Understanding the Three Stages of Compiler Design: Front End, Middle End, and Back End
Compilers are complex programs that transform source code into machine code that can be executed by a computer. The process of compiling is divided into three stages: the front end, the middle end, and the back end. Each stage is responsible for a different set of tasks that are essential to the compilation process.
The front end is the first stage of the compiler, and it is responsible for scanning the input and verifying syntax and semantics according to a specific source language. The front end performs lexical analysis, syntax analysis, and semantic analysis. The front end also performs type checking for statically typed languages by collecting type information. If the input program is syntactically incorrect or has a type error, the front end generates error and/or warning messages, usually identifying the location in the source code where the problem was detected.
Once the front end has verified the syntax and semantics of the input program, it transforms the program into an intermediate representation (IR) for further processing by the middle end. The IR is usually a lower-level representation of the program with respect to the source code.
The middle end performs optimizations on the IR that are independent of the CPU architecture being targeted. This source code/machine code independence enables generic optimizations to be shared between versions of the compiler supporting different languages and target processors. Examples of middle end optimizations are dead-code elimination, reachability analysis, constant propagation, relocation of computation to a less frequently executed place, and specialization of computation based on the context. The middle end produces the “optimized” IR that is used by the back end.
The back end takes the optimized IR from the middle end and generates target-dependent assembly code. The back end performs analysis, transformations, and optimizations that are specific to the target CPU architecture. The back end performs instruction scheduling, which re-orders instructions to keep parallel execution units busy by filling delay slots. The back end also performs register allocation, which assigns variables to CPU registers. The output of the back end is machine code specialized for a particular processor and operating system.
The front/middle/back-end approach makes it possible to combine front ends for different languages with back ends for different CPUs while sharing the optimizations of the middle end. Practical examples of this approach are the GNU Compiler Collection, Clang (LLVM-based C/C++ compiler), and the Amsterdam Compiler Kit, which have multiple front-ends, shared optimizations, and multiple back-ends.
In conclusion, the compilation process is divided into three stages: the front end, the middle end, and the back end. The front end verifies syntax and semantics according to a specific source language, while the middle end performs optimizations on the IR that are independent of the CPU architecture being targeted. The back end generates target-dependent assembly code and performs analysis, transformations, and optimizations that are specific to the target CPU architecture. The front/middle/back-end approach makes it possible to combine front ends for different languages with back ends for different CPUs while sharing the optimizations of the middle end.
