Reducing dynamic energy of set-associative L1 instruction cache by early tag lookup

To minimize the access latency of set-associative caches, the data in all ways are read out in parallel with the tag lookup. However, this is energy inefficient, as only the data from the matching way is used and the others are discarded. This paper proposes an early tag lookup (ETL) technique for L1 instruction caches that determines the matching way one cycle earlier than the cache access, so that only the matching data way need be accessed. ETL incurs no performance penalty and insignificant hardware overhead. Evaluation on a 4-way set-associative L1 instruction cache in 45nm technology shows that ETL reduces the read energy by 68% on average.