Comment by brian-armstrong

Clang supports vector types for C and they're still nowhere near as fast as using intrinsics. For whatever various reasons, compilers tend to do pretty poorly at producing properly vectorized code. I think part of this is that it's hard to encode all the information you need to vectorize properly (this loop will run some power of 2 number of times, these pointers won't alias, etc)

Even using intrinsics is dicey sometimes - compilers spill registers more often than can make sense. I think the ideal situation would be if you could somehow hint at register scheduling without writing asm yourself, but compiler are still a long ways from that.