Speed of math function atan: comparison D and C++

[J-S Caux]

I'm considering shifting a large existing C++ codebase into D 
(it's a scientific code making much use of functions like atan, 
log etc).

I've compared the raw speed of atan between C++ (Apple LLVM 
version 7.3.0 (clang-703.0.29)) and D (dmd v2.079.0, also ldc2 
1.7.0) by doing long loops of such functions.

I can't get the D to run faster than about half the speed of C++.

Are there benchmarks for such scientific functions published 
somewhere?