Compiler-driven cached code compression schemes for embedded ILP processors

During the last 15 years, embedded systems have grown in complexity and performance to rival desktop systems. The architectures of these systems present unique challenges to processor microarchitecture, including instruction encoding and instruction fetch processes. This paper presents new techniques for reducing embedded system code size without reducing functionality. This approach is to extract the pipeline decoder logic for an embedded VLIW processor in software at system development time. The code size reduction is achieved by Huffman compressing or tailor encoding the ISA of the original program. Some interesting results were found. In particular, the degree of compression for the ROM doesn't translate into an improvement in instructions delivered per cycle. Experiments found that when the missprediction penalty of the added Huffman decoder stage was taken into account, a Tailored ISA approach produced higher performance. Methods that compress the entire operation using Huffman encodings, and decompress at ICache hit time still achieved a median performance advantage, while providing higher ROM size savings. All results were generated by an optimizing compiler and tool suite, and presented for an encoding similar to the Intel/HP IA-64 architecture.