A DISE Implementation of Dynamic Code Decompression

Code compression coupled with dynamic decompression is an important technique for both embedded and general-purpose microprocessors. Post-fetch decompression, in which decompression is performed after the compressed instructions have been fetched, allows the instruction cache to store compressed code but requires a highly efficient decompression implementation. We propose implementing post-fetch decompression using dynamic instruction stream editing (DISE), a programmable decoder---similar in structure to those in many IA32 processors---that is used to add functionality to an application by injecting custom code snippets into its fetched instruction stream. A DISE implementation of post-fetch decompression naturally supports customized program-specific decompression dictionaries, enables parameterized decompression allowing similar instruction sequences to share dictionary entries, and uses no decompression-specific hardware. Cycle-level simulation of DISE decompression shows that it can reduce static program size by 35% and execution time by 20%. Parameterized decompression, a feature unique to DISE, accounts for 20% of the code size reduction by making more effective use of the dictionary and allowing PC-relative branches to be included in compressed sequences. DISE-based compression can reduce total energy consumption by 10% and the energy-delay product by as much as 20%. Categories and Subject Descriptors B.3 [Hardware]: Memory Structure