Energy-Effective Instruction Fetch Unit for Wide Issue Processors

Continuing advances in semiconductor technology and demand for higher performance will lead to more powerful, superpipelined and wider issue processors. Instruction caches in such processors will consume a significant fraction of the on-chip energy due to very wide fetch on each cycle. This paper proposes a new energy-effective design of the fetch unit that exploits the fact that not all instructions in a given I-cache fetch line are used due to taken branches. A Fetch Mask Determination unit is proposed to detect which instructions in an I-cache access will actually be used to avoid fetching any of the other instructions. The solution is evaluated for a 4-, 8- and 16-wide issue processor in 100nm technology. Results show an average improvement in the I-cache Energy-Delay product of 20% for the 8-wide issue processor and 33% for the 16-wide issue processor for the SPEC2000, with no negative impact on performance.