In software engineering, retargeting is an attribute of software development tools that have been specifically designed to generate code for more than one computing platform.
A retargetable compiler is a compiler that has been designed to be relatively easy to modify to generate code for different CPU instruction set architectures. The history of this idea dates back to the 1950s when UNCOL was proposed as the universal intermediate language. The Pascal P-compiler is an example of an early widely used retargetable compiler.
The cost of producing a retargetable compiler that generates code of similar quality to a non-retargetable compiler (i.e., one designed to only ever produce code for a single processor) is higher because it is not possible to make use of cpu specific details throughout all phases of compilation. The benefits of a retargetable compiler is that the total cost over multiple cpus is much lower than the combined cost of many individual non-targetable compilers.
Some retargetable compilers, e.g., GCC, become so widely ported and developed that they now include support for many optimizations and machine specific details that the quality of code often surpasses that of non-retargetable compilers on many cpus.
A general-purpose global optimizer followed by machine-specific peephole optimization is a commonly used implementation technique. The optimization of code for some high performance processors requires a detailed and specific knowledge of the architecture and how the instructions are executed. Unless developers invest the large amount of time necessary to write a code generator specifically for an architecture the optimizations performed by a retargetable compiler will only those applicable to generic processor characteristics.
Examples of retargetable compilers:
- Portable C Compiler
- The Small-C compiler
- MPG, the "machine-independent efficient microprogram generator"
Conversely, retargetable assemblers are capable of generating object files of different formats, which is useful in porting assembly language programs to various operating systems that run on the same CPU architecture (such as Windows and Linux on the x86 platform). NASM is one such assembler.
- A Retargetable C Compiler: Design and Implementation (ISBN 0-8053-1670-1)
- Davidson and Fraser; The Design and Application of a Retargetable Peephole Optimizer; ToPLaS v2(2) 191-202 (April 1980)
- Rainer Leupers, Peter Marwedel. "Retargetable Compiler Technology for Embedded Systems". 2001. (ISBN 0-7923-7578-5) p. 63.