By and large, GPUs don't have this anymore, actually (they used to, but this changed many years ago). On modern GPUs your example is "scalarized" and results i

By and large, GPUs don't have this anymore, actually (they used to, but this changed many years ago). On modern GPUs your example is "scalarized" and results in separate instructions for each vector component. Of course execution is still "vectorized", but only in the sense of operating on the data for 16/32/64 threads at once. The reason for this design is that while vec3 and vec4 are common, they are by no means universal in modern shader code. A micro-architecture built on vec3 or vec4 would be under-utilized in the large amounts of shader code that are scalar in the source language. There are some exceptions, e.g. native support for 16-bit vec2 comes naturally on architectures with 32-bit registers. But those are exceptions.