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Clang

From Wikipedia, the free encyclopedia
This article is about the compiler. For other uses, seeClang (disambiguation).
Not to be confused withC (programming language).

Clang
Original author(s)Chris Lattner
Developer(s)LLVM Developer Group
Initial releaseSeptember 26, 2007;17 years ago(2007-09-26)[1]
Stable release
19.1.3[2]Edit this on Wikidata / 30 October 2024;0 days ago(30 October 2024)
Repository
Written inC++
Operating systemUnix-like
PlatformAArch64,ARMv7,IA-32,x86-64,ppc64le[3]
TypeCompiler front end
LicenseApache 2.0with LLVM Exceptions[4][5]
Websiteclang.llvm.org

Clang(/ˈkleɪŋ/)[6]is acompiler front endfor theprogramming languagesC,C++,Objective-C,Objective-C++,and thesoftware frameworksOpenMP,[7]OpenCL,RenderScript,CUDA,SYCL,andHIP.[8]It acts as adrop-in replacementfor theGNU Compiler Collection(GCC), supporting most of itscompiling flagsand unofficial language extensions.[9][10]It includes astatic analyzer,and several code analysis tools.[11]

Clang operates in tandem with theLLVMcompiler back endand has been a subproject of LLVM 2.6 and later.[12]As with LLVM, it isfree and open-source softwareunder theApache 2.0software license.[4][5]Its contributors includeApple,Microsoft,Google,ARM,Sony,Intel,andAMD.

Clang 17, the latest major version of Clang as of October 2023, has full support for all published C++ standards up toC++17,implements most features ofC++20,and has initial support for the upcomingC++23standard.[13]Since v16.0.0, Clang compiles C++ using the GNU++17 dialect by default, which includes features from theC++17standard and conforming GNU extensions.[14]

Background

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In 2005,Apple Inc.made extensive use of LLVM in several commercial products,[15]including theiOS SDKandXcode3.1. AnOpenGLcode compiler forOS Xthat converts OpenGL calls into more fundamental calls forgraphics processing units(GPU) that do not support certain features, was one of the first uses of LLVM. This enabled Apple to support OpenGL on computers usingIntel GMAchipsets, increasing performance on those machines.[16]

The LLVM project originally intended to useGCC's front end. The GCC source code, however, is large and somewhat cumbersome; as one long-time GCC developer put it referring to LLVM, "Trying to make the hippo dance is not really a lot of fun".[17]Besides, Apple software usesObjective-C,which is a low priority for GCC developers. As such, GCC does not integrate smoothly into Apple'sintegrated development environment(IDE).[18]Finally, GCC's license agreement, theGNU General Public License (GPL) version 3,requires developers who distribute extensions or modified versions of GCC to make theirsource codeavailable, but LLVM'spermissive software licensedoesn't require this.[4][5]

For these reasons, Apple developed Clang, a new compiler front end which supports C, Objective-C and C++.[18]In July 2007, the project received the approval for becoming open-source.[19]

Design

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Clang works in tandem with LLVM.[20]The combination of Clang and LLVM provides most of thetoolchainfor replacing the GCCstack.One of Clang's main goals is to provide a library-based architecture,[21]so that the compiler could interoperate with other tools that interact with source code, such asintegrated development environments(IDE). In contrast, GCC works in a compile-link-debugworkflow; integrating it with other tools is not always easy. For instance, GCC uses a step calledfoldthat is key to the overall compile process, which has the side effect of translating the code tree into a form that looks unlike the original source code. If an error is found during or after the fold step, it can be difficult to translate that back into one location in the original source. Besides, vendors using the GCC stack within IDEs must use separate tools to index the code, to provide features likesyntax highlightingandintelligent code completion.

Clang retains more information during the compiling process than GCC, and preserves the overall form of the original code, making it easier to map errors back into the original source. Clang's error reports are more detailed, specific, and machine-readable, so IDEs can index the compiler's output. Modular design of the compiler can offer source code inde xing, syntax checking, and other features normally associated withrapid application developmentsystems. Theparse treeis also more suitable for supporting automatedcode refactoring,as it directly represents the original source code.

Clang compiles only C-like languages, such as C, C++, Objective-C, and Objective-C++. In many cases, Clang can replace GCC as needed, with no other effects on the toolchain as a whole.[citation needed]It supports most of the commonly used GCC options. A Fortran project, Flang was in-progress in 2022. However, for other languages, such asAda,LLVM remains dependent on GCC or another compiler front end.

Flang - Fortran

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TheFlangproject byNvidiaandThe Portland GroupaddsFortransupport.[22]Flang is LLVM's Fortran frontend. It is often referred to as "LLVM Flang" to differentiate itself from "Classic Flang" – these are two separate and independent Fortran compilers. "LLVM Flang" is under active development. Development versions of Flang were in progress as of October 2023and could be downloaded from the LLVM Project.[23]

Performance and GCC compatibility

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Clang compilinghtop

Clang is compatible with GCC.[10]Itscommand-line interfaceshares many of GCC'sflagsand options. Clang implements many GNU language extensions andcompiler intrinsics,some of which are purely for compatibility. For example, even though Clang implements atomic intrinsics which correspond exactly with C11atomics,it also implements GCC's__sync_*intrinsics for compatibility with GCC and theC++ Standard Library(libstdc++). Clang also maintainsapplication binary interface(ABI) compatibility with GCC-generatedobject code.In practice, Clang is a drop-in replacement for GCC.[24]

Clang's developers aim to reducememory footprintand increase compiling speed compared to other compilers, such as GCC. In October 2007, they report that Clang compiled theCarbonlibraries more than twice as fast as GCC, while using about one-sixth GCC's memory and disk space.[25]By 2011, Clang seemed to retain this advantage in compiler performance.[26][27]As of mid-2014, Clang still consistently compiles faster than GCC in a mixed compile time and program performance benchmark.[28]However, by 2019, Clang is significantly slower at compiling the Linux Kernel than GCC while remaining slightly faster at compiling LLVM.[29]

While Clang has historically been faster than GCC at compiling, the output quality has lagged behind. As of 2014, performance of Clang-compiled programs lagged behind performance of the GCC-compiled program, sometimes by large factors (up to 5.5x),[28]replicating earlier reports of slower performance.[26]Both compilers have evolved to increase their performance since then, with the gap narrowing:

  • Comparisons in November 2016 between GCC 4.8.2 versus clang 3.4, on a large harness of test files shows that GCC outperforms clang by approximately 17% on well-optimized source code. Test results are code-specific, and unoptimized C source code can reverse such differences. The two compilers thus seem broadly comparable.[30][unreliable source]
  • Comparisons in 2019 onIntel Ice Lakehas shown that programs generated by Clang 10 has achieved 96% of the performance of GCC 10 over 41 different benchmarks (while winning 22 and losing 19 out of them).[29]
  • In 2023, another comparison conducted four years later revealed that programs compiled using Clang now match the performance of those compiled with GCC. On average, Clang 16 surpasses GCC 13 by 6%.[31]

Interface

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libclangprovides a C interface, providing a relatively small API. Exposed functionality includes: parsing source code into anAST,loading ASTs, traversing the AST, associating source locations with elements within the AST.

Status history

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This table presents only significant steps and releases in Clang history.

Date Highlights
11 July 2007 Clang front-end released under open-source licence
25 February 2009 Clang/LLVM can compile a workingFreeBSDkernel.[32][33]
16 March 2009 Clang/LLVM can compile a workingDragonFly BSDkernel.[34][35]
23 October 2009 Clang 1.0 released, with LLVM 2.6 for the first time.
December 2009 Code generation for C and Objective-C reach production quality. Support for C++ and Objective-C++ still incomplete. Clang C++ can parse GCC 4.2libstdc++and generate working code for non-trivial programs,[20]and can compile itself.[36]
2 February 2010 Clangself-hosting.[37]
20 May 2010 Clang latest version built theBoost C++ librariessuccessfully, and passed nearly all tests.[38]
10 June 2010 Clang/LLVM becomes integral part ofFreeBSD,but default compiler is still GCC.[39]
25 October 2010 Clang/LLVM can compile a working modifiedLinux kernel.[40]
January 2011 Preliminary work completed to support the draftC++0xstandard, with a few of the draft's new features supported in Clang development version.[41][13]
10 February 2011 Clang can compile a workingHotSpotJava virtual machine.[26]
19 January 2012 Clang becomes an optional component inNetBSDcross-platform build system, but GCC is still default.[42]
29 February 2012 Clang 3.0 can rebuild 91.2% of theDebianarchive.[43]
29 February 2012 Clang becomes default compiler inMINIX 3[44]
12 May 2012 Clang/LLVM announced to replace GCC inFreeBSD.[45]
5 November 2012 Clang becomes default compiler inFreeBSD10.x on amd64/i386.[46]
18 February 2013 Clang/LLVM can compile a working modifiedAndroidLinux KernelforNexus 7.[47][48]
19 April 2013 Clang isC++11feature complete.[49]
6 November 2013 Clang isC++14feature complete.[50]
11 September 2014 Clang 3.5 can rebuild 94.3% of the Debian archive. The percentage of failures has dropped by 1.2% per release since January 2013, mainly due to increased compatibility with GCC flags.[51]
October 2016 Clang becomes default compiler forAndroid[52](and later only compiler supported byAndroid NDK[53]).
13 March 2017 Clang 4.0.0released
26 July 2017 Clang becomes default compiler inOpenBSD6.2 on amd64/i386.[54]
7 September 2017 Clang 5.0.0released
19 January 2018 Clang becomes default compiler inOpenBSD6.3 on arm.[55]
5 March 2018 Clang is now used to buildGoogle Chromefor Windows.[56]
8 March 2018 Clang 6.0.0released
5 September 2018 Clang is now used to buildFirefoxfor Windows.[57]
19 September 2018 Clang 7.0.0released
20 March 2019 Clang 8.0.0released
1 July 2019 Clang becomes default compiler inOpenBSD6.6 on mips64.[58]
19 September 2019 Clang 9.0.0releasedwith officialRISC-Vtarget support.[59]
29 February 2020 Clang becomes the only C compiler in theFreeBSDbase system, with the removal ofGCC.[60]
24 March 2020 Clang 10.0.0released
2 April 2020 Clang becomes default compiler inOpenBSD6.7 on powerpc.[61]
12 October 2020 Clang 11.0.0released
21 December 2020 Clang becomes default compiler inOpenBSD6.9 on mips64el.[62]
14 April 2021 Clang 12.0.0released
4 October 2021 Clang 13.0.0released
25 March 2022 Clang 14.0.0released
6 September 2022 Clang 15.0.0released
17 March 2023 Clang 16.0.0released
9 September 2023 Clang 17.0.1released
8 March 2024 Clang 18.1.1released
17 September 2024 Clang 19.1.0released

See also

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References

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  2. ^"LLVM 19.1.3".October 30, 2024.RetrievedOctober 30,2024.
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