The graphics in IVB are not just on the die, but are part of the internal memory ring and thus the shared L3 cache. So CPU cores are able to directly send and receive shared memory from the GPU, and the GPU is able to use the same last-level cache (LLC) as the CPU cores.
This is why you see the simple fill-rate benchmarks of IVB blow away SNB and discrete GPUs.
This is why you see the simple fill-rate benchmarks of IVB blow away SNB and discrete GPUs.