<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>
<ol>
<li><a href="#scf_commenting">Commenting</a></li>
<li><a href="#scf_commentformat">Comment Formatting</a></li>
- <li><a href="#scf_includes">#include Style</a></li>
+ <li><a href="#scf_includes"><tt>#include</tt> Style</a></li>
<li><a href="#scf_codewidth">Source Code Width</a></li>
<li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>
<li><a href="#scf_indentation">Indent Code Consistently</a></li>
<li><a href="#ci_warningerrors">Treat Compiler Warnings Like
Errors</a></li>
<li><a href="#ci_portable_code">Write Portable Code</a></li>
+ <li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>
</ol></li>
</ol></li>
<li><a href="#styleissues">Style Issues</a>
<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_preincrement">Prefer Preincrement</a></li>
- <li><a href="#hl_avoidendl">Avoid std::endl</a></li>
- <li><a href="#hl_exploitcpp">Exploit C++ to its Fullest</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>
<div class="doc_text">
<p>Comments are one critical part of readability and maintainability. Everyone
-knows they should comment, so should you. :) Although we all should probably
+knows they should comment, so should you. Although we all should probably
comment our code more than we do, there are a few very critical places that
documentation is very useful:</p>
<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
// base class for all of the VM instructions.
//
//===----------------------------------------------------------------------===//
-
</pre>
+</div>
<p>A few things to note about this particular format: The "<tt>-*- C++
-*-</tt>" string on the first line is there to tell Emacs that the source file
-is a C++ file, not a C file (Emacs assumes .h files are C files by default [Note
-that tag this is not necessary in .cpp files]). The name of the file is also on
-the first line, along with a very short description of the purpose of the file.
-This is important when printing out code and flipping though lots of pages.</p>
+is a C++ file, not a C file (Emacs assumes .h files are C files by default).
+Note that this tag is not necessary in .cpp files. The name of the file is also
+on the first line, along with a very short description of the purpose of 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
-something sane goes a long ways towards avoiding writing documentation. :)</p>
+something sane goes a long ways towards avoiding writing documentation.</p>
<b>Method information</b>
<ol>
<li>When writing a C code: Obviously if you are writing C code, use C style
- comments. :)</li>
- <li>When writing a header file that may be #included by a C source file.</li>
+ comments.</li>
+ <li>When writing a header file that may be <tt>#include</tt>d by a C source
+ file.</li>
<li>When writing a source file that is used by a tool that only accepts C
style comments.</li>
</ol>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="scf_includes">#include Style</a>
+ <a name="scf_includes"><tt>#include</tt> Style</a>
</div>
<div class="doc_text">
<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
include guards if working on a header file), the <a
-href="hl_dontinclude">minimal</a> list of #includes required by the file should
-be listed. We prefer these #includes to be listed in this order:</p>
+href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
+file should be listed. We prefer these <tt>#include</tt>s to be listed in this
+order:</p>
<ol>
<li><a href="#mmheader">Main Module header</a></li>
<li><a href="#hl_privateheaders">Local/Private Headers</a></li>
- <li>llvm/*</li>
- <li>llvm/Analysis/*</li>
- <li>llvm/Assembly/*</li>
- <li>llvm/Bytecode/*</li>
- <li>llvm/CodeGen/*</li>
+ <li><tt>llvm/*</tt></li>
+ <li><tt>llvm/Analysis/*</tt></li>
+ <li><tt>llvm/Assembly/*</tt></li>
+ <li><tt>llvm/Bytecode/*</tt></li>
+ <li><tt>llvm/CodeGen/*</tt></li>
<li>...</li>
- <li>Support/*</li>
- <li>Config/*</li>
- <li>System #includes</li>
+ <li><tt>Support/*</tt></li>
+ <li><tt>Config/*</tt></li>
+ <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 #include should always
-be included <b>first</b> regardless of where it lives on the file system. By
-including a header file first in the .cpp files that implement the interfaces,
-we ensure that the header does not have any hidden dependencies which are not
-explicitly #included in the header, but should be. It is also a form of
-documentation in the .cpp file to indicate where the interfaces it implements
-are defined.</p>
+which implement an interface defined by a .h file. This <tt>#include</tt>
+should always be included <b>first</b> regardless of where it lives on the file
+system. By including a header file first in the .cpp files that implement the
+interfaces, we ensure that the header does not have any hidden dependencies
+which are not explicitly #included in the header, but should be. It is also a
+form of documentation in the .cpp file to indicate where the interfaces it
+implements are defined.</p>
</div>
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>
<!-- _______________________________________________________________________ -->
syntax of the code slightly. For example, an warning that annoys me occurs when
I write code like this:</p>
+<div class="doc_code">
<pre>
- if (V = getValue()) {
- ..
- }
+if (V = getValue()) {
+ ...
+}
</pre>
+</div>
<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
operator, and that I probably mistyped it. In most cases, I haven't, and I
really don't want the spurious errors. To fix this particular problem, I
rewrite the code like this:</p>
+<div class="doc_code">
<pre>
- if ((V = getValue())) {
- ..
- }
+if ((V = getValue())) {
+ ...
+}
</pre>
+</div>
<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
be fixed by massaging the code appropriately.</p>
<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>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
+</div>
+<div class="doc_text">
+
+<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
+interchangeably. The only difference is when they are used to declare a class:
+<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
+all members public by default.</p>
+
+<p>Unfortunately, not all compilers follow the rules and some will generate
+different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
+declare the symbol. This can lead to problems at link time.</p>
+
+<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
+<b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_dontinclude">#include as Little as Possible</a>
+ <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
</div>
<div class="doc_text">
have to, especially in header files.</p>
<p>But wait, sometimes you need to have the definition of a class to use it, or
-to inherit from it. In these cases go ahead and #include that header file. Be
-aware however that there are many cases where you don't need to have the full
-definition of a class. If you are using a pointer or reference to a class, you
-don't need the header file. If you are simply returning a class instance from a
-prototyped function or method, you don't need it. In fact, for most cases, you
-simply don't need the definition of a class... and not <tt>#include</tt>'ing
-speeds up compilation.</p>
+to inherit from it. In these cases go ahead and <tt>#include</tt> that header
+file. Be aware however that there are many cases where you don't need to have
+the full definition of a class. If you are using a pointer or reference to a
+class, you don't need the header file. If you are simply returning a class
+instance from a prototyped function or method, you don't need it. In fact, for
+most cases, you simply don't need the definition of a class... and not
+<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
<p>Many modules have a complex implementation that causes them to use more than
one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
internal communication interface (helper classes, extra functions, etc) in the
-public module header file. Don't do this. :)</p>
+public module header file. Don't do this.</p>
<p>If you really need to do something like this, put a private header file in
the same directory as the source files, and include it locally. This ensures
</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">
helps the poor debugging make sense of why an assertion is being made and
enforced, and hopefully what to do about it. Here is one complete example:</p>
+<div class="doc_code">
<pre>
- inline Value *getOperand(unsigned i) {
- assert(i < Operands.size() && "getOperand() out of range!");
- return Operands[i];
- }
+inline Value *getOperand(unsigned i) {
+ assert(i < Operands.size() && "getOperand() out of range!");
+ return Operands[i];
+}
</pre>
+</div>
<p>Here are some examples:</p>
+<div class="doc_code">
<pre>
- assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
+assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
- assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
+assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
- assert(idx < getNumSuccessors() && "Successor # out of range!");
+assert(idx < getNumSuccessors() && "Successor # out of range!");
- assert(V1.getType() == V2.getType() && "Constant types must be identical!");
+assert(V1.getType() == V2.getType() && "Constant types must be identical!");
- assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
+assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
</pre>
+</div>
<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="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 "<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="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="hl_preincrement">Prefer Preincrement</a>
+ <a name="ll_preincrement">Prefer Preincrement</a>
</div>
<div class="doc_text">
-<p>Hard fast rule: Preincrement (++X) may be no slower than postincrement (X++)
-and could very well be a lot faster than it. Use preincrementation whenever
-possible.</p>
+<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
+postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
+preincrementation whenever possible.</p>
<p>The semantics of postincrement include making a copy of the value being
incremented, returning it, and then preincrementing the "work value". For
</div>
-
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_avoidendl">Avoid std::endl</a>
+ <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
</div>
<div class="doc_text">
to the output stream specified. In addition to doing this, however, it also
flushes the output stream. In other words, these are equivalent:</p>
+<div class="doc_code">
<pre>
- std::cout << std::endl;
- std::cout << "\n" << std::flush;
+std::cout << std::endl;
+std::cout << '\n' << std::flush;
</pre>
+</div>
<p>Most of the time, you probably have no reason to flush the output stream, so
-it's better to use a literal <tt>"\n"</tt>.</p>
+it's better to use a literal <tt>'\n'</tt>.</p>
</div>
-<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="hl_exploitcpp">Exploit C++ to its Fullest</a>
-</div>
-
-<div class="doc_text">
-
-<p>C++ is a powerful language. With a firm grasp on its capabilities, you can make
-write effective, consise, readable and maintainable code all at the same time.
-By staying consistent, you reduce the amount of special cases that need to be
-remembered. Reducing the total number of lines of code you write is a good way
-to avoid documentation, and avoid giving bugs a place to hide.</p>
-
-<p>For these reasons, come to know and love the contents of your local
-<algorithm> header file. Know about <functional> and what it can do
-for you. C++ is just a tool that wants you to master it. :)</p>
-
-</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<ol>
-<li><a href="http://www.aw.com/product/0,2627,0201924889,00.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>
<p>If you get some free time, and you haven't read them: do so, you might learn
-something. :)</p>
+something.</p>
</div>
<hr>
<address>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
- <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
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