Basically the whole package. The instruction set is pretty big, but this is not much of an issue as you generally just need a subset for a given problem (I gene

Basically the whole package. The instruction set is pretty big, but this is not much of an issue as you generally just need a subset for a given problem (I generally only tweak SSE/AVX code) and it has a lot of if's and but's when it comes to performance, the out-of-order execution also adds a degree of uncertainty, branch prediction and prefetching, a complex(ish) cache... There is a lot of things one can fiddle with in order to improve performance. GPUs are much stricter in their design, which can lead to some headaches when having to completely change the way you formulate a problem, but once you're past that step, the simplicity of the architecture makes it a lot nicer to optimize in my opinion. (And my personal experience has been that the tools are cheaper (free) and more stable as well, but your mileage may vary).