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| <!--*************************************************************************--> |
| <h1>Clang - Features and Goals</h1> |
| <!--*************************************************************************--> |
| |
| <p> |
| This page describes the <a href="index.html#goals">features and goals</a> of |
| Clang in more detail and gives a more broad explanation about what we mean. |
| These features are: |
| </p> |
| |
| <p>End-User Features:</p> |
| |
| <ul> |
| <li><a href="#performance">Fast compiles and low memory use</a></li> |
| <li><a href="#expressivediags">Expressive diagnostics</a></li> |
| <li><a href="#gcccompat">GCC compatibility</a></li> |
| </ul> |
| |
| <p>Utility and Applications:</p> |
| |
| <ul> |
| <li><a href="#libraryarch">Library based architecture</a></li> |
| <li><a href="#diverseclients">Support diverse clients</a></li> |
| <li><a href="#ideintegration">Integration with IDEs</a></li> |
| <li><a href="#license">Use the LLVM 'BSD' License</a></li> |
| </ul> |
| |
| <p>Internal Design and Implementation:</p> |
| |
| <ul> |
| <li><a href="#real">A real-world, production quality compiler</a></li> |
| <li><a href="#simplecode">A simple and hackable code base</a></li> |
| <li><a href="#unifiedparser">A single unified parser for C, Objective C, C++, |
| and Objective C++</a></li> |
| <li><a href="#conformance">Conformance with C/C++/ObjC and their |
| variants</a></li> |
| </ul> |
| |
| <!--*************************************************************************--> |
| <h1><a name="enduser">End-User Features</a></h1> |
| <!--*************************************************************************--> |
| |
| |
| <!--=======================================================================--> |
| <h2><a name="performance">Fast compiles and Low Memory Use</a></h2> |
| <!--=======================================================================--> |
| |
| <p>A major focus of our work on clang is to make it fast, light and scalable. |
| The library-based architecture of clang makes it straight-forward to time and |
| profile the cost of each layer of the stack, and the driver has a number of |
| options for performance analysis.</p> |
| |
| <p>While there is still much that can be done, we find that the clang front-end |
| is significantly quicker than gcc and uses less memory For example, when |
| compiling "Carbon.h" on Mac OS/X, we see that clang is 2.5x faster than GCC:</p> |
| |
| <img class="img_slide" src="feature-compile1.png" width="400" height="300" /> |
| |
| <p>Carbon.h is a monster: it transitively includes 558 files, 12.3M of code, |
| declares 10000 functions, has 2000 struct definitions, 8000 fields, 20000 enum |
| constants, etc (see slide 25+ of the <a href="clang_video-07-25-2007.html">clang |
| talk</a> for more information). It is also #include'd into almost every C file |
| in a GUI app on the Mac, so its compile time is very important.</p> |
| |
| <p>From the slide above, you can see that we can measure the time to preprocess |
| the file independently from the time to parse it, and independently from the |
| time to build the ASTs for the code. GCC doesn't provide a way to measure the |
| parser without AST building (it only provides -fsyntax-only). In our |
| measurements, we find that clang's preprocessor is consistently 40% faster than |
| GCCs, and the parser + AST builder is ~4x faster than GCC's. If you have |
| sources that do not depend as heavily on the preprocessor (or if you |
| use Precompiled Headers) you may see a much bigger speedup from clang. |
| </p> |
| |
| <p>Compile time performance is important, but when using clang as an API, often |
| memory use is even moreso: the less memory the code takes the more code you can |
| fit into memory at a time (useful for whole program analysis tools, for |
| example).</p> |
| |
| <img class="img_slide" src="feature-memory1.png" width="400" height="300" /> |
| |
| <p>Here we see a huge advantage of clang: its ASTs take <b>5x less memory</b> |
| than GCC's syntax trees, despite the fact that clang's ASTs capture far more |
| source-level information than GCC's trees do. This feat is accomplished through |
| the use of carefully designed APIs and efficient representations.</p> |
| |
| <p>In addition to being efficient when pitted head-to-head against GCC in batch |
| mode, clang is built with a <a href="#libraryarch">library based |
| architecture</a> that makes it relatively easy to adapt it and build new tools |
| with it. This means that it is often possible to apply out-of-the-box thinking |
| and novel techniques to improve compilation in various ways.</p> |
| |
| <img class="img_slide" src="feature-compile2.png" width="400" height="300" /> |
| |
| <p>This slide shows how the clang preprocessor can be used to make "distcc" |
| parallelization <b>3x</b> more scalable than when using the GCC preprocessor. |
| "distcc" quickly bottlenecks on the preprocessor running on the central driver |
| machine, so a fast preprocessor is very useful. Comparing the first two bars |
| of each group shows how a ~40% faster preprocessor can reduce preprocessing time |
| of these large C++ apps by about 40% (shocking!).</p> |
| |
| <p>The third bar on the slide is the interesting part: it shows how trivial |
| caching of file system accesses across invocations of the preprocessor allows |
| clang to reduce time spent in the kernel by 10x, making distcc over 3x more |
| scalable. This is obviously just one simple hack, doing more interesting things |
| (like caching tokens across preprocessed files) would yield another substantial |
| speedup.</p> |
| |
| <p>The clean framework-based design of clang means that many things are possible |
| that would be very difficult in other systems, for example incremental |
| compilation, multithreading, intelligent caching, etc. We are only starting |
| to tap the full potential of the clang design.</p> |
| |
| |
| <!--=======================================================================--> |
| <h2><a name="expressivediags">Expressive Diagnostics</a></h2> |
| <!--=======================================================================--> |
| |
| <p>Clang is designed to efficiently capture range information for expressions |
| and statements, which allows it to emit very useful and detailed diagnostic |
| information (e.g. warnings and errors) when a problem is detected.</p> |
| |
| <p>For example, this slide compares the diagnostics emitted by clang (top) to |
| the diagnostics emitted by GCC (middle) for a simple example:</p> |
| |
| <img class="img_slide" src="feature-diagnostics1.png" width="400" height="300"/> |
| |
| <p>As you can see, clang goes beyond tracking just column number information: it |
| is able to highlight the subexpressions involved in a problem, making it much |
| easier to understand the source of the problem in many cases. For example, in |
| the first problem, it tells you <em>why</em> the operand is invalid (it |
| requires a pointer) and what type it really is.</p> |
| |
| <p>In the second error, you can see how clang uses column number information to |
| identify exactly which "+" out of the four on that line is causing the problem. |
| Further, it highlights the subexpressions involved, which can be very useful |
| when a complex subexpression that relies on tricky precedence rules.</p> |
| |
| <p>The example doesn't show it, but clang works very hard to retain typedef |
| information, ensuring that diagnostics print the user types, not the fully |
| expanded (and often huge) types. This is clearly important for C++ code (tell |
| me about "<tt>std::string</tt>", not about "<tt>std::basic_string<char, |
| std::char_traits<char>, std::allocator<char> ></tt>"!), but it is |
| also very useful in C code in some cases as well (e.g. "<tt>__m128"</tt> vs |
| "<tt>float __attribute__((__vector_size__(16)))</tt>").</p> |
| |
| <!--=======================================================================--> |
| <h2><a name="gcccompat">GCC Compatibility</a></h2> |
| <!--=======================================================================--> |
| |
| <p>GCC is currently the defacto-standard open source compiler today, and it |
| routinely compiles a huge volume of code. GCC supports a huge number of |
| extensions and features (many of which are undocumented) and a lot of |
| code and header files depend on these features in order to build.</p> |
| |
| <p>While it would be nice to be able to ignore these extensions and focus on |
| implementing the language standards to the letter, pragmatics force us to |
| support the GCC extensions that see the most use. Many users just want their |
| code to compile, they don't care to argue about whether it is pedantically C99 |
| or not.</p> |
| |
| <p>As mentioned above, all |
| extensions are explicitly recognized as such and marked with extension |
| diagnostics, which can be mapped to warnings, errors, or just ignored. |
| </p> |
| |
| |
| <!--*************************************************************************--> |
| <h1><a name="applications">Utility and Applications</a></h1> |
| <!--*************************************************************************--> |
| |
| <!--=======================================================================--> |
| <h2><a name="libraryarch">Library Based Architecture</a></h2> |
| <!--=======================================================================--> |
| |
| <p>A major design concept for clang is its use of a library-based |
| architecture. In this design, various parts of the front-end can be cleanly |
| divided into separate libraries which can then be mixed up for different needs |
| and uses. In addition, the library-based approach encourages good interfaces |
| and makes it easier for new developers to get involved (because they only need |
| to understand small pieces of the big picture).</p> |
| |
| <blockquote> |
| "The world needs better compiler tools, tools which are built as libraries. |
| This design point allows reuse of the tools in new and novel ways. However, |
| building the tools as libraries isn't enough: they must have clean APIs, be as |
| decoupled from each other as possible, and be easy to modify/extend. This |
| requires clean layering, decent design, and keeping the libraries independent of |
| any specific client."</blockquote> |
| |
| <p> |
| Currently, clang is divided into the following libraries and tool: |
| </p> |
| |
| <ul> |
| <li><b>libsupport</b> - Basic support library, from LLVM.</li> |
| <li><b>libsystem</b> - System abstraction library, from LLVM.</li> |
| <li><b>libbasic</b> - Diagnostics, SourceLocations, SourceBuffer abstraction, |
| file system caching for input source files.</li> |
| <li><b>libast</b> - Provides classes to represent the C AST, the C type system, |
| builtin functions, and various helpers for analyzing and manipulating the |
| AST (visitors, pretty printers, etc).</li> |
| <li><b>liblex</b> - Lexing and preprocessing, identifier hash table, pragma |
| handling, tokens, and macro expansion.</li> |
| <li><b>libparse</b> - Parsing. This library invokes coarse-grained 'Actions' |
| provided by the client (e.g. libsema builds ASTs) but knows nothing about |
| ASTs or other client-specific data structures.</li> |
| <li><b>libsema</b> - Semantic Analysis. This provides a set of parser actions |
| to build a standardized AST for programs.</li> |
| <li><b>libcodegen</b> - Lower the AST to LLVM IR for optimization & code |
| generation.</li> |
| <li><b>librewrite</b> - Editing of text buffers (important for code rewriting |
| transformation, like refactoring).</li> |
| <li><b>libanalysis</b> - Static analysis support.</li> |
| <li><b>clang</b> - A driver program, client of the libraries at various |
| levels.</li> |
| </ul> |
| |
| <p>As an example of the power of this library based design.... If you wanted to |
| build a preprocessor, you would take the Basic and Lexer libraries. If you want |
| an indexer, you would take the previous two and add the Parser library and |
| some actions for indexing. If you want a refactoring, static analysis, or |
| source-to-source compiler tool, you would then add the AST building and |
| semantic analyzer libraries.</p> |
| |
| <p>For more information about the low-level implementation details of the |
| various clang libraries, please see the <a href="docs/InternalsManual.html"> |
| clang Internals Manual</a>.</p> |
| |
| <!--=======================================================================--> |
| <h2><a name="diverseclients">Support Diverse Clients</a></h2> |
| <!--=======================================================================--> |
| |
| <p>Clang is designed and built with many grand plans for how we can use it. The |
| driving force is the fact that we use C and C++ daily, and have to suffer due to |
| a lack of good tools available for it. We believe that the C and C++ tools |
| ecosystem has been significantly limited by how difficult it is to parse and |
| represent the source code for these languages, and we aim to rectify this |
| problem in clang.</p> |
| |
| <p>The problem with this goal is that different clients have very different |
| requirements. Consider code generation, for example: a simple front-end that |
| parses for code generation must analyze the code for validity and emit code |
| in some intermediate form to pass off to a optimizer or backend. Because |
| validity analysis and code generation can largely be done on the fly, there is |
| not hard requirement that the front-end actually build up a full AST for all |
| the expressions and statements in the code. TCC and GCC are examples of |
| compilers that either build no real AST (in the former case) or build a stripped |
| down and simplified AST (in the later case) because they focus primarily on |
| codegen.</p> |
| |
| <p>On the opposite side of the spectrum, some clients (like refactoring) want |
| highly detailed information about the original source code and want a complete |
| AST to describe it with. Refactoring wants to have information about macro |
| expansions, the location of every paren expression '(((x)))' vs 'x', full |
| position information, and much more. Further, refactoring wants to look |
| <em>across the whole program</em> to ensure that it is making transformations |
| that are safe. Making this efficient and getting this right requires a |
| significant amount of engineering and algorithmic work that simply are |
| unnecessary for a simple static compiler.</p> |
| |
| <p>The beauty of the clang approach is that it does not restrict how you use it. |
| In particular, it is possible to use the clang preprocessor and parser to build |
| an extremely quick and light-weight on-the-fly code generator (similar to TCC) |
| that does not build an AST at all. As an intermediate step, clang supports |
| using the current AST generation and semantic analysis code and having a code |
| generation client free the AST for each function after code generation. Finally, |
| clang provides support for building and retaining fully-fledged ASTs, and even |
| supports writing them out to disk.</p> |
| |
| <p>Designing the libraries with clean and simple APIs allows these high-level |
| policy decisions to be determined in the client, instead of forcing "one true |
| way" in the implementation of any of these libraries. Getting this right is |
| hard, and we don't always get it right the first time, but we fix any problems |
| when we realize we made a mistake.</p> |
| |
| <!--=======================================================================--> |
| <h2><a name="ideintegration">Integration with IDEs</h2> |
| <!--=======================================================================--> |
| |
| <p> |
| We believe that Integrated Development Environments (IDE's) are a great way |
| to pull together various pieces of the development puzzle, and aim to make clang |
| work well in such an environment. The chief advantage of an IDE is that they |
| typically have visibility across your entire project and are long-lived |
| processes, whereas stand-alone compiler tools are typically invoked on each |
| individual file in the project, and thus have limited scope.</p> |
| |
| <p>There are many implications of this difference, but a significant one has to |
| do with efficiency and caching: sharing an address space across different files |
| in a project, means that you can use intelligent caching and other techniques to |
| dramatically reduce analysis/compilation time.</p> |
| |
| <p>A further difference between IDEs and batch compiler is that they often |
| impose very different requirements on the front-end: they depend on high |
| performance in order to provide a "snappy" experience, and thus really want |
| techniques like "incremental compilation", "fuzzy parsing", etc. Finally, IDEs |
| often have very different requirements than code generation, often requiring |
| information that a codegen-only frontend can throw away. Clang is |
| specifically designed and built to capture this information. |
| </p> |
| |
| |
| <!--=======================================================================--> |
| <h2><a name="license">Use the LLVM 'BSD' License</a></h2> |
| <!--=======================================================================--> |
| |
| <p>We actively indend for clang (and a LLVM as a whole) to be used for |
| commercial projects, and the BSD license is the simplest way to allow this. We |
| feel that the license encourages contributors to pick up the source and work |
| with it, and believe that those individuals and organizations will contribute |
| back their work if they do not want to have to maintain a fork forever (which is |
| time consuming and expensive when merges are involved). Further, nobody makes |
| money on compilers these days, but many people need them to get bigger goals |
| accomplished: it makes sense for everyone to work together.</p> |
| |
| <p>For more information about the LLVM/clang license, please see the <a |
| href="http://llvm.org/docs/DeveloperPolicy.html#license">LLVM License |
| Description</a> for more information.</p> |
| |
| |
| |
| <!--*************************************************************************--> |
| <h1><a name="design">Internal Design and Implementation</a></h1> |
| <!--*************************************************************************--> |
| |
| <!--=======================================================================--> |
| <h2><a name="real">A real-world, production quality compiler</a></h2> |
| <!--=======================================================================--> |
| |
| <p> |
| Clang is designed and built by experienced compiler developers who |
| are increasingly frustrated with the problems that <a |
| href="comparison.html">existing open source compilers</a> have. Clang is |
| carefully and thoughtfully designed and built to provide the foundation of a |
| whole new generation of C/C++/Objective C development tools, and we intend for |
| it to be production quality.</p> |
| |
| <p>Being a production quality compiler means many things: it means being high |
| performance, being solid and (relatively) bug free, and it means eventually |
| being used and depended on by a broad range of people. While we are still in |
| the early development stages, we strongly believe that this will become a |
| reality.</p> |
| |
| <!--=======================================================================--> |
| <h2><a name="simplecode">A simple and hackable code base</a></h2> |
| <!--=======================================================================--> |
| |
| <p>Our goal is to make it possible for anyone with a basic understanding |
| of compilers and working knowledge of the C/C++/ObjC languages to understand and |
| extend the clang source base. A large part of this falls out of our decision to |
| make the AST mirror the languages as closely as possible: you have your friendly |
| if statement, for statement, parenthesis expression, structs, unions, etc, all |
| represented in a simple and explicit way.</p> |
| |
| <p>In addition to a simple design, we work to make the source base approachable |
| by commenting it well, including citations of the language standards where |
| appropriate, and designing the code for simplicity. Beyond that, clang offers |
| a set of AST dumpers, printers, and visualizers that make it easy to put code in |
| and see how it is represented.</p> |
| |
| <!--=======================================================================--> |
| <h2><a name="unifiedparser">A single unified parser for C, Objective C, C++, |
| and Objective C++</a></h2> |
| <!--=======================================================================--> |
| |
| <p>Clang is the "C Language Family Front-end", which means we intend to support |
| the most popular members of the C family. We are convinced that the right |
| parsing technology for this class of languages is a hand-built recursive-descent |
| parser. Because it is plain C++ code, recursive descent makes it very easy for |
| new developers to understand the code, it easily supports ad-hoc rules and other |
| strange hacks required by C/C++, and makes it straight-forward to implement |
| excellent diagnostics and error recovery.</p> |
| |
| <p>We believe that implementing C/C++/ObjC in a single unified parser makes the |
| end result easier to maintain and evolve than maintaining a separate C and C++ |
| parser which must be bugfixed and maintained independently of each other.</p> |
| |
| <!--=======================================================================--> |
| <h2><a name="conformance">Conformance with C/C++/ObjC and their |
| variants</a></h2> |
| <!--=======================================================================--> |
| |
| <p>When you start work on implementing a language, you find out that there is a |
| huge gap between how the language works and how most people understand it to |
| work. This gap is the difference between a normal programmer and a (scary? |
| super-natural?) "language lawyer", who knows the ins and outs of the language |
| and can grok standardese with ease.</p> |
| |
| <p>In practice, being conformant with the languages means that we aim to support |
| the full language, including the dark and dusty corners (like trigraphs, |
| preprocessor arcana, C99 VLAs, etc). Where we support extensions above and |
| beyond what the standard officially allows, we make an effort to explicitly call |
| this out in the code and emit warnings about it (which are disabled by default, |
| but can optionally be mapped to either warnings or errors), allowing you to use |
| clang in "strict" mode if you desire.</p> |
| |
| <p>We also intend to support "dialects" of these languages, such as C89, K&R |
| C, C++'03, Objective-C 2, etc.</p> |
| |
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