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Misha Brukmanc1449e62008-12-10 23:07:02 +0000[diff] [blame]6 <title>LLVM Coding Standards</title>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]7</head>
8<body>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]9
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]10<h1>
Misha Brukmanc1449e62008-12-10 23:07:02 +0000[diff] [blame]11 LLVM Coding Standards
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]12</h1>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]13
14<ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]15 <li><a href="#introduction">Introduction</a></li>
Chris Lattner7ae36bb2001-07-23 20:40:41 +0000[diff] [blame]16 <li><a href="#mechanicalissues">Mechanical Source Issues</a>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]17 <ol>
18 <li><a href="#sourceformating">Source Code Formatting</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]19 <ol>
20 <li><a href="#scf_commenting">Commenting</a></li>
21 <li><a href="#scf_commentformat">Comment Formatting</a></li>
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]22 <li><a href="#scf_includes"><tt>#include</tt> Style</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]23 <li><a href="#scf_codewidth">Source Code Width</a></li>
24 <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>
25 <li><a href="#scf_indentation">Indent Code Consistently</a></li>
26 </ol></li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]27 <li><a href="#compilerissues">Compiler Issues</a>
28 <ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]29 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like
30 Errors</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]31 <li><a href="#ci_portable_code">Write Portable Code</a></li>
Chris Lattner558f5d22010-11-30 19:20:40 +0000[diff] [blame]32 <li><a href="#ci_rtti_exceptions">Do not use RTTI or Exceptions</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame]33 <li><a href="#ci_class_struct">Use of <tt>class</tt>/<tt>struct</tt> Keywords</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]34 </ol></li>
35 </ol></li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]36 <li><a href="#styleissues">Style Issues</a>
37 <ol>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]38 <li><a href="#macro">The High-Level Issues</a>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]39 <ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]40 <li><a href="#hl_module">A Public Header File <b>is</b> a
41 Module</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame]42 <li><a href="#hl_dontinclude"><tt>#include</tt> as Little as Possible</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]43 <li><a href="#hl_privateheaders">Keep "internal" Headers
44 Private</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame]45 <li><a href="#hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]46 Code</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame]47 <li><a href="#hl_else_after_return">Don't use <tt>else</tt> after a
48 <tt>return</tt></a></li>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]49 <li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
50 Functions</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]51 </ol></li>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]52 <li><a href="#micro">The Low-Level Issues</a>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]53 <ol>
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]54 <li><a href="#ll_naming">Name Types, Functions, Variables, and Enumerators Properly</a></li>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]55 <li><a href="#ll_assert">Assert Liberally</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame]56 <li><a href="#ll_ns_std">Do not use '<tt>using namespace std</tt>'</a></li>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]57 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]58 classes in headers</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame]59 <li><a href="#ll_end">Don't evaluate <tt>end()</tt> every time through a
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]60 loop</a></li>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]61 <li><a href="#ll_iostream"><tt>#include &lt;iostream&gt;</tt> is
62 <em>forbidden</em></a></li>
Chris Lattner78ceb3a2010-12-10 00:54:03 +0000[diff] [blame]63 <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a></li>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]64 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]65 </ol></li>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]66
67 <li><a href="#nano">Microscopic Details</a>
68 <ol>
69 <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
70 <li><a href="#micro_preincrement">Prefer Preincrement</a></li>
71 <li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
72 <li><a href="#micro_anonns">Anonymous Namespaces</a></li>
73 </ol></li>
74
75
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]76 </ol></li>
77 <li><a href="#seealso">See Also</a></li>
78</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]79
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]80<div class="doc_author">
Chris Lattner219bd292009-07-22 05:43:01 +0000[diff] [blame]81 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]82</div>
83
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]84
85<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]86<h2>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]87 <a name="introduction">Introduction</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]88</h2>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]89<!-- *********************************************************************** -->
90
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]91<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]92
93<p>This document attempts to describe a few coding standards that are being used
94in the LLVM source tree. Although no coding standards should be regarded as
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]95absolute requirements to be followed in all instances, coding standards can be
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]96useful.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]97
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]98<p>This document intentionally does not prescribe fixed standards for religious
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]99issues such as brace placement and space usage. For issues like this, follow
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]100the golden rule:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]101
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]102<blockquote>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]103
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]104<p><b><a name="goldenrule">If you are adding a significant body of source to a
105project, feel free to use whatever style you are most comfortable with. If you
106are extending, enhancing, or bug fixing already implemented code, use the style
107that is already being used so that the source is uniform and easy to
108follow.</a></b></p>
109
110</blockquote>
111
112<p>The ultimate goal of these guidelines is the increase readability and
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]113maintainability of our common source base. If you have suggestions for topics to
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]114be included, please mail them to <a
115href="mailto:sabre@nondot.org">Chris</a>.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]116
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]117</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]118
119<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]120<h2>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]121 <a name="mechanicalissues">Mechanical Source Issues</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]122</h2>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]123<!-- *********************************************************************** -->
124
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]125<div>
126
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]127<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]128<h3>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]129 <a name="sourceformating">Source Code Formatting</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]130</h3>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]131
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]132<div>
133
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]134<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]135<h4>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]136 <a name="scf_commenting">Commenting</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]137</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]138
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]139<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]140
141<p>Comments are one critical part of readability and maintainability. Everyone
Chris Lattnerb927ca82009-07-12 00:10:24 +0000[diff] [blame]142knows they should comment, so should you. When writing comments, write them as
143English prose, which means they should use proper capitalization, punctuation,
144etc. Although we all should probably
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]145comment our code more than we do, there are a few very critical places that
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]146documentation is very useful:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]147
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]148<h5>File Headers</h5>
149
150<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]151
Reid Spencer669ed452007-07-09 08:04:31 +0000[diff] [blame]152<p>Every source file should have a header on it that describes the basic
153purpose of the file. If a file does not have a header, it should not be
154checked into Subversion. Most source trees will probably have a standard
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]155file header format. The standard format for the LLVM source tree looks like
156this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]157
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]158<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]159<pre>
Chris Lattnerc49fcd12003-10-13 14:58:11 +0000[diff] [blame]160//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
Misha Brukman98117cd2009-01-02 16:58:42 +0000[diff] [blame]161//
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]162// The LLVM Compiler Infrastructure
163//
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]164// This file is distributed under the University of Illinois Open Source
165// License. See LICENSE.TXT for details.
Misha Brukman98117cd2009-01-02 16:58:42 +0000[diff] [blame]166//
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]167//===----------------------------------------------------------------------===//
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]168//
169// This file contains the declaration of the Instruction class, which is the
170// base class for all of the VM instructions.
171//
172//===----------------------------------------------------------------------===//
173</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]174</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]175
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]176<p>A few things to note about this particular format: The "<tt>-*- C++
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]177-*-</tt>" string on the first line is there to tell Emacs that the source file
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]178is a C++ file, not a C file (Emacs assumes <tt>.h</tt> files are C files by default).
179Note that this tag is not necessary in <tt>.cpp</tt> files. The name of the file is also
Misha Brukman69c58892004-07-28 22:37:57 +0000[diff] [blame]180on the first line, along with a very short description of the purpose of the
181file. This is important when printing out code and flipping though lots of
182pages.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]183
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]184<p>The next section in the file is a concise note that defines the license
185that the file is released under. This makes it perfectly clear what terms the
186source code can be distributed under and should not be modified in any way.</p>
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]187
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]188<p>The main body of the description does not have to be very long in most cases.
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]189Here it's only two lines. If an algorithm is being implemented or something
190tricky is going on, a reference to the paper where it is published should be
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]191included, as well as any notes or "gotchas" in the code to watch out for.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]192
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]193</div>
194
195<h5>Class overviews</h5>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]196
Jim Laskeyf55914a2006-07-31 20:18:49 +0000[diff] [blame]197<p>Classes are one fundamental part of a good object oriented design. As such,
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]198a class definition should have a comment block that explains what the class is
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]199used for... if it's not obvious. If it's so completely obvious your grandma
200could figure it out, it's probably safe to leave it out. Naming classes
Misha Brukman3b53a262004-12-04 00:32:12 +0000[diff] [blame]201something sane goes a long ways towards avoiding writing documentation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]202
203
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]204<h5>Method information</h5>
205
206<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]207
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]208<p>Methods defined in a class (as well as any global functions) should also be
Dan Gohman621a4152010-02-26 20:18:32 +0000[diff] [blame]209documented properly. A quick note about what it does and a description of the
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]210borderline behaviour is all that is necessary here (unless something
Dan Gohman621a4152010-02-26 20:18:32 +0000[diff] [blame]211particularly tricky or insidious is going on). The hope is that people can
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]212figure out how to use your interfaces without reading the code itself... that is
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]213the goal metric.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]214
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]215<p>Good things to talk about here are what happens when something unexpected
216happens: does the method return null? Abort? Format your hard disk?</p>
217
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]218</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]219
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]220</div>
221
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]222<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]223<h4>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]224 <a name="scf_commentformat">Comment Formatting</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]225</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]226
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]227<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]228
229<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]230require less typing, don't have nesting problems, etc. There are a few cases
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]231when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]232
233<ol>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]234 <li>When writing C code: Obviously if you are writing C code, use C style
Misha Brukman3b53a262004-12-04 00:32:12 +0000[diff] [blame]235 comments.</li>
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]236 <li>When writing a header file that may be <tt>#include</tt>d by a C source
237 file.</li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]238 <li>When writing a source file that is used by a tool that only accepts C
239 style comments.</li>
240</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]241
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]242<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
243These nest properly and are better behaved in general than C style comments.</p>
244
245</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]246
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]247<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]248<h4>
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]249 <a name="scf_includes"><tt>#include</tt> Style</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]250</h4>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]251
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]252<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]253
254<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]255include guards if working on a header file), the <a
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]256href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
257file should be listed. We prefer these <tt>#include</tt>s to be listed in this
258order:</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]259
260<ol>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]261 <li><a href="#mmheader">Main Module Header</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]262 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]263 <li><tt>llvm/*</tt></li>
264 <li><tt>llvm/Analysis/*</tt></li>
265 <li><tt>llvm/Assembly/*</tt></li>
Duncan Sands18d52f22010-09-29 20:09:55 +0000[diff] [blame]266 <li><tt>llvm/Bitcode/*</tt></li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]267 <li><tt>llvm/CodeGen/*</tt></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]268 <li>...</li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]269 <li><tt>Support/*</tt></li>
270 <li><tt>Config/*</tt></li>
271 <li>System <tt>#includes</tt></li>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]272</ol>
273
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]274<p>and each category should be sorted by name.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]275
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]276<p><a name="mmheader">The "Main Module Header"</a> file applies to <tt>.cpp</tt> files
277which implement an interface defined by a <tt>.h</tt> file. This <tt>#include</tt>
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]278should always be included <b>first</b> regardless of where it lives on the file
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]279system. By including a header file first in the <tt>.cpp</tt> files that implement the
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]280interfaces, we ensure that the header does not have any hidden dependencies
281which are not explicitly #included in the header, but should be. It is also a
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]282form of documentation in the <tt>.cpp</tt> file to indicate where the interfaces it
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]283implements are defined.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]284
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]285</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]286
287<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]288<h4>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]289 <a name="scf_codewidth">Source Code Width</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]290</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]291
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]292<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]293
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]294<p>Write your code to fit within 80 columns of text. This helps those of us who
295like to print out code and look at your code in an xterm without resizing
296it.</p>
297
Chris Lattner5822e9e2008-07-08 05:12:37 +0000[diff] [blame]298<p>The longer answer is that there must be some limit to the width of the code
299in order to reasonably allow developers to have multiple files side-by-side in
300windows on a modest display. If you are going to pick a width limit, it is
301somewhat arbitrary but you might as well pick something standard. Going with
30290 columns (for example) instead of 80 columns wouldn't add any significant
303value and would be detrimental to printing out code. Also many other projects
304have standardized on 80 columns, so some people have already configured their
305editors for it (vs something else, like 90 columns).</p>
306
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]307<p>This is one of many contentious issues in coding standards, but it is not up
Chris Lattner5822e9e2008-07-08 05:12:37 +0000[diff] [blame]308for debate.</p>
309
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]310</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]311
312<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]313<h4>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]314 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]315</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]316
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]317<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]318
319<p>In all cases, prefer spaces to tabs in source files. People have different
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]320preferred indentation levels, and different styles of indentation that they
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]321like; this is fine. What isn't fine is that different editors/viewers expand
322tabs out to different tab stops. This can cause your code to look completely
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]323unreadable, and it is not worth dealing with.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]324
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]325<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]326style of existing code if you are modifying and extending it. If you like four
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]327spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
328with two spaces of indentation. Also, do not reindent a whole source file: it
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]329makes for incredible diffs that are absolutely worthless.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]330
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]331</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]332
333<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]334<h4>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]335 <a name="scf_indentation">Indent Code Consistently</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]336</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]337
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]338<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]339
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]340<p>Okay, in your first year of programming you were told that indentation is
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]341important. If you didn't believe and internalize this then, now is the time.
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]342Just do it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]343
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]344</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]345
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]346</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]347
348<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]349<h3>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]350 <a name="compilerissues">Compiler Issues</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]351</h3>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]352
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]353<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]354
355<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]356<h4>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]357 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]358</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]359
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]360<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]361
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]362<p>If your code has compiler warnings in it, something is wrong &mdash; you
363aren't casting values correctly, your have "questionable" constructs in your
364code, or you are doing something legitimately wrong. Compiler warnings can
365cover up legitimate errors in output and make dealing with a translation unit
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]366difficult.</p>
367
368<p>It is not possible to prevent all warnings from all compilers, nor is it
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]369desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]370a good thorough set of warnings, and stick to it. At least in the case of
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]371<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]372syntax of the code slightly. For example, a warning that annoys me occurs when
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]373I write code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]374
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]375<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]376<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]377if (V = getValue()) {
378 ...
379}
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]380</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]381</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]382
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]383<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
384operator, and that I probably mistyped it. In most cases, I haven't, and I
385really don't want the spurious errors. To fix this particular problem, I
386rewrite the code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]387
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]388<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]389<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]390if ((V = getValue())) {
391 ...
392}
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]393</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]394</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]395
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]396<p>which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]397be fixed by massaging the code appropriately.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]398
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]399<p>These are the <tt>gcc</tt> warnings that I prefer to enable:</p>
400
401<div class="doc_code">
402<pre>
403-Wall -Winline -W -Wwrite-strings -Wno-unused
404</pre>
405</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]406
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]407</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]408
409<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]410<h4>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]411 <a name="ci_portable_code">Write Portable Code</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]412</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]413
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]414<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]415
416<p>In almost all cases, it is possible and within reason to write completely
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]417portable code. If there are cases where it isn't possible to write portable
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]418code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]419
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]420<p>In practice, this means that you shouldn't assume much about the host
Chris Lattner221b2392010-11-16 22:19:06 +0000[diff] [blame]421compiler, and Visual Studio tends to be the lowest common denominator.
422If advanced features are used, they should only be an implementation detail of
423a library which has a simple exposed API, and preferably be buried in
424libSystem.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]425
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]426</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]427
Reid Spencerb40df562004-09-23 16:03:48 +0000[diff] [blame]428<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]429<h4>
Chris Lattner558f5d22010-11-30 19:20:40 +0000[diff] [blame]430<a name="ci_rtti_exceptions">Do not use RTTI or Exceptions</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]431</h4>
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]432<div>
Chris Lattner558f5d22010-11-30 19:20:40 +0000[diff] [blame]433
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]434<p>In an effort to reduce code and executable size, LLVM does not use RTTI
435(e.g. <tt>dynamic_cast&lt;&gt;</tt>) or exceptions. These two language features
436violate the general C++ principle of <i>"you only pay for what you use"</i>,
437causing executable bloat even if exceptions are never used in the code base, or
438if RTTI is never used for a class. Because of this, we turn them off globally
439in the code.</p>
Chris Lattner558f5d22010-11-30 19:20:40 +0000[diff] [blame]440
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]441<p>That said, LLVM does make extensive use of a hand-rolled form of RTTI that
442use templates like <a href="ProgrammersManual.html#isa"><tt>isa&lt;&gt;</tt>,
443<tt>cast&lt;&gt;</tt>, and <tt>dyn_cast&lt;&gt;</tt></a>. This form of RTTI is
444opt-in and can be added to any class. It is also substantially more efficient
445than <tt>dynamic_cast&lt;&gt;</tt>.</p>
Chris Lattner558f5d22010-11-30 19:20:40 +0000[diff] [blame]446
447</div>
448
449<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]450<h4>
Reid Spencerb40df562004-09-23 16:03:48 +0000[diff] [blame]451<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]452</h4>
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]453<div>
Misha Brukmanedcc78a2004-10-26 15:45:13 +0000[diff] [blame]454
455<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
456interchangeably. The only difference is when they are used to declare a class:
457<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
458all members public by default.</p>
459
460<p>Unfortunately, not all compilers follow the rules and some will generate
461different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
462declare the symbol. This can lead to problems at link time.</p>
463
464<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]465<b>all</b> members are public and the type is a C++
466<a href="http://en.wikipedia.org/wiki/Plain_old_data_structure">POD</a> type, in
467which case <tt>struct</tt> is allowed.</p>
Misha Brukmanedcc78a2004-10-26 15:45:13 +0000[diff] [blame]468
Reid Spencerb40df562004-09-23 16:03:48 +0000[diff] [blame]469</div>
470
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]471</div>
472
473</div>
474
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]475<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]476<h2>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]477 <a name="styleissues">Style Issues</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]478</h2>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]479<!-- *********************************************************************** -->
480
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]481<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]482
483<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]484<h3>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]485 <a name="macro">The High-Level Issues</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]486</h3>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]487<!-- ======================================================================= -->
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]488
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]489<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]490
491<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]492<h4>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]493 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]494</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]495
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]496<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]497
498<p>C++ doesn't do too well in the modularity department. There is no real
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]499encapsulation or data hiding (unless you use expensive protocol classes), but it
500is what we have to work with. When you write a public header file (in the LLVM
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]501source tree, they live in the top level "<tt>include</tt>" directory), you are
502defining a module of functionality.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]503
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]504<p>Ideally, modules should be completely independent of each other, and their
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]505header files should only <tt>#include</tt> the absolute minimum number of
506headers possible. A module is not just a class, a function, or a
507namespace: <a href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's
508a collection of these</a> that defines an interface. This interface may be
509several functions, classes, or data structures, but the important issue is how
510they work together.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]511
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]512<p>In general, a module should be implemented by one or more <tt>.cpp</tt>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]513files. Each of these <tt>.cpp</tt> files should include the header that defines
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]514their interface first. This ensures that all of the dependences of the module
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]515header have been properly added to the module header itself, and are not
516implicit. System headers should be included after user headers for a
517translation unit.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]518
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]519</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]520
521<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]522<h4>
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]523 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]524</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]525
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]526<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]527
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]528<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
529have to, especially in header files.</p>
530
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]531<p>But wait! Sometimes you need to have the definition of a class to use it, or
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]532to inherit from it. In these cases go ahead and <tt>#include</tt> that header
533file. Be aware however that there are many cases where you don't need to have
534the full definition of a class. If you are using a pointer or reference to a
535class, you don't need the header file. If you are simply returning a class
536instance from a prototyped function or method, you don't need it. In fact, for
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]537most cases, you simply don't need the definition of a class. And not
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]538<tt>#include</tt>'ing speeds up compilation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]539
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]540<p>It is easy to try to go too overboard on this recommendation, however. You
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]541<b>must</b> include all of the header files that you are using &mdash; you can
542include them either directly or indirectly (through another header file). To
543make sure that you don't accidentally forget to include a header file in your
544module header, make sure to include your module header <b>first</b> in the
545implementation file (as mentioned above). This way there won't be any hidden
546dependencies that you'll find out about later.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]547
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]548</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]549
550<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]551<h4>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]552 <a name="hl_privateheaders">Keep "Internal" Headers Private</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]553</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]554
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]555<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]556
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]557<p>Many modules have a complex implementation that causes them to use more than
558one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
559internal communication interface (helper classes, extra functions, etc) in the
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]560public module header file. Don't do this!</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]561
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]562<p>If you really need to do something like this, put a private header file in
563the same directory as the source files, and include it locally. This ensures
564that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]565
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]566<p>Note however, that it's okay to put extra implementation methods in a public
567class itself. Just make them private (or protected) and all is well.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]568
569</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]570
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]571<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]572<h4>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame]573 <a name="hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify Code</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]574</h4>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]575
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]576<div>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]577
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]578<p>When reading code, keep in mind how much state and how many previous
579decisions have to be remembered by the reader to understand a block of code.
580Aim to reduce indentation where possible when it doesn't make it more difficult
581to understand the code. One great way to do this is by making use of early
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]582exits and the <tt>continue</tt> keyword in long loops. As an example of using
583an early exit from a function, consider this "bad" code:</p>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]584
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]585<div class="doc_code">
586<pre>
587Value *DoSomething(Instruction *I) {
588 if (!isa&lt;TerminatorInst&gt;(I) &amp;&amp;
589 I-&gt;hasOneUse() &amp;&amp; SomeOtherThing(I)) {
590 ... some long code ....
591 }
592
593 return 0;
594}
595</pre>
596</div>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]597
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]598<p>This code has several problems if the body of the '<tt>if</tt>' is large.
599When you're looking at the top of the function, it isn't immediately clear that
600this <em>only</em> does interesting things with non-terminator instructions, and
601only applies to things with the other predicates. Second, it is relatively
602difficult to describe (in comments) why these predicates are important because
603the <tt>if</tt> statement makes it difficult to lay out the comments. Third,
604when you're deep within the body of the code, it is indented an extra level.
605Finally, when reading the top of the function, it isn't clear what the result is
606if the predicate isn't true; you have to read to the end of the function to know
607that it returns null.</p>
Matthijs Kooijmaned58a972008-07-30 12:14:10 +0000[diff] [blame]608
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]609<p>It is much preferred to format the code like this:</p>
610
611<div class="doc_code">
612<pre>
613Value *DoSomething(Instruction *I) {
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]614 // Terminators never need 'something' done to them because ...
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]615 if (isa&lt;TerminatorInst&gt;(I))
616 return 0;
617
618 // We conservatively avoid transforming instructions with multiple uses
619 // because goats like cheese.
620 if (!I-&gt;hasOneUse())
621 return 0;
622
623 // This is really just here for example.
624 if (!SomeOtherThing(I))
625 return 0;
626
627 ... some long code ....
628}
629</pre>
630</div>
631
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]632<p>This fixes these problems. A similar problem frequently happens in <tt>for</tt>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]633loops. A silly example is something like this:</p>
634
635<div class="doc_code">
636<pre>
637 for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; ++II) {
638 if (BinaryOperator *BO = dyn_cast&lt;BinaryOperator&gt;(II)) {
639 Value *LHS = BO-&gt;getOperand(0);
640 Value *RHS = BO-&gt;getOperand(1);
641 if (LHS != RHS) {
642 ...
643 }
644 }
645 }
646</pre>
647</div>
648
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]649<p>When you have very, very small loops, this sort of structure is fine. But if
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]650it exceeds more than 10-15 lines, it becomes difficult for people to read and
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]651understand at a glance. The problem with this sort of code is that it gets very
652nested very quickly. Meaning that the reader of the code has to keep a lot of
653context in their brain to remember what is going immediately on in the loop,
654because they don't know if/when the <tt>if</tt> conditions will have elses etc.
655It is strongly preferred to structure the loop like this:</p>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]656
657<div class="doc_code">
658<pre>
659 for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; ++II) {
660 BinaryOperator *BO = dyn_cast&lt;BinaryOperator&gt;(II);
661 if (!BO) continue;
662
663 Value *LHS = BO-&gt;getOperand(0);
664 Value *RHS = BO-&gt;getOperand(1);
665 if (LHS == RHS) continue;
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]666
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]667 ...
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]668 }
669</pre>
670</div>
671
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]672<p>This has all the benefits of using early exits for functions: it reduces
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]673nesting of the loop, it makes it easier to describe why the conditions are true,
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]674and it makes it obvious to the reader that there is no <tt>else</tt> coming up
675that they have to push context into their brain for. If a loop is large, this
676can be a big understandability win.</p>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]677
678</div>
679
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]680<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]681<h4>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame]682 <a name="hl_else_after_return">Don't use <tt>else</tt> after a <tt>return</tt></a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]683</h4>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]684
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]685<div>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]686
687<p>For similar reasons above (reduction of indentation and easier reading),
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]688please do not use '<tt>else</tt>' or '<tt>else if</tt>' after something that
689interrupts control flow &mdash; like <tt>return</tt>, <tt>break</tt>,
690<tt>continue</tt>, <tt>goto</tt>, etc. For example, this is <em>bad</em>:</p>
691
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]692<div class="doc_code">
693<pre>
694 case 'J': {
695 if (Signed) {
696 Type = Context.getsigjmp_bufType();
697 if (Type.isNull()) {
698 Error = ASTContext::GE_Missing_sigjmp_buf;
699 return QualType();
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]700 <b>} else {
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]701 break;
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]702 }</b>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]703 } else {
704 Type = Context.getjmp_bufType();
705 if (Type.isNull()) {
706 Error = ASTContext::GE_Missing_jmp_buf;
707 return QualType();
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]708 <b>} else {
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]709 break;
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]710 }</b>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]711 }
712 }
713 }
714</pre>
715</div>
716
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]717<p>It is better to write it like this:</p>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]718
719<div class="doc_code">
720<pre>
721 case 'J':
722 if (Signed) {
723 Type = Context.getsigjmp_bufType();
724 if (Type.isNull()) {
725 Error = ASTContext::GE_Missing_sigjmp_buf;
726 return QualType();
727 }
728 } else {
729 Type = Context.getjmp_bufType();
730 if (Type.isNull()) {
731 Error = ASTContext::GE_Missing_jmp_buf;
732 return QualType();
733 }
734 }
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]735 <b>break;</b>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]736</pre>
737</div>
738
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]739<p>Or better yet (in this case) as:</p>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]740
741<div class="doc_code">
742<pre>
743 case 'J':
744 if (Signed)
745 Type = Context.getsigjmp_bufType();
746 else
747 Type = Context.getjmp_bufType();
748
749 if (Type.isNull()) {
750 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
751 ASTContext::GE_Missing_jmp_buf;
752 return QualType();
753 }
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]754 <b>break;</b>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]755</pre>
756</div>
757
758<p>The idea is to reduce indentation and the amount of code you have to keep
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]759track of when reading the code.</p>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]760
761</div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]762
763<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]764<h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]765 <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]766</h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]767
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]768<div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]769
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]770<p>It is very common to write small loops that just compute a boolean value.
771There are a number of ways that people commonly write these, but an example of
772this sort of thing is:</p>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]773
774<div class="doc_code">
775<pre>
776 <b>bool FoundFoo = false;</b>
777 for (unsigned i = 0, e = BarList.size(); i != e; ++i)
778 if (BarList[i]-&gt;isFoo()) {
779 <b>FoundFoo = true;</b>
780 break;
781 }
782
783 <b>if (FoundFoo) {</b>
784 ...
785 }
786</pre>
787</div>
788
789<p>This sort of code is awkward to write, and is almost always a bad sign.
790Instead of this sort of loop, we strongly prefer to use a predicate function
791(which may be <a href="#micro_anonns">static</a>) that uses
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]792<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]793the code to be structured like this:</p>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]794
795<div class="doc_code">
796<pre>
797/// ListContainsFoo - Return true if the specified list has an element that is
798/// a foo.
799static bool ListContainsFoo(const std::vector&lt;Bar*&gt; &amp;List) {
800 for (unsigned i = 0, e = List.size(); i != e; ++i)
801 if (List[i]-&gt;isFoo())
802 return true;
803 return false;
804}
805...
806
807 <b>if (ListContainsFoo(BarList)) {</b>
808 ...
809 }
810</pre>
811</div>
812
813<p>There are many reasons for doing this: it reduces indentation and factors out
814code which can often be shared by other code that checks for the same predicate.
815More importantly, it <em>forces you to pick a name</em> for the function, and
816forces you to write a comment for it. In this silly example, this doesn't add
817much value. However, if the condition is complex, this can make it a lot easier
818for the reader to understand the code that queries for this predicate. Instead
Chris Lattner71d8f3b2009-07-22 16:30:39 +0000[diff] [blame]819of being faced with the in-line details of how we check to see if the BarList
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]820contains a foo, we can trust the function name and continue reading with better
821locality.</p>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]822
823</div>
824
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]825</div>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]826
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]827<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]828<h3>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]829 <a name="micro">The Low-Level Issues</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]830</h3>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]831<!-- ======================================================================= -->
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]832
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]833<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]834
835<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]836<h4>
837 <a name="ll_naming">
838 Name Types, Functions, Variables, and Enumerators Properly
839 </a>
840</h4>
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]841
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]842<div>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]843
844<p>Poorly-chosen names can mislead the reader and cause bugs. We cannot stress
845enough how important it is to use <em>descriptive</em> names. Pick names that
846match the semantics and role of the underlying entities, within reason. Avoid
847abbreviations unless they are well known. After picking a good name, make sure
848to use consistent capitalization for the name, as inconsistency requires clients
849to either memorize the APIs or to look it up to find the exact spelling.</p>
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]850
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]851<p>In general, names should be in camel case (e.g. <tt>TextFileReader</tt>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]852and <tt>isLValue()</tt>). Different kinds of declarations have different
853rules:</p>
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]854
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]855<ul>
Chris Lattner78ceb3a2010-12-10 00:54:03 +0000[diff] [blame]856<li><p><b>Type names</b> (including classes, structs, enums, typedefs, etc)
857 should be nouns and start with an upper-case letter (e.g.
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]858 <tt>TextFileReader</tt>).</p></li>
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]859
Chris Lattner78ceb3a2010-12-10 00:54:03 +0000[diff] [blame]860<li><p><b>Function names</b> should be verb phrases (as they represent
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]861 actions), and command-like function should be imperative. The name should
862 be camel case, and start with a lower case letter (e.g. <tt>openFile()</tt>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]863 or <tt>isFoo()</tt>).</p></li>
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]864
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]865<li><p><b>Enum declarations</b> (e.g. <tt>enum Foo {...}</tt>) are types, so
866 they should follow the naming conventions for types. A common use for enums
867 is as a discriminator for a union, or an indicator of a subclass. When an
868 enum is used for something like this, it should have a <tt>Kind</tt> suffix
869 (e.g. <tt>ValueKind</tt>).</p></li>
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]870
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]871<li><p><b>Enumerators</b> (e.g. <tt>enum { Foo, Bar }</tt>) and <b>public member
872 variables</b> should start with an upper-case letter, just like types.
873 Unless the enumerators are defined in their own small namespace or inside a
874 class, enumerators should have a prefix corresponding to the enum
875 declaration name. For example, <tt>enum ValueKind { ... };</tt> may contain
876 enumerators like <tt>VK_Argument</tt>, <tt>VK_BasicBlock</tt>, etc.
877 Enumerators that are just convenience constants are exempt from the
878 requirement for a prefix. For instance:</p>
879
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]880<div class="doc_code">
881<pre>
882enum {
883 MaxSize = 42,
884 Density = 12
885};
886</pre>
887</div>
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]888</li>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]889
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]890</ul>
891
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]892<p>As an exception, classes that mimic STL classes can have member names in
893STL's style of lower-case words separated by underscores (e.g. <tt>begin()</tt>,
894<tt>push_back()</tt>, and <tt>empty()</tt>).</p>
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]895
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]896<p>Here are some examples of good and bad names:</p>
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]897
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]898<div class="doc_code">
899<pre>
900class VehicleMaker {
901 ...
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]902 Factory&lt;Tire&gt; F; // Bad -- abbreviation and non-descriptive.
903 Factory&lt;Tire&gt; Factory; // Better.
904 Factory&lt;Tire&gt; TireFactory; // Even better -- if VehicleMaker has more than one
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]905 // kind of factories.
906};
907
908Vehicle MakeVehicle(VehicleType Type) {
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]909 VehicleMaker M; // Might be OK if having a short life-span.
910 Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information.
Chris Lattner69ad7972010-12-10 00:52:35 +0000[diff] [blame]911 Light headlight = M.makeLight("head"); // Good -- descriptive.
Zhanyong Wan7fcd4dc2010-12-02 05:10:07 +0000[diff] [blame]912 ...
913}
914</pre>
915</div>
916
917</div>
918
919
920<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]921<h4>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]922 <a name="ll_assert">Assert Liberally</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]923</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]924
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]925<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]926
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]927<p>Use the "<tt>assert</tt>" macro to its fullest. Check all of your
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]928preconditions and assumptions, you never know when a bug (not necessarily even
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]929yours) might be caught early by an assertion, which reduces debugging time
930dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already
931included by the header files you are using, so it doesn't cost anything to use
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]932it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]933
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]934<p>To further assist with debugging, make sure to put some kind of error message
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]935in the assertion statement, which is printed if the assertion is tripped. This
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]936helps the poor debugger make sense of why an assertion is being made and
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]937enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]938
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]939<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]940<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]941inline Value *getOperand(unsigned i) {
942 assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
943 return Operands[i];
944}
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]945</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]946</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]947
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]948<p>Here are more examples:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]949
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]950<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]951<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]952assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]953
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]954assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]955
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]956assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]957
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]958assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]959
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]960assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]961</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]962</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]963
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]964<p>You get the idea.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]965
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]966<p>Please be aware that, when adding assert statements, not all compilers are aware of
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]967the semantics of the assert. In some places, asserts are used to indicate a piece of
968code that should not be reached. These are typically of the form:</p>
969
970<div class="doc_code">
971<pre>
Dan Gohmanf0032762008-11-24 17:18:39 +0000[diff] [blame]972assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]973</pre>
974</div>
975
976<p>When used in a function that returns a value, they should be followed with a return
977statement and a comment indicating that this line is never reached. This will prevent
978a compiler which is unable to deduce that the assert statement never returns from
979generating a warning.</p>
980
981<div class="doc_code">
982<pre>
Dan Gohmanf0032762008-11-24 17:18:39 +0000[diff] [blame]983assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]984// Not reached
985return 0;
986</pre>
987</div>
988
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]989<p>Another issue is that values used only by assertions will produce an "unused
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]990value" warning when assertions are disabled. For example, this code will
991warn:</p>
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]992
993<div class="doc_code">
994<pre>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]995unsigned Size = V.size();
996assert(Size &gt; 42 &amp;&amp; "Vector smaller than it should be");
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]997
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]998bool NewToSet = Myset.insert(Value);
999assert(NewToSet &amp;&amp; "The value shouldn't be in the set yet");
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]1000</pre>
1001</div>
1002
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1003<p>These are two interesting different cases. In the first case, the call to
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]1004V.size() is only useful for the assert, and we don't want it executed when
1005assertions are disabled. Code like this should move the call into the assert
1006itself. In the second case, the side effects of the call must happen whether
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1007the assert is enabled or not. In this case, the value should be cast to void to
1008disable the warning. To be specific, it is preferred to write the code like
1009this:</p>
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]1010
1011<div class="doc_code">
1012<pre>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1013assert(V.size() &gt; 42 &amp;&amp; "Vector smaller than it should be");
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]1014
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1015bool NewToSet = Myset.insert(Value); (void)NewToSet;
1016assert(NewToSet &amp;&amp; "The value shouldn't be in the set yet");
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]1017</pre>
1018</div>
1019
1020
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1021</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1022
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]1023<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1024<h4>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1025 <a name="ll_ns_std">Do Not Use '<tt>using namespace std</tt>'</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1026</h4>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]1027
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1028<div>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1029
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]1030<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]1031namespace with an "<tt>std::</tt>" prefix, rather than rely on
1032"<tt>using namespace std;</tt>".</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]1033
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]1034<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
Misha Brukman2d33c542008-12-11 19:37:04 +0000[diff] [blame]1035the namespace of any source file that <tt>#include</tt>s the header. This is
1036clearly a bad thing.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]1037
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1038<p>In implementation files (e.g. <tt>.cpp</tt> files), the rule is more of a stylistic
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]1039rule, but is still important. Basically, using explicit namespace prefixes
1040makes the code <b>clearer</b>, because it is immediately obvious what facilities
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1041are being used and where they are coming from. And <b>more portable</b>, because
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]1042namespace clashes cannot occur between LLVM code and other namespaces. The
1043portability rule is important because different standard library implementations
1044expose different symbols (potentially ones they shouldn't), and future revisions
1045to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
1046such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]1047
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]1048<p>The exception to the general rule (i.e. it's not an exception for
1049the <tt>std</tt> namespace) is for implementation files. For example, all of
1050the code in the LLVM project implements code that lives in the 'llvm' namespace.
Chris Lattnereddd9692010-11-17 19:47:20 +0000[diff] [blame]1051As such, it is ok, and actually clearer, for the <tt>.cpp</tt> files to have a
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1052'<tt>using namespace llvm;</tt>' directive at the top, after the
Chris Lattnereddd9692010-11-17 19:47:20 +0000[diff] [blame]1053<tt>#include</tt>s. This reduces indentation in the body of the file for source
1054editors that indent based on braces, and keeps the conceptual context cleaner.
1055The general form of this rule is that any <tt>.cpp</tt> file that implements
1056code in any namespace may use that namespace (and its parents'), but should not
1057use any others.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]1058
1059</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1060
1061<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1062<h4>
1063 <a name="ll_virtual_anch">
1064 Provide a Virtual Method Anchor for Classes in Headers
1065 </a>
1066</h4>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1067
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1068<div>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1069
1070<p>If a class is defined in a header file and has a v-table (either it has
1071virtual methods or it derives from classes with virtual methods), it must
1072always have at least one out-of-line virtual method in the class. Without
Misha Brukman2d33c542008-12-11 19:37:04 +0000[diff] [blame]1073this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
1074that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
1075increasing link times.</p>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1076
1077</div>
1078
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]1079<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1080<h4>
Nick Lewycky7ac01992011-02-20 02:03:04 +0000[diff] [blame]1081 <a name="ll_end">Don't evaluate <tt>end()</tt> every time through a loop</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1082</h4>
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]1083
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1084<div>
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]1085
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1086<p>Because C++ doesn't have a standard "<tt>foreach</tt>" loop (though it can be
1087emulated with macros and may be coming in C++'0x) we end up writing a lot of
1088loops that manually iterate from begin to end on a variety of containers or
1089through other data structures. One common mistake is to write a loop in this
1090style:</p>
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]1091
1092<div class="doc_code">
1093<pre>
1094 BasicBlock *BB = ...
1095 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
1096 ... use I ...
1097</pre>
1098</div>
1099
1100<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
1101every time through the loop. Instead of writing the loop like this, we strongly
1102prefer loops to be written so that they evaluate it once before the loop starts.
1103A convenient way to do this is like so:</p>
1104
1105<div class="doc_code">
1106<pre>
1107 BasicBlock *BB = ...
1108 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
1109 ... use I ...
1110</pre>
1111</div>
1112
1113<p>The observant may quickly point out that these two loops may have different
1114semantics: if the container (a basic block in this case) is being mutated, then
1115"<tt>BB->end()</tt>" may change its value every time through the loop and the
1116second loop may not in fact be correct. If you actually do depend on this
Chris Lattner09bc65e2009-06-30 06:20:03 +0000[diff] [blame]1117behavior, please write the loop in the first form and add a comment indicating
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]1118that you did it intentionally.</p>
1119
1120<p>Why do we prefer the second form (when correct)? Writing the loop in the
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1121first form has two problems. First it may be less efficient than evaluating it
1122at the start of the loop. In this case, the cost is probably minor &mdash; a
1123few extra loads every time through the loop. However, if the base expression is
1124more complex, then the cost can rise quickly. I've seen loops where the end
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]1125expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
Chris Lattnera44f87f2009-06-30 06:27:54 +0000[diff] [blame]1126lookups really aren't cheap. By writing it in the second form consistently, you
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]1127eliminate the issue entirely and don't even have to think about it.</p>
1128
Chris Lattner09bc65e2009-06-30 06:20:03 +0000[diff] [blame]1129<p>The second (even bigger) issue is that writing the loop in the first form
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]1130hints to the reader that the loop is mutating the container (a fact that a
1131comment would handily confirm!). If you write the loop in the second form, it
1132is immediately obvious without even looking at the body of the loop that the
1133container isn't being modified, which makes it easier to read the code and
1134understand what it does.</p>
1135
1136<p>While the second form of the loop is a few extra keystrokes, we do strongly
1137prefer it.</p>
1138
1139</div>
1140
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1141<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1142<h4>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1143 <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is Forbidden</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1144</h4>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1145
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1146<div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1147
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1148<p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
1149hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
1150support clients using LLVM libraries as part of larger systems. In particular,
1151we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1152the static constructors are run whenever an application starts up that uses the
1153dynamic library. There are two problems with this:</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1154
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1155<ol>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1156 <li>The time to run the static c'tors impacts startup time of applications
1157 &mdash; a critical time for GUI apps.</li>
1158
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1159 <li>The static c'tors cause the app to pull many extra pages of memory off the
1160 disk: both the code for the static c'tors in each <tt>.o</tt> file and the
1161 small amount of data that gets touched. In addition, touched/dirty pages
1162 put more pressure on the VM system on low-memory machines.</li>
1163</ol>
1164
1165<p>Note that using the other stream headers (<tt>&lt;sstream&gt;</tt> for
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1166example) is not problematic in this regard &mdash;
1167just <tt>&lt;iostream&gt;</tt>. However, <tt>raw_ostream</tt> provides various
1168APIs that are better performing for almost every use than <tt>std::ostream</tt>
1169style APIs. <b>Therefore new code should always
Chris Lattner983c5922009-08-23 21:53:47 +0000[diff] [blame]1170use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or
1171the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1172
1173</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1174
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1175
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1176<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1177<h4>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1178 <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1179</h4>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1180
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1181<div>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1182
1183<p>LLVM includes a lightweight, simple, and efficient stream implementation
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1184in <tt>llvm/Support/raw_ostream.h</tt>, which provides all of the common
1185features of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt>
1186instead of <tt>ostream</tt>.</p>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1187
1188<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
1189be forward declared as <tt>class raw_ostream</tt>. Public headers should
1190generally not include the <tt>raw_ostream</tt> header, but use forward
1191declarations and constant references to <tt>raw_ostream</tt> instances.</p>
1192
1193</div>
1194
1195
1196<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1197<h4>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1198 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1199</h4>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1200
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1201<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1202
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1203<p>The <tt>std::endl</tt> modifier, when used with <tt>iostreams</tt> outputs a
1204newline to the output stream specified. In addition to doing this, however, it
1205also flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1206
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1207<div class="doc_code">
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1208<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1209std::cout &lt;&lt; std::endl;
1210std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1211</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1212</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1213
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1214<p>Most of the time, you probably have no reason to flush the output stream, so
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1215it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1216
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1217</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1218
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1219</div>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]1220
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1221<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1222<h3>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1223 <a name="nano">Microscopic Details</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1224</h3>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1225<!-- ======================================================================= -->
1226
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1227<div>
1228
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1229<p>This section describes preferred low-level formatting guidelines along with
1230reasoning on why we prefer them.</p>
1231
1232<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1233<h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1234 <a name="micro_spaceparen">Spaces Before Parentheses</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1235</h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1236
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1237<div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1238
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1239<p>We prefer to put a space before an open parenthesis only in control flow
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1240statements, but not in normal function call expressions and function-like
1241macros. For example, this is good:</p>
1242
1243<div class="doc_code">
1244<pre>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1245<b>if (</b>x) ...
1246<b>for (</b>i = 0; i != 100; ++i) ...
1247<b>while (</b>llvm_rocks) ...
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1248
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1249<b>somefunc(</b>42);
1250<b><a href="#ll_assert">assert</a>(</b>3 != 4 &amp;&amp; "laws of math are failing me");
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1251
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1252a = <b>foo(</b>42, 92) + <b>bar(</b>x);
1253</pre>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1254</div>
1255
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1256<p>and this is bad:</p>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1257
1258<div class="doc_code">
1259<pre>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1260<b>if(</b>x) ...
1261<b>for(</b>i = 0; i != 100; ++i) ...
1262<b>while(</b>llvm_rocks) ...
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1263
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1264<b>somefunc (</b>42);
1265<b><a href="#ll_assert">assert</a> (</b>3 != 4 &amp;&amp; "laws of math are failing me");
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1266
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1267a = <b>foo (</b>42, 92) + <b>bar (</b>x);
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1268</pre>
1269</div>
1270
1271<p>The reason for doing this is not completely arbitrary. This style makes
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1272control flow operators stand out more, and makes expressions flow better. The
1273function call operator binds very tightly as a postfix operator. Putting a
1274space after a function name (as in the last example) makes it appear that the
1275code might bind the arguments of the left-hand-side of a binary operator with
1276the argument list of a function and the name of the right side. More
1277specifically, it is easy to misread the "a" example as:</p>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1278
1279<div class="doc_code">
1280<pre>
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1281a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1282</pre>
1283</div>
1284
Bill Wendlingb8602032010-12-21 03:31:05 +0000[diff] [blame]1285<p>when skimming through the code. By avoiding a space in a function, we avoid
1286this misinterpretation.</p>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1287
1288</div>
1289
1290<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1291<h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1292 <a name="micro_preincrement">Prefer Preincrement</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1293</h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1294
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1295<div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1296
1297<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
1298postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
1299preincrementation whenever possible.</p>
1300
1301<p>The semantics of postincrement include making a copy of the value being
1302incremented, returning it, and then preincrementing the "work value". For
1303primitive types, this isn't a big deal... but for iterators, it can be a huge
1304issue (for example, some iterators contains stack and set objects in them...
1305copying an iterator could invoke the copy ctor's of these as well). In general,
1306get in the habit of always using preincrement, and you won't have a problem.</p>
1307
1308</div>
1309
1310<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1311<h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1312 <a name="micro_namespaceindent">Namespace Indentation</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1313</h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1314
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1315<div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1316
1317<p>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1318In general, we strive to reduce indentation wherever possible. This is useful
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1319because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
1320wrapping horribly, but also because it makes it easier to understand the code.
1321Namespaces are a funny thing: they are often large, and we often desire to put
1322lots of stuff into them (so they can be large). Other times they are tiny,
1323because they just hold an enum or something similar. In order to balance this,
1324we use different approaches for small versus large namespaces.
1325</p>
1326
1327<p>
1328If a namespace definition is small and <em>easily</em> fits on a screen (say,
1329less than 35 lines of code), then you should indent its body. Here's an
1330example:
1331</p>
1332
1333<div class="doc_code">
1334<pre>
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1335namespace llvm {
1336 namespace X86 {
1337 /// RelocationType - An enum for the x86 relocation codes. Note that
1338 /// the terminology here doesn't follow x86 convention - word means
1339 /// 32-bit and dword means 64-bit.
1340 enum RelocationType {
1341 /// reloc_pcrel_word - PC relative relocation, add the relocated value to
1342 /// the value already in memory, after we adjust it for where the PC is.
1343 reloc_pcrel_word = 0,
1344
1345 /// reloc_picrel_word - PIC base relative relocation, add the relocated
1346 /// value to the value already in memory, after we adjust it for where the
1347 /// PIC base is.
1348 reloc_picrel_word = 1,
1349
1350 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
1351 /// add the relocated value to the value already in memory.
1352 reloc_absolute_word = 2,
1353 reloc_absolute_dword = 3
1354 };
1355 }
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1356}
1357</pre>
1358</div>
1359
1360<p>Since the body is small, indenting adds value because it makes it very clear
1361where the namespace starts and ends, and it is easy to take the whole thing in
1362in one "gulp" when reading the code. If the blob of code in the namespace is
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1363larger (as it typically is in a header in the <tt>llvm</tt> or <tt>clang</tt> namespaces), do not
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1364indent the code, and add a comment indicating what namespace is being closed.
1365For example:</p>
1366
1367<div class="doc_code">
1368<pre>
1369namespace llvm {
1370namespace knowledge {
1371
1372/// Grokable - This class represents things that Smith can have an intimate
1373/// understanding of and contains the data associated with it.
1374class Grokable {
1375...
1376public:
1377 explicit Grokable() { ... }
1378 virtual ~Grokable() = 0;
1379
1380 ...
1381
1382};
1383
1384} // end namespace knowledge
1385} // end namespace llvm
1386</pre>
1387</div>
1388
1389<p>Because the class is large, we don't expect that the reader can easily
1390understand the entire concept in a glance, and the end of the file (where the
1391namespaces end) may be a long ways away from the place they open. As such,
1392indenting the contents of the namespace doesn't add any value, and detracts from
1393the readability of the class. In these cases it is best to <em>not</em> indent
1394the contents of the namespace.</p>
1395
1396</div>
1397
1398<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1399<h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1400 <a name="micro_anonns">Anonymous Namespaces</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1401</h4>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1402
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1403<div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1404
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1405<p>After talking about namespaces in general, you may be wondering about
1406anonymous namespaces in particular.
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1407Anonymous namespaces are a great language feature that tells the C++ compiler
1408that the contents of the namespace are only visible within the current
1409translation unit, allowing more aggressive optimization and eliminating the
1410possibility of symbol name collisions. Anonymous namespaces are to C++ as
1411"static" is to C functions and global variables. While "static" is available
1412in C++, anonymous namespaces are more general: they can make entire classes
1413private to a file.</p>
1414
1415<p>The problem with anonymous namespaces is that they naturally want to
1416encourage indentation of their body, and they reduce locality of reference: if
1417you see a random function definition in a C++ file, it is easy to see if it is
1418marked static, but seeing if it is in an anonymous namespace requires scanning
1419a big chunk of the file.</p>
1420
1421<p>Because of this, we have a simple guideline: make anonymous namespaces as
1422small as possible, and only use them for class declarations. For example, this
1423is good:</p>
1424
1425<div class="doc_code">
1426<pre>
1427<b>namespace {</b>
1428 class StringSort {
1429 ...
1430 public:
1431 StringSort(...)
1432 bool operator&lt;(const char *RHS) const;
1433 };
1434<b>} // end anonymous namespace</b>
1435
1436static void Helper() {
1437 ...
1438}
1439
1440bool StringSort::operator&lt;(const char *RHS) const {
1441 ...
1442}
1443
1444</pre>
1445</div>
1446
1447<p>This is bad:</p>
1448
1449
1450<div class="doc_code">
1451<pre>
1452<b>namespace {</b>
1453class StringSort {
1454...
1455public:
1456 StringSort(...)
1457 bool operator&lt;(const char *RHS) const;
1458};
1459
1460void Helper() {
1461 ...
1462}
1463
1464bool StringSort::operator&lt;(const char *RHS) const {
1465 ...
1466}
1467
1468<b>} // end anonymous namespace</b>
1469
1470</pre>
1471</div>
1472
1473
1474<p>This is bad specifically because if you're looking at "Helper" in the middle
1475of a large C++ file, that you have no immediate way to tell if it is local to
1476the file. When it is marked static explicitly, this is immediately obvious.
1477Also, there is no reason to enclose the definition of "operator&lt;" in the
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1478namespace just because it was declared there.
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1479</p>
1480
1481</div>
1482
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1483</div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1484
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1485</div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1486
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1487<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1488<h2>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1489 <a name="seealso">See Also</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1490</h2>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1491<!-- *********************************************************************** -->
1492
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]1493<div>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1494
1495<p>A lot of these comments and recommendations have been culled for other
1496sources. Two particularly important books for our work are:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1497
1498<ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1499
Chris Lattner788ee282007-11-09 21:49:08 +0000[diff] [blame]1500<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
1501C++</a> by Scott Meyers. Also
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]1502interesting and useful are "More Effective C++" and "Effective STL" by the same
1503author.</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1504
Chris Lattner788ee282007-11-09 21:49:08 +0000[diff] [blame]1505<li>Large-Scale C++ Software Design by John Lakos</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1506
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1507</ol>
1508
1509<p>If you get some free time, and you haven't read them: do so, you might learn
Misha Brukman3b53a262004-12-04 00:32:12 +0000[diff] [blame]1510something.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1511
1512</div>
1513
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1514<!-- *********************************************************************** -->
1515
1516<hr>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1517<address>
1518 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]1519 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1520 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]1521 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1522
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1523 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]1524 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1525 Last modified: $Date$
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1526</address>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1527
1528</body>
1529</html>