Woeful performance of D compared to C++

[Sean Kelly]

I tried running these under Tango with DMD on Win32 (as it's the setup I 
currently have).  Here are my slightly altered programs to make the two 
a bit more comparable.  First, the D code:

import tango.stdc.stdlib;
import tango.stdc.stdio;

void main() {
     const uint n = 10_000_000;
     ubyte doors;
     uint wins, wins_switching;

     for (uint i; i < n; ++i) {
         doors |= cast(ubyte)(1 << rand() % 3);

         if (doors & 1) {
             ++wins;
         } else {
             ++wins_switching;
         }

         doors = 0;
     }

     printf("Wins switching: %d [%f%%]\n", wins_switching,
              (wins_switching / cast(double) n) * 100);
     printf("Wins without switching: %d [%f%%]\n", wins,
              (wins / cast(double) n) * 100);
}

And now the C++ code:

#include <cstdlib>
#include <cstdio>

int main() {
     unsigned char doors = 0;
     const unsigned int n = 10000000;
     unsigned int wins = 0, wins_switching = 0;

     for (unsigned int i = 0; i < n; ++i) {
         unsigned char r = 1 << (rand() % 3);
         doors |= r; // place the car behind a random door

         if (doors & 1) { // choose zero'th door, same as random choice
             ++wins;
         } else {
             ++wins_switching;
         }

         doors ^= r; // zero the door with car
     }

     const double d = n / 100;

     printf("Wins switching: %d [%f%%]\n", wins_switching,
              (wins_switching / (double) n) * 100);
     printf("Wins without switching: %d [%f%%]\n", wins,
              (wins / (double) n) * 100);
}

C:> dmd -O -inline -release dtest
C:> dmc -o ctest.cpp

Here are the results for three runs of the D app:

Execution time: 1.323 s
Execution time: 1.005 s
Execution time: 1.125 s

And three runs of the C++ app:

Execution time: 1.149 s
Execution time: 1.202 s
Execution time: 1.304 s

The numbers above aren't quite as accurate as those using "time" on 
Unix, but they're sufficient for a rough comparison.  That said, DMD and 
DMC perform pretty much the same once the variable of IOStreams vs. 
writefln is removed.


Sean