[Rd] Fast way to call an R function from C++?

[King Jiefei]

Hi,

I'm looking for a most efficient way to call an R function from C++ in a
package. I know there are two functions (`R_forceAndCall` and `Rf_eval`)
that can do the "call" part, but both are slow compared to calling the same
function in R. I also try to use Rcpp and it is the worse one. Here is my
test code:

C++ code:
```
// [[Rcpp::export]]
SEXP C_test1(SEXP f, SEXP x) {
SEXP call =PROTECT(Rf_lang2(f, x));
SEXP val = R_forceAndCall(call, 1, R_GlobalEnv);
UNPROTECT(1);
return val;
}

// [[Rcpp::export]]
SEXP C_test2(SEXP expr, SEXP env) {
SEXP val = Rf_eval(expr, env);
return val;
}

// [[Rcpp::export]]
SEXP C_test3(SEXP f,SEXP x) {
Function fun(f);
return fun(x);
}
```

R code:
```
testFunc<-function(x){
  x=x^2
  return(x)
}
evn=new.env()
evn$x=x
expr=quote(testFunc(evn$x))

testFunc(evn$x)
C_test1(testFunc, evn$x)
C_test2(expr,evn)
C_test3(testFunc,evn$x)
```

For the results, I run each function 1,000,000 times:

   - testFunc : 0.47 sec
   - C_test1 : 2.46 sec
   - C_test2 : 2.74 sec
   - C_test3 : 18.86 sec

It is clear to see that calling an R function in R is the fast one, it is
about 5X faster than ` R_forceAndCall ` and ` Rf_eval`. the latter two
functions have a similar performance and using Rcpp is the worst one. Is it
expected? Why is calling an R function from C++ much slower than calling
the function from R? Is there any faster way to do the function call in C++?

Best,
Jiefei

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