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-</tr></table>
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+ <title>LLVM Coding Standards</title>
+</head>
+<body>
+
+<div class="doc_title">
+ LLVM Coding Standards
+</div>
<ol>
- <li><a href="#introduction">Introduction</a>
+ <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><a href="#scf_commentformat">Comment Formatting</a>
- <li><a href="#scf_includes">#include Style</a>
- <li><a href="#scf_codewidth">Source Code Width</a>
- <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a>
- <li><a href="#scf_indentation">Indent Code Consistently</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><a href="#ci_cpp_features">Which C++ features can I use?</a>
- <li><a href="#ci_portable_code">Write Portable Code</a>
- </ol>
- </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><a href="#hl_dontinclude">#include as Little as Possible</a>
- <li><a href="#hl_privateheaders">Keep "internal" Headers Private</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="#hl_earlyexit">Use Early Exits and 'continue' to Simplify
+ Code</a></li>
+ <li><a href="#hl_else_after_return">Don't use "else" after a
+ return</a></li>
+ <li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
+ Functions</a></li>
+ </ol></li>
<li><a href="#micro">The Low Level Issues</a>
<ol>
- <li><a href="#hl_assert">Assert Liberally</a>
- <li><a href="#hl_preincrement">Prefer Preincrement</a>
- <li><a href="#hl_avoidendl">Avoid endl</a>
- <li><a href="#hl_exploitcpp">Exploit C++ to its Fullest</a>
- </ol>
- <li><a href="#iterators">Writing Iterators</a>
- </ol>
- <li><a href="#seealso">See Also</a>
-</ol><p>
+ <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_end">Don't evaluate end() every time through a
+ loop</a></li>
+ <li><a href="#ll_iostream"><tt>#include <iostream></tt> is
+ <em>forbidden</em></a></li>
+ <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
+ <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li>
+ </ol></li>
+
+ <li><a href="#nano">Microscopic Details</a>
+ <ol>
+ <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
+ <li><a href="#micro_preincrement">Prefer Preincrement</a></li>
+ <li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
+ <li><a href="#micro_anonns">Anonymous Namespaces</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>
+</div>
<!-- *********************************************************************** -->
-</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0><tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
-<a name="introduction">Introduction
-</b></font></td></tr></table><ul>
+<div class="doc_section">
+ <a name="introduction">Introduction</a>
+</div>
<!-- *********************************************************************** -->
-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
+<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>
+useful.</p>
-This document intentionally does not prescribe fixed standards for religious
+<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:
+the golden rule:</p>
+
+<blockquote>
-<a name="goldenrule">
-<blockquote><b>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.</b></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>
-The ultimate goal of these guidelines is the increase readability and
+</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>
+be included, please mail them to <a
+href="mailto:sabre@nondot.org">Chris</a>.</p>
+</div>
<!-- *********************************************************************** -->
-</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0><tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
-<a name="mechanicalissues">Mechanical Source Issues
-</b></font></td></tr></table><ul>
+<div class="doc_section">
+ <a name="mechanicalissues">Mechanical Source Issues</a>
+</div>
<!-- *********************************************************************** -->
<!-- ======================================================================= -->
-</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td> </td><td width="100%"> <font color="#EEEEFF" face="Georgia,Palatino"><b>
-<a name="sourceformating">Source Code Formatting
-</b></font></td></tr></table><ul>
-
+<div class="doc_subsection">
+ <a name="sourceformating">Source Code Formatting</a>
+</div>
<!-- _______________________________________________________________________ -->
-</ul><a name="scf_commenting"><h4><hr size=0>Commenting</h4><ul>
+<div class="doc_subsubsection">
+ <a name="scf_commenting">Commenting</a>
+</div>
+
+<div class="doc_text">
-Comments are one critical part of readability and maintainability. Everyone
-knows they should comment, so should you. :) Although we all should probably
+<p>Comments are one critical part of readability and maintainability. Everyone
+knows they should comment, so should you. When writing comments, write them as
+English prose, which means they should use proper capitalization, punctuation,
+etc. 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>
+documentation is very useful:</p>
-<ol>
-<h4><li>File Headers</h4>
-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 file header format. The
-standard format for the LLVM source tree looks like this:<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>
-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>
+<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>
-The main body of the description does not have to be very long in most cases.
+<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>
-
+included, as well as any notes or "gotchas" in the code to watch out for.</p>
-<h4><li>Class overviews</h4>
+<b>Class overviews</b>
-Classes are one fundemental part of a good object oriented design. As such, a
-class definition should have a comment block that explains what the class is
+<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>
-<h4><li>Method information</h4>
+<b>Method information</b>
-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
+<p>Methods defined in a class (as well as any global functions) should also be
+documented properly. A quick note about what it does and 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
+particularly tricky or insidious 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>
+the goal metric.</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>
-</ol>
+<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>
<!-- _______________________________________________________________________ -->
-</ul><a name="scf_commentformat"><h4><hr size=0>Comment Formatting</h4><ul>
+<div class="doc_subsubsection">
+ <a name="scf_commentformat">Comment Formatting</a>
+</div>
-In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
+<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>
+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>When writing a header file that may be #included by a C source file.
-<li>When writing a source file that is used by a tool that only accepts C style
-comments.
-</ol><p>
+ <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>
-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>
<!-- _______________________________________________________________________ -->
-</ul><a name="scf_includes"><h4><hr size=0>#include Style</h4><ul>
+<div class="doc_subsubsection">
+ <a name="scf_includes"><tt>#include</tt> Style</a>
+</div>
-Immediately after the <a href="#scf_commenting">header file comment</a> (and
+<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><a href="#hl_privateheaders">Local/Private Headers</a>
-<li>llvm/*
-<li>llvm/Analysis/*
-<li>llvm/Assembly/*
-<li>llvm/Bytecode/*
-<li>llvm/CodeGen/*
-<li>...
-<li>Support/*
-<li>Config/*
-<li>System #includes
+ <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>
-... and each catagory should be sorted by name.<p>
+<p>... and each category should be sorted by name.</p>
-<a name="mmheader">The "Main Module Header" 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>
+<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>
<!-- _______________________________________________________________________ -->
-</ul><a name="scf_codewidth"><h4><hr size=0>Source Code Width</h4><ul>
+<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>
-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>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>
<!-- _______________________________________________________________________ -->
-</ul><a name="scf_spacestabs"><h4><hr size=0>Use Spaces Instead of Tabs</h4><ul>
+<div class="doc_subsubsection">
+ <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
+</div>
+
+<div class="doc_text">
-In all cases, prefer spaces to tabs in source files. People have different
-prefered indentation levels, and different styles of indentation that they
+<p>In all cases, prefer spaces to tabs in source files. People have different
+preferred 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>
+unreadable, and it is not worth dealing with.</p>
-As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
+
<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>
+makes for incredible diffs that are absolutely worthless.</p>
+</div>
<!-- _______________________________________________________________________ -->
-</ul><a name="scf_indentation"><h4><hr size=0>Indent Code Consistently</h4><ul>
+<div class="doc_subsubsection">
+ <a name="scf_indentation">Indent Code Consistently</a>
+</div>
-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 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>
<!-- ======================================================================= -->
-</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td> </td><td width="100%"> <font color="#EEEEFF" face="Georgia,Palatino"><b>
-<a name="compilerissues">Compiler Issues
-</b></font></td></tr></table><ul>
+<div class="doc_subsection">
+ <a name="compilerissues">Compiler Issues</a>
+</div>
<!-- _______________________________________________________________________ -->
-</ul><a name="ci_warningerrors"><h4><hr size=0>Treat Compiler Warnings Like Errors</h4><ul>
+<div class="doc_subsubsection">
+ <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
+</div>
-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>
+<div class="doc_text">
-It is not possible to prevent all warnings from all compilers, nor is it
+<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>
+I write code like this:</p>
+<div class="doc_code">
<pre>
- if (V = getValue()) {
- ..
- }
-</pre><p>
+if (V = getValue()) {
+ ...
+}
+</pre>
+</div>
-<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>
+<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><p>
+if ((V = getValue())) {
+ ...
+}
+</pre>
+</div>
-...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can be
-fixed by massaging the code appropriately.<p>
+<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
+be fixed by massaging the code appropriately.</p>
-These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall -Winline
--W -Wwrite-strings -Wno-unused</tt><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>
<!-- _______________________________________________________________________ -->
-</ul><a name="ci_cpp_features"><h4><hr size=0>Which C++ features can I use?</h4><ul>
-
-Compilers are finally catching up to the C++ standard. Most compilers implement
-most features, so you can use just about any features that you would like. In
-the LLVM source tree, I have chosen to not use these features:<p>
+<div class="doc_subsubsection">
+ <a name="ci_portable_code">Write Portable Code</a>
+</div>
-<ol>
-<li>Exceptions: Exceptions are very useful for error reporting and handling
-exceptional conditions. I do not use them in LLVM because they do have an
-associated performance impact (by restricting restructuring of code), and parts
-of LLVM are designed for performance critical purposes.<p>
-
-Just like most of the rules in this document, this isn't a hard and fast
-requirement. Exceptions are used in the Parser, because it simplifies error
-reporting <b>significantly</b>, and the LLVM parser is not at all in the
-critical path.<p>
+<div class="doc_text">
-<li>RTTI: RTTI has a large cost in terms of executable size, and compilers are
-not yet very good at stomping out "dead" class information blocks. Because of
-this, typeinfo and dynamic cast are not used.
-</ol><p>
+<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>
-Other features, such as templates (without partial specialization) can be used
-freely. The general goal is to have clear, consise, performant code... if a
-technique assists with that then use it.<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>
<!-- _______________________________________________________________________ -->
-</ul><a name="ci_portable_code"><h4><hr size=0>Write Portable Code</h4><ul>
+<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">
-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 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>
-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>
+<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 and the type is a C++ "POD" type, in which case
+<tt>struct</tt> is allowed.</p>
+</div>
<!-- *********************************************************************** -->
-</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0><tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
-<a name="styleissues">Style Issues
-</b></font></td></tr></table><ul>
+<div class="doc_section">
+ <a name="styleissues">Style Issues</a>
+</div>
<!-- *********************************************************************** -->
<!-- ======================================================================= -->
-</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td> </td><td width="100%"> <font color="#EEEEFF" face="Georgia,Palatino"><b>
-<a name="macro">The High Level Issues
-</b></font></td></tr></table><ul>
+<div class="doc_subsection">
+ <a name="macro">The High Level Issues</a>
+</div>
+<!-- ======================================================================= -->
<!-- _______________________________________________________________________ -->
-</ul><a name="hl_module"><h4><hr size=0>A Public Header File <b>is</b> a Module</h4><ul>
+<div class="doc_subsubsection">
+ <a name="hl_module">A Public Header File <b>is</b> a Module</a>
+</div>
+
+<div class="doc_text">
-C++ doesn't do too well in the modularity department. There is no real
+<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>
+module of functionality.</p>
-Ideally, modules should be completely independent of each other, and their
+<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>
+together.</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>
+<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>
<!-- _______________________________________________________________________ -->
-</ul><a name="hl_dontinclude"><h4><hr size=0>#include as Little as Possible</h4><ul>
-
-<tt>#include</tt> hurts compile time performance. Don't do it unless you have
-to, especially in header files.<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>
-
-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
+<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
+accidentally 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>
+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="hl_earlyexit">Use Early Exits and 'continue' to Simplify Code</a>
+</div>
+
+<div class="doc_text">
+
+<p>When reading code, keep in mind how much state and how many previous
+decisions have to be remembered by the reader to understand a block of code.
+Aim to reduce indentation where possible when it doesn't make it more difficult
+to understand the code. One great way to do this is by making use of early
+exits and the 'continue' keyword in long loops. As an example of using an early
+exit from a function, consider this "bad" code:</p>
+
+<div class="doc_code">
+<pre>
+Value *DoSomething(Instruction *I) {
+ if (!isa<TerminatorInst>(I) &&
+ I->hasOneUse() && SomeOtherThing(I)) {
+ ... some long code ....
+ }
+
+ return 0;
+}
+</pre>
+</div>
+
+<p>This code has several problems if the body of the 'if' is large. When you're
+looking at the top of the function, it isn't immediately clear that this
+<em>only</em> does interesting things with non-terminator instructions, and only
+applies to things with the other predicates. Second, it is relatively difficult
+to describe (in comments) why these predicates are important because the if
+statement makes it difficult to lay out the comments. Third, when you're deep
+within the body of the code, it is indented an extra level. Finally, when
+reading the top of the function, it isn't clear what the result is if the
+predicate isn't true, you have to read to the end of the function to know that
+it returns null.</p>
+
+<p>It is much preferred to format the code like this:</p>
+
+<div class="doc_code">
+<pre>
+Value *DoSomething(Instruction *I) {
+ // Terminators never need 'something' done to them because, ...
+ if (isa<TerminatorInst>(I))
+ return 0;
+
+ // We conservatively avoid transforming instructions with multiple uses
+ // because goats like cheese.
+ if (!I->hasOneUse())
+ return 0;
+
+ // This is really just here for example.
+ if (!SomeOtherThing(I))
+ return 0;
+
+ ... some long code ....
+}
+</pre>
+</div>
+
+<p>This fixes these problems. A similar problem frequently happens in for
+loops. A silly example is something like this:</p>
+
+<div class="doc_code">
+<pre>
+ for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
+ if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
+ Value *LHS = BO->getOperand(0);
+ Value *RHS = BO->getOperand(1);
+ if (LHS != RHS) {
+ ...
+ }
+ }
+ }
+</pre>
+</div>
+
+<p>When you have very very small loops, this sort of structure is fine, but if
+it exceeds more than 10-15 lines, it becomes difficult for people to read and
+understand at a glance.
+The problem with this sort of code is that it gets very nested very quickly,
+meaning that the reader of the code has to keep a lot of context in their brain
+to remember what is going immediately on in the loop, because they don't know
+if/when the if conditions will have elses etc. It is strongly preferred to
+structure the loop like this:</p>
+
+<div class="doc_code">
+<pre>
+ for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
+ BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
+ if (!BO) continue;
+
+ Value *LHS = BO->getOperand(0);
+ Value *RHS = BO->getOperand(1);
+ if (LHS == RHS) continue;
+ }
+</pre>
+</div>
+
+<p>This has all the benefits of using early exits from functions: it reduces
+nesting of the loop, it makes it easier to describe why the conditions are true,
+and it makes it obvious to the reader that there is no "else" coming up that
+they have to push context into their brain for. If a loop is large, this can
+be a big understandability win.</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="hl_else_after_return">Don't use "else" after a return</a>
+</div>
+
+<div class="doc_text">
+
+<p>For similar reasons above (reduction of indentation and easier reading),
+ please do not use "else" or "else if" after something that interrupts
+ control flow like return, break, continue, goto, etc. For example, this is
+ "bad":</p>
+
+<div class="doc_code">
+<pre>
+ case 'J': {
+ if (Signed) {
+ Type = Context.getsigjmp_bufType();
+ if (Type.isNull()) {
+ Error = ASTContext::GE_Missing_sigjmp_buf;
+ return QualType();
+ } else {
+ break;
+ }
+ } else {
+ Type = Context.getjmp_bufType();
+ if (Type.isNull()) {
+ Error = ASTContext::GE_Missing_jmp_buf;
+ return QualType();
+ } else {
+ break;
+ }
+ }
+ }
+ }
+</pre>
+</div>
+
+<p>It is better to write this something like:</p>
+
+<div class="doc_code">
+<pre>
+ case 'J':
+ if (Signed) {
+ Type = Context.getsigjmp_bufType();
+ if (Type.isNull()) {
+ Error = ASTContext::GE_Missing_sigjmp_buf;
+ return QualType();
+ }
+ } else {
+ Type = Context.getjmp_bufType();
+ if (Type.isNull()) {
+ Error = ASTContext::GE_Missing_jmp_buf;
+ return QualType();
+ }
+ }
+ break;
+</pre>
+</div>
+<p>Or better yet (in this case), as:</p>
+
+<div class="doc_code">
+<pre>
+ case 'J':
+ if (Signed)
+ Type = Context.getsigjmp_bufType();
+ else
+ Type = Context.getjmp_bufType();
+
+ if (Type.isNull()) {
+ Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
+ ASTContext::GE_Missing_jmp_buf;
+ return QualType();
+ }
+ break;
+</pre>
+</div>
+
+<p>The idea is to reduce indentation and the amount of code you have to keep
+ track of when reading the code.</p>
+
+</div>
<!-- _______________________________________________________________________ -->
-</ul><a name="hl_privateheaders"><h4><hr size=0>Keep "internal" Headers Private</h4><ul>
+<div class="doc_subsubsection">
+ <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
+</div>
-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>
+<div class="doc_text">
-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>It is very common to write small loops that just compute a boolean
+ value. There are a number of ways that people commonly write these, but an
+ example of this sort of thing is:</p>
+
+<div class="doc_code">
+<pre>
+ <b>bool FoundFoo = false;</b>
+ for (unsigned i = 0, e = BarList.size(); i != e; ++i)
+ if (BarList[i]->isFoo()) {
+ <b>FoundFoo = true;</b>
+ break;
+ }
+
+ <b>if (FoundFoo) {</b>
+ ...
+ }
+</pre>
+</div>
+
+<p>This sort of code is awkward to write, and is almost always a bad sign.
+Instead of this sort of loop, we strongly prefer to use a predicate function
+(which may be <a href="#micro_anonns">static</a>) that uses
+<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
+the code to be structured like this:
+</p>
+
+
+<div class="doc_code">
+<pre>
+/// ListContainsFoo - Return true if the specified list has an element that is
+/// a foo.
+static bool ListContainsFoo(const std::vector<Bar*> &List) {
+ for (unsigned i = 0, e = List.size(); i != e; ++i)
+ if (List[i]->isFoo())
+ return true;
+ return false;
+}
+...
+
+ <b>if (ListContainsFoo(BarList)) {</b>
+ ...
+ }
+</pre>
+</div>
-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>
+<p>There are many reasons for doing this: it reduces indentation and factors out
+code which can often be shared by other code that checks for the same predicate.
+More importantly, it <em>forces you to pick a name</em> for the function, and
+forces you to write a comment for it. In this silly example, this doesn't add
+much value. However, if the condition is complex, this can make it a lot easier
+for the reader to understand the code that queries for this predicate. Instead
+of being faced with the in-line details of how we check to see if the BarList
+contains a foo, we can trust the function name and continue reading with better
+locality.</p>
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="micro">The Low Level Issues</a>
+</div>
<!-- ======================================================================= -->
-</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td> </td><td width="100%"> <font color="#EEEEFF" face="Georgia,Palatino"><b>
-<a name="micro">The Low Level Issues
-</b></font></td></tr></table><ul>
<!-- _______________________________________________________________________ -->
-</ul><a name="hl_assert"><h4><hr size=0>Assert Liberally</h4><ul>
+<div class="doc_subsubsection">
+ <a name="ll_assert">Assert Liberally</a>
+</div>
+
+<div class="doc_text">
-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
+<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
+preconditions and assumptions, you never know when a bug (not necessarily 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>
+it.</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
+<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>
+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>
-Here are some examples:
+<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!");
-</pre><p>
+assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
+</pre>
+</div>
-You get the idea...<p>
+<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>
<!-- _______________________________________________________________________ -->
-</ul><a name="hl_preincrement"><h4><hr size=0>Prefer Preincrement</h4><ul>
+<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>
-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>
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="ll_virtual_anch">Provide a virtual method anchor for classes
+ in headers</a>
+</div>
-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 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_end">Don't evaluate end() every time through a loop</a>
+</div>
+
+<div class="doc_text">
+
+<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
+with macros and may be coming in C++'0x) we end up writing a lot of loops that
+manually iterate from begin to end on a variety of containers or through other
+data structures. One common mistake is to write a loop in this style:</p>
+
+<div class="doc_code">
+<pre>
+ BasicBlock *BB = ...
+ for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
+ ... use I ...
+</pre>
+</div>
+
+<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
+every time through the loop. Instead of writing the loop like this, we strongly
+prefer loops to be written so that they evaluate it once before the loop starts.
+A convenient way to do this is like so:</p>
+
+<div class="doc_code">
+<pre>
+ BasicBlock *BB = ...
+ for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
+ ... use I ...
+</pre>
+</div>
+
+<p>The observant may quickly point out that these two loops may have different
+semantics: if the container (a basic block in this case) is being mutated, then
+"<tt>BB->end()</tt>" may change its value every time through the loop and the
+second loop may not in fact be correct. If you actually do depend on this
+behavior, please write the loop in the first form and add a comment indicating
+that you did it intentionally.</p>
+
+<p>Why do we prefer the second form (when correct)? Writing the loop in the
+first form has two problems: First it may be less efficient than evaluating it
+at the start of the loop. In this case, the cost is probably minor: a few extra
+loads every time through the loop. However, if the base expression is more
+complex, then the cost can rise quickly. I've seen loops where the end
+expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
+lookups really aren't cheap. By writing it in the second form consistently, you
+eliminate the issue entirely and don't even have to think about it.</p>
+
+<p>The second (even bigger) issue is that writing the loop in the first form
+hints to the reader that the loop is mutating the container (a fact that a
+comment would handily confirm!). If you write the loop in the second form, it
+is immediately obvious without even looking at the body of the loop that the
+container isn't being modified, which makes it easier to read the code and
+understand what it does.</p>
+
+<p>While the second form of the loop is a few extra keystrokes, we do strongly
+prefer it.</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 not problematic in this regard (just <tt><iostream></tt>).
+However, raw_ostream provides various APIs that are better performing for almost
+every use than std::ostream style APIs, so you should just use it for new
+code.</p>
+
+<p><b>New code should always
+use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or
+the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p>
+
+</div>
<!-- _______________________________________________________________________ -->
-</ul><a name="hl_avoidendl"><h4><hr size=0>Avoid endl</h4><ul>
+<div class="doc_subsubsection">
+ <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
+</div>
-The <tt>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_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>
- cout << endl;
- cout << "\n" << 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>
-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>
<!-- _______________________________________________________________________ -->
-</ul><a name="hl_exploitcpp"><h4><hr size=0>Exploit C++ to its Fullest</h4><ul>
+<div class="doc_subsubsection">
+ <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
+</div>
+
+<div class="doc_text">
-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>LLVM includes a lightweight, simple, and efficient stream implementation
+in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
+of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> instead
+of <tt>ostream</tt>.</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>
+<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
+be forward declared as <tt>class raw_ostream</tt>. Public headers should
+generally not include the <tt>raw_ostream</tt> header, but use forward
+declarations and constant references to <tt>raw_ostream</tt> instances.</p>
+</div>
<!-- ======================================================================= -->
-</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td> </td><td width="100%"> <font color="#EEEEFF" face="Georgia,Palatino"><b>
-<a name="iterators">Writing Iterators
-</b></font></td></tr></table><ul>
+<div class="doc_subsection">
+ <a name="nano">Microscopic Details</a>
+</div>
+<!-- ======================================================================= -->
+
+<p>This section describes preferred low-level formatting guidelines along with
+reasoning on why we prefer them.</p>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="micro_spaceparen">Spaces Before Parentheses</a>
+</div>
+
+<div class="doc_text">
-Here's a pretty good summary of how to write your own data structure iterators
-in a way that is compatible with the STL, and with a lot of other code out there
-(slightly edited by Chris):<p>
+<p>We prefer to put a space before a parentheses only in control flow
+statements, but not in normal function call expressions and function-like
+macros. For example, this is good:</p>
+<div class="doc_code">
<pre>
-From: Ross Smith <ross.s@ihug.co.nz>
-Newsgroups: comp.lang.c++.moderated
-Subject: Writing iterators (was: Re: Non-template functions that take iterators)
-Date: 28 Jun 2001 12:07:10 -0400
-
-Andre Majorel wrote:
-> Any pointers handy on "writing STL-compatible iterators for
-> dummies ?"
-
-I'll give it a try...
-
-The usual situation requiring user-defined iterators is that you have
-a type that bears some resemblance to an STL container, and you want
-to provide iterators so it can be used with STL algorithms. You need
-to ask three questions:
-
-First, is this simply a wrapper for an underlying collection of
-objects that's held somewhere as a real STL container, or is it a
-"virtual container" for which iteration is (under the hood) more
-complicated than simply incrementing some underlying iterator (or
-pointer or index or whatever)? In the former case you can frequently
-get away with making your container's iterators simply typedefs for
-those of the underlying container; your begin() function would call
-member_container.begin(), and so on.
-
-Second, do you only need read-only iterators, or do you need separate
-read-only (const) and read-write (non-const) iterators?
-
-Third, which kind of iterator (input, output, forward, bidirectional,
-or random access) is appropriate? If you're familiar with the
-properties of the iterator types (if not, visit
-<a href="http://www.sgi.com/tech/stl/">http://www.sgi.com/tech/stl/</a>), the appropriate choice should be
-obvious from the semantics of the container.
-
-I'll start with forward iterators, as the simplest case that's likely
-to come up in normal code. Input and output iterators have some odd
-properties and rarely need to be implemented in user code; I'll leave
-them out of discussion. Bidirectional and random access iterators are
-covered below.
-
-The exact behaviour of a forward iterator is spelled out in the
-Standard in terms of a set of expressions with specified behaviour,
-rather than a set of member functions, which leaves some leeway in how
-you actually implement it. Typically it looks something like this
-(I'll start with the const-iterator-only situation):
-
- #include <iterator>
-
- class container {
- public:
- typedef something_or_other value_type;
- class const_iterator:
- public std::iterator<std::forward_iterator_tag, value_type> {
- friend class container;
- public:
- const value_type& operator*() const;
- const value_type* operator->() const;
- const_iterator& operator++();
- const_iterator operator++(int);
- friend bool operator==(const_iterator lhs,
- const_iterator rhs);
- friend bool operator!=(const_iterator lhs,
- const_iterator rhs);
- private:
- //...
- };
- //...
- };
+ <b>if (</b>x) ...
+ <b>for (</b>i = 0; i != 100; ++i) ...
+ <b>while (</b>llvm_rocks) ...
-An iterator should always be derived from an instantiation of the
-std::iterator template. The iterator's life cycle functions
-(constructors, destructor, and assignment operator) aren't declared
-here; in most cases the compiler-generated ones are sufficient. The
-container needs to be a friend of the iterator so that the container's
-begin() and end() functions can fill in the iterator's private members
-with the appropriate values.
-
-<i>[Chris's Note: I prefer to not make my iterators friends. Instead, two
-ctor's are provided for the iterator class: one to start at the end of the
-container, and one at the beginning. Typically this is done by providing
-two constructors with different signatures.]</i>
-
-There are normally only three member functions that need nontrivial
-implementations; the rest are just boilerplate.
-
- const container::value_type&
- container::const_iterator::operator*() const {
- // find the element and return a reference to it
- }
+ <b>somefunc(</b>42);
+ <b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me");
+
+ a = <b>foo(</b>42, 92) + <b>bar(</b>x);
+ </pre>
+</div>
- const container::value_type*
- container::const_iterator::operator->() const {
- return &**this;
- }
+<p>... and this is bad:</p>
-If there's an underlying real container, operator*() can just return a
-reference to the appropriate element. If there's no actual container
-and the elements need to be generated on the fly -- what I think of as
-a "virtual container" -- things get a bit more complicated; you'll
-probably need to give the iterator a value_type member object, and
-fill it in when you need to. This might be done as part of the
-increment operator (below), or if the operation is nontrivial, you
-might choose the "lazy" approach and only generate the actual value
-when one of the dereferencing operators is called.
-
-The operator->() function is just boilerplate around a call to
-operator*().
-
- container::const_iterator&
- container::const_iterator::operator++() {
- // the incrementing logic goes here
- return *this;
- }
+<div class="doc_code">
+<pre>
+ <b>if(</b>x) ...
+ <b>for(</b>i = 0; i != 100; ++i) ...
+ <b>while(</b>llvm_rocks) ...
+
+ <b>somefunc (</b>42);
+ <b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me");
+
+ a = <b>foo (</b>42, 92) + <b>bar (</b>x);
+</pre>
+</div>
+
+<p>The reason for doing this is not completely arbitrary. This style makes
+ control flow operators stand out more, and makes expressions flow better. The
+ function call operator binds very tightly as a postfix operator. Putting
+ a space after a function name (as in the last example) makes it appear that
+ the code might bind the arguments of the left-hand-side of a binary operator
+ with the argument list of a function and the name of the right side. More
+ specifically, it is easy to misread the "a" example as:</p>
+
+<div class="doc_code">
+<pre>
+ a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
+</pre>
+</div>
- container::const_iterator
- container::const_iterator::operator++(int) {
- const_iterator old(*this);
- ++*this;
- return old;
- }
+<p>... when skimming through the code. By avoiding a space in a function, we
+avoid this misinterpretation.</p>
-Again, the incrementing logic will usually be trivial if there's a
-real container involved, more complicated if you're working with a
-virtual container. In particular, watch out for what happens when you
-increment past the last valid item -- this needs to produce an
-iterator that will compare equal to container.end(), and making this
-work is often nontrivial for virtual containers.
+</div>
-The post-increment function is just boilerplate again (and
-incidentally makes it obvious why all the experts recommend using
-pre-increment wherever possible).
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="micro_preincrement">Prefer Preincrement</a>
+</div>
- bool operator==(container::const_iterator lhs,
- container::const_iterator rhs) {
- // equality comparison goes here
- }
+<div class="doc_text">
- bool operator!=(container::const_iterator lhs,
- container::const_iterator rhs) {
- return !(lhs == rhs);
- }
+<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>
-For a real container, the equality comparison will usually just
-compare the underlying iterators (or pointers or indices or whatever).
-The semantics of comparisons for virtual container iterators are often
-tricky. Remember that iterator comparison only needs to be defined for
-iterators into the same container, so you can often simplify things by
-taking for granted that lhs and rhs both point into the same container
-object. Again, the second function is just boilerplate.
-
-It's a matter of taste whether iterator arguments are passed by value
-or reference; I've shown tham passed by value to reduce clutter, but
-if the iterator contains several data members, passing by reference
-may be better.
-
-That convers the const-iterator-only situation. When we need separate
-const and mutable iterators, one small complication is added beyond
-the simple addition of a second class.
-
- class container {
- public:
- typedef something_or_other value_type;
- class const_iterator;
- class iterator:
- public std::iterator<std::forward_iterator_tag, value_type> {
- friend class container;
- friend class container::const_iterator;
- public:
- value_type& operator*() const;
- value_type* operator->() const;
- iterator& operator++();
- iterator operator++(int);
- friend bool operator==(iterator lhs, iterator rhs);
- friend bool operator!=(iterator lhs, iterator rhs);
- private:
- //...
- };
- class const_iterator:
- public std::iterator<std::forward_iterator_tag, value_type> {
- friend class container;
- public:
- const_iterator();
- const_iterator(const iterator& i);
- const value_type& operator*() const;
- const value_type* operator->() const;
- const_iterator& operator++();
- const_iterator operator++(int);
- friend bool operator==(const_iterator lhs,
- const_iterator rhs);
- friend bool operator!=(const_iterator lhs,
- const_iterator rhs);
- private:
- //...
- };
- //...
- };
+<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>
-There needs to be a conversion from iterator to const_iterator (so
-that mixed-type operations, such as comparison between an iterator and
-a const_iterator, will work). This is done here by giving
-const_iterator a conversion constructor from iterator (equivalently,
-we could have given iterator an operator const_iterator()), which
-requires const_iterator to be a friend of iterator, so it can copy its
-data members. (It also requires the addition of an explicit default
-constructor to const_iterator, since the existence of another
-user-defined constructor inhibits the compiler-defined one.)
-
-Bidirectional iterators add just two member functions to forward
-iterators:
-
- class iterator:
- public std::iterator<std::bidirectional_iterator_tag, value_type> {
- public:
- //...
- iterator& operator--();
- iterator operator--(int);
- //...
- };
+</div>
-I won't detail the implementations, they're obvious variations on
-operator++().
-
-Random access iterators add several more member and friend functions:
-
- class iterator:
- public std::iterator<std::random_access_iterator_tag, value_type> {
- public:
- //...
- iterator& operator+=(difference_type rhs);
- iterator& operator-=(difference_type rhs);
- friend iterator operator+(iterator lhs, difference_type rhs);
- friend iterator operator+(difference_type lhs, iterator rhs);
- friend iterator operator-(iterator lhs, difference_type rhs);
- friend difference_type operator-(iterator lhs, iterator rhs);
- friend bool operator<(iterator lhs, iterator rhs);
- friend bool operator>(iterator lhs, iterator rhs);
- friend bool operator<=(iterator lhs, iterator rhs);
- friend bool operator>=(iterator lhs, iterator rhs);
- //...
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="micro_namespaceindent">Namespace Indentation</a>
+</div>
+
+<div class="doc_text">
+
+<p>
+In general, we strive to reduce indentation where ever possible. This is useful
+because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
+wrapping horribly, but also because it makes it easier to understand the code.
+Namespaces are a funny thing: they are often large, and we often desire to put
+lots of stuff into them (so they can be large). Other times they are tiny,
+because they just hold an enum or something similar. In order to balance this,
+we use different approaches for small versus large namespaces.
+</p>
+
+<p>
+If a namespace definition is small and <em>easily</em> fits on a screen (say,
+less than 35 lines of code), then you should indent its body. Here's an
+example:
+</p>
+
+<div class="doc_code">
+<pre>
+namespace llvm {
+ namespace X86 {
+ /// RelocationType - An enum for the x86 relocation codes. Note that
+ /// the terminology here doesn't follow x86 convention - word means
+ /// 32-bit and dword means 64-bit.
+ enum RelocationType {
+ /// reloc_pcrel_word - PC relative relocation, add the relocated value to
+ /// the value already in memory, after we adjust it for where the PC is.
+ reloc_pcrel_word = 0,
+
+ /// reloc_picrel_word - PIC base relative relocation, add the relocated
+ /// value to the value already in memory, after we adjust it for where the
+ /// PIC base is.
+ reloc_picrel_word = 1,
+
+ /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
+ /// add the relocated value to the value already in memory.
+ reloc_absolute_word = 2,
+ reloc_absolute_dword = 3
};
+ }
+}
+</pre>
+</div>
- container::iterator&
- container::iterator::operator+=(container::difference_type rhs) {
- // add rhs to iterator position
- return *this;
- }
+<p>Since the body is small, indenting adds value because it makes it very clear
+where the namespace starts and ends, and it is easy to take the whole thing in
+in one "gulp" when reading the code. If the blob of code in the namespace is
+larger (as it typically is in a header in the llvm or clang namespaces), do not
+indent the code, and add a comment indicating what namespace is being closed.
+For example:</p>
- container::iterator&
- container::iterator::operator-=(container::difference_type rhs) {
- // subtract rhs from iterator position
- return *this;
- }
+<div class="doc_code">
+<pre>
+namespace llvm {
+namespace knowledge {
+
+/// Grokable - This class represents things that Smith can have an intimate
+/// understanding of and contains the data associated with it.
+class Grokable {
+...
+public:
+ explicit Grokable() { ... }
+ virtual ~Grokable() = 0;
+
+ ...
+
+};
+
+} // end namespace knowledge
+} // end namespace llvm
+</pre>
+</div>
- container::iterator operator+(container::iterator lhs,
- container::difference_type rhs) {
- return iterator(lhs) += rhs;
- }
+<p>Because the class is large, we don't expect that the reader can easily
+understand the entire concept in a glance, and the end of the file (where the
+namespaces end) may be a long ways away from the place they open. As such,
+indenting the contents of the namespace doesn't add any value, and detracts from
+the readability of the class. In these cases it is best to <em>not</em> indent
+the contents of the namespace.</p>
- container::iterator operator+(container::difference_type lhs,
- container::iterator rhs) {
- return iterator(rhs) += lhs;
- }
+</div>
- container::iterator operator-(container::iterator lhs,
- container::difference_type rhs) {
- return iterator(lhs) -= rhs;
- }
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="micro_anonns">Anonymous Namespaces</a>
+</div>
+
+<div class="doc_text">
+
+<p>After talking about namespaces in general, you may be wondering about
+anonymous namespaces in particular.
+Anonymous namespaces are a great language feature that tells the C++ compiler
+that the contents of the namespace are only visible within the current
+translation unit, allowing more aggressive optimization and eliminating the
+possibility of symbol name collisions. Anonymous namespaces are to C++ as
+"static" is to C functions and global variables. While "static" is available
+in C++, anonymous namespaces are more general: they can make entire classes
+private to a file.</p>
+
+<p>The problem with anonymous namespaces is that they naturally want to
+encourage indentation of their body, and they reduce locality of reference: if
+you see a random function definition in a C++ file, it is easy to see if it is
+marked static, but seeing if it is in an anonymous namespace requires scanning
+a big chunk of the file.</p>
+
+<p>Because of this, we have a simple guideline: make anonymous namespaces as
+small as possible, and only use them for class declarations. For example, this
+is good:</p>
+
+<div class="doc_code">
+<pre>
+<b>namespace {</b>
+ class StringSort {
+ ...
+ public:
+ StringSort(...)
+ bool operator<(const char *RHS) const;
+ };
+<b>} // end anonymous namespace</b>
- container::difference_type operator-(container::iterator lhs,
- container::iterator rhs) {
- // calculate distance between iterators
- }
+static void Helper() {
+ ...
+}
- bool operator<(container::iterator lhs, container::iterator rhs) {
- // perform less-than comparison
- }
+bool StringSort::operator<(const char *RHS) const {
+ ...
+}
- bool operator>(container::iterator lhs, container::iterator rhs) {
- return rhs < lhs;
- }
+</pre>
+</div>
- bool operator<=(container::iterator lhs, container::iterator rhs) {
- return !(rhs < lhs);
- }
+<p>This is bad:</p>
- bool operator>=(container::iterator lhs, container::iterator rhs) {
- return !(lhs < rhs);
- }
-Four of the functions (operator+=(), operator-=(), the second
-operator-(), and operator<()) are nontrivial; the rest are
-boilerplate.
-
-One feature of the above code that some experts may disapprove of is
-the declaration of all the free functions as friends, when in fact
-only a few of them need direct access to the iterator's private data.
-I originally got into the habit of doing this simply to keep the
-declarations together; declaring some functions inside the class and
-some outside seemed awkward. Since then, though, I've been told that
-there's a subtle difference in the way name lookup works for functions
-declared inside a class (as friends) and outside, so keeping them
-together in the class is probably a good idea for practical as well as
-aesthetic reasons.
-
-I hope all this is some help to anyone who needs to write their own
-STL-like containers and iterators.
-
---
-Ross Smith <ross.s@ihug.co.nz> The Internet Group, Auckland, New Zealand
+<div class="doc_code">
+<pre>
+<b>namespace {</b>
+class StringSort {
+...
+public:
+ StringSort(...)
+ bool operator<(const char *RHS) const;
+};
+
+void Helper() {
+ ...
+}
+
+bool StringSort::operator<(const char *RHS) const {
+ ...
+}
+
+<b>} // end anonymous namespace</b>
+
</pre>
+</div>
+
+
+<p>This is bad specifically because if you're looking at "Helper" in the middle
+of a large C++ file, that you have no immediate way to tell if it is local to
+the file. When it is marked static explicitly, this is immediately obvious.
+Also, there is no reason to enclose the definition of "operator<" in the
+namespace just because it was declared there.
+</p>
+
+</div>
+
<!-- *********************************************************************** -->
-</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0><tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
-<a name="seealso">See Also
-</b></font></td></tr></table><ul>
+<div class="doc_section">
+ <a name="seealso">See Also</a>
+</div>
<!-- *********************************************************************** -->
-A lot of these comments and recommendations have been culled for other sources.
-Two particularly important books for our work are:<p>
+<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.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>.
-<li><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++ Software Design</a> by John Lakos
-</ol><p>
-If you get some free time, and you haven't read them: do so, you might learn
-something. :)
+<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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-<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
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+ <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+ <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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projects / oota-llvm.git / blobdiff