In its current state you don't want to be using SIMD with dmd because the generated assembly will be significantly slower then if you just use the default FPU math.
That may be true for some kinds of code, but it isn't true int general. For example, see the comparison of pfft's performance when built with 64 bit DMD using SIMD and without SIMD:
http://i.imgur.com/kYYI9R9.png This benchmark was run on a core i5 2500K on 64 bit Debian Wheezy.
