The computer is going to be called the Post-K super because it will succeed Japan's K Supercomputer. The K machine is the fifth fastest in the world and can manage 10.5 PFLOPS, needs 12MW of power, and is built out of 705,000 Sparc64 VIIIfx cores.
But the Post-K will have 100 times more application performance than the K Supercomputer and manage 1,000 PFLOPS when it is switched on in 2020. The fastest known super in the world now is China's 125.4 PFLOPS Sunway TaihuLight machine which uses Chinese chips.
The Post-K will be the fastest known computer an ARM-powered system. It is not difficult really because there are no other ARM supercomputers out there.
The custom-designed supercomputer CPU powering the Post-K will run ARMv8 code which has been optimized o speed up the adding up. Fujitsu is an ARM licensee and is a big fan of classic RISC architectures. It used to use the Sparc64 and has now gone to the 64-bit ARM.