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A compiler is a computer program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

 (or set of programs) that transforms source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

 written in a programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

 (the source language) into another computer language (the target language, often having a binary form known as object code
Object code
Object code, or sometimes object module, is what a computer compiler produces. In a general sense object code is a sequence of statements in a computer language, usually a machine code language....

). The most common reason for wanting to transform source code is to create an executable
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...

 program.

The name "compiler" is primarily used for programs that translate source code from a high-level programming language
High-level programming language
A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...

 to a lower level language (e.g., assembly language
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

 or machine code
Machine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...

). If the compiled program can run on a computer whose CPU or operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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
Decompiler
A decompiler is the name given to a computer program that performs, as far as possible, the reverse operation to that of a compiler. That is, it translates a file containing information at a relatively low level of abstraction into a form having a higher level of abstraction...

. A program that translates between high-level languages is usually called a language translator
Translator (computing)
A Translator is a computer program that translates one programming language instruction into anotherprogramming language instruction without the loss of original meaning. OR, the translator will translate X...

, source to source translator, or language converter. A language rewriter
Rewriting
In mathematics, computer science, and logic, rewriting covers a wide range of methods of replacing subterms of a formula with other terms. What is considered are rewriting systems...

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
Lexical analysis
In computer science, lexical analysis is the process of converting a sequence of characters into a sequence of tokens. A program or function which performs lexical analysis is called a lexical analyzer, lexer or scanner...

, preprocessing, parsing
Parsing
In computer science and linguistics, parsing, or, more formally, syntactic analysis, is the process of analyzing a text, made of a sequence of tokens , to determine its grammatical structure with respect to a given formal grammar...

, semantic analysis (Syntax-directed translation
Syntax-directed translation
In computer programming, Syntax-directed translation is a method of translating a string into a sequence of actions by attaching one such action to each rule of a grammar. Thus, parsing a string of the grammar produces a sequence of rule applications...

), 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 a lot of time ensuring the correctness of their software
Compiler correctness
In computing, compiler correctness is the branch of software engineering that deals with trying to show that a compiler behaves according to its language specification...

.

The term compiler-compiler
Compiler-compiler
A compiler-compiler or compiler generator is a tool that creates a parser, interpreter, or compiler from some form of formal description of a language and machine...

 is sometimes used to refer to a parser generator, a tool often used to help create the lexer
Lexical analysis
In computer science, lexical analysis is the process of converting a sequence of characters into a sequence of tokens. A program or function which performs lexical analysis is called a lexical analyzer, lexer or scanner...

 and parser.

History



Software for early computers was primarily written in assembly language for many years. Higher level programming languages were not invented until the benefits of being able to reuse software on different kinds of CPUs started to become significantly greater than the cost of writing a compiler. The very limited memory
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....

 capacity of early computers also created many technical problems when implementing a compiler.

Towards the end of the 1950s, machine-independent programming languages were first proposed. Subsequently, several experimental compilers were developed. The first compiler was written by Grace Hopper
Grace Hopper
Rear Admiral Grace Murray Hopper was an American computer scientist and United States Navy officer. A pioneer in the field, she was one of the first programmers of the Harvard Mark I computer, and developed the first compiler for a computer programming language...

, in 1952, for the A-0 programming language
A-0 programming language
The A-0 system , written by Grace Hopper in 1951 and 1952 for the UNIVAC I, was the first compiler ever developed for an electronic computer. The A-0 functioned more as a loader or linker than the modern notion of a compiler. A program was specified as a sequence of subroutines and arguments...

. The FORTRAN
Fortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...

 team led by John Backus
John Backus
John Warner Backus was an American computer scientist. He directed the team that invented the first widely used high-level programming language and was the inventor of the Backus-Naur form , the almost universally used notation to define formal language syntax.He also did research in...

 at IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 is generally credited as having introduced the first complete compiler in 1957. COBOL
COBOL
COBOL is one of the oldest programming languages. Its name is an acronym for COmmon Business-Oriented Language, defining its primary domain in business, finance, and administrative systems for companies and governments....

 was an early language to be compiled on multiple architectures, in 1960.

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
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

s and the increasing complexity of computer architectures, compilers have become more and more complex.

Early compilers were written in assembly language. The first self-hosting
Self-hosting
The term self-hosting was coined to refer to the use of a computer program as part of the toolchain or operating system that produces new versions of that same program—for example, a compiler that can compile its own source code. Self-hosting software is commonplace on personal computers and larger...

compiler — capable of compiling its own source code in a high-level language — was created for Lisp
Lisp programming language
Lisp is a family of computer programming languages with a long history and a distinctive, fully parenthesized syntax. Originally specified in 1958, Lisp is the second-oldest high-level programming language in widespread use today; only Fortran is older...

 by Tim Hart and Mike Levin at MIT
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...

 in 1962. Since the 1970s it has become common practice to implement a compiler in the language it compiles, although both Pascal
Pascal (programming language)
Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal...

 and C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

 have been popular choices for implementation language. Building a self-hosting compiler is a bootstrapping
Bootstrapping (compilers)
In computer science, bootstrapping is the process of writing a compiler in the target programming language which it is intended to compile...

 problem—the first such compiler for a language must be compiled either by a compiler written in a different language, or (as in Hart and Levin's Lisp compiler) compiled by running the compiler in an interpreter
Interpreter (computing)
In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language...

.

Compilers in education


Compiler construction and compiler optimization
Compiler optimization
Compiler optimization is the process of tuning the output of a compiler to minimize or maximize some attributes of an executable computer program. The most common requirement is to minimize the time taken to execute a program; a less common one is to minimize the amount of memory occupied...

 are taught at universities and schools as part of the computer science
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

 curriculum. Such courses are usually supplemented with the implementation of a compiler for an educational programming language
Educational programming language
An educational programming language is a programming language that is designed primarily as a learning instrument and not so much as a tool for writing programs for real-world work.-Learning paths:...

. A well-documented example is Niklaus Wirth
Niklaus Wirth
Niklaus Emil Wirth is a Swiss computer scientist, best known for designing several programming languages, including Pascal, and for pioneering several classic topics in software engineering. In 1984 he won the Turing Award for developing a sequence of innovative computer languages.-Biography:Wirth...

's PL/0
PL/0
At least two programming languages are known as PL/0. One is a subset of IBM's general-purpose programming language PL/I.The other PL/0, covered here, is similar to but much simpler than the general-purpose programming language Pascal, intended as an educational programming language. It serves as...

 compiler, which Wirth used to teach compiler construction in the 1970s. 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)
  2. The use of a recursive descent parser
    Recursive descent parser
    A recursive descent parser is a top-down parser built from a set of mutually-recursive procedures where each such procedure usually implements one of the production rules of the grammar...

  3. The use of EBNF to specify the syntax of a language
  4. A code generator producing portable P-code
    P-code
    P-code can refer to:* Precompiled code, for example Java Byte code, MATLAB .p-files, etc.* p-code machine * Code used in the UCSD p-System...

  5. The use of T-diagrams in the formal description of the bootstrapping
    Bootstrapping (compilers)
    In computer science, bootstrapping is the process of writing a compiler in the target programming language which it is intended to compile...

     problem

Compilation


Compilers enabled the development of programs that are machine-independent. Before the development of FORTRAN (FORmula TRANslator), the first higher-level language, in the 1950s, machine-dependent assembly language
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

 was widely used. While assembly language produces more reusable and relocatable programs than machine code on the same architecture, it has to be modified or rewritten if the program is to be executed on different hardware architecture.

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

The structure of a compiler


Compilers bridge source programs in high-level languages with the underlying hardware. A compiler requires 1) determining the correctness of the syntax of programs, 2) generating correct and efficient object code, 3) run-time organization, and 4) formatting output according to assembler and/or linker conventions. A compiler consists of three main parts: the frontend, the middle-end, and the backend.

The front end checks whether the program is correctly written in terms of the programming language syntax and semantics. Here legal and illegal programs are recognized. Errors are reported, if any, in a useful way. Type checking is also performed by collecting type information. The frontend then generates an intermediate representation or IR of the source code for processing by the middle-end.

The middle end is where optimization takes place. Typical transformations for optimization are 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. The middle-end generates another IR for the following backend. Most optimization efforts are focused on this part.

The back end is responsible for translating the IR from the middle-end into assembly code. The target instruction(s) are chosen for each IR instruction. Register allocation
Register allocation
In compiler optimization, register allocation is the process of assigning a large number of target program variables onto a small number of CPU registers...

 assigns processor register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...

s for the program variables where possible. The backend utilizes the hardware by figuring out how to keep parallel execution unit
Execution unit
In computer engineering, an execution unit is a part of a CPU that performs the operations and calculations called for by the Branch Unit, which receives data from the CPU...

s busy, filling delay slots, and so on. Although most algorithms for optimization are in NP
NP (complexity)
In computational complexity theory, NP is one of the most fundamental complexity classes.The abbreviation NP refers to "nondeterministic polynomial time."...

, heuristic techniques are well-developed.

Compiler output


One classification of compilers is by the platform
Platform (computing)
A computing platform includes some sort of hardware architecture and a software framework , where the combination allows software, particularly application software, to run...

 on which their generated code executes. This is known as the target platform.

A native or hosted compiler is one which 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
Cross compiler
A cross compiler is a compiler capable of creating executable code for a platform other than the one on which the compiler is run. Cross compiler tools are used to generate executables for embedded system or multiple platforms. It is used to compile for a platform upon which it is not feasible to...

 is designed to run on a different platform. Cross compilers are often used when developing software for embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...

s that are not intended to support a software development environment.

The output of a compiler that produces code for a virtual machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...

 (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 target of a compiler may itself be a high-level programming language
High-level programming language
A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...

.
C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

, often viewed as some sort of portable assembler, can also be the target language of a compiler.
E.g.: Cfront
Cfront
Cfront was the original compiler for C++ from around 1983, which converted C++ to C; developed by Bjarne Stroustrup. The preprocessor did not understand all of the language and much of the code was written via translations. Cfront had a complete parser, built symbol tables, and built a tree for...

, the original compiler for C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

 used C as target language.

Compiled versus interpreted languages


Higher-level programming languages are generally divided for convenience into compiled language
Compiled language
A compiled language is a programming language whose implementations are typically compilers , and not interpreters ....

s and interpreted language
Interpreted language
Interpreted language is a programming language in which programs are 'indirectly' executed by an interpreter program. This can be contrasted with a compiled language which is converted into machine code and then 'directly' executed by the host CPU...

s. However, in practice there is rarely anything about a language that requires it to be exclusively compiled or exclusively interpreted, although it is possible to design languages that 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.

Modern trends toward just-in-time compilation
Just-in-time compilation
In computing, just-in-time compilation , also known as dynamic translation, is a method to improve the runtime performance of computer programs. Historically, computer programs had two modes of runtime operation, either interpreted or static compilation...

 and bytecode interpretation
Bytecode
Bytecode, also known as p-code , is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software interpreter as well as being suitable for further compilation into machine code...

 at times blur the traditional categorizations of compilers and interpreters.

Interpretation cannot completely replace compilation and assembly: even though an interpreter can itself be interpreted, a directly executed programme is needed somewhere at the bottom of the stack.

Some language specifications spell out that implementations must include a compilation facility; for example, Common Lisp
Common Lisp
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...

. 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 special evaluation function. To implement these features in a compiled language, programs must usually be shipped with a runtime library
Runtime library
In computer programming, a runtime library is a special program library used by a compiler, to implement functions built into a programming language, during the execution of a computer program...

 that includes a version of the compiler itself.

Hardware compilation


The output of some compilers may target hardware
Hardware
Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

 at a very low level, for example a Field Programmable Gate Array (FPGA) or structured Application-specific integrated circuit
Application-specific integrated circuit
An application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC...

 (ASIC). 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 are not instructions that are executed in sequence - only an interconnection of transistors or lookup tables.
For example, XST is the Xilinx Synthesis Tool used for configuring FPGAs. Similar tools are available from Altera, Synplicity, Synopsys and other vendors.

Compiler construction


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
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

 with back ends for different CPUs. Practical examples of this approach are the GNU Compiler Collection
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...

, LLVM, and the Amsterdam Compiler Kit
Amsterdam Compiler Kit
The Amsterdam Compiler Kit is a fast, lightweight and retargetable compiler suite and toolchain written by Andrew Tanenbaum and Ceriel Jacobs, and is MINIX's native toolchain. The ACK was originally closed-source software , but in April 2003 it was released under an open source BSD license...

, which have multiple front-ends, shared analysis and multiple back-ends.

One-pass versus multi-pass compilers


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

The ability to compile in a single pass
One-pass compiler
In computer programming, a one-pass compiler is a compiler that passes through the source code of each compilation unit only once. In other words, a one-pass compiler does not "look back" at code it previously processed. Another term sometimes used is narrow compiler, which emphasizes the limited...

 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 multi-pass compiler
Multi-pass compiler
A multi-pass compiler is a type of compiler that processes the source code or abstract syntax tree of a program several times. This is in contrast to a one-pass compiler, which traverses the program only once. Each pass takes the result of the previous pass as the input, and creates an intermediate...

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 (programming language)
Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal...

).

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

The disadvantage of compiling in a single pass is that it is not possible to perform many of the sophisticated optimizations
Compiler optimization
Compiler optimization is the process of tuning the output of a compiler to minimize or maximize some attributes of an executable computer program. The most common requirement is to minimize the time taken to execute a program; a less common one is to minimize the amount of memory occupied...

 needed to generate high quality code. It can be difficult to count exactly how many passes an optimizing compiler makes. For instance, different phases of optimization may analyse one expression many times but only analyse another expression once.

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

While the typical multi-pass compiler outputs machine code from its final pass, there are several other types:
  • A "source-to-source compiler
    Source-to-source compiler
    A source-to-source compiler is a type of compiler that takes a high level programming 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...

    " 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
    Automatic parallelization
    Automatic parallelization, also auto parallelization, autoparallelization, or parallelization, the last one of which implies automation when used in context, refers to converting sequential code into multi-threaded or vectorized code in order to utilize multiple processors simultaneously in a...

     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
    OpenMP
    OpenMP is an API that supports multi-platform shared memory multiprocessing programming in C, C++, and Fortran, on most processor architectures and operating systems, including Linux, Unix, AIX, Solaris, Mac OS X, and Microsoft Windows platforms...

    ) or language constructs (e.g. Fortran's DOALL statements).
  • Stage compiler that compiles to assembly language of a theoretical machine, like some Prolog
    Prolog
    Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...

     implementations
    • This Prolog machine is also known as the Warren Abstract Machine
      Warren abstract machine
      In 1983, David H. D. Warren designed an abstract machine for the execution of Prolog consisting of a memory architecture and an instruction set. This design became known as the Warren Abstract Machine and has become the de facto standard target for Prolog compilers.-Purpose:The purpose of...

       (or WAM).
    • Bytecode compilers for Java
      Java (programming language)
      Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

      , Python, and many more are also a subtype of this.
  • Just-in-time compiler
    Just-in-time compilation
    In computing, just-in-time compilation , also known as dynamic translation, is a method to improve the runtime performance of computer programs. Historically, computer programs had two modes of runtime operation, either interpreted or static compilation...

    , used by Smalltalk and Java systems, and also by Microsoft .NET's Common Intermediate Language
    Common Intermediate Language
    Common Intermediate Language is the lowest-level human-readable programming language defined by the Common Language Infrastructure specification and is used by the .NET Framework and Mono...

     (CIL)
    • Applications are delivered in bytecode, which is compiled to native machine code just prior to execution.

Front end


The front end analyzes the source code to build an internal representation of the program, called the intermediate representation or IR. It also manages the symbol table
Symbol table
In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier in a program's source code is associated with information relating to its declaration or appearance in the source, such as its type, scope level and...

, a data structure mapping each symbol in the source code to associated information such as location, type and scope. This is done over several phases, which includes some of the following:
  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
    Top-down parsing
    Top-down parsing is a type of parsing strategy where in one first looks at the highest level of the parse tree and works down the parse tree by using the rewriting rules of a formal grammar...

    , recursive-descent
    Recursive descent parser
    A recursive descent parser is a top-down parser built from a set of mutually-recursive procedures where each such procedure usually implements one of the production rules of the grammar...

    , 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
    Atlas Autocode
    Atlas Autocode was a programming language developed around 1965 at Manchester University for the Atlas Computer. It was developed by Tony Brooker and Derrick Morris as an improvement on the ALGOL programming languages, removing some of Algol's poorer features such as "passing parameters by name"...

    , and Imp
    Edinburgh IMP
    Edinburgh IMP is a development of ATLAS Autocode, initially developed around 1966-1969 at Edinburgh University, Scotland. IMP was a general-purpose programming language which was used heavily for systems programming....

     (and some implementations of ALGOL
    ALGOL
    ALGOL is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years...

     and Coral 66) are examples of stropped languages which compilers would have a Line Reconstruction phase.
  2. Lexical analysis
    Lexical analysis
    In computer science, lexical analysis is the process of converting a sequence of characters into a sequence of tokens. A program or function which performs lexical analysis is called a lexical analyzer, lexer or scanner...

     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
    Identifier
    An identifier is a name that identifies either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object , or physical [noncountable] substance...

     or symbol name
    Symbol
    A symbol is something which represents an idea, a physical entity or a process but is distinct from it. The purpose of a symbol is to communicate meaning. For example, a red octagon may be a symbol for "STOP". On a map, a picture of a tent might represent a campsite. Numerals are symbols for...

    . The token syntax is typically a regular language
    Regular language
    In theoretical computer science and formal language theory, a regular language is a formal language that can be expressed using regular expression....

    , so a finite state automaton constructed from a regular expression
    Regular expression
    In computing, a regular expression provides a concise and flexible means for "matching" strings of text, such as particular characters, words, or patterns of characters. Abbreviations for "regular expression" include "regex" and "regexp"...

     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.
  3. Preprocessing
    Preprocessor
    In computer science, a preprocessor is a program that processes its input data to produce output that is used as input to another program. The output is said to be a preprocessed form of the input data, which is often used by some subsequent programs like compilers...

    . Some languages, e.g., C
    C (programming language)
    C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

    , 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.
  4. Syntax analysis involves parsing
    Parsing
    In computer science and linguistics, parsing, or, more formally, syntactic analysis, is the process of analyzing a text, made of a sequence of tokens , to determine its grammatical structure with respect to a given formal grammar...

     the token sequence to identify the syntactic structure of the program. This phase typically builds a parse tree
    Parse tree
    A concrete syntax tree or parse tree or parsing treeis an ordered, rooted tree that represents the syntactic structure of a string according to some formal grammar. In a parse tree, the interior nodes are labeled by non-terminals of the grammar, while the leaf nodes are labeled by terminals of the...

    , which replaces the linear sequence of tokens with a tree structure built according to the rules of a formal grammar
    Formal grammar
    A formal grammar is a set of formation rules for strings in a formal language. The rules describe how to form strings from the language's alphabet that are valid according to the language's syntax...

     which define the language's syntax. The parse tree is often analyzed, augmented, and transformed by later phases in the compiler.
  5. Semantic analysis is the phase in which the compiler adds semantic information to the parse tree
    Parse tree
    A concrete syntax tree or parse tree or parsing treeis an ordered, rooted tree that represents the syntactic structure of a string according to some formal grammar. In a parse tree, the interior nodes are labeled by non-terminals of the grammar, while the leaf nodes are labeled by terminals of the...

     and builds the symbol table. This phase performs semantic checks such as type checking (checking for type errors), or object binding
    Object binding
    Several object binding times exist in object oriented systems. Java, for example, has late binding leading to more loosely coupled systems ....

     (associating variable and function references with their definitions), or definite assignment
    Definite assignment analysis
    In computer science, definite assignment analysis is a data-flow analysis used by compilers to conservatively ensure that a variable or location is always assigned to before it is used.-Motivation:...

     (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
    In computer science and linguistics, parsing, or, more formally, syntactic analysis, is the process of analyzing a text, made of a sequence of tokens , to determine its grammatical structure with respect to a given formal grammar...

     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.

Back end


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 chain
    Use-define chain
    A Use-Definition Chain is a data structure that consists of a use, U, of a variable, and all the definitions, D, of that variable that can reach that use without any other intervening definitions...

    s, dependence analysis
    Dependence analysis
    In compiler theory, dependence analysis produces execution-order constraints between statements/instructions. Broadly speaking, a statement S2 depends on S1 if S1 must be executed before S2...

    , alias analysis
    Alias analysis
    Alias analysis is a technique in compiler theory, used to determine if a storage location may be accessed in more than one way. Two pointers are said to be aliased if they point to the same location....

    , pointer analysis
    Pointer analysis
    In computer science pointer analysis, or points-to analysis, is a static code analysis technique that establishes which pointers, or heap references, can point to which variables or storage locations. It is often a component of more complex analyses such as escape analysis...

    , escape analysis
    Escape analysis
    In programming language compiler optimization theory, escape analysis is a method for determining the dynamic scope of pointers. It is related to pointer analysis and shape analysis....

     etc. Accurate analysis is the basis for any compiler optimization. The call graph
    Call graph
    A call graph is a directed graph that represents calling relationships between subroutines in a computer program. Specifically, each node represents a procedure and each edge indicates that procedure f calls procedure g...

     and control flow graph
    Control flow graph
    A control flow graph in computer science is a representation, using graph notation, of all paths that might be traversed through a program during its execution.- Overview :...

     are usually also built during the analysis phase.
  2. Optimization
    Compiler optimization
    Compiler optimization is the process of tuning the output of a compiler to minimize or maximize some attributes of an executable computer program. The most common requirement is to minimize the time taken to execute a program; a less common one is to minimize the amount of memory occupied...

    : the intermediate language representation is transformed into functionally equivalent but faster (or smaller) forms. Popular optimizations are inline expansion
    Inline expansion
    In computing, inline expansion, or inlining, is a manual or compiler optimization that replaces a function call site with the body of the callee. This optimization may improve time and space usage at runtime, at the possible cost of increasing the final size of the program In computing, inline...

    , dead code elimination
    Dead code elimination
    In compiler theory, dead code elimination is a compiler optimization to remove code which does not affect the program results. Removing such code has two benefits: it shrinks program size, an important...

    , constant propagation, loop transformation, register allocation
    Register allocation
    In compiler optimization, register allocation is the process of assigning a large number of target program variables onto a small number of CPU registers...

     and even automatic parallelization
    Automatic parallelization
    Automatic parallelization, also auto parallelization, autoparallelization, or parallelization, the last one of which implies automation when used in context, refers to converting sequential code into multi-threaded or vectorized code in order to utilize multiple processors simultaneously in a...

    .
  3. Code generation: the transformed intermediate language is translated into the output language, usually the native machine language of the system. This involves resource and storage decisions, such as deciding which variables to fit into registers and memory and the selection and scheduling of appropriate machine instructions along with their associated addressing modes (see also Sethi-Ullman algorithm
    Sethi-Ullman algorithm
    In computer science, the Sethi–Ullman algorithm is an algorithm named after Ravi Sethi and Jeffrey D. Ullman, its inventors, for translating abstract syntax trees into machine code that uses as few instructions as possible.-Overview:...

    ). Debug data may also need to be generated to facilitate debugging
    Debugging
    Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...

    .


Compiler analysis is the prerequisite for any compiler optimization, and they tightly work together. For example, dependence analysis
Dependence analysis
In compiler theory, dependence analysis produces execution-order constraints between statements/instructions. Broadly speaking, a statement S2 depends on S1 if S1 must be executed before S2...

 is crucial for loop transformation.

In addition, the scope of compiler analysis and optimizations vary greatly, from as small as a basic block
Basic block
In computing, a basic block is a portion of the code within a program with certain desirable properties that make it highly amenable to analysis. Compilers usually decompose programs into their basic blocks as a first step in the analysis process...

 to the procedure/function level, or even over the whole program (interprocedural optimization
Interprocedural optimization
Interprocedural optimization is a collection of compiler techniques used in computer programming to improve performance in programs containing many frequently used functions of small or medium length...

). 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
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...

, IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

, SGI
Silicon Graphics
Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark...

, Intel, Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

, and Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...

. The open source GCC
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...

 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
Open64
Open64 is an open source, optimizing compiler for the Itanium and x86-64 microprocessor architectures. It derives from the SGI compilers for the MIPS R10000 processor, called MIPSPro. It was initially released in 2000 as GNU GPL software under the name Pro64. The following year, University of...

, 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.

Compiler correctness



Compiler correctness
Compiler correctness
In computing, compiler correctness is the branch of software engineering that deals with trying to show that a compiler behaves according to its language specification...

 is the branch of software engineering that deals with trying to show that a compiler behaves according to its language specification
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

. Techniques include developing the compiler using formal methods
Formal methods
In computer science and software engineering, formal methods are a particular kind of mathematically-based techniques for the specification, development and verification of software and hardware systems...

 and using rigorous testing (often called compiler validation) on an existing compiler.

Related techniques


Assembly language
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

 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
Disassembler
A disassembler is a computer program that translates machine language into assembly language—the inverse operation to that of an assembler. A disassembler differs from a decompiler, which targets a high-level language rather than an assembly language...

.

A program that translates from a low level language to a higher level one is a decompiler
Decompiler
A decompiler is the name given to a computer program that performs, as far as possible, the reverse operation to that of a compiler. That is, it translates a file containing information at a relatively low level of abstraction into a form having a higher level of abstraction...

.

A program that translates between high-level languages is usually called a language translator, source to source translator, language converter, or language rewriter
Rewriting
In mathematics, computer science, and logic, rewriting covers a wide range of methods of replacing subterms of a formula with other terms. What is considered are rewriting systems...

. The last term is usually applied to translations that do not involve a change of language.

International conferences and organizations


Every year, the European Joint Conferences on Theory and Practice of Software (ETAPS) sponsors the International Conference on Compiler Construction (CC), with papers from both the academic and industrial sectors.

See also


  • Abstract interpretation
    Abstract interpretation
    In computer science, abstract interpretation is a theory of sound approximation of the semantics of computer programs, based on monotonic functions over ordered sets, especially lattices. It can be viewed as a partial execution of a computer program which gains information about its semantics In...

  • Attribute grammar
    Attribute grammar
    An attribute grammar is a formal way to define attributes for the productions of a formal grammar, associating these attributes to values. The evaluation occurs in the nodes of the abstract syntax tree, when the language is processed by some parser or compiler....

  • Binary recompiler
    Binary recompiler
    Binary recompiler is a software that takes executable binaries as input, analyzes the structure, applies transformations and optimizations, and outputs new optimized executable binaries. First introduced by Gary Kildall in the 1980s....

  • Bottom-up parsing
    Bottom-up parsing
    Bottom-up parsing is a strategy for analyzing unknown information that attempts to identify the most fundamental units first, and then to infer higher-order structures from them...

  • Byzantine fault tolerance
    Byzantine fault tolerance
    Byzantine fault tolerance is a sub-field of fault tolerance research inspired by the Byzantine Generals' Problem, which is a generalized version of the Two Generals' Problem....

  • Compile farm
    Compile farm
    A compile farm is a server farm, a collection of one or more servers, which has been set up to compile computer programs remotely for various reasons...

  • Compiler-compiler
    Compiler-compiler
    A compiler-compiler or compiler generator is a tool that creates a parser, interpreter, or compiler from some form of formal description of a language and machine...

     (or Parser generator)
  • Compiler correctness
    Compiler correctness
    In computing, compiler correctness is the branch of software engineering that deals with trying to show that a compiler behaves according to its language specification...

  • Decompiler
    Decompiler
    A decompiler is the name given to a computer program that performs, as far as possible, the reverse operation to that of a compiler. That is, it translates a file containing information at a relatively low level of abstraction into a form having a higher level of abstraction...

  • History of compiler writing
    History of compiler writing
    In computing, a compiler is a computer program that transforms source code written in a programming language or computer language , into another computer language...

  • Just-in-time compilation
    Just-in-time compilation
    In computing, just-in-time compilation , also known as dynamic translation, is a method to improve the runtime performance of computer programs. Historically, computer programs had two modes of runtime operation, either interpreted or static compilation...

  • Linker
  • List of compilers
  • List of important publications in computer science#Compilers
  • Metacompilation
  • Semantics encoding
    Semantics encoding
    A semantics encoding is a translation between formal languages. For programmers, the most familiar form of encoding is the compilation of a programming language into machine code or byte-code. Conversion between document formats are also forms of encoding. Compilation of TeX or LaTeX documents to...

  • Transcompiler
    Transcompiler
    A transcompiler is a special compiler that translates the source code of a programming language into the source code of another programming language, e.g...


External links