D-Programming-Language/druntime — pull request #290 by ibuclaw (2012-08-16 22:50:36+00:00)
Title: __simd implementation available only if D_SIMD.

As per the docs:

```D_SIMD - Vector extensions (__vector and __simd) are supported```
http://dlang.org/version.html#PredefinedVersions

The likes of other compilers (such as GDC) may support __vector, but not __simd, so the __simd implementation should be reserved for compilers that use the predefined version D_SIMD.