An efficient compiler technique for code size reduction using reduced bit-width ISAs

For many embedded applications, program code size is a critical design factor. One promising approach for reducing code size is to employ a "dual instruction set", where processor architectures support a normal (usually 32 bit) Instruction Set, and a narrow, space-efficient (usually 16 bit) Instruction Set with a limited set of opcodes and access to a limited set of registers. This future, however, requires compilers that can reduce code size by compiling for both Instruction Sets. Existing compiler techniques operate at the function-level granularity and are unable to make the trade-off between increased register pressure (resulting in more spills) and decreased code size. We present a profitability based compiler heuristic that operates at the instruction-level granularity and is able to effectively take advantage: of both Instruction Sets. We also demonstrate improved code size reduction, for the MIPS 32/16 bit ISA, using our technique. Our approach more than doubles the code size reduction achieved by existing compilers.