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Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]6 <title>Clang - Features and Goals</title>
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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
74<img class="img_slide" src="feature-compile1.png" width="400" height="300" />
75
76<p>Carbon.h is a monster: it transitively includes 558 files, 12.3M of code,
77declares 10000 functions, has 2000 struct definitions, 8000 fields, 20000 enum
78constants, etc (see slide 25+ of the <a href="clang_video-07-25-2007.html">clang
79talk</a> for more information). It is also #include'd into almost every C file
80in a GUI app on the Mac, so its compile time is very important.</p>
81
82<p>From the slide above, you can see that we can measure the time to preprocess
83the file independently from the time to parse it, and independently from the
84time to build the ASTs for the code. GCC doesn't provide a way to measure the
85parser without AST building (it only provides -fsyntax-only). In our
86measurements, we find that clang's preprocessor is consistently 40% faster than
87GCCs, and the parser + AST builder is ~4x faster than GCC's. If you have
88sources that do not depend as heavily on the preprocessor (or if you
89use Precompiled Headers) you may see a much bigger speedup from clang.
90</p>
91
92<p>Compile time performance is important, but when using clang as an API, often
93memory use is even moreso: the less memory the code takes the more code you can
94fit into memory at a time (useful for whole program analysis tools, for
95example).</p>
96
97<img class="img_slide" src="feature-memory1.png" width="400" height="300" />
98
99<p>Here we see a huge advantage of clang: its ASTs take <b>5x less memory</b>
100than GCC's syntax trees, despite the fact that clang's ASTs capture far more
101source-level information than GCC's trees do. This feat is accomplished through
102the use of carefully designed APIs and efficient representations.</p>
103
104<p>In addition to being efficient when pitted head-to-head against GCC in batch
105mode, clang is built with a <a href="#libraryarch">library based
106architecture</a> that makes it relatively easy to adapt it and build new tools
107with it. This means that it is often possible to apply out-of-the-box thinking
108and novel techniques to improve compilation in various ways.</p>
109
110<img class="img_slide" src="feature-compile2.png" width="400" height="300" />
111
112<p>This slide shows how the clang preprocessor can be used to make "distcc"
113parallelization <b>3x</b> more scalable than when using the GCC preprocessor.
114"distcc" quickly bottlenecks on the preprocessor running on the central driver
115machine, so a fast preprocessor is very useful. Comparing the first two bars
116of each group shows how a ~40% faster preprocessor can reduce preprocessing time
117of these large C++ apps by about 40% (shocking!).</p>
118
119<p>The third bar on the slide is the interesting part: it shows how trivial
120caching of file system accesses across invocations of the preprocessor allows
121clang to reduce time spent in the kernel by 10x, making distcc over 3x more
122scalable. This is obviously just one simple hack, doing more interesting things
123(like caching tokens across preprocessed files) would yield another substantial
124speedup.</p>
125
126<p>The clean framework-based design of clang means that many things are possible
127that would be very difficult in other systems, for example incremental
128compilation, multithreading, intelligent caching, etc. We are only starting
129to tap the full potential of the clang design.</p>
130
131
132<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]133<h3><a name="expressivediags">Expressive Diagnostics</a></h3>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]134<!--=======================================================================-->
135
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame]136<p>In addition to being fast and functional, we aim to make Clang extremely user
137friendly. As far as a command-line compiler goes, this basically boils down to
138making the diagnostics (error and warning messages) generated by the compiler
Chris Lattner9ef36922009-03-19 22:03:42 +0000[diff] [blame]139be as useful as possible. There are several ways that we do this, but the
140most important are pinpointing exactly what is wrong in the program,
Chris Lattner202a7422009-03-19 18:56:04 +0000[diff] [blame]141highlighting related information so that it is easy to understand at a glance,
142and making the wording as clear as possible.</p>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]143
Chris Lattner202a7422009-03-19 18:56:04 +0000[diff] [blame]144<p>Here is one simple example that illustrates the difference between a typical
145GCC and Clang diagnostic:</p>
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame]146
147<pre>
148 $ <b>gcc-4.2 -fsyntax-only t.c</b>
149 t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
150 $ <b>clang -fsyntax-only t.c</b>
151 t.c:7:39: error: invalid operands to binary expression ('int' and 'struct A')
152 <font color="darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</font>
153 <font color="blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</font>
154</pre>
155
156<p>Here you can see that you don't even need to see the original source code to
157understand what is wrong based on the Clang error: Because clang prints a
158caret, you know exactly <em>which</em> plus it is complaining about. The range
159information highlights the left and right side of the plus which makes it
160immediately obvious what the compiler is talking about, which is very useful for
Chris Lattner9ef36922009-03-19 22:03:42 +0000[diff] [blame]161cases involving precedence issues and many other situations.</p>
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame]162
Chris Lattner9ef36922009-03-19 22:03:42 +0000[diff] [blame]163<p>Clang diagnostics are very polished and have many features. For more
Chris Lattner202a7422009-03-19 18:56:04 +0000[diff] [blame]164information and examples, please see the <a href="diagnostics.html">Expressive
165Diagnostics</a> page.</p>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]166
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]167<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]168<h3><a name="gcccompat">GCC Compatibility</a></h3>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]169<!--=======================================================================-->
170
171<p>GCC is currently the defacto-standard open source compiler today, and it
172routinely compiles a huge volume of code. GCC supports a huge number of
173extensions and features (many of which are undocumented) and a lot of
174code and header files depend on these features in order to build.</p>
175
176<p>While it would be nice to be able to ignore these extensions and focus on
177implementing the language standards to the letter, pragmatics force us to
178support the GCC extensions that see the most use. Many users just want their
179code to compile, they don't care to argue about whether it is pedantically C99
180or not.</p>
181
182<p>As mentioned above, all
183extensions are explicitly recognized as such and marked with extension
184diagnostics, which can be mapped to warnings, errors, or just ignored.
185</p>
186
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]187
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]188<!--*************************************************************************-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]189<h2><a name="applications">Utility and Applications</a></h2>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]190<!--*************************************************************************-->
191
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]192<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]193<h3><a name="libraryarch">Library Based Architecture</a></h3>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]194<!--=======================================================================-->
195
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]196<p>A major design concept for clang is its use of a library-based
197architecture. In this design, various parts of the front-end can be cleanly
198divided into separate libraries which can then be mixed up for different needs
199and uses. In addition, the library-based approach encourages good interfaces
200and makes it easier for new developers to get involved (because they only need
201to understand small pieces of the big picture).</p>
202
203<blockquote>
204"The world needs better compiler tools, tools which are built as libraries.
205This design point allows reuse of the tools in new and novel ways. However,
206building the tools as libraries isn't enough: they must have clean APIs, be as
207decoupled from each other as possible, and be easy to modify/extend. This
208requires clean layering, decent design, and keeping the libraries independent of
209any specific client."</blockquote>
210
211<p>
212Currently, clang is divided into the following libraries and tool:
213</p>
214
215<ul>
216<li><b>libsupport</b> - Basic support library, from LLVM.</li>
217<li><b>libsystem</b> - System abstraction library, from LLVM.</li>
218<li><b>libbasic</b> - Diagnostics, SourceLocations, SourceBuffer abstraction,
219 file system caching for input source files.</li>
220<li><b>libast</b> - Provides classes to represent the C AST, the C type system,
221 builtin functions, and various helpers for analyzing and manipulating the
222 AST (visitors, pretty printers, etc).</li>
223<li><b>liblex</b> - Lexing and preprocessing, identifier hash table, pragma
224 handling, tokens, and macro expansion.</li>
225<li><b>libparse</b> - Parsing. This library invokes coarse-grained 'Actions'
226 provided by the client (e.g. libsema builds ASTs) but knows nothing about
227 ASTs or other client-specific data structures.</li>
228<li><b>libsema</b> - Semantic Analysis. This provides a set of parser actions
229 to build a standardized AST for programs.</li>
230<li><b>libcodegen</b> - Lower the AST to LLVM IR for optimization &amp; code
231 generation.</li>
232<li><b>librewrite</b> - Editing of text buffers (important for code rewriting
233 transformation, like refactoring).</li>
234<li><b>libanalysis</b> - Static analysis support.</li>
Chris Lattner909a8832009-07-10 05:07:59 +0000[diff] [blame]235<li><b><a href="docs/libIndex.html">libindex</a></b> - Cross-translation-unit infrastructure and indexing support.</li>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]236<li><b>clang</b> - A driver program, client of the libraries at various
237 levels.</li>
238</ul>
239
240<p>As an example of the power of this library based design.... If you wanted to
241build a preprocessor, you would take the Basic and Lexer libraries. If you want
242an indexer, you would take the previous two and add the Parser library and
243some actions for indexing. If you want a refactoring, static analysis, or
244source-to-source compiler tool, you would then add the AST building and
245semantic analyzer libraries.</p>
246
247<p>For more information about the low-level implementation details of the
248various clang libraries, please see the <a href="docs/InternalsManual.html">
249clang Internals Manual</a>.</p>
250
251<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]252<h3><a name="diverseclients">Support Diverse Clients</a></h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]253<!--=======================================================================-->
254
255<p>Clang is designed and built with many grand plans for how we can use it. The
256driving force is the fact that we use C and C++ daily, and have to suffer due to
257a lack of good tools available for it. We believe that the C and C++ tools
258ecosystem has been significantly limited by how difficult it is to parse and
259represent the source code for these languages, and we aim to rectify this
260problem in clang.</p>
261
262<p>The problem with this goal is that different clients have very different
263requirements. Consider code generation, for example: a simple front-end that
264parses for code generation must analyze the code for validity and emit code
265in some intermediate form to pass off to a optimizer or backend. Because
266validity analysis and code generation can largely be done on the fly, there is
267not hard requirement that the front-end actually build up a full AST for all
268the expressions and statements in the code. TCC and GCC are examples of
269compilers that either build no real AST (in the former case) or build a stripped
270down and simplified AST (in the later case) because they focus primarily on
271codegen.</p>
272
273<p>On the opposite side of the spectrum, some clients (like refactoring) want
274highly detailed information about the original source code and want a complete
275AST to describe it with. Refactoring wants to have information about macro
276expansions, the location of every paren expression '(((x)))' vs 'x', full
277position information, and much more. Further, refactoring wants to look
278<em>across the whole program</em> to ensure that it is making transformations
279that are safe. Making this efficient and getting this right requires a
280significant amount of engineering and algorithmic work that simply are
281unnecessary for a simple static compiler.</p>
282
283<p>The beauty of the clang approach is that it does not restrict how you use it.
284In particular, it is possible to use the clang preprocessor and parser to build
285an extremely quick and light-weight on-the-fly code generator (similar to TCC)
286that does not build an AST at all. As an intermediate step, clang supports
287using the current AST generation and semantic analysis code and having a code
288generation client free the AST for each function after code generation. Finally,
289clang provides support for building and retaining fully-fledged ASTs, and even
290supports writing them out to disk.</p>
291
292<p>Designing the libraries with clean and simple APIs allows these high-level
293policy decisions to be determined in the client, instead of forcing "one true
294way" in the implementation of any of these libraries. Getting this right is
295hard, and we don't always get it right the first time, but we fix any problems
296when we realize we made a mistake.</p>
297
298<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]299<h3><a name="ideintegration">Integration with IDEs</h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]300<!--=======================================================================-->
301
302<p>
303We believe that Integrated Development Environments (IDE's) are a great way
304to pull together various pieces of the development puzzle, and aim to make clang
305work well in such an environment. The chief advantage of an IDE is that they
306typically have visibility across your entire project and are long-lived
307processes, whereas stand-alone compiler tools are typically invoked on each
308individual file in the project, and thus have limited scope.</p>
309
310<p>There are many implications of this difference, but a significant one has to
311do with efficiency and caching: sharing an address space across different files
312in a project, means that you can use intelligent caching and other techniques to
313dramatically reduce analysis/compilation time.</p>
314
315<p>A further difference between IDEs and batch compiler is that they often
316impose very different requirements on the front-end: they depend on high
317performance in order to provide a "snappy" experience, and thus really want
318techniques like "incremental compilation", "fuzzy parsing", etc. Finally, IDEs
319often have very different requirements than code generation, often requiring
320information that a codegen-only frontend can throw away. Clang is
321specifically designed and built to capture this information.
322</p>
323
324
325<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]326<h3><a name="license">Use the LLVM 'BSD' License</a></h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]327<!--=======================================================================-->
328
329<p>We actively indend for clang (and a LLVM as a whole) to be used for
330commercial projects, and the BSD license is the simplest way to allow this. We
331feel that the license encourages contributors to pick up the source and work
332with it, and believe that those individuals and organizations will contribute
333back their work if they do not want to have to maintain a fork forever (which is
334time consuming and expensive when merges are involved). Further, nobody makes
335money on compilers these days, but many people need them to get bigger goals
336accomplished: it makes sense for everyone to work together.</p>
337
338<p>For more information about the LLVM/clang license, please see the <a
339href="http://llvm.org/docs/DeveloperPolicy.html#license">LLVM License
340Description</a> for more information.</p>
341
342
343
344<!--*************************************************************************-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]345<h2><a name="design">Internal Design and Implementation</a></h2>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]346<!--*************************************************************************-->
347
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]348<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]349<h3><a name="real">A real-world, production quality compiler</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]350<!--=======================================================================-->
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]351
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]352<p>
Chris Lattnercddb2af2007-12-10 18:56:37 +0000[diff] [blame]353Clang is designed and built by experienced compiler developers who
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]354are increasingly frustrated with the problems that <a
355href="comparison.html">existing open source compilers</a> have. Clang is
356carefully and thoughtfully designed and built to provide the foundation of a
357whole new generation of C/C++/Objective C development tools, and we intend for
Chris Lattnercddb2af2007-12-10 18:56:37 +0000[diff] [blame]358it to be production quality.</p>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]359
360<p>Being a production quality compiler means many things: it means being high
361performance, being solid and (relatively) bug free, and it means eventually
362being used and depended on by a broad range of people. While we are still in
363the early development stages, we strongly believe that this will become a
364reality.</p>
365
366<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]367<h3><a name="simplecode">A simple and hackable code base</a></h3>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]368<!--=======================================================================-->
369
370<p>Our goal is to make it possible for anyone with a basic understanding
371of compilers and working knowledge of the C/C++/ObjC languages to understand and
372extend the clang source base. A large part of this falls out of our decision to
373make the AST mirror the languages as closely as possible: you have your friendly
374if statement, for statement, parenthesis expression, structs, unions, etc, all
375represented in a simple and explicit way.</p>
376
377<p>In addition to a simple design, we work to make the source base approachable
378by commenting it well, including citations of the language standards where
379appropriate, and designing the code for simplicity. Beyond that, clang offers
380a set of AST dumpers, printers, and visualizers that make it easy to put code in
381and see how it is represented.</p>
382
383<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]384<h3><a name="unifiedparser">A single unified parser for C, Objective C, C++,
385and Objective C++</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]386<!--=======================================================================-->
387
388<p>Clang is the "C Language Family Front-end", which means we intend to support
389the most popular members of the C family. We are convinced that the right
390parsing technology for this class of languages is a hand-built recursive-descent
391parser. Because it is plain C++ code, recursive descent makes it very easy for
392new developers to understand the code, it easily supports ad-hoc rules and other
393strange hacks required by C/C++, and makes it straight-forward to implement
394excellent diagnostics and error recovery.</p>
395
396<p>We believe that implementing C/C++/ObjC in a single unified parser makes the
397end result easier to maintain and evolve than maintaining a separate C and C++
398parser which must be bugfixed and maintained independently of each other.</p>
399
400<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]401<h3><a name="conformance">Conformance with C/C++/ObjC and their
402 variants</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]403<!--=======================================================================-->
404
405<p>When you start work on implementing a language, you find out that there is a
406huge gap between how the language works and how most people understand it to
407work. This gap is the difference between a normal programmer and a (scary?
408super-natural?) "language lawyer", who knows the ins and outs of the language
409and can grok standardese with ease.</p>
410
411<p>In practice, being conformant with the languages means that we aim to support
412the full language, including the dark and dusty corners (like trigraphs,
413preprocessor arcana, C99 VLAs, etc). Where we support extensions above and
414beyond what the standard officially allows, we make an effort to explicitly call
415this out in the code and emit warnings about it (which are disabled by default,
416but can optionally be mapped to either warnings or errors), allowing you to use
417clang in "strict" mode if you desire.</p>
418
419<p>We also intend to support "dialects" of these languages, such as C89, K&amp;R
420C, C++'03, Objective-C 2, etc.</p>
421
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]422</div>
423</body>
Chris Lattnerbafc68f2007-10-06 05:48:57 +0000[diff] [blame]424</html>