1f4a2713aSLionel Sambuc<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2f4a2713aSLionel Sambuc "http://www.w3.org/TR/html4/strict.dtd"> 3f4a2713aSLionel Sambuc<!-- Material used from: HTML 4.01 specs: http://www.w3.org/TR/html401/ --> 4f4a2713aSLionel Sambuc<html> 5f4a2713aSLionel Sambuc<head> 6f4a2713aSLionel Sambuc <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 7f4a2713aSLionel Sambuc <title>Comparing clang to other open source compilers</title> 8f4a2713aSLionel Sambuc <link type="text/css" rel="stylesheet" href="menu.css"> 9f4a2713aSLionel Sambuc <link type="text/css" rel="stylesheet" href="content.css"> 10f4a2713aSLionel Sambuc</head> 11f4a2713aSLionel Sambuc<body> 12f4a2713aSLionel Sambuc <!--#include virtual="menu.html.incl"--> 13f4a2713aSLionel Sambuc <div id="content"> 14f4a2713aSLionel Sambuc <h1>Clang vs Other Open Source Compilers</h1> 15f4a2713aSLionel Sambuc 16f4a2713aSLionel Sambuc <p>Building an entirely new compiler front-end is a big task, and it isn't 17f4a2713aSLionel Sambuc always clear to people why we decided to do this. Here we compare clang 18f4a2713aSLionel Sambuc and its goals to other open source compiler front-ends that are 19f4a2713aSLionel Sambuc available. We restrict the discussion to very specific objective points 20f4a2713aSLionel Sambuc to avoid controversy where possible. Also, software is infinitely 21f4a2713aSLionel Sambuc mutable, so we don't talk about little details that can be fixed with 22f4a2713aSLionel Sambuc a reasonable amount of effort: we'll talk about issues that are 23f4a2713aSLionel Sambuc difficult to fix for architectural or political reasons.</p> 24f4a2713aSLionel Sambuc 25f4a2713aSLionel Sambuc <p>The goal of this list is to describe how differences in goals lead to 26f4a2713aSLionel Sambuc different strengths and weaknesses, not to make some compiler look bad. 27f4a2713aSLionel Sambuc This will hopefully help you to evaluate whether using clang is a good 28f4a2713aSLionel Sambuc idea for your personal goals. Because we don't know specifically what 29f4a2713aSLionel Sambuc <em>you</em> want to do, we describe the features of these compilers in 30f4a2713aSLionel Sambuc terms of <em>our</em> goals: if you are only interested in static 31f4a2713aSLionel Sambuc analysis, you may not care that something lacks codegen support, for 32f4a2713aSLionel Sambuc example.</p> 33f4a2713aSLionel Sambuc 34*0a6a1f1dSLionel Sambuc <p>Please email <a href="get_involved.html">cfe-dev</a> if you think we should add another compiler to this 35f4a2713aSLionel Sambuc list or if you think some characterization is unfair here.</p> 36f4a2713aSLionel Sambuc 37f4a2713aSLionel Sambuc <ul> 38f4a2713aSLionel Sambuc <li><a href="#gcc">Clang vs GCC</a> (GNU Compiler Collection)</li> 39f4a2713aSLionel Sambuc <li><a href="#elsa">Clang vs Elsa</a> (Elkhound-based C++ Parser)</li> 40f4a2713aSLionel Sambuc <li><a href="#pcc">Clang vs PCC</a> (Portable C Compiler)</li> 41f4a2713aSLionel Sambuc </ul> 42f4a2713aSLionel Sambuc 43f4a2713aSLionel Sambuc 44f4a2713aSLionel Sambuc <!--=====================================================================--> 45f4a2713aSLionel Sambuc <h2><a name="gcc">Clang vs GCC (GNU Compiler Collection)</a></h2> 46f4a2713aSLionel Sambuc <!--=====================================================================--> 47f4a2713aSLionel Sambuc 48f4a2713aSLionel Sambuc <p>Pro's of GCC vs clang:</p> 49f4a2713aSLionel Sambuc 50f4a2713aSLionel Sambuc <ul> 51f4a2713aSLionel Sambuc <li>GCC supports languages that clang does not aim to, such as Java, Ada, 52*0a6a1f1dSLionel Sambuc FORTRAN, Go, etc.</li> 53f4a2713aSLionel Sambuc <li>GCC supports more targets than LLVM.</li> 54*0a6a1f1dSLionel Sambuc <li>GCC supports many language extensions, some of which are not implemented 55*0a6a1f1dSLionel Sambuc by Clang. For instance, in C mode, GCC supports 56*0a6a1f1dSLionel Sambuc <a href="http://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html">nested 57*0a6a1f1dSLionel Sambuc functions</a> and has an 58*0a6a1f1dSLionel Sambuc <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37428">undocumented 59*0a6a1f1dSLionel Sambuc extension allowing VLAs in structs</a>. 60f4a2713aSLionel Sambuc </ul> 61f4a2713aSLionel Sambuc 62f4a2713aSLionel Sambuc <p>Pro's of clang vs GCC:</p> 63f4a2713aSLionel Sambuc 64f4a2713aSLionel Sambuc <ul> 65f4a2713aSLionel Sambuc <li>The Clang ASTs and design are intended to be <a 66f4a2713aSLionel Sambuc href="features.html#simplecode">easily understandable</a> by 67f4a2713aSLionel Sambuc anyone who is familiar with the languages involved and who has a basic 68f4a2713aSLionel Sambuc understanding of how a compiler works. GCC has a very old codebase 69f4a2713aSLionel Sambuc which presents a steep learning curve to new developers.</li> 70f4a2713aSLionel Sambuc <li>Clang is designed as an API from its inception, allowing it to be reused 71f4a2713aSLionel Sambuc by source analysis tools, refactoring, IDEs (etc) as well as for code 72f4a2713aSLionel Sambuc generation. GCC is built as a monolithic static compiler, which makes 73f4a2713aSLionel Sambuc it extremely difficult to use as an API and integrate into other tools. 74f4a2713aSLionel Sambuc Further, its historic design and <a 75f4a2713aSLionel Sambuc href="http://gcc.gnu.org/ml/gcc/2007-11/msg00460.html">current</a> 76f4a2713aSLionel Sambuc <a href="http://gcc.gnu.org/ml/gcc/2004-12/msg00888.html">policy</a> 77f4a2713aSLionel Sambuc makes it difficult to decouple the front-end from the rest of the 78f4a2713aSLionel Sambuc compiler. </li> 79f4a2713aSLionel Sambuc <li>Various GCC design decisions make it very difficult to reuse: its build 80f4a2713aSLionel Sambuc system is difficult to modify, you can't link multiple targets into one 81f4a2713aSLionel Sambuc binary, you can't link multiple front-ends into one binary, it uses a 82f4a2713aSLionel Sambuc custom garbage collector, uses global variables extensively, is not 83f4a2713aSLionel Sambuc reentrant or multi-threadable, etc. Clang has none of these problems. 84f4a2713aSLionel Sambuc </li> 85f4a2713aSLionel Sambuc <li>Clang does not implicitly simplify code as it parses it like GCC does. 86f4a2713aSLionel Sambuc Doing so causes many problems for source analysis tools: as one simple 87f4a2713aSLionel Sambuc example, if you write "x-x" in your source code, the GCC AST will 88f4a2713aSLionel Sambuc contain "0", with no mention of 'x'. This is extremely bad for a 89f4a2713aSLionel Sambuc refactoring tool that wants to rename 'x'.</li> 90f4a2713aSLionel Sambuc <li>Clang can serialize its AST out to disk and read it back into another 91f4a2713aSLionel Sambuc program, which is useful for whole program analysis. GCC does not have 92f4a2713aSLionel Sambuc this. GCC's PCH mechanism (which is just a dump of the compiler 93f4a2713aSLionel Sambuc memory image) is related, but is architecturally only 94f4a2713aSLionel Sambuc able to read the dump back into the exact same executable as the one 95f4a2713aSLionel Sambuc that produced it (it is not a structured format).</li> 96f4a2713aSLionel Sambuc <li>Clang is <a href="features.html#performance">much faster and uses far 97f4a2713aSLionel Sambuc less memory</a> than GCC.</li> 98f4a2713aSLionel Sambuc <li>Clang aims to provide extremely clear and concise diagnostics (error and 99f4a2713aSLionel Sambuc warning messages), and includes support for <a 100f4a2713aSLionel Sambuc href="diagnostics.html">expressive diagnostics</a>. GCC's warnings are 101f4a2713aSLionel Sambuc sometimes acceptable, but are often confusing and it does not support 102f4a2713aSLionel Sambuc expressive diagnostics. Clang also preserves typedefs in diagnostics 103f4a2713aSLionel Sambuc consistently, showing macro expansions and many other features.</li> 104f4a2713aSLionel Sambuc <li>GCC is licensed under the GPL license. <a href="features.html#license"> 105f4a2713aSLionel Sambuc clang uses a BSD license,</a> which allows it to be embedded in 106f4a2713aSLionel Sambuc software that is not GPL-licensed.</li> 107f4a2713aSLionel Sambuc <li>Clang inherits a number of features from its use of LLVM as a backend, 108f4a2713aSLionel Sambuc including support for a bytecode representation for intermediate code, 109f4a2713aSLionel Sambuc pluggable optimizers, link-time optimization support, Just-In-Time 110f4a2713aSLionel Sambuc compilation, ability to link in multiple code generators, etc.</li> 111*0a6a1f1dSLionel Sambuc <li><a href="compatibility.html#cxx">Clang's support for C++</a> is more 112*0a6a1f1dSLionel Sambuc compliant than GCC's in many ways.</li> 113*0a6a1f1dSLionel Sambuc <li>Clang supports 114*0a6a1f1dSLionel Sambuc <a href="http://clang.llvm.org/docs/LanguageExtensions.html">many language 115*0a6a1f1dSLionel Sambuc extensions</a>, some of which are not implemented by GCC. For instance, 116*0a6a1f1dSLionel Sambuc Clang provides attributes for checking thread safety and extended vector 117*0a6a1f1dSLionel Sambuc types.</li> 118f4a2713aSLionel Sambuc </ul> 119f4a2713aSLionel Sambuc 120f4a2713aSLionel Sambuc <!--=====================================================================--> 121f4a2713aSLionel Sambuc <h2><a name="elsa">Clang vs Elsa (Elkhound-based C++ Parser)</a></h2> 122f4a2713aSLionel Sambuc <!--=====================================================================--> 123f4a2713aSLionel Sambuc 124f4a2713aSLionel Sambuc <p>Pro's of Elsa vs clang:</p> 125f4a2713aSLionel Sambuc 126f4a2713aSLionel Sambuc <ul> 127f4a2713aSLionel Sambuc <li>Elsa's parser and AST is designed to be easily extensible by adding 128f4a2713aSLionel Sambuc grammar rules. Clang has a very simple and easily hackable parser, 129f4a2713aSLionel Sambuc but requires you to write C++ code to do it.</li> 130f4a2713aSLionel Sambuc </ul> 131f4a2713aSLionel Sambuc 132f4a2713aSLionel Sambuc <p>Pro's of clang vs Elsa:</p> 133f4a2713aSLionel Sambuc 134f4a2713aSLionel Sambuc <ul> 135f4a2713aSLionel Sambuc <li>Clang's C and C++ support is far more mature and practically useful than 136f4a2713aSLionel Sambuc Elsa's, and includes many C++'11 features.</li> 137f4a2713aSLionel Sambuc <li>The Elsa community is extremely small and major development work seems 138f4a2713aSLionel Sambuc to have ceased in 2005. Work continued to be used by other small 139f4a2713aSLionel Sambuc projects (e.g. Oink), but Oink is apparently dead now too. Clang has a 140f4a2713aSLionel Sambuc vibrant community including developers that 141f4a2713aSLionel Sambuc are paid to work on it full time. In practice this means that you can 142f4a2713aSLionel Sambuc file bugs against Clang and they will often be fixed for you. If you 143f4a2713aSLionel Sambuc use Elsa, you are (mostly) on your own for bug fixes and feature 144f4a2713aSLionel Sambuc enhancements.</li> 145f4a2713aSLionel Sambuc <li>Elsa is not built as a stack of reusable libraries like clang is. It is 146f4a2713aSLionel Sambuc very difficult to use part of Elsa without the whole front-end. For 147f4a2713aSLionel Sambuc example, you cannot use Elsa to parse C/ObjC code without building an 148f4a2713aSLionel Sambuc AST. You can do this in Clang and it is much faster than building an 149f4a2713aSLionel Sambuc AST.</li> 150f4a2713aSLionel Sambuc <li>Elsa does not have an integrated preprocessor, which makes it extremely 151f4a2713aSLionel Sambuc difficult to accurately map from a source location in the AST back to 152f4a2713aSLionel Sambuc its original position before preprocessing. Like GCC, it does not keep 153f4a2713aSLionel Sambuc track of macro expansions.</li> 154f4a2713aSLionel Sambuc <li>Elsa is even slower and uses more memory than GCC, which itself requires 155f4a2713aSLionel Sambuc far more space and time than clang.</li> 156f4a2713aSLionel Sambuc <li>Elsa only does partial semantic analysis. It is intended to work on 157f4a2713aSLionel Sambuc code that is already validated by GCC, so it does not do many semantic 158f4a2713aSLionel Sambuc checks required by the languages it implements.</li> 159f4a2713aSLionel Sambuc <li>Elsa does not support Objective-C.</li> 160f4a2713aSLionel Sambuc <li>Elsa does not support native code generation.</li> 161f4a2713aSLionel Sambuc </ul> 162f4a2713aSLionel Sambuc 163f4a2713aSLionel Sambuc 164f4a2713aSLionel Sambuc <!--=====================================================================--> 165f4a2713aSLionel Sambuc <h2><a name="pcc">Clang vs PCC (Portable C Compiler)</a></h2> 166f4a2713aSLionel Sambuc <!--=====================================================================--> 167f4a2713aSLionel Sambuc 168f4a2713aSLionel Sambuc <p>Pro's of PCC vs clang:</p> 169f4a2713aSLionel Sambuc 170f4a2713aSLionel Sambuc <ul> 171f4a2713aSLionel Sambuc <li>The PCC source base is very small and builds quickly with just a C 172f4a2713aSLionel Sambuc compiler.</li> 173f4a2713aSLionel Sambuc </ul> 174f4a2713aSLionel Sambuc 175f4a2713aSLionel Sambuc <p>Pro's of clang vs PCC:</p> 176f4a2713aSLionel Sambuc 177f4a2713aSLionel Sambuc <ul> 178f4a2713aSLionel Sambuc <li>PCC dates from the 1970's and has been dormant for most of that time. 179f4a2713aSLionel Sambuc The clang + llvm communities are very active.</li> 180f4a2713aSLionel Sambuc <li>PCC doesn't support Objective-C or C++ and doesn't aim to support 181f4a2713aSLionel Sambuc C++.</li> 182f4a2713aSLionel Sambuc <li>PCC's code generation is very limited compared to LLVM. It produces very 183f4a2713aSLionel Sambuc inefficient code and does not support many important targets.</li> 184f4a2713aSLionel Sambuc <li>Like Elsa, PCC's does not have an integrated preprocessor, making it 185f4a2713aSLionel Sambuc extremely difficult to use it for source analysis tools.</li> 186f4a2713aSLionel Sambuc </ul> 187f4a2713aSLionel Sambuc </div> 188f4a2713aSLionel Sambuc</body> 189f4a2713aSLionel Sambuc</html> 190