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<title>Comparing clang to other open source compilers</title>
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<h1>Clang vs Other Open Source Compilers</h1>
<p>Building an entirely new compiler front-end is a big task, and it isn't
always clear to people why we decided to do this. Here we compare clang
and its goals to other open source compiler front-ends that are
available. We restrict the discussion to very specific objective points
to avoid controversy where possible. Also, software is infinitely
mutable, so we don't talk about little details that can be fixed with
a reasonable amount of effort: we'll talk about issues that are
difficult to fix for architectural or political reasons.</p>
<p>The goal of this list is to describe how differences in goals lead to
different strengths and weaknesses, not to make some compiler look bad.
This will hopefully help you to evaluate whether using clang is a good
idea for your personal goals. Because we don't know specifically what
<em>you</em> want to do, we describe the features of these compilers in
terms of <em>our</em> goals: if you are only interested in static
analysis, you may not care that something lacks codegen support, for
example.</p>
<p>Please email <a href="get_involved.html">cfe-dev</a> if you think we should add another compiler to this
list or if you think some characterization is unfair here.</p>
<ul>
<li><a href="#gcc">Clang vs GCC</a> (GNU Compiler Collection)</li>
<li><a href="#elsa">Clang vs Elsa</a> (Elkhound-based C++ Parser)</li>
<li><a href="#pcc">Clang vs PCC</a> (Portable C Compiler)</li>
</ul>
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<h2><a name="gcc">Clang vs GCC (GNU Compiler Collection)</a></h2>
<!--=====================================================================-->
<p>Pro's of GCC vs clang:</p>
<ul>
<li>GCC supports languages that clang does not aim to, such as Java, Ada,
FORTRAN, Go, etc.</li>
<li>GCC supports more targets than LLVM.</li>
<li>GCC supports many language extensions, some of which are not implemented
by Clang. For instance, in C mode, GCC supports
<a href="http://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html">nested
functions</a> and has an
<a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37428">undocumented
extension allowing VLAs in structs</a>.
</ul>
<p>Pro's of clang vs GCC:</p>
<ul>
<li>The Clang ASTs and design are intended to be <a
href="features.html#simplecode">easily understandable</a> by
anyone who is familiar with the languages involved and who has a basic
understanding of how a compiler works. GCC has a very old codebase
which presents a steep learning curve to new developers.</li>
<li>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 <a
href="http://gcc.gnu.org/ml/gcc/2007-11/msg00460.html">current</a>
<a href="http://gcc.gnu.org/ml/gcc/2004-12/msg00888.html">policy</a>
makes it difficult to decouple the front-end from the rest of the
compiler. </li>
<li>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.
</li>
<li>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'.</li>
<li>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).</li>
<li>Clang is <a href="features.html#performance">much faster and uses far
less memory</a> than GCC.</li>
<li>Clang aims to provide extremely clear and concise diagnostics (error and
warning messages), and includes support for <a
href="diagnostics.html">expressive diagnostics</a>. GCC's warnings are
sometimes acceptable, but are often confusing and it does not support
expressive diagnostics. Clang also preserves typedefs in diagnostics
consistently, showing macro expansions and many other features.</li>
<li>GCC is licensed under the GPL license. <a href="features.html#license">
clang uses a BSD license,</a> which allows it to be embedded in
software that is not GPL-licensed.</li>
<li>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.</li>
<li><a href="compatibility.html#cxx">Clang's support for C++</a> is more
compliant than GCC's in many ways.</li>
<li>Clang supports
<a href="http://clang.llvm.org/docs/LanguageExtensions.html">many language
extensions</a>, some of which are not implemented by GCC. For instance,
Clang provides attributes for checking thread safety and extended vector
types.</li>
</ul>
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<h2><a name="elsa">Clang vs Elsa (Elkhound-based C++ Parser)</a></h2>
<!--=====================================================================-->
<p>Pro's of Elsa vs clang:</p>
<ul>
<li>Elsa's parser and AST is designed to be easily extensible by adding
grammar rules. Clang has a very simple and easily hackable parser,
but requires you to write C++ code to do it.</li>
</ul>
<p>Pro's of clang vs Elsa:</p>
<ul>
<li>Clang's C and C++ support is far more mature and practically useful than
Elsa's, and includes many C++'11 features.</li>
<li>The Elsa community is extremely small and major development work seems
to have ceased in 2005. Work continued to be used by other small
projects (e.g. Oink), but Oink is apparently dead now too. Clang has a
vibrant community including developers that
are paid to work on it full time. In practice this means that you can
file bugs against Clang and they will often be fixed for you. If you
use Elsa, you are (mostly) on your own for bug fixes and feature
enhancements.</li>
<li>Elsa is not built as a stack of reusable libraries like clang is. It is
very difficult to use part of Elsa without the whole front-end. For
example, you cannot use Elsa to parse C/ObjC code without building an
AST. You can do this in Clang and it is much faster than building an
AST.</li>
<li>Elsa does not have an integrated preprocessor, which makes it extremely
difficult to accurately map from a source location in the AST back to
its original position before preprocessing. Like GCC, it does not keep
track of macro expansions.</li>
<li>Elsa is even slower and uses more memory than GCC, which itself requires
far more space and time than clang.</li>
<li>Elsa only does partial semantic analysis. It is intended to work on
code that is already validated by GCC, so it does not do many semantic
checks required by the languages it implements.</li>
<li>Elsa does not support Objective-C.</li>
<li>Elsa does not support native code generation.</li>
</ul>
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<h2><a name="pcc">Clang vs PCC (Portable C Compiler)</a></h2>
<!--=====================================================================-->
<p>Pro's of PCC vs clang:</p>
<ul>
<li>The PCC source base is very small and builds quickly with just a C
compiler.</li>
</ul>
<p>Pro's of clang vs PCC:</p>
<ul>
<li>PCC dates from the 1970's and has been dormant for most of that time.
The clang + llvm communities are very active.</li>
<li>PCC doesn't support Objective-C or C++ and doesn't aim to support
C++.</li>
<li>PCC's code generation is very limited compared to LLVM. It produces very
inefficient code and does not support many important targets.</li>
<li>Like Elsa, PCC's does not have an integrated preprocessor, making it
extremely difficult to use it for source analysis tools.</li>
</ul>
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