Removing and restoring control flow with the Value State Dependence Graph

This thesis studies the practicality of compiling with only data flow information. Specifically, we focus on the challenges that arise when using the Value State Dependence Graph (VSDG) as an intermediate representation (IR). We perform a detailed survey of IRs in the literature in order to discover trends over time, and we classify them by their features in a taxonomy. We see how the VSDG fits into the IR landscape, and look at the divide between academia and the 'real world' in terms of compiler technology. Since most data flow IRs cannot be constructed for irreducible programs, we perform an empirical study of irreducibility in current versions of open source software, and then compare them with older versions of the same software. We also study machine-generated C code from a variety of different software tools. We show that irreducibility is no longer a problem, and is becoming less so with time. We then address the problem of constructing the VSDG. Since previous approaches in the literature have been poorly documented or ignored altogether, we give our approach to constructing the VSDG from a common IR: the Control Flow Graph. We show how our approach is independent of the source and target language, how it is able to handle unstructured control flow, and how it is able to transform irreducible programs on the fly. Once the VSDG is constructed, we implement Lawrence's proceduralisation algorithm in order to encode an evaluation strategy whilst translating the program into a parallel representation: the Program Dependence Graph. From here, we implement scheduling and then code generation using the LLVM compiler. We compare our compiler framework against several existing compilers, and show how removing control flow with the VSDG and then restoring it later can produce high quality code. We also examine specific situations where the VSDG can put pressure on existing code generators. Our results show that the VSDG represents a radically different, yet practical, approach to compilation