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1.14 |
=head1 LIBECB - e-C-Builtins |
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=head2 ABOUT LIBECB |
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Libecb is currently a simple header file that doesn't require any |
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configuration to use or include in your project. |
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1.16 |
It's part of the e-suite of libraries, other members of which include |
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1.14 |
libev and libeio. |
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Its homepage can be found here: |
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http://software.schmorp.de/pkg/libecb |
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It mainly provides a number of wrappers around GCC built-ins, together |
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with replacement functions for other compilers. In addition to this, |
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1.16 |
it provides a number of other lowlevel C utilities, such as endianness |
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1.14 |
detection, byte swapping or bit rotations. |
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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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1.17 |
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1.14 |
More might come. |
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1.3 |
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=head2 ABOUT THE HEADER |
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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: |
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#include <ecb.h> |
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The header should work fine for both C and C++ compilation, and gives you |
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all of F<inttypes.h> in addition to the ECB symbols. |
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1.16 |
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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It also currently includes everything from F<inttypes.h>. |
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=head2 ABOUT THIS MANUAL / CONVENTIONS |
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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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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. |
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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 |
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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 |
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refers to any kind of boolean value, not a specific type. |
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1.1 |
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=head2 GCC ATTRIBUTES |
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A major part of libecb deals with GCC attributes. These are additional |
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attributes that you cna 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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While GCC allows declarations to show up in many surprising places, |
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but not in many expeted places, the safest way is to put attribute |
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declarations before the whole declaration: |
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ecb_const int mysqrt (int a); |
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ecb_unused int i; |
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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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int i ecb_unused; |
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1.3 |
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1.1 |
=over 4 |
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1.2 |
=item ecb_attribute ((attrs...)) |
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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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Example: use the C<deprecated> attribute on a function. |
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ecb_attribute((__deprecated__)) void |
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do_not_use_me_anymore (void); |
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1.3 |
=item ecb_unused |
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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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1.15 |
{ |
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int var ecb_unused; |
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1.3 |
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1.15 |
#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 |
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} |
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1.3 |
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1.2 |
=item ecb_noinline |
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1.9 |
Prevent a function from being inlined - it might be optimised away, but |
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1.3 |
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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1.2 |
=item ecb_noreturn |
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1.17 |
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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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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} |
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sf-exg |
1.19 |
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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1.2 |
=item ecb_const |
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sf-exg |
1.19 |
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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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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It is best suited for functions in the sense of mathematical functions, |
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1.19 |
such as a function returning the square root of its input argument. |
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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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See C<ecb_pure> for a slightly less restrictive class of functions. |
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1.2 |
=item ecb_pure |
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1.17 |
Similar to C<ecb_const>, declares a function that has no side |
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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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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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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. |
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1.2 |
=item ecb_hot |
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1.17 |
This declares a function as "hot" with regards to the cache - the function |
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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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The compiler reacts by trying to place hot functions near to each other in |
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memory. |
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sf-exg |
1.19 |
Whether a function is hot or not often depends on the whole program, |
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1.17 |
and less on the function itself. C<ecb_cold> is likely more useful in |
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practise. |
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1.2 |
=item ecb_cold |
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1.17 |
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 |
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cache. |
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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 |
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leading to calls to those functions can automatically be marked as if |
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sf-exg |
1.19 |
C<ecb_unlikely> had been used to reach them. |
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1.17 |
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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. |
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1.2 |
=item ecb_artificial |
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1.17 |
Declares the function as "artificial", in this case meaning that this |
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function is not really mean to be a function, but more like an accessor |
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- many methods in C++ classes are mere accessor functions, and having a |
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crash reported in such a method, or single-stepping through them, is not |
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usually so helpful, especially when it's inlined to just a few instructions. |
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Marking them as artificial will instruct the debugger about just this, |
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leading to happier debugging and thus happier lives. |
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Example: in some kind of smart-pointer class, mark the pointer accessor as |
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artificial, so that the whole class acts more like a pointer and less like |
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some C++ abstraction monster. |
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template<typename T> |
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struct my_smart_ptr |
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{ |
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T *value; |
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ecb_artificial |
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operator T *() |
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{ |
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return value; |
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} |
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}; |
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1.2 |
=back |
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1.1 |
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=head2 OPTIMISATION HINTS |
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=over 4 |
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1.14 |
=item bool ecb_is_constant(expr) |
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1.1 |
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1.3 |
Returns true iff the expression can be deduced to be a compile-time |
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constant, and false otherwise. |
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For example, when you have a C<rndm16> function that returns a 16 bit |
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random number, and you have a function that maps this to a range from |
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1.5 |
0..n-1, then you could use this inline function in a header file: |
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1.3 |
|
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ecb_inline uint32_t |
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rndm (uint32_t n) |
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{ |
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1.6 |
return (n * (uint32_t)rndm16 ()) >> 16; |
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1.3 |
} |
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However, for powers of two, you could use a normal mask, but that is only |
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worth it if, at compile time, you can detect this case. This is the case |
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when the passed number is a constant and also a power of two (C<n & (n - |
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1) == 0>): |
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ecb_inline uint32_t |
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rndm (uint32_t n) |
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{ |
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return is_constant (n) && !(n & (n - 1)) |
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? rndm16 () & (num - 1) |
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1.6 |
: (n * (uint32_t)rndm16 ()) >> 16; |
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1.3 |
} |
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1.14 |
=item bool ecb_expect (expr, value) |
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1.1 |
|
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1.7 |
Evaluates C<expr> and returns it. In addition, it tells the compiler that |
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the C<expr> evaluates to C<value> a lot, which can be used for static |
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branch optimisations. |
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1.1 |
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1.7 |
Usually, you want to use the more intuitive C<ecb_likely> and |
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C<ecb_unlikely> functions instead. |
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1.1 |
|
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1.15 |
=item bool ecb_likely (cond) |
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1.1 |
|
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1.15 |
=item bool ecb_unlikely (cond) |
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1.1 |
|
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1.7 |
These two functions expect a expression that is true or false and return |
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C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
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other conditional statement, it will not change the program: |
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/* these two do the same thing */ |
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if (some_condition) ...; |
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if (ecb_likely (some_condition)) ...; |
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However, by using C<ecb_likely>, you tell the compiler that the condition |
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sf-exg |
1.11 |
is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
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1.7 |
true). |
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1.9 |
For example, when you check for a null pointer and expect this to be a |
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rare, exceptional, case, then use C<ecb_unlikely>: |
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1.7 |
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void my_free (void *ptr) |
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{ |
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if (ecb_unlikely (ptr == 0)) |
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return; |
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} |
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Consequent use of these functions to mark away exceptional cases or to |
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tell the compiler what the hot path through a function is can increase |
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performance considerably. |
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A very good example is in a function that reserves more space for some |
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memory block (for example, inside an implementation of a string stream) - |
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1.9 |
each time something is added, you have to check for a buffer overrun, but |
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1.7 |
you expect that most checks will turn out to be false: |
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/* make sure we have "size" extra room in our buffer */ |
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ecb_inline void |
300 |
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reserve (int size) |
301 |
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{ |
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if (ecb_unlikely (current + size > end)) |
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real_reserve_method (size); /* presumably noinline */ |
304 |
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} |
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1.14 |
=item bool ecb_assume (cond) |
307 |
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1.7 |
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Try to tell the compiler that some condition is true, even if it's not |
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obvious. |
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This can be used to teach the compiler about invariants or other |
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conditions that might improve code generation, but which are impossible to |
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deduce form the code itself. |
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For example, the example reservation function from the C<ecb_unlikely> |
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description could be written thus (only C<ecb_assume> was added): |
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318 |
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ecb_inline void |
319 |
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reserve (int size) |
320 |
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{ |
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if (ecb_unlikely (current + size > end)) |
322 |
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real_reserve_method (size); /* presumably noinline */ |
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ecb_assume (current + size <= end); |
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} |
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If you then call this function twice, like this: |
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reserve (10); |
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reserve (1); |
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Then the compiler I<might> be able to optimise out the second call |
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completely, as it knows that C<< current + 1 > end >> is false and the |
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call will never be executed. |
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=item bool ecb_unreachable () |
337 |
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This function does nothing itself, except tell the compiler that it will |
339 |
root |
1.9 |
never be executed. Apart from suppressing a warning in some cases, this |
340 |
root |
1.7 |
function can be used to implement C<ecb_assume> or similar functions. |
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342 |
root |
1.14 |
=item bool ecb_prefetch (addr, rw, locality) |
343 |
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1.7 |
|
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Tells the compiler to try to prefetch memory at the given C<addr>ess |
345 |
root |
1.10 |
for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
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1.7 |
C<0> means that there will only be one access later, C<3> means that |
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the data will likely be accessed very often, and values in between mean |
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something... in between. The memory pointed to by the address does not |
349 |
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need to be accessible (it could be a null pointer for example), but C<rw> |
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and C<locality> must be compile-time constants. |
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An obvious way to use this is to prefetch some data far away, in a big |
353 |
root |
1.9 |
array you loop over. This prefetches memory some 128 array elements later, |
354 |
root |
1.7 |
in the hope that it will be ready when the CPU arrives at that location. |
355 |
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356 |
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int sum = 0; |
357 |
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358 |
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for (i = 0; i < N; ++i) |
359 |
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{ |
360 |
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sum += arr [i] |
361 |
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ecb_prefetch (arr + i + 128, 0, 0); |
362 |
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} |
363 |
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It's hard to predict how far to prefetch, and most CPUs that can prefetch |
365 |
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are often good enough to predict this kind of behaviour themselves. It |
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gets more interesting with linked lists, especially when you do some fair |
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processing on each list element: |
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for (node *n = start; n; n = n->next) |
370 |
|
|
{ |
371 |
|
|
ecb_prefetch (n->next, 0, 0); |
372 |
|
|
... do medium amount of work with *n |
373 |
|
|
} |
374 |
|
|
|
375 |
|
|
After processing the node, (part of) the next node might already be in |
376 |
|
|
cache. |
377 |
root |
1.1 |
|
378 |
root |
1.2 |
=back |
379 |
root |
1.1 |
|
380 |
|
|
=head2 BIT FIDDLING / BITSTUFFS |
381 |
|
|
|
382 |
root |
1.4 |
=over 4 |
383 |
|
|
|
384 |
root |
1.3 |
=item bool ecb_big_endian () |
385 |
|
|
|
386 |
|
|
=item bool ecb_little_endian () |
387 |
|
|
|
388 |
sf-exg |
1.11 |
These two functions return true if the byte order is big endian |
389 |
|
|
(most-significant byte first) or little endian (least-significant byte |
390 |
|
|
first) respectively. |
391 |
|
|
|
392 |
root |
1.24 |
On systems that are neither, their return values are unspecified. |
393 |
|
|
|
394 |
root |
1.3 |
=item int ecb_ctz32 (uint32_t x) |
395 |
|
|
|
396 |
sf-exg |
1.11 |
Returns the index of the least significant bit set in C<x> (or |
397 |
root |
1.24 |
equivalently the number of bits set to 0 before the least significant bit |
398 |
|
|
set), starting from 0. If C<x> is 0 the result is undefined. A common use |
399 |
|
|
case is to compute the integer binary logarithm, i.e., C<floor (log2 |
400 |
|
|
(n))>. For example: |
401 |
sf-exg |
1.11 |
|
402 |
root |
1.15 |
ecb_ctz32 (3) = 0 |
403 |
|
|
ecb_ctz32 (6) = 1 |
404 |
sf-exg |
1.11 |
|
405 |
root |
1.3 |
=item int ecb_popcount32 (uint32_t x) |
406 |
|
|
|
407 |
sf-exg |
1.11 |
Returns the number of bits set to 1 in C<x>. For example: |
408 |
|
|
|
409 |
root |
1.15 |
ecb_popcount32 (7) = 3 |
410 |
|
|
ecb_popcount32 (255) = 8 |
411 |
sf-exg |
1.11 |
|
412 |
root |
1.8 |
=item uint32_t ecb_bswap16 (uint32_t x) |
413 |
|
|
|
414 |
root |
1.3 |
=item uint32_t ecb_bswap32 (uint32_t x) |
415 |
|
|
|
416 |
root |
1.21 |
These two functions return the value of the 16-bit (32-bit) value C<x> |
417 |
|
|
after reversing the order of bytes (0x11223344 becomes 0x44332211). |
418 |
sf-exg |
1.13 |
|
419 |
root |
1.3 |
=item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
420 |
|
|
|
421 |
|
|
=item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
422 |
|
|
|
423 |
root |
1.22 |
These two functions return the value of C<x> after rotating all the bits |
424 |
sf-exg |
1.11 |
by C<count> positions to the right or left respectively. |
425 |
|
|
|
426 |
root |
1.20 |
Current GCC versions understand these functions and usually compile them |
427 |
|
|
to "optimal" code (e.g. a single C<roll> on x86). |
428 |
|
|
|
429 |
root |
1.3 |
=back |
430 |
root |
1.1 |
|
431 |
|
|
=head2 ARITHMETIC |
432 |
|
|
|
433 |
root |
1.3 |
=over 4 |
434 |
|
|
|
435 |
root |
1.14 |
=item x = ecb_mod (m, n) |
436 |
root |
1.3 |
|
437 |
root |
1.14 |
Returns the positive remainder of the modulo operation between C<m> and |
438 |
root |
1.24 |
C<n>, using floored division. Unlike the C modulo operator C<%>, this |
439 |
|
|
function ensures that the return value is always positive and that the two |
440 |
|
|
numbers I<m> and I<m' = m + i * n> result in the same value modulo I<n> - |
441 |
|
|
the C<%> operator usually has a behaviour change at C<m = 0>. |
442 |
root |
1.14 |
|
443 |
sf-exg |
1.23 |
C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
444 |
root |
1.14 |
negatable, that is, both C<m> and C<-m> must be representable in its |
445 |
|
|
type. |
446 |
sf-exg |
1.11 |
|
447 |
root |
1.24 |
Current GCC versions compile this into an efficient branchless sequence on |
448 |
|
|
many systems. |
449 |
|
|
|
450 |
|
|
For example, when you want to rotate forward through the members of an |
451 |
|
|
array for increasing C<m> (which might be negative), then you should use |
452 |
|
|
C<ecb_mod>, as the C<%> operator might give either negative results, or |
453 |
|
|
change direction for negative values: |
454 |
|
|
|
455 |
|
|
for (m = -100; m <= 100; ++m) |
456 |
|
|
int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
457 |
|
|
|
458 |
root |
1.3 |
=back |
459 |
root |
1.1 |
|
460 |
|
|
=head2 UTILITY |
461 |
|
|
|
462 |
root |
1.3 |
=over 4 |
463 |
|
|
|
464 |
sf-exg |
1.23 |
=item element_count = ecb_array_length (name) |
465 |
root |
1.3 |
|
466 |
sf-exg |
1.13 |
Returns the number of elements in the array C<name>. For example: |
467 |
|
|
|
468 |
|
|
int primes[] = { 2, 3, 5, 7, 11 }; |
469 |
|
|
int sum = 0; |
470 |
|
|
|
471 |
|
|
for (i = 0; i < ecb_array_length (primes); i++) |
472 |
|
|
sum += primes [i]; |
473 |
|
|
|
474 |
root |
1.3 |
=back |
475 |
root |
1.1 |
|
476 |
|
|
|