| <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" |
| "http://www.w3.org/TR/html4/strict.dtd"> |
| <!-- Material used from: HTML 4.01 specs: http://www.w3.org/TR/html401/ --> |
| <html> |
| <head> |
| <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> |
| <title>Comparing Clang to other open source compilers</title> |
| <link type="text/css" rel="stylesheet" href="menu.css"> |
| <link type="text/css" rel="stylesheet" href="content.css"> |
| </head> |
| <body> |
| <!--#include virtual="menu.html.incl"--> |
| <div id="content"> |
| <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> |
| |
| |
| <!--=====================================================================--> |
| <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="https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html">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 has been designed from the start to provide extremely clear and |
| concise diagnostics (error and warning messages), and includes support |
| for <a href="diagnostics.html">expressive diagnostics</a>. |
| Modern versions of GCC have made significant advances in this area, |
| incorporating various Clang features such as preserving typedefs in |
| diagnostics and showing macro expansions, but GCC is still catching |
| up.</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> |
| |
| <!--=====================================================================--> |
| <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> |
| |
| |
| <!--=====================================================================--> |
| <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 and 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> |
| </div> |
| </body> |
| </html> |
