What's the purpose of a compiler backend?

The purpose of a compiler backend is to generate machine code from the intermediate representation produced by the compiler frontend.

The compiler backend, also known as the code generator, is the second half of the compiler structure. It takes the intermediate representation (IR) produced by the compiler frontend and translates it into machine code that can be executed by the target machine. This process is crucial in the overall compilation process as it bridges the gap between high-level programming languages and the low-level machine language.

The backend of a compiler is responsible for several tasks. Firstly, it performs optimisation on the IR to improve the efficiency of the generated code. This can involve removing redundant code, reordering instructions to make better use of the processor's execution units, or replacing slow operations with faster ones. The goal is to produce code that runs as quickly and efficiently as possible, while still producing the same results as the original program.

Next, the backend maps the IR to the instruction set of the target machine. This involves selecting the appropriate machine instructions to perform each operation in the IR, and arranging them in the correct order. The backend must also allocate registers for storing temporary values during computation. This is a complex task, as the backend must balance the need to minimise memory usage with the need to avoid unnecessary data movement between registers and memory.

Finally, the backend generates the actual machine code. This is the binary code that can be directly executed by the processor. The backend must ensure that this code adheres to the calling conventions of the target machine, which dictate how functions are called and how parameters are passed between them.

In summary, the compiler backend plays a crucial role in the compilation process. It takes the high-level, platform-independent code produced by the frontend and transforms it into efficient, low-level machine code that can be executed on the target machine. This involves a series of complex tasks, including optimisation, instruction selection, register allocation, and code generation.