Superoptimisation : provably optimal code generation using answer set programming

Code optimisation in modern compilers is an accepted misnomer for performance im­ provement some of the time. The code that compilers generate is often significantly improved, but it is unlikely to produce optimal sequences of instructions; and if it does, it will not be possible to determine that they are indeed optimal. None of the existing approaches, or techniques for creating new optimisations, is likely to change this state of play. Superoptimisation is a radical approach to generating provably optimal code, that per­ forms searches over the space of all possible instructions. Rather than starting with naively generated code and improving it, a superoptimiser starts with the specification of a function and performs a directed search for an optimal sequence of instructions that fulfils this specification. In this thesis, we present TOAST, the Total Optimisation using Answer Set Techno­ logy system, a provably optimal code generation system that applies superoptimising techniques to optimise acyclic integer-based code for modern microprocessor architec­ tures. TOAST utilises Answer Set Programming (ASP), a declarative logic program­ ming language, as an expressive modelling and efficient computational framework to solve the optimal code generation problem. We demonstrate the validity of the approach of superoptimisation using Answer Set Programming by optimising code sequences for two 32-bit RISC architectures, the MIPS R2000 and the SPARC V8. We also present an application of the TOAST sys­ tem as a peephole optimiser, by generating libraries of equivalence classes of all op­ timal instruction sequences of a given length for a specific microprocessor architecture. While this is a computationally expensive process, it only ever needs to be performed once per architecture. We also provide significant benchmarks for the performance of state of the art domain solver tools, further demonstrating the applicability of Answer Set Programming in modelling complex real-world problems.