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Revision 1.38 by sf-exg, Thu Aug 25 16:06:08 2011 UTC vs.
Revision 1.75 by root, Sat Dec 28 08:01:05 2019 UTC

54only a generic name is used (C<expr>, C<cond>, C<value> and so on), then 54only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
55the corresponding function relies on C to implement the correct types, and 55the corresponding function relies on C to implement the correct types, and
56is usually implemented as a macro. Specifically, a "bool" in this manual 56is usually implemented as a macro. Specifically, a "bool" in this manual
57refers to any kind of boolean value, not a specific type. 57refers to any kind of boolean value, not a specific type.
58 58
59=head2 TYPES / TYPE SUPPORT
60
61ecb.h makes sure that the following types are defined (in the expected way):
62
63 int8_t uint8_t int16_t uint16_t
64 int32_t uint32_t int64_t uint64_t
65 intptr_t uintptr_t
66
67The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
68platform (currently C<4> or C<8>) and can be used in preprocessor
69expressions.
70
71For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>.
72
73=head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
74
75All the following symbols expand to an expression that can be tested in
76preprocessor instructions as well as treated as a boolean (use C<!!> to
77ensure it's either C<0> or C<1> if you need that).
78
79=over 4
80
81=item ECB_C
82
83True if the implementation defines the C<__STDC__> macro to a true value,
84while not claiming to be C++.
85
86=item ECB_C99
87
88True if the implementation claims to be compliant to C99 (ISO/IEC
899899:1999) or any later version, while not claiming to be C++.
90
91Note that later versions (ECB_C11) remove core features again (for
92example, variable length arrays).
93
94=item ECB_C11, ECB_C17
95
96True if the implementation claims to be compliant to C11/C17 (ISO/IEC
979899:2011, :20187) or any later version, while not claiming to be C++.
98
99=item ECB_CPP
100
101True if the implementation defines the C<__cplusplus__> macro to a true
102value, which is typically true for C++ compilers.
103
104=item ECB_CPP11, ECB_CPP14, ECB_CPP17
105
106True if the implementation claims to be compliant to C++11/C++14/C++17
107(ISO/IEC 14882:2011, :2014, :2017) or any later version.
108
109=item ECB_GCC_VERSION (major, minor)
110
111Expands to a true value (suitable for testing in by the preprocessor)
112if the compiler used is GNU C and the version is the given version, or
113higher.
114
115This macro tries to return false on compilers that claim to be GCC
116compatible but aren't.
117
118=item ECB_EXTERN_C
119
120Expands to C<extern "C"> in C++, and a simple C<extern> in C.
121
122This 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
128These two macros can be used to wrap multiple C<extern "C"> definitions -
129they expand to nothing in C.
130
131They 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
142If this evaluates to a true value (suitable for testing in by the
143preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
144and double/binary64 representations internally I<and> the endianness of
145both types match the endianness of C<uint32_t> and C<uint64_t>.
146
147This means you can just copy the bits of a C<float> (or C<double>) to an
148C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
149without having to think about format or endianness.
150
151This is true for basically all modern platforms, although F<ecb.h> might
152not be able to deduce this correctly everywhere and might err on the safe
153side.
154
155=item ECB_AMD64, ECB_AMD64_X32
156
157These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
158ABI, respectively, and undefined elsewhere.
159
160The designers of the new X32 ABI for some inexplicable reason decided to
161make it look exactly like amd64, even though it's completely incompatible
162to that ABI, breaking about every piece of software that assumed that
163C<__x86_64> stands for, well, the x86-64 ABI, making these macros
164necessary.
165
166=back
167
168=head2 MACRO TRICKERY
169
170=over 4
171
172=item ECB_CONCAT (a, b)
173
174Expands any macros in C<a> and C<b>, then concatenates the result to form
175a single token. This is mainly useful to form identifiers from components,
176e.g.:
177
178 #define S1 str
179 #define S2 cpy
180
181 ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
182
183=item ECB_STRINGIFY (arg)
184
185Expands any macros in C<arg> and returns the stringified version of
186it. This is mainly useful to get the contents of a macro in string form,
187e.g.:
188
189 #define SQL_LIMIT 100
190 sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
191
192=item ECB_STRINGIFY_EXPR (expr)
193
194Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
195is a valid expression. This is useful to catch typos or cases where the
196macro isn't available:
197
198 #include <errno.h>
199
200 ECB_STRINGIFY (EDOM); // "33" (on my system at least)
201 ECB_STRINGIFY_EXPR (EDOM); // "33"
202
203 // now imagine we had a typo:
204
205 ECB_STRINGIFY (EDAM); // "EDAM"
206 ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
207
208=back
209
59=head2 GCC ATTRIBUTES 210=head2 ATTRIBUTES
60 211
61A major part of libecb deals with GCC attributes. These are additional 212A major part of libecb deals with additional attributes that can be
62attributes that you can assign to functions, variables and sometimes even 213assigned to functions, variables and sometimes even types - much like
63types - much like C<const> or C<volatile> in C. 214C<const> or C<volatile> in C. They are implemented using either GCC
64 215attributes or other compiler/language specific features. Attributes
65While GCC allows declarations to show up in many surprising places,
66but not in many expected places, the safest way is to put attribute
67declarations before the whole declaration: 216declarations must be put before the whole declaration:
68 217
69 ecb_const int mysqrt (int a); 218 ecb_const int mysqrt (int a);
70 ecb_unused int i; 219 ecb_unused int i;
71 220
72For variables, it is often nicer to put the attribute after the name, and
73avoid multiple declarations using commas:
74
75 int i ecb_unused;
76
77=over 4 221=over 4
78
79=item ecb_attribute ((attrs...))
80
81A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
82nothing on other compilers, so the effect is that only GCC sees these.
83
84Example: use the C<deprecated> attribute on a function.
85
86 ecb_attribute((__deprecated__)) void
87 do_not_use_me_anymore (void);
88 222
89=item ecb_unused 223=item ecb_unused
90 224
91Marks a function or a variable as "unused", which simply suppresses a 225Marks a function or a variable as "unused", which simply suppresses a
92warning by GCC when it detects it as unused. This is useful when you e.g. 226warning by GCC when it detects it as unused. This is useful when you e.g.
93declare a variable but do not always use it: 227declare a variable but do not always use it:
94 228
95 { 229 {
96 int var ecb_unused; 230 ecb_unused int var;
97 231
98 #ifdef SOMECONDITION 232 #ifdef SOMECONDITION
99 var = ...; 233 var = ...;
100 return var; 234 return var;
101 #else 235 #else
102 return 0; 236 return 0;
103 #endif 237 #endif
104 } 238 }
105 239
240=item ecb_deprecated
241
242Similar to C<ecb_unused>, but marks a function, variable or type as
243deprecated. This makes some compilers warn when the type is used.
244
245=item ecb_deprecated_message (message)
246
247Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
248used instead of a generic depreciation message when the object is being
249used.
250
106=item ecb_inline 251=item ecb_inline
107 252
108This is not actually an attribute, but you use it like one. It expands 253Expands either to (a compiler-specific equivalent of) C<static inline> or
109either to C<static inline> or to just C<static>, if inline isn't 254to just C<static>, if inline isn't supported. It should be used to declare
110supported. It should be used to declare functions that should be inlined, 255functions that should be inlined, for code size or speed reasons.
111for code size or speed reasons.
112 256
113Example: inline this function, it surely will reduce codesize. 257Example: inline this function, it surely will reduce codesize.
114 258
115 ecb_inline int 259 ecb_inline int
116 negmul (int a, int b) 260 negmul (int a, int b)
118 return - (a * b); 262 return - (a * b);
119 } 263 }
120 264
121=item ecb_noinline 265=item ecb_noinline
122 266
123Prevent a function from being inlined - it might be optimised away, but 267Prevents a function from being inlined - it might be optimised away, but
124not inlined into other functions. This is useful if you know your function 268not inlined into other functions. This is useful if you know your function
125is rarely called and large enough for inlining not to be helpful. 269is rarely called and large enough for inlining not to be helpful.
126 270
127=item ecb_noreturn 271=item ecb_noreturn
128 272
138 } 282 }
139 283
140In this case, the compiler would probably be smart enough to deduce it on 284In this case, the compiler would probably be smart enough to deduce it on
141its own, so this is mainly useful for declarations. 285its own, so this is mainly useful for declarations.
142 286
287=item ecb_restrict
288
289Expands to the C<restrict> keyword or equivalent on compilers that support
290them, and to nothing on others. Must be specified on a pointer type or
291an array index to indicate that the memory doesn't alias with any other
292restricted pointer in the same scope.
293
294Example: multiply a vector, and allow the compiler to parallelise the
295loop, because it knows it doesn't overwrite input values.
296
297 void
298 multiply (ecb_restrict float *src,
299 ecb_restrict float *dst,
300 int len, float factor)
301 {
302 int i;
303
304 for (i = 0; i < len; ++i)
305 dst [i] = src [i] * factor;
306 }
307
143=item ecb_const 308=item ecb_const
144 309
145Declares that the function only depends on the values of its arguments, 310Declares that the function only depends on the values of its arguments,
146much like a mathematical function. It specifically does not read or write 311much like a mathematical function. It specifically does not read or write
147any memory any arguments might point to, global variables, or call any 312any memory any arguments might point to, global variables, or call any
207functions only called in exceptional or rare cases. 372functions only called in exceptional or rare cases.
208 373
209=item ecb_artificial 374=item ecb_artificial
210 375
211Declares the function as "artificial", in this case meaning that this 376Declares the function as "artificial", in this case meaning that this
212function is not really mean to be a function, but more like an accessor 377function is not really meant to be a function, but more like an accessor
213- many methods in C++ classes are mere accessor functions, and having a 378- many methods in C++ classes are mere accessor functions, and having a
214crash reported in such a method, or single-stepping through them, is not 379crash reported in such a method, or single-stepping through them, is not
215usually so helpful, especially when it's inlined to just a few instructions. 380usually so helpful, especially when it's inlined to just a few instructions.
216 381
217Marking them as artificial will instruct the debugger about just this, 382Marking them as artificial will instruct the debugger about just this,
237 402
238=head2 OPTIMISATION HINTS 403=head2 OPTIMISATION HINTS
239 404
240=over 4 405=over 4
241 406
407=item ECB_OPTIMIZE_SIZE
408
409Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
410can also be defined before including F<ecb.h>, in which case it will be
411unchanged.
412
242=item bool ecb_is_constant(expr) 413=item bool ecb_is_constant (expr)
243 414
244Returns true iff the expression can be deduced to be a compile-time 415Returns true iff the expression can be deduced to be a compile-time
245constant, and false otherwise. 416constant, and false otherwise.
246 417
247For example, when you have a C<rndm16> function that returns a 16 bit 418For example, when you have a C<rndm16> function that returns a 16 bit
265 return is_constant (n) && !(n & (n - 1)) 436 return is_constant (n) && !(n & (n - 1))
266 ? rndm16 () & (num - 1) 437 ? rndm16 () & (num - 1)
267 : (n * (uint32_t)rndm16 ()) >> 16; 438 : (n * (uint32_t)rndm16 ()) >> 16;
268 } 439 }
269 440
270=item bool ecb_expect (expr, value) 441=item ecb_expect (expr, value)
271 442
272Evaluates C<expr> and returns it. In addition, it tells the compiler that 443Evaluates C<expr> and returns it. In addition, it tells the compiler that
273the C<expr> evaluates to C<value> a lot, which can be used for static 444the C<expr> evaluates to C<value> a lot, which can be used for static
274branch optimisations. 445branch optimisations.
275 446
322 { 493 {
323 if (ecb_expect_false (current + size > end)) 494 if (ecb_expect_false (current + size > end))
324 real_reserve_method (size); /* presumably noinline */ 495 real_reserve_method (size); /* presumably noinline */
325 } 496 }
326 497
327=item bool ecb_assume (cond) 498=item ecb_assume (cond)
328 499
329Try to tell the compiler that some condition is true, even if it's not 500Tries to tell the compiler that some condition is true, even if it's not
330obvious. 501obvious. This is not a function, but a statement: it cannot be used in
502another expression.
331 503
332This can be used to teach the compiler about invariants or other 504This can be used to teach the compiler about invariants or other
333conditions that might improve code generation, but which are impossible to 505conditions that might improve code generation, but which are impossible to
334deduce form the code itself. 506deduce form the code itself.
335 507
352 524
353Then the compiler I<might> be able to optimise out the second call 525Then the compiler I<might> be able to optimise out the second call
354completely, as it knows that C<< current + 1 > end >> is false and the 526completely, as it knows that C<< current + 1 > end >> is false and the
355call will never be executed. 527call will never be executed.
356 528
357=item bool ecb_unreachable () 529=item ecb_unreachable ()
358 530
359This function does nothing itself, except tell the compiler that it will 531This function does nothing itself, except tell the compiler that it will
360never be executed. Apart from suppressing a warning in some cases, this 532never be executed. Apart from suppressing a warning in some cases, this
361function can be used to implement C<ecb_assume> or similar functions. 533function can be used to implement C<ecb_assume> or similar functionality.
362 534
363=item bool ecb_prefetch (addr, rw, locality) 535=item ecb_prefetch (addr, rw, locality)
364 536
365Tells the compiler to try to prefetch memory at the given C<addr>ess 537Tells the compiler to try to prefetch memory at the given C<addr>ess
366for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of 538for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
367C<0> means that there will only be one access later, C<3> means that 539C<0> means that there will only be one access later, C<3> means that
368the data will likely be accessed very often, and values in between mean 540the data will likely be accessed very often, and values in between mean
369something... in between. The memory pointed to by the address does not 541something... in between. The memory pointed to by the address does not
370need to be accessible (it could be a null pointer for example), but C<rw> 542need to be accessible (it could be a null pointer for example), but C<rw>
371and C<locality> must be compile-time constants. 543and C<locality> must be compile-time constants.
372 544
545This is a statement, not a function: you cannot use it as part of an
546expression.
547
373An obvious way to use this is to prefetch some data far away, in a big 548An obvious way to use this is to prefetch some data far away, in a big
374array you loop over. This prefetches memory some 128 array elements later, 549array you loop over. This prefetches memory some 128 array elements later,
375in the hope that it will be ready when the CPU arrives at that location. 550in the hope that it will be ready when the CPU arrives at that location.
376 551
377 int sum = 0; 552 int sum = 0;
425For example: 600For example:
426 601
427 ecb_ctz32 (3) = 0 602 ecb_ctz32 (3) = 0
428 ecb_ctz32 (6) = 1 603 ecb_ctz32 (6) = 1
429 604
605=item bool ecb_is_pot32 (uint32_t x)
606
607=item bool ecb_is_pot64 (uint32_t x)
608
609Returns true iff C<x> is a power of two or C<x == 0>.
610
611For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
612
430=item int ecb_ld32 (uint32_t x) 613=item int ecb_ld32 (uint32_t x)
431 614
432=item int ecb_ld64 (uint64_t x) 615=item int ecb_ld64 (uint64_t x)
433 616
434Returns the index of the most significant bit set in C<x>, or the number 617Returns the index of the most significant bit set in C<x>, or the number
455For example: 638For example:
456 639
457 ecb_popcount32 (7) = 3 640 ecb_popcount32 (7) = 3
458 ecb_popcount32 (255) = 8 641 ecb_popcount32 (255) = 8
459 642
643=item uint8_t ecb_bitrev8 (uint8_t x)
644
645=item uint16_t ecb_bitrev16 (uint16_t x)
646
647=item uint32_t ecb_bitrev32 (uint32_t x)
648
649Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
650and so on.
651
652Example:
653
654 ecb_bitrev8 (0xa7) = 0xea
655 ecb_bitrev32 (0xffcc4411) = 0x882233ff
656
460=item uint32_t ecb_bswap16 (uint32_t x) 657=item uint32_t ecb_bswap16 (uint32_t x)
461 658
462=item uint32_t ecb_bswap32 (uint32_t x) 659=item uint32_t ecb_bswap32 (uint32_t x)
463 660
464=item uint64_t ecb_bswap64 (uint64_t x) 661=item uint64_t ecb_bswap64 (uint64_t x)
488(C<ecb_rotl>). 685(C<ecb_rotl>).
489 686
490Current GCC versions understand these functions and usually compile them 687Current GCC versions understand these functions and usually compile them
491to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on 688to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
492x86). 689x86).
690
691=back
692
693=head2 FLOATING POINT FIDDLING
694
695=over 4
696
697=item ECB_INFINITY [-UECB_NO_LIBM]
698
699Evaluates to positive infinity if supported by the platform, otherwise to
700a truly huge number.
701
702=item ECB_NAN [-UECB_NO_LIBM]
703
704Evaluates to a quiet NAN if supported by the platform, otherwise to
705C<ECB_INFINITY>.
706
707=item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
708
709Same as C<ldexpf>, but always available.
710
711=item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
712
713=item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
714
715=item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
716
717These functions each take an argument in the native C<float> or C<double>
718type and return the IEEE 754 bit representation of it (binary16/half,
719binary32/single or binary64/double precision).
720
721The bit representation is just as IEEE 754 defines it, i.e. the sign bit
722will be the most significant bit, followed by exponent and mantissa.
723
724This function should work even when the native floating point format isn't
725IEEE compliant, of course at a speed and code size penalty, and of course
726also within reasonable limits (it tries to convert NaNs, infinities and
727denormals, but will likely convert negative zero to positive zero).
728
729On all modern platforms (where C<ECB_STDFP> is true), the compiler should
730be able to optimise away this function completely.
731
732These functions can be helpful when serialising floats to the network - you
733can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
734
735Another use for these functions is to manipulate floating point values
736directly.
737
738Silly example: toggle the sign bit of a float.
739
740 /* On gcc-4.7 on amd64, */
741 /* this results in a single add instruction to toggle the bit, and 4 extra */
742 /* instructions to move the float value to an integer register and back. */
743
744 x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
745
746=item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
747
748=item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
749
750=item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
751
752The reverse operation of the previous function - takes the bit
753representation of an IEEE binary16, binary32 or binary64 number (half,
754single or double precision) and converts it to the native C<float> or
755C<double> format.
756
757This function should work even when the native floating point format isn't
758IEEE compliant, of course at a speed and code size penalty, and of course
759also within reasonable limits (it tries to convert normals and denormals,
760and might be lucky for infinities, and with extraordinary luck, also for
761negative zero).
762
763On all modern platforms (where C<ECB_STDFP> is true), the compiler should
764be able to optimise away this function completely.
765
766=item uint16_t ecb_binary32_to_binary16 (uint32_t x)
767
768=item uint32_t ecb_binary16_to_binary32 (uint16_t x)
769
770Convert a IEEE binary32/single precision to binary16/half format, and vice
771versa, handling all details (round-to-nearest-even, subnormals, infinity
772and NaNs) correctly.
773
774These are functions are available under C<-DECB_NO_LIBM>, since
775they do not rely on the platform floating point format. The
776C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
777usually what you want.
493 778
494=back 779=back
495 780
496=head2 ARITHMETIC 781=head2 ARITHMETIC
497 782
548 for (i = 0; i < ecb_array_length (primes); i++) 833 for (i = 0; i < ecb_array_length (primes); i++)
549 sum += primes [i]; 834 sum += primes [i];
550 835
551=back 836=back
552 837
838=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
553 839
840These symbols need to be defined before including F<ecb.h> the first time.
841
842=over 4
843
844=item ECB_NO_THREADS
845
846If F<ecb.h> is never used from multiple threads, then this symbol can
847be defined, in which case memory fences (and similar constructs) are
848completely removed, leading to more efficient code and fewer dependencies.
849
850Setting this symbol to a true value implies C<ECB_NO_SMP>.
851
852=item ECB_NO_SMP
853
854The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
855multiple threads, but never concurrently (e.g. if the system the program
856runs on has only a single CPU with a single core, no hyperthreading and so
857on), then this symbol can be defined, leading to more efficient code and
858fewer dependencies.
859
860=item ECB_NO_LIBM
861
862When defined to C<1>, do not export any functions that might introduce
863dependencies on the math library (usually called F<-lm>) - these are
864marked with [-UECB_NO_LIBM].
865
866=back
867
868=head1 UNDOCUMENTED FUNCTIONALITY
869
870F<ecb.h> is full of undocumented functionality as well, some of which is
871intended to be internal-use only, some of which we forgot to document, and
872some of which we hide because we are not sure we will keep the interface
873stable.
874
875While you are welcome to rummage around and use whatever you find useful
876(we can't stop you), keep in mind that we will change undocumented
877functionality in incompatible ways without thinking twice, while we are
878considerably more conservative with documented things.
879
880=head1 AUTHORS
881
882C<libecb> is designed and maintained by:
883
884 Emanuele Giaquinta <e.giaquinta@glauco.it>
885 Marc Alexander Lehmann <schmorp@schmorp.de>
886
887

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