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<META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
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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>
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<p>Building an entirely new compiler front-end is a big task, and it isn't
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always clear to people why we decided to do this. Here we compare clang
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and its goals to other open source compiler front-ends that are
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available. We restrict the discussion to very specific objective points
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to avoid controversy where possible. Also, software is infinitely
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mutable, so we don't talk about little details that can be fixed with
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a reasonable amount of effort: we'll talk about issues that are
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difficult to fix for architectural or political reasons.</p>
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<p>The goal of this list is to describe how differences in goals lead to
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different strengths and weaknesses, not to make some compiler look bad.
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This will hopefully help you to evaluate whether using clang is a good
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idea for your personal goals. Because we don't know specifically what
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<em>you</em> want to do, we describe the features of these compilers in
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terms of <em>our</em> goals: if you are only interested in static
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analysis, you may not care that something lacks codegen support, for
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example.</p>
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<p>Please email cfe-dev if you think we should add another compiler to this
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list or if you think some characterization is unfair here.</p>
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<ul>
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<li><a href="#gcc">Clang vs GCC</a> (GNU Compiler Collection)</li>
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<li><a href="#elsa">Clang vs Elsa</a> (Elkhound-based C++ Parser)</li>
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<li><a href="#pcc">Clang vs PCC</a> (Portable C Compiler)</li>
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</ul>
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<!--=====================================================================-->
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<h2><a name="gcc">Clang vs GCC (GNU Compiler Collection)</a></h2>
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<!--=====================================================================-->
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<p>Pro's of GCC vs clang:</p>
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<ul>
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<li>GCC supports languages that clang does not aim to, such as Java, Ada,
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FORTRAN, etc.</li>
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<li>GCC front-ends are very mature and already support C/C++/ObjC and all
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the variants we are interested in. <a href="cxx_status.html">clang's
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support for C++</a> in particular is nowhere near what GCC supports.</li>
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<li>GCC's codegen is much more mature than clang's right now and supports
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more targets than LLVM.</li>
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<li>GCC is popular and widely adopted.</li>
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<li>GCC does not require a C++ compiler to build it.</li>
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</ul>
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<p>Pro's of clang vs GCC:</p>
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<ul>
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<li>The Clang ASTs and design are intended to be <a
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href="features.html#simplecode">easily understandable</a> by
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anyone who is familiar with the languages involved and who has a basic
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understanding of how a compiler works. GCC has a very old codebase
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which presents a steep learning curve to new developers.</li>
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<li>Clang is designed as an API from its inception, allowing it to be reused
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by source analysis tools, refactoring, IDEs (etc) as well as for code
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generation. GCC is built as a monolithic static compiler, which makes
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it extremely difficult to use as an API and integrate into other tools.
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Further, its historic design and <a
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href="http://gcc.gnu.org/ml/gcc/2007-11/msg00460.html">current</a>
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<a href="http://gcc.gnu.org/ml/gcc/2004-12/msg00888.html">policy</a>
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makes it difficult to decouple the front-end from the rest of the
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compiler. </li>
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<li>Various GCC design decisions make it very difficult to reuse: its build
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system is difficult to modify, you can't link multiple targets into one
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binary, you can't link multiple front-ends into one binary, it uses a
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custom garbage collector, uses global variables extensively, is not
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reentrant or multi-threadable, etc. Clang has none of these problems.
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</li>
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<li>For every token, clang tracks information about where it was written and
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where it was ultimately expanded into if it was involved in a macro.
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GCC does not track information about macro instantiations when parsing
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source code. This makes it very difficult for source rewriting tools
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(e.g. for refactoring) to work in the presence of (even simple)
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macros.</li>
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<li>Clang does not implicitly simplify code as it parses it like GCC does.
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Doing so causes many problems for source analysis tools: as one simple
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example, if you write "x-x" in your source code, the GCC AST will
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contain "0", with no mention of 'x'. This is extremely bad for a
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refactoring tool that wants to rename 'x'.</li>
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<li>Clang can serialize its AST out to disk and read it back into another
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program, which is useful for whole program analysis. GCC does not have
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this. GCC's PCH mechanism (which is just a dump of the compiler
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memory image) is related, but is architecturally only
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able to read the dump back into the exact same executable as the one
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that produced it (it is not a structured format).</li>
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<li>Clang is <a href="features.html#performance">much faster and uses far
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less memory</a> than GCC.</li>
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<li>Clang aims to provide extremely clear and concise diagnostics (error and
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warning messages), and includes support for <a
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href="features.html#expressivediags">expressive diagnostics</a>. GCC's
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warnings are acceptable, but are often confusing and it does not support
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expressive diagnostics. Clang also preserves typedefs in diagnostics
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consistently.</li>
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<li>GCC is licensed under the GPL license. clang uses a BSD license, which
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allows it to be used by projects that do not themselves want to be
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GPL.</li>
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<li>Clang inherits a number of features from its use of LLVM as a backend,
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including support for a bytecode representation for intermediate code,
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pluggable optimizers, link-time optimization support, Just-In-Time
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compilation, ability to link in multiple code generators, etc.</li>
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</ul>
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<!--=====================================================================-->
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<h2><a name="elsa">Clang vs Elsa (Elkhound-based C++ Parser)</a></h2>
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<!--=====================================================================-->
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<p>Pro's of Elsa vs clang:</p>
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<ul>
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<li>Elsa's support for C++ is far beyond what clang provides. If you need
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C++ support in the next year, Elsa is a great way to get it. That said,
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Elsa is missing important support for templates and other pieces: for
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example, it is not capable of compiling the GCC STL headers from any
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version newer than GCC 3.4.</li>
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<li>Elsa's parser and AST is designed to be easily extensible by adding
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grammar rules. Clang has a very simple and easily hackable parser,
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but requires you to write C++ code to do it.</li>
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</ul>
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<p>Pro's of clang vs Elsa:</p>
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<ul>
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<li>The Elsa community is extremely small and major development work seems
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to have ceased in 2005, though it continues to be used by other small
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projects
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(e.g. Oink). Clang has a vibrant community including developers that
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are paid to work on it full time. In practice this means that you can
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file bugs against Clang and they will often be fixed for you. If you
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use Elsa, you are (mostly) on your own for bug fixes and feature
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enhancements.</li>
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<li>Elsa is not built as a stack of reusable libraries like clang is. It is
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very difficult to use part of Elsa without the whole front-end. For
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example, you cannot use Elsa to parse C/ObjC code without building an
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AST. You can do this in Clang and it is much faster than building an
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AST.</li>
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<li>Elsa does not have an integrated preprocessor, which makes it extremely
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difficult to accurately map from a source location in the AST back to
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its original position before preprocessing. Like GCC, it does not keep
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track of macro expansions.</li>
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<li>Elsa is even slower and uses more memory than GCC, which itself requires
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far more space and time than clang.</li>
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<li>Elsa only does partial semantic analysis. It is intended to work on
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code that is already validated by GCC, so it does not do many semantic
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checks required by the languages it implements.</li>
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<li>Elsa does not support Objective-C.</li>
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<li>Elsa does not support native code generation.</li>
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</ul>
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<p>Note that there is a fork of Elsa known as "Pork". It addresses some of
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these shortcomings by loosely integrating a preprocessor. This allows it
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to map from a source location in the AST to the original position before
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preprocessing, providing it better support for static analysis and
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refactoring. Note that Pork is in stasis now too.</p>
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<!--=====================================================================-->
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<h2><a name="pcc">Clang vs PCC (Portable C Compiler)</a></h2>
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<!--=====================================================================-->
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<p>Pro's of PCC vs clang:</p>
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<ul>
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<li>The PCC source base is very small and builds quickly with just a C
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compiler.</li>
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</ul>
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<p>Pro's of clang vs PCC:</p>
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<ul>
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<li>PCC dates from the 1970's and has been dormant for most of that time.
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The clang + llvm communities are very active.</li>
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<li>PCC doesn't support C99, Objective-C, and doesn't aim to support
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C++.</li>
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<li>PCC's code generation is very limited compared to LLVM. It produces very
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inefficient code and does not support many important targets.</li>
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<li>Like Elsa, PCC's does not have an integrated preprocessor, making it
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extremely difficult to use it for source analysis tools.</li>
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</div>
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