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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>
Reid Spencerb40df562004-09-23 16:03:48 +0000[diff] [blame]32 <li><a href="#ci_class_struct">Use of class/struct 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>
37 <li><a href="#macro">The High Level Issues</a>
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>
41 <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
42 <li><a href="#hl_privateheaders">Keep "internal" Headers
43 Private</a></li>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]44 <li><a href="#hl_earlyexit">Use Early Exits and 'continue' to Simplify
45 Code</a></li>
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]46 <li><a href="#hl_else_after_return">Don't use "else" after a
47 return</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 Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]51 <li><a href="#micro">The Low Level Issues</a>
52 <ol>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]53 <li><a href="#ll_assert">Assert Liberally</a></li>
54 <li><a href="#ll_ns_std">Do not use 'using namespace std'</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>
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]57 <li><a href="#ll_end">Don't evaluate end() every time through a
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>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]61 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
Daniel Dunbare8530a32009-07-24 23:04:51 +0000[diff] [blame]62 <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</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
Misha Brukman69c58892004-07-28 22:37:57 +0000[diff] [blame]170is a C++ file, not a C file (Emacs assumes .h files are C files by default).
171Note that this tag is not necessary in .cpp files. The name of the file is also
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>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]247 <li><a href="#mmheader">Main Module header</a></li>
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
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]262<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]263which implement an interface defined by a .h file. This <tt>#include</tt>
264should always be included <b>first</b> regardless of where it lives on the file
265system. By including a header file first in the .cpp files that implement the
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
268form of documentation in the .cpp file to indicate where the interfaces it
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
326<p>Okay, 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">
437 <a name="macro">The High Level Issues</a>
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
465their interface first. This ensure that all of the dependences of the module
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">
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]525 <a name="hl_earlyexit">Use Early Exits and 'continue' 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
534exits and the 'continue' keyword in long loops. As an example of using an early
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
584<p>This fixes these problems. A similar problem frequently happens in for
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;
619 }
620</pre>
621</div>
622
623<p>This has all the benefits of using early exits from functions: it reduces
624nesting of the loop, it makes it easier to describe why the conditions are true,
625and it makes it obvious to the reader that there is no "else" coming up that
626they have to push context into their brain for. If a loop is large, this can
627be a big understandability win.</p>
628
629</div>
630
Chris Lattner9eb7e0a2009-07-28 22:54:04 +0000[diff] [blame]631<!-- _______________________________________________________________________ -->
632<div class="doc_subsubsection">
633 <a name="hl_else_after_return">Don't use "else" after a return</a>
634</div>
635
636<div class="doc_text">
637
638<p>For similar reasons above (reduction of indentation and easier reading),
639 please do not use "else" or "else if" after something that interrupts
640 control flow like return, break, continue, goto, etc. For example, this is
641 "bad":</p>
642
643<div class="doc_code">
644<pre>
645 case 'J': {
646 if (Signed) {
647 Type = Context.getsigjmp_bufType();
648 if (Type.isNull()) {
649 Error = ASTContext::GE_Missing_sigjmp_buf;
650 return QualType();
651 } else {
652 break;
653 }
654 } else {
655 Type = Context.getjmp_bufType();
656 if (Type.isNull()) {
657 Error = ASTContext::GE_Missing_jmp_buf;
658 return QualType();
659 } else {
660 break;
661 }
662 }
663 }
664 }
665</pre>
666</div>
667
668<p>It is better to write this something like:</p>
669
670<div class="doc_code">
671<pre>
672 case 'J':
673 if (Signed) {
674 Type = Context.getsigjmp_bufType();
675 if (Type.isNull()) {
676 Error = ASTContext::GE_Missing_sigjmp_buf;
677 return QualType();
678 }
679 } else {
680 Type = Context.getjmp_bufType();
681 if (Type.isNull()) {
682 Error = ASTContext::GE_Missing_jmp_buf;
683 return QualType();
684 }
685 }
686 break;
687</pre>
688</div>
689
690<p>Or better yet (in this case), as:</p>
691
692<div class="doc_code">
693<pre>
694 case 'J':
695 if (Signed)
696 Type = Context.getsigjmp_bufType();
697 else
698 Type = Context.getjmp_bufType();
699
700 if (Type.isNull()) {
701 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
702 ASTContext::GE_Missing_jmp_buf;
703 return QualType();
704 }
705 break;
706</pre>
707</div>
708
709<p>The idea is to reduce indentation and the amount of code you have to keep
710 track of when reading the code.</p>
711
712</div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]713
714<!-- _______________________________________________________________________ -->
715<div class="doc_subsubsection">
716 <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
717</div>
718
719<div class="doc_text">
720
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]721<p>It is very common to write small loops that just compute a boolean
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]722 value. There are a number of ways that people commonly write these, but an
723 example of this sort of thing is:</p>
724
725<div class="doc_code">
726<pre>
727 <b>bool FoundFoo = false;</b>
728 for (unsigned i = 0, e = BarList.size(); i != e; ++i)
729 if (BarList[i]-&gt;isFoo()) {
730 <b>FoundFoo = true;</b>
731 break;
732 }
733
734 <b>if (FoundFoo) {</b>
735 ...
736 }
737</pre>
738</div>
739
740<p>This sort of code is awkward to write, and is almost always a bad sign.
741Instead of this sort of loop, we strongly prefer to use a predicate function
742(which may be <a href="#micro_anonns">static</a>) that uses
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]743<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
744the code to be structured like this:
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]745</p>
746
747
748<div class="doc_code">
749<pre>
750/// ListContainsFoo - Return true if the specified list has an element that is
751/// a foo.
752static bool ListContainsFoo(const std::vector&lt;Bar*&gt; &amp;List) {
753 for (unsigned i = 0, e = List.size(); i != e; ++i)
754 if (List[i]-&gt;isFoo())
755 return true;
756 return false;
757}
758...
759
760 <b>if (ListContainsFoo(BarList)) {</b>
761 ...
762 }
763</pre>
764</div>
765
766<p>There are many reasons for doing this: it reduces indentation and factors out
767code which can often be shared by other code that checks for the same predicate.
768More importantly, it <em>forces you to pick a name</em> for the function, and
769forces you to write a comment for it. In this silly example, this doesn't add
770much value. However, if the condition is complex, this can make it a lot easier
771for the reader to understand the code that queries for this predicate. Instead
Chris Lattner71d8f3b2009-07-22 16:30:39 +0000[diff] [blame]772of 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]773contains a foo, we can trust the function name and continue reading with better
774locality.</p>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]775
776</div>
777
778
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]779<!-- ======================================================================= -->
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]780<div class="doc_subsection">
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]781 <a name="micro">The Low Level Issues</a>
782</div>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]783<!-- ======================================================================= -->
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]784
785
786<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]787<div class="doc_subsubsection">
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]788 <a name="ll_assert">Assert Liberally</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]789</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]790
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]791<div class="doc_text">
792
793<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]794preconditions and assumptions, you never know when a bug (not necessarily even
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]795yours) might be caught early by an assertion, which reduces debugging time
796dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already
797included 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]798it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]799
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]800<p>To further assist with debugging, make sure to put some kind of error message
801in the assertion statement (which is printed if the assertion is tripped). This
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]802helps the poor debugging make sense of why an assertion is being made and
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]803enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]804
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]805<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]806<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]807inline Value *getOperand(unsigned i) {
808 assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
809 return Operands[i];
810}
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]811</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]812</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]813
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]814<p>Here are some examples:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]815
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]816<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]817<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]818assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]819
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]820assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]821
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]822assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]823
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]824assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]825
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]826assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]827</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]828</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]829
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]830<p>You get the idea...</p>
831
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]832<p>Please be aware when adding assert statements that not all compilers are aware of
833the semantics of the assert. In some places, asserts are used to indicate a piece of
834code that should not be reached. These are typically of the form:</p>
835
836<div class="doc_code">
837<pre>
Dan Gohmanf0032762008-11-24 17:18:39 +0000[diff] [blame]838assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]839</pre>
840</div>
841
842<p>When used in a function that returns a value, they should be followed with a return
843statement and a comment indicating that this line is never reached. This will prevent
844a compiler which is unable to deduce that the assert statement never returns from
845generating a warning.</p>
846
847<div class="doc_code">
848<pre>
Dan Gohmanf0032762008-11-24 17:18:39 +0000[diff] [blame]849assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]850// Not reached
851return 0;
852</pre>
853</div>
854
Chris Lattner97928d12010-11-12 00:19:41 +0000[diff] [blame]855<p>Another issue is that values used only by assertions will produce an "unused
856 value" warning when assertions are disabled. For example, this code will warn:
857</p>
858
859<div class="doc_code">
860<pre>
861 unsigned Size = V.size();
862 assert(Size &gt; 42 &amp;&amp; "Vector smaller than it should be");
863
864 bool NewToSet = Myset.insert(Value);
865 assert(NewToSet &amp;&amp; "The value shouldn't be in the set yet");
866</pre>
867</div>
868
869<p>These are two interesting different cases: in the first case, the call to
870V.size() is only useful for the assert, and we don't want it executed when
871assertions are disabled. Code like this should move the call into the assert
872itself. In the second case, the side effects of the call must happen whether
873the assert is enabled or not. In this case, the value should be cast to void
874to disable the warning. To be specific, it is preferred to write the code
875like this:</p>
876
877<div class="doc_code">
878<pre>
879 assert(V.size() &gt; 42 &amp;&amp; "Vector smaller than it should be");
880
881 bool NewToSet = Myset.insert(Value); (void)NewToSet;
882 assert(NewToSet &amp;&amp; "The value shouldn't be in the set yet");
883</pre>
884</div>
885
886
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]887</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]888
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]889<!-- _______________________________________________________________________ -->
890<div class="doc_subsubsection">
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]891 <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]892</div>
893
894<div class="doc_text">
895<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]896namespace with an "<tt>std::</tt>" prefix, rather than rely on
897"<tt>using namespace std;</tt>".</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]898
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]899<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
Misha Brukman2d33c542008-12-11 19:37:04 +0000[diff] [blame]900the namespace of any source file that <tt>#include</tt>s the header. This is
901clearly a bad thing.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]902
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]903<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
904rule, but is still important. Basically, using explicit namespace prefixes
905makes the code <b>clearer</b>, because it is immediately obvious what facilities
906are being used and where they are coming from, and <b>more portable</b>, because
907namespace clashes cannot occur between LLVM code and other namespaces. The
908portability rule is important because different standard library implementations
909expose different symbols (potentially ones they shouldn't), and future revisions
910to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
911such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]912
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]913<p>The exception to the general rule (i.e. it's not an exception for
914the <tt>std</tt> namespace) is for implementation files. For example, all of
915the code in the LLVM project implements code that lives in the 'llvm' namespace.
916As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
917namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
918general form of this rule is that any .cpp file that implements code in any
919namespace may use that namespace (and its parents'), but should not use any
920others.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]921
922</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]923
924<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]925<div class="doc_subsubsection">
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]926 <a name="ll_virtual_anch">Provide a virtual method anchor for classes
927 in headers</a>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]928</div>
929
930<div class="doc_text">
931
932<p>If a class is defined in a header file and has a v-table (either it has
933virtual methods or it derives from classes with virtual methods), it must
934always have at least one out-of-line virtual method in the class. Without
Misha Brukman2d33c542008-12-11 19:37:04 +0000[diff] [blame]935this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
936that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
937increasing link times.</p>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]938
939</div>
940
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]941<!-- _______________________________________________________________________ -->
942<div class="doc_subsubsection">
943 <a name="ll_end">Don't evaluate end() every time through a loop</a>
944</div>
945
946<div class="doc_text">
947
948<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
949with macros and may be coming in C++'0x) we end up writing a lot of loops that
950manually iterate from begin to end on a variety of containers or through other
951data structures. One common mistake is to write a loop in this style:</p>
952
953<div class="doc_code">
954<pre>
955 BasicBlock *BB = ...
956 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
957 ... use I ...
958</pre>
959</div>
960
961<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
962every time through the loop. Instead of writing the loop like this, we strongly
963prefer loops to be written so that they evaluate it once before the loop starts.
964A convenient way to do this is like so:</p>
965
966<div class="doc_code">
967<pre>
968 BasicBlock *BB = ...
969 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
970 ... use I ...
971</pre>
972</div>
973
974<p>The observant may quickly point out that these two loops may have different
975semantics: if the container (a basic block in this case) is being mutated, then
976"<tt>BB->end()</tt>" may change its value every time through the loop and the
977second loop may not in fact be correct. If you actually do depend on this
Chris Lattner09bc65e2009-06-30 06:20:03 +0000[diff] [blame]978behavior, please write the loop in the first form and add a comment indicating
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]979that you did it intentionally.</p>
980
981<p>Why do we prefer the second form (when correct)? Writing the loop in the
982first form has two problems: First it may be less efficient than evaluating it
983at the start of the loop. In this case, the cost is probably minor: a few extra
984loads every time through the loop. However, if the base expression is more
985complex, then the cost can rise quickly. I've seen loops where the end
986expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
Chris Lattnera44f87f2009-06-30 06:27:54 +0000[diff] [blame]987lookups really aren't cheap. By writing it in the second form consistently, you
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]988eliminate the issue entirely and don't even have to think about it.</p>
989
Chris Lattner09bc65e2009-06-30 06:20:03 +0000[diff] [blame]990<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]991hints to the reader that the loop is mutating the container (a fact that a
992comment would handily confirm!). If you write the loop in the second form, it
993is immediately obvious without even looking at the body of the loop that the
994container isn't being modified, which makes it easier to read the code and
995understand what it does.</p>
996
997<p>While the second form of the loop is a few extra keystrokes, we do strongly
998prefer it.</p>
999
1000</div>
1001
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1002<!-- _______________________________________________________________________ -->
1003<div class="doc_subsubsection">
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1004 <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is forbidden</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1005</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1006
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1007<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1008
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1009<p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
1010hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
1011support clients using LLVM libraries as part of larger systems. In particular,
1012we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
1013the static c'tors are run whenever an application start up that uses the dynamic
1014library. There are two problems with this:</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1015
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1016<ol>
1017 <li>The time to run the static c'tors impacts startup time of
1018 applications&mdash;a critical time for GUI apps.</li>
1019 <li>The static c'tors cause the app to pull many extra pages of memory off the
1020 disk: both the code for the static c'tors in each <tt>.o</tt> file and the
1021 small amount of data that gets touched. In addition, touched/dirty pages
1022 put more pressure on the VM system on low-memory machines.</li>
1023</ol>
1024
1025<p>Note that using the other stream headers (<tt>&lt;sstream&gt;</tt> for
Chris Lattner983c5922009-08-23 21:53:47 +0000[diff] [blame]1026example) is not problematic in this regard (just <tt>&lt;iostream&gt;</tt>).
1027However, raw_ostream provides various APIs that are better performing for almost
1028every use than std::ostream style APIs, so you should just use it for new
1029code.</p>
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1030
Chris Lattner983c5922009-08-23 21:53:47 +0000[diff] [blame]1031<p><b>New code should always
1032use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or
1033the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1034
1035</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1036
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1037
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1038<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1039<div class="doc_subsubsection">
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]1040 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1041</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1042
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1043<div class="doc_text">
1044
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]1045<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
1046to the output stream specified. In addition to doing this, however, it also
1047flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1048
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1049<div class="doc_code">
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1050<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1051std::cout &lt;&lt; std::endl;
1052std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1053</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]1054</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1055
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1056<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]1057it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1058
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1059</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]1060
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]1061
Daniel Dunbare8530a32009-07-24 23:04:51 +0000[diff] [blame]1062<!-- _______________________________________________________________________ -->
1063<div class="doc_subsubsection">
1064 <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
1065</div>
1066
1067<div class="doc_text">
1068
1069<p>LLVM includes a lightweight, simple, and efficient stream implementation
1070in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
Daniel Dunbar26fe8662009-07-24 23:54:34 +0000[diff] [blame]1071of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> instead
1072of <tt>ostream</tt>.</p>
Daniel Dunbare8530a32009-07-24 23:04:51 +0000[diff] [blame]1073
Daniel Dunbar26fe8662009-07-24 23:54:34 +0000[diff] [blame]1074<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
Daniel Dunbare8530a32009-07-24 23:04:51 +0000[diff] [blame]1075be forward declared as <tt>class raw_ostream</tt>. Public headers should
1076generally not include the <tt>raw_ostream</tt> header, but use forward
1077declarations and constant references to <tt>raw_ostream</tt> instances.</p>
1078
1079</div>
1080
1081
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1082<!-- ======================================================================= -->
1083<div class="doc_subsection">
1084 <a name="nano">Microscopic Details</a>
1085</div>
1086<!-- ======================================================================= -->
1087
1088<p>This section describes preferred low-level formatting guidelines along with
1089reasoning on why we prefer them.</p>
1090
1091<!-- _______________________________________________________________________ -->
1092<div class="doc_subsubsection">
1093 <a name="micro_spaceparen">Spaces Before Parentheses</a>
1094</div>
1095
1096<div class="doc_text">
1097
1098<p>We prefer to put a space before a parentheses only in control flow
1099statements, but not in normal function call expressions and function-like
1100macros. For example, this is good:</p>
1101
1102<div class="doc_code">
1103<pre>
1104 <b>if (</b>x) ...
1105 <b>for (</b>i = 0; i != 100; ++i) ...
1106 <b>while (</b>llvm_rocks) ...
1107
1108 <b>somefunc(</b>42);
1109 <b><a href="#ll_assert">assert</a>(</b>3 != 4 &amp;&amp; "laws of math are failing me");
1110
1111 a = <b>foo(</b>42, 92) + <b>bar(</b>x);
1112 </pre>
1113</div>
1114
1115<p>... and this is bad:</p>
1116
1117<div class="doc_code">
1118<pre>
1119 <b>if(</b>x) ...
1120 <b>for(</b>i = 0; i != 100; ++i) ...
1121 <b>while(</b>llvm_rocks) ...
1122
1123 <b>somefunc (</b>42);
1124 <b><a href="#ll_assert">assert</a> (</b>3 != 4 &amp;&amp; "laws of math are failing me");
1125
1126 a = <b>foo (</b>42, 92) + <b>bar (</b>x);
1127</pre>
1128</div>
1129
1130<p>The reason for doing this is not completely arbitrary. This style makes
1131 control flow operators stand out more, and makes expressions flow better. The
1132 function call operator binds very tightly as a postfix operator. Putting
1133 a space after a function name (as in the last example) makes it appear that
1134 the code might bind the arguments of the left-hand-side of a binary operator
1135 with the argument list of a function and the name of the right side. More
1136 specifically, it is easy to misread the "a" example as:</p>
1137
1138<div class="doc_code">
1139<pre>
1140 a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
1141</pre>
1142</div>
1143
1144<p>... when skimming through the code. By avoiding a space in a function, we
1145avoid this misinterpretation.</p>
1146
1147</div>
1148
1149<!-- _______________________________________________________________________ -->
1150<div class="doc_subsubsection">
1151 <a name="micro_preincrement">Prefer Preincrement</a>
1152</div>
1153
1154<div class="doc_text">
1155
1156<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
1157postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
1158preincrementation whenever possible.</p>
1159
1160<p>The semantics of postincrement include making a copy of the value being
1161incremented, returning it, and then preincrementing the "work value". For
1162primitive types, this isn't a big deal... but for iterators, it can be a huge
1163issue (for example, some iterators contains stack and set objects in them...
1164copying an iterator could invoke the copy ctor's of these as well). In general,
1165get in the habit of always using preincrement, and you won't have a problem.</p>
1166
1167</div>
1168
1169<!-- _______________________________________________________________________ -->
1170<div class="doc_subsubsection">
1171 <a name="micro_namespaceindent">Namespace Indentation</a>
1172</div>
1173
1174<div class="doc_text">
1175
1176<p>
1177In general, we strive to reduce indentation where ever possible. This is useful
1178because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
1179wrapping horribly, but also because it makes it easier to understand the code.
1180Namespaces are a funny thing: they are often large, and we often desire to put
1181lots of stuff into them (so they can be large). Other times they are tiny,
1182because they just hold an enum or something similar. In order to balance this,
1183we use different approaches for small versus large namespaces.
1184</p>
1185
1186<p>
1187If a namespace definition is small and <em>easily</em> fits on a screen (say,
1188less than 35 lines of code), then you should indent its body. Here's an
1189example:
1190</p>
1191
1192<div class="doc_code">
1193<pre>
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1194namespace llvm {
1195 namespace X86 {
1196 /// RelocationType - An enum for the x86 relocation codes. Note that
1197 /// the terminology here doesn't follow x86 convention - word means
1198 /// 32-bit and dword means 64-bit.
1199 enum RelocationType {
1200 /// reloc_pcrel_word - PC relative relocation, add the relocated value to
1201 /// the value already in memory, after we adjust it for where the PC is.
1202 reloc_pcrel_word = 0,
1203
1204 /// reloc_picrel_word - PIC base relative relocation, add the relocated
1205 /// value to the value already in memory, after we adjust it for where the
1206 /// PIC base is.
1207 reloc_picrel_word = 1,
1208
1209 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
1210 /// add the relocated value to the value already in memory.
1211 reloc_absolute_word = 2,
1212 reloc_absolute_dword = 3
1213 };
1214 }
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1215}
1216</pre>
1217</div>
1218
1219<p>Since the body is small, indenting adds value because it makes it very clear
1220where the namespace starts and ends, and it is easy to take the whole thing in
1221in one "gulp" when reading the code. If the blob of code in the namespace is
1222larger (as it typically is in a header in the llvm or clang namespaces), do not
1223indent the code, and add a comment indicating what namespace is being closed.
1224For example:</p>
1225
1226<div class="doc_code">
1227<pre>
1228namespace llvm {
1229namespace knowledge {
1230
1231/// Grokable - This class represents things that Smith can have an intimate
1232/// understanding of and contains the data associated with it.
1233class Grokable {
1234...
1235public:
1236 explicit Grokable() { ... }
1237 virtual ~Grokable() = 0;
1238
1239 ...
1240
1241};
1242
1243} // end namespace knowledge
1244} // end namespace llvm
1245</pre>
1246</div>
1247
1248<p>Because the class is large, we don't expect that the reader can easily
1249understand the entire concept in a glance, and the end of the file (where the
1250namespaces end) may be a long ways away from the place they open. As such,
1251indenting the contents of the namespace doesn't add any value, and detracts from
1252the readability of the class. In these cases it is best to <em>not</em> indent
1253the contents of the namespace.</p>
1254
1255</div>
1256
1257<!-- _______________________________________________________________________ -->
1258<div class="doc_subsubsection">
1259 <a name="micro_anonns">Anonymous Namespaces</a>
1260</div>
1261
1262<div class="doc_text">
1263
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1264<p>After talking about namespaces in general, you may be wondering about
1265anonymous namespaces in particular.
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1266Anonymous namespaces are a great language feature that tells the C++ compiler
1267that the contents of the namespace are only visible within the current
1268translation unit, allowing more aggressive optimization and eliminating the
1269possibility of symbol name collisions. Anonymous namespaces are to C++ as
1270"static" is to C functions and global variables. While "static" is available
1271in C++, anonymous namespaces are more general: they can make entire classes
1272private to a file.</p>
1273
1274<p>The problem with anonymous namespaces is that they naturally want to
1275encourage indentation of their body, and they reduce locality of reference: if
1276you see a random function definition in a C++ file, it is easy to see if it is
1277marked static, but seeing if it is in an anonymous namespace requires scanning
1278a big chunk of the file.</p>
1279
1280<p>Because of this, we have a simple guideline: make anonymous namespaces as
1281small as possible, and only use them for class declarations. For example, this
1282is good:</p>
1283
1284<div class="doc_code">
1285<pre>
1286<b>namespace {</b>
1287 class StringSort {
1288 ...
1289 public:
1290 StringSort(...)
1291 bool operator&lt;(const char *RHS) const;
1292 };
1293<b>} // end anonymous namespace</b>
1294
1295static void Helper() {
1296 ...
1297}
1298
1299bool StringSort::operator&lt;(const char *RHS) const {
1300 ...
1301}
1302
1303</pre>
1304</div>
1305
1306<p>This is bad:</p>
1307
1308
1309<div class="doc_code">
1310<pre>
1311<b>namespace {</b>
1312class StringSort {
1313...
1314public:
1315 StringSort(...)
1316 bool operator&lt;(const char *RHS) const;
1317};
1318
1319void Helper() {
1320 ...
1321}
1322
1323bool StringSort::operator&lt;(const char *RHS) const {
1324 ...
1325}
1326
1327<b>} // end anonymous namespace</b>
1328
1329</pre>
1330</div>
1331
1332
1333<p>This is bad specifically because if you're looking at "Helper" in the middle
1334of a large C++ file, that you have no immediate way to tell if it is local to
1335the file. When it is marked static explicitly, this is immediately obvious.
1336Also, there is no reason to enclose the definition of "operator&lt;" in the
Chris Lattner59fec6a2009-07-22 16:54:14 +0000[diff] [blame]1337namespace just because it was declared there.
Chris Lattnerd283cb12009-07-22 05:40:54 +0000[diff] [blame]1338</p>
1339
1340</div>
1341
1342
1343
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1344<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1345<div class="doc_section">
1346 <a name="seealso">See Also</a>
1347</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1348<!-- *********************************************************************** -->
1349
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1350<div class="doc_text">
1351
1352<p>A lot of these comments and recommendations have been culled for other
1353sources. Two particularly important books for our work are:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1354
1355<ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1356
Chris Lattner788ee282007-11-09 21:49:08 +0000[diff] [blame]1357<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
1358C++</a> by Scott Meyers. Also
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]1359interesting and useful are "More Effective C++" and "Effective STL" by the same
1360author.</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1361
Chris Lattner788ee282007-11-09 21:49:08 +0000[diff] [blame]1362<li>Large-Scale C++ Software Design by John Lakos</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1363
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1364</ol>
1365
1366<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]1367something.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1368
1369</div>
1370
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]1371<!-- *********************************************************************** -->
1372
1373<hr>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1374<address>
1375 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]1376 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1377 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]1378 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]1379
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1380 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]1381 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]1382 Last modified: $Date$
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]1383</address>
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1386</html>