How to tune numerical D? (matrix multiplication is faster in g++ vs gdc)

[jerro]

> A bit better version:
> http://codepad.org/jhbYxEgU
>
> I think this code is good compared to the original (there are 
> better algorithms).

You can make it much faster even without really changing the 
algorithm. Just by reversing the order of inner two loops like 
this:

void matrixMult2(in int[][] m1, in int[][] m2, int[][] m3) pure 
nothrow {
     foreach (immutable i; 0 .. m1.length)
         foreach (immutable k; 0 .. m2[0].length)
             foreach (immutable j; 0 .. m3[0].length)
                 m3[i][j] += m1[i][k] * m2[k][j];
}

you can make the code much more cache friendly (because now you 
aren't iterating any matrix by column in the inner loop) and also 
allow the compiler to do auto vectorization. matrixMul2() takes 
2.6 seconds on my machine and matrixMul()takes 72 seconds (both 
compiled with  gdmd -O -inline -release -noboundscheck -mavx).

This isn't really relevant to the comparison with C++ in this 
thread, I just thought it may be useful for anyone writing matrix 
code.