1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3<!-- Material used from: HTML 4.01 specs: http://www.w3.org/TR/html401/ --> 4<html> 5<head> 6 <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 7 <title>Comparing clang to other open source compilers</title> 8 <link type="text/css" rel="stylesheet" href="menu.css"> 9 <link type="text/css" rel="stylesheet" href="content.css"> 10</head> 11<body> 12 <!--#include virtual="menu.html.incl"--> 13 <div id="content"> 14 <h1>Clang vs Other Open Source Compilers</h1> 15 16 <p>Building an entirely new compiler front-end is a big task, and it isn't 17 always clear to people why we decided to do this. Here we compare clang 18 and its goals to other open source compiler front-ends that are 19 available. We restrict the discussion to very specific objective points 20 to avoid controversy where possible. Also, software is infinitely 21 mutable, so we don't talk about little details that can be fixed with 22 a reasonable amount of effort: we'll talk about issues that are 23 difficult to fix for architectural or political reasons.</p> 24 25 <p>The goal of this list is to describe how differences in goals lead to 26 different strengths and weaknesses, not to make some compiler look bad. 27 This will hopefully help you to evaluate whether using clang is a good 28 idea for your personal goals. Because we don't know specifically what 29 <em>you</em> want to do, we describe the features of these compilers in 30 terms of <em>our</em> goals: if you are only interested in static 31 analysis, you may not care that something lacks codegen support, for 32 example.</p> 33 34 <p>Please email <a href="get_involved.html">cfe-dev</a> if you think we should add another compiler to this 35 list or if you think some characterization is unfair here.</p> 36 37 <ul> 38 <li><a href="#gcc">Clang vs GCC</a> (GNU Compiler Collection)</li> 39 <li><a href="#elsa">Clang vs Elsa</a> (Elkhound-based C++ Parser)</li> 40 <li><a href="#pcc">Clang vs PCC</a> (Portable C Compiler)</li> 41 </ul> 42 43 44 <!--=====================================================================--> 45 <h2><a name="gcc">Clang vs GCC (GNU Compiler Collection)</a></h2> 46 <!--=====================================================================--> 47 48 <p>Pro's of GCC vs clang:</p> 49 50 <ul> 51 <li>GCC supports languages that clang does not aim to, such as Java, Ada, 52 FORTRAN, Go, etc.</li> 53 <li>GCC supports more targets than LLVM.</li> 54 <li>GCC supports many language extensions, some of which are not implemented 55 by Clang. For instance, in C mode, GCC supports 56 <a href="http://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html">nested 57 functions</a> and has an 58 <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37428">undocumented 59 extension allowing VLAs in structs</a>. 60 </ul> 61 62 <p>Pro's of clang vs GCC:</p> 63 64 <ul> 65 <li>The Clang ASTs and design are intended to be <a 66 href="features.html#simplecode">easily understandable</a> by 67 anyone who is familiar with the languages involved and who has a basic 68 understanding of how a compiler works. GCC has a very old codebase 69 which presents a steep learning curve to new developers.</li> 70 <li>Clang is designed as an API from its inception, allowing it to be reused 71 by source analysis tools, refactoring, IDEs (etc) as well as for code 72 generation. GCC is built as a monolithic static compiler, which makes 73 it extremely difficult to use as an API and integrate into other tools. 74 Further, its historic design and <a 75 href="http://gcc.gnu.org/ml/gcc/2007-11/msg00460.html">current</a> 76 <a href="http://gcc.gnu.org/ml/gcc/2004-12/msg00888.html">policy</a> 77 makes it difficult to decouple the front-end from the rest of the 78 compiler. </li> 79 <li>Various GCC design decisions make it very difficult to reuse: its build 80 system is difficult to modify, you can't link multiple targets into one 81 binary, you can't link multiple front-ends into one binary, it uses a 82 custom garbage collector, uses global variables extensively, is not 83 reentrant or multi-threadable, etc. Clang has none of these problems. 84 </li> 85 <li>Clang does not implicitly simplify code as it parses it like GCC does. 86 Doing so causes many problems for source analysis tools: as one simple 87 example, if you write "x-x" in your source code, the GCC AST will 88 contain "0", with no mention of 'x'. This is extremely bad for a 89 refactoring tool that wants to rename 'x'.</li> 90 <li>Clang can serialize its AST out to disk and read it back into another 91 program, which is useful for whole program analysis. GCC does not have 92 this. GCC's PCH mechanism (which is just a dump of the compiler 93 memory image) is related, but is architecturally only 94 able to read the dump back into the exact same executable as the one 95 that produced it (it is not a structured format).</li> 96 <li>Clang is <a href="features.html#performance">much faster and uses far 97 less memory</a> than GCC.</li> 98 <li>Clang aims to provide extremely clear and concise diagnostics (error and 99 warning messages), and includes support for <a 100 href="diagnostics.html">expressive diagnostics</a>. GCC's warnings are 101 sometimes acceptable, but are often confusing and it does not support 102 expressive diagnostics. Clang also preserves typedefs in diagnostics 103 consistently, showing macro expansions and many other features.</li> 104 <li>GCC is licensed under the GPL license. <a href="features.html#license"> 105 clang uses a BSD license,</a> which allows it to be embedded in 106 software that is not GPL-licensed.</li> 107 <li>Clang inherits a number of features from its use of LLVM as a backend, 108 including support for a bytecode representation for intermediate code, 109 pluggable optimizers, link-time optimization support, Just-In-Time 110 compilation, ability to link in multiple code generators, etc.</li> 111 <li><a href="compatibility.html#cxx">Clang's support for C++</a> is more 112 compliant than GCC's in many ways.</li> 113 <li>Clang supports 114 <a href="http://clang.llvm.org/docs/LanguageExtensions.html">many language 115 extensions</a>, some of which are not implemented by GCC. For instance, 116 Clang provides attributes for checking thread safety and extended vector 117 types.</li> 118 </ul> 119 120 <!--=====================================================================--> 121 <h2><a name="elsa">Clang vs Elsa (Elkhound-based C++ Parser)</a></h2> 122 <!--=====================================================================--> 123 124 <p>Pro's of Elsa vs clang:</p> 125 126 <ul> 127 <li>Elsa's parser and AST is designed to be easily extensible by adding 128 grammar rules. Clang has a very simple and easily hackable parser, 129 but requires you to write C++ code to do it.</li> 130 </ul> 131 132 <p>Pro's of clang vs Elsa:</p> 133 134 <ul> 135 <li>Clang's C and C++ support is far more mature and practically useful than 136 Elsa's, and includes many C++'11 features.</li> 137 <li>The Elsa community is extremely small and major development work seems 138 to have ceased in 2005. Work continued to be used by other small 139 projects (e.g. Oink), but Oink is apparently dead now too. Clang has a 140 vibrant community including developers that 141 are paid to work on it full time. In practice this means that you can 142 file bugs against Clang and they will often be fixed for you. If you 143 use Elsa, you are (mostly) on your own for bug fixes and feature 144 enhancements.</li> 145 <li>Elsa is not built as a stack of reusable libraries like clang is. It is 146 very difficult to use part of Elsa without the whole front-end. For 147 example, you cannot use Elsa to parse C/ObjC code without building an 148 AST. You can do this in Clang and it is much faster than building an 149 AST.</li> 150 <li>Elsa does not have an integrated preprocessor, which makes it extremely 151 difficult to accurately map from a source location in the AST back to 152 its original position before preprocessing. Like GCC, it does not keep 153 track of macro expansions.</li> 154 <li>Elsa is even slower and uses more memory than GCC, which itself requires 155 far more space and time than clang.</li> 156 <li>Elsa only does partial semantic analysis. It is intended to work on 157 code that is already validated by GCC, so it does not do many semantic 158 checks required by the languages it implements.</li> 159 <li>Elsa does not support Objective-C.</li> 160 <li>Elsa does not support native code generation.</li> 161 </ul> 162 163 164 <!--=====================================================================--> 165 <h2><a name="pcc">Clang vs PCC (Portable C Compiler)</a></h2> 166 <!--=====================================================================--> 167 168 <p>Pro's of PCC vs clang:</p> 169 170 <ul> 171 <li>The PCC source base is very small and builds quickly with just a C 172 compiler.</li> 173 </ul> 174 175 <p>Pro's of clang vs PCC:</p> 176 177 <ul> 178 <li>PCC dates from the 1970's and has been dormant for most of that time. 179 The clang + llvm communities are very active.</li> 180 <li>PCC doesn't support Objective-C or C++ and doesn't aim to support 181 C++.</li> 182 <li>PCC's code generation is very limited compared to LLVM. It produces very 183 inefficient code and does not support many important targets.</li> 184 <li>Like Elsa, PCC's does not have an integrated preprocessor, making it 185 extremely difficult to use it for source analysis tools.</li> 186 </ul> 187 </div> 188</body> 189</html> 190