Clang doesn't guarantee that; it runs everything through its optimizer, which means you can use an intrinsic and not get the instruction it's named after.

The other big general issue is memory aliasing. If you're working on 8-bit data then C says that means it aliases everything and it's going to deoptimize your other memory accesses. Conversely if you're freely switching types like SIMD tends to do, that's against the aliasing rules.