middleend

Middleend (or middle end) of a compiler refers to the portion of the compiler that sits between the front-end and the back-end. Its primary role is to transform a source program into an intermediate representation (IR) and to optimize and prepare it for code generation. The middle-end is designed to be largely target-agnostic, performing analyses and transformations that are valid across multiple architectures before any machine-specific code is produced.

Typical tasks include semantic checks, type resolution, and building symbol tables, followed by IR construction. The middle-end applies a sequence of optimizations such as constant propagation, dead code elimination, inlining, loop transformations, and data-flow analyses (e.g., reaching definitions, live variable analysis) to improve performance without altering observable semantics. Many middle-ends use an intermediate representation in SSA form because it simplifies data-flow reasoning and optimization across function boundaries.

The middle-end produces a cleaned, optimized IR that the back-end lowers to machine code. The back-end handles target-specific aspects such as instruction selection, register allocation, and scheduling. In practice, compilers like LLVM separate front-end, middle-end, and back-end layers to enable reusable optimizations independent of architecture, while other compiler architectures implement similar division with varying responsibilities.

The concept is used in compiler architecture models and in discussions of design methodology; while specifics vary, the middle-end remains the stage responsible for analysis, transformation, and optimization of the IR before code generation.