[Rcpp-devel] Cost of function pointer dereferencing

[Davor Cubranic]

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, <romain at r-enthusiasts.com> <romain 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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