How fast is D compared to C++?

Asked 2011-02-28T13:06:37.363

141

I like some features of D, but would be interested if they come with a runtime penalty?

To compare, I implemented a simple program that computes scalar products of many short vectors both in C++ and in D. The result is surprising:

D: 18.9 s [see below for final runtime]
C++: 3.8 s

Is C++ really almost five times as fast or did I make a mistake in the D program?

I compiled C++ with g++ -O3 (gcc-snapshot 2011-02-19) and D with dmd -O (dmd 2.052) on a moderate recent linux desktop. The results are reproducible over several runs and standard deviations negligible.

Here the C++ program:

#include <iostream>
#include <random>
#include <chrono>
#include <string>

#include <vector>
#include <array>

typedef std::chrono::duration<long, std::ratio<1, 1000>> millisecs;
template <typename _T>
long time_since(std::chrono::time_point<_T>& time) {
      long tm = std::chrono::duration_cast<millisecs>( std::chrono::system_clock::now() - time).count();
  time = std::chrono::system_clock::now();
  return tm;
}

const long N = 20000;
const int size = 10;

typedef int value_type;
typedef long long result_type;
typedef std::vector<value_type> vector_t;
typedef typename vector_t::size_type size_type;

inline value_type scalar_product(const vector_t& x, const vector_t& y) {
  value_type res = 0;
  size_type siz = x.size();
  for (size_type i = 0; i < siz; ++i)
    res += x[i] * y[i];
  return res;
}

int main() {
  auto tm_before = std::chrono::system_clock::now();

  // 1. allocate and fill randomly many short vectors
  vector_t* xs = new vector_t [N];
  for (int i = 0; i < N; ++i) {
    xs[i] = vector_t(size);
      }
  std::cerr << "allocation: " << time_since(tm_before) << " ms" << std::endl;

  std::mt19937 rnd_engine;
  std::uniform_int_distribution<value_type> runif_gen(-1000, 10


                    
                    
                        
                        c++
                        
                        performance
                        
                        runtime
                        
                        d
                        
                    
                    
                        
                        
                        
                        
                        
                        Edit
                        
                        
                        
                            
                        
                        
                        Report



        
            2 Answers
            
            
                
                    
                        
                            
                            
                        
                        69
                        
                            
                            
                        
                        
                    
                    
                        
                        To enable all optimizations and disable all safety checks, compile your D program with the following DMD flags:

-O -inline -release -noboundscheck


EDIT: I've tried your programs with g++, dmd and gdc. dmd does lag behind, but gdc achieves performance very close to g++. The commandline I used was gdmd -O -release -inline (gdmd is a wrapper around gdc which accepts dmd options).

Looking at the assembler listing, it looks like neither dmd nor gdc inlined scalar_product, but g++/gdc did emit MMX instructions, so they might be auto-vectorizing the loop.

                        
                        
                            answered 2011-02-28T13:14:39.837
                            
                        
                        
                            
                                
                                
                            
                        
                        
                            
                            
                            
                                
                                
                            
                            
                        
                    
                
            
            
            
                
                    
                        
                            
                            
                        
                        31
                        
                            
                            
                        
                        
                    
                    
                        
                        This is a very instructive thread, thanks for all the work to the OP and helpers.

One note - this test is not assessing the general question of abstraction/feature penalty or even that of backend quality. It focuses on virtually one optimization (loop optimization). I think it's fair to say that gcc's backend is somewhat more refined than dmd's, but it would be a mistake to assume that the gap between them is as large for all tasks.

                        
                        
                            answered 2011-03-02T19:25:36.643
                            
                        
                        
                            
                                
                                
                            
                        
                        
                            
                            
                            
                                
                                
                            
                            
                        
                    
                
            
            
        

        
        
            Your Answer