Compiler

This article is about software to translate computer languages. For the manga, seeCompiler (manga).

"Compile" and "Compiling" redirect here. For the software company, seeCompile (company).For other uses, seeCompilation.

Incomputing,acompileris acomputer programthattranslatescomputer code written in oneprogramming language(thesourcelanguage) into another language (thetargetlanguage). The name "compiler" is primarily used for programs that translatesource codefrom ahigh-level programming languageto alow-level programming language(e.g.assembly language,object code,ormachine code) to create anexecutableprogram.

There are many different types of compilers which produce output in different useful forms. Across-compilerproduces code for a differentCPUoroperating systemthan the one on which the cross-compiler itself runs. Abootstrap compileris often a temporary compiler, used for compiling a more permanent or better optimised compiler for a language.

Related software includedecompilers,programs that translate from low-level languages to higher level ones; programs that translate between high-level languages, usually calledsource-to-source compilersortranspilers;languagerewriters,usually programs that translate the form ofexpressionswithout a change of language; andcompiler-compilers,compilers that produce compilers (or parts of them), 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:preprocessing,lexical analysis,parsing,semantic analysis(syntax-directed translation), conversion of input programs to anintermediate representation,code optimizationandmachine specific code generation.Compilers generally implement these phases as modular components, promoting efficient design and correctness oftransformationsof 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 ensurecompiler correctness.

Compilers are not the only language processor used to transform source programs. Aninterpreteris 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

Main article:History of compiler construction

A diagram of the operation of a typical multi-language, multi-target compiler

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. Limitedmemorycapacity 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 areformal languagesthat are strictly defined by their syntax andsemanticswhich 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(BNF) describes the syntax of "sentences" of a language. It was developed byJohn Backusand used for the syntax ofAlgol 60.The ideas derive from thecontext-free grammarconcepts by linguistNoam Chomsky."BNF and its extensions have become standard tools for describing the syntax of programming notations. In many cases, parts of compilers are generated automatically from a BNF description."

Between 1942 and 1945,Konrad Zusedesigned the first (algorithmic) programming language for computers calledPlankalkül( "Plan Calculus" ). Zuse also envisioned aPlanfertigungsgerät( "Plan assembly device" ) to automatically translate the mathematical formulation of a program into machine-readablepunched film stock.While no actual implementation occurred until the 1970s, it presented concepts later seen inAPLdesigned by Ken Iverson in the late 1950s.APL is a language for mathematical computations.

Between 1949 and 1951,Heinz RutishauserproposedSuperplan,a high-level language and automatic translator.His ideas were later refined byFriedrich L. BauerandKlaus Samelson.

High-level language design during the formative years of digital computing provided useful programming tools for a variety of applications:

• FORTRAN(Formula Translation) for engineering and science applications is considered to be one of the first actually implemented high-level languages and first optimizing compiler.
• COBOL(Common Business-Oriented Language) evolved fromA-0andFLOW-MATICto become the dominant high-level language for business applications.
• LISP(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:

• May 1952:Grace Hopper's team atRemington Randwrote the compiler for theA-0programming language (and coined the termcompilerto describe it),although the A-0 compiler functioned more as a loader or linker than the modern notion of a full compiler.
• 1952, before September:AnAutocodecompiler developed byAlick Glenniefor theManchester Mark Icomputer at the University of Manchester is considered by some to be the first compiled programming language.
• 1954–1957:A team led byJohn BackusatIBMdevelopedFORTRANwhich 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 ofCOBOL.The COBOL design drew on A-0 and FLOW-MATIC. By the early 1960s COBOL was compiled on multiple architectures.
• 1958–1960:Algol 58was the precursor toALGOL 60.It introducedcode blocks,a key advance in the rise ofstructured programming.ALGOL 60 was the first language to implementnested functiondefinitions withlexical scope.It includedrecursion.Its syntax was defined usingBNF.ALGOL 60 inspired many languages that followed it.Tony Hoareremarked: "... it was not only an improvement on its predecessors but also on nearly all its successors."
• 1958–1962:John McCarthyatMITdesignedLISP.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,BLISS,B,andC.

BCPL(Basic Combined Programming Language) designed in 1966 byMartin Richardsat 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 researchbut also provided a basis for the design of B and C languages.

BLISS(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(Multiplexed Information and Computing Service), a time-sharing operating system project, involvedMIT,Bell Labs,General Electric(laterHoneywell) and was led byFernando Corbatófrom MIT.Multics was written in thePL/Ilanguage 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 languageBbased on BCPL concepts, written byDennis RitchieandKen Thompson.Ritchie created a boot-strapping compiler for B and wroteUnics(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 ofCbased 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(OOP) offered some interesting possibilities for application development and maintenance. OOP concepts go further back but were part ofLISPandSimulalanguage science.Bell Labs became interested in OOP with the development ofC++.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 newerprogramming languagesand the increasing complexity of computer architectures, compilers became more complex.

DARPA(Defense Advanced Research Projects Agency) sponsored a compiler project with Wulf's CMU research team in 1970. The Production Quality Compiler-CompilerPQCCdesign 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) 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 languageAda.

The AdaSTONEMANdocumentformalized the program support environment (APSE) along with the kernel (KAPSE) and minimal (MAPSE). An Ada interpreter NYU/ED supported development and standardization efforts with the American National Standards Institute (ANSI) and the International Standards Organization (ISO). Initial Ada compiler development by the U.S. Military Services included the compilers in a complete integrated design environment along the lines of theSTONEMANdocument. Army and Navy worked on the Ada Language System (ALS) project targeted to DEC/VAX architecture while the Air Force started on the Ada Integrated Environment (AIE) targeted to IBM 370 series. While the projects did not provide the desired results, they did contribute to the overall effort on Ada development.

Other Ada compiler efforts got underway in Britain at the University of York and in Germany at the University of Karlsruhe. In the U. S., Verdix (later acquired by Rational) delivered the Verdix Ada Development System (VADS) to the Army. VADS provided a set of development tools including a compiler. Unix/VADS could be hosted on a variety of Unix platforms such as DEC Ultrix and the Sun 3/60 Solaris targeted to Motorola 68020 in an Army CECOM evaluation.There were soon many Ada compilers available that passed the Ada Validation tests. The Free Software Foundation GNU project developed theGNU Compiler Collection(GCC) which provides a core capability to support multiple languages and targets. The Ada versionGNATis 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 anintegrated development environment.

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 andparallel computingwere 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. As a result, 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 asingle passhas classically been seen as a benefit because it simplifies the job of writing a compiler and one-pass compilers generally perform compilations faster thanmulti-pass compilers.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).

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 sophisticatedoptimizationsneeded 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

Compiler design

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.

• Thefront endscans the input and verifies syntax and semantics according to a specific source language. Forstatically typed languagesit performstype checkingby 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 anintermediate representation(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.
• Themiddle endperforms 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) or unreachable code (reachability analysis), discovery and propagation of constant values (constant propagation), 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.
• Theback endtakes 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, performingregister allocationin the process. The back end performsinstruction scheduling,which re-orders instructions to keep parallelexecution unitsbusy by fillingdelay slots.Although most optimization problems areNP-hard,heuristictechniques for solving them are well-developed and 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 differentCPUswhile sharing the optimizations of the middle end.Practical examples of this approach are theGNU Compiler Collection,Clang(LLVM-based C/C++ compiler),and theAmsterdam Compiler Kit,which have multiple front-ends, shared optimizations and multiple back-ends.

Front end

Lexerandparserexample forC.Starting from the sequence of characters "if(net>0.0)total+=net*(1.0+tax/100.0);",the scanner composes a sequence oftokens,and categorizes each of them, for example asidentifier,reserved word,number literal,oroperator.The latter sequence is transformed by the parser into asyntax tree,which is then treated by the remaining compiler phases. The scanner and parser handles theregularand properlycontext-freeparts of thegrammar for C,respectively.

The front end analyzes the source code to build an internal representation of the program, called theintermediate representation(IR). It also manages thesymbol 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 ascannerless parser,it was traditionally implemented and analyzed as several phases, which may execute sequentially or concurrently. This method is favored due to its modularity andseparation of concerns.Most commonly, the frontend is broken into three phases:lexical analysis(also known as lexing or scanning),syntax analysis(also known as scanning or parsing), andsemantic 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 usuallycontext-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 usingattribute grammars.These phases themselves can be further broken down: lexing as scanning and evaluating, and parsing as building aconcrete syntax tree(CST, parse tree) and then transforming it into anabstract syntax tree(AST, syntax tree). In some cases additional phases are used, notablyline reconstructionandpreprocessing,but these are rare.

The main phases of the front end include the following:

• Line reconstructionconverts the input character sequence to a canonical form ready for the parser. Languages whichstroptheir keywords or allow arbitrary spaces within identifiers require this phase. Thetop-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 AutocodeandImp(and some implementations ofALGOLandCoral 66) are examples of stropped languages whose compilers would have aLine Reconstructionphase.
• Preprocessingsupportsmacrosubstitution andconditional 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 asSchemesupport macro substitutions based on syntactic forms.
• Lexical analysis(also known aslexingortokenization) breaks the source code text into a sequence of small pieces calledlexical tokens.This phase can be divided into two stages: thescanning,which segments the input text into syntactic units calledlexemesand assigns them a category; and theevaluating,which converts lexemes into a processed value. A token is a pair consisting of atoken nameand an optionaltoken value.Common token categories may include identifiers, keywords, separators, operators, literals and comments, although the set of token categories varies in differentprogramming languages.The lexeme syntax is typically aregular language,so afinite state automatonconstructed from aregular expressioncan be used to recognize it. The software doing lexical analysis is called alexical analyzer.This may not be a separate step—it can be combined with the parsing step inscannerless parsing,in which case parsing is done at the character level, not the token level.
• Syntax analysis(also known asparsing) involvesparsingthe token sequence to identify the syntactic structure of the program. This phase typically builds aparse tree,which replaces the linear sequence of tokens with a tree structure built according to the rules of aformal grammarwhich define the language's syntax. The parse tree is often analyzed, augmented, and transformed by later phases in the compiler.
• Semantic analysisadds semantic information to theparse treeand builds thesymbol table.This phase performs semantic checks such astype checking(checking for type errors), orobject binding(associating variable and function references with their definitions), ordefinite 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 theparsingphase, and logically precedes thecode generationphase, 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 asoptimizer,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:

• Analysis:This is the gathering of program information from the intermediate representation derived from the input;data-flow analysisis used to builduse-define chains,together withdependence analysis,alias analysis,pointer analysis,escape analysis,etc. Accurate analysis is the basis for any compiler optimization. Thecontrol-flow graphof every compiled function and thecall graphof the program are usually also built during the analysis phase.
• Optimization:the intermediate language representation is transformed into functionally equivalent but faster (or smaller) forms. Popular optimizations areinline expansion,dead-code elimination,constant propagation,loop transformationand evenautomatic parallelization.

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

The scope of compiler analysis and optimizations vary greatly; their scope may range from operating within abasic 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 optimizationsare 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 optimizationrequires 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 fromHP,IBM,SGI,Intel,Microsoft,andSun Microsystems.Thefree softwareGCCwas 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 isOpen64,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 forcode 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 ispeephole optimizations,which rewrites short sequences of assembler instructions into more efficient instructions.
• Code generation:the transformed intermediate language is translated into the output language, usually the nativemachine languageof the system. This involves resource and storage decisions, such as deciding which variables to fit intoregistersand memory and theselectionandschedulingof appropriate machine instructions along with their associatedaddressing modes(see alsoSethi–Ullman algorithm). Debug data may also need to be generated to facilitatedebugging.

Compiler correctness

Main article:Compiler correctness

Compiler correctnessis the branch of software engineering that deals with trying to show that a compiler behaves according to itslanguage specification.Techniques include developing the compiler usingformal methodsand using rigorous testing (often called compiler validation) on an existing compiler.

Compiled versus interpreted languages

Higher-level programming languages usually appear with a type oftranslationin mind: either designed ascompiled languageorinterpreted language.However, in practice there is rarely anything about a language thatrequiresit 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,BASICis 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 set of directly executed machine instructions is needed somewhere at the bottom of the execution stack (seemachine 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 towardjust-in-time compilationandbytecode interpretationat times blur the traditional categorizations of compilers and interpreters even further.

Some language specifications spell out that implementationsmustinclude 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,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 specialevaluation function.To implement these features in a compiled language, programs must usually be shipped with aruntime librarythat includes a version of the compiler itself.

Types

One classification of compilers is by theplatformon which their generated code executes. This is known as thetarget platform.

Anativeorhostedcompiler 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 across compileris designed to run on a different platform. Cross compilers are often used when developing software forembedded systemsthat are not intended to support a software development environment.

The output of a compiler that produces code for avirtual 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 ahigh-level programming language.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 forC++,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, soindent styleand creating pretty C intermediate code are ignored. Some of the features of C that make it a good target language include the#linedirective, which can be generated by the compiler to supportdebuggingof 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 compilersare a type of compiler that takes a high-level language as its input and outputs a high-level language. For example, anautomatic parallelizingcompiler 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'sDOALLstatements). Other terms for a source-to-source compiler are transcompiler or transpiler.
• Bytecodecompilers compile to assembly language of a theoretical machine, like somePrologimplementations
  • This Prolog machine is also known as theWarren Abstract Machine(or WAM).
  • Bytecode compilers forJava,Pythonare also examples of this category.
• Just-in-time compilers(JIT compiler) defer compilation until runtime. JIT compilers exist for many modern languages includingPython,JavaScript,Smalltalk,Java,Microsoft.NET'sCommon 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-independentintermediate representation.A bytecode interpreter executes the bytecode, but the JIT compiler will translate the bytecode to machine code when increased performance is necessary.
• Hardware compilers(also known as synthesis tools) are compilers whose input is ahardware description languageand whose output is a description, in the form of anetlistor otherwise, of a hardware configuration.
  • The output of these compilers targetcomputer hardwareat a very low level, for example afield-programmable gate array(FPGA) or structuredapplication-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 oftransistorsorlookup 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.
• A program that translates from a low-level language to a higher level one is adecompiler.
• A program that translates into an object code format that is not supported on the compilation machine is called across compilerand is commonly used to prepare code for execution on embedded software applications.
• A program that rewrites object code back into the same type of object code while applying optimisations and transformations is abinary recompiler.

Assemblers,which translate human readableassembly languageto themachine codeinstructions executed by hardware, are not considered compilers.(The inverse program that translates machine code to assembly language is called adisassembler.)

See also

• Abstract interpretation
• Bottom-up parsing
• Compile and go system
• Compile farm
• List of compilers
• List of important publications in computer science § Compilers
• Metacompilation
• Program transformation

Further reading

• Aho, Alfred V.;Sethi, Ravi;Ullman, Jeffrey D.(1986).Compilers: Principles, Techniques, and Tools(1st ed.).Addison-Wesley.ISBN9780201100884.
• Allen, Frances E.(September 1981). "A History of Language Processor Technology in IBM".IBM Journal of Research and Development.25(5).IBM:535–548.doi:10.1147/rd.255.0535.
• Allen, Randy;Kennedy, Ken(2001).Optimizing Compilers for Modern Architectures.Morgan Kaufmann Publishers.ISBN978-1-55860-286-1.
• Appel, Andrew Wilson(2002).Modern Compiler Implementation in Java(2nd ed.).Cambridge University Press.ISBN978-0-521-82060-8.
• Appel, Andrew Wilson(1998).Modern Compiler Implementation in ML.Cambridge University Press.ISBN978-0-521-58274-2.
• Bornat, Richard(1979).Understanding and Writing Compilers: A Do It Yourself Guide(PDF).Macmillan Publishing.ISBN978-0-333-21732-0.Archived fromthe original(PDF)on 15 June 2007.Retrieved11 April2007.
• Calingaert, Peter (1979).Horowitz, Ellis(ed.).Assemblers, Compilers, and Program Translation.Computer software engineering series (1st printing, 1st ed.). Potomac, Maryland:Computer Science Press, Inc.ISBN0-914894-23-4.ISSN0888-2088.LCCN78-21905.Retrieved20 March2020.(2+xiv+270+6 pages)
• Cooper, Keith Daniel; Torczon, Linda (2012).Engineering a compiler(2nd ed.). Amsterdam, Netherlands: Elsevier/Morgan Kaufmann. p. 8.ISBN978-0-12088478-0.OCLC714113472.
• Gries, David(1971).Compiler Construction for Digital Computers(in English, Spanish, Japanese, Chinese, Italian, and Russian). New York: John Wiley and Sons.ISBN0-471-32776-X.The first text on compiler construction.
• McKeeman, William Marshall;Horning, James J.;Wortman, David B. (1970).A Compiler Generator.Englewood Cliffs, NJ:Prentice-Hall.ISBN978-0-13-155077-3.
• Muchnick, Steven(1997).Advanced Compiler Design and Implementation.Morgan Kaufmann Publishers.ISBN978-1-55860-320-2.
• Scott, Michael Lee(2005).Programming Language Pragmatics(2nd ed.).Morgan Kaufmann.ISBN978-0-12-633951-2.
• Srikant, Y. N.; Shankar, Priti (2003).The Compiler Design Handbook: Optimizations and Machine Code Generation.CRC Press.ISBN978-0-8493-1240-3.
• Terry, Patrick D. (1997).Compilers and Compiler Generators: An Introduction with C++.International Thomson Computer Press.ISBN978-1-85032-298-6.
• Wirth, Niklaus(1996).Compiler Construction(PDF).Addison-Wesley.ISBN978-0-201-40353-4.Archived fromthe original(PDF)on 17 February 2017.Retrieved24 April2012.
• LLVM community."The LLVM Target-Independent Code Generator".LLVM Documentation.Retrieved17 June2016.
• Compiler textbook referencesA collection of references to mainstream Compiler Construction Textbooks

External links

• CompilersatCurlie
• Incremental Approach to Compiler Construction– a PDF tutorial
• Basics of Compiler Designat theWayback Machine(archived 15 May 2018)
• Short animationonYouTubeexplaining the key conceptual difference between compilers and interpreters
• Syntax Analysis & LL1 ParsingonYouTube
• Let's Build a Compiler,by Jack Crenshaw
• Forum about compiler developmentat theWayback Machine(archived 10 October 2014)