It's been about ten years since I did any kernel work, but last I checked Linux doesn't use most of the fancy features of the CPU to avoid trashing more registers than necessary. If you know you don't do any floating point operations, for example, you know you don't need to save/restore the floating point registers on interrupts. The same might go for things like SSE.

Those instructions tend to be larger, so it may partly explain the difference in average instruction size. The same explanation would apply to why the Radeon driver has a higher average.

I wonder if that flag does anything at the instruction encoding level