|Original author(s)||Chris Lattner and others|
|Developer(s)||Apple Inc. and others|
|Initial release||September 26, 2007|
5.0.0 / 7 September 2017
|License||University of Illinois/NCSA Open Source License|
Clang // is a compiler front end for the programming languages C, C++, Objective-C, Objective-C++, OpenMP, OpenCL, and CUDA. It uses LLVM as its back end and has been part of the LLVM release cycle since LLVM 2.6.
It is designed to be able to replace the full GNU Compiler Collection (GCC). Its contributors include Apple, Microsoft, Google, ARM, Sony, Intel and Advanced Micro Devices (AMD). It is open-source software, with source code released under the University of Illinois/NCSA License, a permissive free software licence.
One of the first uses of LLVM was an OpenGL code compiler for OS X that converts OpenGL calls into more fundamental calls for graphics processing units (GPU) that do not support certain features. This allowed Apple to support the entire OpenGL application programming interface (API) on computers using Intel Graphics Media Accelerator (GMA) chipsets, increasing performance on those machines. For GPUs that support it, the code is compiled to exploit fully the underlying hardware, but on GMA machines, LLVM compiles the same OpenGL code into subroutines to ensure continued proper function.
LLVM was intended originally to use GCC's front end, but GCC turned out to cause some problems for developers of LLVM and at Apple. The GCC source code is a large and somewhat cumbersome system for developers to work with; as one long-time GCC developer put it, "Trying to make the hippo dance is not really a lot of fun".
Apple software makes heavy use of Objective-C, but the Objective-C front-end in GCC is a low priority for GCC developers. Also, GCC does not integrate smoothly into Apple's IDE. Finally, GCC is licensed under the GNU General Public License (GPL) version 3, which requires developers who distribute extensions for, or modified versions of, GCC to make their source code available, whereas LLVM has a BSD-like license which does not require users to release their source code changes when publishing compiled binaries of those changes.
Clang is intended to work atop LLVM. The combination of Clang and LLVM provides most of the toolchain, to allow replacing the full GCC stack. Because it is built with a library-based design, like the rest of LLVM, Clang is easy to embed into other applications. This is one reason why most OpenCL implementations are built with Clang and LLVM.
One of Clang's main goals is to provide a library-based architecture, to allow the compiler to be more tightly tied to tools that interact with source code, such as an integrated development environment (IDE) graphical user interface (GUI). In contrast, GCC is designed to work in a classic compile-link-debug cycle, and integrating it with other tools is not always easy. For instance, GCC uses a step called fold that 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. Also, vendors using the GCC stack within IDEs use separate tools to index the code, to provide features like syntax highlighting and autocomplete.
Clang is designed to retain more information during the compiling process than GCC, and to preserve the overall form of the original code. The goal of this is to make it easier to map errors back into the original source. The error reports offered by Clang are also aimed to be more detailed and specific, as well as machine-readable, so IDEs can index the output of the compiler during compiling. Modular design of the compiler can offer source code indexing, syntax checking, and other features normally associated with rapid application development systems. The parse tree is also more suitable for supporting automated code refactoring, as it directly represents the original source code.
Clang is a compiler for only C-like languages, including C, C++, Objective-C, Objective-C++, OpenCL, and CUDA. For other languages, like Ada, LLVM remains dependent on GCC or another compiler frontend. In many cases, Clang can be used or swapped out for GCC as needed, with no other effects on the toolchain as a whole. It supports most of the commonly used GCC options. An unofficial sub-project Flang by Nvidia added Fortran support.
Performance and GCC compatibility
Clang is designed to be highly compatible with GCC. Clang's command-line interface is similar to and shares many flags and options with GCC. Clang implements many GNU language extensions and enables them by default. Clang implements many GCC compiler intrinsics purely for compatibility. For example, even though Clang implements atomic intrinsics which correspond exactly with C11 atomics, it also implements GCC's __sync_* intrinsics for compatibility with GCC and libstdc++. Clang also maintains ABI compatibility with GCC-generated object code. In practice Clang can often be used as a drop-in replacement for GCC.
Clang's developers aim to reduce memory footprint and increase compilation speed compared to competing compilers, such as GCC. In October 2007, they report that Clang compiled the Carbon libraries more than twice as fast as GCC, while using about one-sixth GCC's memory and disk space. However, as of 2011 this was not a typical result. As of mid-2014, Clang won more than a third of the benchmarks, with GCC winning most. As of 2014, performance of Clang-compiled programs lagged behind performance of the GCC-compiled program, sometimes by large factors (up to 5.5x),, replicating earlier reports of slower performance.
More recent comparisons indicate that both compilers have evolved to increase their performance. As of GCC 4.8.1 versus clang 3.3, on a large harness of test files, clang had improved significantly, outperforming GCC by ~ 25%. Test results are code-specific, and optimized c source-code can reverse such differences. The two compilers now seem broadly comparable. 
This table presents only significant steps and releases in Clang history.
|11 July 2007||Clang frontend released under open-source license|
|25 February 2009||Clang/LLVM can compile a working FreeBSD kernel.|
|16 March 2009||Clang/LLVM can compile a working DragonFly BSD kernel.|
|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 4.2 libstdc++ and generate working code for non-trivial programs, and can compile itself.|
|2 February 2010||Clang self-hosting.|
|20 February 2010||HelenOS source code is modified to compile with Clang successfully, and passes all kernel and user space regression tests on IA-32 processors.|
|20 May 2010||Clang latest version built the Boost C++ libraries successfully, and passed nearly all tests.|
|10 June 2010||Clang/LLVM becomes integral part of FreeBSD, but default compiler is still GCC.|
|25 October 2010||Clang/LLVM can compile a working modified Linux kernel.|
|January 2011||Preliminary work completed to support the draft C++0x standard, with a few of the draft's new features supported in Clang development version.|
|10 February 2011||Clang can compile a working HotSpot Java virtual machine.|
|19 January 2012||Clang becomes an optional component in NetBSD cross-platform build system, but GCC is still default.|
|29 February 2012||Clang 3.0 can rebuild 91.2% of the Debian archive.|
|29 February 2012||Clang becomes default compiler in MINIX 3|
|12 May 2012||Clang/LLVM announced to replace GCC in FreeBSD.|
|5 November 2012||Clang becomes default compiler in FreeBSD 10.x.|
|18 February 2013||Clang/LLVM can compile a working modified Android Linux Kernel for Nexus 7.|
|19 April 2013||Clang is C++11 feature complete.|
|6 November 2013||Clang is C++14 feature complete.|
|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.|
|13 March 2017||Clang 4.0.0 Released|
|26 July 2017||Clang becomes default compiler in OpenBSD 6.2.|
|7 September 2017||Clang 5.0.0 Released|
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While the overall GCC compatibility is excellent and the compile times are impressive, the performance of the generated code is still lacking behind a recent GCC version.
- "Benchmarking LLVM & Clang Against GCC 4.5". Phoronix. 21 April 2010. Retrieved 13 February 2011.
Binaries from LLVM-GCC and Clang both struggled to compete with GCC 4.5.0 in the timed HMMer benchmark of a Pfam database search. LLVM-GCC and Clang were about 23% slower(...)Though LLVM / Clang isn't the performance champion at this point, both components continue to be under very active development and there will hopefully be more news to report in the coming months
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- Du Toit, Stefanus. "Clang is C++11 feature complete as of *just now*!".
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- Ledru, Sylvestre. "Rebuild of Debian using Clang 3.5.0".
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