Compilation Framework for Code Size Reduction using Reduced Bit-width ISAs

Aviral Shrivastava , Partha Biswas , Ashok Halambi , Nikil Dutt , and Alex Nicolau

ACM TODAES: ACM Transactions on Design Automation of Electronic Systems

Abstract: 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 feature however, requires compilers that can reduce code size by compiling for both Instruction Sets. Existing compiler techniques operate at the routine-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 compilation framework for such dual instruction sets, which uses a profitability based compiler heuristic that operates at the instruction-level granularity and is able to effectively take advantage of both Instruction Sets. We demonstrate consistent and improved code size reduction (on average 22%), for the MIPS 32/16 bit ISA. We also show that the code compression obtained by this "dual instruction set" technique is heavily dependent on the application characteristics and the narrow Instruction Set itself.