[Rcpp-devel] Cost of function pointer dereferencing

[romain at r-enthusiasts.com]

Thanks. I'll use that. 

 Le jeu 23/12/10 22:51, "Davor Cubranic" cubranic at stat.ubc.ca a écrit:
> After a bit of googling, I figured out how to write the templated functor.
> It's very simple:
> incw <- '
> template<double Func(double) >
> struct Ftor {
> inline double operator () (double x) const {
> return Func(x);
> }
> };
> Ftor<exp> ftor;
> '
> fw <- cxxfunction( , '
> double x ;
> for( int j=0; j<1000; j++){
> for( int i=0; i<1000000; i++){
> x = ftor(2.0) ; 
> }
> }
> return wrap( x ) ;
> ', plugin = "Rcpp", includes = incw , verbose=TRUE)
> 
> And it's still (insignificantly) faster than the static call:
> 
> test replications elapsed  relative user.self sys.self user.child
> 3 functor            1   0.463  1.000000     0.464    0.001          0
> 1  static            1   0.468  1.010799     0.464    0.000          0
> 2 pointer            1  15.960 34.470842    15.824    0.017          0
> 
> Davor
> 
> 
> On 2010-12-23, at 9:54 AM, Davor Cubranic wrote:
> 
> > I tried using a functor, calling 'exp' in its
> operator(), and it was even faster than static (albeit very
> slightly):
> 
> >      test replications elapsed  relative user.self
> sys.self user.child
> 3  functor            1   0.465  1.000000     0.464 
>   0.000          0
> 1   static            1   0.472  1.015054     0.464 
>   0.001          0
> 2  pointer            1  15.855 34.096774    15.815 
>   0.007          0
> 
> > Just add 'fw' below to your benchmark:
> > 
> > incw <- '
> >   class Ftor {
> >   public:
> >     inline double operator () (double x) const
> {
>       return exp(x);
> >     }
> >   };
> >   const Ftor ftor = Ftor();
> > '
> > fw <- cxxfunction( , '
> > double x ;
> > for( int j=0; j<1000; j++){
> > for( int i=0; i<1000000; i++){
> >   x = ftor(2.0) ; 
> > }
> > }
> > return wrap( x ) ;
> > ', plugin = "Rcpp", includes = incw ,
> verbose=TRUE)
> 
> > Maybe you could create a functor for each basic
> function and use that in a common loop implementation.
> 
> > I also tried templatizing Ftor on the function it
> delegates to, so that you could do something like Ftor<exp>(), but I
> couldn't get it to compile. Your template-fu is far better than mine, so
> maybe you'll be able to wrangle it into shape and have even more succint
> code.
> 
> > Davor
> > 
> > 
> > On 2010-12-23, at 8:45 AM, <roma
> in at r-enthusiasts.com> <roma
> in at r-enthusiasts.com> wrote:
> 
> >> Hello, 
> >> 
> >> Since I've been playing with efficiency of
> various operators for numeric vectors (+,-,/,*), I'm now looking at other
> functions such as exp, sqrt, etc ...
>> 
> >> The way we currently implement exp for vectors
> requires that we keep a function pointer for the atomic exp and that we
> dereference the function pointer each time. It turns out that this has some
> cost : 
>> 
> >>    test replications elapsed relative user.self
> sys.self user.child sys.child
>> 1  static            1   0.691  1.00000    
> 0.691    0.000          0         0
>> 2 pointer            1  11.157 16.14616   
> 11.144    0.012          0         0
>> 
> >> Reproduced by this code: 
> >> 
> >> require( Rcpp )
> >> require( inline )
> >> 
> >> fx <- cxxfunction( , '
> >> double x ;
> >> for( int j=0; j<1000; j++){
> >> for( int i=0; i<1000000; i++){
> >>   x = exp(2.0) ; 
> >> }
> >> }
> >> return wrap( x ) ;
> >> ', plugin = "Rcpp" )
> >> 
> >> inc <- '
> >>   double (*fun)(double) = exp ;
> >> '
> >> fy <- cxxfunction( , '
> >> double x ;
> >> for( int j=0; j<1000; j++){
> >> for( int i=0; i<1000000; i++){
> >>   x = (*fun)(2.0) ; 
> >> }
> >> }
> >> return wrap( x ) ;
> >> ', plugin = "Rcpp", includes = inc )
> >> 
> >> 
> >> require( rbenchmark )
> >> benchmark( 
> >>   order = "relative", 
> >>   static = fx(), 
> >>   pointer = fy(),
> >>   replications = 1L
> >> )
> >> 
> >> Romain
> >> 
> >> 
> >> 
> >> 
> >>
> _______________________________________________
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>> 
> >
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