TheInfoList

In
computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and software. It has sci ...

, a compiler is a
computer program In imperative programming, a computer program is a sequence of instructions in a programming language that a computer can execute or interpret. In declarative programming, a ''computer program'' is a Set (mathematics), set of instructions. A comp ...
that translates computer code written in one
programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from an alphabet (computer science) ...

(the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that translate
source code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and ...

from a
high-level programming language In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
to a
lower level language A low-level programming language is a programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are ...
(e.g.
assembly language In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, gene ...
,
object code In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
, or
machine code In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
) to create an
executable In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and ...
program.
Compilers: Principles, Techniques, and Tools ''Compilers: Principles, Techniques, and Tools'' is a computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their appl ...
by Alfred V. Aho, Ravi Sethi, Jeffrey D. Ullman - Second Edition, 2007
There are many different types of compilers which produce output in different useful forms. A ''
cross-compiler A cross compiler is a compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). ...
'' produces code for a different
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of ...

or
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

than the one on which the cross-compiler itself runs. A ''
bootstrap compiler In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , a ...
'' is often a temporary compiler, used for compiling a more permanent or better optimised compiler for a language. Related software include, a program that translates from a low-level language to a higher level one is a ''
decompiler A decompiler is a computer program A computer program is a collection of instructions that can be executed by a computer to perform a specific task. A computer program is usually written by a computer programmer in a programming language ...
'' ; a program that translates between high-level languages, usually called a ''
source-to-source compiler A source-to-source translator, source-to-source compiler (S2S compiler), transcompiler, or transpiler is a type of translator Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, sourc ...
'' or ''transpiler''. A language '' rewriter'' is usually a program that translates the form of
expressions Expression may refer to: Linguistics * Expression (linguistics), a word, phrase, or sentence * Fixed expression, a form of words with a specific meaning * Idiom, a type of fixed expression * Metaphor#Common types, Metaphorical expression, a parti ...
without a change of language. A ''
compiler-compiler In computer science, a compiler-compiler or compiler generator is a programming tool that creates a parsing, parser, interpreter (computer software), interpreter, or compiler from some form of formal description of a programming language and mach ...

'' is a compiler that produces a compiler (or part of one), often in a generic and reusable way so as to be able to produce many differing compilers. A compiler is likely to perform some or all of the following operations, often called phases: ,
lexical analysis In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
,
parsing Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other obj ...

, semantic analysis (
syntax-directed translationSyntax-directed translation refers to a method of compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language ( ...
), conversion of input programs to an
intermediate representation An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive for further processing, such as Compiler optimization, optimization a ...
,
code optimization In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of com ...
and code generation. Compilers generally implement these phases as modular components, promoting efficient design and correctness of
transformation Transformation may refer to: Science and mathematics In biology and medicine * Metamorphosis, the biological process of changing physical form after birth or hatching * Malignant transformation, the process of cells becoming cancerous * Transf ...
s of source input to target output. Program faults caused by incorrect compiler behavior can be very difficult to track down and work around; therefore, compiler implementers invest significant effort to ensure
compiler correctnessIn computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and software. ...
. Compilers are not the only language processor used to transform source programs. An interpreter is computer software that transforms and then executes the indicated operations. The translation process influences the design of computer languages, which leads to a preference of compilation or interpretation. In theory, a programming language can have both a compiler and an interpreter. In practice, programming languages tend to be associated with just one (a compiler or an interpreter).

# History

Theoretical computing concepts developed by scientists, mathematicians, and engineers formed the basis of digital modern computing development during World War II. Primitive binary languages evolved because digital devices only understand ones and zeros and the circuit patterns in the underlying machine architecture. In the late 1940s, assembly languages were created to offer a more workable abstraction of the computer architectures. Limited
memory Memory is the faculty of the brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exis ...
capacity of early computers led to substantial technical challenges when the first compilers were designed. Therefore, the compilation process needed to be divided into several small programs. The front end programs produce the analysis products used by the back end programs to generate target code. As computer technology provided more resources, compiler designs could align better with the compilation process. It is usually more productive for a programmer to use a high-level language, so the development of high-level languages followed naturally from the capabilities offered by digital computers. High-level languages are
formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from an alphabet (computer science), alphabet and are well-formedness, well-formed a ...
s that are strictly defined by their syntax and
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another ...
which form the high-level language architecture. Elements of these formal languages include: * ''Alphabet'', any finite set of symbols; * ''String'', a finite sequence of symbols; * ''Language'', any set of strings on an alphabet. The sentences in a language may be defined by a set of rules called a grammar.
Backus–Naur form In computer science, Backus–Naur form or Backus normal form (BNF) is a metasyntax notation for context-free grammars, often used to describe the Syntax (programming languages), syntax of Formal language#Programming languages, languages used in c ...
(BNF) describes the syntax of "sentences" of a language and was used for the syntax of Algol 60 by
John Backus John Warner Backus (December 3, 1924 – March 17, 2007) was an American computer scientist . He directed the team that invented and implemented Fortran, FORTRAN, the first widely used high-level programming language, and was the inventor of ...

. The ideas derive from the
context-free grammar In formal language theory, a context-free grammar (CFG) is a formal grammar whose Production (computer science), production rules are of the form :A\ \to\ \alpha with A a ''single'' nonterminal symbol, and \alpha a string of Terminal and nonterm ...
concepts by
Noam Chomsky Avram Noam Chomsky (born December 7, 1928) is an American linguist Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gesture ...

, a linguist. "BNF and its extensions have become standard tools for describing the syntax of programming notations, and in many cases parts of compilers are generated automatically from a BNF description." In the 1940s,
Konrad Zuse Konrad Zuse (; 22 June 1910 – 18 December 1995) was a German civil engineer A civil engineer is a person who practices civil engineering Civil engineering is a Regulation and licensure in engineering, professional engineering discipli ...

designed an algorithmic programming language called
Plankalkül Plankalkül () is a programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are used in computer p ...
("Plan Calculus"). While no actual implementation occurred until the 1970s, it presented concepts later seen in APL designed by Ken Iverson in the late 1950s. APL is a language for mathematical computations. High-level language design during the formative years of digital computing provided useful programming tools for a variety of applications: *
FORTRAN Fortran (; formerly FORTRAN) is a general-purpose, compiled language, compiled imperative programming, imperative programming language that is especially suited to numerical analysis, numeric computation and computational science, scientific com ...
(Formula Translation) for engineering and science applications is considered to be the first high-level language. *
COBOL COBOL (; an acronym An acronym is a word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (lingui ...

(Common Business-Oriented Language) evolved from A-0 and
FLOW-MATIC FLOW-MATIC, originally known as B-0 (Business Language version 0), was the first English-like data processing language. It was developed for the UNIVAC I The UNIVAC I (UNIVersal Automatic Computer I) was the first general-purpose electronic digit ...
to become the dominant high-level language for business applications. *
LISP Lisp (historically LISP) is a family of programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbo ...
(List Processor) for symbolic computation. Compiler technology evolved from the need for a strictly defined transformation of the high-level source program into a low-level target program for the digital computer. The compiler could be viewed as a front end to deal with the analysis of the source code and a back end to synthesize the analysis into the target code. Optimization between the front end and back end could produce more efficient target code. Some early milestones in the development of compiler technology: * 1952: An
Autocode Autocode is the name of a family of "simplified coding systems", later called programming languages, devised in the 1950s and 1960s for a series of digital computer A computer is a machine A machine is a man-made device that uses powe ...
compiler developed by
Alick Glennie Alick Edwards Glennie (1925–2003) was a British people, British computer scientist, most famous for having developed Autocode, which many people regard as the first ever computer compiler.Knuth, Donald E.; Pardo, Luis Trabb, "Early development o ...
for the Manchester Mark I computer at the University of Manchester is considered by some to be the first compiled programming language. * 1952:
Grace Hopper Grace Brewster Murray Hopper (; December 9, 1906 – January 1, 1992) was an American computer scientist A computer scientist is a person who has acquired the knowledge of computer science Computer science deals with the theoretical f ...

's team at
Remington Rand Remington Rand was an early American business machine manufacturer, best known originally as a typewriter Video showing the operation of a typewriter A typewriter is a mechanical Mechanical may refer to: Machine * Mechanical system A ...
wrote the compiler for the A-0 programming language (and coined the term ''compiler'' to describe it), although the A-0 compiler functioned more as a loader or linker than the modern notion of a full compiler. * 1954–1957: A team led by
John Backus John Warner Backus (December 3, 1924 – March 17, 2007) was an American computer scientist . He directed the team that invented and implemented Fortran, FORTRAN, the first widely used high-level programming language, and was the inventor of ...

at
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

developed
FORTRAN Fortran (; formerly FORTRAN) is a general-purpose, compiled language, compiled imperative programming, imperative programming language that is especially suited to numerical analysis, numeric computation and computational science, scientific com ...
which is usually considered the first high-level language. In 1957, they completed a FORTRAN compiler that is generally credited as having introduced the first unambiguously complete compiler. * 1959: The Conference on Data Systems Language (CODASYL) initiated development of
COBOL COBOL (; an acronym An acronym is a word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (lingui ...

. The COBOL design drew on A-0 and FLOW-MATIC. By the early 1960s COBOL was compiled on multiple architectures. * 1958–1962:
John McCarthyJohn McCarthy may refer to: Government * John George MacCarthy (1829–1892), Member of Parliament for Mallow constituency, 1874–1880 * John McCarthy (Irish politician) (1862–1893), Member of Parliament for the Mid Tipperary constituency, 189 ...
at
MIT Massachusetts Institute of Technology (MIT) is a private land-grant research university A research university is a university A university ( la, universitas, 'a whole') is an educational institution, institution of higher education, hi ...

designed
LISP Lisp (historically LISP) is a family of programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbo ...
. The symbol processing capabilities provided useful features for artificial intelligence research. In 1962, LISP 1.5 release noted some tools: an interpreter written by Stephen Russell and Daniel J. Edwards, a compiler and assembler written by Tim Hart and Mike Levin. Early operating systems and software were written in assembly language. In the 1960s and early 1970s, the use of high-level languages for system programming was still controversial due to resource limitations. However, several research and industry efforts began the shift toward high-level systems programming languages, for example,
BCPL BCPL ("Basic Combined Programming Language") is a procedural, imperative Imperative may refer to: *Imperative mood, a grammatical mood (or mode) expressing commands, direct requests, and prohibitions *Imperative programming, a programming pa ...
,
BLISS BLISS is a system programming language A system programming language is a programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/outp ...

, B, and C.
BCPL BCPL ("Basic Combined Programming Language") is a procedural, imperative Imperative may refer to: *Imperative mood, a grammatical mood (or mode) expressing commands, direct requests, and prohibitions *Imperative programming, a programming pa ...
(Basic Combined Programming Language) designed in 1966 by Martin Richards at the University of Cambridge was originally developed as a compiler writing tool. Several compilers have been implemented, Richards' book provides insights to the language and its compiler. BCPL was not only an influential systems programming language that is still used in research but also provided a basis for the design of B and C languages.
BLISS BLISS is a system programming language A system programming language is a programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/outp ...

(Basic Language for Implementation of System Software) was developed for a Digital Equipment Corporation (DEC) PDP-10 computer by W.A. Wulf's Carnegie Mellon University (CMU) research team. The CMU team went on to develop BLISS-11 compiler one year later in 1970.
Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experi ...

(Multiplexed Information and Computing Service), a time-sharing operating system project, involved
MIT Massachusetts Institute of Technology (MIT) is a private land-grant research university A research university is a university A university ( la, universitas, 'a whole') is an educational institution, institution of higher education, hi ...

,
Bell Labs Nokia Bell Labs (formerly named Bell Labs Innovations (1996–2007), AT&T Bell Laboratories (1984–1996) and Bell Telephone Laboratories (1925–1984)) is an American industrial research and scientific development company A company, ab ...
,
General Electric General Electric Company (GE) is an American Multinational corporation, multinational Conglomerate (company), conglomerate incorporated in New York State and headquartered in Boston. Until 2021, the company operated through GE Aviation, aviat ...
(later
Honeywell Honeywell International Inc. is an American public company, publicly traded, multinational corporation, multinational conglomerate (company), conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four are ...

) and was led by Fernando Corbató from MIT. Multics was written in the
PL/I PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative Imperative may refer to: *Imperative mood, a grammatical mood (or mode) expressing commands, direct requests, and prohibitions *Imperative prog ...
language developed by IBM and IBM User Group. IBM's goal was to satisfy business, scientific, and systems programming requirements. There were other languages that could have been considered but PL/I offered the most complete solution even though it had not been implemented. For the first few years of the Multics project, a subset of the language could be compiled to assembly language with the Early PL/I (EPL) compiler by Doug McIlory and Bob Morris from Bell Labs. EPL supported the project until a boot-strapping compiler for the full PL/I could be developed. Bell Labs left the Multics project in 1969, and developed a system programming language B based on BCPL concepts, written by
Dennis Ritchie Dennis MacAlistair Ritchie (September 9, 1941 – October 12, 2011) was an American . He created the and, with long-time colleague , the and . Ritchie and Thompson were awarded the from the in 1983, the from the in 1990 and the from in ...

and
Ken Thompson Kenneth Lane Thompson (born February 4, 1943) is an American pioneer of computer science. Thompson worked at Bell Labs for most of his career where he designed and implemented the original Unix operating system. He also invented the B (programmi ...

. Ritchie created a boot-strapping compiler for B and wrote Unics (Uniplexed Information and Computing Service) operating system for a PDP-7 in B. Unics eventually became spelled Unix. Bell Labs started the development and expansion of C based on B and BCPL. The BCPL compiler had been transported to Multics by Bell Labs and BCPL was a preferred language at Bell Labs. Initially, a front-end program to Bell Labs' B compiler was used while a C compiler was developed. In 1971, a new PDP-11 provided the resource to define extensions to B and rewrite the compiler. By 1973 the design of C language was essentially complete and the Unix kernel for a PDP-11 was rewritten in C. Steve Johnson started development of Portable C Compiler (PCC) to support retargeting of C compilers to new machines.
Object-oriented programming Object-oriented programming (OOP) is a programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mai ...
(OOP) offered some interesting possibilities for application development and maintenance. OOP concepts go further back but were part of
LISP Lisp (historically LISP) is a family of programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbo ...

and
Simula Simula is the name of two simulation A simulation is the imitation of the operation of a real-world process or system over time. Simulations require the use of models; the model represents the key characteristics or behaviors of the select ...
language science. At Bell Labs, the development of
C++ C++ () is a general-purpose programming language In computer software, a general-purpose programming language is a programming language dedicated to a general-purpose, designed to be used for writing software in a wide variety of application ...

became interested in OOP. C++ was first used in 1980 for systems programming. The initial design leveraged C language systems programming capabilities with Simula concepts. Object-oriented facilities were added in 1983. The Cfront program implemented a C++ front-end for C84 language compiler. In subsequent years several C++ compilers were developed as C++ popularity grew. 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 A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from an alphabet (computer science) ...

s and the increasing complexity of computer architectures, compilers became more complex.
DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Ad ...
(Defense Advanced Research Projects Agency) sponsored a compiler project with Wulf's CMU research team in 1970. The Production Quality Compiler-Compiler
PQCC The Production Quality Compiler-Compiler Project (or PQCC) was a long-term project led by William Wulf William Allan Wulf (born December 8, 1939) is a computer scientist notable for his work in programming language A programming language is a f ...
design would produce a Production Quality Compiler (PQC) from formal definitions of source language and the target. PQCC tried to extend the term compiler-compiler beyond the traditional meaning as a parser generator (e.g.,
Yacc Yacc (Yet Another Compiler-Compiler) is a computer program A computer program is a collection of instructions that can be executed by a computer to perform a specific task. A computer program is usually written by a computer programmer in a ...
) without much success. PQCC might more properly be referred to as a compiler generator. PQCC research into code generation process sought to build a truly automatic compiler-writing system. The effort discovered and designed the phase structure of the PQC. The BLISS-11 compiler provided the initial structure. The phases included analyses (front end), intermediate translation to virtual machine (middle end), and translation to the target (back end). TCOL was developed for the PQCC research to handle language specific constructs in the intermediate representation. Variations of TCOL supported various languages. The PQCC project investigated techniques of automated compiler construction. The design concepts proved useful in optimizing compilers and compilers for the (since 1995, object-oriented) programming language
Ada Ada may refer to: Places Africa * Ada Foah Ada Foah is a town on the southeast coast of Ghana, where the Volta River meets the Atlantic Ocean. The town is located along the Volta River, off of the Accra-Aflao motorway. Known for Palm tree, pal ...
GNU Compiler Collection The GNU Compiler Collection (GCC) is an optimizing compiler In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an ...
(GCC) which provides a core capability to support multiple languages and targets. The Ada version
GNAT GNAT is a free-software Free software (or libre software) is computer software Software is a collection of Instruction (computer science), instructions and data (computing), data that tell a computer how to work. This is in contrast t ...

is one of the most widely used Ada compilers. GNAT is free but there is also commercial support, for example, AdaCore, was founded in 1994 to provide commercial software solutions for Ada. GNAT Pro includes the GNU GCC based GNAT with a tool suite to provide an
integrated development environment An integrated development environment (IDE) is a software application Application software (app for short) is computing software designed to carry out a specific task other than one relating to the operation of the computer itself, typically ...
. High-level languages continued to drive compiler research and development. Focus areas included optimization and automatic code generation. Trends in programming languages and development environments influenced compiler technology. More compilers became included in language distributions (PERL, Java Development Kit) and as a component of an IDE (VADS, Eclipse, Ada Pro). The interrelationship and interdependence of technologies grew. The advent of web services promoted growth of web languages and scripting languages. Scripts trace back to the early days of Command Line Interfaces (CLI) where the user could enter commands to be executed by the system. User Shell concepts developed with languages to write shell programs. Early Windows designs offered a simple batch programming capability. The conventional transformation of these language used an interpreter. While not widely used, Bash and Batch compilers have been written. More recently sophisticated interpreted languages became part of the developers tool kit. Modern scripting languages include PHP, Python, Ruby and Lua. (Lua is widely used in game development.) All of these have interpreter and compiler support. "When the field of compiling began in the late 50s, its focus was limited to the translation of high-level language programs into machine code ... The compiler field is increasingly intertwined with other disciplines including computer architecture, programming languages, formal methods, software engineering, and computer security." The "Compiler Research: The Next 50 Years" article noted the importance of object-oriented languages and Java. Security and parallel computing were cited among the future research targets.

# Compiler construction

A compiler implements a formal transformation from a high-level source program to a low-level target program. Compiler design can define an end-to-end solution or tackle a defined subset that interfaces with other compilation tools e.g. preprocessors, assemblers, linkers. Design requirements include rigorously defined interfaces both internally between compiler components and externally between supporting toolsets. In the early days, the approach taken to compiler design was directly affected by the complexity of the computer language to be processed, the experience of the person(s) designing it, and the resources available. Resource limitations led to the need to pass through the source code more than once. A compiler for a relatively simple language written by one person might be a single, monolithic piece of software. However, as the source language grows in complexity the design may be split into a number of interdependent phases. Separate phases provide design improvements that focus development on the functions in the compilation process.

## 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 much 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 single pass has classically been seen as a benefit because it simplifies the job of writing a compiler and one-pass compilers generally perform compilations faster than s. Thus, partly driven by the resource limitations of early systems, many early languages were specifically designed so that they could be compiled in a single pass (e.g.,
Pascal Pascal, Pascal's or PASCAL may refer to: People and fictional characters * Pascal (given name), including a list of people with the name * Pascal (surname), including a list of people and fictional characters with the name ** Blaise Pascal, French ...
). 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 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.

## Three-stage compiler structure

Regardless of the exact number of phases in the compiler design, the phases can be assigned to one of three stages. The stages include a front end, a middle end, and a back end. * The ''front end'' scans the input and verifies syntax and semantics according to a specific source language. For statically typed languages it performs
type checking In programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from a ...
by collecting type information. If the input program is syntactically incorrect or has a type error, it generates error and/or warning messages, usually identifying the location in the source code where the problem was detected; in some cases the actual error may be (much) earlier in the program. Aspects of the front end include lexical analysis, syntax analysis, and semantic analysis. The front end transforms the input program into an
intermediate representation An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive for further processing, such as Compiler optimization, optimization a ...
(IR) for further processing by the middle end. This IR is usually a lower-level representation of the program with respect to the source code. * The ''middle end'' performs optimizations on the IR that are independent of the CPU architecture being targeted. 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. Examples of middle end optimizations are removal of useless (
dead code elimination In compiler theory In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "c ...
) or unreachable code ( reachability analysis), discovery and propagation of constant values (
constant propagation Constant folding and constant propagation are related compiler optimizations used by many modern compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language ...
), relocation of computation to a less frequently executed place (e.g., out of a loop), or specialization of computation based on the context, eventually producing the "optimized" IR that is used by the back end. * The ''back end'' takes the optimized IR from the middle end. It may perform more analysis, transformations and optimizations that are specific for the target CPU architecture. The back end generates the target-dependent assembly code, performing
register allocation In compiler optimization In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hard ...
in the process. The back end performs
instruction scheduling In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algorit ...
, which re-orders instructions to keep parallel
execution unit Capital punishment, also known as the death penalty, is the state-sanctioned killing of a person as punishment for a crime. The sentence ordering that someone is punished with the death penalty is called a death sentence, and the act of ca ...
s busy by filling
delay slot{{unreferenced, date=April 2013 In computer architecture, a delay slot is an instruction slot being executed without the effects of a preceding instruction. The most common form is a single arbitrary instruction located immediately after a branch ...
s. Although most optimization problems are
NP-hard In computational complexity theory Computational complexity theory focuses on classifying computational problem In theoretical computer science An artistic representation of a Turing machine. Turing machines are used to model general computin ...
,
heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be Mathematical optimisation, optimal, perfect, or Rationality, rational, but is nevertheless ...
techniques for solving them are well-developed and currently implemented in production-quality compilers. Typically the output of a back end is machine code specialized for a particular processor and operating system. This front/middle/back-end approach makes it possible to combine front ends for different languages with back ends for different
CPUs A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controllin ...

while sharing the optimizations of the middle end. Practical examples of this approach are the
GNU Compiler Collection The GNU Compiler Collection (GCC) is an optimizing compiler In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an ...
,
Clang Clang is a compiler front end In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both com ...
(
LLVM LLVM is a set of compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The ...
-based C/C++ compiler), and the
Amsterdam Compiler Kit The Amsterdam Compiler Kit (ACK) is a retargetable compiler 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. Compilers ...
, which have multiple front-ends, shared optimizations and multiple back-ends.

### Front end

The front end analyzes the source code to build an internal representation of the program, called the
intermediate representation An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive for further processing, such as Compiler optimization, optimization a ...
(IR). It also manages the
symbol table In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
, a data structure mapping each symbol in the source code to associated information such as location, type and scope. While the frontend can be a single monolithic function or program, as in a scannerless parser, it was traditionally implemented and analyzed as several phases, which may execute sequentially or concurrently. This method is favored due to its modularity and
separation of concerns In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of compu ...
. Most commonly today, the frontend is broken into three phases:
lexical analysis In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
(also known as lexing or scanning),
syntax analysis In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. The traditional areas of linguistic analysis includ ...
(also known as scanning or parsing), and semantic analysis. Lexing and parsing comprise the syntactic analysis (word syntax and phrase syntax, respectively), and in simple cases, these modules (the lexer and parser) can be automatically generated from a grammar for the language, though in more complex cases these require manual modification. The lexical grammar and phrase grammar are usually
context-free grammar In formal language theory, a context-free grammar (CFG) is a formal grammar whose Production (computer science), production rules are of the form :A\ \to\ \alpha with A a ''single'' nonterminal symbol, and \alpha a string of Terminal and nonterm ...
s, which simplifies analysis significantly, with context-sensitivity handled at the semantic analysis phase. The semantic analysis phase is generally more complex and written by hand, but can be partially or fully automated using
attribute grammarAn attribute grammar is a formal way to define attributes for the productions of a formal grammar In formal language theory, a grammar (when the context is not given, often called a formal grammar for clarity) describes how to form strings from a l ...
s. These phases themselves can be further broken down: lexing as scanning and evaluating, and parsing as building a
concrete syntax tree A parse tree or parsing tree or derivation tree or concrete syntax tree is an ordered, rooted tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most ...
(CST, parse tree) and then transforming it into an abstract syntax tree (AST, syntax tree). In some cases additional phases are used, notably ''line reconstruction'' and ''preprocessing,'' but these are rare. The main phases of the front end include the following: * ' converts the input character sequence to a canonical form ready for the parser. Languages which stropping (syntax), strop their keywords or allow arbitrary spaces within identifiers require this phase. 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 ALGOL and Coral 66) are examples of stropped languages whose compilers would have a ''Line Reconstruction'' phase. * ''Preprocessor, Preprocessing'' 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. * ''Lexical analysis'' (also known as ''lexing'' or ''tokenization'') breaks the source code text into a sequence of small pieces called ''lexical tokens''. This phase can be divided into two stages: the ''scanning'', which segments the input text into syntactic units called ''lexemes'' and assigns them a category; and the ''evaluating'', which converts lexemes into a processed value. A token is a pair consisting of a ''token name'' and an optional ''token value''. Common token categories may include identifiers, keywords, separators, operators, literals and comments, although the set of token categories varies in different
programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from an alphabet (computer science) ...

s. The lexeme syntax is typically a regular language, so a finite state automaton constructed from a regular expression can be used to recognize it. The software doing lexical analysis is called a lexical analyzer. This may not be a separate step—it can be combined with the parsing step in scannerless parsing, in which case parsing is done at the character level, not the token level. * ''Syntax analysis'' (also known as ''parsing'') involves
parsing Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other obj ...

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 (compilers), Semantic analysis'' adds semantic information to the parse tree and builds the
symbol table In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
. This phase performs semantic checks such as
type checking In programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from a ...
(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 Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other obj ...

phase, and logically precedes the code generation phase, though it is often possible to fold multiple phases into one pass over the code in a compiler implementation.

### Middle end

The middle end, also known as ''optimizer,'' performs optimizations on the intermediate representation in order to improve the performance and the quality of the produced machine code. The middle end contains those optimizations that are independent of the CPU architecture being targeted. The main phases of the middle end include the following: * Compiler analysis, Analysis: This is the gathering of program information from the intermediate representation derived from the input; data-flow analysis is used to build use-define chains, together with dependence analysis, alias analysis, pointer analysis, escape analysis, etc. Accurate analysis is the basis for any compiler optimization. The control-flow graph of every compiled function and the call graph of the program 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 In compiler theory In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "c ...
,
constant propagation Constant folding and constant propagation are related compiler optimizations used by many modern compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language ...
, loop transformation and even automatic parallelization. Compiler analysis is the prerequisite for any compiler optimization, and they tightly work together. For example, dependence analysis is crucial for loop transformation. The scope of compiler analysis and optimizations vary greatly; their scope may range from operating within a basic block, to whole procedures, or even the whole program. There is a trade-off between the granularity of the optimizations and the cost of compilation. For example, peephole optimizations are fast to perform during compilation but only affect a small local fragment of the code, and can be performed independently of the context in which the code fragment appears. In contrast, interprocedural optimization requires more compilation time and memory space, but enable optimizations that are only possible by considering the behavior of multiple functions simultaneously. Interprocedural analysis and optimizations are common in modern commercial compilers from Hewlett-Packard, HP,
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

, Silicon Graphics, SGI, Intel, Microsoft, and Sun Microsystems. The free software 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.

### Back end

The back end is responsible for the CPU architecture specific optimizations and for code generation''.'' The main phases of the back end include the following: * ''Machine dependent optimizations'': optimizations that depend on the details of the CPU architecture that the compiler targets. A prominent example is peephole optimizations, which rewrites short sequences of assembler instructions into more efficient instructions. * ''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 Register allocation, registers and memory and the Instruction selection, selection and Instruction scheduling, scheduling of appropriate machine instructions along with their associated addressing modes (see also Sethi–Ullman algorithm). Debug data may also need to be generated to facilitate debugging.

## Compiler correctness

Compiler correctness is the branch of software engineering that deals with trying to show that a compiler behaves according to its programming language, language specification. Techniques include developing the compiler using formal methods and using rigorous testing (often called compiler validation) on an existing compiler.

# Compiled versus interpreted languages

Higher-level programming languages usually appear with a type of Translator (computing), translation in mind: either designed as compiled language or interpreted language. 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 rely on re-interpretation at run time. The categorization usually reflects the most popular or widespread implementations of a language – for instance, BASIC is sometimes called an interpreted language, and C a compiled one, despite the existence of BASIC compilers and C interpreters. Interpretation does not replace compilation completely. It only hides it from the user and makes it gradual. Even though an interpreter can itself be interpreted, a directly executed program is needed somewhere at the bottom of the execution stack (see machine language). Furthermore, for optimization compilers can contain interpreter functionality, and interpreters may include ahead of time compilation techniques. For example, where an expression can be executed during compilation and the results inserted into the output program, then it prevents it having to be recalculated each time the program runs, which can greatly speed up the final program. Modern trends toward just-in-time compilation and bytecode, bytecode interpretation at times blur the traditional categorizations of compilers and interpreters even further. 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 eval, 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.

# Types

One classification of compilers is by the Computing platform, 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 systems 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. The lower level language that is the target of a compiler may itself be a
high-level programming language In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
. C, viewed by some as a sort of portable assembly language, is frequently the target language of such compilers. For example, Cfront, the original compiler for
C++ C++ () is a general-purpose programming language In computer software, a general-purpose programming language is a programming language dedicated to a general-purpose, designed to be used for writing software in a wide variety of application ...

, used C as its target language. The C code generated by such a compiler is usually not intended to be readable and maintained by humans, so indent style and creating pretty C intermediate code are ignored. Some of the features of C that make it a good target language include the C preprocessor#Special macros and directives, #line directive, which can be generated by the compiler to support debugging of the original source, and the wide platform support available with C compilers. While a common compiler type outputs machine code, there are many other types: *Source-to-source compilers are 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 DOALL statements). Other terms for source-to-source compilers are language translator, language converter, or language Rewriting, rewriter. The last term is usually applied to translations that do not involve a change of language. * Bytecode compilers compile 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 (programming language), Java, Python (programming language), Python are also examples of this category. *Just-in-time compilation, Just-in-time compilers (JIT compiler) defer compilation until runtime. JIT compilers exist for many modern languages including Python (programming language), Python, JavaScript, Smalltalk, Java (programming language), Java, Microsoft .NET Framework, .NET's Common Intermediate Language (CIL) and others. A JIT compiler generally runs inside an interpreter. When the interpreter detects that a code path is "hot", meaning it is executed frequently, the JIT compiler will be invoked and compile the "hot" code for increased performance. ** For some languages, such as Java, applications are first compiled using a bytecode compiler and delivered in a machine-independent
intermediate representation An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive for further processing, such as Compiler optimization, optimization a ...
. A bytecode interpreter executes the bytecode, but the JIT compiler will translate the bytecode to machine code when increased performance is necessary. * silicon compiler, Hardware compilers (also known as synthesis tools) are compilers whose input is a hardware description language and whose output is a description, in the form of a netlist or otherwise, of a hardware configuration. ** The output of these compilers target computer 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 compilers, because the source code they compile effectively controls the final configuration of the hardware and how it operates. The output of the compilation is only an interconnection of transistors or lookup tables. ** An example of hardware compiler is XST, the Xilinx Synthesis Tool used for configuring FPGAs. Similar tools are available from Altera, Synplicity, Synopsys and other hardware vendors. * An ''assembler'' is a program that compiles human readable
assembly language In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, gene ...
to
machine code In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a particular task. Programming involves tasks such as analysis, ge ...
, the actual instructions executed by hardware. The inverse program that translates machine code to assembly language is called a disassembler. * A program that translates from a low-level language to a higher level one is a
decompiler A decompiler is a computer program A computer program is a collection of instructions that can be executed by a computer to perform a specific task. A computer program is usually written by a computer programmer in a programming language ...
. * A program that translates into an object code format that is not supported on the compilation machine is called a cross compiler and is commonly used to prepare code for embedded applications. * A program that rewrites object code back into the same type of object code while applying optimisations and transformations is a binary recompiler.

* Abstract interpretation * Bottom-up parsing * Compile and go loader * Compile farm * List of compilers * * Metacompilation