Blame - www/comparison.html - fp2-dev/platform/external/clang

Chris Lattner

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2007-12-10 01:44:24 +0000

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<title>Comparing clang to other compilers</title>

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<h1>Clang vs Other 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 technical points

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to avoid controversy where possible. Also, software is infinitely

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mutable, so we avoid mentioning anything that would be easy to fix.</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 specific goals.</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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<h2><a name="gcc">Clang vs GCC (GNU Compiler Collection)</a></h2>

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<p>Pros 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. clang's support for C++ in

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particular is nowhere near what GCC supports.</li>

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<li>GCC is popular and widely adopted.</li>

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</ul>

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<p>Cons of GCC vs clang:</p>

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<ul>

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<li>GCC has a very old codebase which presents a steep learning curve to new

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developers. The Clang ASTs and design are intended to be easily

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understandable to anyone who is familiar with the languages involved

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and have a basic understanding of how a compiler works.</li>

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<li>GCC is built as a monolithic static compiler, which makes it extremely

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difficult to use as an API and integrate into other tools (e.g. an IDE).

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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> was

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intended to make it difficult to decouple the front-end from

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the rest of the compiler. Clang is designed as an API from its

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inception.</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>GCC does not track information about macro instantiations when parsing

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source code, this makes it very difficult for static analysis and

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refactoring tools to work in the presense of (even simple) macros.</li>

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<li>GCC simplifies code as it parses it. As one simple example, if you

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write "x-x" in your source code, the GCC AST will contain "0", with no

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mention of x. This is extremely bad for a refactoring tool that wants

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to rename 'x' for example.</li>

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<li>GCC does not have a way to serialize the AST of a file out to disk and

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read it back into another program. Its PCH mechanism is architecturally

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only able to read the dump back into the exact same binary.</li>

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<li>GCC is <a href="features.html#performance">very slow and uses a large

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amount of memory</a>.</li>

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<li>The diagnostics produced by GCC are acceptable, but are often confusing

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and it does not support <a

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href="features.html#expressivediags">expressive diagnostics</a>.</li>

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<li>GCC is licensed under the GPL license, which makes it difficult to use

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for projects that do not themselves want to be GPL. clang uses a BSD

license.</li>

</ul>

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<h2><a name="elsa">Clang vs Elsa (Elkhound-based C++ Parser)</a></h2>

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<p>Pros 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 composable by adding

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grammar rules. Clang has a very simple and easily extensible parser,

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but requires you to write C++ code to extend it.</li>

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</ul>

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<p>Cons of Elsa vs clang:</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 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.</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. Likewise, it does not keep

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track of macro expansions.</li>

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<li>Elsa is slower and uses more memory than GCC, which requires far more

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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>

</ul>

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<h2><a name="pcc">Clang vs PCC (Portable C Compiler)</a></h2>

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<p>Pros 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

compiler.</li>

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<p>Cons of PCC vs clang:</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 community are very active.</li>

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<li>PCC doesn't support Objective-C and doesn't aim to support 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>PCC's does not have an integrated preprocessor, so it is extremely

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difficult to use it for source analysis tools.</li>

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