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finish the features description. 

llvm-svn: 44787
This commit is contained in:
Chris Lattner 2007-12-10 08:12:49 +00:00
parent ba61806ef1
commit e396aaae28
2 changed files with 174 additions and 45 deletions
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clang/www/content.css
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@ -17,11 +17,6 @@ table,tr,td {
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@ -15,7 +15,10 @@
<div id="content">
<!--*************************************************************************-->
<h1>Clang - Features and Goals</h1>
<!--*************************************************************************-->
<p>
This page describes the <a href="index.html#goals">features and goals</a> of
Clang in more detail and gives a more broad explanation about what we mean.
@ -25,12 +28,22 @@ These features are:
<p>End-User Features:</p>
<ul>
<li><a href="#performance">High Performance and Low Memory Use</a></li>
<li><a href="#expressivediags">Expressive Diagnostics</a></a></li>
<li><a href="#performance">High performance and low memory use</a></li>
<li><a href="#expressivediags">Expressive diagnostics</a></a></li>
<li><a href="#gcccompat">GCC compatibility</a></li>
</ul>
<p>Driving Goals and Internal Design:</p>
<p>Utility and Applications:</p>
<ul>
<li><a href="#libraryarch">Library based architecture</a></li>
<li><a href="#diverseclients">Support diverse clients</a></li>
<li><a href="#ideintegration">Integration with IDEs</a></li>
<li><a href="#license">Use the LLVM 'BSD' License</a></li>
</ul>
<p>Internal Design and Implementation:</p>
<ul>
<li><a href="#real">A real-world, production quality compiler</a></li>
<li><a href="#simplecode">A simple and hackable code base</a></li>
@ -40,9 +53,9 @@ These features are:
variants</a></li>
</ul>
<!--=======================================================================-->
<!--*************************************************************************-->
<h1>End-User Features</h1>
<!--=======================================================================-->
<!--*************************************************************************-->
<!--=======================================================================-->
@ -169,10 +182,165 @@ diagnostics, which can be mapped to warnings, errors, or just ignored.
</p>
<!--*************************************************************************-->
<h1>Utility and Applications</h1>
<!--*************************************************************************-->
<!--=======================================================================-->
<h1>Driving Goals and Internal Design</h1>
<h2><a name="libraryarch">Library Based Architecture</a></h2>
<!--=======================================================================-->
<p>A major design concept for clang is its use of a library-based
architecture. In this design, various parts of the front-end can be cleanly
divided into separate libraries which can then be mixed up for different needs
and uses. In addition, the library-based approach encourages good interfaces
and makes it easier for new developers to get involved (because they only need
to understand small pieces of the big picture).</p>
<blockquote>
"The world needs better compiler tools, tools which are built as libraries.
This design point allows reuse of the tools in new and novel ways. However,
building the tools as libraries isn't enough: they must have clean APIs, be as
decoupled from each other as possible, and be easy to modify/extend. This
requires clean layering, decent design, and keeping the libraries independent of
any specific client."</blockquote>
<p>
Currently, clang is divided into the following libraries and tool:
</p>
<ul>
<li><b>libsupport</b> - Basic support library, from LLVM.</li>
<li><b>libsystem</b> - System abstraction library, from LLVM.</li>
<li><b>libbasic</b> - Diagnostics, SourceLocations, SourceBuffer abstraction,
file system caching for input source files.</li>
<li><b>libast</b> - Provides classes to represent the C AST, the C type system,
builtin functions, and various helpers for analyzing and manipulating the
AST (visitors, pretty printers, etc).</li>
<li><b>liblex</b> - Lexing and preprocessing, identifier hash table, pragma
handling, tokens, and macro expansion.</li>
<li><b>libparse</b> - Parsing. This library invokes coarse-grained 'Actions'
provided by the client (e.g. libsema builds ASTs) but knows nothing about
ASTs or other client-specific data structures.</li>
<li><b>libsema</b> - Semantic Analysis. This provides a set of parser actions
to build a standardized AST for programs.</li>
<li><b>libcodegen</b> - Lower the AST to LLVM IR for optimization &amp; code
generation.</li>
<li><b>librewrite</b> - Editing of text buffers (important for code rewriting
transformation, like refactoring).</li>
<li><b>libanalysis</b> - Static analysis support.</li>
<li><b>clang</b> - A driver program, client of the libraries at various
levels.</li>
</ul>
<p>As an example of the power of this library based design.... If you wanted to
build a preprocessor, you would take the Basic and Lexer libraries. If you want
an indexer, you would take the previous two and add the Parser library and
some actions for indexing. If you want a refactoring, static analysis, or
source-to-source compiler tool, you would then add the AST building and
semantic analyzer libraries.</p>
<p>For more information about the low-level implementation details of the
various clang libraries, please see the <a href="docs/InternalsManual.html">
clang Internals Manual</a>.</p>
<!--=======================================================================-->
<h2><a name="diverseclients">Support Diverse Clients</a></h2>
<!--=======================================================================-->
<p>Clang is designed and built with many grand plans for how we can use it. The
driving force is the fact that we use C and C++ daily, and have to suffer due to
a lack of good tools available for it. We believe that the C and C++ tools
ecosystem has been significantly limited by how difficult it is to parse and
represent the source code for these languages, and we aim to rectify this
problem in clang.</p>
<p>The problem with this goal is that different clients have very different
requirements. Consider code generation, for example: a simple front-end that
parses for code generation must analyze the code for validity and emit code
in some intermediate form to pass off to a optimizer or backend. Because
validity analysis and code generation can largely be done on the fly, there is
not hard requirement that the front-end actually build up a full AST for all
the expressions and statements in the code. TCC and GCC are examples of
compilers that either build no real AST (in the former case) or build a stripped
down and simplified AST (in the later case) because they focus primarily on
codegen.</p>
<p>On the opposite side of the spectrum, some clients (like refactoring) want
highly detailed information about the original source code and want a complete
AST to describe it with. Refactoring wants to have information about macro
expansions, the location of every paren expression '(((x)))' vs 'x', full
position information, and much more. Further, refactoring wants to look
<em>across the whole program</em> to ensure that it is making transformations
that are safe. Making this efficient and getting this right requires a
significant amount of engineering and algorithmic work that simply are
unnecessary for a simple static compiler.</p>
<p>The beauty of the clang approach is that it does not restrict how you use it.
In particular, it is possible to use the clang preprocessor and parser to build
an extremely quick and light-weight on-the-fly code generator (similar to TCC)
that does not build an AST at all. As an intermediate step, clang supports
using the current AST generation and semantic analysis code and having a code
generation client free the AST for each function after code generation. Finally,
clang provides support for building and retaining fully-fledged ASTs, and even
supports writing them out to disk.</p>
<p>Designing the libraries with clean and simple APIs allows these high-level
policy decisions to be determined in the client, instead of forcing "one true
way" in the implementation of any of these libraries. Getting this right is
hard, and we don't always get it right the first time, but we fix any problems
when we realize we made a mistake.</p>
<!--=======================================================================-->
<h2><a name="ideintegration">Integration with IDEs</h2>
<!--=======================================================================-->
<p>
We believe that Integrated Development Environments (IDE's) are a great way
to pull together various pieces of the development puzzle, and aim to make clang
work well in such an environment. The chief advantage of an IDE is that they
typically have visibility across your entire project and are long-lived
processes, whereas stand-alone compiler tools are typically invoked on each
individual file in the project, and thus have limited scope.</p>
<p>There are many implications of this difference, but a significant one has to
do with efficiency and caching: sharing an address space across different files
in a project, means that you can use intelligent caching and other techniques to
dramatically reduce analysis/compilation time.</p>
<p>A further difference between IDEs and batch compiler is that they often
impose very different requirements on the front-end: they depend on high
performance in order to provide a "snappy" experience, and thus really want
techniques like "incremental compilation", "fuzzy parsing", etc. Finally, IDEs
often have very different requirements than code generation, often requiring
information that a codegen-only frontend can throw away. Clang is
specifically designed and built to capture this information.
</p>
<!--=======================================================================-->
<h2><a name="license">Use the LLVM 'BSD' License</a></h2>
<!--=======================================================================-->
<p>We actively indend for clang (and a LLVM as a whole) to be used for
commercial projects, and the BSD license is the simplest way to allow this. We
feel that the license encourages contributors to pick up the source and work
with it, and believe that those individuals and organizations will contribute
back their work if they do not want to have to maintain a fork forever (which is
time consuming and expensive when merges are involved). Further, nobody makes
money on compilers these days, but many people need them to get bigger goals
accomplished: it makes sense for everyone to work together.</p>
<p>For more information about the LLVM/clang license, please see the <a
href="http://llvm.org/docs/DeveloperPolicy.html#license">LLVM License
Description</a> for more information.</p>
<!--*************************************************************************-->
<h1>Internal Design and Implementation</h1>
<!--*************************************************************************-->
<!--=======================================================================-->
<h2><a name="real">A real-world, production quality compiler</a></h2>
<!--=======================================================================-->
@ -247,40 +415,6 @@ clang in "strict" mode if you desire.</p>
<p>We also intend to support "dialects" of these languages, such as C89, K&amp;R
C, C++'03, Objective-C 2, etc.</p>
<!--=======================================================================-->
<h2><a name="libraryarch">Library based architecture</a></h2>
<!--=======================================================================-->
A major design concept for the LLVM front-end involves using a library based architecture. In this library based architecture, various parts of the front-end can be cleanly divided into separate libraries which can then be mixed up for different needs and uses. In addition, the library based approach makes it much easier for new developers to get involved and extend LLVM to do new and unique things. In the words of Chris,
<blockquote>
"The world needs better compiler tools, tools which are built as libraries.
This design point allows reuse of the tools in new and novel ways. However,
building the tools as libraries isn't enough: they must have clean APIs, be as
decoupled from each other as possible, and be easy to modify/extend. This
requires clean layering, decent design, and keeping the libraries independent of
any specific client."</blockquote>
Currently, the LLVM front-end is divided into the following libraries:
<ul>
<li>libsupport - Basic support library, reused from LLVM.
<li>libsystem - System abstraction library, reused from LLVM.
<li>libbasic - Diagnostics, SourceLocations, SourceBuffer abstraction, file system caching for input source files. <span class="weak_txt">(depends on above libraries)</span>
<li>libast - Provides classes to represent the C AST, the C type system, builtin functions, and various helpers for analyzing and manipulating the AST (visitors, pretty printers, etc). <span class="weak_txt">(depends on above libraries)</span>
<li>liblex - C/C++/ObjC lexing and preprocessing, identifier hash table, pragma handling, tokens, and macros. <span class="weak_txt">(depends on above libraries)</span>
<li>libparse - Parsing and local semantic analysis. This library invokes coarse-grained 'Actions' provided by the client to do stuff (e.g. libsema builds ASTs). <span class="weak_txt">(depends on above libraries)</span>
<li>libsema - Provides a set of parser actions to build a standardized AST for programs. AST's are 'streamed' out a top-level declaration at a time, allowing clients to use decl-at-a-time processing, build up entire translation units, or even build 'whole program' ASTs depending on how they use the APIs. <span class="weak_txt">(depends on libast and libparse)</span>
<li>libcodegen - Lower the AST to LLVM IR for optimization &amp; codegen. <span class="weak_txt">(depends on libast)</span>
<li>librewrite - Editing of text buffers, depends on libast.</li>
<li>libanalysis - Static analysis support, depends on libast.</li>
<li><b>clang</b> - An example driver, client of the libraries at various levels. <span class="weak_txt">(depends on above libraries, and LLVM VMCore)</span>
</ul>
As an example of the power of this library based design.... If you wanted to build a preprocessor, you would take the Basic and Lexer libraries. If you want an indexer, you would take the previous two and add the Parser library and some actions for indexing. If you want a refactoring, static analysis, or source-to-source compiler tool, you would then add the AST building and semantic analyzer libraries.
In the end, LLVM's library based design will provide developers with many more possibilities.
<h2>Better Integration with IDEs</h2>
Another design goal of Clang is to integrate extremely well with IDEs. IDEs often have very different requirements than code generation, often requiring information that a codegen-only frontend can throw away. Clang is specifically designed and built to capture this information.
</div>
</body>
</html>