c++ - A simple test case between clang++/g++/gfortran -


i ran across question on scicomp involves computing sum. there, can see c++ , similar fortran implementation. interestingly saw fortran version faster 32%.

i thought, not sure result , tried regenerate situation. here (very slightly) different codes ran:

c++

#include <iostream> #include <complex> #include <cmath> #include <iomanip>  int main () {     const double alpha = 1;     std::cout.precision(16);      std::complex<double> sum = 0;     const std::complex<double> = std::complex<double>(1,1)/std::sqrt(2.);     (unsigned int k=1; k<10000000; ++k)     {         sum += std::pow(a, k)*std::pow(k, -alpha);          if (k % 1000000 == 0)             std::cout << k << ' ' << sum << std::endl;     }      return 0; }

fortran

implicit none integer, parameter :: dp = kind(0.d0) complex(dp), parameter :: i_ = (0, 1)  real(dp) :: alpha = 1 complex(dp) :: s = 0 integer :: k k = 1, 10000000     s = s + ((i_+1)/sqrt(2._dp))**k * k**(-alpha)     if (modulo(k, 1000000) == 0) print *, k, s end end

i compile above codes using gcc 4.6.3 , clang 3.0 on ubuntu 12.04 lts machine -o3 flag. here's timings:

time ./a.out

gfortran

real    0m1.538s user    0m1.536s sys     0m0.000s

g++

real    0m2.225s user    0m2.228s sys     0m0.000s

clang

real    0m1.250s user    0m1.244s sys     0m0.004s

interestingly can see fortran code faster c++ same 32% when gcc used. using clang, however, can see c++ code runs faster 19%. here questions:

  1. why g++ generated code slower gfortran? since same compiler family mean (this) fortran code can translated faster code? case fortran vs c++?
  2. why clang doing here? there fortran front-end llvm compiler? if there, code generated 1 faster?

update:

using -ffast-math -o3 options generates following results:

gfortran

real    0m1.515s user    0m1.512s sys     0m0.000s

g++

real    0m1.478s user    0m1.476s sys     0m0.000s

clang

real    0m1.253s user    0m1.252s sys     0m0.000s

npw g++ version running fast gfortran , still clang faster both. adding -fcx-fortran-rules above options not change results

i believe problem in output part. well-known c++ streams (std::cout) inefficient. while different compilers may optimize this, idea rewrite critical performance parts using c printf function instead of std::cout.


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