--- libecb/ecb.pod 2011/05/26 23:32:41 1.16 +++ libecb/ecb.pod 2011/06/01 00:57:14 1.26 @@ -17,6 +17,10 @@ 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 @@ -54,7 +58,21 @@ =head2 GCC ATTRIBUTES -blabla where to put, what others +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 or C 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 @@ -93,16 +111,113 @@ =item ecb_noreturn +Marks a function as "not returning, ever". Some typical functions that +don't return are C or C (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 for a slightly less restrictive class of functions. + =item ecb_pure +Similar to C, declares a function that has no side +effects. Unlike C, 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 +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 or C. A +function that calculates the MD5 sum of some input and updates some MD5 +state passed as argument would I 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 is likely more useful in +practise. + =item ecb_cold +The opposite of C - 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 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 + struct my_smart_ptr + { + T *value; + + ecb_artificial + operator T *() + { + return value; + } + }; + =back =head2 OPTIMISATION HINTS @@ -274,13 +389,15 @@ (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) Returns the index of the least significant bit set in C (or -equivalently the number of bits set to 0 before the least significant -bit set), starting from 0. If C is 0 the result is undefined. A -common use case is to compute the integer binary logarithm, i.e., -floor(log2(n)). For example: +equivalently the number of bits set to 0 before the least significant bit +set), starting from 0. If C is 0 the result is undefined. A common use +case is to compute the integer binary logarithm, i.e., C. For example: ecb_ctz32 (3) = 0 ecb_ctz32 (6) = 1 @@ -296,16 +413,19 @@ =item uint32_t ecb_bswap32 (uint32_t x) -These two functions return the value of the 16-bit (32-bit) variable -C after reversing the order of bytes. +These two functions return the value of the 16-bit (32-bit) value C +after reversing the order of bytes (0x11223344 becomes 0x44332211). =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) -These two functions return the value of C after shifting all the bits +These two functions return the value of C after rotating all the bits by C positions to the right or left respectively. +Current GCC versions understand these functions and usually compile them +to "optimal" code (e.g. a single C on x86). + =back =head2 ARITHMETIC @@ -314,13 +434,29 @@ =item x = ecb_mod (m, n) -Returns the positive remainder of the modulo operation between C and -C. Unlike the C modulo operator C<%>, this function ensures that the -return value is always positive). +Returns C modulo C, which is the same as the positive remainder +of the division operation between C and C, 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 and +I result in the same value modulo I - in other words, +C implements the mathematical modulo operation, which is missing +in the language. -C must be strictly positive (i.e. C<< >1 >>), while C must be +C must be strictly positive (i.e. C<< >= 1 >>), while C must be negatable, that is, both C and C<-m> must be representable in its -type. +type (this typically includes 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 +many systems. + +For example, when you want to rotate forward through the members of an +array for increasing C (which might be negative), then you should use +C, 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))]; =back @@ -328,7 +464,7 @@ =over 4 -=item element_count = ecb_array_length (name) [MACRO] +=item element_count = ecb_array_length (name) Returns the number of elements in the array C. For example: