Matrix Multiplication benchmark

[Matthias Pleh]

I've created a simple 4x4 Matrix struct and have made some tests, with 
surprising results. (although I've heard from similar results in c++)

struct mat4 {
   float[4][4] m;
   mat4 opMul(in mat4 _m);
}
mat4 mul(in mat4 m1, in mat4 m2);


I've tested this 2 multiplication functions (overloaded and free).

Result (100_000 times, windows7 64bit, 32bit build):

DMD: (dmd -O -inline -release)
opMul:   20 msecs
mul  : 1355 msecs

GDC: (gdc -m32 -O3 -frelease)
opMul:   10 msecs
mul  : 1481 msecs


Why this huge difference in performance?