[Rcpp-devel] Passing an exposed function to another C++ function as a parameter

[Dirk Eddelbuettel]

Hi Manuel,

On 3 August 2011 at 08:22, Manuel Castejón Limas wrote:
| 
|     Dear all,
| 
|     I'm rewriting the AMORE package using Rcpp --- in fact it's more like I'm
|     having a lot of fun while rewriting the AMORE package thanks to Rcpp.
|     Nevertheless, I'm facing this little problem that I hope it would be pretty
|     easy for you to solve.
|      
| 
|     Let's consider the C++ function Tanh_f0 
| 
|     double
| 
|     Tanh_f0(double inducedLocalField)
| 
|       {
| 
|         return tanh(inducedLocalField);
| 
|       }
| 
| 
|     After compilation using inline, Tanh_f0 is exposed to R using Module and
|     accessible through actMod$Tanh_f0
| 
| 
|     actMod$Tanh_f0(0.1)
| 
|     # [1] 0.09966799
| 
| 
|     Now, I want to pass actMod$Tanh_f0 as a parameter of a function in order
|     to use the original C++ function Tanh_f0.
| 
|    
|    
|     testCode <- '  Rcpp::Function fx (myfun);
| 
|     double result = as<double>(fx(x)); 
| 
|     return wrap( result );
| 
|         '
| 
|     usingTanhf0 <- cfunction(sig=signature(myfun="function", x="numeric"), body
|     =testCode,…)
| 
| 
|     And that indeed works, but when compared to using a simple tanh function it
|     shows quite a bad performance.
| 
| 
| 
|     testCode <- '  double value = as<double>(x);
| 
|     double result = tanh(value); 
| 
|     return wrap( result );
| 
|         '
| 
| 
|     usingTanh <- cfunction(sig=signature(x="numeric"), body=testCode, … )
| 
|     benchmark(usingTanhf0(myfun=actMod$Tanh_f0, x=0.1) , usingTanh( x=0.1),
|     columns=c("test", "replications", "elapsed", "relative"), order="relative",
|     replications=1000)
| 
|     #                                           test
|     replications elapsed relative
| 
|     # 2                           usingTanh(x = 0.1)        
|     1000   0.004        1
| 
|     # 1 usingTanhf0(myfun = actMod$Tanh_f0, x = 0.1)         1000   0.080
|           20

( I have a hard time with the formatting of your email. That table was
essentially unreadable. I even looked at Gmane's rendering at 
http://article.gmane.org/gmane.comp.lang.r.rcpp/2301  which is also
broken.  Can you work on your email setup? )

Profiling is a good idea. What we see here is that the ease of use comes at
some overhead for the wrapping and unwrapping it implies.  Such is life --
Modules has some cost.

We do of course try to eliminate any obvious source of underperformance so
there is always scope for profiling and debugging. Help is always appreciated.
 
|     Looks like having to go the R way to get access to Tanh_f0 has a high cost.
|     In order to be faster, it would be great if I could have access to the
|     original Tanh_f0 which I guess is pointed to by  actMod$Tanh_f0, may be at
|     the address 0x100153d30 ?

Why don't you just access   double tanh(double)   itself?  If Tanh_f0 a
mockup for the real function you want to use this way?
 
|     actMod$Tanh_f0
| 
|     # internal C++ function <0x100153d30>
| 
|     #     signature : double Tanh_f0(double)
| 
| 
|     After having had a look at RppDE sources, I guess the solution would be to
|     pass the function as an external pointer; something like :
| 
| 
|     testCode <- ' 
| 
|     typedef double (*funPtr)(double) ;
| 
|     Rcpp::XPtr<  funPtr > fx (myfun);
| 
|     double result = (*fx)(0.1); 
| 
|     return wrap( result );
| 
|     '
| 
|     testCodefun <- cfunction(sig=signature(myfun="C++Function"), body=
|     testCode,…)
| 
|     result <- testCodefun(myfun=actMod$Tanh_f0)
| 
| 
|     But this does NOT work and crashes R.
| 
|     Any hints?

No sorry, I don't have time to look through this now but XPtr per se is well
tested and reliable.  It is meant to be used with external objects -- not
necessarily function pointers.
 
|     Thank you for your patience in reading such long message!

Pleasure, and sorry I have no immediate fix for you.

Dirk

|     Manuel
| 
| 
| 
| 
| 
| 
| ----------------------------------------------------------------------
| _______________________________________________
| Rcpp-devel mailing list
| Rcpp-devel at lists.r-forge.r-project.org
| https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel

-- 
Gauss once played himself in a zero-sum game and won $50.
                      -- #11 at http://www.gaussfacts.com