Why is D slower than LuaJIT?

[Andrei Alexandrescu]

On 12/22/10 11:06 PM, Andreas Mayer wrote:
> Andrei Alexandrescu Wrote:
>
>> Andreas, any chance you could run this on your machine and compare it
>> with Lua? (I don't have Lua installed.) Thanks!
>
> Your version: 40 ms (iota and baseline give the same timings)
> LuaJIT with map calls removed: 21 ms
>
> Interesting results.

Cool, thanks. I also tested against this C++ baseline:

#include <stdio.h>

int main() {
      const double limit = 10000000.0;
      double result = 0.0;
      for (double i = 0; i != limit; ++i) {
        result += i;
      }
      printf("%f\n", result);
}

The baseline (compiled with -O3) runs in 21 ms on my machine, which 
means (if my and Andreas' machines are similar in performance) that Lua 
has essentially native performance for this loop and D has an issue in 
code generation that makes it 2x slower. I think this could be filed as 
a performance bug for dmd.

I'm thinking what to do about iota, which has good features but exacts 
too much cost on tight loop performance. One solution would be to define 
iota to be the simple, forward range that I defined as Iota2 in my 
previous post. Then, we need a different name for the full-fledged iota 
(random-access, has known length, iterates through the same numbers 
forward and backward etc). Ideas?


Andrei