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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 many compiler built-ins,
16 together with replacement functions for other compilers. In addition
17 to this, it provides a number of other lowlevel C utilities, such as
18 endianness detection, byte swapping or bit rotations.
19
20 Or in other words, things that should be built into any standard C
21 system, but aren't, implemented as efficient as possible with GCC (clang,
22 msvc...), and still 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
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++, i..e C, but not 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 Note that many C++20 features will likely have their own feature test
116 macros (see e.g. L<http://eel.is/c++draft/cpp.predefined#1.8>).
117
118 =item ECB_OPTIMIZE_SIZE
119
120 Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
121 can also be defined before including F<ecb.h>, in which case it will be
122 unchanged.
123
124 =item ECB_GCC_VERSION (major, minor)
125
126 Expands to a true value (suitable for testing by the preprocessor) if the
127 compiler used is GNU C and the version is the given version, or higher.
128
129 This macro tries to return false on compilers that claim to be GCC
130 compatible but aren't.
131
132 =item ECB_EXTERN_C
133
134 Expands to C<extern "C"> in C++, and a simple C<extern> in C.
135
136 This can be used to declare a single external C function:
137
138 ECB_EXTERN_C int printf (const char *format, ...);
139
140 =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
141
142 These two macros can be used to wrap multiple C<extern "C"> definitions -
143 they expand to nothing in C.
144
145 They are most useful in header files:
146
147 ECB_EXTERN_C_BEG
148
149 int mycfun1 (int x);
150 int mycfun2 (int x);
151
152 ECB_EXTERN_C_END
153
154 =item ECB_STDFP
155
156 If this evaluates to a true value (suitable for testing by the
157 preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
158 and double/binary64 representations internally I<and> the endianness of
159 both types match the endianness of C<uint32_t> and C<uint64_t>.
160
161 This means you can just copy the bits of a C<float> (or C<double>) to an
162 C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
163 without having to think about format or endianness.
164
165 This is true for basically all modern platforms, although F<ecb.h> might
166 not be able to deduce this correctly everywhere and might err on the safe
167 side.
168
169 =item ECB_64BIT_NATIVE
170
171 Evaluates to a true value (suitable for both preprocessor and C code
172 testing) if 64 bit integer types on this architecture are evaluated
173 "natively", that is, with similar speeds as 32 bit integerss. While 64 bit
174 integer support is very common (and in fatc required by libecb), 32 bit
175 cpus have to emulate operations on them, so you might want to avoid them.
176
177 =item ECB_AMD64, ECB_AMD64_X32
178
179 These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
180 ABI, respectively, and undefined elsewhere.
181
182 The designers of the new X32 ABI for some inexplicable reason decided to
183 make it look exactly like amd64, even though it's completely incompatible
184 to that ABI, breaking about every piece of software that assumed that
185 C<__x86_64> stands for, well, the x86-64 ABI, making these macros
186 necessary.
187
188 =back
189
190 =head2 MACRO TRICKERY
191
192 =over
193
194 =item ECB_CONCAT (a, b)
195
196 Expands any macros in C<a> and C<b>, then concatenates the result to form
197 a single token. This is mainly useful to form identifiers from components,
198 e.g.:
199
200 #define S1 str
201 #define S2 cpy
202
203 ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
204
205 =item ECB_STRINGIFY (arg)
206
207 Expands any macros in C<arg> and returns the stringified version of
208 it. This is mainly useful to get the contents of a macro in string form,
209 e.g.:
210
211 #define SQL_LIMIT 100
212 sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
213
214 =item ECB_STRINGIFY_EXPR (expr)
215
216 Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
217 is a valid expression. This is useful to catch typos or cases where the
218 macro isn't available:
219
220 #include <errno.h>
221
222 ECB_STRINGIFY (EDOM); // "33" (on my system at least)
223 ECB_STRINGIFY_EXPR (EDOM); // "33"
224
225 // now imagine we had a typo:
226
227 ECB_STRINGIFY (EDAM); // "EDAM"
228 ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
229
230 =back
231
232 =head2 ATTRIBUTES
233
234 A major part of libecb deals with additional attributes that can be
235 assigned to functions, variables and sometimes even types - much like
236 C<const> or C<volatile> in C. They are implemented using either GCC
237 attributes or other compiler/language specific features. Attributes
238 declarations must be put before the whole declaration:
239
240 ecb_const int mysqrt (int a);
241 ecb_unused int i;
242
243 =over
244
245 =item ecb_unused
246
247 Marks a function or a variable as "unused", which simply suppresses a
248 warning by the compiler when it detects it as unused. This is useful when
249 you e.g. declare a variable but do not always use it:
250
251 {
252 ecb_unused int var;
253
254 #ifdef SOMECONDITION
255 var = ...;
256 return var;
257 #else
258 return 0;
259 #endif
260 }
261
262 =item ecb_deprecated
263
264 Similar to C<ecb_unused>, but marks a function, variable or type as
265 deprecated. This makes some compilers warn when the type is used.
266
267 =item ecb_deprecated_message (message)
268
269 Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
270 used instead of a generic depreciation message when the object is being
271 used.
272
273 =item ecb_inline
274
275 Expands either to (a compiler-specific equivalent of) C<static inline> or
276 to just C<static>, if inline isn't supported. It should be used to declare
277 functions that should be inlined, for code size or speed reasons.
278
279 Example: inline this function, it surely will reduce codesize.
280
281 ecb_inline int
282 negmul (int a, int b)
283 {
284 return - (a * b);
285 }
286
287 =item ecb_noinline
288
289 Prevents a function from being inlined - it might be optimised away, but
290 not inlined into other functions. This is useful if you know your function
291 is rarely called and large enough for inlining not to be helpful.
292
293 =item ecb_noreturn
294
295 Marks a function as "not returning, ever". Some typical functions that
296 don't return are C<exit> or C<abort> (which really works hard to not
297 return), and now you can make your own:
298
299 ecb_noreturn void
300 my_abort (const char *errline)
301 {
302 puts (errline);
303 abort ();
304 }
305
306 In this case, the compiler would probably be smart enough to deduce it on
307 its own, so this is mainly useful for declarations.
308
309 =item ecb_restrict
310
311 Expands to the C<restrict> keyword or equivalent on compilers that support
312 them, and to nothing on others. Must be specified on a pointer type or
313 an array index to indicate that the memory doesn't alias with any other
314 restricted pointer in the same scope.
315
316 Example: multiply a vector, and allow the compiler to parallelise the
317 loop, because it knows it doesn't overwrite input values.
318
319 void
320 multiply (ecb_restrict float *src,
321 ecb_restrict float *dst,
322 int len, float factor)
323 {
324 int i;
325
326 for (i = 0; i < len; ++i)
327 dst [i] = src [i] * factor;
328 }
329
330 =item ecb_const
331
332 Declares that the function only depends on the values of its arguments,
333 much like a mathematical function. It specifically does not read or write
334 any memory any arguments might point to, global variables, or call any
335 non-const functions. It also must not have any side effects.
336
337 Such a function can be optimised much more aggressively by the compiler -
338 for example, multiple calls with the same arguments can be optimised into
339 a single call, which wouldn't be possible if the compiler would have to
340 expect any side effects.
341
342 It is best suited for functions in the sense of mathematical functions,
343 such as a function returning the square root of its input argument.
344
345 Not suited would be a function that calculates the hash of some memory
346 area you pass in, prints some messages or looks at a global variable to
347 decide on rounding.
348
349 See C<ecb_pure> for a slightly less restrictive class of functions.
350
351 =item ecb_pure
352
353 Similar to C<ecb_const>, declares a function that has no side
354 effects. Unlike C<ecb_const>, the function is allowed to examine global
355 variables and any other memory areas (such as the ones passed to it via
356 pointers).
357
358 While these functions cannot be optimised as aggressively as C<ecb_const>
359 functions, they can still be optimised away in many occasions, and the
360 compiler has more freedom in moving calls to them around.
361
362 Typical examples for such functions would be C<strlen> or C<memcmp>. A
363 function that calculates the MD5 sum of some input and updates some MD5
364 state passed as argument would I<NOT> be pure, however, as it would modify
365 some memory area that is not the return value.
366
367 =item ecb_hot
368
369 This declares a function as "hot" with regards to the cache - the function
370 is used so often, that it is very beneficial to keep it in the cache if
371 possible.
372
373 The compiler reacts by trying to place hot functions near to each other in
374 memory.
375
376 Whether a function is hot or not often depends on the whole program,
377 and less on the function itself. C<ecb_cold> is likely more useful in
378 practise.
379
380 =item ecb_cold
381
382 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
383 the cache, or in other words, this function is not called often, or not at
384 speed-critical times, and keeping it in the cache might be a waste of said
385 cache.
386
387 In addition to placing cold functions together (or at least away from hot
388 functions), this knowledge can be used in other ways, for example, the
389 function will be optimised for size, as opposed to speed, and codepaths
390 leading to calls to those functions can automatically be marked as if
391 C<ecb_expect_false> had been used to reach them.
392
393 Good examples for such functions would be error reporting functions, or
394 functions only called in exceptional or rare cases.
395
396 =item ecb_artificial
397
398 Declares the function as "artificial", in this case meaning that this
399 function is not really meant to be a function, but more like an accessor
400 - many methods in C++ classes are mere accessor functions, and having a
401 crash reported in such a method, or single-stepping through them, is not
402 usually so helpful, especially when it's inlined to just a few instructions.
403
404 Marking them as artificial will instruct the debugger about just this,
405 leading to happier debugging and thus happier lives.
406
407 Example: in some kind of smart-pointer class, mark the pointer accessor as
408 artificial, so that the whole class acts more like a pointer and less like
409 some C++ abstraction monster.
410
411 template<typename T>
412 struct my_smart_ptr
413 {
414 T *value;
415
416 ecb_artificial
417 operator T *()
418 {
419 return value;
420 }
421 };
422
423 =back
424
425 =head2 OPTIMISATION HINTS
426
427 =over
428
429 =item bool ecb_is_constant (expr)
430
431 Returns true iff the expression can be deduced to be a compile-time
432 constant, and false otherwise.
433
434 For example, when you have a C<rndm16> function that returns a 16 bit
435 random number, and you have a function that maps this to a range from
436 0..n-1, then you could use this inline function in a header file:
437
438 ecb_inline uint32_t
439 rndm (uint32_t n)
440 {
441 return (n * (uint32_t)rndm16 ()) >> 16;
442 }
443
444 However, for powers of two, you could use a normal mask, but that is only
445 worth it if, at compile time, you can detect this case. This is the case
446 when the passed number is a constant and also a power of two (C<n & (n -
447 1) == 0>):
448
449 ecb_inline uint32_t
450 rndm (uint32_t n)
451 {
452 return is_constant (n) && !(n & (n - 1))
453 ? rndm16 () & (num - 1)
454 : (n * (uint32_t)rndm16 ()) >> 16;
455 }
456
457 =item ecb_expect (expr, value)
458
459 Evaluates C<expr> and returns it. In addition, it tells the compiler that
460 the C<expr> evaluates to C<value> a lot, which can be used for static
461 branch optimisations.
462
463 Usually, you want to use the more intuitive C<ecb_expect_true> and
464 C<ecb_expect_false> functions instead.
465
466 =item bool ecb_expect_true (cond)
467
468 =item bool ecb_expect_false (cond)
469
470 These two functions expect a expression that is true or false and return
471 C<1> or C<0>, respectively, so when used in the condition of an C<if> or
472 other conditional statement, it will not change the program:
473
474 /* these two do the same thing */
475 if (some_condition) ...;
476 if (ecb_expect_true (some_condition)) ...;
477
478 However, by using C<ecb_expect_true>, you tell the compiler that the
479 condition is likely to be true (and for C<ecb_expect_false>, that it is
480 unlikely to be true).
481
482 For example, when you check for a null pointer and expect this to be a
483 rare, exceptional, case, then use C<ecb_expect_false>:
484
485 void my_free (void *ptr)
486 {
487 if (ecb_expect_false (ptr == 0))
488 return;
489 }
490
491 Consequent use of these functions to mark away exceptional cases or to
492 tell the compiler what the hot path through a function is can increase
493 performance considerably.
494
495 You might know these functions under the name C<likely> and C<unlikely>
496 - while these are common aliases, we find that the expect name is easier
497 to understand when quickly skimming code. If you wish, you can use
498 C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
499 C<ecb_expect_false> - these are simply aliases.
500
501 A very good example is in a function that reserves more space for some
502 memory block (for example, inside an implementation of a string stream) -
503 each time something is added, you have to check for a buffer overrun, but
504 you expect that most checks will turn out to be false:
505
506 /* make sure we have "size" extra room in our buffer */
507 ecb_inline void
508 reserve (int size)
509 {
510 if (ecb_expect_false (current + size > end))
511 real_reserve_method (size); /* presumably noinline */
512 }
513
514 =item ecb_assume (cond)
515
516 Tries to tell the compiler that some condition is true, even if it's not
517 obvious. This is not a function, but a statement: it cannot be used in
518 another expression.
519
520 This can be used to teach the compiler about invariants or other
521 conditions that might improve code generation, but which are impossible to
522 deduce form the code itself.
523
524 For example, the example reservation function from the C<ecb_expect_false>
525 description could be written thus (only C<ecb_assume> was added):
526
527 ecb_inline void
528 reserve (int size)
529 {
530 if (ecb_expect_false (current + size > end))
531 real_reserve_method (size); /* presumably noinline */
532
533 ecb_assume (current + size <= end);
534 }
535
536 If you then call this function twice, like this:
537
538 reserve (10);
539 reserve (1);
540
541 Then the compiler I<might> be able to optimise out the second call
542 completely, as it knows that C<< current + 1 > end >> is false and the
543 call will never be executed.
544
545 =item ecb_unreachable ()
546
547 This function does nothing itself, except tell the compiler that it will
548 never be executed. Apart from suppressing a warning in some cases, this
549 function can be used to implement C<ecb_assume> or similar functionality.
550
551 =item ecb_prefetch (addr, rw, locality)
552
553 Tells the compiler to try to prefetch memory at the given C<addr>ess
554 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
555 C<0> means that there will only be one access later, C<3> means that
556 the data will likely be accessed very often, and values in between mean
557 something... in between. The memory pointed to by the address does not
558 need to be accessible (it could be a null pointer for example), but C<rw>
559 and C<locality> must be compile-time constants.
560
561 This is a statement, not a function: you cannot use it as part of an
562 expression.
563
564 An obvious way to use this is to prefetch some data far away, in a big
565 array you loop over. This prefetches memory some 128 array elements later,
566 in the hope that it will be ready when the CPU arrives at that location.
567
568 int sum = 0;
569
570 for (i = 0; i < N; ++i)
571 {
572 sum += arr [i]
573 ecb_prefetch (arr + i + 128, 0, 0);
574 }
575
576 It's hard to predict how far to prefetch, and most CPUs that can prefetch
577 are often good enough to predict this kind of behaviour themselves. It
578 gets more interesting with linked lists, especially when you do some fair
579 processing on each list element:
580
581 for (node *n = start; n; n = n->next)
582 {
583 ecb_prefetch (n->next, 0, 0);
584 ... do medium amount of work with *n
585 }
586
587 After processing the node, (part of) the next node might already be in
588 cache.
589
590 =back
591
592 =head2 BIT FIDDLING / BIT WIZARDRY
593
594 =over
595
596 =item bool ecb_big_endian ()
597
598 =item bool ecb_little_endian ()
599
600 These two functions return true if the byte order is big endian
601 (most-significant byte first) or little endian (least-significant byte
602 first) respectively.
603
604 On systems that are neither, their return values are unspecified.
605
606 =item int ecb_ctz32 (uint32_t x)
607
608 =item int ecb_ctz64 (uint64_t x)
609
610 =item int ecb_ctz (T x) [C++]
611
612 Returns the index of the least significant bit set in C<x> (or
613 equivalently the number of bits set to 0 before the least significant bit
614 set), starting from 0. If C<x> is 0 the result is undefined.
615
616 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
617
618 The overloaded C++ C<ecb_ctz> function supports C<uint8_t>, C<uint16_t>,
619 C<uint32_t> and C<uint64_t> types.
620
621 For example:
622
623 ecb_ctz32 (3) = 0
624 ecb_ctz32 (6) = 1
625
626 =item bool ecb_is_pot32 (uint32_t x)
627
628 =item bool ecb_is_pot64 (uint32_t x)
629
630 =item bool ecb_is_pot (T x) [C++]
631
632 Returns true iff C<x> is a power of two or C<x == 0>.
633
634 For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
635
636 The overloaded C++ C<ecb_is_pot> function supports C<uint8_t>, C<uint16_t>,
637 C<uint32_t> and C<uint64_t> types.
638
639 =item int ecb_ld32 (uint32_t x)
640
641 =item int ecb_ld64 (uint64_t x)
642
643 =item int ecb_ld64 (T x) [C++]
644
645 Returns the index of the most significant bit set in C<x>, or the number
646 of digits the number requires in binary (so that C<< 2**ld <= x <
647 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
648 to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
649 example to see how many bits a certain number requires to be encoded.
650
651 This function is similar to the "count leading zero bits" function, except
652 that that one returns how many zero bits are "in front" of the number (in
653 the given data type), while C<ecb_ld> returns how many bits the number
654 itself requires.
655
656 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
657
658 The overloaded C++ C<ecb_ld> function supports C<uint8_t>, C<uint16_t>,
659 C<uint32_t> and C<uint64_t> types.
660
661 =item int ecb_popcount32 (uint32_t x)
662
663 =item int ecb_popcount64 (uint64_t x)
664
665 =item int ecb_popcount (T x) [C++]
666
667 Returns the number of bits set to 1 in C<x>.
668
669 For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
670
671 The overloaded C++ C<ecb_popcount> function supports C<uint8_t>, C<uint16_t>,
672 C<uint32_t> and C<uint64_t> types.
673
674 For example:
675
676 ecb_popcount32 (7) = 3
677 ecb_popcount32 (255) = 8
678
679 =item uint8_t ecb_bitrev8 (uint8_t x)
680
681 =item uint16_t ecb_bitrev16 (uint16_t x)
682
683 =item uint32_t ecb_bitrev32 (uint32_t x)
684
685 =item T ecb_bitrev (T x) [C++]
686
687 Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
688 and so on.
689
690 The overloaded C++ C<ecb_bitrev> function supports C<uint8_t>, C<uint16_t> and C<uint32_t> types.
691
692 Example:
693
694 ecb_bitrev8 (0xa7) = 0xea
695 ecb_bitrev32 (0xffcc4411) = 0x882233ff
696
697 =item T ecb_bitrev (T x) [C++]
698
699 Overloaded C++ bitrev function.
700
701 C<T> must be one of C<uint8_t>, C<uint16_t> or C<uint32_t>.
702
703 =item uint32_t ecb_bswap16 (uint32_t x)
704
705 =item uint32_t ecb_bswap32 (uint32_t x)
706
707 =item uint64_t ecb_bswap64 (uint64_t x)
708
709 =item T ecb_bswap (T x)
710
711 These functions return the value of the 16-bit (32-bit, 64-bit) value
712 C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
713 C<ecb_bswap32>).
714
715 The overloaded C++ C<ecb_bswap> function supports C<uint8_t>, C<uint16_t>,
716 C<uint32_t> and C<uint64_t> types.
717
718 =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
719
720 =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
721
722 =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
723
724 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
725
726 =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
727
728 =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
729
730 =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
731
732 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
733
734 These two families of functions return the value of C<x> after rotating
735 all the bits by C<count> positions to the right (C<ecb_rotr>) or left
736 (C<ecb_rotl>).
737
738 Current GCC/clang versions understand these functions and usually compile
739 them to "optimal" code (e.g. a single C<rol> or a combination of C<shld>
740 on x86).
741
742 =item T ecb_rotl (T x, unsigned int count) [C++]
743
744 =item T ecb_rotr (T x, unsigned int count) [C++]
745
746 Overloaded C++ rotl/rotr functions.
747
748 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
749
750 =back
751
752 =head2 HOST ENDIANNESS CONVERSION
753
754 =over
755
756 =item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v)
757
758 =item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v)
759
760 =item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v)
761
762 =item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v)
763
764 =item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v)
765
766 =item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v)
767
768 Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order.
769
770 The naming convention is C<ecb_>(C<be>|C<le>)C<_u>C<16|32|64>C<_to_host>,
771 where C<be> and C<le> stand for big endian and little endian, respectively.
772
773 =item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v)
774
775 =item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v)
776
777 =item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v)
778
779 =item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v)
780
781 =item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v)
782
783 =item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v)
784
785 Like above, but converts I<from> host byte order to the specified
786 endianness.
787
788 =back
789
790 In C++ the following additional template functions are supported:
791
792 =over
793
794 =item T ecb_be_to_host (T v)
795
796 =item T ecb_le_to_host (T v)
797
798 =item T ecb_host_to_be (T v)
799
800 =item T ecb_host_to_le (T v)
801
802 =back
803
804 These functions work like their C counterparts, above, but use templates,
805 which make them useful in generic code.
806
807 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>
808 (so unlike their C counterparts, there is a version for C<uint8_t>, which
809 again can be useful in generic code).
810
811 =head2 UNALIGNED LOAD/STORE
812
813 These function load or store unaligned multi-byte values.
814
815 =over
816
817 =item uint_fast16_t ecb_peek_u16_u (const void *ptr)
818
819 =item uint_fast32_t ecb_peek_u32_u (const void *ptr)
820
821 =item uint_fast64_t ecb_peek_u64_u (const void *ptr)
822
823 These functions load an unaligned, unsigned 16, 32 or 64 bit value from
824 memory.
825
826 =item uint_fast16_t ecb_peek_be_u16_u (const void *ptr)
827
828 =item uint_fast32_t ecb_peek_be_u32_u (const void *ptr)
829
830 =item uint_fast64_t ecb_peek_be_u64_u (const void *ptr)
831
832 =item uint_fast16_t ecb_peek_le_u16_u (const void *ptr)
833
834 =item uint_fast32_t ecb_peek_le_u32_u (const void *ptr)
835
836 =item uint_fast64_t ecb_peek_le_u64_u (const void *ptr)
837
838 Like above, but additionally convert from big endian (C<be>) or little
839 endian (C<le>) byte order to host byte order while doing so.
840
841 =item ecb_poke_u16_u (void *ptr, uint16_t v)
842
843 =item ecb_poke_u32_u (void *ptr, uint32_t v)
844
845 =item ecb_poke_u64_u (void *ptr, uint64_t v)
846
847 These functions store an unaligned, unsigned 16, 32 or 64 bit value to
848 memory.
849
850 =item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v)
851
852 =item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v)
853
854 =item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v)
855
856 =item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v)
857
858 =item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v)
859
860 =item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v)
861
862 Like above, but additionally convert from host byte order to big endian
863 (C<be>) or little endian (C<le>) byte order while doing so.
864
865 =back
866
867 In C++ the following additional template functions are supported:
868
869 =over
870
871 =item T ecb_peek<T> (const void *ptr)
872
873 =item T ecb_peek_be<T> (const void *ptr)
874
875 =item T ecb_peek_le<T> (const void *ptr)
876
877 =item T ecb_peek_u<T> (const void *ptr)
878
879 =item T ecb_peek_be_u<T> (const void *ptr)
880
881 =item T ecb_peek_le_u<T> (const void *ptr)
882
883 Similarly to their C counterparts, these functions load an unsigned 8, 16,
884 32 or 64 bit value from memory, with optional conversion from big/little
885 endian.
886
887 Since the type cannot be deduced, it has to be specified explicitly, e.g.
888
889 uint_fast16_t v = ecb_peek<uint16_t> (ptr);
890
891 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
892
893 Unlike their C counterparts, these functions support 8 bit quantities
894 (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
895 all of which hopefully makes them more useful in generic code.
896
897 =item ecb_poke (void *ptr, T v)
898
899 =item ecb_poke_be (void *ptr, T v)
900
901 =item ecb_poke_le (void *ptr, T v)
902
903 =item ecb_poke_u (void *ptr, T v)
904
905 =item ecb_poke_be_u (void *ptr, T v)
906
907 =item ecb_poke_le_u (void *ptr, T v)
908
909 Again, similarly to their C counterparts, these functions store an
910 unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to
911 big/little endian.
912
913 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
914
915 Unlike their C counterparts, these functions support 8 bit quantities
916 (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
917 all of which hopefully makes them more useful in generic code.
918
919 =back
920
921 =head2 FAST INTEGER TO STRING
922
923 Libecb defines a set of very fast integer to decimal string (or integer
924 to ascii, short C<i2a>) functions. These work by converting the integer
925 to a fixed point representation and then successively multiplying out
926 the topmost digits. Unlike some other, also very fast, libraries, ecb's
927 algorithm should be completely branchless per digit, and does not rely on
928 the presence of special cpu functions (such as clz).
929
930 There is a high level API that takes an C<int32_t>, C<uint32_t>,
931 C<int64_t> or C<uint64_t> as argument, and a low-level API, which is
932 harder to use but supports slightly more formatting options.
933
934 =head3 HIGH LEVEL API
935
936 The high level API consists of four functions, one each for C<int32_t>,
937 C<uint32_t>, C<int64_t> and C<uint64_t>:
938
939 =over
940
941 =item ECB_I2A_I32_DIGITS (=11)
942
943 =item char *ecb_i2a_u32 (char *ptr, uint32_t value)
944
945 Takes an C<uint32_t> I<value> and formats it as a decimal number starting
946 at I<ptr>, using at most C<ECB_I2A_I32_DIGITS> characters. Returns a
947 pointer to just after the generated string, where you would normally put
948 the temrinating C<0> character. This function outputs the minimum number
949 of digits.
950
951 =item ECB_I2A_U32_DIGITS (=10)
952
953 =item char *ecb_i2a_i32 (char *ptr, int32_t value)
954
955 Same as C<ecb_i2a_u32>, but formats a C<int32_t> value, including a minus
956 sign if needed.
957
958 =item ECB_I2A_I64_DIGITS (=20)
959
960 =item char *ecb_i2a_u64 (char *ptr, uint64_t value)
961
962 =item ECB_I2A_U64_DIGITS (=21)
963
964 =item char *ecb_i2a_i64 (char *ptr, int64_t value)
965
966 Similar to their 32 bit counterparts, these take a 64 bit argument.
967
968 =item ECB_I2A_MAX_DIGITS (=21)
969
970 Instead of using a type specific length macro, youi can just use
971 C<ECB_I2A_MAX_DIGITS>, which is good enough for any C<ecb_i2a> function.
972
973 =back
974
975 =head3 LOW-LEVEL API
976
977 The functions above use a number of low-level APIs which have some strict
978 limitaitons, but cna be used as building blocks (study of C<ecb_i2a_i32>
979 and related cunctions is recommended).
980
981 There are three families of functions: functions that convert a number
982 to a fixed number of digits with leading zeroes (C<ecb_i2a_0N>, C<0>
983 for "leading zeroes"), functions that generate up to N digits, skipping
984 leading zeroes (C<_N>), and functions that can generate more digits, but
985 the leading digit has limited range (C<_xN>).
986
987 None of the functions deal with negative numbera.
988
989 =over
990
991 =item char *ecb_i2a_02 (char *ptr, uint32_t value) // 32 bit
992
993 =item char *ecb_i2a_03 (char *ptr, uint32_t value) // 32 bit
994
995 =item char *ecb_i2a_04 (char *ptr, uint32_t value) // 32 bit
996
997 =item char *ecb_i2a_05 (char *ptr, uint32_t value) // 64 bit
998
999 =item char *ecb_i2a_06 (char *ptr, uint32_t value) // 64 bit
1000
1001 =item char *ecb_i2a_07 (char *ptr, uint32_t value) // 64 bit
1002
1003 =item char *ecb_i2a_08 (char *ptr, uint32_t value) // 64 bit
1004
1005 =item char *ecb_i2a_09 (char *ptr, uint32_t value) // 64 bit
1006
1007 The C<< ecb_i2a_0I<N> > functions take an unsigned I<value> and convert
1008 them to exactly I<N> digits, returning a pointer to the first character
1009 after the digits. The I<value> must be in range. The functions marked with
1010 I<32 bit> do their calculations internally in 32 bit, the ones marked with
1011 I<64 bit> internally use 64 bit integers, which might be slow on 32 bit
1012 architectures (the high level API decides on 32 vs. 64 bit versions using
1013 C<ECB_64BIT_NATIVE>).
1014
1015 =item char *ecb_i2a_2 (char *ptr, uint32_t value) // 32 bit
1016
1017 =item char *ecb_i2a_3 (char *ptr, uint32_t value) // 32 bit
1018
1019 =item char *ecb_i2a_4 (char *ptr, uint32_t value) // 32 bit
1020
1021 =item char *ecb_i2a_5 (char *ptr, uint32_t value) // 64 bit
1022
1023 =item char *ecb_i2a_6 (char *ptr, uint32_t value) // 64 bit
1024
1025 =item char *ecb_i2a_7 (char *ptr, uint32_t value) // 64 bit
1026
1027 =item char *ecb_i2a_8 (char *ptr, uint32_t value) // 64 bit
1028
1029 =item char *ecb_i2a_9 (char *ptr, uint32_t value) // 64 bit
1030
1031 Similarly, the C<< ecb_i2a_I<N> > functions take an unsigned I<value>
1032 and convert them to at most I<N> digits, suppressing leading zeroes, and
1033 returning a pointer to the first character after the digits.
1034
1035 =item ECB_I2A_MAX_X5 (=59074)
1036
1037 =item char *ecb_i2a_x5 (char *ptr, uint32_t value) // 32 bit
1038
1039 =item ECB_I2A_MAX_X10 (=2932500665)
1040
1041 =item char *ecb_i2a_x10 (char *ptr, uint32_t value) // 64 bit
1042
1043 The C<< ecb_i2a_xI<N> >> functions are similar to the C<< ecb_i2a_I<N> >
1044 functions, but they can generate one digit more, as long as the number
1045 is within range, which is given by the symbols C<ECB_I2A_MAX_X5> (almost
1046 16 bit range) and C<ECB_I2A_MAX_X10> (a bit more than 31 bit range),
1047 respectively.
1048
1049 For example, the sigit part of a 32 bit signed integer just fits into the
1050 C<ECB_I2A_MAX_X10> range, so while C<ecb_i2a_x10> cannot convert a 10
1051 digit number, it can convert all 32 bit signed numbers. Sadly, it's not
1052 good enough for 32 bit unsigned numbers.
1053
1054 =back
1055
1056 =head2 FLOATING POINT FIDDLING
1057
1058 =over
1059
1060 =item ECB_INFINITY [-UECB_NO_LIBM]
1061
1062 Evaluates to positive infinity if supported by the platform, otherwise to
1063 a truly huge number.
1064
1065 =item ECB_NAN [-UECB_NO_LIBM]
1066
1067 Evaluates to a quiet NAN if supported by the platform, otherwise to
1068 C<ECB_INFINITY>.
1069
1070 =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
1071
1072 Same as C<ldexpf>, but always available.
1073
1074 =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
1075
1076 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
1077
1078 =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
1079
1080 These functions each take an argument in the native C<float> or C<double>
1081 type and return the IEEE 754 bit representation of it (binary16/half,
1082 binary32/single or binary64/double precision).
1083
1084 The bit representation is just as IEEE 754 defines it, i.e. the sign bit
1085 will be the most significant bit, followed by exponent and mantissa.
1086
1087 This function should work even when the native floating point format isn't
1088 IEEE compliant, of course at a speed and code size penalty, and of course
1089 also within reasonable limits (it tries to convert NaNs, infinities and
1090 denormals, but will likely convert negative zero to positive zero).
1091
1092 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1093 be able to optimise away this function completely.
1094
1095 These functions can be helpful when serialising floats to the network - you
1096 can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
1097
1098 Another use for these functions is to manipulate floating point values
1099 directly.
1100
1101 Silly example: toggle the sign bit of a float.
1102
1103 /* On gcc-4.7 on amd64, */
1104 /* this results in a single add instruction to toggle the bit, and 4 extra */
1105 /* instructions to move the float value to an integer register and back. */
1106
1107 x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
1108
1109 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
1110
1111 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
1112
1113 =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
1114
1115 The reverse operation of the previous function - takes the bit
1116 representation of an IEEE binary16, binary32 or binary64 number (half,
1117 single or double precision) and converts it to the native C<float> or
1118 C<double> format.
1119
1120 This function should work even when the native floating point format isn't
1121 IEEE compliant, of course at a speed and code size penalty, and of course
1122 also within reasonable limits (it tries to convert normals and denormals,
1123 and might be lucky for infinities, and with extraordinary luck, also for
1124 negative zero).
1125
1126 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1127 be able to optimise away this function completely.
1128
1129 =item uint16_t ecb_binary32_to_binary16 (uint32_t x)
1130
1131 =item uint32_t ecb_binary16_to_binary32 (uint16_t x)
1132
1133 Convert a IEEE binary32/single precision to binary16/half format, and vice
1134 versa, handling all details (round-to-nearest-even, subnormals, infinity
1135 and NaNs) correctly.
1136
1137 These are functions are available under C<-DECB_NO_LIBM>, since
1138 they do not rely on the platform floating point format. The
1139 C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
1140 usually what you want.
1141
1142 =back
1143
1144 =head2 ARITHMETIC
1145
1146 =over
1147
1148 =item x = ecb_mod (m, n)
1149
1150 Returns C<m> modulo C<n>, which is the same as the positive remainder
1151 of the division operation between C<m> and C<n>, using floored
1152 division. Unlike the C remainder operator C<%>, this function ensures that
1153 the return value is always positive and that the two numbers I<m> and
1154 I<m' = m + i * n> result in the same value modulo I<n> - in other words,
1155 C<ecb_mod> implements the mathematical modulo operation, which is missing
1156 in the language.
1157
1158 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
1159 negatable, that is, both C<m> and C<-m> must be representable in its
1160 type (this typically excludes the minimum signed integer value, the same
1161 limitation as for C</> and C<%> in C).
1162
1163 Current GCC/clang versions compile this into an efficient branchless
1164 sequence on almost all CPUs.
1165
1166 For example, when you want to rotate forward through the members of an
1167 array for increasing C<m> (which might be negative), then you should use
1168 C<ecb_mod>, as the C<%> operator might give either negative results, or
1169 change direction for negative values:
1170
1171 for (m = -100; m <= 100; ++m)
1172 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
1173
1174 =item x = ecb_div_rd (val, div)
1175
1176 =item x = ecb_div_ru (val, div)
1177
1178 Returns C<val> divided by C<div> rounded down or up, respectively.
1179 C<val> and C<div> must have integer types and C<div> must be strictly
1180 positive. Note that these functions are implemented with macros in C
1181 and with function templates in C++.
1182
1183 =back
1184
1185 =head2 UTILITY
1186
1187 =over
1188
1189 =item element_count = ecb_array_length (name)
1190
1191 Returns the number of elements in the array C<name>. For example:
1192
1193 int primes[] = { 2, 3, 5, 7, 11 };
1194 int sum = 0;
1195
1196 for (i = 0; i < ecb_array_length (primes); i++)
1197 sum += primes [i];
1198
1199 =back
1200
1201 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
1202
1203 These symbols need to be defined before including F<ecb.h> the first time.
1204
1205 =over
1206
1207 =item ECB_NO_THREADS
1208
1209 If F<ecb.h> is never used from multiple threads, then this symbol can
1210 be defined, in which case memory fences (and similar constructs) are
1211 completely removed, leading to more efficient code and fewer dependencies.
1212
1213 Setting this symbol to a true value implies C<ECB_NO_SMP>.
1214
1215 =item ECB_NO_SMP
1216
1217 The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
1218 multiple threads, but never concurrently (e.g. if the system the program
1219 runs on has only a single CPU with a single core, no hyperthreading and so
1220 on), then this symbol can be defined, leading to more efficient code and
1221 fewer dependencies.
1222
1223 =item ECB_NO_LIBM
1224
1225 When defined to C<1>, do not export any functions that might introduce
1226 dependencies on the math library (usually called F<-lm>) - these are
1227 marked with [-UECB_NO_LIBM].
1228
1229 =back
1230
1231 =head1 UNDOCUMENTED FUNCTIONALITY
1232
1233 F<ecb.h> is full of undocumented functionality as well, some of which is
1234 intended to be internal-use only, some of which we forgot to document, and
1235 some of which we hide because we are not sure we will keep the interface
1236 stable.
1237
1238 While you are welcome to rummage around and use whatever you find useful
1239 (we can't stop you), keep in mind that we will change undocumented
1240 functionality in incompatible ways without thinking twice, while we are
1241 considerably more conservative with documented things.
1242
1243 =head1 AUTHORS
1244
1245 C<libecb> is designed and maintained by:
1246
1247 Emanuele Giaquinta <e.giaquinta@glauco.it>
1248 Marc Alexander Lehmann <schmorp@schmorp.de>
1249
1250