Editing Retargeting (section)

==Compilers==
A '''retargetable compiler''' is a [[compiler]] that has been designed to be relatively easy to modify to generate code for different [[CPU]] [[instruction set architecture]]s.  The history of this idea dates back to the 1950s when [[UNCOL]] was proposed as the universal intermediate language.  The Pascal [http://www.software.ac.uk/blog/2013-08-19-heroes-software-engineering 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., [[GNU Compiler Collection|GCC]], became 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.<ref>[http://www.well.com/~cwf/pro/Davidson%20and%20Fraser.%20The%20design%20and%20application%20of%20a%20retargetable%20peephole%20optimizer.pdf Davidson and Fraser; The Design and Application of a Retargetable Peephole Optimizer; ToPLaS v2(2) 191-202 (April 1980)]</ref>  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 be those applicable to generic processor characteristics.

A retargetable compiler is a kind of [[cross compiler]].
Often (but not always) a retargetable compiler is [[software portability|portable]] (the compiler itself can run on several different CPUs) and [[Self-hosting (compilers)|self-hosting]].

Examples of retargetable compilers:
*[[GNU Compiler Collection|GCC]]
*[[Amsterdam Compiler Kit|ACK]]
*[[Little C compiler|lcc]]
*[[VBCC]]
*[[Portable C Compiler]]
*[[Small Device C Compiler|SDCC]]
*[[LLVM]]
* The [[Small-C]] compiler
* MPG, the "machine-independent efficient microprogram generator"<ref>
Rainer Leupers, Peter Marwedel.
[https://books.google.com/books?id=P_oBnK5xkywC "Retargetable Compiler Technology for Embedded Systems"].
2001.
({{ISBN|0-7923-7578-5}})
p. 63.
</ref>