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|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<link rel="stylesheet" href="llvm.css" type="text/css">
<title>LLVM Coding Standards</title>
</head>
<body>
<div class="doc_title">
LLVM Coding Standards
</div>
<ol>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#mechanicalissues">Mechanical Source Issues</a>
<ol>
<li><a href="#sourceformating">Source Code Formatting</a>
<ol>
<li><a href="#scf_commenting">Commenting</a></li>
<li><a href="#scf_commentformat">Comment Formatting</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>
</ol></li>
<li><a href="#compilerissues">Compiler Issues</a>
<ol>
<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>
<ol>
<li><a href="#macro">The High Level Issues</a>
<ol>
<li><a href="#hl_module">A Public Header File <b>is</b> a
Module</a></li>
<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="#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> and
<a href="mailto:void@nondot.org">Bill Wendling</a></p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="introduction">Introduction</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document attempts to describe a few coding standards that are being used
in the LLVM source tree. Although no coding standards should be regarded as
absolute requirements to be followed in all instances, coding standards can be
useful.</p>
<p>This document intentionally does not prescribe fixed standards for religious
issues such as brace placement and space usage. For issues like this, follow
the golden rule:</p>
<blockquote>
<p><b><a name="goldenrule">If you are adding a significant body of source to a
project, feel free to use whatever style you are most comfortable with. If you
are extending, enhancing, or bug fixing already implemented code, use the style
that is already being used so that the source is uniform and easy to
follow.</a></b></p>
</blockquote>
<p>The ultimate goal of these guidelines is the increase readability and
maintainability of our common source base. If you have suggestions for topics to
be included, please mail them to <a
href="mailto:sabre@nondot.org">Chris</a>.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="mechanicalissues">Mechanical Source Issues</a>
</div>
<!-- *********************************************************************** -->
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="sourceformating">Source Code Formatting</a>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="scf_commenting">Commenting</a>
</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
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 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 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 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 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
tricky is going on, a reference to the paper where it is published should be
included, as well as any notes or "gotchas" in the code to watch out for.</p>
<b>Class overviews</b>
<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>
<b>Method information</b>
<p>Methods defined in a class (as well as any global functions) should also be
documented properly. A quick note about what it does any a description of the
borderline behaviour is all that is necessary here (unless something
particularly tricky or insideous is going on). The hope is that people can
figure out how to use your interfaces without reading the code itself... that is
the goal metric.</p>
<p>Good things to talk about here are what happens when something unexpected
happens: does the method return null? Abort? Format your hard disk?</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="scf_commentformat">Comment Formatting</a>
</div>
<div class="doc_text">
<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
require less typing, don't have nesting problems, etc. There are a few cases
when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
<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 <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>
<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
These nest properly and are better behaved in general than C style comments.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<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 <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><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><tt>Support/*</tt></li>
<li><tt>Config/*</tt></li>
<li>System <tt>#includes</tt></li>
</ol>
<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>
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>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="scf_codewidth">Source Code Width</a>
</div>
<div class="doc_text">
<p>Write your code to fit within 80 columns of text. This helps those of us who
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>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
</div>
<div class="doc_text">
<p>In all cases, prefer spaces to tabs in source files. People have different
prefered indentation levels, and different styles of indentation that they
like... this is fine. What isn't is that different editors/viewers expand tabs
out to different tab stops. This can cause your code to look completely
unreadable, and it is not worth dealing with.</p>
<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
style of existing code if your are modifying and extending it. If you like four
spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
with two spaces of indentation. Also, do not reindent a whole source file: it
makes for incredible diffs that are absolutely worthless.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="scf_indentation">Indent Code Consistently</a>
</div>
<div class="doc_text">
<p>Okay, your first year of programming you were told that indentation is
important. If you didn't believe and internalize this then, now is the time.
Just do it.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="compilerissues">Compiler Issues</a>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
</div>
<div class="doc_text">
<p>If your code has compiler warnings in it, something is wrong: you aren't
casting values correctly, your have "questionable" constructs in your code, or
you are doing something legitimately wrong. Compiler warnings can cover up
legitimate errors in output and make dealing with a translation unit
difficult.</p>
<p>It is not possible to prevent all warnings from all compilers, nor is it
desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
a good thorough set of warnings, and stick to them. At least in the case of
<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
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()) {
...
}
</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())) {
...
}
</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>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
-Winline -W -Wwrite-strings -Wno-unused</tt></p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ci_portable_code">Write Portable Code</a>
</div>
<div class="doc_text">
<p>In almost all cases, it is possible and within reason to write completely
portable code. If there are cases where it isn't possible to write portable
code, isolate it behind a well defined (and well documented) interface.</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. 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_section">
<a name="styleissues">Style Issues</a>
</div>
<!-- *********************************************************************** -->
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="macro">The High Level Issues</a>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="hl_module">A Public Header File <b>is</b> a Module</a>
</div>
<div class="doc_text">
<p>C++ doesn't do too well in the modularity department. There is no real
encapsulation or data hiding (unless you use expensive protocol classes), but it
is what we have to work with. When you write a public header file (in the LLVM
source tree, they live in the top level "include" directory), you are defining a
module of functionality.</p>
<p>Ideally, modules should be completely independent of each other, and their
header files should only include the absolute minimum number of headers
possible. A module is not just a class, a function, or a namespace: <a
href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
of these</a> that defines an interface. This interface may be several
functions, classes or data structures, but the important issue is how they work
together.</p>
<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
files. Each of these <tt>.cpp</tt> files should include the header that defines
their interface first. This ensure that all of the dependences of the module
header have been properly added to the module header itself, and are not
implicit. System headers should be included after user headers for a
translation unit.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
</div>
<div class="doc_text">
<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
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 <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 -- 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
above). This way there won't be any hidden dependencies that you'll find out
about later...</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="hl_privateheaders">Keep "internal" Headers Private</a>
</div>
<div class="doc_text">
<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>
<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
that your private interface remains private and undisturbed by outsiders.</p>
<p>Note however, that it's okay to put extra implementation methods a public
class itself... just make them private (or protected), and all is well.</p>
</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>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ll_assert">Assert Liberally</a>
</div>
<div class="doc_text">
<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
preconditions and assumptions, you never know when a bug (not neccesarily even
yours) might be caught early by an assertion, which reduces debugging time
dramatically. The "<tt><cassert></tt>" header file is probably already
included by the header files you are using, so it doesn't cost anything to use
it.</p>
<p>To further assist with debugging, make sure to put some kind of error message
in the assertion statement (which is printed if the assertion is tripped). This
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];
}
</pre>
</div>
<p>Here are some examples:</p>
<div class="doc_code">
<pre>
assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
assert(idx < getNumSuccessors() && "Successor # out of range!");
assert(V1.getType() == V2.getType() && "Constant types must be identical!");
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="ll_preincrement">Prefer Preincrement</a>
</div>
<div class="doc_text">
<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
primitive types, this isn't a big deal... but for iterators, it can be a huge
issue (for example, some iterators contains stack and set objects in them...
copying an iterator could invoke the copy ctor's of these as well). In general,
get in the habit of always using preincrement, and you won't have a problem.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
</div>
<div class="doc_text">
<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
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;
</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>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="seealso">See Also</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>A lot of these comments and recommendations have been culled for other
sources. Two particularly important books for our work are:</p>
<ol>
<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>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>
</div>
<!-- *********************************************************************** -->
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