300
301 Expands to the C<restrict> keyword or equivalent on compilers that support
302 them, and to nothing on others. Must be specified on a pointer type or
303 an array index to indicate that the memory doesn't alias with any other
304 restricted pointer in the same scope.
305
306 Example: multiply a vector, and allow the compiler to parallelise the
307 loop, because it knows it doesn't overwrite input values.
308
309 void
310 multiply (ecb_restrict float *src,
311 ecb_restrict float *dst,
312 int len, float factor)
313 {
314 int i;
315
316 for (i = 0; i < len; ++i)
317 dst [i] = src [i] * factor;
318 }
319
320 =item ecb_const
321
322 Declares that the function only depends on the values of its arguments,
323 much like a mathematical function. It specifically does not read or write
324 any memory any arguments might point to, global variables, or call any
325 non-const functions. It also must not have any side effects.
326
327 Such a function can be optimised much more aggressively by the compiler -
328 for example, multiple calls with the same arguments can be optimised into
329 a single call, which wouldn't be possible if the compiler would have to
330 expect any side effects.
331
332 It is best suited for functions in the sense of mathematical functions,
333 such as a function returning the square root of its input argument.
334
335 Not suited would be a function that calculates the hash of some memory
336 area you pass in, prints some messages or looks at a global variable to
337 decide on rounding.
338
339 See C<ecb_pure> for a slightly less restrictive class of functions.
340
341 =item ecb_pure
342
343 Similar to C<ecb_const>, declares a function that has no side
344 effects. Unlike C<ecb_const>, the function is allowed to examine global
345 variables and any other memory areas (such as the ones passed to it via
346 pointers).
347
348 While these functions cannot be optimised as aggressively as C<ecb_const>
349 functions, they can still be optimised away in many occasions, and the
350 compiler has more freedom in moving calls to them around.
351
352 Typical examples for such functions would be C<strlen> or C<memcmp>. A
353 function that calculates the MD5 sum of some input and updates some MD5
354 state passed as argument would I<NOT> be pure, however, as it would modify
355 some memory area that is not the return value.
356
357 =item ecb_hot
358
359 This declares a function as "hot" with regards to the cache - the function
360 is used so often, that it is very beneficial to keep it in the cache if
361 possible.
362
363 The compiler reacts by trying to place hot functions near to each other in
364 memory.
365
366 Whether a function is hot or not often depends on the whole program,
367 and less on the function itself. C<ecb_cold> is likely more useful in
368 practise.
369
370 =item ecb_cold
371
372 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
373 the cache, or in other words, this function is not called often, or not at
374 speed-critical times, and keeping it in the cache might be a waste of said
375 cache.
376
377 In addition to placing cold functions together (or at least away from hot
378 functions), this knowledge can be used in other ways, for example, the
379 function will be optimised for size, as opposed to speed, and codepaths
380 leading to calls to those functions can automatically be marked as if
381 C<ecb_expect_false> had been used to reach them.
382
383 Good examples for such functions would be error reporting functions, or
384 functions only called in exceptional or rare cases.
385
386 =item ecb_artificial
387
388 Declares the function as "artificial", in this case meaning that this
389 function is not really meant to be a function, but more like an accessor
390 - many methods in C++ classes are mere accessor functions, and having a
391 crash reported in such a method, or single-stepping through them, is not
392 usually so helpful, especially when it's inlined to just a few instructions.
393
394 Marking them as artificial will instruct the debugger about just this,
395 leading to happier debugging and thus happier lives.
396
397 Example: in some kind of smart-pointer class, mark the pointer accessor as
398 artificial, so that the whole class acts more like a pointer and less like
399 some C++ abstraction monster.
400
401 template<typename T>
402 struct my_smart_ptr
403 {
404 T *value;
405
406 ecb_artificial
407 operator T *()
408 {
409 return value;
410 }
411 };
412
413 =back
414
415 =head2 OPTIMISATION HINTS
416
417 =over 4
418
419 =item bool ecb_is_constant (expr)
420
421 Returns true iff the expression can be deduced to be a compile-time
422 constant, and false otherwise.
423
424 For example, when you have a C<rndm16> function that returns a 16 bit
425 random number, and you have a function that maps this to a range from
426 0..n-1, then you could use this inline function in a header file:
427
428 ecb_inline uint32_t
429 rndm (uint32_t n)
430 {
431 return (n * (uint32_t)rndm16 ()) >> 16;
432 }
433
434 However, for powers of two, you could use a normal mask, but that is only
435 worth it if, at compile time, you can detect this case. This is the case
436 when the passed number is a constant and also a power of two (C<n & (n -
437 1) == 0>):
438
439 ecb_inline uint32_t
440 rndm (uint32_t n)
441 {
442 return is_constant (n) && !(n & (n - 1))
443 ? rndm16 () & (num - 1)
444 : (n * (uint32_t)rndm16 ()) >> 16;
445 }
446
447 =item ecb_expect (expr, value)
448
449 Evaluates C<expr> and returns it. In addition, it tells the compiler that
450 the C<expr> evaluates to C<value> a lot, which can be used for static
451 branch optimisations.
452
453 Usually, you want to use the more intuitive C<ecb_expect_true> and
454 C<ecb_expect_false> functions instead.
455
456 =item bool ecb_expect_true (cond)
457
458 =item bool ecb_expect_false (cond)
459
460 These two functions expect a expression that is true or false and return
461 C<1> or C<0>, respectively, so when used in the condition of an C<if> or
462 other conditional statement, it will not change the program:
463
464 /* these two do the same thing */
465 if (some_condition) ...;
466 if (ecb_expect_true (some_condition)) ...;
467
468 However, by using C<ecb_expect_true>, you tell the compiler that the
469 condition is likely to be true (and for C<ecb_expect_false>, that it is
470 unlikely to be true).
471
472 For example, when you check for a null pointer and expect this to be a
473 rare, exceptional, case, then use C<ecb_expect_false>:
474
475 void my_free (void *ptr)
476 {
477 if (ecb_expect_false (ptr == 0))
478 return;
479 }
480
481 Consequent use of these functions to mark away exceptional cases or to
482 tell the compiler what the hot path through a function is can increase
483 performance considerably.
484
485 You might know these functions under the name C<likely> and C<unlikely>
486 - while these are common aliases, we find that the expect name is easier
487 to understand when quickly skimming code. If you wish, you can use
488 C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
489 C<ecb_expect_false> - these are simply aliases.
490
491 A very good example is in a function that reserves more space for some
492 memory block (for example, inside an implementation of a string stream) -
493 each time something is added, you have to check for a buffer overrun, but
494 you expect that most checks will turn out to be false:
495
496 /* make sure we have "size" extra room in our buffer */
497 ecb_inline void
498 reserve (int size)
499 {
500 if (ecb_expect_false (current + size > end))
501 real_reserve_method (size); /* presumably noinline */
502 }
503
504 =item ecb_assume (cond)
505
506 Tries to tell the compiler that some condition is true, even if it's not
507 obvious. This is not a function, but a statement: it cannot be used in
508 another expression.
509
510 This can be used to teach the compiler about invariants or other
511 conditions that might improve code generation, but which are impossible to
512 deduce form the code itself.
513
514 For example, the example reservation function from the C<ecb_expect_false>
515 description could be written thus (only C<ecb_assume> was added):
516
517 ecb_inline void
518 reserve (int size)
519 {
520 if (ecb_expect_false (current + size > end))
521 real_reserve_method (size); /* presumably noinline */
522
523 ecb_assume (current + size <= end);
524 }
525
526 If you then call this function twice, like this:
527
528 reserve (10);
529 reserve (1);
530
531 Then the compiler I<might> be able to optimise out the second call
532 completely, as it knows that C<< current + 1 > end >> is false and the
533 call will never be executed.
534
535 =item ecb_unreachable ()
536
537 This function does nothing itself, except tell the compiler that it will
538 never be executed. Apart from suppressing a warning in some cases, this
539 function can be used to implement C<ecb_assume> or similar functionality.
540
541 =item ecb_prefetch (addr, rw, locality)
542
543 Tells the compiler to try to prefetch memory at the given C<addr>ess
544 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
545 C<0> means that there will only be one access later, C<3> means that
546 the data will likely be accessed very often, and values in between mean
547 something... in between. The memory pointed to by the address does not
548 need to be accessible (it could be a null pointer for example), but C<rw>
549 and C<locality> must be compile-time constants.
550
551 This is a statement, not a function: you cannot use it as part of an
552 expression.
553
554 An obvious way to use this is to prefetch some data far away, in a big
555 array you loop over. This prefetches memory some 128 array elements later,
556 in the hope that it will be ready when the CPU arrives at that location.
557
558 int sum = 0;
559
560 for (i = 0; i < N; ++i)
561 {
562 sum += arr [i]
563 ecb_prefetch (arr + i + 128, 0, 0);
564 }
565
566 It's hard to predict how far to prefetch, and most CPUs that can prefetch
567 are often good enough to predict this kind of behaviour themselves. It
568 gets more interesting with linked lists, especially when you do some fair
569 processing on each list element:
570
571 for (node *n = start; n; n = n->next)
572 {
573 ecb_prefetch (n->next, 0, 0);
574 ... do medium amount of work with *n
575 }
576
577 After processing the node, (part of) the next node might already be in
578 cache.
579
580 =back
581
582 =head2 BIT FIDDLING / BIT WIZARDRY
583
584 =over 4
585
586 =item bool ecb_big_endian ()
587
588 =item bool ecb_little_endian ()
589
590 These two functions return true if the byte order is big endian
591 (most-significant byte first) or little endian (least-significant byte
592 first) respectively.
593
594 On systems that are neither, their return values are unspecified.
595
596 =item int ecb_ctz32 (uint32_t x)
597
598 =item int ecb_ctz64 (uint64_t x)
599
600 =item int ecb_ctz (T x) [C++]
601
602 Returns the index of the least significant bit set in C<x> (or
603 equivalently the number of bits set to 0 before the least significant bit
604 set), starting from 0. If C<x> is 0 the result is undefined.
605
606 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
607
608 The overloaded C++ C<ecb_ctz> function supports C<uint8_t>, C<uint16_t>,
609 C<uint32_t> and C<uint64_t> types.
610
611 For example:
612
613 ecb_ctz32 (3) = 0
614 ecb_ctz32 (6) = 1
615
616 =item bool ecb_is_pot32 (uint32_t x)
617
618 =item bool ecb_is_pot64 (uint32_t x)
619
620 =item bool ecb_is_pot (T x) [C++]
621
622 Returns true iff C<x> is a power of two or C<x == 0>.
623
624 For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
625
626 The overloaded C++ C<ecb_is_pot> function supports C<uint8_t>, C<uint16_t>,
627 C<uint32_t> and C<uint64_t> types.
628
629 =item int ecb_ld32 (uint32_t x)
630
631 =item int ecb_ld64 (uint64_t x)
632
633 =item int ecb_ld64 (T x) [C++]
634
635 Returns the index of the most significant bit set in C<x>, or the number
636 of digits the number requires in binary (so that C<< 2**ld <= x <
637 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
638 to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
639 example to see how many bits a certain number requires to be encoded.
640
641 This function is similar to the "count leading zero bits" function, except
642 that that one returns how many zero bits are "in front" of the number (in
643 the given data type), while C<ecb_ld> returns how many bits the number
644 itself requires.
645
646 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
647
648 The overloaded C++ C<ecb_ld> function supports C<uint8_t>, C<uint16_t>,
649 C<uint32_t> and C<uint64_t> types.
650
651 =item int ecb_popcount32 (uint32_t x)
652
653 =item int ecb_popcount64 (uint64_t x)
654
655 =item int ecb_popcount (T x) [C++]
656
657 Returns the number of bits set to 1 in C<x>.
658
659 For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
660
661 The overloaded C++ C<ecb_popcount> function supports C<uint8_t>, C<uint16_t>,
662 C<uint32_t> and C<uint64_t> types.
663
664 For example:
665
666 ecb_popcount32 (7) = 3
667 ecb_popcount32 (255) = 8
668
669 =item uint8_t ecb_bitrev8 (uint8_t x)
670
671 =item uint16_t ecb_bitrev16 (uint16_t x)
672
673 =item uint32_t ecb_bitrev32 (uint32_t x)
674
675 =item T ecb_bitrev (T x) [C++]
676
677 Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
678 and so on.
679
680 The overloaded C++ C<ecb_bitrev> function supports C<uint8_t>, C<uint16_t> and C<uint32_t> types.
681
682 Example:
683
684 ecb_bitrev8 (0xa7) = 0xea
685 ecb_bitrev32 (0xffcc4411) = 0x882233ff
686
687 =item T ecb_bitrev (T x) [C++]
688
689 Overloaded C++ bitrev function.
690
691 C<T> must be one of C<uint8_t>, C<uint16_t> or C<uint32_t>.
692
693 =item uint32_t ecb_bswap16 (uint32_t x)
694
695 =item uint32_t ecb_bswap32 (uint32_t x)
696
697 =item uint64_t ecb_bswap64 (uint64_t x)
698
699 =item T ecb_bswap (T x)
700
701 These functions return the value of the 16-bit (32-bit, 64-bit) value
702 C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
703 C<ecb_bswap32>).
704
705 The overloaded C++ C<ecb_bswap> function supports C<uint8_t>, C<uint16_t>,
706 C<uint32_t> and C<uint64_t> types.
707
708 =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
709
710 =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
711
712 =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
713
714 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
715
716 =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
717
718 =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
719
720 =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
721
722 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
723
724 These two families of functions return the value of C<x> after rotating
725 all the bits by C<count> positions to the right (C<ecb_rotr>) or left
726 (C<ecb_rotl>).
727
728 Current GCC versions understand these functions and usually compile them
729 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
730 x86).
731
732 =item T ecb_rotl (T x, unsigned int count) [C++]
733
734 =item T ecb_rotr (T x, unsigned int count) [C++]
735
736 Overloaded C++ rotl/rotr functions.
737
738 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
739
740 =back
741
742 =head2 HOST ENDIANNESS CONVERSION
743
744 =over 4
745
746 =item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v)
747
748 =item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v)
749
750 =item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v)
751
752 =item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v)
753
754 =item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v)
755
756 =item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v)
757
758 Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order.
759
760 The naming convention is C<ecb_>(C<be>|C<le>)C<_u>C<16|32|64>C<_to_host>,
761 where C<be> and C<le> stand for big endian and little endian, respectively.
762
763 =item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v)
764
765 =item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v)
766
767 =item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v)
768
769 =item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v)
770
771 =item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v)
772
773 =item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v)
774
775 Like above, but converts I<from> host byte order to the specified
776 endianness.
777
778 =back
779
780 In C++ the following additional template functions are supported:
781
782 =over 4
783
784 =item T ecb_be_to_host (T v)
785
786 =item T ecb_le_to_host (T v)
787
788 =item T ecb_host_to_be (T v)
789
790 =item T ecb_host_to_le (T v)
791
792 These functions work like their C counterparts, above, but use templates,
793 which make them useful in generic code.
794
795 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>
796 (so unlike their C counterparts, there is a version for C<uint8_t>, which
797 again can be useful in generic code).
798
799 =head2 UNALIGNED LOAD/STORE
800
801 These function load or store unaligned multi-byte values.
802
803 =over 4
804
805 =item uint_fast16_t ecb_peek_u16_u (const void *ptr)
806
807 =item uint_fast32_t ecb_peek_u32_u (const void *ptr)
808
809 =item uint_fast64_t ecb_peek_u64_u (const void *ptr)
810
811 These functions load an unaligned, unsigned 16, 32 or 64 bit value from
812 memory.
813
814 =item uint_fast16_t ecb_peek_be_u16_u (const void *ptr)
815
816 =item uint_fast32_t ecb_peek_be_u32_u (const void *ptr)
817
818 =item uint_fast64_t ecb_peek_be_u64_u (const void *ptr)
819
820 =item uint_fast16_t ecb_peek_le_u16_u (const void *ptr)
821
822 =item uint_fast32_t ecb_peek_le_u32_u (const void *ptr)
823
824 =item uint_fast64_t ecb_peek_le_u64_u (const void *ptr)
825
826 Like above, but additionally convert from big endian (C<be>) or little
827 endian (C<le>) byte order to host byte order while doing so.
828
829 =item ecb_poke_u16_u (void *ptr, uint16_t v)
830
831 =item ecb_poke_u32_u (void *ptr, uint32_t v)
832
833 =item ecb_poke_u64_u (void *ptr, uint64_t v)
834
835 These functions store an unaligned, unsigned 16, 32 or 64 bit value to
836 memory.
837
838 =item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v)
839
840 =item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v)
841
842 =item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v)
843
844 =item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v)
845
846 =item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v)
847
848 =item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v)
849
850 Like above, but additionally convert from host byte order to big endian
851 (C<be>) or little endian (C<le>) byte order while doing so.
852
853 =back
854
855 In C++ the following additional template functions are supported:
856
857 =over 4
858
859 =item T ecb_peek<T> (const void *ptr)
860
861 =item T ecb_peek_be<T> (const void *ptr)
862
863 =item T ecb_peek_le<T> (const void *ptr)
864
865 =item T ecb_peek_u<T> (const void *ptr)
866
867 =item T ecb_peek_be_u<T> (const void *ptr)
868
869 =item T ecb_peek_le_u<T> (const void *ptr)
870
871 Similarly to their C counterparts, these functions load an unsigned 8, 16,
872 32 or 64 bit value from memory, with optional conversion from big/little
873 endian.
874
875 Since the type cannot be deduced, it has to be specified explicitly, e.g.
876
877 uint_fast16_t v = ecb_peek<uint16_t> (ptr);
878
879 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
880
881 Unlike their C counterparts, these functions support 8 bit quantities
882 (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
883 all of which hopefully makes them more useful in generic code.
884
885 =item ecb_poke (void *ptr, T v)
886
887 =item ecb_poke_be (void *ptr, T v)
888
889 =item ecb_poke_le (void *ptr, T v)
890
891 =item ecb_poke_u (void *ptr, T v)
892
893 =item ecb_poke_be_u (void *ptr, T v)
894
895 =item ecb_poke_le_u (void *ptr, T v)
896
897 Again, similarly to their C counterparts, these functions store an
898 unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to
899 big/little endian.
900
901 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
902
903 Unlike their C counterparts, these functions support 8 bit quantities
904 (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
905 all of which hopefully makes them more useful in generic code.
906
907 =back
908
909 =head2 FLOATING POINT FIDDLING
910
911 =over 4
912
913 =item ECB_INFINITY [-UECB_NO_LIBM]
914
915 Evaluates to positive infinity if supported by the platform, otherwise to
916 a truly huge number.
917
918 =item ECB_NAN [-UECB_NO_LIBM]
919
920 Evaluates to a quiet NAN if supported by the platform, otherwise to
921 C<ECB_INFINITY>.
922
923 =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
924
925 Same as C<ldexpf>, but always available.
926
927 =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
928
929 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
930
931 =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
932
933 These functions each take an argument in the native C<float> or C<double>
934 type and return the IEEE 754 bit representation of it (binary16/half,
935 binary32/single or binary64/double precision).
936
937 The bit representation is just as IEEE 754 defines it, i.e. the sign bit
938 will be the most significant bit, followed by exponent and mantissa.
939
940 This function should work even when the native floating point format isn't
941 IEEE compliant, of course at a speed and code size penalty, and of course
942 also within reasonable limits (it tries to convert NaNs, infinities and
943 denormals, but will likely convert negative zero to positive zero).
944
945 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
946 be able to optimise away this function completely.
947
948 These functions can be helpful when serialising floats to the network - you
949 can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
950
951 Another use for these functions is to manipulate floating point values
952 directly.
953
954 Silly example: toggle the sign bit of a float.
955
956 /* On gcc-4.7 on amd64, */
957 /* this results in a single add instruction to toggle the bit, and 4 extra */
958 /* instructions to move the float value to an integer register and back. */
959
960 x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
961
962 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
963
964 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
965
966 =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
967
968 The reverse operation of the previous function - takes the bit
969 representation of an IEEE binary16, binary32 or binary64 number (half,
970 single or double precision) and converts it to the native C<float> or
971 C<double> format.
972
973 This function should work even when the native floating point format isn't
974 IEEE compliant, of course at a speed and code size penalty, and of course
975 also within reasonable limits (it tries to convert normals and denormals,
976 and might be lucky for infinities, and with extraordinary luck, also for
977 negative zero).
978
979 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
980 be able to optimise away this function completely.
981
982 =item uint16_t ecb_binary32_to_binary16 (uint32_t x)
983
984 =item uint32_t ecb_binary16_to_binary32 (uint16_t x)
985
986 Convert a IEEE binary32/single precision to binary16/half format, and vice
987 versa, handling all details (round-to-nearest-even, subnormals, infinity
988 and NaNs) correctly.
989
990 These are functions are available under C<-DECB_NO_LIBM>, since
991 they do not rely on the platform floating point format. The
992 C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
993 usually what you want.
994
995 =back
996
997 =head2 ARITHMETIC
998
999 =over 4
1000
1001 =item x = ecb_mod (m, n)
1002
1003 Returns C<m> modulo C<n>, which is the same as the positive remainder
1004 of the division operation between C<m> and C<n>, using floored
1005 division. Unlike the C remainder operator C<%>, this function ensures that
1006 the return value is always positive and that the two numbers I<m> and
1007 I<m' = m + i * n> result in the same value modulo I<n> - in other words,
1008 C<ecb_mod> implements the mathematical modulo operation, which is missing
1009 in the language.
1010
1011 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
1012 negatable, that is, both C<m> and C<-m> must be representable in its
1013 type (this typically excludes the minimum signed integer value, the same
1014 limitation as for C</> and C<%> in C).
1015
1016 Current GCC versions compile this into an efficient branchless sequence on
1017 almost all CPUs.
1018
1019 For example, when you want to rotate forward through the members of an
1020 array for increasing C<m> (which might be negative), then you should use
1021 C<ecb_mod>, as the C<%> operator might give either negative results, or
1022 change direction for negative values:
1023
1024 for (m = -100; m <= 100; ++m)
1025 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
1026
1027 =item x = ecb_div_rd (val, div)
1028
1029 =item x = ecb_div_ru (val, div)
1030
1031 Returns C<val> divided by C<div> rounded down or up, respectively.
1032 C<val> and C<div> must have integer types and C<div> must be strictly
1033 positive. Note that these functions are implemented with macros in C
1034 and with function templates in C++.
1035
1036 =back
1037
1038 =head2 UTILITY
1039
1040 =over 4
1041
1042 =item element_count = ecb_array_length (name)
1043
1044 Returns the number of elements in the array C<name>. For example:
1045
1046 int primes[] = { 2, 3, 5, 7, 11 };
1047 int sum = 0;
1048
1049 for (i = 0; i < ecb_array_length (primes); i++)
1050 sum += primes [i];
1051
1052 =back
1053
1054 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
1055
1056 These symbols need to be defined before including F<ecb.h> the first time.
1057
1058 =over 4
1059
1060 =item ECB_NO_THREADS
1061
1062 If F<ecb.h> is never used from multiple threads, then this symbol can
1063 be defined, in which case memory fences (and similar constructs) are
1064 completely removed, leading to more efficient code and fewer dependencies.
1065
1066 Setting this symbol to a true value implies C<ECB_NO_SMP>.
1067
1068 =item ECB_NO_SMP
1069
1070 The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
1071 multiple threads, but never concurrently (e.g. if the system the program
1072 runs on has only a single CPU with a single core, no hyperthreading and so
1073 on), then this symbol can be defined, leading to more efficient code and
1074 fewer dependencies.
1075
1076 =item ECB_NO_LIBM
1077
1078 When defined to C<1>, do not export any functions that might introduce
1079 dependencies on the math library (usually called F<-lm>) - these are
1080 marked with [-UECB_NO_LIBM].
1081
1082 =back
1083
1084 =head1 UNDOCUMENTED FUNCTIONALITY
1085
1086 F<ecb.h> is full of undocumented functionality as well, some of which is
1087 intended to be internal-use only, some of which we forgot to document, and
1088 some of which we hide because we are not sure we will keep the interface
1089 stable.
1090
1091 While you are welcome to rummage around and use whatever you find useful
1092 (we can't stop you), keep in mind that we will change undocumented
1093 functionality in incompatible ways without thinking twice, while we are
1094 considerably more conservative with documented things.
1095
1096 =head1 AUTHORS
1097
1098 C<libecb> is designed and maintained by:
1099
1100 Emanuele Giaquinta <e.giaquinta@glauco.it>
1101 Marc Alexander Lehmann <schmorp@schmorp.de>
1102
1103