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7 <title>Comparing clang to other open source compilers</title>
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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>
Chris Lattner356afa52009-05-01 01:42:13 +0000[diff] [blame]53 <li>GCC front-ends are very mature and already support C++.
54 <a href="cxx_status.html">clang's support for C++</a> is nowhere near
55 what GCC supports.</li>
Cedric Venet3d658642009-02-14 20:20:19 +0000[diff] [blame]56 <li>GCC supports more targets than LLVM.</li>
57 <li>GCC is popular and widely adopted.</li>
58 <li>GCC does not require a C++ compiler to build it.</li>
59 </ul>
60
61 <p>Pro's of clang vs GCC:</p>
62
63 <ul>
64 <li>The Clang ASTs and design are intended to be <a
65 href="features.html#simplecode">easily understandable</a> by
66 anyone who is familiar with the languages involved and who has a basic
67 understanding of how a compiler works. GCC has a very old codebase
68 which presents a steep learning curve to new developers.</li>
69 <li>Clang is designed as an API from its inception, allowing it to be reused
70 by source analysis tools, refactoring, IDEs (etc) as well as for code
71 generation. GCC is built as a monolithic static compiler, which makes
72 it extremely difficult to use as an API and integrate into other tools.
73 Further, its historic design and <a
74 href="http://gcc.gnu.org/ml/gcc/2007-11/msg00460.html">current</a>
75 <a href="http://gcc.gnu.org/ml/gcc/2004-12/msg00888.html">policy</a>
76 makes it difficult to decouple the front-end from the rest of the
77 compiler. </li>
78 <li>Various GCC design decisions make it very difficult to reuse: its build
79 system is difficult to modify, you can't link multiple targets into one
80 binary, you can't link multiple front-ends into one binary, it uses a
81 custom garbage collector, uses global variables extensively, is not
82 reentrant or multi-threadable, etc. Clang has none of these problems.
83 </li>
84 <li>For every token, clang tracks information about where it was written and
85 where it was ultimately expanded into if it was involved in a macro.
86 GCC does not track information about macro instantiations when parsing
87 source code. This makes it very difficult for source rewriting tools
88 (e.g. for refactoring) to work in the presence of (even simple)
89 macros.</li>
90 <li>Clang does not implicitly simplify code as it parses it like GCC does.
91 Doing so causes many problems for source analysis tools: as one simple
92 example, if you write "x-x" in your source code, the GCC AST will
93 contain "0", with no mention of 'x'. This is extremely bad for a
94 refactoring tool that wants to rename 'x'.</li>
95 <li>Clang can serialize its AST out to disk and read it back into another
96 program, which is useful for whole program analysis. GCC does not have
97 this. GCC's PCH mechanism (which is just a dump of the compiler
98 memory image) is related, but is architecturally only
99 able to read the dump back into the exact same executable as the one
100 that produced it (it is not a structured format).</li>
101 <li>Clang is <a href="features.html#performance">much faster and uses far
102 less memory</a> than GCC.</li>
103 <li>Clang aims to provide extremely clear and concise diagnostics (error and
104 warning messages), and includes support for <a
Chris Lattner7eee9f72009-03-19 18:57:33 +0000[diff] [blame]105 href="diagnostics.html">expressive diagnostics</a>. GCC's warnings are
106 sometimes acceptable, but are often confusing and it does not support
Cedric Venet3d658642009-02-14 20:20:19 +0000[diff] [blame]107 expressive diagnostics. Clang also preserves typedefs in diagnostics
Chris Lattner7eee9f72009-03-19 18:57:33 +0000[diff] [blame]108 consistently, showing macro expansions and many other features.</li>
Cedric Venet3d658642009-02-14 20:20:19 +0000[diff] [blame]109 <li>GCC is licensed under the GPL license. clang uses a BSD license, which
110 allows it to be used by projects that do not themselves want to be
111 GPL.</li>
112 <li>Clang inherits a number of features from its use of LLVM as a backend,
113 including support for a bytecode representation for intermediate code,
114 pluggable optimizers, link-time optimization support, Just-In-Time
115 compilation, ability to link in multiple code generators, etc.</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 support for C++ is far beyond what clang provides. If you need
126 C++ support in the next year, Elsa is a great way to get it. That said,
127 Elsa is missing important support for templates and other pieces: for
128 example, it is not capable of compiling the GCC STL headers from any
129 version newer than GCC 3.4.</li>
130 <li>Elsa's parser and AST is designed to be easily extensible by adding
131 grammar rules. Clang has a very simple and easily hackable parser,
132 but requires you to write C++ code to do it.</li>
133 </ul>
134
135 <p>Pro's of clang vs Elsa:</p>
136
137 <ul>
138 <li>The Elsa community is extremely small and major development work seems
Chris Lattnerb76c2322009-06-26 04:10:17 +0000[diff] [blame^]139 to have ceased in 2005. Work continued to be used by other small
140 projects (e.g. Oink), but Oink is apparently dead now too. Clang has a
141 vibrant community including developers that
Cedric Venet3d658642009-02-14 20:20:19 +0000[diff] [blame]142 are paid to work on it full time. In practice this means that you can
143 file bugs against Clang and they will often be fixed for you. If you
144 use Elsa, you are (mostly) on your own for bug fixes and feature
145 enhancements.</li>
146 <li>Elsa is not built as a stack of reusable libraries like clang is. It is
147 very difficult to use part of Elsa without the whole front-end. For
148 example, you cannot use Elsa to parse C/ObjC code without building an
149 AST. You can do this in Clang and it is much faster than building an
150 AST.</li>
151 <li>Elsa does not have an integrated preprocessor, which makes it extremely
152 difficult to accurately map from a source location in the AST back to
153 its original position before preprocessing. Like GCC, it does not keep
154 track of macro expansions.</li>
155 <li>Elsa is even slower and uses more memory than GCC, which itself requires
156 far more space and time than clang.</li>
157 <li>Elsa only does partial semantic analysis. It is intended to work on
158 code that is already validated by GCC, so it does not do many semantic
159 checks required by the languages it implements.</li>
160 <li>Elsa does not support Objective-C.</li>
161 <li>Elsa does not support native code generation.</li>
162 </ul>
163
164 <p>Note that there is a fork of Elsa known as "Pork". It addresses some of
165 these shortcomings by loosely integrating a preprocessor. This allows it
166 to map from a source location in the AST to the original position before
167 preprocessing, providing it better support for static analysis and
168 refactoring. Note that Pork is in stasis now too.</p>
169
170
171 <!--=====================================================================-->
172 <h2><a name="pcc">Clang vs PCC (Portable C Compiler)</a></h2>
173 <!--=====================================================================-->
174
175 <p>Pro's of PCC vs clang:</p>
176
177 <ul>
178 <li>The PCC source base is very small and builds quickly with just a C
179 compiler.</li>
180 </ul>
181
182 <p>Pro's of clang vs PCC:</p>
183
184 <ul>
185 <li>PCC dates from the 1970's and has been dormant for most of that time.
186 The clang + llvm communities are very active.</li>
187 <li>PCC doesn't support C99, Objective-C, and doesn't aim to support
188 C++.</li>
189 <li>PCC's code generation is very limited compared to LLVM. It produces very
190 inefficient code and does not support many important targets.</li>
191 <li>Like Elsa, PCC's does not have an integrated preprocessor, making it
192 extremely difficult to use it for source analysis tools.</li>
193 </div>
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