Compiler: Difference between revisions

{{Short description|Computer program which translates code from one programming language to another}}

{{About|software to translate computer languages|the manga|Compiler (manga)}}

{{Redirect2|Compile|Compiling|the software company|Compile (company)|other uses|Compilation (disambiguation){{!}}Compilation}}

{{Use dmy dates|date=October 2020}}

In [[computing]], a '''compiler''' is a [[computer program]] that [[Translator (computing)|translates]] computer code written in one [[programming language]] (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that translate [[source code]] from a [[high-level programming language]] to a [[lower level language|low-level programming language]] (e.g. [[assembly language]], [[object code]], or [[machine code]]) to create an [[executable]] program.<ref>{{cite web |author= |date= |title=Encyclopedia: Definition of Compiler |url=https://www.pcmag.com/encyclopedia/term/compiler |access-date=2 July 2022 |work=PCMag.com}}</ref><ref name="dragon">[[Compilers: Principles, Techniques, and Tools]] by Alfred V. Aho, Ravi Sethi, Jeffrey D. Ullman - Second Edition, 2007</ref>{{rp|p1}}<ref name="SUDARSANAM MALIK FUJITA 2002 pp. 506–515">{{cite book | last1=Sudarsanam | first1=Ashok | last2=Malik | first2=Sharad | last3=Fujita | first3=Masahiro | title=Readings in Hardware/Software Co-Design | chapter=A Retargetable Compilation Methodology for Embedded Digital Signal Processors Using a Machine-Dependent Code Optimization Library | publisher=Elsevier | date=2002 | doi=10.1016/b978-155860702-6/50045-4 | pages=506–515 | isbn=9781558607026 | quote=A compiler is a computer program that translates a program written in a high-level language (HLL), such as C, into an equivalent assembly language program [2]. }}</ref>

There are many different types of compilers which produce output in different useful forms. A ''[[cross-compiler]]''  produces code for a different [[Central processing unit|CPU]] or [[operating system]] than the one on which the cross-compiler itself runs.  A ''[[bootstrap compiler]]'' is often a temporary compiler, used for compiling a more permanent or better optimised compiler for a language.

Related software include ''[[decompiler]]s'', programs that translate from low-level languages to higher level ones; programs that translate between high-level languages, usually called ''[[source-to-source compiler]]s'' or ''transpilers''; language ''[[rewriting|rewriter]]s'', usually programs that translate the form of [[Expression (computer science)|expressions]] without a change of language; and ''[[compiler-compiler]]s'', 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: [[Preprocessor|preprocessing]], [[lexical analysis]], [[parser|parsing]], [[Semantic analysis (compilers)|semantic analysis]] ([[syntax-directed translation]]), conversion of input programs to an [[intermediate representation]], [[code optimization]] and [[code generation (compiler)|machine specific code generation]]. Compilers generally implement these phases as modular components, promoting efficient design and correctness of [[program transformation|transformation]]s of source input to target output. Program faults caused by incorrect compiler behavior can be very difficult to track down and work around; therefore, compiler implementers invest significant effort to ensure [[compiler correctness]].<ref name="Sun2016">{{cite book |last1=Sun|first1=Chengnian|last2=Le|first2=Vu|last3=Zhang|first3=Qirun|last4=Su|first4=Zhendong|title=Proceedings of the 25th International Symposium on Software Testing and Analysis |chapter=Toward understanding compiler bugs in GCC and LLVM |date=2016|chapter-url=http://dl.acm.org/citation.cfm?doid=2931037.2931074|publisher=ACM|series=ISSTA 2016|pages=294–305|doi=10.1145/2931037.2931074|isbn=9781450343909|s2cid=8339241}}</ref>

With respect to making source code runnable, an [[interpreter (computing)|interpreter]] provides a similar function as a compiler, but via a different mechanism. An interpreter executes code without converting it to machine code.<ref name="dragon"/>{{rp|p2}} Some interpreters execute source code while others execute an intermediate form such as [[bytecode]].

A program compiled to native code tends to run faster than if interpreted. Environments with a bytecode intermediate form tend toward intermediate speed. [[Just-in-time compilation]] allows for native execution speed with a one-time startup processing time cost.

[[Low-level programming language]]s, such as [[assembly language|assembly]] and [[C (programming language)|C]], are typically compiled, especially when speed is a significant concern, rather than [[cross-platform]] support. For such languages, there are more one-to-one correspondences between the source code and the resulting [[machine code]], making it easier for programmers to control the use of hardware.

In theory, a programming language can be used via either a compiler or an interpreter, but in practice, each language tends to be used with only one or the other. Nonetheless, it is possible to write a compiler for a language that is commonly interpreted. For example, [[Common Lisp]] can be compiled to Java bytecode (then interpreted by the [[Java virtual machine]]), C code (then compiled to native machine code), or directly to native code.

{{Main|History of compiler construction}}

* ''May 1952'': [[Grace Hopper]]'s team at [[Remington Rand]] wrote the compiler for the [[A-0 System|A-0]] programming language (and coined the term ''compiler'' to describe it),<ref>{{cite book |last1=Hopper |first1=Grace Murray |title=Proceedings of the 1952 ACM national meeting (Pittsburgh) on - ACM '52 |chapter=The education of a computer |date=1952 |pages=243–249 |doi=10.1145/609784.609818 |s2cid=10081016|doi-access=free }}</ref><ref>{{cite book |last1=Ridgway |first1=Richard K. |title=Proceedings of the 1952 ACM national meeting (Toronto) on - ACM '52 |chapter=Compiling routines |date=1952 |pages=1–5 |doi=10.1145/800259.808980 |s2cid=14878552|doi-access=free }}</ref><ref>{{cite web | title=List of early compilers and assemblers | url=http://shape-of-code.coding-guidelines.com/2017/05/21/evidence-for-28-possible-compilers-in-1957}}</ref> although the A-0 compiler functioned more as a loader or [[Linker (computing)|linker]] than the modern notion of a full compiler.<ref>{{ cite conference |last=Hopper|first=Grace|title=Keynote Address|doi=10.1145/800025.1198341 |book-title=Proceedings of the ACM SIGPLAN History of Programming Languages (HOPL) conference, June 1978 | url=https://dl.acm.org/doi/pdf/10.1145/800025.1198341|url-access=subscription}}</ref><ref>{{ cite web |last=Bruderer|first=Herbert|title=Did Grace Hopper Create the First Compiler? |date=21 December 2022 | url=https://cacm.acm.org/blogs/blog-cacm/268001-did-grace-hopper-create-the-first-compiler/fulltext}}</ref><ref>{{cite journal |last1=Strawn |first1=George |last2=Strawn |first2=Candace |title=Grace Hopper: Compilers and Cobol | url = https://www.computer.org/csdl/magazine/it/2015/01/mit2015010062/13rRUxCitFF |journal=IT Professional |date=2015 |volume=17 |issue=Jan.-Feb. 2015 |pages=62–64 |doi=10.1109/MITP.2015.6 |url-access=subscription }}</ref>

* ''1952, before September'': An [[Autocode]] compiler developed by [[Alick Glennie]] for the [[Manchester Mark I]] computer at the University of Manchester is considered by some to be the first compiled programming language.<ref>Knuth, Donald E.; Pardo, Luis Trabb, "Early development of programming languages", Encyclopedia of Computer Science and Technology (Marcel Dekker) 7: 419–493</ref>

=== One-pass vis-à-vis multi-pass compilers{{anchor|Single-pass}} ===

[[File:Xxx Scanner and parser example for C.gif|thumb|right|400px|[[Lexical analysis|Lexer]] and [[Parsing|parser]] example for [[C (programming language)|C]]. Starting from the sequence of characters "<code>if(net>0.0)total+=net*(1.0+tax/100.0);</code>", the scanner composes a sequence of [[Lexical analysis#token|tokens]], and categorizes each of them, for example as {{color|#600000|identifier}}, {{color|#606000|reserved word}}, {{color|#006000|number literal}}, or {{color|#000060|operator}}. The latter sequence is transformed by the parser into a [[abstract syntax tree|syntax tree]], which is then treated by the remaining compiler phases. The scanner and parser handles the [[regular grammar|regular]] and properly [[context-free grammar|context-free]] parts of the [[C syntax|grammar for C]], respectively.]]

Furthermore, for optimization compilers can contain interpreter functionality, and interpreters may include ahead of time compilation techniques. For example, where an expression can be executed during compilation and the results inserted into the output program, then it prevents it having to be recalculated each time the program runs, which can greatly speed up the final program. Modern trends toward [[just-in-time compilation]] and [[bytecode|bytecode interpretation]] at times blur the traditional categorizations of compilers and interpreters even further.

** An example of hardware compiler is XST, the Xilinx Synthesis Tool used for configuring FPGAs.<ref>{{cite web|author=Xilinx Staff|date=2009|title=XST Synthesis Overview|publisher=Xilinx, Inc.|url=http://www.xilinx.com/support/documentation/sw_manuals/xilinx11/ise_c_using_xst_for_synthesis.htm|access-date=28 February 2017|url-status=live|archive-url=https://web.archive.org/web/20161102004019/http://www.xilinx.com/support/documentation/sw_manuals/xilinx11/ise_c_using_xst_for_synthesis.htm|archive-date=2 November 2016}}</ref>{{primary source inline|date=March 2017}} Similar tools are available from Altera,<ref>{{cite web|author=Altera Staff|date=2017|title=Spectra-Q™ Engine|publisher=Altera.com|url=https://www.altera.com/products/design-software/fpga-design/quartus-prime/features/spectra-q.html|access-date=28 February 2017|url-status=dead|archive-url=https://web.archive.org/web/20161010221724/https://www.altera.com/products/design-software/fpga-design/quartus-prime/features/spectra-q.html|archive-date=10 October 2016}}</ref>{{primary source inline|date=March 2017}} Synplicity, Synopsys and other hardware vendors.{{citation needed|date=March 2017}}