Post-Link Outlining for Code Size Reduction

Abstract

This paper introduces PLOS, a novel Post-Link Outlining approach designed to enhance code Size reduction for resource-constrained environments. Built on top of a post-link optimizer BOLT, PLOS maintains a holistic view of the whole-program structure and behavior, utilizing runtime information while preserving standard build system flows. The approach includes a granular outlining algorithm that matches and replaces repeated instruction sequences within/across modules and outlined functions, along with careful stack frame management to ensure correct function call handling. By integrating profiling information, PLOS balances the trade-off between code size and execution efficiency. The evaluation using eight MiBench benchmarks on an ARM-based Phytium FCT662 core demonstrates that PLOS achieves a mean code size reduction of 10.88% (up to 43.53%) and 6.61% (up to 14.78%) compared to LLVM’s and GCC's standard optimization, respectively, 1.76% (up to 4.75%) over LLVM’s aggressive code size reduction optimizations, and 2.88% (up to 8.56%) over a link-time outliner. The experimental results also show that PLOS can achieve a favorable balance between code size reduction and performance regression.