[BlackMonday]

Yeah, the bandwith problem is already noticeable with AMD's current APU's even though they use small GPU cores compared to discrete grapics cards. Faster DDR3/4 memory brings noticeable FPS improvements. If they already had HBM they would run circles around Intel (which they probably already do unless the competitor is a Iris Pro with eDRAM).

Could the CPU also profit from the HBM memory? The bandwith is much better than with DDR4 main memory (even if it is 2 or 4 channels), and I would guess the latency as well because it would be on the same die?

[snuxoll]

HBM won't be on-die, but it will be on-package - HBM relies on chip stacking to get the desired throughput in a small surface area, regardless the latency and throughput would stomp system DRAM something awful, and if it's a proper L4 cache then the CPU would benefit as well.

IBM does something similar (though not for graphics) in recent POWER CPU's with the Centaur memory controller(s), they are off-chip memory controllers with a bunch of eDRAM to act as a L4 cache (though the difference here is each system has multiple centaur controllers to handle different DIMM slots). They're able to burst to ~96GB/sec to system memory using this, having a good amount of on-package HBM would probably yield similar gains :)