Communications of the ACM
Intermediate Representation
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Program compilation is a complicated process. A compiler is a software program that translates a high-level source-language program into a form ready to execute on a computer. Early on in the evolution of compilers, designers introduced intermediate representations (IRs, also commonly called intermediate languages) to manage the complexity of the compilation process. The use of an IR as the compiler's internal representation of the program enables the compiler to be broken up into multiple phases and components, thus benefiting from modularity.
An IR is any data structure that can represent the program without loss of information so that its execution can be conducted accurately. It serves as the common interface among the compiler components. Since its use is internal to a compiler, each compiler is free to define the form and details of its IR, and its specification needs to be known only to the compiler writers. Its existence can be transient during the compilation process, or it can be output and handled as text or binary files. An IR should be general so that it is capable of representing programs translated from multiple languages. Compiler writers traditionally refer to the semantic content of programming languages as being high. The semantic content of machine-executable code is considered low because it has retained only enough information from the original program to allow its correct execution. It would be difficult (if not impossible) to recreate the source program from its lower form. The compilation process entails the gradual lowering of the program representation from high-level human programming constructs to low-level real or virtual machine instructions (see Figure 1). In order for an IR to be capable of representing multiple languages, it needs to be closer to the machine level to represent the execution behavior of all the languages. A longer code sequence usually accompanies machine-executable code because it reflects the details of the machines on which execution takes place.
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