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 cfe-dev 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, etc.</li> 53 <li>GCC supports more targets than LLVM.</li> 54 <li>GCC is popular and widely adopted.</li> 55 <li>GCC does not require a C++ compiler to build it.</li> 56 </ul> 57 58 <p>Pro's of clang vs GCC:</p> 59 60 <ul> 61 <li>The Clang ASTs and design are intended to be <a 62 href="features.html#simplecode">easily understandable</a> by 63 anyone who is familiar with the languages involved and who has a basic 64 understanding of how a compiler works. GCC has a very old codebase 65 which presents a steep learning curve to new developers.</li> 66 <li>Clang is designed as an API from its inception, allowing it to be reused 67 by source analysis tools, refactoring, IDEs (etc) as well as for code 68 generation. GCC is built as a monolithic static compiler, which makes 69 it extremely difficult to use as an API and integrate into other tools. 70 Further, its historic design and <a 71 href="http://gcc.gnu.org/ml/gcc/2007-11/msg00460.html">current</a> 72 <a href="http://gcc.gnu.org/ml/gcc/2004-12/msg00888.html">policy</a> 73 makes it difficult to decouple the front-end from the rest of the 74 compiler. </li> 75 <li>Various GCC design decisions make it very difficult to reuse: its build 76 system is difficult to modify, you can't link multiple targets into one 77 binary, you can't link multiple front-ends into one binary, it uses a 78 custom garbage collector, uses global variables extensively, is not 79 reentrant or multi-threadable, etc. Clang has none of these problems. 80 </li> 81 <li>For every token, clang tracks information about where it was written and 82 where it was ultimately expanded into if it was involved in a macro. 83 GCC does not track information about macro instantiations when parsing 84 source code. This makes it very difficult for source rewriting tools 85 (e.g. for refactoring) to work in the presence of (even simple) 86 macros.</li> 87 <li>Clang does not implicitly simplify code as it parses it like GCC does. 88 Doing so causes many problems for source analysis tools: as one simple 89 example, if you write "x-x" in your source code, the GCC AST will 90 contain "0", with no mention of 'x'. This is extremely bad for a 91 refactoring tool that wants to rename 'x'.</li> 92 <li>Clang can serialize its AST out to disk and read it back into another 93 program, which is useful for whole program analysis. GCC does not have 94 this. GCC's PCH mechanism (which is just a dump of the compiler 95 memory image) is related, but is architecturally only 96 able to read the dump back into the exact same executable as the one 97 that produced it (it is not a structured format).</li> 98 <li>Clang is <a href="features.html#performance">much faster and uses far 99 less memory</a> than GCC.</li> 100 <li>Clang aims to provide extremely clear and concise diagnostics (error and 101 warning messages), and includes support for <a 102 href="diagnostics.html">expressive diagnostics</a>. GCC's warnings are 103 sometimes acceptable, but are often confusing and it does not support 104 expressive diagnostics. Clang also preserves typedefs in diagnostics 105 consistently, showing macro expansions and many other features.</li> 106 <li>GCC is licensed under the GPL license. <a href="features.html#license"> 107 clang uses a BSD license,</a> which allows it to be embedded in 108 software that is not GPL-licensed.</li> 109 <li>Clang inherits a number of features from its use of LLVM as a backend, 110 including support for a bytecode representation for intermediate code, 111 pluggable optimizers, link-time optimization support, Just-In-Time 112 compilation, ability to link in multiple code generators, etc.</li> 113 <li><a href="compatibility.html#c++">Clang's support for C++</a> is more 114 compliant than GCC's in many ways (e.g. conformant two phase name 115 lookup).</li> 116 </ul> 117 118 <!--=====================================================================--> 119 <h2><a name="elsa">Clang vs Elsa (Elkhound-based C++ Parser)</a></h2> 120 <!--=====================================================================--> 121 122 <p>Pro's of Elsa vs clang:</p> 123 124 <ul> 125 <li>Elsa's parser and AST is designed to be easily extensible by adding 126 grammar rules. Clang has a very simple and easily hackable parser, 127 but requires you to write C++ code to do it.</li> 128 </ul> 129 130 <p>Pro's of clang vs Elsa:</p> 131 132 <ul> 133 <li>Clang's C and C++ support is far more mature and practically useful than 134 Elsa's, and includes many C++'11 features.</li> 135 <li>The Elsa community is extremely small and major development work seems 136 to have ceased in 2005. Work continued to be used by other small 137 projects (e.g. Oink), but Oink is apparently dead now too. Clang has a 138 vibrant community including developers that 139 are paid to work on it full time. In practice this means that you can 140 file bugs against Clang and they will often be fixed for you. If you 141 use Elsa, you are (mostly) on your own for bug fixes and feature 142 enhancements.</li> 143 <li>Elsa is not built as a stack of reusable libraries like clang is. It is 144 very difficult to use part of Elsa without the whole front-end. For 145 example, you cannot use Elsa to parse C/ObjC code without building an 146 AST. You can do this in Clang and it is much faster than building an 147 AST.</li> 148 <li>Elsa does not have an integrated preprocessor, which makes it extremely 149 difficult to accurately map from a source location in the AST back to 150 its original position before preprocessing. Like GCC, it does not keep 151 track of macro expansions.</li> 152 <li>Elsa is even slower and uses more memory than GCC, which itself requires 153 far more space and time than clang.</li> 154 <li>Elsa only does partial semantic analysis. It is intended to work on 155 code that is already validated by GCC, so it does not do many semantic 156 checks required by the languages it implements.</li> 157 <li>Elsa does not support Objective-C.</li> 158 <li>Elsa does not support native code generation.</li> 159 </ul> 160 161 162 <!--=====================================================================--> 163 <h2><a name="pcc">Clang vs PCC (Portable C Compiler)</a></h2> 164 <!--=====================================================================--> 165 166 <p>Pro's of PCC vs clang:</p> 167 168 <ul> 169 <li>The PCC source base is very small and builds quickly with just a C 170 compiler.</li> 171 </ul> 172 173 <p>Pro's of clang vs PCC:</p> 174 175 <ul> 176 <li>PCC dates from the 1970's and has been dormant for most of that time. 177 The clang + llvm communities are very active.</li> 178 <li>PCC doesn't support Objective-C or C++ and doesn't aim to support 179 C++.</li> 180 <li>PCC's code generation is very limited compared to LLVM. It produces very 181 inefficient code and does not support many important targets.</li> 182 <li>Like Elsa, PCC's does not have an integrated preprocessor, making it 183 extremely difficult to use it for source analysis tools.</li> 184 </ul> 185 </div> 186 </body> 187 </html> 188
