Bounds checking gets in the way for simple cases but anything even slightly more complicated needs to be written extremely carefully in any language, for autovectorisation to work. It is like, essentially, writing explicit SIMD without using intrinsics and without guarantees it will work as desired.

And, that is assuming that the autovectoriser is able to synthesise the desired instructions, e.g. I believe SSE2's packssdw & packusdw ("pack with signed/unsigned saturation") and pmaddwd ("multiply and add packed integers") are useful in a JPEG codec but I find it extremely unlikely that any compiler will autovectorise to them.