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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>GCC front-ends are very mature and already support C/C++/ObjC and all | |
| the variants we are interested in. <a href="cxx_status.html">clang's | |
| support for C++</a> in particular is nowhere near what GCC supports.</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="features.html#expressivediags">expressive diagnostics</a>. GCC's | |
| warnings are acceptable, but are often confusing and it does not support | |
| expressive diagnostics. Clang also preserves typedefs in diagnostics | |
| consistently.</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 support for C++ is far beyond what clang provides. If you need | |
| C++ support in the next year, Elsa is a great way to get it. That said, | |
| Elsa is missing important support for templates and other pieces: for | |
| example, it is not capable of compiling the GCC STL headers from any | |
| version newer than GCC 3.4.</li> | |
| <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, though it continues to be used by other small | |
| projects | |
| (e.g. Oink). 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 C99, Objective-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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