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Diffstat (limited to 'manual/math.texi')
| -rw-r--r-- | manual/math.texi | 64 |
1 files changed, 62 insertions, 2 deletions
diff --git a/manual/math.texi b/manual/math.texi index 2b7cb82f5e..15a2075d33 100644 --- a/manual/math.texi +++ b/manual/math.texi @@ -54,6 +54,7 @@ in case of double using @code{double} is a good compromise. constant. * FP Comparison Functions:: Special functions to compare floating-point numbers. +* FP Function Optimizations:: Fast code or small code. * Trig Functions:: Sine, cosine, and tangent. * Inverse Trig Functions:: Arc sine, arc cosine, and arc tangent. * Exponents and Logarithms:: Also includes square root. @@ -704,6 +705,35 @@ that the comparison for equality and unequality do @emph{not} throw an exception if one of the arguments is an unordered value. +@node FP Function Optimizations +@section Is Fast Code or Small Code preferred? +@cindex Optimization + +If an application uses many floating point function it is often the case +that the costs for the function calls itseld are not neglectable. +Modern processor implementation often can execute the operation itself +very fast but the call means a disturbance of the control flow. + +For this reason the GNU C Library provides optimizations for many of the +frequently used math functions. When the GNU CC is used and the user +activates the optimizer several new inline functions and macros get +defined. These new functions and macros have the same names as the +library function and so get used instead of the later. In case of +inline functions the compiler will decide whether it is reasonable to +use the inline function and this decision is usually correct. + +For the generated code this means that no calls to the library functions +are necessary. This increases the speed significantly. But the +drawback is that the code size increases and this increase is not always +neglectable. + +In cases where the inline functions and macros are not wanted the symbol +@code{__NO_MATH_INLINES} should be defined before any system header is +included. This will make sure only library functions are used. Of +course it can be determined for each single file in the project whether +giving this option is preferred or not. + + @node Trig Functions @section Trigonometric Functions @cindex trigonometric functions @@ -1430,8 +1460,14 @@ the same seed, used in different C libraries or on different CPU types, will give you different random numbers. The GNU library supports the standard @w{ISO C} random number functions -plus another set derived from BSD. We recommend you use the standard -ones, @code{rand} and @code{srand}. +plus two other sets derived from BSD and SVID. We recommend you use the +standard ones, @code{rand} and @code{srand} if only a small number of +random bits are required. The SVID functions provide an interface which +allows better randon number generator algorithms and they return up to +48 random bits in one calls and they also return random floating-point +numbers if wanted. The SVID function might not be available on some BSD +derived systems but since they are required in the XPG they are +available on all Unix-conformant systems. @menu * ISO Random:: @code{rand} and friends. @@ -1478,6 +1514,30 @@ To produce truly random numbers (not just pseudo-random), do @code{srand (time (0))}. @end deftypefun +A completely broken interface was designed by the POSIX.1 committee to +support reproducible random numbers in multi-threaded programs. + +@comment stdlib.h +@comment POSIX.1 +@deftypefun int rand_r (unsigned int *@var{seed}) +This function returns a random number in the range 0 to @code{RAND_MAX} +just as @code{rand} does. But this function does not keep an internal +state for the RNG. Instead the @code{unsigned int} variable pointed to +by the argument @var{seed} is the only state. Before the value is +returned the state will be updated so that the next call will return a +new number. + +I.e., the state of the RNG can only have as much bits as the type +@code{unsigned int} has. This is far too few to provide a good RNG. +This interface is broken by design. + +If the program requires reproducible random numbers in multi-threaded +programs the reentrant SVID functions are probably a better choice. But +these functions are GNU extensions and therefore @code{rand_r}, as being +standardized in POSIX.1, should always be kept as a default method. +@end deftypefun + + @node BSD Random @subsection BSD Random Number Functions |