libecb/ecb.pod

=head1 LIBECB - e-C-Builtins

=head2 ABOUT LIBECB

Libecb is currently a simple header file that doesn't require any
configuration to use or include in your project.

It's part of the e-suite of libraries, other members of which include
libev and libeio.

Its homepage can be found here:

    http://software.schmorp.de/pkg/libecb

It mainly provides a number of wrappers around GCC built-ins, together
with replacement functions for other compilers. In addition to this,
it provides a number of other lowlevel C utilities, such as endianness
detection, byte swapping or bit rotations.

Or in other words, things that should be built into any standard C system,
but aren't, implemented as efficient as possible with GCC, and still
correct with other compilers.

More might come.

=head2 ABOUT THE HEADER

At the moment, all you have to do is copy F<ecb.h> somewhere where your
compiler can find it and include it:

   #include <ecb.h>

The header should work fine for both C and C++ compilation, and gives you
all of F<inttypes.h> in addition to the ECB symbols.

There are currently no object files to link to - future versions might
come with an (optional) object code library to link against, to reduce
code size or gain access to additional features.

It also currently includes everything from F<inttypes.h>.

=head2 ABOUT THIS MANUAL / CONVENTIONS

This manual mainly describes each (public) function available after
including the F<ecb.h> header. The header might define other symbols than
these, but these are not part of the public API, and not supported in any
way.

When the manual mentions a "function" then this could be defined either as
as inline function, a macro, or an external symbol.

When functions use a concrete standard type, such as C<int> or
C<uint32_t>, then the corresponding function works only with that type. If
only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
the corresponding function relies on C to implement the correct types, and
is usually implemented as a macro. Specifically, a "bool" in this manual
refers to any kind of boolean value, not a specific type.

=head2 TYPES / TYPE SUPPORT

ecb.h makes sure that the following types are defined (in the expected way):

   int8_t   uint8_t   int16_t  uint16_t
   int32_t  uint32_t  int64_t  uint64_t
   intptr_t uintptr_t ptrdiff_t

The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
platform (currently C<4> or C<8>).

=head2 GCC ATTRIBUTES

A major part of libecb deals with GCC attributes. These are additional
attributes that you can assign to functions, variables and sometimes even
types - much like C<const> or C<volatile> in C.

While GCC allows declarations to show up in many surprising places,
but not in many expected places, the safest way is to put attribute
declarations before the whole declaration:

   ecb_const int mysqrt (int a);
   ecb_unused int i;

For variables, it is often nicer to put the attribute after the name, and
avoid multiple declarations using commas:

   int i ecb_unused;

=over 4

=item ecb_attribute ((attrs...))

A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
nothing on other compilers, so the effect is that only GCC sees these.

Example: use the C<deprecated> attribute on a function.

  ecb_attribute((__deprecated__)) void
  do_not_use_me_anymore (void);

=item ecb_unused

Marks a function or a variable as "unused", which simply suppresses a
warning by GCC when it detects it as unused. This is useful when you e.g.
declare a variable but do not always use it:

  {
    int var ecb_unused;

    #ifdef SOMECONDITION
       var = ...;
       return var;
    #else
       return 0;
    #endif
  }

=item ecb_inline

This is not actually an attribute, but you use it like one. It expands
either to C<static inline> or to just C<static>, if inline isn't
supported. It should be used to declare functions that should be inlined,
for code size or speed reasons.

Example: inline this function, it surely will reduce codesize.

   ecb_inline int
   negmul (int a, int b)
   {
     return - (a * b);
   }

=item ecb_noinline

Prevent a function from being inlined - it might be optimised away, but
not inlined into other functions. This is useful if you know your function
is rarely called and large enough for inlining not to be helpful.

=item ecb_noreturn

Marks a function as "not returning, ever". Some typical functions that
don't return are C<exit> or C<abort> (which really works hard to not
return), and now you can make your own:

   ecb_noreturn void
   my_abort (const char *errline)
   {
     puts (errline);
     abort ();
   }

In this case, the compiler would probably be smart enough to deduce it on
its own, so this is mainly useful for declarations.

=item ecb_const

Declares that the function only depends on the values of its arguments,
much like a mathematical function. It specifically does not read or write
any memory any arguments might point to, global variables, or call any
non-const functions. It also must not have any side effects.

Such a function can be optimised much more aggressively by the compiler -
for example, multiple calls with the same arguments can be optimised into
a single call, which wouldn't be possible if the compiler would have to
expect any side effects.

It is best suited for functions in the sense of mathematical functions,
such as a function returning the square root of its input argument.

Not suited would be a function that calculates the hash of some memory
area you pass in, prints some messages or looks at a global variable to
decide on rounding.

See C<ecb_pure> for a slightly less restrictive class of functions.

=item ecb_pure

Similar to C<ecb_const>, declares a function that has no side
effects. Unlike C<ecb_const>, the function is allowed to examine global
variables and any other memory areas (such as the ones passed to it via
pointers).

While these functions cannot be optimised as aggressively as C<ecb_const>
functions, they can still be optimised away in many occasions, and the
compiler has more freedom in moving calls to them around.

Typical examples for such functions would be C<strlen> or C<memcmp>. A
function that calculates the MD5 sum of some input and updates some MD5
state passed as argument would I<NOT> be pure, however, as it would modify
some memory area that is not the return value.

=item ecb_hot

This declares a function as "hot" with regards to the cache - the function
is used so often, that it is very beneficial to keep it in the cache if
possible.

The compiler reacts by trying to place hot functions near to each other in
memory.

Whether a function is hot or not often depends on the whole program,
and less on the function itself. C<ecb_cold> is likely more useful in
practise.

=item ecb_cold

The opposite of C<ecb_hot> - declares a function as "cold" with regards to
the cache, or in other words, this function is not called often, or not at
speed-critical times, and keeping it in the cache might be a waste of said
cache.

In addition to placing cold functions together (or at least away from hot
functions), this knowledge can be used in other ways, for example, the
function will be optimised for size, as opposed to speed, and codepaths
leading to calls to those functions can automatically be marked as if
C<ecb_expect_false> had been used to reach them.

Good examples for such functions would be error reporting functions, or
functions only called in exceptional or rare cases.

=item ecb_artificial

Declares the function as "artificial", in this case meaning that this
function is not really mean to be a function, but more like an accessor
- many methods in C++ classes are mere accessor functions, and having a
crash reported in such a method, or single-stepping through them, is not
usually so helpful, especially when it's inlined to just a few instructions.

Marking them as artificial will instruct the debugger about just this,
leading to happier debugging and thus happier lives.

Example: in some kind of smart-pointer class, mark the pointer accessor as
artificial, so that the whole class acts more like a pointer and less like
some C++ abstraction monster.

  template<typename T>
  struct my_smart_ptr
  {
    T *value;

    ecb_artificial
    operator T *()
    {
      return value;
    }
  };

=back

=head2 OPTIMISATION HINTS

=over 4

=item bool ecb_is_constant(expr)

Returns true iff the expression can be deduced to be a compile-time
constant, and false otherwise.

For example, when you have a C<rndm16> function that returns a 16 bit
random number, and you have a function that maps this to a range from
0..n-1, then you could use this inline function in a header file:

  ecb_inline uint32_t
  rndm (uint32_t n)
  {
    return (n * (uint32_t)rndm16 ()) >> 16;
  }

However, for powers of two, you could use a normal mask, but that is only
worth it if, at compile time, you can detect this case. This is the case
when the passed number is a constant and also a power of two (C<n & (n -
1) == 0>):

  ecb_inline uint32_t
  rndm (uint32_t n)
  {
    return is_constant (n) && !(n & (n - 1))
      ? rndm16 () & (num - 1)
      : (n * (uint32_t)rndm16 ()) >> 16;
  }

=item bool ecb_expect (expr, value)

Evaluates C<expr> and returns it. In addition, it tells the compiler that
the C<expr> evaluates to C<value> a lot, which can be used for static
branch optimisations.

Usually, you want to use the more intuitive C<ecb_expect_true> and
C<ecb_expect_false> functions instead.

=item bool ecb_expect_true (cond)

=item bool ecb_expect_false (cond)

These two functions expect a expression that is true or false and return
C<1> or C<0>, respectively, so when used in the condition of an C<if> or
other conditional statement, it will not change the program:

  /* these two do the same thing */
  if (some_condition) ...;
  if (ecb_expect_true (some_condition)) ...;

However, by using C<ecb_expect_true>, you tell the compiler that the
condition is likely to be true (and for C<ecb_expect_false>, that it is
unlikely to be true).

For example, when you check for a null pointer and expect this to be a
rare, exceptional, case, then use C<ecb_expect_false>:

  void my_free (void *ptr)
  {
    if (ecb_expect_false (ptr == 0))
      return;
  }

Consequent use of these functions to mark away exceptional cases or to
tell the compiler what the hot path through a function is can increase
performance considerably.

You might know these functions under the name C<likely> and C<unlikely>
- while these are common aliases, we find that the expect name is easier
to understand when quickly skimming code. If you wish, you can use
C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
C<ecb_expect_false> - these are simply aliases.

A very good example is in a function that reserves more space for some
memory block (for example, inside an implementation of a string stream) -
each time something is added, you have to check for a buffer overrun, but
you expect that most checks will turn out to be false:

  /* make sure we have "size" extra room in our buffer */
  ecb_inline void
  reserve (int size)
  {
    if (ecb_expect_false (current + size > end))
      real_reserve_method (size); /* presumably noinline */
  }

=item bool ecb_assume (cond)

Try to tell the compiler that some condition is true, even if it's not
obvious.

This can be used to teach the compiler about invariants or other
conditions that might improve code generation, but which are impossible to
deduce form the code itself.

For example, the example reservation function from the C<ecb_expect_false>
description could be written thus (only C<ecb_assume> was added):

  ecb_inline void
  reserve (int size)
  {
    if (ecb_expect_false (current + size > end))
      real_reserve_method (size); /* presumably noinline */

    ecb_assume (current + size <= end);
  }

If you then call this function twice, like this:

  reserve (10);
  reserve (1);

Then the compiler I<might> be able to optimise out the second call
completely, as it knows that C<< current + 1 > end >> is false and the
call will never be executed.

=item bool ecb_unreachable ()

This function does nothing itself, except tell the compiler that it will
never be executed. Apart from suppressing a warning in some cases, this
function can be used to implement C<ecb_assume> or similar functions.

=item bool ecb_prefetch (addr, rw, locality)

Tells the compiler to try to prefetch memory at the given C<addr>ess
for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
C<0> means that there will only be one access later, C<3> means that
the data will likely be accessed very often, and values in between mean
something... in between. The memory pointed to by the address does not
need to be accessible (it could be a null pointer for example), but C<rw>
and C<locality> must be compile-time constants.

An obvious way to use this is to prefetch some data far away, in a big
array you loop over. This prefetches memory some 128 array elements later,
in the hope that it will be ready when the CPU arrives at that location.

  int sum = 0;

  for (i = 0; i < N; ++i)
    {
      sum += arr [i]
      ecb_prefetch (arr + i + 128, 0, 0);
    }

It's hard to predict how far to prefetch, and most CPUs that can prefetch
are often good enough to predict this kind of behaviour themselves. It
gets more interesting with linked lists, especially when you do some fair
processing on each list element:

  for (node *n = start; n; n = n->next)
    {
      ecb_prefetch (n->next, 0, 0);
      ... do medium amount of work with *n
    }

After processing the node, (part of) the next node might already be in
cache.

=back

=head2 BIT FIDDLING / BIT WIZARDRY

=over 4

=item bool ecb_big_endian ()

=item bool ecb_little_endian ()

These two functions return true if the byte order is big endian
(most-significant byte first) or little endian (least-significant byte
first) respectively.

On systems that are neither, their return values are unspecified.

=item int ecb_ctz32 (uint32_t x)

=item int ecb_ctz64 (uint64_t x)

Returns the index of the least significant bit set in C<x> (or
equivalently the number of bits set to 0 before the least significant bit
set), starting from 0. If C<x> is 0 the result is undefined.

For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.

For example:

  ecb_ctz32 (3) = 0
  ecb_ctz32 (6) = 1

=item bool ecb_is_pot32 (uint32_t x)

=item bool ecb_is_pot64 (uint32_t x)

Return true iff C<x> is a power of two or C<x == 0>.

For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.

=item int ecb_ld32 (uint32_t x)

=item int ecb_ld64 (uint64_t x)

Returns the index of the most significant bit set in C<x>, or the number
of digits the number requires in binary (so that C<< 2**ld <= x <
2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
example to see how many bits a certain number requires to be encoded.

This function is similar to the "count leading zero bits" function, except
that that one returns how many zero bits are "in front" of the number (in
the given data type), while C<ecb_ld> returns how many bits the number
itself requires.

For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.

=item int ecb_popcount32 (uint32_t x)

=item int ecb_popcount64 (uint64_t x)

Returns the number of bits set to 1 in C<x>.

For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.

For example:

  ecb_popcount32 (7) = 3
  ecb_popcount32 (255) = 8

=item uint8_t  ecb_bitrev8  (uint8_t  x)

=item uint16_t ecb_bitrev16 (uint16_t x)

=item uint32_t ecb_bitrev32 (uint32_t x)

Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
and so on.

Example:

   ecb_bitrev8 (0xa7) = 0xea
   ecb_bitrev32 (0xffcc4411) = 0x882233ff

=item uint32_t ecb_bswap16 (uint32_t x)

=item uint32_t ecb_bswap32 (uint32_t x)

=item uint64_t ecb_bswap64 (uint64_t x)

These functions return the value of the 16-bit (32-bit, 64-bit) value
C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
C<ecb_bswap32>).

=item uint8_t  ecb_rotl8  (uint8_t  x, unsigned int count)

=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)

=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)

=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)

=item uint8_t  ecb_rotr8  (uint8_t  x, unsigned int count)

=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)

=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)

=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)

These two families of functions return the value of C<x> after rotating
all the bits by C<count> positions to the right (C<ecb_rotr>) or left
(C<ecb_rotl>).

Current GCC versions understand these functions and usually compile them
to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
x86).

=back

=head2 ARITHMETIC

=over 4

=item x = ecb_mod (m, n)

Returns C<m> modulo C<n>, which is the same as the positive remainder
of the division operation between C<m> and C<n>, using floored
division. Unlike the C remainder operator C<%>, this function ensures that
the return value is always positive and that the two numbers I<m> and
I<m' = m + i * n> result in the same value modulo I<n> - in other words,
C<ecb_mod> implements the mathematical modulo operation, which is missing
in the language.

C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
negatable, that is, both C<m> and C<-m> must be representable in its
type (this typically excludes the minimum signed integer value, the same
limitation as for C</> and C<%> in C).

Current GCC versions compile this into an efficient branchless sequence on
almost all CPUs.

For example, when you want to rotate forward through the members of an
array for increasing C<m> (which might be negative), then you should use
C<ecb_mod>, as the C<%> operator might give either negative results, or
change direction for negative values:

   for (m = -100; m <= 100; ++m)
     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];

=item x = ecb_div_rd (val, div)

=item x = ecb_div_ru (val, div)

Returns C<val> divided by C<div> rounded down or up, respectively.
C<val> and C<div> must have integer types and C<div> must be strictly
positive. Note that these functions are implemented with macros in C
and with function templates in C++.

=back

=head2 UTILITY

=over 4

=item element_count = ecb_array_length (name)

Returns the number of elements in the array C<name>. For example:

  int primes[] = { 2, 3, 5, 7, 11 };
  int sum = 0;

  for (i = 0; i < ecb_array_length (primes); i++)
    sum += primes [i];

=back


Revision:	1.42
Committed:	Mon May 28 08:54:03 2012 UTC (12 years ago) by root
Branch:	MAIN
Changes since 1.41:	+3 -3 lines
Log Message:	* empty log message *
#	Content
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_t int16_t uint16_t
64	int32_t uint32_t int64_t uint64_t
65	intptr_t uintptr_t ptrdiff_t
66
67	The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
68	platform (currently C<4> or C<8>).
69
70	=head2 GCC ATTRIBUTES
71
72	A major part of libecb deals with GCC attributes. These are additional
73	attributes that you can assign to functions, variables and sometimes even
74	types - much like C<const> or C<volatile> in C.
75
76	While GCC allows declarations to show up in many surprising places,
77	but not in many expected places, the safest way is to put attribute
78	declarations before the whole declaration:
79
80	ecb_const int mysqrt (int a);
81	ecb_unused int i;
82
83	For variables, it is often nicer to put the attribute after the name, and
84	avoid multiple declarations using commas:
85
86	int i ecb_unused;
87
88	=over 4
89
90	=item ecb_attribute ((attrs...))
91
92	A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
93	nothing on other compilers, so the effect is that only GCC sees these.
94
95	Example: use the C<deprecated> attribute on a function.
96
97	ecb_attribute((__deprecated__)) void
98	do_not_use_me_anymore (void);
99
100	=item ecb_unused
101
102	Marks a function or a variable as "unused", which simply suppresses a
103	warning by GCC when it detects it as unused. This is useful when you e.g.
104	declare a variable but do not always use it:
105
106	{
107	int var ecb_unused;
108
109	#ifdef SOMECONDITION
110	var = ...;
111	return var;
112	#else
113	return 0;
114	#endif
115	}
116
117	=item ecb_inline
118
119	This is not actually an attribute, but you use it like one. It expands
120	either to C<static inline> or to just C<static>, if inline isn't
121	supported. It should be used to declare functions that should be inlined,
122	for code size or speed reasons.
123
124	Example: inline this function, it surely will reduce codesize.
125
126	ecb_inline int
127	negmul (int a, int b)
128	{
129	return - (a * b);
130	}
131
132	=item ecb_noinline
133
134	Prevent a function from being inlined - it might be optimised away, but
135	not inlined into other functions. This is useful if you know your function
136	is rarely called and large enough for inlining not to be helpful.
137
138	=item ecb_noreturn
139
140	Marks a function as "not returning, ever". Some typical functions that
141	don't return are C<exit> or C<abort> (which really works hard to not
142	return), and now you can make your own:
143
144	ecb_noreturn void
145	my_abort (const char *errline)
146	{
147	puts (errline);
148	abort ();
149	}
150
151	In this case, the compiler would probably be smart enough to deduce it on
152	its own, so this is mainly useful for declarations.
153
154	=item ecb_const
155
156	Declares that the function only depends on the values of its arguments,
157	much like a mathematical function. It specifically does not read or write
158	any memory any arguments might point to, global variables, or call any
159	non-const functions. It also must not have any side effects.
160
161	Such a function can be optimised much more aggressively by the compiler -
162	for example, multiple calls with the same arguments can be optimised into
163	a single call, which wouldn't be possible if the compiler would have to
164	expect any side effects.
165
166	It is best suited for functions in the sense of mathematical functions,
167	such as a function returning the square root of its input argument.
168
169	Not suited would be a function that calculates the hash of some memory
170	area you pass in, prints some messages or looks at a global variable to
171	decide on rounding.
172
173	See C<ecb_pure> for a slightly less restrictive class of functions.
174
175	=item ecb_pure
176
177	Similar to C<ecb_const>, declares a function that has no side
178	effects. Unlike C<ecb_const>, the function is allowed to examine global
179	variables and any other memory areas (such as the ones passed to it via
180	pointers).
181
182	While these functions cannot be optimised as aggressively as C<ecb_const>
183	functions, they can still be optimised away in many occasions, and the
184	compiler has more freedom in moving calls to them around.
185
186	Typical examples for such functions would be C<strlen> or C<memcmp>. A
187	function that calculates the MD5 sum of some input and updates some MD5
188	state passed as argument would I<NOT> be pure, however, as it would modify
189	some memory area that is not the return value.
190
191	=item ecb_hot
192
193	This declares a function as "hot" with regards to the cache - the function
194	is used so often, that it is very beneficial to keep it in the cache if
195	possible.
196
197	The compiler reacts by trying to place hot functions near to each other in
198	memory.
199
200	Whether a function is hot or not often depends on the whole program,
201	and less on the function itself. C<ecb_cold> is likely more useful in
202	practise.
203
204	=item ecb_cold
205
206	The opposite of C<ecb_hot> - declares a function as "cold" with regards to
207	the cache, or in other words, this function is not called often, or not at
208	speed-critical times, and keeping it in the cache might be a waste of said
209	cache.
210
211	In addition to placing cold functions together (or at least away from hot
212	functions), this knowledge can be used in other ways, for example, the
213	function will be optimised for size, as opposed to speed, and codepaths
214	leading to calls to those functions can automatically be marked as if
215	C<ecb_expect_false> had been used to reach them.
216
217	Good examples for such functions would be error reporting functions, or
218	functions only called in exceptional or rare cases.
219
220	=item ecb_artificial
221
222	Declares the function as "artificial", in this case meaning that this
223	function is not really mean to be a function, but more like an accessor
224	- many methods in C++ classes are mere accessor functions, and having a
225	crash reported in such a method, or single-stepping through them, is not
226	usually so helpful, especially when it's inlined to just a few instructions.
227
228	Marking them as artificial will instruct the debugger about just this,
229	leading to happier debugging and thus happier lives.
230
231	Example: in some kind of smart-pointer class, mark the pointer accessor as
232	artificial, so that the whole class acts more like a pointer and less like
233	some C++ abstraction monster.
234
235	template<typename T>
236	struct my_smart_ptr
237	{
238	T *value;
239
240	ecb_artificial
241	operator T *()
242	{
243	return value;
244	}
245	};
246
247	=back
248
249	=head2 OPTIMISATION HINTS
250
251	=over 4
252
253	=item bool ecb_is_constant(expr)
254
255	Returns true iff the expression can be deduced to be a compile-time
256	constant, and false otherwise.
257
258	For example, when you have a C<rndm16> function that returns a 16 bit
259	random number, and you have a function that maps this to a range from
260	0..n-1, then you could use this inline function in a header file:
261
262	ecb_inline uint32_t
263	rndm (uint32_t n)
264	{
265	return (n * (uint32_t)rndm16 ()) >> 16;
266	}
267
268	However, for powers of two, you could use a normal mask, but that is only
269	worth it if, at compile time, you can detect this case. This is the case
270	when the passed number is a constant and also a power of two (C<n & (n -
271	1) == 0>):
272
273	ecb_inline uint32_t
274	rndm (uint32_t n)
275	{
276	return is_constant (n) && !(n & (n - 1))
277	? rndm16 () & (num - 1)
278	: (n * (uint32_t)rndm16 ()) >> 16;
279	}
280
281	=item bool ecb_expect (expr, value)
282
283	Evaluates C<expr> and returns it. In addition, it tells the compiler that
284	the C<expr> evaluates to C<value> a lot, which can be used for static
285	branch optimisations.
286
287	Usually, you want to use the more intuitive C<ecb_expect_true> and
288	C<ecb_expect_false> functions instead.
289
290	=item bool ecb_expect_true (cond)
291
292	=item bool ecb_expect_false (cond)
293
294	These two functions expect a expression that is true or false and return
295	C<1> or C<0>, respectively, so when used in the condition of an C<if> or
296	other conditional statement, it will not change the program:
297
298	/* these two do the same thing */
299	if (some_condition) ...;
300	if (ecb_expect_true (some_condition)) ...;
301
302	However, by using C<ecb_expect_true>, you tell the compiler that the
303	condition is likely to be true (and for C<ecb_expect_false>, that it is
304	unlikely to be true).
305
306	For example, when you check for a null pointer and expect this to be a
307	rare, exceptional, case, then use C<ecb_expect_false>:
308
309	void my_free (void *ptr)
310	{
311	if (ecb_expect_false (ptr == 0))
312	return;
313	}
314
315	Consequent use of these functions to mark away exceptional cases or to
316	tell the compiler what the hot path through a function is can increase
317	performance considerably.
318
319	You might know these functions under the name C<likely> and C<unlikely>
320	- while these are common aliases, we find that the expect name is easier
321	to understand when quickly skimming code. If you wish, you can use
322	C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
323	C<ecb_expect_false> - these are simply aliases.
324
325	A very good example is in a function that reserves more space for some
326	memory block (for example, inside an implementation of a string stream) -
327	each time something is added, you have to check for a buffer overrun, but
328	you expect that most checks will turn out to be false:
329
330	/* make sure we have "size" extra room in our buffer */
331	ecb_inline void
332	reserve (int size)
333	{
334	if (ecb_expect_false (current + size > end))
335	real_reserve_method (size); /* presumably noinline */
336	}
337
338	=item bool ecb_assume (cond)
339
340	Try to tell the compiler that some condition is true, even if it's not
341	obvious.
342
343	This can be used to teach the compiler about invariants or other
344	conditions that might improve code generation, but which are impossible to
345	deduce form the code itself.
346
347	For example, the example reservation function from the C<ecb_expect_false>
348	description could be written thus (only C<ecb_assume> was added):
349
350	ecb_inline void
351	reserve (int size)
352	{
353	if (ecb_expect_false (current + size > end))
354	real_reserve_method (size); /* presumably noinline */
355
356	ecb_assume (current + size <= end);
357	}
358
359	If you then call this function twice, like this:
360
361	reserve (10);
362	reserve (1);
363
364	Then the compiler I<might> be able to optimise out the second call
365	completely, as it knows that C<< current + 1 > end >> is false and the
366	call will never be executed.
367
368	=item bool ecb_unreachable ()
369
370	This function does nothing itself, except tell the compiler that it will
371	never be executed. Apart from suppressing a warning in some cases, this
372	function can be used to implement C<ecb_assume> or similar functions.
373
374	=item bool ecb_prefetch (addr, rw, locality)
375
376	Tells the compiler to try to prefetch memory at the given C<addr>ess
377	for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
378	C<0> means that there will only be one access later, C<3> means that
379	the data will likely be accessed very often, and values in between mean
380	something... in between. The memory pointed to by the address does not
381	need to be accessible (it could be a null pointer for example), but C<rw>
382	and C<locality> must be compile-time constants.
383
384	An obvious way to use this is to prefetch some data far away, in a big
385	array you loop over. This prefetches memory some 128 array elements later,
386	in the hope that it will be ready when the CPU arrives at that location.
387
388	int sum = 0;
389
390	for (i = 0; i < N; ++i)
391	{
392	sum += arr [i]
393	ecb_prefetch (arr + i + 128, 0, 0);
394	}
395
396	It's hard to predict how far to prefetch, and most CPUs that can prefetch
397	are often good enough to predict this kind of behaviour themselves. It
398	gets more interesting with linked lists, especially when you do some fair
399	processing on each list element:
400
401	for (node *n = start; n; n = n->next)
402	{
403	ecb_prefetch (n->next, 0, 0);
404	... do medium amount of work with *n
405	}
406
407	After processing the node, (part of) the next node might already be in
408	cache.
409
410	=back
411
412	=head2 BIT FIDDLING / BIT WIZARDRY
413
414	=over 4
415
416	=item bool ecb_big_endian ()
417
418	=item bool ecb_little_endian ()
419
420	These two functions return true if the byte order is big endian
421	(most-significant byte first) or little endian (least-significant byte
422	first) respectively.
423
424	On systems that are neither, their return values are unspecified.
425
426	=item int ecb_ctz32 (uint32_t x)
427
428	=item int ecb_ctz64 (uint64_t x)
429
430	Returns the index of the least significant bit set in C<x> (or
431	equivalently the number of bits set to 0 before the least significant bit
432	set), starting from 0. If C<x> is 0 the result is undefined.
433
434	For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
435
436	For example:
437
438	ecb_ctz32 (3) = 0
439	ecb_ctz32 (6) = 1
440
441	=item bool ecb_is_pot32 (uint32_t x)
442
443	=item bool ecb_is_pot64 (uint32_t x)
444
445	Return true iff C<x> is a power of two or C<x == 0>.
446
447	For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
448
449	=item int ecb_ld32 (uint32_t x)
450
451	=item int ecb_ld64 (uint64_t x)
452
453	Returns the index of the most significant bit set in C<x>, or the number
454	of digits the number requires in binary (so that C<< 2**ld <= x <
455	2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
456	to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
457	example to see how many bits a certain number requires to be encoded.
458
459	This function is similar to the "count leading zero bits" function, except
460	that that one returns how many zero bits are "in front" of the number (in
461	the given data type), while C<ecb_ld> returns how many bits the number
462	itself requires.
463
464	For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
465
466	=item int ecb_popcount32 (uint32_t x)
467
468	=item int ecb_popcount64 (uint64_t x)
469
470	Returns the number of bits set to 1 in C<x>.
471
472	For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
473
474	For example:
475
476	ecb_popcount32 (7) = 3
477	ecb_popcount32 (255) = 8
478
479	=item uint8_t ecb_bitrev8 (uint8_t x)
480
481	=item uint16_t ecb_bitrev16 (uint16_t x)
482
483	=item uint32_t ecb_bitrev32 (uint32_t x)
484
485	Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
486	and so on.
487
488	Example:
489
490	ecb_bitrev8 (0xa7) = 0xea
491	ecb_bitrev32 (0xffcc4411) = 0x882233ff
492
493	=item uint32_t ecb_bswap16 (uint32_t x)
494
495	=item uint32_t ecb_bswap32 (uint32_t x)
496
497	=item uint64_t ecb_bswap64 (uint64_t x)
498
499	These functions return the value of the 16-bit (32-bit, 64-bit) value
500	C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
501	C<ecb_bswap32>).
502
503	=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
504
505	=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
506
507	=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
508
509	=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
510
511	=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
512
513	=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
514
515	=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
516
517	=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
518
519	These two families of functions return the value of C<x> after rotating
520	all the bits by C<count> positions to the right (C<ecb_rotr>) or left
521	(C<ecb_rotl>).
522
523	Current GCC versions understand these functions and usually compile them
524	to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
525	x86).
526
527	=back
528
529	=head2 ARITHMETIC
530
531	=over 4
532
533	=item x = ecb_mod (m, n)
534
535	Returns C<m> modulo C<n>, which is the same as the positive remainder
536	of the division operation between C<m> and C<n>, using floored
537	division. Unlike the C remainder operator C<%>, this function ensures that
538	the return value is always positive and that the two numbers I<m> and
539	I<m' = m + i * n> result in the same value modulo I<n> - in other words,
540	C<ecb_mod> implements the mathematical modulo operation, which is missing
541	in the language.
542
543	C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
544	negatable, that is, both C<m> and C<-m> must be representable in its
545	type (this typically excludes the minimum signed integer value, the same
546	limitation as for C</> and C<%> in C).
547
548	Current GCC versions compile this into an efficient branchless sequence on
549	almost all CPUs.
550
551	For example, when you want to rotate forward through the members of an
552	array for increasing C<m> (which might be negative), then you should use
553	C<ecb_mod>, as the C<%> operator might give either negative results, or
554	change direction for negative values:
555
556	for (m = -100; m <= 100; ++m)
557	int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
558
559	=item x = ecb_div_rd (val, div)
560
561	=item x = ecb_div_ru (val, div)
562
563	Returns C<val> divided by C<div> rounded down or up, respectively.
564	C<val> and C<div> must have integer types and C<div> must be strictly
565	positive. Note that these functions are implemented with macros in C
566	and with function templates in C++.
567
568	=back
569
570	=head2 UTILITY
571
572	=over 4
573
574	=item element_count = ecb_array_length (name)
575
576	Returns the number of elements in the array C<name>. For example:
577
578	int primes[] = { 2, 3, 5, 7, 11 };
579	int sum = 0;
580
581	for (i = 0; i < ecb_array_length (primes); i++)
582	sum += primes [i];
583
584	=back
585
586