<html>
<head>
<link rel="stylesheet" href="llvm.css" type="text/css">
- <title>A Few Coding Standards</title>
+ <title>LLVM Coding Standards</title>
</head>
<body>
<div class="doc_title">
- A Few Coding Standards
+ LLVM Coding Standards
</div>
<ol>
<li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
<li><a href="#hl_privateheaders">Keep "internal" Headers
Private</a></li>
+ <li><a href="#ll_iostream"><tt>#include <iostream></tt> is
+ <em>forbidden</em></a></li>
</ol></li>
<li><a href="#micro">The Low Level Issues</a>
<ol>
- <li><a href="#hl_assert">Assert Liberally</a></li>
- <li><a href="#hl_ns_std">Do not use 'using namespace std'</a></li>
- <li><a href="#hl_preincrement">Prefer Preincrement</a></li>
- <li><a href="#hl_avoidendl">Avoid <tt>std::endl</tt></a></li>
+ <li><a href="#ll_assert">Assert Liberally</a></li>
+ <li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
+ <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
+ classes in headers</a></li>
+ <li><a href="#ll_preincrement">Prefer Preincrement</a></li>
+ <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
</ol></li>
</ol></li>
<li><a href="#seealso">See Also</a></li>
</ol>
<div class="doc_author">
- <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+ <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
+ <a href="mailto:void@nondot.org">Bill Wendling</a></p>
</div>
<b>File Headers</b>
-<p>Every source file should have a header on it that
-describes the basic purpose of the file. If a file does not have a header, it
-should not be checked into CVS. Most source trees will probably have a standard
+<p>Every source file should have a header on it that describes the basic
+purpose of the file. If a file does not have a header, it should not be
+checked into Subversion. Most source trees will probably have a standard
file header format. The standard format for the LLVM source tree looks like
this:</p>
<div class="doc_code">
<pre>
//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
-//
+//
// The LLVM Compiler Infrastructure
//
-// This file was developed by the LLVM research group and is distributed under
-// the University of Illinois Open Source License. See LICENSE.TXT for details.
-//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
//===----------------------------------------------------------------------===//
//
// This file contains the declaration of the Instruction class, which is the
file. This is important when printing out code and flipping though lots of
pages.</p>
-<p>The next section in the file is a concise note that defines the license that
-the file is released under. This makes it perfectly clear what terms the source
-code can be distributed under.</p>
+<p>The next section in the file is a concise note that defines the license
+that the file is released under. This makes it perfectly clear what terms the
+source code can be distributed under and should not be modified in any way.</p>
<p>The main body of the description does not have to be very long in most cases.
Here it's only two lines. If an algorithm is being implemented or something
<b>Class overviews</b>
-<p>Classes are one fundemental part of a good object oriented design. As such,
+<p>Classes are one fundamental part of a good object oriented design. As such,
a class definition should have a comment block that explains what the class is
used for... if it's not obvious. If it's so completely obvious your grandma
could figure it out, it's probably safe to leave it out. Naming classes
<li>System <tt>#includes</tt></li>
</ol>
-<p>... and each catagory should be sorted by name.</p>
+<p>... and each category should be sorted by name.</p>
<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
which implement an interface defined by a .h file. This <tt>#include</tt>
like to print out code and look at your code in an xterm without resizing
it.</p>
+<p>The longer answer is that there must be some limit to the width of the code
+in order to reasonably allow developers to have multiple files side-by-side in
+windows on a modest display. If you are going to pick a width limit, it is
+somewhat arbitrary but you might as well pick something standard. Going with
+90 columns (for example) instead of 80 columns wouldn't add any significant
+value and would be detrimental to printing out code. Also many other projects
+have standardized on 80 columns, so some people have already configured their
+editors for it (vs something else, like 90 columns).</p>
+
+<p>This is one of many contentious issues in coding standards, but is not up
+for debate.</p>
+
</div>
<!-- _______________________________________________________________________ -->
<p>In practice, this means that you shouldn't assume much about the host
compiler, including its support for "high tech" features like partial
-specialization of templates. In fact, Visual C++ 6 could be an important target
-for our work in the future, and we don't want to have to rewrite all of our code
-to support it.</p>
+specialization of templates. If these features are used, they should only be
+an implementation detail of a library which has a simple exposed API.</p>
</div>
<tt>#include</tt>'ing speeds up compilation.</p>
<p>It is easy to try to go too overboard on this recommendation, however. You
-<b>must</b> include all of the header files that you are using, either directly
+<b>must</b> include all of the header files that you are using -- you can
+include them either directly
or indirectly (through another header file). To make sure that you don't
accidently forget to include a header file in your module header, make sure to
include your module header <b>first</b> in the implementation file (as mentioned
</div>
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
+</div>
+
+<div class="doc_text">
+
+<p>The use of <tt>#include <iostream></tt> in library files is
+hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
+support clients using LLVM libraries as part of larger systems. In particular,
+we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
+the static c'tors are run whenever an application start up that uses the dynamic
+library. There are two problems with this:</p>
+
+<ol>
+ <li>The time to run the static c'tors impacts startup time of
+ applications—a critical time for GUI apps.</li>
+ <li>The static c'tors cause the app to pull many extra pages of memory off the
+ disk: both the code for the static c'tors in each <tt>.o</tt> file and the
+ small amount of data that gets touched. In addition, touched/dirty pages
+ put more pressure on the VM system on low-memory machines.</li>
+</ol>
+
+<p>Note that using the other stream headers (<tt><sstream></tt> for
+example) is allowed normally, it is just <tt><iostream></tt> that is
+causing problems.</p>
+
+<p>The preferred replacement for stream functionality is the
+<tt>llvm::raw_ostream</tt> class (for writing to output streams of various
+sorts) and the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
+
+</div>
+
+
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="micro">The Low Level Issues</a>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_assert">Assert Liberally</a>
+ <a name="ll_assert">Assert Liberally</a>
</div>
<div class="doc_text">
<p>You get the idea...</p>
+<p>Please be aware when adding assert statements that not all compilers are aware of
+the semantics of the assert. In some places, asserts are used to indicate a piece of
+code that should not be reached. These are typically of the form:</p>
+
+<div class="doc_code">
+<pre>
+assert(0 && "Some helpful error message");
+</pre>
+</div>
+
+<p>When used in a function that returns a value, they should be followed with a return
+statement and a comment indicating that this line is never reached. This will prevent
+a compiler which is unable to deduce that the assert statement never returns from
+generating a warning.</p>
+
+<div class="doc_code">
+<pre>
+assert(0 && "Some helpful error message");
+// Not reached
+return 0;
+</pre>
+</div>
+
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_ns_std">Do not use 'using namespace std'</a>
+ <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
</div>
<div class="doc_text">
<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
-namespace with an "std::" prefix, rather than rely on "using namespace std;".
-</p>
-
-<p> In header files, adding a 'using namespace XXX' directive pollutes the
-namespace of any source file that includes the header. This is clearly a bad
-thing.</p>
-
-<p>In implementation files (e.g. .cpp files) the rule is more of a stylistic
-rule, but is still important. Basically, using explicit namespace prefixes
-makes
-the code <b>more clear</b> - because it is immediately obvious what facilities
-are being used and where they are coming from - and <b>more portable</b> -
-because namespace clashes cannot occur between LLVM code and other namespaces.
-The portability rule is important because different standard library
-implementations expose different symbols (potentially ones they shouldn't) and
-future revisions to the C++ standard will add more symbols to the std
-namespace. As such, we never 'using namespace std;' in LLVM.</p>
-
-<p>The exception to the general rule (i.e. it's not an exception for the std
-namespace) is for implementation files. For example, all of the code in the
-LLVM project implements code that lives in the 'llvm' namespace. As such, it
-is ok, and actually more clear, for the .cpp files to have a 'using namespace
-llvm' directive at their top, after the #includes. The general form of this
-rule is that any .cpp file that implements code in any namespace may use that
-namespace (and its parents), but should not use any others.</p>
+namespace with an "<tt>std::</tt>" prefix, rather than rely on
+"<tt>using namespace std;</tt>".</p>
+
+<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
+the namespace of any source file that <tt>#include</tt>s the header. This is
+clearly a bad thing.</p>
+
+<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
+rule, but is still important. Basically, using explicit namespace prefixes
+makes the code <b>clearer</b>, because it is immediately obvious what facilities
+are being used and where they are coming from, and <b>more portable</b>, because
+namespace clashes cannot occur between LLVM code and other namespaces. The
+portability rule is important because different standard library implementations
+expose different symbols (potentially ones they shouldn't), and future revisions
+to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
+such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
+
+<p>The exception to the general rule (i.e. it's not an exception for
+the <tt>std</tt> namespace) is for implementation files. For example, all of
+the code in the LLVM project implements code that lives in the 'llvm' namespace.
+As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
+namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
+general form of this rule is that any .cpp file that implements code in any
+namespace may use that namespace (and its parents'), but should not use any
+others.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_preincrement">Prefer Preincrement</a>
+ <a name="ll_virtual_anch">Provide a virtual method anchor for classes
+ in headers</a>
+</div>
+
+<div class="doc_text">
+
+<p>If a class is defined in a header file and has a v-table (either it has
+virtual methods or it derives from classes with virtual methods), it must
+always have at least one out-of-line virtual method in the class. Without
+this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
+that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
+increasing link times.</p>
+
+</div>
+
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="ll_preincrement">Prefer Preincrement</a>
</div>
<div class="doc_text">
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_avoidendl">Avoid <tt>std::endl</tt></a>
+ <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
</div>
<div class="doc_text">
</div>
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="seealso">See Also</a>
<ol>
-<li><a href="http://www.aw-bc.com/catalog/academic/product/0,1144,0201310155,00.html">Effective
-C++</a> by Scott Meyers. There is an online version of the book (only some
-chapters though) <a
-href="http://www.awlonline.com/cseng/meyerscddemo/">available as well</a>. Also
+<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
+C++</a> by Scott Meyers. Also
interesting and useful are "More Effective C++" and "Effective STL" by the same
author.</li>
-<li><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++
-Software Design</a> by John Lakos</li>
+<li>Large-Scale C++ Software Design by John Lakos</li>
</ol>
<hr>
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+ <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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