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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>
235<li><b>clang</b> - A driver program, client of the libraries at various
236 levels.</li>
237</ul>
238
239<p>As an example of the power of this library based design.... If you wanted to
240build a preprocessor, you would take the Basic and Lexer libraries. If you want
241an indexer, you would take the previous two and add the Parser library and
242some actions for indexing. If you want a refactoring, static analysis, or
243source-to-source compiler tool, you would then add the AST building and
244semantic analyzer libraries.</p>
245
246<p>For more information about the low-level implementation details of the
247various clang libraries, please see the <a href="docs/InternalsManual.html">
248clang Internals Manual</a>.</p>
249
250<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]251<h3><a name="diverseclients">Support Diverse Clients</a></h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]252<!--=======================================================================-->
253
254<p>Clang is designed and built with many grand plans for how we can use it. The
255driving force is the fact that we use C and C++ daily, and have to suffer due to
256a lack of good tools available for it. We believe that the C and C++ tools
257ecosystem has been significantly limited by how difficult it is to parse and
258represent the source code for these languages, and we aim to rectify this
259problem in clang.</p>
260
261<p>The problem with this goal is that different clients have very different
262requirements. Consider code generation, for example: a simple front-end that
263parses for code generation must analyze the code for validity and emit code
264in some intermediate form to pass off to a optimizer or backend. Because
265validity analysis and code generation can largely be done on the fly, there is
266not hard requirement that the front-end actually build up a full AST for all
267the expressions and statements in the code. TCC and GCC are examples of
268compilers that either build no real AST (in the former case) or build a stripped
269down and simplified AST (in the later case) because they focus primarily on
270codegen.</p>
271
272<p>On the opposite side of the spectrum, some clients (like refactoring) want
273highly detailed information about the original source code and want a complete
274AST to describe it with. Refactoring wants to have information about macro
275expansions, the location of every paren expression '(((x)))' vs 'x', full
276position information, and much more. Further, refactoring wants to look
277<em>across the whole program</em> to ensure that it is making transformations
278that are safe. Making this efficient and getting this right requires a
279significant amount of engineering and algorithmic work that simply are
280unnecessary for a simple static compiler.</p>
281
282<p>The beauty of the clang approach is that it does not restrict how you use it.
283In particular, it is possible to use the clang preprocessor and parser to build
284an extremely quick and light-weight on-the-fly code generator (similar to TCC)
285that does not build an AST at all. As an intermediate step, clang supports
286using the current AST generation and semantic analysis code and having a code
287generation client free the AST for each function after code generation. Finally,
288clang provides support for building and retaining fully-fledged ASTs, and even
289supports writing them out to disk.</p>
290
291<p>Designing the libraries with clean and simple APIs allows these high-level
292policy decisions to be determined in the client, instead of forcing "one true
293way" in the implementation of any of these libraries. Getting this right is
294hard, and we don't always get it right the first time, but we fix any problems
295when we realize we made a mistake.</p>
296
297<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]298<h3><a name="ideintegration">Integration with IDEs</h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]299<!--=======================================================================-->
300
301<p>
302We believe that Integrated Development Environments (IDE's) are a great way
303to pull together various pieces of the development puzzle, and aim to make clang
304work well in such an environment. The chief advantage of an IDE is that they
305typically have visibility across your entire project and are long-lived
306processes, whereas stand-alone compiler tools are typically invoked on each
307individual file in the project, and thus have limited scope.</p>
308
309<p>There are many implications of this difference, but a significant one has to
310do with efficiency and caching: sharing an address space across different files
311in a project, means that you can use intelligent caching and other techniques to
312dramatically reduce analysis/compilation time.</p>
313
314<p>A further difference between IDEs and batch compiler is that they often
315impose very different requirements on the front-end: they depend on high
316performance in order to provide a "snappy" experience, and thus really want
317techniques like "incremental compilation", "fuzzy parsing", etc. Finally, IDEs
318often have very different requirements than code generation, often requiring
319information that a codegen-only frontend can throw away. Clang is
320specifically designed and built to capture this information.
321</p>
322
323
324<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]325<h3><a name="license">Use the LLVM 'BSD' License</a></h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]326<!--=======================================================================-->
327
328<p>We actively indend for clang (and a LLVM as a whole) to be used for
329commercial projects, and the BSD license is the simplest way to allow this. We
330feel that the license encourages contributors to pick up the source and work
331with it, and believe that those individuals and organizations will contribute
332back their work if they do not want to have to maintain a fork forever (which is
333time consuming and expensive when merges are involved). Further, nobody makes
334money on compilers these days, but many people need them to get bigger goals
335accomplished: it makes sense for everyone to work together.</p>
336
337<p>For more information about the LLVM/clang license, please see the <a
338href="http://llvm.org/docs/DeveloperPolicy.html#license">LLVM License
339Description</a> for more information.</p>
340
341
342
343<!--*************************************************************************-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]344<h2><a name="design">Internal Design and Implementation</a></h2>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]345<!--*************************************************************************-->
346
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]347<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]348<h3><a name="real">A real-world, production quality compiler</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]349<!--=======================================================================-->
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]350
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]351<p>
Chris Lattnercddb2af2007-12-10 18:56:37 +0000[diff] [blame]352Clang is designed and built by experienced compiler developers who
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]353are increasingly frustrated with the problems that <a
354href="comparison.html">existing open source compilers</a> have. Clang is
355carefully and thoughtfully designed and built to provide the foundation of a
356whole new generation of C/C++/Objective C development tools, and we intend for
Chris Lattnercddb2af2007-12-10 18:56:37 +0000[diff] [blame]357it to be production quality.</p>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]358
359<p>Being a production quality compiler means many things: it means being high
360performance, being solid and (relatively) bug free, and it means eventually
361being used and depended on by a broad range of people. While we are still in
362the early development stages, we strongly believe that this will become a
363reality.</p>
364
365<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]366<h3><a name="simplecode">A simple and hackable code base</a></h3>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]367<!--=======================================================================-->
368
369<p>Our goal is to make it possible for anyone with a basic understanding
370of compilers and working knowledge of the C/C++/ObjC languages to understand and
371extend the clang source base. A large part of this falls out of our decision to
372make the AST mirror the languages as closely as possible: you have your friendly
373if statement, for statement, parenthesis expression, structs, unions, etc, all
374represented in a simple and explicit way.</p>
375
376<p>In addition to a simple design, we work to make the source base approachable
377by commenting it well, including citations of the language standards where
378appropriate, and designing the code for simplicity. Beyond that, clang offers
379a set of AST dumpers, printers, and visualizers that make it easy to put code in
380and see how it is represented.</p>
381
382<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]383<h3><a name="unifiedparser">A single unified parser for C, Objective C, C++,
384and Objective C++</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]385<!--=======================================================================-->
386
387<p>Clang is the "C Language Family Front-end", which means we intend to support
388the most popular members of the C family. We are convinced that the right
389parsing technology for this class of languages is a hand-built recursive-descent
390parser. Because it is plain C++ code, recursive descent makes it very easy for
391new developers to understand the code, it easily supports ad-hoc rules and other
392strange hacks required by C/C++, and makes it straight-forward to implement
393excellent diagnostics and error recovery.</p>
394
395<p>We believe that implementing C/C++/ObjC in a single unified parser makes the
396end result easier to maintain and evolve than maintaining a separate C and C++
397parser which must be bugfixed and maintained independently of each other.</p>
398
399<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]400<h3><a name="conformance">Conformance with C/C++/ObjC and their
401 variants</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]402<!--=======================================================================-->
403
404<p>When you start work on implementing a language, you find out that there is a
405huge gap between how the language works and how most people understand it to
406work. This gap is the difference between a normal programmer and a (scary?
407super-natural?) "language lawyer", who knows the ins and outs of the language
408and can grok standardese with ease.</p>
409
410<p>In practice, being conformant with the languages means that we aim to support
411the full language, including the dark and dusty corners (like trigraphs,
412preprocessor arcana, C99 VLAs, etc). Where we support extensions above and
413beyond what the standard officially allows, we make an effort to explicitly call
414this out in the code and emit warnings about it (which are disabled by default,
415but can optionally be mapped to either warnings or errors), allowing you to use
416clang in "strict" mode if you desire.</p>
417
418<p>We also intend to support "dialects" of these languages, such as C89, K&amp;R
419C, C++'03, Objective-C 2, etc.</p>
420
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422</body>
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