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=head1 LIBECB - e-C-Builtins |
2 |
|
3 |
=head2 ABOUT LIBECB |
4 |
|
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Libecb is currently a simple header file that doesn't require any |
6 |
configuration to use or include in your project. |
7 |
|
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It's part of the e-suite of libraries, other members of which include |
9 |
libev and libeio. |
10 |
|
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Its homepage can be found here: |
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|
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http://software.schmorp.de/pkg/libecb |
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|
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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 |
|
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Or in other words, things that should be built into any standard C system, |
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but aren't, implemented as efficient as possible with GCC, and still |
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correct with other compilers. |
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|
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More might come. |
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|
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=head2 ABOUT THE HEADER |
27 |
|
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At the moment, all you have to do is copy F<ecb.h> somewhere where your |
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compiler can find it and include it: |
30 |
|
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#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. |
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|
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There are currently no object files to link to - future versions might |
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come with an (optional) object code library to link against, to reduce |
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code size or gain access to additional features. |
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|
40 |
It also currently includes everything from F<inttypes.h>. |
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|
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=head2 ABOUT THIS MANUAL / CONVENTIONS |
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|
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This manual mainly describes each (public) function available after |
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including the F<ecb.h> header. The header might define other symbols than |
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these, but these are not part of the public API, and not supported in any |
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way. |
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|
49 |
When the manual mentions a "function" then this could be defined either as |
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as inline function, a macro, or an external symbol. |
51 |
|
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When functions use a concrete standard type, such as C<int> or |
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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 |
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the corresponding function relies on C to implement the correct types, and |
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is usually implemented as a macro. Specifically, a "bool" in this manual |
57 |
refers to any kind of boolean value, not a specific type. |
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|
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=head2 TYPES / TYPE SUPPORT |
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|
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ecb.h makes sure that the following types are defined (in the expected way): |
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|
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int8_t uint8_t int16_t uint16_t |
64 |
int32_t uint32_t int64_t uint64_t |
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intptr_t uintptr_t ptrdiff_t |
66 |
|
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The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this |
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platform (currently C<4> or C<8>). |
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|
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=head2 LANGUAGE/COMPILER VERSIONS |
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|
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=over 4 |
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|
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=item ECB_C99 |
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|
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Expands to a true value (suitable for testing in by the preprocessor) |
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if the environment claims to be C99 compliant. |
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|
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=item ECB_C11 |
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|
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Expands to a true value (suitable for testing in by the preprocessor) |
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if the environment claims to be C11 compliant. |
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|
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=item ECB_GCC_VERSION(major,minor) |
85 |
|
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Expands to a true value (suitable for testing in by the preprocessor) |
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if the compiler used is GNU C and the version is the givne version, or |
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higher. |
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|
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This macro tries to return false on compilers that claim to be GCC |
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compatible but aren't. |
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|
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=back |
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|
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=head2 GCC ATTRIBUTES |
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|
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A major part of libecb deals with GCC attributes. These are additional |
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attributes that you can assign to functions, variables and sometimes even |
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types - much like C<const> or C<volatile> in C. |
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|
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While GCC allows declarations to show up in many surprising places, |
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but not in many expected places, the safest way is to put attribute |
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declarations before the whole declaration: |
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|
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ecb_const int mysqrt (int a); |
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ecb_unused int i; |
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|
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For variables, it is often nicer to put the attribute after the name, and |
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avoid multiple declarations using commas: |
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|
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int i ecb_unused; |
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|
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=over 4 |
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|
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=item ecb_attribute ((attrs...)) |
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|
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A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to |
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nothing on other compilers, so the effect is that only GCC sees these. |
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|
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Example: use the C<deprecated> attribute on a function. |
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|
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ecb_attribute((__deprecated__)) void |
123 |
do_not_use_me_anymore (void); |
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|
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=item ecb_unused |
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|
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Marks a function or a variable as "unused", which simply suppresses a |
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warning by GCC when it detects it as unused. This is useful when you e.g. |
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declare a variable but do not always use it: |
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|
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{ |
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int var ecb_unused; |
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|
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#ifdef SOMECONDITION |
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var = ...; |
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return var; |
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#else |
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return 0; |
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#endif |
140 |
} |
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|
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=item ecb_inline |
143 |
|
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This is not actually an attribute, but you use it like one. It expands |
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either to C<static inline> or to just C<static>, if inline isn't |
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supported. It should be used to declare functions that should be inlined, |
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for code size or speed reasons. |
148 |
|
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Example: inline this function, it surely will reduce codesize. |
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|
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ecb_inline int |
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negmul (int a, int b) |
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{ |
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return - (a * b); |
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} |
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|
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=item ecb_noinline |
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|
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Prevent a function from being inlined - it might be optimised away, but |
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not inlined into other functions. This is useful if you know your function |
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is rarely called and large enough for inlining not to be helpful. |
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|
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=item ecb_noreturn |
164 |
|
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Marks a function as "not returning, ever". Some typical functions that |
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don't return are C<exit> or C<abort> (which really works hard to not |
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return), and now you can make your own: |
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|
169 |
ecb_noreturn void |
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my_abort (const char *errline) |
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{ |
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puts (errline); |
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abort (); |
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} |
175 |
|
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In this case, the compiler would probably be smart enough to deduce it on |
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its own, so this is mainly useful for declarations. |
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|
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=item ecb_const |
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|
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Declares that the function only depends on the values of its arguments, |
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much like a mathematical function. It specifically does not read or write |
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any memory any arguments might point to, global variables, or call any |
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non-const functions. It also must not have any side effects. |
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|
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Such a function can be optimised much more aggressively by the compiler - |
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for example, multiple calls with the same arguments can be optimised into |
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a single call, which wouldn't be possible if the compiler would have to |
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expect any side effects. |
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|
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It is best suited for functions in the sense of mathematical functions, |
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such as a function returning the square root of its input argument. |
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|
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Not suited would be a function that calculates the hash of some memory |
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area you pass in, prints some messages or looks at a global variable to |
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decide on rounding. |
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|
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See C<ecb_pure> for a slightly less restrictive class of functions. |
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|
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=item ecb_pure |
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|
202 |
Similar to C<ecb_const>, declares a function that has no side |
203 |
effects. Unlike C<ecb_const>, the function is allowed to examine global |
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variables and any other memory areas (such as the ones passed to it via |
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pointers). |
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|
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While these functions cannot be optimised as aggressively as C<ecb_const> |
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functions, they can still be optimised away in many occasions, and the |
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compiler has more freedom in moving calls to them around. |
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|
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Typical examples for such functions would be C<strlen> or C<memcmp>. A |
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function that calculates the MD5 sum of some input and updates some MD5 |
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state passed as argument would I<NOT> be pure, however, as it would modify |
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some memory area that is not the return value. |
215 |
|
216 |
=item ecb_hot |
217 |
|
218 |
This declares a function as "hot" with regards to the cache - the function |
219 |
is used so often, that it is very beneficial to keep it in the cache if |
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possible. |
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|
222 |
The compiler reacts by trying to place hot functions near to each other in |
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memory. |
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|
225 |
Whether a function is hot or not often depends on the whole program, |
226 |
and less on the function itself. C<ecb_cold> is likely more useful in |
227 |
practise. |
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|
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=item ecb_cold |
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|
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The opposite of C<ecb_hot> - declares a function as "cold" with regards to |
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the cache, or in other words, this function is not called often, or not at |
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speed-critical times, and keeping it in the cache might be a waste of said |
234 |
cache. |
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|
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In addition to placing cold functions together (or at least away from hot |
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functions), this knowledge can be used in other ways, for example, the |
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function will be optimised for size, as opposed to speed, and codepaths |
239 |
leading to calls to those functions can automatically be marked as if |
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C<ecb_expect_false> had been used to reach them. |
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|
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Good examples for such functions would be error reporting functions, or |
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functions only called in exceptional or rare cases. |
244 |
|
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=item ecb_artificial |
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|
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Declares the function as "artificial", in this case meaning that this |
248 |
function is not really mean to be a function, but more like an accessor |
249 |
- many methods in C++ classes are mere accessor functions, and having a |
250 |
crash reported in such a method, or single-stepping through them, is not |
251 |
usually so helpful, especially when it's inlined to just a few instructions. |
252 |
|
253 |
Marking them as artificial will instruct the debugger about just this, |
254 |
leading to happier debugging and thus happier lives. |
255 |
|
256 |
Example: in some kind of smart-pointer class, mark the pointer accessor as |
257 |
artificial, so that the whole class acts more like a pointer and less like |
258 |
some C++ abstraction monster. |
259 |
|
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template<typename T> |
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struct my_smart_ptr |
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{ |
263 |
T *value; |
264 |
|
265 |
ecb_artificial |
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operator T *() |
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{ |
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return value; |
269 |
} |
270 |
}; |
271 |
|
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=back |
273 |
|
274 |
=head2 OPTIMISATION HINTS |
275 |
|
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=over 4 |
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|
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=item bool ecb_is_constant(expr) |
279 |
|
280 |
Returns true iff the expression can be deduced to be a compile-time |
281 |
constant, and false otherwise. |
282 |
|
283 |
For example, when you have a C<rndm16> function that returns a 16 bit |
284 |
random number, and you have a function that maps this to a range from |
285 |
0..n-1, then you could use this inline function in a header file: |
286 |
|
287 |
ecb_inline uint32_t |
288 |
rndm (uint32_t n) |
289 |
{ |
290 |
return (n * (uint32_t)rndm16 ()) >> 16; |
291 |
} |
292 |
|
293 |
However, for powers of two, you could use a normal mask, but that is only |
294 |
worth it if, at compile time, you can detect this case. This is the case |
295 |
when the passed number is a constant and also a power of two (C<n & (n - |
296 |
1) == 0>): |
297 |
|
298 |
ecb_inline uint32_t |
299 |
rndm (uint32_t n) |
300 |
{ |
301 |
return is_constant (n) && !(n & (n - 1)) |
302 |
? rndm16 () & (num - 1) |
303 |
: (n * (uint32_t)rndm16 ()) >> 16; |
304 |
} |
305 |
|
306 |
=item bool ecb_expect (expr, value) |
307 |
|
308 |
Evaluates C<expr> and returns it. In addition, it tells the compiler that |
309 |
the C<expr> evaluates to C<value> a lot, which can be used for static |
310 |
branch optimisations. |
311 |
|
312 |
Usually, you want to use the more intuitive C<ecb_expect_true> and |
313 |
C<ecb_expect_false> functions instead. |
314 |
|
315 |
=item bool ecb_expect_true (cond) |
316 |
|
317 |
=item bool ecb_expect_false (cond) |
318 |
|
319 |
These two functions expect a expression that is true or false and return |
320 |
C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
321 |
other conditional statement, it will not change the program: |
322 |
|
323 |
/* these two do the same thing */ |
324 |
if (some_condition) ...; |
325 |
if (ecb_expect_true (some_condition)) ...; |
326 |
|
327 |
However, by using C<ecb_expect_true>, you tell the compiler that the |
328 |
condition is likely to be true (and for C<ecb_expect_false>, that it is |
329 |
unlikely to be true). |
330 |
|
331 |
For example, when you check for a null pointer and expect this to be a |
332 |
rare, exceptional, case, then use C<ecb_expect_false>: |
333 |
|
334 |
void my_free (void *ptr) |
335 |
{ |
336 |
if (ecb_expect_false (ptr == 0)) |
337 |
return; |
338 |
} |
339 |
|
340 |
Consequent use of these functions to mark away exceptional cases or to |
341 |
tell the compiler what the hot path through a function is can increase |
342 |
performance considerably. |
343 |
|
344 |
You might know these functions under the name C<likely> and C<unlikely> |
345 |
- while these are common aliases, we find that the expect name is easier |
346 |
to understand when quickly skimming code. If you wish, you can use |
347 |
C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
348 |
C<ecb_expect_false> - these are simply aliases. |
349 |
|
350 |
A very good example is in a function that reserves more space for some |
351 |
memory block (for example, inside an implementation of a string stream) - |
352 |
each time something is added, you have to check for a buffer overrun, but |
353 |
you expect that most checks will turn out to be false: |
354 |
|
355 |
/* make sure we have "size" extra room in our buffer */ |
356 |
ecb_inline void |
357 |
reserve (int size) |
358 |
{ |
359 |
if (ecb_expect_false (current + size > end)) |
360 |
real_reserve_method (size); /* presumably noinline */ |
361 |
} |
362 |
|
363 |
=item bool ecb_assume (cond) |
364 |
|
365 |
Try to tell the compiler that some condition is true, even if it's not |
366 |
obvious. |
367 |
|
368 |
This can be used to teach the compiler about invariants or other |
369 |
conditions that might improve code generation, but which are impossible to |
370 |
deduce form the code itself. |
371 |
|
372 |
For example, the example reservation function from the C<ecb_expect_false> |
373 |
description could be written thus (only C<ecb_assume> was added): |
374 |
|
375 |
ecb_inline void |
376 |
reserve (int size) |
377 |
{ |
378 |
if (ecb_expect_false (current + size > end)) |
379 |
real_reserve_method (size); /* presumably noinline */ |
380 |
|
381 |
ecb_assume (current + size <= end); |
382 |
} |
383 |
|
384 |
If you then call this function twice, like this: |
385 |
|
386 |
reserve (10); |
387 |
reserve (1); |
388 |
|
389 |
Then the compiler I<might> be able to optimise out the second call |
390 |
completely, as it knows that C<< current + 1 > end >> is false and the |
391 |
call will never be executed. |
392 |
|
393 |
=item bool ecb_unreachable () |
394 |
|
395 |
This function does nothing itself, except tell the compiler that it will |
396 |
never be executed. Apart from suppressing a warning in some cases, this |
397 |
function can be used to implement C<ecb_assume> or similar functions. |
398 |
|
399 |
=item bool ecb_prefetch (addr, rw, locality) |
400 |
|
401 |
Tells the compiler to try to prefetch memory at the given C<addr>ess |
402 |
for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
403 |
C<0> means that there will only be one access later, C<3> means that |
404 |
the data will likely be accessed very often, and values in between mean |
405 |
something... in between. The memory pointed to by the address does not |
406 |
need to be accessible (it could be a null pointer for example), but C<rw> |
407 |
and C<locality> must be compile-time constants. |
408 |
|
409 |
An obvious way to use this is to prefetch some data far away, in a big |
410 |
array you loop over. This prefetches memory some 128 array elements later, |
411 |
in the hope that it will be ready when the CPU arrives at that location. |
412 |
|
413 |
int sum = 0; |
414 |
|
415 |
for (i = 0; i < N; ++i) |
416 |
{ |
417 |
sum += arr [i] |
418 |
ecb_prefetch (arr + i + 128, 0, 0); |
419 |
} |
420 |
|
421 |
It's hard to predict how far to prefetch, and most CPUs that can prefetch |
422 |
are often good enough to predict this kind of behaviour themselves. It |
423 |
gets more interesting with linked lists, especially when you do some fair |
424 |
processing on each list element: |
425 |
|
426 |
for (node *n = start; n; n = n->next) |
427 |
{ |
428 |
ecb_prefetch (n->next, 0, 0); |
429 |
... do medium amount of work with *n |
430 |
} |
431 |
|
432 |
After processing the node, (part of) the next node might already be in |
433 |
cache. |
434 |
|
435 |
=back |
436 |
|
437 |
=head2 BIT FIDDLING / BIT WIZARDRY |
438 |
|
439 |
=over 4 |
440 |
|
441 |
=item bool ecb_big_endian () |
442 |
|
443 |
=item bool ecb_little_endian () |
444 |
|
445 |
These two functions return true if the byte order is big endian |
446 |
(most-significant byte first) or little endian (least-significant byte |
447 |
first) respectively. |
448 |
|
449 |
On systems that are neither, their return values are unspecified. |
450 |
|
451 |
=item int ecb_ctz32 (uint32_t x) |
452 |
|
453 |
=item int ecb_ctz64 (uint64_t x) |
454 |
|
455 |
Returns the index of the least significant bit set in C<x> (or |
456 |
equivalently the number of bits set to 0 before the least significant bit |
457 |
set), starting from 0. If C<x> is 0 the result is undefined. |
458 |
|
459 |
For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>. |
460 |
|
461 |
For example: |
462 |
|
463 |
ecb_ctz32 (3) = 0 |
464 |
ecb_ctz32 (6) = 1 |
465 |
|
466 |
=item bool ecb_is_pot32 (uint32_t x) |
467 |
|
468 |
=item bool ecb_is_pot64 (uint32_t x) |
469 |
|
470 |
Return true iff C<x> is a power of two or C<x == 0>. |
471 |
|
472 |
For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>. |
473 |
|
474 |
=item int ecb_ld32 (uint32_t x) |
475 |
|
476 |
=item int ecb_ld64 (uint64_t x) |
477 |
|
478 |
Returns the index of the most significant bit set in C<x>, or the number |
479 |
of digits the number requires in binary (so that C<< 2**ld <= x < |
480 |
2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is |
481 |
to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for |
482 |
example to see how many bits a certain number requires to be encoded. |
483 |
|
484 |
This function is similar to the "count leading zero bits" function, except |
485 |
that that one returns how many zero bits are "in front" of the number (in |
486 |
the given data type), while C<ecb_ld> returns how many bits the number |
487 |
itself requires. |
488 |
|
489 |
For smaller types than C<uint32_t> you can safely use C<ecb_ld32>. |
490 |
|
491 |
=item int ecb_popcount32 (uint32_t x) |
492 |
|
493 |
=item int ecb_popcount64 (uint64_t x) |
494 |
|
495 |
Returns the number of bits set to 1 in C<x>. |
496 |
|
497 |
For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>. |
498 |
|
499 |
For example: |
500 |
|
501 |
ecb_popcount32 (7) = 3 |
502 |
ecb_popcount32 (255) = 8 |
503 |
|
504 |
=item uint8_t ecb_bitrev8 (uint8_t x) |
505 |
|
506 |
=item uint16_t ecb_bitrev16 (uint16_t x) |
507 |
|
508 |
=item uint32_t ecb_bitrev32 (uint32_t x) |
509 |
|
510 |
Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1 |
511 |
and so on. |
512 |
|
513 |
Example: |
514 |
|
515 |
ecb_bitrev8 (0xa7) = 0xea |
516 |
ecb_bitrev32 (0xffcc4411) = 0x882233ff |
517 |
|
518 |
=item uint32_t ecb_bswap16 (uint32_t x) |
519 |
|
520 |
=item uint32_t ecb_bswap32 (uint32_t x) |
521 |
|
522 |
=item uint64_t ecb_bswap64 (uint64_t x) |
523 |
|
524 |
These functions return the value of the 16-bit (32-bit, 64-bit) value |
525 |
C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in |
526 |
C<ecb_bswap32>). |
527 |
|
528 |
=item uint8_t ecb_rotl8 (uint8_t x, unsigned int count) |
529 |
|
530 |
=item uint16_t ecb_rotl16 (uint16_t x, unsigned int count) |
531 |
|
532 |
=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
533 |
|
534 |
=item uint64_t ecb_rotl64 (uint64_t x, unsigned int count) |
535 |
|
536 |
=item uint8_t ecb_rotr8 (uint8_t x, unsigned int count) |
537 |
|
538 |
=item uint16_t ecb_rotr16 (uint16_t x, unsigned int count) |
539 |
|
540 |
=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
541 |
|
542 |
=item uint64_t ecb_rotr64 (uint64_t x, unsigned int count) |
543 |
|
544 |
These two families of functions return the value of C<x> after rotating |
545 |
all the bits by C<count> positions to the right (C<ecb_rotr>) or left |
546 |
(C<ecb_rotl>). |
547 |
|
548 |
Current GCC versions understand these functions and usually compile them |
549 |
to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on |
550 |
x86). |
551 |
|
552 |
=back |
553 |
|
554 |
=head2 ARITHMETIC |
555 |
|
556 |
=over 4 |
557 |
|
558 |
=item x = ecb_mod (m, n) |
559 |
|
560 |
Returns C<m> modulo C<n>, which is the same as the positive remainder |
561 |
of the division operation between C<m> and C<n>, using floored |
562 |
division. Unlike the C remainder operator C<%>, this function ensures that |
563 |
the return value is always positive and that the two numbers I<m> and |
564 |
I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
565 |
C<ecb_mod> implements the mathematical modulo operation, which is missing |
566 |
in the language. |
567 |
|
568 |
C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
569 |
negatable, that is, both C<m> and C<-m> must be representable in its |
570 |
type (this typically excludes the minimum signed integer value, the same |
571 |
limitation as for C</> and C<%> in C). |
572 |
|
573 |
Current GCC versions compile this into an efficient branchless sequence on |
574 |
almost all CPUs. |
575 |
|
576 |
For example, when you want to rotate forward through the members of an |
577 |
array for increasing C<m> (which might be negative), then you should use |
578 |
C<ecb_mod>, as the C<%> operator might give either negative results, or |
579 |
change direction for negative values: |
580 |
|
581 |
for (m = -100; m <= 100; ++m) |
582 |
int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
583 |
|
584 |
=item x = ecb_div_rd (val, div) |
585 |
|
586 |
=item x = ecb_div_ru (val, div) |
587 |
|
588 |
Returns C<val> divided by C<div> rounded down or up, respectively. |
589 |
C<val> and C<div> must have integer types and C<div> must be strictly |
590 |
positive. Note that these functions are implemented with macros in C |
591 |
and with function templates in C++. |
592 |
|
593 |
=back |
594 |
|
595 |
=head2 UTILITY |
596 |
|
597 |
=over 4 |
598 |
|
599 |
=item element_count = ecb_array_length (name) |
600 |
|
601 |
Returns the number of elements in the array C<name>. For example: |
602 |
|
603 |
int primes[] = { 2, 3, 5, 7, 11 }; |
604 |
int sum = 0; |
605 |
|
606 |
for (i = 0; i < ecb_array_length (primes); i++) |
607 |
sum += primes [i]; |
608 |
|
609 |
=back |
610 |
|
611 |
=head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF |
612 |
|
613 |
These symbols need to be defined before including F<ecb.h> the first time. |
614 |
|
615 |
=over 4 |
616 |
|
617 |
=item ECB_NO_THRADS |
618 |
|
619 |
If F<ecb.h> is never used from multiple threads, then this symbol can |
620 |
be defined, in which case memory fences (and similar constructs) are |
621 |
completely removed, leading to more efficient code and fewer dependencies. |
622 |
|
623 |
Setting this symbol to a true value implies C<ECB_NO_SMP>. |
624 |
|
625 |
=item ECB_NO_SMP |
626 |
|
627 |
The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from |
628 |
multiple threads, but never concurrently (e.g. if the system the program |
629 |
runs on has only a single CPU with a single core, no hyperthreading and so |
630 |
on), then this symbol can be defined, leading to more efficient code and |
631 |
fewer dependencies. |
632 |
|
633 |
=back |
634 |
|
635 |
|