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A compiler is a computer program (or set of programs) that transforms source code written in a programming language (the source language) into another computer language (the target language, often having a binary form known as object code).[1] 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). If the compiled program can run on a computer whose CPU or operating system is different from the one on which the compiler runs, the compiler is known as a cross-compiler. 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, or language converter. A language 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 (Syntax-directed translation), code generation, and code optimization.

Program faults caused by incorrect compiler behavior can be very difficult to track down and work around; therefore, compiler implementors invest significant effort to ensure compiler correctness.

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


Software for early computers was primarily written in assembly language. Higher level programming languages were not invented until the benefits of being able to reuse software on different kinds of CPUs started to become significantly greater than the costs of writing a compiler. The limited memory capacity of early computers led to substantial technical challenges when the first compilers were being designed.

Towards the end of the 1950s, machine-independent programming languages were first proposed. Subsequently several experimental compilers were developed. It is believed that the first compiler was written by Grace Hopper, in 1952, for the A-0 programming language.The A-0 functioned more as a loader or linker than the modern notion of a compiler. The first autocode and its compiler were developed by Alick Glennie in 1952 for the Mark 1 computer at the University of Manchester and is considered by some to be the first compiled programming language. The FORTRAN 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.[2]

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

Early compilers were written in assembly language. The first self-hosting compiler – capable of compiling its own source code in a high-level language – was created in 1962 for Lisp by Tim Hart and Mike Levin at MIT.[3] Since the 1970s it has become common practice to implement a compiler in the language it compiles, although both Pascal and C have been popular choices for implementation language. Building a self-hosting compiler is a bootstrapping problem—the first such compiler for a language must be compiled either by hand or 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.

Compilers in education[edit]

Compiler construction and compiler optimization are taught at universities and schools as part of a computer science curriculum.[4] 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.[5] In spite of its simplicity, the PL/0 compiler introduced several influential concepts to the field:

  1. Program development by stepwise refinement (also the title of a 1971 paper by Wirth)[6]
  2. The use of a recursive descent parser
  3. The use of EBNF to specify the syntax of a language
  4. A code generator producing portable P-code
  5. The use of T-diagrams[7] in the formal description of the bootstrapping problem


Compilers enabled the development of programs that are machine-independent. Before the development of FORTRAN, the first higher-level language, in the 1950s, machine-dependent assembly language was widely used. While assembly language produces more abstraction than machine code on the same architecture, just as with machine code, it has to be modified or rewritten if the program is to be executed on different computer hardware architecture.

With the advent of high-level programming languages that followed FORTRAN, such as COBOL, C, and BASIC, programmers could write machine-independent source programs. A compiler translates the high-level source programs into target programs in machine languages for the specific hardware. Once the target program is generated, the user can execute the program.

Structure of a compiler[edit]

Compilers bridge source programs in high-level languages with the underlying hardware. A compiler verifies code syntax, generates efficient object code, performs run-time organization, and formats the output according to assembler and linker conventions. A compiler consists of:

  1. The front end: Verifies syntax and semantics, and generates an intermediate representation or IR of the source code for processing by the middle-end. Performs type checking by collecting type information. Generates errors and warning, if any, in a useful way.
  2. The middle end: Performs optimizations, including removal of useless or unreachable code, discovery and propagation of constant values, relocation of computation to a less frequently executed place (e.g., out of a loop), or specialization of computation based on the context. Generates another IR for the backend.
  3. The back end: Generates the assembly code, performing register allocation in process. (Assigns processor registers for the program variables where possible.) Optimizes target code utilization of the hardware by figuring out how to keep parallel execution units busy, filling delay slots. Although most algorithms for optimization are in NP, heuristic techniques are well-developed.

Compiler output[edit]




The lower level language that is the target of a compiler may itself be a high-level programming language. C, often viewed as some sort of portable assembler, can also be the target language of a compiler. E.g.: Cfront, the original compiler for C++ used C as target language. The C created by such a compiler is usually not intended to be read and maintained by humans. So indent style and pretty C intermediate code are irrelevant. Some features of C turn it into a good target language. E.g.: C code with #line directives can be generated to support debugging of the original source.

Compiled versus interpreted languages[edit]


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 stack (see machine language). Modern trends toward just-in-time compilation and bytecode interpretation at times blur the traditional categorizations of compilers and interpreters.


Hardware compilation[edit]

The output of some compilers may target computer hardware at a very low level, for example a Field Programmable Gate Array (FPGA) or structured Application-specific integrated circuit (ASIC).[8] Such compilers are said to be hardware compilers or synthesis tools because the source code they compile effectively controls the final configuration of the hardware and how it operates; the output of the compilation is 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 construction[edit]

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 (e.g., 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 which these two ends meet is 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 languages with back ends for different CPUs. 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[edit]

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.





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 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).
  • Stage compiler that compiles to assembly language of a theoretical machine, like some Prolog implementations
  • 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[edit]

The compiler frontend 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.

While the frontend can be a single monolithic function or program, as in a scannerless parser, it is more commonly implemented and analyzed as several phases, which may execute sequentially or concurrently. This is particularly done for good engineering: modularity and separation of concerns. Most commonly today this is done as three phases: lexing, 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 or writing by hand. The lexical grammar and phrase grammar are usually context-free grammars, 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 grammars. These phases themselves can be further broken down – lexing as scanning and evaluating, parsing as first building a concrete syntax tree (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. A detailed list of possible phases includes:

  1. Line reconstruction: Languages which 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, 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 Imp (and some implementations of ALGOL and Coral 66) are examples of stropped languages which compilers would have a Line Reconstruction phase.
  2. 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, identifier or 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. 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.
  3. Preprocessing. Some languages, e.g., C, require a preprocessing phase which supports 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 support macro substitutions based on syntactic forms.

Back end[edit]

The term back end is sometimes confused with 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:

  1. 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 chains, 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.
  2. 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 and even automatic parallelization.


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 HP, IBM, SGI, Intel, Microsoft, and Sun Microsystems. The open source 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.


Compiler correctness[edit]

Compiler correctness is the branch of software engineering that deals with trying to show that a compiler behaves according to its language specification.[9] Techniques include developing the compiler using formal methods and using rigorous testing (often called compiler validation) on an existing compiler.

Related techniques[edit]

Assembly language is a type of low-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 between high-level languages is usually called a language translator, source to source translator, language converter, or language rewriter. The last term is usually applied to translations that do not involve a change of language.


International conferences and organizations[edit]

A number of conferences in the field of programming languages present advances in compiler construction as one of their main topics.

ACM SIGPLAN supports a number of conferences, including:

The European Joint Conferences on Theory and Practice of Software (ETAPS) sponsors the International Conference on Compiler Construction, with papers from both the academic and industrial sectors.[10]

Asian Symposium on Programming Languages and Systems (APLAS) is organized by the Asian Association for Foundation of Software (AAFS).

See also[edit]


  1. ^ "Definition of:compiler". 
  2. ^ "IP: The World's First COBOL Compilers". 12 June 1997. 
  3. ^ T. Hart and M. Levin. "The New Compiler, AIM-39 - CSAIL Digital Archive - Artificial Intelligence Laboratory Series". 
  4. ^ Chakraborty, P., Saxena, P. C., Katti, C. P., Pahwa, G., Taneja, S. A new practicum in compiler construction. Computer Applications in Engineering Education, In Press.
  5. ^ "The PL/0 compiler/interpreter". 
  6. ^ "The ACM Digital Library". 
  7. ^ 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, Chapter 3. T-diagrams are also now used to describe client-server interconnectivity on the World Wide Web: cf. Patrick Closhen, et al. 1997: T-Diagrams as Visual Language to Illustrate WWW Technology, Darmstadt University of Technology, Darmstadt, Germany
  8. ^ "A Fast Routability-Driven Router for FPGAs". Department of Electrical and Computer Engineering, University of Toronto. 
  9. ^ Chlipala, Adam. "Syntactic Proofs of Compositional Compiler Correctness". Harvard University Cambridge, Massachusetts, USA. 
  10. ^ ETAPS - European Joint Conferences on Theory and Practice of Software. Cf. "CC" (Compiler Construction) subsection.


  1. Compiler textbook references A collection of references to mainstream Compiler Construction Textbooks[dead link]
  2. Aho, Alfred V.; Sethi, Ravi; Ullman, Jeffrey D. (1986). Compilers: Principles, Techniques, and Tools (1st ed.). Addison-Wesley. ISBN 9780201100884. 
  3. Allen, Frances E. (September 1981). "A History of Language Processor Technology in IBM". IBM Journal of Research and Development (IBM) 25 (5). (subscription required (help)). 
  4. Allen, Randy; Kennedy, Ken (2001). Optimizing Compilers for Modern Architectures. Morgan Kaufmann Publishers. ISBN 1-55860-286-0. 
  5. Appel, Andrew Wilson (2002). Modern Compiler Implementation in Java (2nd ed.). Cambridge University Press. ISBN 0-521-82060-X. 
  6. Appel, Andrew Wilson (1998). Modern Compiler Implementation in ML. Cambridge University Press. ISBN 0-521-58274-1. 
  7. Bornat, Richard (1979). Understanding and Writing Compilers: A Do It Yourself Guide. Macmillan Publishing. ISBN 0-333-21732-2. 
  8. Cooper, Keith D.; Torczon, Linda (2004). Engineering a Compiler. Morgan Kaufmann. ISBN 1-55860-699-8. 
  9. Leverett, Bruce W.; Cattell, R. G. G.; Newcomer, Joseph M.; Hobbs, S.O.; Reiner, A.H.; Schatz, B.R.; Wulf, W.A. (August 1980). "An Overview of the Production – Quality Compiler – Compiler Project". Computer (Carnegie-Mellon University) 13 (8): 38–49. doi:10.1109/MC.1980.1653748. ISSN 0018-9162. (subscription required (help)). 
  10. McKeeman, William Marshall; Horning, James J.; Wortman, David B. (1970). A Compiler Generator. Englewood Cliffs, NJ: Prentice-Hall. ISBN 0-13-155077-2. 
  11. Muchnick, Steven (1997). Advanced Compiler Design and Implementation. Morgan Kaufmann Publishers. ISBN 1-55860-320-4. 
  12. Scott, Michael Lee (2005). Programming Language Pragmatics (2nd ed.). Morgan Kaufmann. ISBN 0-12-633951-1. 
  13. Srikant, Y. N.; Shankar, Priti (2003). The Compiler Design Handbook: Optimizations and Machine Code Generation. CRC Press. ISBN 0-8493-1240-X. 
  14. Terry, Patrick D. (1997). Compilers and Compiler Generators: An Introduction with C++. International Thomson Computer Press. ISBN 1-85032-298-8. 
  15. Wirth, Niklaus url= (1996). Compiler Construction. Addison-Wesley. ISBN 0-201-40353-6. 

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