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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
139be as useful as possible. There are several ways that we do this. This section
140talks about the experience provided by the command line compiler, contrasting
141Clang output to GCC 4.2's output in several examples.
142<!--
143Other clients
144that embed Clang and extract equivalent information through internal APIs.-->
145</p>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]146
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame^]147<h4>Column Numbers and Caret Diagnostics</h4>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]148
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame^]149<p>First, all diagnostics produced by clang include full column number
150information, and use this to print "caret diagnostics". This is a feature
151provided by many commercial compilers, but is generally missing from open source
152compilers. This is nice because it makes it very easy to understand exactly
153what is wrong in a particular piece of code, an example is:</p>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]154
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame^]155<pre>
156 $ <b>gcc-4.2 -fsyntax-only -Wformat format-strings.c</b>
157 format-strings.c:91: warning: too few arguments for format
158 $ <b>clang -fsyntax-only format-strings.c</b>
159 format-strings.c:91:13: warning: '.*' specified field precision is missing a matching 'int' argument
160 <font color="darkgreen"> printf("%.*d");</font>
161 <font color="blue"> ^</font>
162</pre>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]163
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame^]164<p>The caret (the blue "^" character) exactly shows where the problem is, even
165inside of the string. This makes it really easy to jump to the problem and
166helps when multiple instances of the same character occur on a line. We'll
167revisit this more in following examples.</p>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]168
Chris Lattner13cc2352009-03-19 06:52:51 +0000[diff] [blame^]169<h4>Range Highlighting for Related Text</h4>
170
171<p>Clang captures and accurately tracks range information for expressions,
172statements, and other constructs in your program and uses this to make
173diagnostics highlight related information. For example, here's a somewhat
174nonsensical example to illustrate this:</p>
175
176<pre>
177 $ <b>gcc-4.2 -fsyntax-only t.c</b>
178 t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
179 $ <b>clang -fsyntax-only t.c</b>
180 t.c:7:39: error: invalid operands to binary expression ('int' and 'struct A')
181 <font color="darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</font>
182 <font color="blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</font>
183</pre>
184
185<p>Here you can see that you don't even need to see the original source code to
186understand what is wrong based on the Clang error: Because clang prints a
187caret, you know exactly <em>which</em> plus it is complaining about. The range
188information highlights the left and right side of the plus which makes it
189immediately obvious what the compiler is talking about, which is very useful for
190cases involving precedence issues and many other cases.</p>
191
192<h4>Precision in Wording</h4>
193
194<p>A detail is that we have tried really hard to make the diagnostics that come
195out of clang contain exactly the pertinent information about what is wrong and
196why. In the example above, we tell you what the inferred types are for
197the left and right hand sides, and we don't repeat what is obvious from the
198caret (that this is a "binary +"). Many other examples abound, here is a simple
199one:</p>
200
201<pre>
202 $ <b>gcc-4.2 -fsyntax-only t.c</b>
203 t.c:5: error: invalid type argument of 'unary *'
204 $ <b>clang -fsyntax-only t.c</b>
205 t.c:5:11: error: indirection requires pointer operand ('int' invalid)
206 <font color="darkgreen"> int y = *SomeA.X;</font>
207 <font color="blue"> ^~~~~~~~</font>
208</pre>
209
210<p>In this example, not only do we tell you that there is a problem with the *
211and point to it, we say exactly why and tell you what the type is (in case it is
212a complicated subexpression, such as a call to an overloaded function). This
213sort of attention to detail makes it much easier to understand and fix problems
214quickly.</p>
215
216<h4>No Pretty Printing of Expressions in Diagnostics</h4>
217
218<p>Since Clang has range highlighting, it never needs to pretty print your code
219back out to you. This is particularly bad in G++ (which often emits errors
220containing lowered vtable references), but even GCC can produce
221inscrutible error messages in some cases when it tries to do this. In this
222example P and Q have type "int*":</p>
223
224<pre>
225 $ <b>gcc-4.2 -fsyntax-only t.c</b>
226 #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function
227 $ <b>clang -fsyntax-only t.c</b>
228 t.c:12:8: error: called object type 'int' is not a function or function pointer
229 <font color="darkgreen"> (P-Q)();</font>
230 <font color="blue"> ~~~~~^</font>
231</pre>
232
233
234<h4>Typedef Preservation and Selective Unwrapping</h4>
235
236<p>Many programmers use high-level user defined types, typedefs, and other
237syntactic sugar to refer to types in their program. This is useful because they
238can abbreviate otherwise very long types and it is useful to preserve the
239typename in diagnostics. However, sometimes very simple typedefs can wrap
240trivial types and it is important to strip off the typedef to understand what
241is going on. Clang aims to handle both cases well.<p>
242
243<p>For example, here is an example that shows where it is important to preserve
244a typedef in C:</p>
245
246<pre>
247 $ <b>gcc-4.2 -fsyntax-only t.c</b>
248 t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *')
249 $ <b>clang -fsyntax-only t.c</b>
250 t.c:15:11: error: can't convert between vector values of different size ('__m128' and 'int const *')
251 <font color="darkgreen"> myvec[1]/P;</font>
252 <font color="blue"> ~~~~~~~~^~</font>
253</pre>
254
255<p>Here the type printed by GCC isn't even valid, but if the error were about a
256very long and complicated type (as often happens in C++) the error message would
257be ugly just because it was long and hard to read. Here's an example where it
258is useful for the compiler to expose underlying details of a typedef:</p>
259
260<pre>
261 $ <b>gcc-4.2 -fsyntax-only t.c</b>
262 t.c:13: error: request for member 'x' in something not a structure or union
263 $ <b>clang -fsyntax-only t.c</b>
264 t.c:13:9: error: member reference base type 'pid_t' (aka 'int') is not a structure or union
265 <font color="darkgreen"> myvar = myvar.x;</font>
266 <font color="blue"> ~~~~~ ^</font>
267</pre>
268
269<p>If the user was somehow confused about how the system "pid_t" typedef is
270defined, Clang helpfully displays it with "aka".</p>
271
272<h4>Automatic Macro Expansion</h4>
273
274<p>Many errors happen in macros that are sometimes deeply nested. With
275traditional compilers, you need to dig deep into the definition of the macro to
276understand how you got into trouble. Here's a simple example that shows how
277Clang helps you out:</p>
278
279<pre>
280 $ <b>gcc-4.2 -fsyntax-only t.c</b>
281 t.c: In function 'test':
282 t.c:80: error: invalid operands to binary &lt; (have 'struct mystruct' and 'float')
283 $ <b>clang -fsyntax-only t.c</b>
284 t.c:80:3: error: invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))
285 <font color="darkgreen"> X = MYMAX(P, F);</font>
286 <font color="blue"> ^~~~~~~~~~~</font>
287 t.c:76:94: note: instantiated from:
288 <font color="darkgreen">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a &lt; __b ? __b : __a; })</font>
289 <font color="blue"> ~~~ ^ ~~~</font>
290</pre>
291
292<p>This shows how clang automatically prints instantiation information and
293nested range information for diagnostics as they are instantiated through macros
294and also shows how some of the other pieces work in a bigger example. Here's
295another real world warning that occurs in the "window" Unix package (which
296implements the "wwopen" class of APIs):</p>
297
298<pre>
299 $ <b>clang -fsyntax-only t.c</b>
300 t.c:22:2: warning: type specifier missing, defaults to 'int'
301 <font color="darkgreen"> ILPAD();</font>
302 <font color="blue"> ^</font>
303 t.c:17:17: note: instantiated from:
304 <font color="darkgreen">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</font>
305 <font color="blue"> ^</font>
306 t.c:14:2: note: instantiated from:
307 <font color="darkgreen"> register i; \</font>
308 <font color="blue"> ^</font>
309</pre>
310
311<p>In practice, we've found that this is actually more useful in multiply nested
312macros that in simple ones.</p>
313
314
315<h4>C++ Fun Examples</h4>
316
317<p>...</p>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]318
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]319<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]320<h3><a name="gcccompat">GCC Compatibility</a></h3>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]321<!--=======================================================================-->
322
323<p>GCC is currently the defacto-standard open source compiler today, and it
324routinely compiles a huge volume of code. GCC supports a huge number of
325extensions and features (many of which are undocumented) and a lot of
326code and header files depend on these features in order to build.</p>
327
328<p>While it would be nice to be able to ignore these extensions and focus on
329implementing the language standards to the letter, pragmatics force us to
330support the GCC extensions that see the most use. Many users just want their
331code to compile, they don't care to argue about whether it is pedantically C99
332or not.</p>
333
334<p>As mentioned above, all
335extensions are explicitly recognized as such and marked with extension
336diagnostics, which can be mapped to warnings, errors, or just ignored.
337</p>
338
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]339
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]340<!--*************************************************************************-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]341<h2><a name="applications">Utility and Applications</a></h2>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]342<!--*************************************************************************-->
343
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]344<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]345<h3><a name="libraryarch">Library Based Architecture</a></h3>
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]346<!--=======================================================================-->
347
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]348<p>A major design concept for clang is its use of a library-based
349architecture. In this design, various parts of the front-end can be cleanly
350divided into separate libraries which can then be mixed up for different needs
351and uses. In addition, the library-based approach encourages good interfaces
352and makes it easier for new developers to get involved (because they only need
353to understand small pieces of the big picture).</p>
354
355<blockquote>
356"The world needs better compiler tools, tools which are built as libraries.
357This design point allows reuse of the tools in new and novel ways. However,
358building the tools as libraries isn't enough: they must have clean APIs, be as
359decoupled from each other as possible, and be easy to modify/extend. This
360requires clean layering, decent design, and keeping the libraries independent of
361any specific client."</blockquote>
362
363<p>
364Currently, clang is divided into the following libraries and tool:
365</p>
366
367<ul>
368<li><b>libsupport</b> - Basic support library, from LLVM.</li>
369<li><b>libsystem</b> - System abstraction library, from LLVM.</li>
370<li><b>libbasic</b> - Diagnostics, SourceLocations, SourceBuffer abstraction,
371 file system caching for input source files.</li>
372<li><b>libast</b> - Provides classes to represent the C AST, the C type system,
373 builtin functions, and various helpers for analyzing and manipulating the
374 AST (visitors, pretty printers, etc).</li>
375<li><b>liblex</b> - Lexing and preprocessing, identifier hash table, pragma
376 handling, tokens, and macro expansion.</li>
377<li><b>libparse</b> - Parsing. This library invokes coarse-grained 'Actions'
378 provided by the client (e.g. libsema builds ASTs) but knows nothing about
379 ASTs or other client-specific data structures.</li>
380<li><b>libsema</b> - Semantic Analysis. This provides a set of parser actions
381 to build a standardized AST for programs.</li>
382<li><b>libcodegen</b> - Lower the AST to LLVM IR for optimization &amp; code
383 generation.</li>
384<li><b>librewrite</b> - Editing of text buffers (important for code rewriting
385 transformation, like refactoring).</li>
386<li><b>libanalysis</b> - Static analysis support.</li>
387<li><b>clang</b> - A driver program, client of the libraries at various
388 levels.</li>
389</ul>
390
391<p>As an example of the power of this library based design.... If you wanted to
392build a preprocessor, you would take the Basic and Lexer libraries. If you want
393an indexer, you would take the previous two and add the Parser library and
394some actions for indexing. If you want a refactoring, static analysis, or
395source-to-source compiler tool, you would then add the AST building and
396semantic analyzer libraries.</p>
397
398<p>For more information about the low-level implementation details of the
399various clang libraries, please see the <a href="docs/InternalsManual.html">
400clang Internals Manual</a>.</p>
401
402<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]403<h3><a name="diverseclients">Support Diverse Clients</a></h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]404<!--=======================================================================-->
405
406<p>Clang is designed and built with many grand plans for how we can use it. The
407driving force is the fact that we use C and C++ daily, and have to suffer due to
408a lack of good tools available for it. We believe that the C and C++ tools
409ecosystem has been significantly limited by how difficult it is to parse and
410represent the source code for these languages, and we aim to rectify this
411problem in clang.</p>
412
413<p>The problem with this goal is that different clients have very different
414requirements. Consider code generation, for example: a simple front-end that
415parses for code generation must analyze the code for validity and emit code
416in some intermediate form to pass off to a optimizer or backend. Because
417validity analysis and code generation can largely be done on the fly, there is
418not hard requirement that the front-end actually build up a full AST for all
419the expressions and statements in the code. TCC and GCC are examples of
420compilers that either build no real AST (in the former case) or build a stripped
421down and simplified AST (in the later case) because they focus primarily on
422codegen.</p>
423
424<p>On the opposite side of the spectrum, some clients (like refactoring) want
425highly detailed information about the original source code and want a complete
426AST to describe it with. Refactoring wants to have information about macro
427expansions, the location of every paren expression '(((x)))' vs 'x', full
428position information, and much more. Further, refactoring wants to look
429<em>across the whole program</em> to ensure that it is making transformations
430that are safe. Making this efficient and getting this right requires a
431significant amount of engineering and algorithmic work that simply are
432unnecessary for a simple static compiler.</p>
433
434<p>The beauty of the clang approach is that it does not restrict how you use it.
435In particular, it is possible to use the clang preprocessor and parser to build
436an extremely quick and light-weight on-the-fly code generator (similar to TCC)
437that does not build an AST at all. As an intermediate step, clang supports
438using the current AST generation and semantic analysis code and having a code
439generation client free the AST for each function after code generation. Finally,
440clang provides support for building and retaining fully-fledged ASTs, and even
441supports writing them out to disk.</p>
442
443<p>Designing the libraries with clean and simple APIs allows these high-level
444policy decisions to be determined in the client, instead of forcing "one true
445way" in the implementation of any of these libraries. Getting this right is
446hard, and we don't always get it right the first time, but we fix any problems
447when we realize we made a mistake.</p>
448
449<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]450<h3><a name="ideintegration">Integration with IDEs</h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]451<!--=======================================================================-->
452
453<p>
454We believe that Integrated Development Environments (IDE's) are a great way
455to pull together various pieces of the development puzzle, and aim to make clang
456work well in such an environment. The chief advantage of an IDE is that they
457typically have visibility across your entire project and are long-lived
458processes, whereas stand-alone compiler tools are typically invoked on each
459individual file in the project, and thus have limited scope.</p>
460
461<p>There are many implications of this difference, but a significant one has to
462do with efficiency and caching: sharing an address space across different files
463in a project, means that you can use intelligent caching and other techniques to
464dramatically reduce analysis/compilation time.</p>
465
466<p>A further difference between IDEs and batch compiler is that they often
467impose very different requirements on the front-end: they depend on high
468performance in order to provide a "snappy" experience, and thus really want
469techniques like "incremental compilation", "fuzzy parsing", etc. Finally, IDEs
470often have very different requirements than code generation, often requiring
471information that a codegen-only frontend can throw away. Clang is
472specifically designed and built to capture this information.
473</p>
474
475
476<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]477<h3><a name="license">Use the LLVM 'BSD' License</a></h3>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]478<!--=======================================================================-->
479
480<p>We actively indend for clang (and a LLVM as a whole) to be used for
481commercial projects, and the BSD license is the simplest way to allow this. We
482feel that the license encourages contributors to pick up the source and work
483with it, and believe that those individuals and organizations will contribute
484back their work if they do not want to have to maintain a fork forever (which is
485time consuming and expensive when merges are involved). Further, nobody makes
486money on compilers these days, but many people need them to get bigger goals
487accomplished: it makes sense for everyone to work together.</p>
488
489<p>For more information about the LLVM/clang license, please see the <a
490href="http://llvm.org/docs/DeveloperPolicy.html#license">LLVM License
491Description</a> for more information.</p>
492
493
494
495<!--*************************************************************************-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]496<h2><a name="design">Internal Design and Implementation</a></h2>
Chris Lattneread27db2007-12-10 08:12:49 +0000[diff] [blame]497<!--*************************************************************************-->
498
Chris Lattner1a380a02007-12-10 07:14:08 +0000[diff] [blame]499<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]500<h3><a name="real">A real-world, production quality compiler</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]501<!--=======================================================================-->
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]502
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]503<p>
Chris Lattnercddb2af2007-12-10 18:56:37 +0000[diff] [blame]504Clang is designed and built by experienced compiler developers who
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]505are increasingly frustrated with the problems that <a
506href="comparison.html">existing open source compilers</a> have. Clang is
507carefully and thoughtfully designed and built to provide the foundation of a
508whole new generation of C/C++/Objective C development tools, and we intend for
Chris Lattnercddb2af2007-12-10 18:56:37 +0000[diff] [blame]509it to be production quality.</p>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]510
511<p>Being a production quality compiler means many things: it means being high
512performance, being solid and (relatively) bug free, and it means eventually
513being used and depended on by a broad range of people. While we are still in
514the early development stages, we strongly believe that this will become a
515reality.</p>
516
517<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]518<h3><a name="simplecode">A simple and hackable code base</a></h3>
Chris Lattnerb5604af2007-12-10 07:23:52 +0000[diff] [blame]519<!--=======================================================================-->
520
521<p>Our goal is to make it possible for anyone with a basic understanding
522of compilers and working knowledge of the C/C++/ObjC languages to understand and
523extend the clang source base. A large part of this falls out of our decision to
524make the AST mirror the languages as closely as possible: you have your friendly
525if statement, for statement, parenthesis expression, structs, unions, etc, all
526represented in a simple and explicit way.</p>
527
528<p>In addition to a simple design, we work to make the source base approachable
529by commenting it well, including citations of the language standards where
530appropriate, and designing the code for simplicity. Beyond that, clang offers
531a set of AST dumpers, printers, and visualizers that make it easy to put code in
532and see how it is represented.</p>
533
534<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]535<h3><a name="unifiedparser">A single unified parser for C, Objective C, C++,
536and Objective C++</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]537<!--=======================================================================-->
538
539<p>Clang is the "C Language Family Front-end", which means we intend to support
540the most popular members of the C family. We are convinced that the right
541parsing technology for this class of languages is a hand-built recursive-descent
542parser. Because it is plain C++ code, recursive descent makes it very easy for
543new developers to understand the code, it easily supports ad-hoc rules and other
544strange hacks required by C/C++, and makes it straight-forward to implement
545excellent diagnostics and error recovery.</p>
546
547<p>We believe that implementing C/C++/ObjC in a single unified parser makes the
548end result easier to maintain and evolve than maintaining a separate C and C++
549parser which must be bugfixed and maintained independently of each other.</p>
550
551<!--=======================================================================-->
Ted Kremenek3b61b152008-06-17 06:35:36 +0000[diff] [blame]552<h3><a name="conformance">Conformance with C/C++/ObjC and their
553 variants</a></h3>
Chris Lattner6908f302007-12-10 05:52:05 +0000[diff] [blame]554<!--=======================================================================-->
555
556<p>When you start work on implementing a language, you find out that there is a
557huge gap between how the language works and how most people understand it to
558work. This gap is the difference between a normal programmer and a (scary?
559super-natural?) "language lawyer", who knows the ins and outs of the language
560and can grok standardese with ease.</p>
561
562<p>In practice, being conformant with the languages means that we aim to support
563the full language, including the dark and dusty corners (like trigraphs,
564preprocessor arcana, C99 VLAs, etc). Where we support extensions above and
565beyond what the standard officially allows, we make an effort to explicitly call
566this out in the code and emit warnings about it (which are disabled by default,
567but can optionally be mapped to either warnings or errors), allowing you to use
568clang in "strict" mode if you desire.</p>
569
570<p>We also intend to support "dialects" of these languages, such as C89, K&amp;R
571C, C++'03, Objective-C 2, etc.</p>
572
Chris Lattner7a274392007-10-06 05:23:00 +0000[diff] [blame]573</div>
574</body>
Chris Lattnerbafc68f2007-10-06 05:48:57 +0000[diff] [blame]575</html>