The Three-Stage Compiler Structure

This modular three-stage design provides a crucial separation of concerns. The front end is language-dependent but machine-independent; it understands the syntax and semantics of a specific language like C++ or Rust. Its output, the Intermediate Representation (IR), is an abstract, machine-agnostic data structure like an Abstract Syntax Tree (AST) or Three-Address Code. This decouples the source language from the target machine.

The middle end is largely language- and machine-independent. It takes the IR and applies a series of optimization passes, such as dead code elimination, constant folding, and loop optimizations. Because it operates on the generic IR, these complex optimizations can be written once and applied to any language that can be compiled to that IR.

Finally, the back end is machine-dependent but language-independent. It takes the optimized IR and performs instruction selection, register allocation, and instruction scheduling to generate efficient machine code for a specific target architecture, like x86-64 or ARM. This structure makes it possible to build compilers that support M languages and N targets by writing M front ends and N back ends, rather than M*N individual compilers. This principle is exemplified by modern compiler infrastructures like GCC and LLVM.