A SIMD optimization framework for retargetable compilers

Abstract

Retargetable C compilers are currently widely used to quickly obtain compiler support for new embedded processors and to perform early processor architecture exploration. A partially inherent problem of the retargetable compilation approach, though, is the limited code quality as compared to hand-written compilers or assembly code due to the lack of dedicated optimizations techniques. This problem can be circumvented by designing flexible, retargetable code optimization techniques that apply to a certain range of target architectures. This article focuses on target machines with SIMD instruction support, a common feature in embedded processors for multimedia applications. However, SIMD optimization is known to be a difficult task since SIMD architectures are largely nonuniform, support only a limited set of data types and impose several memory alignment constraints. Additionally, such techniques require complicated loop transformations, which are tailored to the SIMD architecture in order to exhibit the necessary amount of parallelism in the code. Thus, integrating the SIMD optimization and the required loop transformations together in a single retargeting formalism is an ambitious challenge. In this article, we present an efficient and quickly retargetable SIMD code optimization framework that is integrated into an industrial retargetable C compiler. Experimental results for different processors demonstrate that the proposed technique applies to real-life target machines and that it produces code quality improvements close to the theoretical limit.