external/bsd/llvm/dist/clang/www/comparison.html

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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 <a href="get_involved.html">cfe-dev</a> 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, Go, etc.</li>
  53     <li>GCC supports more targets than LLVM.</li>
  54     <li>GCC supports many language extensions, some of which are not implemented
  55     by Clang. For instance, in C mode, GCC supports
  56     <a href="http://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html">nested
  57     functions</a> and has an
  58     <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37428">undocumented
  59     extension allowing VLAs in structs</a>.
  60     </ul>
  61
  62     <p>Pro's of clang vs GCC:</p>
  63
  64     <ul>
  65     <li>The Clang ASTs and design are intended to be <a
  66         href="features.html#simplecode">easily understandable</a> by
  67         anyone who is familiar with the languages involved and who has a basic
  68         understanding of how a compiler works.  GCC has a very old codebase
  69         which presents a steep learning curve to new developers.</li>
  70     <li>Clang is designed as an API from its inception, allowing it to be reused
  71         by source analysis tools, refactoring, IDEs (etc) as well as for code
  72         generation.  GCC is built as a monolithic static compiler, which makes
  73         it extremely difficult to use as an API and integrate into other tools.
  74         Further, its historic design and <a
  75         href="http://gcc.gnu.org/ml/gcc/2007-11/msg00460.html">current</a>
  76         <a href="http://gcc.gnu.org/ml/gcc/2004-12/msg00888.html">policy</a>
  77         makes it difficult to decouple the front-end from the rest of the
  78         compiler. </li>
  79     <li>Various GCC design decisions make it very difficult to reuse: its build
  80         system is difficult to modify, you can't link multiple targets into one
  81         binary, you can't link multiple front-ends into one binary, it uses a
  82         custom garbage collector, uses global variables extensively, is not
  83         reentrant or multi-threadable, etc.  Clang has none of these problems.
  84         </li>
  85     <li>Clang does not implicitly simplify code as it parses it like GCC does.
  86         Doing so causes many problems for source analysis tools: as one simple
  87         example, if you write "x-x" in your source code, the GCC AST will
  88         contain "0", with no mention of 'x'.  This is extremely bad for a
  89         refactoring tool that wants to rename 'x'.</li>
  90     <li>Clang can serialize its AST out to disk and read it back into another
  91         program, which is useful for whole program analysis.  GCC does not have
  92         this.  GCC's PCH mechanism (which is just a dump of the compiler
  93         memory image) is related, but is architecturally only
  94         able to read the dump back into the exact same executable as the one
  95         that produced it (it is not a structured format).</li>
  96     <li>Clang is <a href="features.html#performance">much faster and uses far
  97         less memory</a> than GCC.</li>
  98     <li>Clang aims to provide extremely clear and concise diagnostics (error and
  99         warning messages), and includes support for <a
 100         href="diagnostics.html">expressive diagnostics</a>.  GCC's warnings are
 101         sometimes acceptable, but are often confusing and it does not support
 102         expressive diagnostics.  Clang also preserves typedefs in diagnostics
 103         consistently, showing macro expansions and many other features.</li>
 104     <li>GCC is licensed under the GPL license. <a href="features.html#license">
 105         clang uses a BSD license,</a> which allows it to be embedded in
 106         software that is not GPL-licensed.</li>
 107     <li>Clang inherits a number of features from its use of LLVM as a backend,
 108         including support for a bytecode representation for intermediate code,
 109         pluggable optimizers, link-time optimization support, Just-In-Time
 110         compilation, ability to link in multiple code generators, etc.</li>
 111     <li><a href="compatibility.html#cxx">Clang's support for C++</a> is more
 112         compliant than GCC's in many ways.</li>
 113     <li>Clang supports
 114         <a href="http://clang.llvm.org/docs/LanguageExtensions.html">many language
 115         extensions</a>, some of which are not implemented by GCC. For instance,
 116         Clang provides attributes for checking thread safety and extended vector
 117         types.</li>
 118     </ul>
 119
 120     <!--=====================================================================-->
 121     <h2><a name="elsa">Clang vs Elsa (Elkhound-based C++ Parser)</a></h2>
 122     <!--=====================================================================-->
 123
 124     <p>Pro's of Elsa vs clang:</p>
 125
 126     <ul>
 127     <li>Elsa's parser and AST is designed to be easily extensible by adding
 128         grammar rules.  Clang has a very simple and easily hackable parser,
 129         but requires you to write C++ code to do it.</li>
 130     </ul>
 131
 132     <p>Pro's of clang vs Elsa:</p>
 133
 134     <ul>
 135     <li>Clang's C and C++ support is far more mature and practically useful than
 136         Elsa's, and includes many C++'11 features.</li>
 137     <li>The Elsa community is extremely small and major development work seems
 138         to have ceased in 2005. Work continued to be used by other small
 139         projects (e.g. Oink), but Oink is apparently dead now too.  Clang has a
 140         vibrant community including developers that
 141         are paid to work on it full time.  In practice this means that you can
 142         file bugs against Clang and they will often be fixed for you.  If you
 143         use Elsa, you are (mostly) on your own for bug fixes and feature
 144         enhancements.</li>
 145     <li>Elsa is not built as a stack of reusable libraries like clang is.  It is
 146         very difficult to use part of Elsa without the whole front-end.  For
 147         example, you cannot use Elsa to parse C/ObjC code without building an
 148         AST.  You can do this in Clang and it is much faster than building an
 149         AST.</li>
 150     <li>Elsa does not have an integrated preprocessor, which makes it extremely
 151         difficult to accurately map from a source location in the AST back to
 152         its original position before preprocessing.  Like GCC, it does not keep
 153         track of macro expansions.</li>
 154     <li>Elsa is even slower and uses more memory than GCC, which itself requires
 155         far more space and time than clang.</li>
 156     <li>Elsa only does partial semantic analysis.  It is intended to work on
 157         code that is already validated by GCC, so it does not do many semantic
 158         checks required by the languages it implements.</li>
 159     <li>Elsa does not support Objective-C.</li>
 160     <li>Elsa does not support native code generation.</li>
 161     </ul>
 162
 163
 164     <!--=====================================================================-->
 165     <h2><a name="pcc">Clang vs PCC (Portable C Compiler)</a></h2>
 166     <!--=====================================================================-->
 167
 168     <p>Pro's of PCC vs clang:</p>
 169
 170     <ul>
 171     <li>The PCC source base is very small and builds quickly with just a C
 172         compiler.</li>
 173     </ul>
 174
 175     <p>Pro's of clang vs PCC:</p>
 176
 177     <ul>
 178     <li>PCC dates from the 1970's and has been dormant for most of that time.
 179         The clang + llvm communities are very active.</li>
 180     <li>PCC doesn't support Objective-C or C++ and doesn't aim to support
 181         C++.</li>
 182     <li>PCC's code generation is very limited compared to LLVM.  It produces very
 183         inefficient code and does not support many important targets.</li>
 184     <li>Like Elsa, PCC's does not have an integrated preprocessor, making it
 185         extremely difficult to use it for source analysis tools.</li>
 186     </ul>
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