In my experience, I always write a clear, idiomic C code along with my intrinsics-based vectorized function, alo png with comment on tricky part (such as using packus for clamping thing to 0-255). You got clear code, and also optimized code (you might want C code anyway for pre-sse2/mmx and non-x86). Downside is that you are to maintained multiple copy of code, but if you have the version optimized for each sse2/sse3/sse4.1/avx2 anyway it is not really that more hassle.