They tend to stumble over data dependencies and for floating point math they can't reorder instructions as well (unless you explicitly disable IEEE754 complianc

They tend to stumble over data dependencies and for floating point math they can't reorder instructions as well (unless you explicitly disable IEEE754 compliance). There are a number of structured approaches to rewriting code to be SIMD friendly, but you do need a degree of explicitness to get what you want out of the compiler.