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diff --git a/contrib/llvm/tools/clang/www/features.html b/contrib/llvm/tools/clang/www/features.html deleted file mode 100644 index 6034ae7..0000000 --- a/contrib/llvm/tools/clang/www/features.html +++ /dev/null @@ -1,424 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> -<head> - <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /> - <title>Clang - Features and Goals</title> - <link type="text/css" rel="stylesheet" href="menu.css" /> - <link type="text/css" rel="stylesheet" href="content.css" /> - <style type="text/css"> -</style> -</head> -<body> - -<!--#include virtual="menu.html.incl"--> - -<div id="content"> - -<!--*************************************************************************--> -<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> - -<!--*************************************************************************--> -<h2><a name="enduser">End-User Features</a></h2> -<!--*************************************************************************--> - - -<!--=======================================================================--> -<h3><a name="performance">Fast compiles and Low Memory Use</a></h3> -<!--=======================================================================--> - -<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> - - -<!--=======================================================================--> -<h3><a name="expressivediags">Expressive Diagnostics</a></h3> -<!--=======================================================================--> - -<p>In addition to being fast and functional, we aim to make Clang extremely user -friendly. As far as a command-line compiler goes, this basically boils down to -making the diagnostics (error and warning messages) generated by the compiler -be as useful as possible. There are several ways that we do this, but the -most important are pinpointing exactly what is wrong in the program, -highlighting related information so that it is easy to understand at a glance, -and making the wording as clear as possible.</p> - -<p>Here is one simple example that illustrates the difference between a typical -GCC and Clang diagnostic:</p> - -<pre> - $ <b>gcc-4.2 -fsyntax-only t.c</b> - t.c:7: error: invalid operands to binary + (have 'int' and 'struct A') - $ <b>clang -fsyntax-only t.c</b> - t.c:7:39: error: invalid operands to binary expression ('int' and 'struct A') - <font color="darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</font> - <font color="blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</font> -</pre> - -<p>Here you can see that you don't even need to see the original source code to -understand what is wrong based on the Clang error: Because clang prints a -caret, you know exactly <em>which</em> plus it is complaining about. The range -information highlights the left and right side of the plus which makes it -immediately obvious what the compiler is talking about, which is very useful for -cases involving precedence issues and many other situations.</p> - -<p>Clang diagnostics are very polished and have many features. For more -information and examples, please see the <a href="diagnostics.html">Expressive -Diagnostics</a> page.</p> - -<!--=======================================================================--> -<h3><a name="gcccompat">GCC Compatibility</a></h3> -<!--=======================================================================--> - -<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> - - -<!--*************************************************************************--> -<h2><a name="applications">Utility and Applications</a></h2> -<!--*************************************************************************--> - -<!--=======================================================================--> -<h3><a name="libraryarch">Library Based Architecture</a></h3> -<!--=======================================================================--> - -<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><a href="docs/libIndex.html">libindex</a></b> - Cross-translation-unit infrastructure and indexing 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> - -<!--=======================================================================--> -<h3><a name="diverseclients">Support Diverse Clients</a></h3> -<!--=======================================================================--> - -<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> - -<!--=======================================================================--> -<h3><a name="ideintegration">Integration with IDEs</h3> -<!--=======================================================================--> - -<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> - - -<!--=======================================================================--> -<h3><a name="license">Use the LLVM 'BSD' License</a></h3> -<!--=======================================================================--> - -<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> - - - -<!--*************************************************************************--> -<h2><a name="design">Internal Design and Implementation</a></h2> -<!--*************************************************************************--> - -<!--=======================================================================--> -<h3><a name="real">A real-world, production quality compiler</a></h3> -<!--=======================================================================--> - -<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> - -<!--=======================================================================--> -<h3><a name="simplecode">A simple and hackable code base</a></h3> -<!--=======================================================================--> - -<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> - -<!--=======================================================================--> -<h3><a name="unifiedparser">A single unified parser for C, Objective C, C++, -and Objective C++</a></h3> -<!--=======================================================================--> - -<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> - -<!--=======================================================================--> -<h3><a name="conformance">Conformance with C/C++/ObjC and their - variants</a></h3> -<!--=======================================================================--> - -<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> - -</div> -</body> -</html> |
