A compiler is a computer program that translates a computer program written in one computer language (called the source language) into a program written in another computer language (called the output or the target language).
Most compilers translate source code written in a high level language to object code or machine language that may be directly executed by a computer or a virtual machine. However, translation from a low level language to a high level one is also possible; this is normally known as a decompiler if it is reconstructing a high level language which (could have) generated the low level language. Compilers also exist which translate from one high level language to another, or sometimes to an intermediate language that still needs further processing; these are sometimes known as cascaders.
Typical compilers output so-called objectss that basically contain machine code augmented by information about the name and location of entry points and external calls (to functions not contained in the object). A set of object files, which need not have all come from a single compiler provided that the compilers used share a common output format, may then be linked together to create the final executable which can be run directly by a user.
A compiler may produce code intended to run on the same type of computer and operating system ("platform") as the compiler itself runs on. This is sometimes called a native-code compiler. Alternatively, it might produce code designed to run on a different platform. This is known as a cross compiler. Cross compilers are very useful when bringing up a new hardware platform for the first time (see bootstrapping). A "source to source compiler" is a type of compiler that takes a high level language as its input and outputs a high level language. For example, an automatic parallelizing compiler will frequently take in a high level language program as an input and then transform the code and annotate it with parallel code annotations (e.g. OpenMP) or language constructs (e.g. FORTRAN DOALL statements).
Many modern compilers share a common 'two stage' design. The front end translates the source language into an intermediate representation. The second stage is the back end, which works with the internal representation to produce code in the output language. The front end and back end may operate as separate passes, or the front end may call the back end as a subroutine, passing it the intermediate representation.
This approach mitigates complexity separating the concerns of the front end, which typically revolve around language semantics, error checking, and the like, from the concerns of the back end, which concentrates on producing output that is both efficient and correct. It also has the advantage of allowing the use of a single back end for multiple source languages, and similarly allows the use of different back ends for different targets.
Certain languages, due to the design of the language and certain rules placed on the declaration of variables and other objects used, and the predeclaration of executable procedures prior to reference or use, are capable of being compiled in a single pass. The Pascal programming language is well known for this capability, and in fact many Pascal compilers are themselves written in the Pascal language because of the rigid specification of the language and the capability to use a single pass to compile Pascal language programs.
Syntax analysis - Identifying syntactic structures of source code. It only focuses on the structure. In other words, it identifies the order of tokens and understand hierarchical structures in code. This phase is also called parsing.
Semantic analysis is to recognize the meaning of program code and start to prepare for output. In that phase, type checking is done and most of compiler errors show up.
Optimization - the intermediate language representation is transformed into functionally equivalent but faster (or smaller) forms.
Code generation - the transformed intermediate language is translated into the output language, usually the native machine language of the system. This involves resource and storage decisions, such as deciding which variables to fit into registers and memory and the selection and scheduling of appropriate machine instructions.
During the 1990s a large number of free compilers and compiler development tools have been developed for all kinds of languages, both as part of the GNU project and other open-source initiatives. Some of them are considered to be of high quality and their free source code makes a nice read for anyone interested in modern compiler concepts.
by Alfred V. Aho, Ravi Sethi, and Jeffrey D. Ullman (ISBN 0201100886) is considered to be the standard authority on compiler basics, and makes a good primer for the techniques mentioned above. (It is often called the Dragon Book because of the picture on its cover showing a Knight of Programming fighting the Dragon of Compiler Design.) External link to publisher's catalog entry
(ISBN 0333217322) by Richard Bornat is an unusually helpful book, being one of the few that adequately explains the recursive generation of machine instructions from a parse-tree. Having learnt his subject in the early days of mainframes and minicomputers, the author has many useful insights that more recent books often fail to convey.
