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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 cfe-dev 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, etc.</li> |
| <li><a href="cxx_status.html">Clang support for C++</a> is more compliant |
| than GCC's in many ways, but is not as mature as GCC's. GCC has several |
| C++'0x features that Clang does not yet support (e.g. variadic |
| templates).</li> |
| <li>GCC supports more targets than LLVM.</li> |
| <li>GCC is popular and widely adopted.</li> |
| <li>GCC does not require a C++ compiler to build it.</li> |
| </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>For every token, clang tracks information about where it was written and |
| where it was ultimately expanded into if it was involved in a macro. |
| GCC does not track information about macro instantiations when parsing |
| source code. This makes it very difficult for source rewriting tools |
| (e.g. for refactoring) to work in the presence of (even simple) |
| macros.</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. clang uses a BSD license, which |
| allows it to be used by projects that do not themselves want to be |
| GPL.</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> |
| </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>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> |
| |
| <p>Note that there is a fork of Elsa known as "Pork". It addresses some of |
| these shortcomings by loosely integrating a preprocessor. This allows it |
| to map from a source location in the AST to the original position before |
| preprocessing, providing it better support for static analysis and |
| refactoring. Note that Pork is in stasis now too.</p> |
| |
| |
| <!--=====================================================================--> |
| <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> |
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