A dictionary construction technique for code compression systems with echo instructions

Abstract

Dictionary compression mechanisms identify redundant sequences of instructions that occur in a program. The sequences are extracted and copied to a dictionary. Each sequence is then replaced with a codeword that acts as an index into the dictionary, thereby enabling decompression of the program at runtime. The problem of optimally organizing a dictionary consisting solely of redundant sequences in order to maximize compression has long been known to be NP-Complete [23]. This paper addresses the problem of dictionary construction when redundant code fragments are represented as Data Flow Graphs ( DFGs ) rather than linear sequences of instructions. Since there are generally multiple legal schedules for a given DFG G , a compiler must determine a schedule for G so that other DFGs that are subgraphs of G can reference some substring of G 's final code sequence. This reduces the size of the dictionary, and in turn, the size of the compressed program. Our experiments with 10 MediaBench [18] applications yielded reductions in dictionary size ranging from 21.14% to 29.76% compared to a naïve approach.