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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
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]10<div class="doc_title">
Misha Brukmanc1449e62008-12-10 23:07:02 +0000[diff] [blame]11 LLVM Coding Standards
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]12</div>
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>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]32 <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]33 </ol></li>
34 </ol></li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]35 <li><a href="#styleissues">Style Issues</a>
36 <ol>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]37 <li><a href="#macro">The High-Level Issues</a>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]38 <ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]39 <li><a href="#hl_module">A Public Header File <b>is</b> a
40 Module</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]41 <li><a href="#hl_dontinclude"><tt>#include</tt> as Little as Possible</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]42 <li><a href="#hl_privateheaders">Keep "internal" Headers
43 Private</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]44 <li><a href="#hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]45 Code</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]46 <li><a href="#hl_else_after_return">Don't use <tt>else</tt> after a
47 <tt>return</tt></a></li>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]48 <li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
49 Functions</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]50 </ol></li>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]51 <li><a href="#micro">The Low-Level Issues</a>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]52 <ol>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]53 <li><a href="#ll_assert">Assert Liberally</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]54 <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]55 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]56 classes in headers</a></li>
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]57 <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]58 loop</a></li>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]59 <li><a href="#ll_iostream"><tt>#include &lt;iostream&gt;</tt> is
60 <em>forbidden</em></a></li>
Daniel Dunbare8530a32009-07-24 23:04:51 +0000[diff] [blame]61 <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]62 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]63 </ol></li>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]64
65 <li><a href="#nano">Microscopic Details</a>
66 <ol>
67 <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
68 <li><a href="#micro_preincrement">Prefer Preincrement</a></li>
69 <li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
70 <li><a href="#micro_anonns">Anonymous Namespaces</a></li>
71 </ol></li>
72
73
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]74 </ol></li>
75 <li><a href="#seealso">See Also</a></li>
76</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]77
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]78<div class="doc_author">
Chris Lattner219bd292009-07-22 05:43:01 +0000[diff] [blame]79 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]80</div>
81
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]82
83<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]84<div class="doc_section">
85 <a name="introduction">Introduction</a>
86</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]87<!-- *********************************************************************** -->
88
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]89<div class="doc_text">
90
91<p>This document attempts to describe a few coding standards that are being used
92in the LLVM source tree. Although no coding standards should be regarded as
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]93absolute requirements to be followed in all instances, coding standards can be
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]94useful.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]95
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]96<p>This document intentionally does not prescribe fixed standards for religious
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]97issues such as brace placement and space usage. For issues like this, follow
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]98the golden rule:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]99
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]100<blockquote>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]101
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]102<p><b><a name="goldenrule">If you are adding a significant body of source to a
103project, feel free to use whatever style you are most comfortable with. If you
104are extending, enhancing, or bug fixing already implemented code, use the style
105that is already being used so that the source is uniform and easy to
106follow.</a></b></p>
107
108</blockquote>
109
110<p>The ultimate goal of these guidelines is the increase readability and
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]111maintainability of our common source base. If you have suggestions for topics to
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]112be included, please mail them to <a
113href="mailto:sabre@nondot.org">Chris</a>.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]114
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]115</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]116
117<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]118<div class="doc_section">
119 <a name="mechanicalissues">Mechanical Source Issues</a>
120</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]121<!-- *********************************************************************** -->
122
123<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]124<div class="doc_subsection">
125 <a name="sourceformating">Source Code Formatting</a>
126</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]127
128<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]129<div class="doc_subsubsection">
130 <a name="scf_commenting">Commenting</a>
131</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]132
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]133<div class="doc_text">
134
135<p>Comments are one critical part of readability and maintainability. Everyone
Chris Lattnerb927ca82009-07-12 00:10:24 +0000[diff] [blame]136knows they should comment, so should you. When writing comments, write them as
137English prose, which means they should use proper capitalization, punctuation,
138etc. Although we all should probably
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]139comment our code more than we do, there are a few very critical places that
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]140documentation is very useful:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]141
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]142<b>File Headers</b>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]143
Reid Spencer669ed452007-07-09 08:04:31 +0000[diff] [blame]144<p>Every source file should have a header on it that describes the basic
145purpose of the file. If a file does not have a header, it should not be
146checked into Subversion. Most source trees will probably have a standard
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]147file header format. The standard format for the LLVM source tree looks like
148this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]149
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]150<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]151<pre>
Chris Lattnerc49fcd12003-10-13 14:58:11 +0000[diff] [blame]152//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
Misha Brukman98117cd2009-01-02 16:58:42 +0000[diff] [blame]153//
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]154// The LLVM Compiler Infrastructure
155//
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]156// This file is distributed under the University of Illinois Open Source
157// License. See LICENSE.TXT for details.
Misha Brukman98117cd2009-01-02 16:58:42 +0000[diff] [blame]158//
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]159//===----------------------------------------------------------------------===//
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]160//
161// This file contains the declaration of the Instruction class, which is the
162// base class for all of the VM instructions.
163//
164//===----------------------------------------------------------------------===//
165</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]166</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]167
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]168<p>A few things to note about this particular format: The "<tt>-*- C++
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]169-*-</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]170is a C++ file, not a C file (Emacs assumes <tt>.h</tt> files are C files by default).
171Note 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]172on the first line, along with a very short description of the purpose of the
173file. This is important when printing out code and flipping though lots of
174pages.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]175
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]176<p>The next section in the file is a concise note that defines the license
177that the file is released under. This makes it perfectly clear what terms the
178source code can be distributed under and should not be modified in any way.</p>
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]179
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]180<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]181Here it's only two lines. If an algorithm is being implemented or something
182tricky is going on, a reference to the paper where it is published should be
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]183included, 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]184
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]185<b>Class overviews</b>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]186
Jim Laskeyf55914a2006-07-31 20:18:49 +0000[diff] [blame]187<p>Classes are one fundamental part of a good object oriented design. As such,
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]188a class definition should have a comment block that explains what the class is
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]189used for... if it's not obvious. If it's so completely obvious your grandma
190could figure it out, it's probably safe to leave it out. Naming classes
Misha Brukman3b53a262004-12-04 00:32:12 +0000[diff] [blame]191something sane goes a long ways towards avoiding writing documentation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]192
193
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]194<b>Method information</b>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]195
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]196<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]197documented properly. A quick note about what it does and a description of the
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]198borderline behaviour is all that is necessary here (unless something
Dan Gohman621a4152010-02-26 20:18:32 +0000[diff] [blame]199particularly tricky or insidious is going on). The hope is that people can
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]200figure out how to use your interfaces without reading the code itself... that is
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]201the goal metric.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]202
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]203<p>Good things to talk about here are what happens when something unexpected
204happens: does the method return null? Abort? Format your hard disk?</p>
205
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]206</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]207
208<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]209<div class="doc_subsubsection">
210 <a name="scf_commentformat">Comment Formatting</a>
211</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]212
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]213<div class="doc_text">
214
215<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]216require less typing, don't have nesting problems, etc. There are a few cases
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]217when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]218
219<ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]220 <li>When writing a C code: Obviously if you are writing C code, use C style
Misha Brukman3b53a262004-12-04 00:32:12 +0000[diff] [blame]221 comments.</li>
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]222 <li>When writing a header file that may be <tt>#include</tt>d by a C source
223 file.</li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]224 <li>When writing a source file that is used by a tool that only accepts C
225 style comments.</li>
226</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]227
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]228<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
229These nest properly and are better behaved in general than C style comments.</p>
230
231</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]232
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]233<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]234<div class="doc_subsubsection">
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]235 <a name="scf_includes"><tt>#include</tt> Style</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]236</div>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]237
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]238<div class="doc_text">
239
240<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]241include guards if working on a header file), the <a
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]242href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
243file should be listed. We prefer these <tt>#include</tt>s to be listed in this
244order:</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]245
246<ol>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]247 <li><a href="#mmheader">Main Module Header</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]248 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]249 <li><tt>llvm/*</tt></li>
250 <li><tt>llvm/Analysis/*</tt></li>
251 <li><tt>llvm/Assembly/*</tt></li>
Duncan Sands18d52f22010-09-29 20:09:55 +0000[diff] [blame]252 <li><tt>llvm/Bitcode/*</tt></li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]253 <li><tt>llvm/CodeGen/*</tt></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]254 <li>...</li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]255 <li><tt>Support/*</tt></li>
256 <li><tt>Config/*</tt></li>
257 <li>System <tt>#includes</tt></li>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]258</ol>
259
Nick Lewycky822293a2008-11-29 20:13:25 +0000[diff] [blame]260<p>... and each category should be sorted by name.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]261
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]262<p><a name="mmheader">The "Main Module Header"</a> file applies to <tt>.cpp</tt> files
263which implement an interface defined by a <tt>.h</tt> file. This <tt>#include</tt>
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]264should always be included <b>first</b> regardless of where it lives on the file
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]265system. 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]266interfaces, we ensure that the header does not have any hidden dependencies
267which are not explicitly #included in the header, but should be. It is also a
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]268form of documentation in the <tt>.cpp</tt> file to indicate where the interfaces it
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]269implements are defined.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]270
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]271</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]272
273<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]274<div class="doc_subsubsection">
275 <a name="scf_codewidth">Source Code Width</a>
276</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]277
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]278<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]279
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]280<p>Write your code to fit within 80 columns of text. This helps those of us who
281like to print out code and look at your code in an xterm without resizing
282it.</p>
283
Chris Lattner5822e9e2008-07-08 05:12:37 +0000[diff] [blame]284<p>The longer answer is that there must be some limit to the width of the code
285in order to reasonably allow developers to have multiple files side-by-side in
286windows on a modest display. If you are going to pick a width limit, it is
287somewhat arbitrary but you might as well pick something standard. Going with
28890 columns (for example) instead of 80 columns wouldn't add any significant
289value and would be detrimental to printing out code. Also many other projects
290have standardized on 80 columns, so some people have already configured their
291editors for it (vs something else, like 90 columns).</p>
292
293<p>This is one of many contentious issues in coding standards, but is not up
294for debate.</p>
295
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]296</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]297
298<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]299<div class="doc_subsubsection">
300 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
301</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]302
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]303<div class="doc_text">
304
305<p>In all cases, prefer spaces to tabs in source files. People have different
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]306preferred indentation levels, and different styles of indentation that they
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]307like... this is fine. What isn't is that different editors/viewers expand tabs
308out to different tab stops. This can cause your code to look completely
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]309unreadable, and it is not worth dealing with.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]310
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]311<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]312style of existing code if your are modifying and extending it. If you like four
313spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
314with two spaces of indentation. Also, do not reindent a whole source file: it
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]315makes for incredible diffs that are absolutely worthless.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]316
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]317</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]318
319<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]320<div class="doc_subsubsection">
321 <a name="scf_indentation">Indent Code Consistently</a>
322</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]323
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]324<div class="doc_text">
325
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]326<p>Okay, in your first year of programming you were told that indentation is
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]327important. If you didn't believe and internalize this then, now is the time.
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]328Just do it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]329
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]330</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]331
332
333<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]334<div class="doc_subsection">
335 <a name="compilerissues">Compiler Issues</a>
336</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]337
338
339<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]340<div class="doc_subsubsection">
341 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
342</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]343
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]344<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]345
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]346<p>If your code has compiler warnings in it, something is wrong: you aren't
347casting values correctly, your have "questionable" constructs in your code, or
348you are doing something legitimately wrong. Compiler warnings can cover up
349legitimate errors in output and make dealing with a translation unit
350difficult.</p>
351
352<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]353desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
354a good thorough set of warnings, and stick to them. At least in the case of
355<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
356syntax of the code slightly. For example, an warning that annoys me occurs when
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]357I write code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]358
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]359<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]360<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]361if (V = getValue()) {
362 ...
363}
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]364</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]365</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]366
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]367<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
368operator, and that I probably mistyped it. In most cases, I haven't, and I
369really don't want the spurious errors. To fix this particular problem, I
370rewrite the code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]371
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]372<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]373<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]374if ((V = getValue())) {
375 ...
376}
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]377</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]378</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]379
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]380<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
381be fixed by massaging the code appropriately.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]382
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]383<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
384-Winline -W -Wwrite-strings -Wno-unused</tt></p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]385
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]386</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]387
388<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]389<div class="doc_subsubsection">
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]390 <a name="ci_portable_code">Write Portable Code</a>
391</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]392
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]393<div class="doc_text">
394
395<p>In almost all cases, it is possible and within reason to write completely
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]396portable code. If there are cases where it isn't possible to write portable
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]397code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]398
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]399<p>In practice, this means that you shouldn't assume much about the host
Chris Lattner221b2392010-11-16 22:19:06 +0000[diff] [blame]400compiler, and Visual Studio tends to be the lowest common denominator.
401If advanced features are used, they should only be an implementation detail of
402a library which has a simple exposed API, and preferably be buried in
403libSystem.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]404
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]405</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]406
Reid Spencerb40df562004-09-23 16:03:48 +0000[diff] [blame]407<!-- _______________________________________________________________________ -->
408<div class="doc_subsubsection">
409<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
410</div>
411<div class="doc_text">
Misha Brukmanedcc78a2004-10-26 15:45:13 +0000[diff] [blame]412
413<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
414interchangeably. The only difference is when they are used to declare a class:
415<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
416all members public by default.</p>
417
418<p>Unfortunately, not all compilers follow the rules and some will generate
419different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
420declare the symbol. This can lead to problems at link time.</p>
421
422<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
Chris Lattner57310202009-08-11 17:38:15 +0000[diff] [blame]423<b>all</b> members are public and the type is a C++ "POD" type, in which case
424<tt>struct</tt> is allowed.</p>
Misha Brukmanedcc78a2004-10-26 15:45:13 +0000[diff] [blame]425
Reid Spencerb40df562004-09-23 16:03:48 +0000[diff] [blame]426</div>
427
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]428<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]429<div class="doc_section">
430 <a name="styleissues">Style Issues</a>
431</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]432<!-- *********************************************************************** -->
433
434
435<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]436<div class="doc_subsection">
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]437 <a name="macro">The High-Level Issues</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]438</div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]439<!-- ======================================================================= -->
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]440
441
442<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]443<div class="doc_subsubsection">
444 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
445</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]446
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]447<div class="doc_text">
448
449<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]450encapsulation or data hiding (unless you use expensive protocol classes), but it
451is what we have to work with. When you write a public header file (in the LLVM
452source tree, they live in the top level "include" directory), you are defining a
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]453module of functionality.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]454
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]455<p>Ideally, modules should be completely independent of each other, and their
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]456header files should only include the absolute minimum number of headers
457possible. A module is not just a class, a function, or a namespace: <a
458href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
459of these</a> that defines an interface. This interface may be several
460functions, classes or data structures, but the important issue is how they work
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]461together.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]462
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]463<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
464files. Each of these <tt>.cpp</tt> files should include the header that defines
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]465their interface first. This ensures that all of the dependences of the module
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]466header have been properly added to the module header itself, and are not
467implicit. System headers should be included after user headers for a
468translation unit.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]469
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]470</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]471
472<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]473<div class="doc_subsubsection">
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]474 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]475</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]476
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]477<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]478
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]479<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
480have to, especially in header files.</p>
481
482<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]483to inherit from it. In these cases go ahead and <tt>#include</tt> that header
484file. Be aware however that there are many cases where you don't need to have
485the full definition of a class. If you are using a pointer or reference to a
486class, you don't need the header file. If you are simply returning a class
487instance from a prototyped function or method, you don't need it. In fact, for
488most cases, you simply don't need the definition of a class... and not
489<tt>#include</tt>'ing speeds up compilation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]490
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]491<p>It is easy to try to go too overboard on this recommendation, however. You
Chris Lattner5da59b12007-02-10 18:35:31 +0000[diff] [blame]492<b>must</b> include all of the header files that you are using -- you can
493include them either directly
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]494or indirectly (through another header file). To make sure that you don't
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]495accidentally forget to include a header file in your module header, make sure to
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]496include your module header <b>first</b> in the implementation file (as mentioned
497above). This way there won't be any hidden dependencies that you'll find out
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]498about later...</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]499
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]500</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]501
502<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]503<div class="doc_subsubsection">
504 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
505</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]506
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]507<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]508
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]509<p>Many modules have a complex implementation that causes them to use more than
510one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
511internal communication interface (helper classes, extra functions, etc) in the
Misha Brukman3b53a262004-12-04 00:32:12 +0000[diff] [blame]512public module header file. Don't do this.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]513
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]514<p>If you really need to do something like this, put a private header file in
515the same directory as the source files, and include it locally. This ensures
516that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]517
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]518<p>Note however, that it's okay to put extra implementation methods a public
519class itself... just make them private (or protected), and all is well.</p>
520
521</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]522
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]523<!-- _______________________________________________________________________ -->
524<div class="doc_subsubsection">
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]525 <a name="hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify Code</a>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]526</div>
527
528<div class="doc_text">
529
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]530<p>When reading code, keep in mind how much state and how many previous
531decisions have to be remembered by the reader to understand a block of code.
532Aim to reduce indentation where possible when it doesn't make it more difficult
533to understand the code. One great way to do this is by making use of early
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]534exits and the <tt>continue</tt> keyword in long loops. As an example of using an early
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]535exit from a function, consider this "bad" code:</p>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]536
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]537<div class="doc_code">
538<pre>
539Value *DoSomething(Instruction *I) {
540 if (!isa&lt;TerminatorInst&gt;(I) &amp;&amp;
541 I-&gt;hasOneUse() &amp;&amp; SomeOtherThing(I)) {
542 ... some long code ....
543 }
544
545 return 0;
546}
547</pre>
548</div>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]549
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]550<p>This code has several problems if the body of the 'if' is large. When you're
551looking at the top of the function, it isn't immediately clear that this
552<em>only</em> does interesting things with non-terminator instructions, and only
553applies to things with the other predicates. Second, it is relatively difficult
554to describe (in comments) why these predicates are important because the if
555statement makes it difficult to lay out the comments. Third, when you're deep
556within the body of the code, it is indented an extra level. Finally, when
557reading the top of the function, it isn't clear what the result is if the
558predicate isn't true, you have to read to the end of the function to know that
559it returns null.</p>
Matthijs Kooijmaned58a972008-07-30 12:14:10 +0000[diff] [blame]560
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]561<p>It is much preferred to format the code like this:</p>
562
563<div class="doc_code">
564<pre>
565Value *DoSomething(Instruction *I) {
566 // Terminators never need 'something' done to them because, ...
567 if (isa&lt;TerminatorInst&gt;(I))
568 return 0;
569
570 // We conservatively avoid transforming instructions with multiple uses
571 // because goats like cheese.
572 if (!I-&gt;hasOneUse())
573 return 0;
574
575 // This is really just here for example.
576 if (!SomeOtherThing(I))
577 return 0;
578
579 ... some long code ....
580}
581</pre>
582</div>
583
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]584<p>This fixes these problems. A similar problem frequently happens in <tt>for</tt>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]585loops. A silly example is something like this:</p>
586
587<div class="doc_code">
588<pre>
589 for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; ++II) {
590 if (BinaryOperator *BO = dyn_cast&lt;BinaryOperator&gt;(II)) {
591 Value *LHS = BO-&gt;getOperand(0);
592 Value *RHS = BO-&gt;getOperand(1);
593 if (LHS != RHS) {
594 ...
595 }
596 }
597 }
598</pre>
599</div>
600
601<p>When you have very very small loops, this sort of structure is fine, but if
602it exceeds more than 10-15 lines, it becomes difficult for people to read and
603understand at a glance.
604The problem with this sort of code is that it gets very nested very quickly,
605meaning that the reader of the code has to keep a lot of context in their brain
606to remember what is going immediately on in the loop, because they don't know
607if/when the if conditions will have elses etc. It is strongly preferred to
608structure the loop like this:</p>
609
610<div class="doc_code">
611<pre>
612 for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; ++II) {
613 BinaryOperator *BO = dyn_cast&lt;BinaryOperator&gt;(II);
614 if (!BO) continue;
615
616 Value *LHS = BO-&gt;getOperand(0);
617 Value *RHS = BO-&gt;getOperand(1);
618 if (LHS == RHS) continue;
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]619 ...
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]620 }
621</pre>
622</div>
623
624<p>This has all the benefits of using early exits from functions: it reduces
625nesting of the loop, it makes it easier to describe why the conditions are true,
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]626and it makes it obvious to the reader that there is no <tt>else</tt> coming up that
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]627they have to push context into their brain for. If a loop is large, this can
628be a big understandability win.</p>
629
630</div>
631
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]632<!-- _______________________________________________________________________ -->
633<div class="doc_subsubsection">
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]634 <a name="hl_else_after_return">Don't use <tt>else</tt> after a <tt>return</tt></a>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]635</div>
636
637<div class="doc_text">
638
639<p>For similar reasons above (reduction of indentation and easier reading),
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]640 please do not use <tt>else</tt> or '<tt>else if</tt>' after something that interrupts
641 control flow like <tt>return</tt>, <tt>break</tt>, <tt>continue</tt>, <tt>goto</tt>, etc. For example, this is
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]642 "bad":</p>
643
644<div class="doc_code">
645<pre>
646 case 'J': {
647 if (Signed) {
648 Type = Context.getsigjmp_bufType();
649 if (Type.isNull()) {
650 Error = ASTContext::GE_Missing_sigjmp_buf;
651 return QualType();
652 } else {
653 break;
654 }
655 } else {
656 Type = Context.getjmp_bufType();
657 if (Type.isNull()) {
658 Error = ASTContext::GE_Missing_jmp_buf;
659 return QualType();
660 } else {
661 break;
662 }
663 }
664 }
665 }
666</pre>
667</div>
668
669<p>It is better to write this something like:</p>
670
671<div class="doc_code">
672<pre>
673 case 'J':
674 if (Signed) {
675 Type = Context.getsigjmp_bufType();
676 if (Type.isNull()) {
677 Error = ASTContext::GE_Missing_sigjmp_buf;
678 return QualType();
679 }
680 } else {
681 Type = Context.getjmp_bufType();
682 if (Type.isNull()) {
683 Error = ASTContext::GE_Missing_jmp_buf;
684 return QualType();
685 }
686 }
687 break;
688</pre>
689</div>
690
691<p>Or better yet (in this case), as:</p>
692
693<div class="doc_code">
694<pre>
695 case 'J':
696 if (Signed)
697 Type = Context.getsigjmp_bufType();
698 else
699 Type = Context.getjmp_bufType();
700
701 if (Type.isNull()) {
702 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
703 ASTContext::GE_Missing_jmp_buf;
704 return QualType();
705 }
706 break;
707</pre>
708</div>
709
710<p>The idea is to reduce indentation and the amount of code you have to keep
711 track of when reading the code.</p>
712
713</div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]714
715<!-- _______________________________________________________________________ -->
716<div class="doc_subsubsection">
717 <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
718</div>
719
720<div class="doc_text">
721
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]722<p>It is very common to write small loops that just compute a boolean
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]723 value. There are a number of ways that people commonly write these, but an
724 example of this sort of thing is:</p>
725
726<div class="doc_code">
727<pre>
728 <b>bool FoundFoo = false;</b>
729 for (unsigned i = 0, e = BarList.size(); i != e; ++i)
730 if (BarList[i]-&gt;isFoo()) {
731 <b>FoundFoo = true;</b>
732 break;
733 }
734
735 <b>if (FoundFoo) {</b>
736 ...
737 }
738</pre>
739</div>
740
741<p>This sort of code is awkward to write, and is almost always a bad sign.
742Instead of this sort of loop, we strongly prefer to use a predicate function
743(which may be <a href="#micro_anonns">static</a>) that uses
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]744<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
745the code to be structured like this:
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]746</p>
747
748
749<div class="doc_code">
750<pre>
751/// ListContainsFoo - Return true if the specified list has an element that is
752/// a foo.
753static bool ListContainsFoo(const std::vector&lt;Bar*&gt; &amp;List) {
754 for (unsigned i = 0, e = List.size(); i != e; ++i)
755 if (List[i]-&gt;isFoo())
756 return true;
757 return false;
758}
759...
760
761 <b>if (ListContainsFoo(BarList)) {</b>
762 ...
763 }
764</pre>
765</div>
766
767<p>There are many reasons for doing this: it reduces indentation and factors out
768code which can often be shared by other code that checks for the same predicate.
769More importantly, it <em>forces you to pick a name</em> for the function, and
770forces you to write a comment for it. In this silly example, this doesn't add
771much value. However, if the condition is complex, this can make it a lot easier
772for the reader to understand the code that queries for this predicate. Instead
Chris Lattner71d8f3b2009-07-22 16:30:39 +0000[diff] [blame]773of 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]774contains a foo, we can trust the function name and continue reading with better
775locality.</p>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]776
777</div>
778
779
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]780<!-- ======================================================================= -->
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]781<div class="doc_subsection">
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]782 <a name="micro">The Low-Level Issues</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]783</div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]784<!-- ======================================================================= -->
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]785
786
787<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]788<div class="doc_subsubsection">
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]789 <a name="ll_assert">Assert Liberally</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]790</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]791
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]792<div class="doc_text">
793
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]794<p>Use the "<tt>assert</tt>" macro to its fullest. Check all of your
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]795preconditions and assumptions, you never know when a bug (not necessarily even
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]796yours) might be caught early by an assertion, which reduces debugging time
797dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already
798included 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]799it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]800
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]801<p>To further assist with debugging, make sure to put some kind of error message
802in the assertion statement (which is printed if the assertion is tripped). This
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]803helps the poor debugger make sense of why an assertion is being made and
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]804enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]805
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]806<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]807<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]808inline Value *getOperand(unsigned i) {
809 assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
810 return Operands[i];
811}
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]812</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]813</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]814
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]815<p>Here are some examples:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]816
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]817<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]818<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]819assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]820
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]821assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]822
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]823assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]824
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]825assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]826
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]827assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]828</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]829</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]830
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]831<p>You get the idea...</p>
832
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]833<p>Please be aware when adding assert statements that not all compilers are aware of
834the semantics of the assert. In some places, asserts are used to indicate a piece of
835code that should not be reached. These are typically of the form:</p>
836
837<div class="doc_code">
838<pre>
Dan Gohmanf0032762008-11-24 17:18:39 +0000[diff] [blame]839assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]840</pre>
841</div>
842
843<p>When used in a function that returns a value, they should be followed with a return
844statement and a comment indicating that this line is never reached. This will prevent
845a compiler which is unable to deduce that the assert statement never returns from
846generating a warning.</p>
847
848<div class="doc_code">
849<pre>
Dan Gohmanf0032762008-11-24 17:18:39 +0000[diff] [blame]850assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]851// Not reached
852return 0;
853</pre>
854</div>
855
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]856<p>Another issue is that values used only by assertions will produce an "unused
857 value" warning when assertions are disabled. For example, this code will warn:
858</p>
859
860<div class="doc_code">
861<pre>
862 unsigned Size = V.size();
863 assert(Size &gt; 42 &amp;&amp; "Vector smaller than it should be");
864
865 bool NewToSet = Myset.insert(Value);
866 assert(NewToSet &amp;&amp; "The value shouldn't be in the set yet");
867</pre>
868</div>
869
870<p>These are two interesting different cases: in the first case, the call to
871V.size() is only useful for the assert, and we don't want it executed when
872assertions are disabled. Code like this should move the call into the assert
873itself. In the second case, the side effects of the call must happen whether
874the assert is enabled or not. In this case, the value should be cast to void
875to disable the warning. To be specific, it is preferred to write the code
876like this:</p>
877
878<div class="doc_code">
879<pre>
880 assert(V.size() &gt; 42 &amp;&amp; "Vector smaller than it should be");
881
882 bool NewToSet = Myset.insert(Value); (void)NewToSet;
883 assert(NewToSet &amp;&amp; "The value shouldn't be in the set yet");
884</pre>
885</div>
886
887
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]888</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]889
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]890<!-- _______________________________________________________________________ -->
891<div class="doc_subsubsection">
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]892 <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]893</div>
894
895<div class="doc_text">
896<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]897namespace with an "<tt>std::</tt>" prefix, rather than rely on
898"<tt>using namespace std;</tt>".</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]899
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]900<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
Misha Brukman2d33c542008-12-11 19:37:04 +0000[diff] [blame]901the namespace of any source file that <tt>#include</tt>s the header. This is
902clearly a bad thing.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]903
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]904<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]905rule, but is still important. Basically, using explicit namespace prefixes
906makes the code <b>clearer</b>, because it is immediately obvious what facilities
907are being used and where they are coming from, and <b>more portable</b>, because
908namespace clashes cannot occur between LLVM code and other namespaces. The
909portability rule is important because different standard library implementations
910expose different symbols (potentially ones they shouldn't), and future revisions
911to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
912such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]913
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]914<p>The exception to the general rule (i.e. it's not an exception for
915the <tt>std</tt> namespace) is for implementation files. For example, all of
916the code in the LLVM project implements code that lives in the 'llvm' namespace.
Chris Lattnereddd9692010-11-17 19:47:20 +0000[diff] [blame]917As such, it is ok, and actually clearer, for the <tt>.cpp</tt> files to have a
918'<tt>using namespace llvm</tt>' directive at their top, after the
919<tt>#include</tt>s. This reduces indentation in the body of the file for source
920editors that indent based on braces, and keeps the conceptual context cleaner.
921The general form of this rule is that any <tt>.cpp</tt> file that implements
922code in any namespace may use that namespace (and its parents'), but should not
923use any others.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]924
925</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]926
927<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]928<div class="doc_subsubsection">
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]929 <a name="ll_virtual_anch">Provide a virtual method anchor for classes
930 in headers</a>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]931</div>
932
933<div class="doc_text">
934
935<p>If a class is defined in a header file and has a v-table (either it has
936virtual methods or it derives from classes with virtual methods), it must
937always have at least one out-of-line virtual method in the class. Without
Misha Brukman2d33c542008-12-11 19:37:04 +0000[diff] [blame]938this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
939that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
940increasing link times.</p>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]941
942</div>
943
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]944<!-- _______________________________________________________________________ -->
945<div class="doc_subsubsection">
Zhanyong Wancafe0b42010-11-23 05:03:07 +0000[diff] [blame^]946 <a name="ll_end">Don't evaluate <tt>end()</tt> every time through a loop</a>
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]947</div>
948
949<div class="doc_text">
950
951<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
952with macros and may be coming in C++'0x) we end up writing a lot of loops that
953manually iterate from begin to end on a variety of containers or through other
954data structures. One common mistake is to write a loop in this style:</p>
955
956<div class="doc_code">
957<pre>
958 BasicBlock *BB = ...
959 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
960 ... use I ...
961</pre>
962</div>
963
964<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
965every time through the loop. Instead of writing the loop like this, we strongly
966prefer loops to be written so that they evaluate it once before the loop starts.
967A convenient way to do this is like so:</p>
968
969<div class="doc_code">
970<pre>
971 BasicBlock *BB = ...
972 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
973 ... use I ...
974</pre>
975</div>
976
977<p>The observant may quickly point out that these two loops may have different
978semantics: if the container (a basic block in this case) is being mutated, then
979"<tt>BB->end()</tt>" may change its value every time through the loop and the
980second loop may not in fact be correct. If you actually do depend on this
Chris Lattner09bc65e2009-06-30 06:20:03 +0000[diff] [blame]981behavior, please write the loop in the first form and add a comment indicating
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]982that you did it intentionally.</p>
983
984<p>Why do we prefer the second form (when correct)? Writing the loop in the
985first form has two problems: First it may be less efficient than evaluating it
986at the start of the loop. In this case, the cost is probably minor: a few extra
987loads every time through the loop. However, if the base expression is more
988complex, then the cost can rise quickly. I've seen loops where the end
989expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
Chris Lattnera44f87f2009-06-30 06:27:54 +0000[diff] [blame]990lookups really aren't cheap. By writing it in the second form consistently, you
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]991eliminate the issue entirely and don't even have to think about it.</p>
992
Chris Lattner09bc65e2009-06-30 06:20:03 +0000[diff] [blame]993<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]994hints to the reader that the loop is mutating the container (a fact that a
995comment would handily confirm!). If you write the loop in the second form, it
996is immediately obvious without even looking at the body of the loop that the
997container isn't being modified, which makes it easier to read the code and
998understand what it does.</p>
999
1000<p>While the second form of the loop is a few extra keystrokes, we do strongly
1001prefer it.</p>
1002
1003</div>
1004
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1005<!-- _______________________________________________________________________ -->
1006<div class="doc_subsubsection">
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1007 <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is forbidden</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1008</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1009
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1010<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1011
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1012<p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
1013hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
1014support clients using LLVM libraries as part of larger systems. In particular,
1015we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
1016the static c'tors are run whenever an application start up that uses the dynamic
1017library. There are two problems with this:</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1018
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1019<ol>
1020 <li>The time to run the static c'tors impacts startup time of
1021 applications&mdash;a critical time for GUI apps.</li>
1022 <li>The static c'tors cause the app to pull many extra pages of memory off the
1023 disk: both the code for the static c'tors in each <tt>.o</tt> file and the
1024 small amount of data that gets touched. In addition, touched/dirty pages
1025 put more pressure on the VM system on low-memory machines.</li>
1026</ol>
1027
1028<p>Note that using the other stream headers (<tt>&lt;sstream&gt;</tt> for
Chris Lattner983c5922009-08-23 21:53:47 +0000[diff] [blame]1029example) is not problematic in this regard (just <tt>&lt;iostream&gt;</tt>).
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1030However, <tt>raw_ostream</tt> provides various APIs that are better performing for almost
1031every use than <tt>std::ostream</tt> style APIs.
1032<b>Therefore new code should always
Chris Lattner983c5922009-08-23 21:53:47 +0000[diff] [blame]1033use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or
1034the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1035
1036</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1037
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1038
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1039<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1040<div class="doc_subsubsection">
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1041 <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
1042</div>
1043
1044<div class="doc_text">
1045
1046<p>LLVM includes a lightweight, simple, and efficient stream implementation
1047in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
1048of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> instead
1049of <tt>ostream</tt>.</p>
1050
1051<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
1052be forward declared as <tt>class raw_ostream</tt>. Public headers should
1053generally not include the <tt>raw_ostream</tt> header, but use forward
1054declarations and constant references to <tt>raw_ostream</tt> instances.</p>
1055
1056</div>
1057
1058
1059<!-- _______________________________________________________________________ -->
1060<div class="doc_subsubsection">
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1061 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1062</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1063
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1064<div class="doc_text">
1065
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]1066<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
1067to the output stream specified. In addition to doing this, however, it also
1068flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1069
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1070<div class="doc_code">
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1071<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1072std::cout &lt;&lt; std::endl;
1073std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1074</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1075</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1076
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1077<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]1078it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1079
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1080</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1081
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]1082
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1083<!-- ======================================================================= -->
1084<div class="doc_subsection">
1085 <a name="nano">Microscopic Details</a>
1086</div>
1087<!-- ======================================================================= -->
1088
1089<p>This section describes preferred low-level formatting guidelines along with
1090reasoning on why we prefer them.</p>
1091
1092<!-- _______________________________________________________________________ -->
1093<div class="doc_subsubsection">
1094 <a name="micro_spaceparen">Spaces Before Parentheses</a>
1095</div>
1096
1097<div class="doc_text">
1098
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1099<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]1100statements, but not in normal function call expressions and function-like
1101macros. For example, this is good:</p>
1102
1103<div class="doc_code">
1104<pre>
1105 <b>if (</b>x) ...
1106 <b>for (</b>i = 0; i != 100; ++i) ...
1107 <b>while (</b>llvm_rocks) ...
1108
1109 <b>somefunc(</b>42);
1110 <b><a href="#ll_assert">assert</a>(</b>3 != 4 &amp;&amp; "laws of math are failing me");
1111
1112 a = <b>foo(</b>42, 92) + <b>bar(</b>x);
1113 </pre>
1114</div>
1115
1116<p>... and this is bad:</p>
1117
1118<div class="doc_code">
1119<pre>
1120 <b>if(</b>x) ...
1121 <b>for(</b>i = 0; i != 100; ++i) ...
1122 <b>while(</b>llvm_rocks) ...
1123
1124 <b>somefunc (</b>42);
1125 <b><a href="#ll_assert">assert</a> (</b>3 != 4 &amp;&amp; "laws of math are failing me");
1126
1127 a = <b>foo (</b>42, 92) + <b>bar (</b>x);
1128</pre>
1129</div>
1130
1131<p>The reason for doing this is not completely arbitrary. This style makes
1132 control flow operators stand out more, and makes expressions flow better. The
1133 function call operator binds very tightly as a postfix operator. Putting
1134 a space after a function name (as in the last example) makes it appear that
1135 the code might bind the arguments of the left-hand-side of a binary operator
1136 with the argument list of a function and the name of the right side. More
1137 specifically, it is easy to misread the "a" example as:</p>
1138
1139<div class="doc_code">
1140<pre>
1141 a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
1142</pre>
1143</div>
1144
1145<p>... when skimming through the code. By avoiding a space in a function, we
1146avoid this misinterpretation.</p>
1147
1148</div>
1149
1150<!-- _______________________________________________________________________ -->
1151<div class="doc_subsubsection">
1152 <a name="micro_preincrement">Prefer Preincrement</a>
1153</div>
1154
1155<div class="doc_text">
1156
1157<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
1158postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
1159preincrementation whenever possible.</p>
1160
1161<p>The semantics of postincrement include making a copy of the value being
1162incremented, returning it, and then preincrementing the "work value". For
1163primitive types, this isn't a big deal... but for iterators, it can be a huge
1164issue (for example, some iterators contains stack and set objects in them...
1165copying an iterator could invoke the copy ctor's of these as well). In general,
1166get in the habit of always using preincrement, and you won't have a problem.</p>
1167
1168</div>
1169
1170<!-- _______________________________________________________________________ -->
1171<div class="doc_subsubsection">
1172 <a name="micro_namespaceindent">Namespace Indentation</a>
1173</div>
1174
1175<div class="doc_text">
1176
1177<p>
Chris Lattner684cf812010-11-17 17:14:55 +0000[diff] [blame]1178In general, we strive to reduce indentation wherever possible. This is useful
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1179because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
1180wrapping horribly, but also because it makes it easier to understand the code.
1181Namespaces are a funny thing: they are often large, and we often desire to put
1182lots of stuff into them (so they can be large). Other times they are tiny,
1183because they just hold an enum or something similar. In order to balance this,
1184we use different approaches for small versus large namespaces.
1185</p>
1186
1187<p>
1188If a namespace definition is small and <em>easily</em> fits on a screen (say,
1189less than 35 lines of code), then you should indent its body. Here's an
1190example:
1191</p>
1192
1193<div class="doc_code">
1194<pre>
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1195namespace llvm {
1196 namespace X86 {
1197 /// RelocationType - An enum for the x86 relocation codes. Note that
1198 /// the terminology here doesn't follow x86 convention - word means
1199 /// 32-bit and dword means 64-bit.
1200 enum RelocationType {
1201 /// reloc_pcrel_word - PC relative relocation, add the relocated value to
1202 /// the value already in memory, after we adjust it for where the PC is.
1203 reloc_pcrel_word = 0,
1204
1205 /// reloc_picrel_word - PIC base relative relocation, add the relocated
1206 /// value to the value already in memory, after we adjust it for where the
1207 /// PIC base is.
1208 reloc_picrel_word = 1,
1209
1210 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
1211 /// add the relocated value to the value already in memory.
1212 reloc_absolute_word = 2,
1213 reloc_absolute_dword = 3
1214 };
1215 }
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1216}
1217</pre>
1218</div>
1219
1220<p>Since the body is small, indenting adds value because it makes it very clear
1221where the namespace starts and ends, and it is easy to take the whole thing in
1222in 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]1223larger (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]1224indent the code, and add a comment indicating what namespace is being closed.
1225For example:</p>
1226
1227<div class="doc_code">
1228<pre>
1229namespace llvm {
1230namespace knowledge {
1231
1232/// Grokable - This class represents things that Smith can have an intimate
1233/// understanding of and contains the data associated with it.
1234class Grokable {
1235...
1236public:
1237 explicit Grokable() { ... }
1238 virtual ~Grokable() = 0;
1239
1240 ...
1241
1242};
1243
1244} // end namespace knowledge
1245} // end namespace llvm
1246</pre>
1247</div>
1248
1249<p>Because the class is large, we don't expect that the reader can easily
1250understand the entire concept in a glance, and the end of the file (where the
1251namespaces end) may be a long ways away from the place they open. As such,
1252indenting the contents of the namespace doesn't add any value, and detracts from
1253the readability of the class. In these cases it is best to <em>not</em> indent
1254the contents of the namespace.</p>
1255
1256</div>
1257
1258<!-- _______________________________________________________________________ -->
1259<div class="doc_subsubsection">
1260 <a name="micro_anonns">Anonymous Namespaces</a>
1261</div>
1262
1263<div class="doc_text">
1264
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1265<p>After talking about namespaces in general, you may be wondering about
1266anonymous namespaces in particular.
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1267Anonymous namespaces are a great language feature that tells the C++ compiler
1268that the contents of the namespace are only visible within the current
1269translation unit, allowing more aggressive optimization and eliminating the
1270possibility of symbol name collisions. Anonymous namespaces are to C++ as
1271"static" is to C functions and global variables. While "static" is available
1272in C++, anonymous namespaces are more general: they can make entire classes
1273private to a file.</p>
1274
1275<p>The problem with anonymous namespaces is that they naturally want to
1276encourage indentation of their body, and they reduce locality of reference: if
1277you see a random function definition in a C++ file, it is easy to see if it is
1278marked static, but seeing if it is in an anonymous namespace requires scanning
1279a big chunk of the file.</p>
1280
1281<p>Because of this, we have a simple guideline: make anonymous namespaces as
1282small as possible, and only use them for class declarations. For example, this
1283is good:</p>
1284
1285<div class="doc_code">
1286<pre>
1287<b>namespace {</b>
1288 class StringSort {
1289 ...
1290 public:
1291 StringSort(...)
1292 bool operator&lt;(const char *RHS) const;
1293 };
1294<b>} // end anonymous namespace</b>
1295
1296static void Helper() {
1297 ...
1298}
1299
1300bool StringSort::operator&lt;(const char *RHS) const {
1301 ...
1302}
1303
1304</pre>
1305</div>
1306
1307<p>This is bad:</p>
1308
1309
1310<div class="doc_code">
1311<pre>
1312<b>namespace {</b>
1313class StringSort {
1314...
1315public:
1316 StringSort(...)
1317 bool operator&lt;(const char *RHS) const;
1318};
1319
1320void Helper() {
1321 ...
1322}
1323
1324bool StringSort::operator&lt;(const char *RHS) const {
1325 ...
1326}
1327
1328<b>} // end anonymous namespace</b>
1329
1330</pre>
1331</div>
1332
1333
1334<p>This is bad specifically because if you're looking at "Helper" in the middle
1335of a large C++ file, that you have no immediate way to tell if it is local to
1336the file. When it is marked static explicitly, this is immediately obvious.
1337Also, there is no reason to enclose the definition of "operator&lt;" in the
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1338namespace just because it was declared there.
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1339</p>
1340
1341</div>
1342
1343
1344
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1345<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1346<div class="doc_section">
1347 <a name="seealso">See Also</a>
1348</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1349<!-- *********************************************************************** -->
1350
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1351<div class="doc_text">
1352
1353<p>A lot of these comments and recommendations have been culled for other
1354sources. Two particularly important books for our work are:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1355
1356<ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1357
Chris Lattner788ee282007-11-09 21:49:08 +0000[diff] [blame]1358<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
1359C++</a> by Scott Meyers. Also
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]1360interesting and useful are "More Effective C++" and "Effective STL" by the same
1361author.</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1362
Chris Lattner788ee282007-11-09 21:49:08 +0000[diff] [blame]1363<li>Large-Scale C++ Software Design by John Lakos</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1364
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1365</ol>
1366
1367<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]1368something.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1369
1370</div>
1371
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1372<!-- *********************************************************************** -->
1373
1374<hr>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1375<address>
1376 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]1377 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1378 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]1379 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]1380
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1381 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]1382 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1383 Last modified: $Date$
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1384</address>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1385
1386</body>
1387</html>