Gitiles
Code Review Sign In
gerrit-public.fairphone.software / platform / external / clang / 060ddb0173ba91cd400f073ed0bd1f9b9c3a4d50 / . / www / features.html
blob: d55391a34fb48af7ff44a3c4eaa6e7b172a55fd6 [file] [log] [blame]
Chris Lattnerce90ba62007-12-10 05:20:47 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]3<html>
4<head>
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]5 <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]6 <title>Clang - Features and Goals</title>
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]7 <link type="text/css" rel="stylesheet" href="menu.css">
8 <link type="text/css" rel="stylesheet" href="content.css">
Chris Lattnerce90ba62007-12-10 05:20:47 +0000[diff] [blame]9 <style type="text/css">
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]10</style>
11</head>
12<body>
Chris Lattnerce90ba62007-12-10 05:20:47 +0000[diff] [blame]13
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]14<!--#include virtual="menu.html.incl"-->
Chris Lattnerce90ba62007-12-10 05:20:47 +0000[diff] [blame]15
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]16<div id="content">
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]17
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]18<!--*************************************************************************-->
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]19<h1>Clang - Features and Goals</h1>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]20<!--*************************************************************************-->
21
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]22<p>
23This page describes the <a href="index.html#goals">features and goals</a> of
24Clang in more detail and gives a more broad explanation about what we mean.
25These features are:
26</p>
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]27
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]28<p>End-User Features:</p>
29
30<ul>
Chris Lattnerde9a4f52007-12-13 05:42:27 +0000[diff] [blame]31<li><a href="#performance">Fast compiles and low memory use</a></li>
Chris Lattnercf086ea2007-12-10 08:19:29 +0000[diff] [blame]32<li><a href="#expressivediags">Expressive diagnostics</a></li>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]33<li><a href="#gcccompat">GCC compatibility</a></li>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]34</ul>
35
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]36<p>Utility and Applications:</p>
37
38<ul>
39<li><a href="#libraryarch">Library based architecture</a></li>
40<li><a href="#diverseclients">Support diverse clients</a></li>
41<li><a href="#ideintegration">Integration with IDEs</a></li>
42<li><a href="#license">Use the LLVM 'BSD' License</a></li>
43</ul>
44
45<p>Internal Design and Implementation:</p>
46
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]47<ul>
48<li><a href="#real">A real-world, production quality compiler</a></li>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]49<li><a href="#simplecode">A simple and hackable code base</a></li>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]50<li><a href="#unifiedparser">A single unified parser for C, Objective C, C++,
51 and Objective C++</a></li>
52<li><a href="#conformance">Conformance with C/C++/ObjC and their
53 variants</a></li>
54</ul>
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]55
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]56<!--*************************************************************************-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]57<h2><a name="enduser">End-User Features</a></h2>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]58<!--*************************************************************************-->
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]59
60
61<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]62<h3><a name="performance">Fast compiles and Low Memory Use</a></h3>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]63<!--=======================================================================-->
64
65<p>A major focus of our work on clang is to make it fast, light and scalable.
66The library-based architecture of clang makes it straight-forward to time and
67profile the cost of each layer of the stack, and the driver has a number of
68options for performance analysis.</p>
69
70<p>While there is still much that can be done, we find that the clang front-end
71is significantly quicker than gcc and uses less memory For example, when
72compiling "Carbon.h" on Mac OS/X, we see that clang is 2.5x faster than GCC:</p>
73
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]74<img class="img_slide" src="feature-compile1.png" width="400" height="300"
75 alt="Time to parse carbon.h: -fsyntax-only">
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]76
77<p>Carbon.h is a monster: it transitively includes 558 files, 12.3M of code,
78declares 10000 functions, has 2000 struct definitions, 8000 fields, 20000 enum
79constants, etc (see slide 25+ of the <a href="clang_video-07-25-2007.html">clang
80talk</a> for more information). It is also #include'd into almost every C file
81in a GUI app on the Mac, so its compile time is very important.</p>
82
83<p>From the slide above, you can see that we can measure the time to preprocess
84the file independently from the time to parse it, and independently from the
85time to build the ASTs for the code. GCC doesn't provide a way to measure the
86parser without AST building (it only provides -fsyntax-only). In our
87measurements, we find that clang's preprocessor is consistently 40% faster than
88GCCs, and the parser + AST builder is ~4x faster than GCC's. If you have
89sources that do not depend as heavily on the preprocessor (or if you
90use Precompiled Headers) you may see a much bigger speedup from clang.
91</p>
92
93<p>Compile time performance is important, but when using clang as an API, often
94memory use is even moreso: the less memory the code takes the more code you can
95fit into memory at a time (useful for whole program analysis tools, for
96example).</p>
97
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]98<img class="img_slide" src="feature-memory1.png" width="400" height="300"
99 alt="Space">
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]100
101<p>Here we see a huge advantage of clang: its ASTs take <b>5x less memory</b>
102than GCC's syntax trees, despite the fact that clang's ASTs capture far more
103source-level information than GCC's trees do. This feat is accomplished through
104the use of carefully designed APIs and efficient representations.</p>
105
106<p>In addition to being efficient when pitted head-to-head against GCC in batch
107mode, clang is built with a <a href="#libraryarch">library based
108architecture</a> that makes it relatively easy to adapt it and build new tools
109with it. This means that it is often possible to apply out-of-the-box thinking
110and novel techniques to improve compilation in various ways.</p>
111
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]112<img class="img_slide" src="feature-compile2.png" width="400" height="300"
113 alt="Preprocessor Speeds: GCC 4.2 vs clang-all">
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]114
115<p>This slide shows how the clang preprocessor can be used to make "distcc"
116parallelization <b>3x</b> more scalable than when using the GCC preprocessor.
117"distcc" quickly bottlenecks on the preprocessor running on the central driver
118machine, so a fast preprocessor is very useful. Comparing the first two bars
119of each group shows how a ~40% faster preprocessor can reduce preprocessing time
120of these large C++ apps by about 40% (shocking!).</p>
121
122<p>The third bar on the slide is the interesting part: it shows how trivial
123caching of file system accesses across invocations of the preprocessor allows
124clang to reduce time spent in the kernel by 10x, making distcc over 3x more
125scalable. This is obviously just one simple hack, doing more interesting things
126(like caching tokens across preprocessed files) would yield another substantial
127speedup.</p>
128
129<p>The clean framework-based design of clang means that many things are possible
130that would be very difficult in other systems, for example incremental
131compilation, multithreading, intelligent caching, etc. We are only starting
132to tap the full potential of the clang design.</p>
133
134
135<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]136<h3><a name="expressivediags">Expressive Diagnostics</a></h3>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]137<!--=======================================================================-->
138
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame]139<p>In addition to being fast and functional, we aim to make Clang extremely user
140friendly. As far as a command-line compiler goes, this basically boils down to
141making the diagnostics (error and warning messages) generated by the compiler
Chris Lattner9ef36922009-03-19 22:03:42 +0000[diff] [blame]142be as useful as possible. There are several ways that we do this, but the
143most important are pinpointing exactly what is wrong in the program,
Chris Lattner202a7422009-03-19 18:56:04 +0000[diff] [blame]144highlighting related information so that it is easy to understand at a glance,
145and making the wording as clear as possible.</p>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]146
Chris Lattner202a7422009-03-19 18:56:04 +0000[diff] [blame]147<p>Here is one simple example that illustrates the difference between a typical
148GCC and Clang diagnostic:</p>
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame]149
150<pre>
151 $ <b>gcc-4.2 -fsyntax-only t.c</b>
152 t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
153 $ <b>clang -fsyntax-only t.c</b>
154 t.c:7:39: error: invalid operands to binary expression ('int' and 'struct A')
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]155 <span style="color:darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span>
156 <span style="color:blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</span>
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame]157</pre>
158
159<p>Here you can see that you don't even need to see the original source code to
160understand what is wrong based on the Clang error: Because clang prints a
161caret, you know exactly <em>which</em> plus it is complaining about. The range
162information highlights the left and right side of the plus which makes it
163immediately obvious what the compiler is talking about, which is very useful for
Chris Lattner9ef36922009-03-19 22:03:42 +0000[diff] [blame]164cases involving precedence issues and many other situations.</p>
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame]165
Chris Lattner9ef36922009-03-19 22:03:42 +0000[diff] [blame]166<p>Clang diagnostics are very polished and have many features. For more
Chris Lattner202a7422009-03-19 18:56:04 +0000[diff] [blame]167information and examples, please see the <a href="diagnostics.html">Expressive
168Diagnostics</a> page.</p>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]169
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]170<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]171<h3><a name="gcccompat">GCC Compatibility</a></h3>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]172<!--=======================================================================-->
173
174<p>GCC is currently the defacto-standard open source compiler today, and it
175routinely compiles a huge volume of code. GCC supports a huge number of
176extensions and features (many of which are undocumented) and a lot of
177code and header files depend on these features in order to build.</p>
178
179<p>While it would be nice to be able to ignore these extensions and focus on
180implementing the language standards to the letter, pragmatics force us to
181support the GCC extensions that see the most use. Many users just want their
182code to compile, they don't care to argue about whether it is pedantically C99
183or not.</p>
184
185<p>As mentioned above, all
186extensions are explicitly recognized as such and marked with extension
187diagnostics, which can be mapped to warnings, errors, or just ignored.
188</p>
189
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]190
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]191<!--*************************************************************************-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]192<h2><a name="applications">Utility and Applications</a></h2>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]193<!--*************************************************************************-->
194
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]195<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]196<h3><a name="libraryarch">Library Based Architecture</a></h3>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]197<!--=======================================================================-->
198
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]199<p>A major design concept for clang is its use of a library-based
200architecture. In this design, various parts of the front-end can be cleanly
201divided into separate libraries which can then be mixed up for different needs
202and uses. In addition, the library-based approach encourages good interfaces
203and makes it easier for new developers to get involved (because they only need
204to understand small pieces of the big picture).</p>
205
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]206<blockquote><p>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]207"The world needs better compiler tools, tools which are built as libraries.
208This design point allows reuse of the tools in new and novel ways. However,
209building the tools as libraries isn't enough: they must have clean APIs, be as
210decoupled from each other as possible, and be easy to modify/extend. This
211requires clean layering, decent design, and keeping the libraries independent of
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]212any specific client."</p></blockquote>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]213
214<p>
215Currently, clang is divided into the following libraries and tool:
216</p>
217
218<ul>
219<li><b>libsupport</b> - Basic support library, from LLVM.</li>
220<li><b>libsystem</b> - System abstraction library, from LLVM.</li>
221<li><b>libbasic</b> - Diagnostics, SourceLocations, SourceBuffer abstraction,
222 file system caching for input source files.</li>
223<li><b>libast</b> - Provides classes to represent the C AST, the C type system,
224 builtin functions, and various helpers for analyzing and manipulating the
225 AST (visitors, pretty printers, etc).</li>
226<li><b>liblex</b> - Lexing and preprocessing, identifier hash table, pragma
227 handling, tokens, and macro expansion.</li>
228<li><b>libparse</b> - Parsing. This library invokes coarse-grained 'Actions'
229 provided by the client (e.g. libsema builds ASTs) but knows nothing about
230 ASTs or other client-specific data structures.</li>
231<li><b>libsema</b> - Semantic Analysis. This provides a set of parser actions
232 to build a standardized AST for programs.</li>
233<li><b>libcodegen</b> - Lower the AST to LLVM IR for optimization &amp; code
234 generation.</li>
235<li><b>librewrite</b> - Editing of text buffers (important for code rewriting
236 transformation, like refactoring).</li>
237<li><b>libanalysis</b> - Static analysis support.</li>
238<li><b>clang</b> - A driver program, client of the libraries at various
239 levels.</li>
240</ul>
241
242<p>As an example of the power of this library based design.... If you wanted to
243build a preprocessor, you would take the Basic and Lexer libraries. If you want
244an indexer, you would take the previous two and add the Parser library and
245some actions for indexing. If you want a refactoring, static analysis, or
246source-to-source compiler tool, you would then add the AST building and
247semantic analyzer libraries.</p>
248
249<p>For more information about the low-level implementation details of the
250various clang libraries, please see the <a href="docs/InternalsManual.html">
251clang Internals Manual</a>.</p>
252
253<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]254<h3><a name="diverseclients">Support Diverse Clients</a></h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]255<!--=======================================================================-->
256
257<p>Clang is designed and built with many grand plans for how we can use it. The
258driving force is the fact that we use C and C++ daily, and have to suffer due to
259a lack of good tools available for it. We believe that the C and C++ tools
260ecosystem has been significantly limited by how difficult it is to parse and
261represent the source code for these languages, and we aim to rectify this
262problem in clang.</p>
263
264<p>The problem with this goal is that different clients have very different
265requirements. Consider code generation, for example: a simple front-end that
266parses for code generation must analyze the code for validity and emit code
267in some intermediate form to pass off to a optimizer or backend. Because
268validity analysis and code generation can largely be done on the fly, there is
269not hard requirement that the front-end actually build up a full AST for all
270the expressions and statements in the code. TCC and GCC are examples of
271compilers that either build no real AST (in the former case) or build a stripped
272down and simplified AST (in the later case) because they focus primarily on
273codegen.</p>
274
275<p>On the opposite side of the spectrum, some clients (like refactoring) want
276highly detailed information about the original source code and want a complete
277AST to describe it with. Refactoring wants to have information about macro
278expansions, the location of every paren expression '(((x)))' vs 'x', full
279position information, and much more. Further, refactoring wants to look
280<em>across the whole program</em> to ensure that it is making transformations
281that are safe. Making this efficient and getting this right requires a
282significant amount of engineering and algorithmic work that simply are
283unnecessary for a simple static compiler.</p>
284
285<p>The beauty of the clang approach is that it does not restrict how you use it.
286In particular, it is possible to use the clang preprocessor and parser to build
287an extremely quick and light-weight on-the-fly code generator (similar to TCC)
288that does not build an AST at all. As an intermediate step, clang supports
289using the current AST generation and semantic analysis code and having a code
290generation client free the AST for each function after code generation. Finally,
291clang provides support for building and retaining fully-fledged ASTs, and even
292supports writing them out to disk.</p>
293
294<p>Designing the libraries with clean and simple APIs allows these high-level
295policy decisions to be determined in the client, instead of forcing "one true
296way" in the implementation of any of these libraries. Getting this right is
297hard, and we don't always get it right the first time, but we fix any problems
298when we realize we made a mistake.</p>
299
300<!--=======================================================================-->
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]301<h3 id="ideintegration">Integration with IDEs</h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]302<!--=======================================================================-->
303
304<p>
305We believe that Integrated Development Environments (IDE's) are a great way
306to pull together various pieces of the development puzzle, and aim to make clang
307work well in such an environment. The chief advantage of an IDE is that they
308typically have visibility across your entire project and are long-lived
309processes, whereas stand-alone compiler tools are typically invoked on each
310individual file in the project, and thus have limited scope.</p>
311
312<p>There are many implications of this difference, but a significant one has to
313do with efficiency and caching: sharing an address space across different files
314in a project, means that you can use intelligent caching and other techniques to
315dramatically reduce analysis/compilation time.</p>
316
317<p>A further difference between IDEs and batch compiler is that they often
318impose very different requirements on the front-end: they depend on high
319performance in order to provide a "snappy" experience, and thus really want
320techniques like "incremental compilation", "fuzzy parsing", etc. Finally, IDEs
321often have very different requirements than code generation, often requiring
322information that a codegen-only frontend can throw away. Clang is
323specifically designed and built to capture this information.
324</p>
325
326
327<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]328<h3><a name="license">Use the LLVM 'BSD' License</a></h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]329<!--=======================================================================-->
330
Chris Lattner81921cc2010-11-14 07:30:46 +0000[diff] [blame]331<p>We actively intend for clang (and LLVM as a whole) to be used for
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]332commercial projects, and the BSD license is the simplest way to allow this. We
333feel that the license encourages contributors to pick up the source and work
334with it, and believe that those individuals and organizations will contribute
335back their work if they do not want to have to maintain a fork forever (which is
336time consuming and expensive when merges are involved). Further, nobody makes
337money on compilers these days, but many people need them to get bigger goals
338accomplished: it makes sense for everyone to work together.</p>
339
340<p>For more information about the LLVM/clang license, please see the <a
341href="http://llvm.org/docs/DeveloperPolicy.html#license">LLVM License
342Description</a> for more information.</p>
343
344
345
346<!--*************************************************************************-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]347<h2><a name="design">Internal Design and Implementation</a></h2>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]348<!--*************************************************************************-->
349
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]350<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]351<h3><a name="real">A real-world, production quality compiler</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]352<!--=======================================================================-->
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]353
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]354<p>
Chris Lattnercddb2af2007-12-10 18:56:37 +0000[diff] [blame]355Clang is designed and built by experienced compiler developers who
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]356are increasingly frustrated with the problems that <a
357href="comparison.html">existing open source compilers</a> have. Clang is
358carefully and thoughtfully designed and built to provide the foundation of a
359whole new generation of C/C++/Objective C development tools, and we intend for
Chris Lattnercddb2af2007-12-10 18:56:37 +0000[diff] [blame]360it to be production quality.</p>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]361
362<p>Being a production quality compiler means many things: it means being high
363performance, being solid and (relatively) bug free, and it means eventually
364being used and depended on by a broad range of people. While we are still in
365the early development stages, we strongly believe that this will become a
366reality.</p>
367
368<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]369<h3><a name="simplecode">A simple and hackable code base</a></h3>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]370<!--=======================================================================-->
371
372<p>Our goal is to make it possible for anyone with a basic understanding
373of compilers and working knowledge of the C/C++/ObjC languages to understand and
374extend the clang source base. A large part of this falls out of our decision to
375make the AST mirror the languages as closely as possible: you have your friendly
376if statement, for statement, parenthesis expression, structs, unions, etc, all
377represented in a simple and explicit way.</p>
378
379<p>In addition to a simple design, we work to make the source base approachable
380by commenting it well, including citations of the language standards where
381appropriate, and designing the code for simplicity. Beyond that, clang offers
382a set of AST dumpers, printers, and visualizers that make it easy to put code in
383and see how it is represented.</p>
384
385<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]386<h3><a name="unifiedparser">A single unified parser for C, Objective C, C++,
387and Objective C++</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]388<!--=======================================================================-->
389
390<p>Clang is the "C Language Family Front-end", which means we intend to support
391the most popular members of the C family. We are convinced that the right
392parsing technology for this class of languages is a hand-built recursive-descent
393parser. Because it is plain C++ code, recursive descent makes it very easy for
394new developers to understand the code, it easily supports ad-hoc rules and other
395strange hacks required by C/C++, and makes it straight-forward to implement
396excellent diagnostics and error recovery.</p>
397
398<p>We believe that implementing C/C++/ObjC in a single unified parser makes the
399end result easier to maintain and evolve than maintaining a separate C and C++
400parser which must be bugfixed and maintained independently of each other.</p>
401
402<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]403<h3><a name="conformance">Conformance with C/C++/ObjC and their
404 variants</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]405<!--=======================================================================-->
406
407<p>When you start work on implementing a language, you find out that there is a
408huge gap between how the language works and how most people understand it to
409work. This gap is the difference between a normal programmer and a (scary?
410super-natural?) "language lawyer", who knows the ins and outs of the language
411and can grok standardese with ease.</p>
412
413<p>In practice, being conformant with the languages means that we aim to support
414the full language, including the dark and dusty corners (like trigraphs,
415preprocessor arcana, C99 VLAs, etc). Where we support extensions above and
416beyond what the standard officially allows, we make an effort to explicitly call
417this out in the code and emit warnings about it (which are disabled by default,
418but can optionally be mapped to either warnings or errors), allowing you to use
419clang in "strict" mode if you desire.</p>
420
421<p>We also intend to support "dialects" of these languages, such as C89, K&amp;R
422C, C++'03, Objective-C 2, etc.</p>
423
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]424</div>
425</body>
Chris Lattnerbafc68f2007-10-06 05:48:57 +0000[diff] [blame]426</html>