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428 lines
22 KiB
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<title>Clang - Features and Goals</title>
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<!--*************************************************************************-->
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<h1>Clang - Features and Goals</h1>
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<!--*************************************************************************-->
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<p>
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This page describes the <a href="index.html#goals">features and goals</a> of
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Clang in more detail and gives a more broad explanation about what we mean.
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These features are:
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</p>
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<p>End-User Features:</p>
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<ul>
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<li><a href="#performance">Fast compiles and low memory use</a></li>
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<li><a href="#expressivediags">Expressive diagnostics</a></li>
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<li><a href="#gcccompat">GCC compatibility</a></li>
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</ul>
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<p>Utility and Applications:</p>
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<ul>
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<li><a href="#libraryarch">Library based architecture</a></li>
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<li><a href="#diverseclients">Support diverse clients</a></li>
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<li><a href="#ideintegration">Integration with IDEs</a></li>
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<li><a href="#license">Use the LLVM 'BSD' License</a></li>
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</ul>
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<p>Internal Design and Implementation:</p>
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<ul>
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<li><a href="#real">A real-world, production quality compiler</a></li>
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<li><a href="#simplecode">A simple and hackable code base</a></li>
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<li><a href="#unifiedparser">A single unified parser for C, Objective C, C++,
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and Objective C++</a></li>
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<li><a href="#conformance">Conformance with C/C++/ObjC and their
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variants</a></li>
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</ul>
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<!--*************************************************************************-->
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<h2><a name="enduser">End-User Features</a></h2>
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<!--*************************************************************************-->
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<!--=======================================================================-->
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<h3><a name="performance">Fast compiles and Low Memory Use</a></h3>
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<!--=======================================================================-->
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<p>A major focus of our work on clang is to make it fast, light and scalable.
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The library-based architecture of clang makes it straight-forward to time and
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profile the cost of each layer of the stack, and the driver has a number of
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options for performance analysis.</p>
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<p>While there is still much that can be done, we find that the clang front-end
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is significantly quicker than gcc and uses less memory For example, when
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compiling "Carbon.h" on Mac OS/X, we see that clang is 2.5x faster than GCC:</p>
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<img class="img_slide" src="feature-compile1.png" width="400" height="300"
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alt="Time to parse carbon.h: -fsyntax-only">
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<p>Carbon.h is a monster: it transitively includes 558 files, 12.3M of code,
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declares 10000 functions, has 2000 struct definitions, 8000 fields, 20000 enum
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constants, etc (see slide 25+ of the <a href="clang_video-07-25-2007.html">clang
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talk</a> for more information). It is also #include'd into almost every C file
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in a GUI app on the Mac, so its compile time is very important.</p>
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<p>From the slide above, you can see that we can measure the time to preprocess
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the file independently from the time to parse it, and independently from the
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time to build the ASTs for the code. GCC doesn't provide a way to measure the
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parser without AST building (it only provides -fsyntax-only). In our
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measurements, we find that clang's preprocessor is consistently 40% faster than
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GCCs, and the parser + AST builder is ~4x faster than GCC's. If you have
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sources that do not depend as heavily on the preprocessor (or if you
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use Precompiled Headers) you may see a much bigger speedup from clang.
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</p>
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<p>Compile time performance is important, but when using clang as an API, often
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memory use is even moreso: the less memory the code takes the more code you can
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fit into memory at a time (useful for whole program analysis tools, for
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example).</p>
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<img class="img_slide" src="feature-memory1.png" width="400" height="300"
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alt="Space">
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<p>Here we see a huge advantage of clang: its ASTs take <b>5x less memory</b>
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than GCC's syntax trees, despite the fact that clang's ASTs capture far more
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source-level information than GCC's trees do. This feat is accomplished through
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the use of carefully designed APIs and efficient representations.</p>
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<p>In addition to being efficient when pitted head-to-head against GCC in batch
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mode, clang is built with a <a href="#libraryarch">library based
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architecture</a> that makes it relatively easy to adapt it and build new tools
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with it. This means that it is often possible to apply out-of-the-box thinking
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and novel techniques to improve compilation in various ways.</p>
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<img class="img_slide" src="feature-compile2.png" width="400" height="300"
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alt="Preprocessor Speeds: GCC 4.2 vs clang-all">
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<p>This slide shows how the clang preprocessor can be used to make "distcc"
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parallelization <b>3x</b> more scalable than when using the GCC preprocessor.
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"distcc" quickly bottlenecks on the preprocessor running on the central driver
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machine, so a fast preprocessor is very useful. Comparing the first two bars
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of each group shows how a ~40% faster preprocessor can reduce preprocessing time
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of these large C++ apps by about 40% (shocking!).</p>
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<p>The third bar on the slide is the interesting part: it shows how trivial
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caching of file system accesses across invocations of the preprocessor allows
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clang to reduce time spent in the kernel by 10x, making distcc over 3x more
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scalable. This is obviously just one simple hack, doing more interesting things
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(like caching tokens across preprocessed files) would yield another substantial
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speedup.</p>
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<p>The clean framework-based design of clang means that many things are possible
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that would be very difficult in other systems, for example incremental
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compilation, multithreading, intelligent caching, etc. We are only starting
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to tap the full potential of the clang design.</p>
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<!--=======================================================================-->
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<h3><a name="expressivediags">Expressive Diagnostics</a></h3>
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<!--=======================================================================-->
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<p>In addition to being fast and functional, we aim to make Clang extremely user
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friendly. As far as a command-line compiler goes, this basically boils down to
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making the diagnostics (error and warning messages) generated by the compiler
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be as useful as possible. There are several ways that we do this, but the
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most important are pinpointing exactly what is wrong in the program,
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highlighting related information so that it is easy to understand at a glance,
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and making the wording as clear as possible.</p>
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<p>Here is one simple example that illustrates the difference between a typical
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GCC and Clang diagnostic:</p>
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<pre>
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$ <b>gcc-4.2 -fsyntax-only t.c</b>
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t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
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$ <b>clang -fsyntax-only t.c</b>
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t.c:7:39: error: invalid operands to binary expression ('int' and 'struct A')
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<span style="color:darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span>
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<span style="color:blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</span>
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</pre>
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<p>Here you can see that you don't even need to see the original source code to
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understand what is wrong based on the Clang error: Because clang prints a
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caret, you know exactly <em>which</em> plus it is complaining about. The range
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information highlights the left and right side of the plus which makes it
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immediately obvious what the compiler is talking about, which is very useful for
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cases involving precedence issues and many other situations.</p>
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<p>Clang diagnostics are very polished and have many features. For more
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information and examples, please see the <a href="diagnostics.html">Expressive
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Diagnostics</a> page.</p>
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<!--=======================================================================-->
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<h3><a name="gcccompat">GCC Compatibility</a></h3>
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<!--=======================================================================-->
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<p>GCC is currently the defacto-standard open source compiler today, and it
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routinely compiles a huge volume of code. GCC supports a huge number of
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extensions and features (many of which are undocumented) and a lot of
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code and header files depend on these features in order to build.</p>
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<p>While it would be nice to be able to ignore these extensions and focus on
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implementing the language standards to the letter, pragmatics force us to
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support the GCC extensions that see the most use. Many users just want their
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code to compile, they don't care to argue about whether it is pedantically C99
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or not.</p>
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<p>As mentioned above, all
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extensions are explicitly recognized as such and marked with extension
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diagnostics, which can be mapped to warnings, errors, or just ignored.
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</p>
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<!--*************************************************************************-->
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<h2><a name="applications">Utility and Applications</a></h2>
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<!--*************************************************************************-->
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<!--=======================================================================-->
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<h3><a name="libraryarch">Library Based Architecture</a></h3>
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<!--=======================================================================-->
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<p>A major design concept for clang is its use of a library-based
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architecture. In this design, various parts of the front-end can be cleanly
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divided into separate libraries which can then be mixed up for different needs
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and uses. In addition, the library-based approach encourages good interfaces
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and makes it easier for new developers to get involved (because they only need
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to understand small pieces of the big picture).</p>
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<blockquote><p>
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"The world needs better compiler tools, tools which are built as libraries.
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This design point allows reuse of the tools in new and novel ways. However,
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building the tools as libraries isn't enough: they must have clean APIs, be as
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decoupled from each other as possible, and be easy to modify/extend. This
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requires clean layering, decent design, and keeping the libraries independent of
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any specific client."</p></blockquote>
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<p>
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Currently, clang is divided into the following libraries and tool:
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</p>
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<ul>
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<li><b>libsupport</b> - Basic support library, from LLVM.</li>
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<li><b>libsystem</b> - System abstraction library, from LLVM.</li>
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<li><b>libbasic</b> - Diagnostics, SourceLocations, SourceBuffer abstraction,
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file system caching for input source files.</li>
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<li><b>libast</b> - Provides classes to represent the C AST, the C type system,
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builtin functions, and various helpers for analyzing and manipulating the
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AST (visitors, pretty printers, etc).</li>
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<li><b>liblex</b> - Lexing and preprocessing, identifier hash table, pragma
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handling, tokens, and macro expansion.</li>
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<li><b>libparse</b> - Parsing. This library invokes coarse-grained 'Actions'
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provided by the client (e.g. libsema builds ASTs) but knows nothing about
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ASTs or other client-specific data structures.</li>
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<li><b>libsema</b> - Semantic Analysis. This provides a set of parser actions
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to build a standardized AST for programs.</li>
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<li><b>libcodegen</b> - Lower the AST to LLVM IR for optimization & code
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generation.</li>
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<li><b>librewrite</b> - Editing of text buffers (important for code rewriting
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transformation, like refactoring).</li>
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<li><b>libanalysis</b> - Static analysis support.</li>
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<li><b>clang</b> - A driver program, client of the libraries at various
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levels.</li>
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</ul>
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<p>As an example of the power of this library based design.... If you wanted to
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build a preprocessor, you would take the Basic and Lexer libraries. If you want
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an indexer, you would take the previous two and add the Parser library and
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some actions for indexing. If you want a refactoring, static analysis, or
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source-to-source compiler tool, you would then add the AST building and
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semantic analyzer libraries.</p>
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<p>For more information about the low-level implementation details of the
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various clang libraries, please see the <a href="docs/InternalsManual.html">
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clang Internals Manual</a>.</p>
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<!--=======================================================================-->
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<h3><a name="diverseclients">Support Diverse Clients</a></h3>
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<!--=======================================================================-->
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<p>Clang is designed and built with many grand plans for how we can use it. The
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driving force is the fact that we use C and C++ daily, and have to suffer due to
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a lack of good tools available for it. We believe that the C and C++ tools
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ecosystem has been significantly limited by how difficult it is to parse and
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represent the source code for these languages, and we aim to rectify this
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problem in clang.</p>
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<p>The problem with this goal is that different clients have very different
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requirements. Consider code generation, for example: a simple front-end that
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parses for code generation must analyze the code for validity and emit code
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in some intermediate form to pass off to a optimizer or backend. Because
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validity analysis and code generation can largely be done on the fly, there is
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not hard requirement that the front-end actually build up a full AST for all
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the expressions and statements in the code. TCC and GCC are examples of
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compilers that either build no real AST (in the former case) or build a stripped
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down and simplified AST (in the later case) because they focus primarily on
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codegen.</p>
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<p>On the opposite side of the spectrum, some clients (like refactoring) want
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highly detailed information about the original source code and want a complete
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AST to describe it with. Refactoring wants to have information about macro
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expansions, the location of every paren expression '(((x)))' vs 'x', full
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position information, and much more. Further, refactoring wants to look
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<em>across the whole program</em> to ensure that it is making transformations
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that are safe. Making this efficient and getting this right requires a
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significant amount of engineering and algorithmic work that simply are
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unnecessary for a simple static compiler.</p>
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<p>The beauty of the clang approach is that it does not restrict how you use it.
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In particular, it is possible to use the clang preprocessor and parser to build
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an extremely quick and light-weight on-the-fly code generator (similar to TCC)
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that does not build an AST at all. As an intermediate step, clang supports
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using the current AST generation and semantic analysis code and having a code
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generation client free the AST for each function after code generation. Finally,
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clang provides support for building and retaining fully-fledged ASTs, and even
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supports writing them out to disk.</p>
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<p>Designing the libraries with clean and simple APIs allows these high-level
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policy decisions to be determined in the client, instead of forcing "one true
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way" in the implementation of any of these libraries. Getting this right is
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hard, and we don't always get it right the first time, but we fix any problems
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when we realize we made a mistake.</p>
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<!--=======================================================================-->
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<h3 id="ideintegration">Integration with IDEs</h3>
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<!--=======================================================================-->
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<p>
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We believe that Integrated Development Environments (IDE's) are a great way
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to pull together various pieces of the development puzzle, and aim to make clang
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work well in such an environment. The chief advantage of an IDE is that they
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typically have visibility across your entire project and are long-lived
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processes, whereas stand-alone compiler tools are typically invoked on each
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individual file in the project, and thus have limited scope.</p>
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<p>There are many implications of this difference, but a significant one has to
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do with efficiency and caching: sharing an address space across different files
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in a project, means that you can use intelligent caching and other techniques to
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dramatically reduce analysis/compilation time.</p>
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<p>A further difference between IDEs and batch compiler is that they often
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impose very different requirements on the front-end: they depend on high
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performance in order to provide a "snappy" experience, and thus really want
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techniques like "incremental compilation", "fuzzy parsing", etc. Finally, IDEs
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often have very different requirements than code generation, often requiring
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information that a codegen-only frontend can throw away. Clang is
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specifically designed and built to capture this information.
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</p>
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<!--=======================================================================-->
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<h3><a name="license">Use the LLVM 'BSD' License</a></h3>
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<!--=======================================================================-->
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<p>We actively intend for clang (and LLVM as a whole) to be used for
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commercial projects, not only as a stand-alone compiler but also as a library
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embedded inside a proprietary application. The BSD license is the simplest way
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to allow this. We feel that the license encourages contributors to pick up the
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source and work with it, and believe that those individuals and organizations
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will contribute back their work if they do not want to have to maintain a fork
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forever (which is time consuming and expensive when merges are involved).
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Further, nobody makes money on compilers these days, but many people need them
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to get bigger goals accomplished: it makes sense for everyone to work
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together.</p>
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<p>For more information about the LLVM/clang license, please see the <a
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href="http://llvm.org/docs/DeveloperPolicy.html#license">LLVM License
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Description</a> for more information.</p>
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<!--*************************************************************************-->
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<h2><a name="design">Internal Design and Implementation</a></h2>
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<!--*************************************************************************-->
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<!--=======================================================================-->
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<h3><a name="real">A real-world, production quality compiler</a></h3>
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<!--=======================================================================-->
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<p>
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Clang is designed and built by experienced compiler developers who
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are increasingly frustrated with the problems that <a
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href="comparison.html">existing open source compilers</a> have. Clang is
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carefully and thoughtfully designed and built to provide the foundation of a
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whole new generation of C/C++/Objective C development tools, and we intend for
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it to be production quality.</p>
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<p>Being a production quality compiler means many things: it means being high
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performance, being solid and (relatively) bug free, and it means eventually
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being used and depended on by a broad range of people. While we are still in
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the early development stages, we strongly believe that this will become a
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reality.</p>
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<!--=======================================================================-->
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<h3><a name="simplecode">A simple and hackable code base</a></h3>
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<!--=======================================================================-->
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<p>Our goal is to make it possible for anyone with a basic understanding
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of compilers and working knowledge of the C/C++/ObjC languages to understand and
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extend the clang source base. A large part of this falls out of our decision to
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make the AST mirror the languages as closely as possible: you have your friendly
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if statement, for statement, parenthesis expression, structs, unions, etc, all
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represented in a simple and explicit way.</p>
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<p>In addition to a simple design, we work to make the source base approachable
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by commenting it well, including citations of the language standards where
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appropriate, and designing the code for simplicity. Beyond that, clang offers
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a set of AST dumpers, printers, and visualizers that make it easy to put code in
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and see how it is represented.</p>
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<!--=======================================================================-->
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<h3><a name="unifiedparser">A single unified parser for C, Objective C, C++,
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and Objective C++</a></h3>
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<!--=======================================================================-->
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<p>Clang is the "C Language Family Front-end", which means we intend to support
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the most popular members of the C family. We are convinced that the right
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parsing technology for this class of languages is a hand-built recursive-descent
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parser. Because it is plain C++ code, recursive descent makes it very easy for
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new developers to understand the code, it easily supports ad-hoc rules and other
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strange hacks required by C/C++, and makes it straight-forward to implement
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excellent diagnostics and error recovery.</p>
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<p>We believe that implementing C/C++/ObjC in a single unified parser makes the
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end result easier to maintain and evolve than maintaining a separate C and C++
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parser which must be bugfixed and maintained independently of each other.</p>
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<!--=======================================================================-->
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<h3><a name="conformance">Conformance with C/C++/ObjC and their
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variants</a></h3>
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<!--=======================================================================-->
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<p>When you start work on implementing a language, you find out that there is a
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huge gap between how the language works and how most people understand it to
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work. This gap is the difference between a normal programmer and a (scary?
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super-natural?) "language lawyer", who knows the ins and outs of the language
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and can grok standardese with ease.</p>
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<p>In practice, being conformant with the languages means that we aim to support
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the full language, including the dark and dusty corners (like trigraphs,
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preprocessor arcana, C99 VLAs, etc). Where we support extensions above and
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beyond what the standard officially allows, we make an effort to explicitly call
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this out in the code and emit warnings about it (which are disabled by default,
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but can optionally be mapped to either warnings or errors), allowing you to use
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clang in "strict" mode if you desire.</p>
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<p>We also intend to support "dialects" of these languages, such as C89, K&R
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C, C++'03, Objective-C 2, etc.</p>
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