src/third_party/llvm-project/clang/www/features.html - cobalt - Git at Google

 <!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. Many detailed benchmarks can be found online.</p>

 <p>Compile time performance is important, but when using clang as an API, often
 memory use is even more so: 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>

 <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>


 <!--=======================================================================-->
 <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')
   <span style="color:darkgreen">  return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span>
   <span style="color:blue">                       ~~~~~~~~~~~~~~ ^ ~~~~~</span>
 </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><p>
 "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."</p></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 &amp; 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>

 <!--=======================================================================-->
 <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 id="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 intend for clang (and LLVM as a whole) to be used for
 commercial projects, not only as a stand-alone compiler but also as a library
 embedded inside a proprietary application.  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&amp;R
 C, C++'03, Objective-C 2, etc.</p>

 </div>
 </body>
 </html>
	<!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. Many detailed benchmarks can be found online.</p>

	<p>Compile time performance is important, but when using clang as an API, often
	memory use is even more so: 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>

	<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>


	<!--=======================================================================-->
	<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')
	<span style="color:darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span>
	<span style="color:blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</span>
	</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><p>
	"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."</p></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>

	<!--=======================================================================-->
	<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 id="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 intend for clang (and LLVM as a whole) to be used for
	commercial projects, not only as a stand-alone compiler but also as a library
	embedded inside a proprietary application. 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>