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{{otheruses4|the computing term|the anime|Compiler (anime)}}
{{Expert-subject|Computer_science|date=December 2008}}
[[Image:Compiler.svg|right|thumb|350px|A diagram of the operation of a typical multi-language, multi-target compiler.]]

A '''compiler''' is a  [[computer program]] (or set of programs) that transforms [[source code]] written in a [[programming language|computer language]] (the ''source language'') into another computer language (the ''target language'', often having a binary form known as ''[[object code]]'').  The most common reason for wanting to transform source code is to create an [[executable]] program.

The name "compiler" is primarily used for programs that translate source code from a [[high-level programming language]] to a lower level language (e.g., [[assembly language]] or [[machine code]]). A program that translates from a low level language to a higher level one is a ''[[decompiler]]''.  A program that translates between high-level languages is usually called a ''[[Translator (computing)|language translator]]'', ''source to source translator'', or ''language converter''. A ''language [[rewriting|rewriter]]'' is usually a program that translates the form of expressions without a change of language.

A compiler is likely to perform many or all of the following operations: [[lexical analysis]], [[preprocessing]], [[parsing]], semantic analysis, [[code generation (compiler)|code generation]], and [[code optimization]].

Program faults caused by incorrect compiler behavior can be very difficult to track down and work around and compiler implementors invest a lot of time ensuring the [[compiler correctness|correctness of their software]].

The term [[compiler-compiler]] is sometimes used to refer to a [[parser generator]], a tool often used to help create the [[lexical analysis|lexer]] and [[parser]].

==History==
{{Main|History of compiler writing}}
Software for early computers was primarily written in assembly language for many years. Higher level programming languages were not invented until the benefits of being able to reuse software on different kinds of [[CPU]]s started to become significantly greater than the cost of writing a compiler.  The very limited [[Computer storage|memory]] capacity of early computers also created many technical problems when implementing a compiler.

Towards the end of the 1950s, machine-independent programming languages were first proposed. Subsequently, several experimental compilers were developed. The first compiler was written by [[Grace Hopper]], in 1952, for the [[A-0 programming language]]. The [[FORTRAN]]<!-- ###here (only), upper-case FORTRAN is correct, as it was the name used at the time, and on IBM's early compilers ###--> team led by [[John Backus]] at [[IBM]] is generally credited as having introduced the first complete compiler, in 1957. [[COBOL]] was an early language to be compiled on multiple architectures, in 1960.<ref>[http://www.interesting-people.org/archives/interesting-people/199706/msg00011.html IP: "The World's First COBOL Compilers" -- 12 June 1997]</ref>

In many application domains the idea of using a higher level language quickly caught on.  Because of the expanding functionality supported by newer [[programming language]]s and the increasing complexity of computer architectures, compilers have become more and more complex.

Early compilers were written in assembly language. The first ''[[self-hosting]]'' compiler &mdash; capable of compiling its own source code in a high-level language &mdash; was created for [[Lisp programming language|Lisp]] by Tim Hart and Mike Levin at [[Massachusetts Institute of Technology|MIT]] in 1962.<ref>[ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-039.pdf T. Hart and M. Levin "The New Compiler", AIM-39] CSAIL Digital Archive - Artificial Intelligence Laboratory Series</ref> Since the 1970s it has become common practice to implement a compiler in the language it compiles, although both [[Pascal (programming language)|Pascal]] and [[C (programming language)|C]] have been popular choices for implementation language. Building a self-hosting compiler is a [[bootstrapping (compilers)|bootstrapping]] problem—the first such compiler for a language must be compiled either by a compiler written in a different language, or (as in Hart and Levin's Lisp compiler) compiled by running the compiler in an [[Interpreter (computing)|interpreter]].

=== Compilers in education ===
Compiler construction and [[compiler optimization]] are taught at universities and schools as part of the [[computer science]] curriculum. Such courses are usually supplemented with the implementation of a compiler for an [[educational programming language]]. A well-documented example is [[Niklaus Wirth]]'s [[PL/0]] compiler, which Wirth used to teach compiler construction in the 1970s.<ref>[http://www.246.dk/pl0.html "The PL/0 compiler/interpreter"]</ref> In spite of its simplicity, the PL/0 compiler introduced several influential concepts to the field:

# Program development by stepwise refinement (also the title of a 1971 paper by Wirth<ref>[http://www.acm.org/classics/dec95/ Book description at the ACM Digital Library]</ref>)
# The use of a [[recursive descent parser]]
# The use of [[EBNF]] to specify the syntax of a language
# A [[code generation (compiler)|code generator]] producing portable [[P-code]]
# The use of [[T-diagram]]s<ref>T diagrams were first introduced for describing bootstrapping and cross-compiling compilers in McKeeman et al. ''A Compiler Generator'' (1971). Conway described the broader concept before that with his [[UNCOL]] in 1958, to which Bratman added in 1961: H. Bratman, “An alternate form of the ´UNCOL diagram´“, Comm. ACM 4 (March 1961) 3, p. 142. Later on, others, including P.D. Terry, gave an explanation and usage of T-diagrams in their textbooks on the topic of compiler construction. Cf. Terry, 1997, [http://scifac.ru.ac.za/compilers/cha03g.htm Chapter 3]. T-diagrams are also now used to describe client-server interconnectivity on the World Wide Web: cf. Patrick Closhen, et al. 1997: [http://pu.rbg.informatik.tu-darmstadt.de/docs/HJH-19990217-etal-T-diagrams.doc ''T-Diagrams as Visual Language to Illustrate WWW Technology''], Darmstadt University of Technology, Darmstadt, Germany</ref> in the formal description of the [[bootstrapping (compilers)|bootstrapping]] problem

== Compiler output ==

One classification of compilers is by the [[platform (computing)|platform]] on which their generated code executes. This is known as the ''target platform.''
  
A ''native'' or ''hosted'' compiler is one whose output is intended to directly run on the same type of computer and operating system that the compiler itself runs on. The output of a [[cross compiler]] is designed to run on a different platform.  Cross compilers are often used when developing software for [[embedded system]]s that are not intended to support a software development environment.

The output of a compiler that produces code for a [[virtual machine]] (VM) may or may not be executed on the same platform as the compiler that produced it.  For this reason such compilers are not usually classified as native or cross compilers.

=== Compiled versus interpreted languages ===

Higher-level programming languages are generally divided for convenience into [[compiled language]]s and [[interpreted language]]s. However, in practice there is rarely anything about a language that ''requires'' it to be exclusively compiled, or exclusively interpreted; although it is possible to design languages that may be inherently interpretive. The categorization usually reflects the most popular or widespread implementations of a language &mdash; for instance, BASIC is sometimes called an interpreted language, and C a compiled one, despite the existence of BASIC compilers and C interpreters.

Modern trends toward [[just-in-time compilation]] and [[bytecode|bytecode interpretation]] at times blur the traditional categorizations of compilers and interpreters.

Some language specifications spell out that implementations ''must'' include a compilation facility; for example, [[Common Lisp]]. However, there is nothing inherent in the definition of Common Lisp that stops it from being interpreted. Other languages have features that are very easy to implement in an interpreter, but make writing a compiler much harder; for example, [[APL (programming language)|APL]], [[SNOBOL4]], and many scripting languages allow programs to construct arbitrary source code at runtime with regular string operations, and then execute that code by passing it to a special evaluation function. To implement these features in a compiled language, programs must usually be shipped with a [[runtime library]] that includes a version of the compiler itself.

=== Hardware compilation ===

The output of some compilers may target [[hardware]] at a very low level, for example a [[Field Programmable Gate Array]] (FPGA) or structured [[Application-specific integrated circuit]] (ASIC).  Such compilers are said to be ''[[hardware compiler]]s'' or synthesis tools because the programs they compile effectively control the final configuration of the hardware and how it operates; the output of the compilation are not instructions that are executed in sequence - only an interconnection of transistors or lookup tables.
For example, XST is the Xilinx Synthesis Tool used for configuring FPGAs. Similar tools are available from Altera, Synplicity, Synopsys and other vendors.

== Compiler design ==

In the early days, the approach taken to compiler design used to be directly affected by the complexity of the processing, the experience of the person(s) designing it, and the resources available.

A compiler for a relatively simple language written by one person might be a single, monolithic piece of software.  When the source language is large and complex, and high quality output is required the design may be split into a number of relatively independent phases.  Having separate phases means development can be parceled up into small parts and given to different people.  It also becomes much easier to replace a single phase by an improved one, or to insert new phases later (eg, additional optimizations).

The division of the compilation processes into phases was championed by the [[Production Quality Compiler-Compiler Project]] (PQCC) at [[Carnegie Mellon]] University.  This project introduced the terms ''front end'', ''middle end'', and ''back end''.

All but the smallest of compilers have more than two phases.  However, these phases are usually regarded as being part of the front end or the back end.  The point at where these two ''ends'' meet is always open to debate.  The front end is generally considered to be where syntactic and semantic processing takes place, along with translation to a lower level of representation (than source code).

The middle end is usually designed to perform optimizations on a form other than the source code or machine code.  This source code/machine code independence is intended to enable generic optimizations to be shared between versions of the compiler supporting different languages and target processors.

The back end takes the output from the middle.  It may perform more analysis, transformations and optimizations that are for a particular computer.  Then, it generates code for a particular processor and OS.

This front-end/middle/back-end approach makes it possible to combine front ends for different [[programming language|languages]] with back ends for different [[CPU]]s.  Practical examples of this approach are the [[GNU Compiler Collection]], [[LLVM]], and the [[Amsterdam Compiler Kit]], which have multiple front-ends, shared analysis and multiple back-ends.

===One-pass versus multi-pass compilers===
Classifying compilers by number of passes has its background in the hardware resource limitations of computers.  Compiling involves performing lots of work and early computers did not have enough memory to contain one program that did all of this work.  So compilers were split up into smaller programs which each made a pass over the source (or some representation of it) performing some of the required analysis and translations.

The ability to compile in a [[one-pass compiler|single pass]] is often seen as a benefit because it simplifies the job of writing a compiler and one pass compilers generally compile faster than [[multi-pass compiler]]s.  Many languages were designed so that they could be compiled in a single pass (e.g., [[Pascal (programming language)|Pascal]]).

In some cases the design of a language feature may require a compiler to perform more than one pass over the source.  For instance, consider a declaration appearing on line 20 of the source which affects the translation of a statement appearing on line 10.  In this case, the first pass needs to gather information about declarations appearing after statements that they affect, with the actual translation happening during a subsequent pass.

The disadvantage of compiling in a single pass is that it is not possible to perform many of the sophisticated [[compiler optimization|optimizations]] needed to generate high quality code.  It can be difficult to count exactly how many passes an optimizing compiler makes.  For instance, different phases of optimization may analyse one expression many times but only analyse another expression once.

Splitting a compiler up into small programs is a technique used by researchers interested in producing provably correct compilers.  Proving the correctness of a set of small programs often requires less effort than proving the correctness of a larger, single, equivalent program.

While the typical multi-pass compiler outputs machine code from its final pass, there are several other types:

*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 parallelization|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's <code>DOALL</code> statements).
*[[Stage compiler]] that compiles to assembly language of a theoretical machine, like some [[Prolog]] implementations
**This Prolog machine is also known as the [[Warren Abstract Machine]] (or WAM). Bytecode compilers for Java, [[Python language|Python]], and many more are also a subtype of this.
*[[Just-in-time compilation|Just-in-time compiler]], used by Smalltalk and Java systems, and also by Microsoft .Net's [[Common Intermediate Language]] (CIL)
**Applications are delivered in bytecode, which is compiled to native machine code just prior to execution.

=== Front end ===

The front end analyzes the source code to build an internal representation of the program, called the [[intermediate representation]] or ''IR''.  It also manages the [[symbol table]], a data structure mapping each symbol in the source code to associated information such as location, type and scope.  This is done over several phases, which includes some of the following:

# '''Line reconstruction'''.  Languages which [[stropping|strop]] their keywords or allow arbitrary spaces within identifiers require a phase before parsing, which converts the input character sequence to a canonical form ready for the parser.  The [[top-down parsing|top-down]], [[recursive descent parser|recursive-descent]], table-driven parsers used in the 1960s typically read the source one character at a time and did not require a separate tokenizing phase.  [[Atlas Autocode]], and [[Edinburgh IMP|Imp]] (and some implementations of [[Algol60|Algol]] and [[CORAL66|Coral66]]) are examples of stropped languages whose compilers would have a ''Line Reconstruction'' phase.
# [[Lexical analysis]] breaks the source code text into small pieces called ''tokens''.  Each token is a single atomic unit of the language, for instance a [[keyword (computing)|keyword]], [[identifier]] or [[symbol|symbol name]].  The token syntax is typically a [[regular language]], so a [[finite state automaton]] constructed from a [[regular expression]] can be used to recognize it.  This phase is also called lexing or scanning, and the software doing lexical analysis is called a [[lexical analyzer]] or scanner.
# [[Preprocessor|Preprocessing]].  Some languages, e.g., [[C (programming language)|C]], require a preprocessing phase which supports [[Macro (computer science)|macro]] substitution and conditional compilation. Typically the preprocessing phase occurs before syntactic or semantic analysis; e.g. in the case of C, the preprocessor manipulates lexical tokens rather than syntactic forms. However, some languages such as [[Scheme (programming language)|Scheme]] support macro substitutions based on syntactic forms.
# [[Syntax analysis]] involves [[parsing]] the token sequence to identify the syntactic structure of the program. This phase typically builds a [[parse tree]], which replaces the linear sequence of tokens with a tree structure built according to the rules of a [[formal grammar]] which define the language's syntax. The parse tree is often analyzed, augmented, and transformed by later phases in the compiler. 
# Semantic analysis is the phase in which the compiler adds semantic information to the [[parse tree]] and builds the symbol table. This phase performs semantic checks such as [[type checking]] (checking for type errors), or [[object binding]] (associating variable and function references with their definitions), or [[definite assignment analysis|definite assignment]] (requiring all local variables to be initialized before use), rejecting incorrect programs or issuing warnings. Semantic analysis usually requires a complete parse tree, meaning that this phase logically follows the [[parsing]] phase, and logically precedes the [[code generation (compiler)|code generation]] phase, though it is often possible to fold multiple phases into one pass over the code in a compiler implementation.

===Back end===

The term ''back end'' is sometimes confused with ''[[code generation (compiler)|code generator]]'' because of the overlapped functionality of generating assembly code. Some literature uses ''middle end'' to distinguish the generic analysis and optimization phases in the back end from the machine-dependent code generators. 

The main phases of the back end include the following:

# [[Compiler analysis|Analysis]]: This is the gathering of program information from the intermediate representation derived from the input. Typical analyses are [[data flow analysis]] to build [[use-define chain]]s, [[dependence analysis]], [[alias analysis]], [[pointer analysis]], [[escape analysis]] etc. Accurate analysis is the basis for any compiler optimization. The [[call graph]] and [[control flow graph]] are usually also built during the analysis phase.
# [[Compiler optimization|Optimization]]: the intermediate language representation is transformed into functionally equivalent but faster (or smaller) forms. Popular optimizations are [[inline expansion]], [[dead code elimination]], [[constant propagation]], [[loop transformation]], [[register allocation]] or even [[automatic parallelization]].
# [[Code generation (compiler)|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 along with their associated addressing modes (see also [[Sethi-Ullman algorithm]]).

Compiler analysis is the prerequisite for any compiler optimization, and they tightly work together. For example, [[dependence analysis]] is crucial for [[loop transformation]]. 

In addition, the scope of compiler analysis and optimizations vary greatly, from as small as a [[basic block]] to the procedure/function level, or even over the whole program ([[interprocedural optimization]]). Obviously, a compiler can potentially do a better job using a broader view. But that broad view is not free: large scope analysis and optimizations are very costly in terms of compilation time and memory space; this is especially true for interprocedural analysis and optimizations.

Interprocedural analysis and optimizations are common in modern commercial compilers from [[Hewlett-Packard|HP]], [[IBM]], [[Silicon Graphics|SGI]], [[Intel]], [[Microsoft]], and [[Sun Microsystems]]. The open source [[GNU Compiler Collection|GCC]] was criticized for a long time for lacking powerful interprocedural optimizations, but it is changing in this respect. Another open source compiler with full analysis and optimization infrastructure is [[Open64]], which is used by many organizations for research and commercial purposes.

Due to the extra time and space needed for compiler analysis and optimizations, some compilers skip them by default. Users have to use compilation options to explicitly tell the compiler which optimizations should be enabled.

== Related techniques ==

[[Assembly language]] is not a high-level language and a program that compiles it is more commonly known as an ''assembler'', with the inverse program known as a ''[[disassembler]]''.

A program that translates from a low level language to a higher level one is a ''[[decompiler]]''.  

A program that translates between high-level languages is usually called a ''language translator'', ''source to source translator'', ''language converter'', or ''language [[rewriting|rewriter]]''. The last term is usually applied to translations that do not involve a change of language.

== International conferences and organizations ==
<!-- [[International Conference on Compiler Construction]] and [[European Joint Conferences on Theory and Practice of Software]] redirect here -->
Every year, the '''European Joint Conferences on Theory and Practice of Software''' (ETAPS) sponsors the '''International Conference on Compiler Construction''' (CC), with papers from both the academic and industrial sectors.<ref>[http://www.etaps.org/ ETAPS] - European Joint Conferences on Theory and Practice of Software. Cf. "CC" (Compiler Construction) subsection.</ref>

== See also ==
* [[Abstract interpretation]]
* [[Attribute grammar]]
* [[Bottom-up parsing]]
* [[Compiler-compiler]]
* [[Error avalanche]]
* [[History of compiler writing]]
* [[Just-in-time compilation]]
* [[Linker (computing)|Linker]]
* [[List of compilers]]
* [[List of important publications in computer science#Compilers]]
* [[Metacompilation]]
* [[Semantics encoding]]

== Notes ==
{{reflist|2}}

== References ==
{{refbegin|2}}
* [http://www.informatik.uni-trier.de/~ley/db/books/compiler/index.html Compiler textbook references] A collection of references to mainstream Compiler Construction Textbooks
* [[Alfred V. Aho|Aho, Alfred V.]]; [[Ravi Sethi|Sethi, Ravi]]; and [[Jeffrey D. Ullman|Ullman, Jeffrey D.]], ''[[Compilers: Principles, Techniques, and Tools|Compilers: Principles, Techniques and Tools]]'' (ISBN 0-201-10088-6) [http://www.aw.com/catalog/academic/product/0,4096,0201100886,00.html link to publisher].  Also known as “The Dragon Book.”
* [[Frances E. Allen|Allen, Frances E.]], [http://www.research.ibm.com/journal/rd/255/ibmrd2505Q.pdf "A History of Language Processor Technology in IBM"], ''IBM Journal of Research and Development'', v.25, no.5, September 1981.
* Allen, Randy; and [[Ken Kennedy (computer scientist)|Kennedy, Ken]], ''Optimizing Compilers for Modern Architectures'', [[Morgan Kaufmann Publishers]], 2001. ISBN 1-55860-286-0 
* [[Andrew Appel|Appel, Andrew Wilson]]
** ''Modern Compiler Implementation in Java'', 2nd edition. [[Cambridge University Press]], 2002. ISBN 0-521-82060-X
** [http://books.google.com/books?id=8APOYafUt-oC&printsec=frontcover ''Modern Compiler Implementation in ML''], Cambridge University Press, 1998. ISBN 0521582741
*  [[Richard Bornat|Bornat, Richard]], [http://www.cs.mdx.ac.uk/staffpages/r_bornat/books/compiling.pdf ''Understanding and Writing Compilers: A Do It Yourself Guide''], Macmillan Publishing, 1979. ISBN 0-333-21732-2
* Cooper, Keith D., and Torczon, Linda, '' Engineering a Compiler'', Morgan Kaufmann, 2004, ISBN 1-55860-699-8.
* Leverett; Cattel; Hobbs; Newcomer; Reiner; Schatz; Wulf, ''An Overview of the Production Quality Compiler-Compiler Project'', in ''[[Computer (magazine)|Computer]]'' 13(8):38-49 (August 1980)
* McKeeman, William Marshall; [[Jim Horning|Horning, James J.]]; Wortman, David B., [http://www.cs.toronto.edu/XPL/ ''A Compiler Generator''], Englewood Cliffs, N.J. : Prentice-Hall, 1970. ISBN 0131550772
* [[Steven Muchnick|Muchnick, Steven]], [http://books.google.com/books?id=Pq7pHwG1_OkC&printsec=frontcover&source=gbs_summary_r&cad=0 ''Advanced Compiler Design and Implementation''], Morgan Kaufmann Publishers, 1997. ISBN 1-55860-320-4
* [[Michael L. Scott|Scott, Michael Lee]], [http://books.google.com/books?id=4LMtA2wOsPcC&printsec=frontcover&dq=Programming+Language+Pragmatics ''Programming Language Pragmatics''], Morgan Kaufmann, 2005, 2nd edition, 912 pages. ISBN 0-12-633951-1 ([http://www.cs.rochester.edu/~scott/pragmatics/ The author's site on this book]).
* Srikant, Y. N.; Shankar, Priti, [http://books.google.com/books?id=0K_jIsgyNpoC&printsec=frontcover ''The Compiler Design Handbook: Optimizations and Machine Code Generation''], CRC Press, 2003. ISBN 084931240X
* Terry, Patrick D., [http://scifac.ru.ac.za/compilers/conts.htm ''Compilers and Compiler Generators: An Introduction with C++''], International Thomson Computer Press, 1997. ISBN 1850322988,
* [[Niklaus Wirth|Wirth, Niklaus]], [http://www.oberon2005.ru/book/ccnw2005e.pdf''Compiler Construction''] (ISBN 0-201-40353-6), Addison-Wesley, 1996, 176 pages. Revised November 2005.
{{refend}}

==External links==
{{Wiktionary|compiler}}
{{Wikipedia-Books|Compiler construction}}
{{Wikibooks|Compiler construction}}
* [http://compilers.iecc.com/ The comp.compilers newsgroup and RSS feed]
* [http://www.jiscmail.ac.uk/lists/hwcomp.html Hardware compilation mailing list]
* [http://www.onyxbits.de/content/blog/patrick/introduction-compiler-construction-using-flex-and-yacc Practical introduction to compiler construction using flex and yacc]

{{Cat also|Compiler| Computer libraries | Programming language implementation}}

[[Category:Compilers|*]]
[[Category:Compiler theory]]

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====References====
* Adapted from the [[Wikipedia]] article, "Compiler" [http://en.wikipedia.org/wiki/Compiler http://en.wikipedia.org/wiki/Compiler], used under the [[GNU Free Documentation License]]