编译器GCC与Clang的异同

GCC：GNU(Gnu‘s Not Unix)编译器套装(GNU Compiler Collection，GCC)，指一套编程语言编译器，以GPL及LGPL许可证所发行的自由软件，也是GNU项目的关键部分，也是GNU工具链的主要组成部分之一。GCC(特别是其中的C语言编译器)也常被认为是跨平台编译器的事实标准。1985年由理查德·马修·斯托曼开始发展，现在由自由软件基金会负责维护工作。GCC原本用C开发，后来因为LLVM、Clang的崛起，它更快地将开发语言转换为C++。

GCC支持的语言：原名为GNU C语言编译器(GNU C Compiler)，因为它原本只能处理C语言。GCC在发布后很快地得到扩展，变得可处理C++。之后也变得可处理Fortran、Pascal、Objective-C、Java、Ada，Go与其他语言。

许多操作系统，包括许多类Unix系统，如Linux及BSD家族都采用GCC作为标准编译器。苹果电脑预装的Mac OS X操作系统也采用这个编译器。

GCC目前由世界各地不同的数个程序员小组维护。它是移植到最多中央处理器架构以及最多操作系统的编译器。由于GCC已成为GNU系统的官方编译器(包括GNU/Linux家族)，它也成为编译与创建其他操作系统的主要编译器，包括BSD家族、Mac OS X、NeXTSTEP与BeOS。

GCC通常是跨平台软件的编译器首选。有别于一般局限于特定系统与运行环境的编译器，GCC在所有平台上都使用同一个前端处理程序，产生一样的中介码，因此此中介码在各个其他平台上使用GCC编译，有很大的机会可得到正确无误的输出程序。

GCC支持的主要处理器架构：ARM、x86、x86-64、MIPS、PowerPC等。

        GCC结构：GCC的外部接口长得像一个标准的Unix编译器。用户在命令行下键入gcc之程序名，以及一些命令参数，以便决定每个输入文件使用的个别语言编译器，并为输出代码使用适合此硬件平台的汇编语言编译器，并且选择性地运行连接器以制造可执行的程序。每个语言编译器都是独立程序，此程序可处理输入的源代码，并输出汇编语言码。全部的语言编译器都拥有共通的中介架构：一个前端解析匹配此语言的源代码，并产生一抽象语法树，以及一翻译此语法树成为GCC的寄存器转换语言的后端。编译器最优化与静态代码解析技术在此阶段应用于代码上。最后，适用于此硬件架构的汇编语言代码以杰克·戴维森与克里斯·弗雷泽发明的算法产出。

几乎全部的GCC都由C/C++写成，除了Ada前端大部分以Ada写成。

        Clang：是一个C、C++、Objective-C和Objective-C++编程语言的编译器前端。它采用了底层虚拟机(LLVM)作为其后端。它的目标是提供一个GNU编译器套装(GCC)的替代品。作者是克里斯·拉特纳(Chris Lattner)，在苹果公司的赞助支持下进行开发，而源代码授权是使用类BSD的伊利诺伊大学厄巴纳-香槟分校开源码许可。Clang主要由C++编写。

Clang项目包括Clang前端和Clang静态分析器等。这个软件项目在2005年由苹果电脑发起，是LLVM(Low Level Virtual Machine)编译器工具集的前端(front-end)，目的是输出代码对应的抽象语法树(Abstract Syntax Tree, AST)，并将代码编译成LLVM Bitcode。接着在后端(back-end)使用LLVM编译成平台相关的机器语言。

Clang本身性能优异，其生成的AST所耗用掉的内存仅仅是GCC的20%左右。2014年1月发行的FreeBSD10.0版将Clang/LLVM作为默认编译器。

        Clang性能：测试证明Clang编译Objective-C代码时速度为GCC的3倍，还能针对用户发生的编译错误准确地给出建议。

        GCC与Clang区别：

        GCC特性：除支持C/C++/ Objective-C/Objective-C++语言外，还是支持Java/Ada/Fortran/Go等；当前的Clang的C++支持落后于GCC；支持更多平台；更流行，广泛使用，支持完备。

        Clang特性：编译速度快；内存占用小；兼容GCC；设计清晰简单、容易理解，易于扩展增强；基于库的模块化设计，易于IDE集成；出错提示更友好。

        Clang采用的license是BSD，而GCC是GPLv3。

        它们使用的宏不同：

        (1)、GCC定义的宏包括：

__GNUC__
__GNUC_MINOR__
__GNUC_PATCHLEVEL__
__GNUG__

__clang__
__clang_major__
__clang_minor__
__clang_patchlevel__

Pro‘s of GCC vs clang:

(1)、GCC supports languages that clang does not aim to, such as Java, Ada, FORTRAN, Go, etc.

(2)、GCC supports more targets than LLVM.

(3)、GCC supports many language extensions, some of which are not implemented by Clang. For instance, in C mode, GCC supports nested functions and has an extension allowing VLAs in structs.

Pro‘s of clangvs GCC:

(1)、The Clang ASTs and design are intended to be easily understandable by anyone who is familiar with the languages involved and who has a basic understanding of how acompiler works. GCC has a very old codebase which presents a steep learning curve to new developers.

(2)、Clang is designed as an API from its inception, allowing it to be reused by source analysis tools, refactoring, IDEs (etc) as well as for code generation. GCC is built as a monolithic static compiler, which makes it extremely difficult to use as an API and integrate into other tools. Further, its historic design and current policy makes it difficult to decouple the front-end from the rest ofthe compiler.

(3)、Various GCC design decisions make it very difficult to reuse: its build system is difficult to modify, you can‘t link multiple targets into one binary, you can‘t link multiple front-ends into one binary, it uses a custom garbage collector, uses global variables extensively, is not reentrant or multi-threadable, etc. Clang has none of these problems.

(4)、Clang does not implicitly simplify code as it parses it like GCC does. Doing so causes many problems for source analysis tools: as one simple example, if you write"x-x" in your source code, the GCC AST will contain "0",with no mention of ‘x‘. This is extremely bad for a refactoring tool that wants to rename ‘x‘.

(5)、Clang can serialize its AST out to disk and read it back into another program, which is useful for whole program analysis. GCC does not have this. GCC‘s PCH mechanism(which is just a dump of the compiler memory image) is related, but is architecturally only able to read the dump back into the exact same executable as the one that produced it (it is not a structured format).

(6)、Clang is much faster and uses far less memory than GCC.

(7)、Clang has been designed from the start to provide extremely clear and concise diagnostics(error and warning messages), and includes support for expressive diagnostics.Modern versions of GCC have made significant advances in this area,incorporating various Clang features such as preserving typedefs in diagnostics and showing macro expansions, but GCC is still catching up.

(8)、GCC is licensed under the GPL license. clang uses a BSD license, which allows it to be embedded in software that is not GPL-licensed.

(9)、Clang inherits a number of features from its use of LLVM as a backend, including support for a bytecode representation for intermediate code, pluggable optimizers, link-time optimization support, Just-In-Time compilation, ability to link in multiple code generators, etc.

(10)、Clang‘s support for C++ is more compliant than GCC‘s in many ways.

(11)、Clang supports many language extensions, some of which are not implemented by GCC. For instance, Clang provides attributes for checking thread safety and extended vector types.

以上内容主要整理自： GCC维基百科  、  Clang维基百科 、 clang.llvm.org