I've used ISPC before, as well as enoki (sort of like ISPC-in-c++), and found that they have a lot of sharp performance edges. My experience with both was that

I've used ISPC before, as well as enoki (sort of like ISPC-in-c++), and found that they have a lot of sharp performance edges. My experience with both was that as I moved away from the super classic SIMD cases, the more I ran into crazy compiler cliffs where tiny tweaks would blow up the codegen. In each case I gave up, reimplemented what I wanted directly in c++ (the second time using anger fog's wonderful vector class library), and easily got the results I wanted without a ton of finagling the compiler and libraries.