The GEM Optimizing Compiler System

1 Abstract The GEM compiler system is the technology Digital is using to build state-of-the-art compiler products for a variety of languages and hardware /software platforms. Portable, modular software components with carefully specified interfaces simplify the engineering of diverse compilers. A single optimizer, independent of the language and the target platform, transforms the intermediate language generated by the front end into a semantically equivalent form that executes faster on the target machine. The GEM system supports a range of languages and has been successfully retargeted and rehosted for the Alpha AXP and MIPS architectures and for several operating environments. In the past, Digital has made major investments in optimizing compilers that were specifically directed at one hardware platform, namely VAX computers. When Digital began broadening its hardware offerings to include reduced instruction set computer (RISC) architectures, it became clear that new optimization technology was needed, as well as a new strategy for leveraging investments in compiler technology across an increasing number of hardware platforms. This paper presents a technical description of the GEM compiler technology that Digital uses to generate compiler products for a wide range of hardware and software combinations. We begin with an explanation of the GEM strategy of leveraging investments by using portable, modular software components to build compiler products. The bulk of the paper describes the GEM optimizer and code generator technologies, with a focus on how they address challenges posed by the Alpha AXP architecture.[1] We then move to a discussion of compiler engineering and conclude with an overview of some planned enhancements to the software. 2 GEM Compiler Architecture Because of the many hardware platforms available, often with multiple operating systems and a variety of languages offered on those platforms, building a compiler from scratch for each combination is no longer feasible. To simplify the engineering of diverse compilers, GEM compiler products share a basic architecture. The compiler is divided into several major components, in effect, the fundamental building blocks from which a compiler is constructed. The interfaces among these components are carefully specified. The major components of a GEM compiler are the front end, the optimizer, the code generator, and the compiler shell. The logical division of GEM components and the range of GEM support is shown in Figure 1. Note that the host is the computer on which the compiler runs, and the