A Flexible Code Compression Scheme using Partitioned Look-Up Tables

Rapport, 2008

Wide instruction formats make it possible to control microarchitecture resources more finely by enabling more parallelism (VLIW) or by utilizing the microarchitecture more efficiently by exposing the control to the compiler. Unfortunately, wide instructions impose a higher pressure on the memory system due to an increased instruction-fetch bandwidth and a larger code working set/footprint. This paper presents a code compression scheme that allows the compiler to select what subset of the wide instruction set to use in each program phase at the granularity of basic blocks based on a profiling methodology. The decompression engine comprises a set of tables that convert a narrow instruction into a wide instruction in a dynamic fashion. The paper also presents a method for how to configure and dimension the decompression engine and how to generate a compressed program with embedded instructions that dynamically manage the tables in the decompression engine. We find that the 77 control bits in the original FlexCore instruction format can be reduced to 32 bits offering a compression of 58% and a modest performance overhead of less than 1% for management of the decompression tables.