Code Compression for VLIW Embedded Systems Using a Self-Generating Table

We propose a new class of methods for VLIW code compression using variable-sized branch blocks with self-generating tables. Code compression traditionally works on fixed-sized blocks with its efficiency limited by their small size. A branch block, a series of instructions between two consecutive possible branch targets, provides larger blocks for code compression. We compare three methods for compressing branch blocks: table-based, Lempel-Ziv-Welch (LZW)-based and selective code compression. Our approaches are fully adaptive and generate the coding table on-the-fly during compression and decompression. When encountering a branch target, the coding table is cleared to ensure correctness. Decompression requires a simple table lookup and updates the coding table when necessary. When decoding sequentially, the table-based method produces 4 bytes per iteration while the LZW-based methods provide 8 bytes peak and 1.82 bytes average decompression bandwidth. Compared to Huffman's 1 byte and variable-to-fixed (V2F)'s 13-bit peak performance, our methods have higher decoding bandwidth and a comparable compression ratio. Parallel decompression could also be applied to our methods, which is more suitable for VLIW architectures.