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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>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]44 <li><a href="#ll_iostream"><tt>#include &lt;iostream&gt;</tt> is
45 <em>forbidden</em></a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]46 </ol></li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]47 <li><a href="#micro">The Low Level Issues</a>
48 <ol>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]49 <li><a href="#ll_assert">Assert Liberally</a></li>
50 <li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]51 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]52 classes in headers</a></li>
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]53 <li><a href="#ll_end">Don't evaluate end() every time through a
54 loop</a></li>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]55 <li><a href="#ll_preincrement">Prefer Preincrement</a></li>
56 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]57 </ol></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]58 </ol></li>
59 <li><a href="#seealso">See Also</a></li>
60</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]61
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]62<div class="doc_author">
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]63 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
64 <a href="mailto:void@nondot.org">Bill Wendling</a></p>
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]65</div>
66
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]67
68<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]69<div class="doc_section">
70 <a name="introduction">Introduction</a>
71</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]72<!-- *********************************************************************** -->
73
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]74<div class="doc_text">
75
76<p>This document attempts to describe a few coding standards that are being used
77in the LLVM source tree. Although no coding standards should be regarded as
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]78absolute requirements to be followed in all instances, coding standards can be
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]79useful.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]80
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]81<p>This document intentionally does not prescribe fixed standards for religious
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]82issues such as brace placement and space usage. For issues like this, follow
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]83the golden rule:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]84
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]85<blockquote>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]86
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]87<p><b><a name="goldenrule">If you are adding a significant body of source to a
88project, feel free to use whatever style you are most comfortable with. If you
89are extending, enhancing, or bug fixing already implemented code, use the style
90that is already being used so that the source is uniform and easy to
91follow.</a></b></p>
92
93</blockquote>
94
95<p>The ultimate goal of these guidelines is the increase readability and
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]96maintainability of our common source base. If you have suggestions for topics to
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]97be included, please mail them to <a
98href="mailto:sabre@nondot.org">Chris</a>.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]99
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]100</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]101
102<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]103<div class="doc_section">
104 <a name="mechanicalissues">Mechanical Source Issues</a>
105</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]106<!-- *********************************************************************** -->
107
108<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]109<div class="doc_subsection">
110 <a name="sourceformating">Source Code Formatting</a>
111</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]112
113<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]114<div class="doc_subsubsection">
115 <a name="scf_commenting">Commenting</a>
116</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]117
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]118<div class="doc_text">
119
120<p>Comments are one critical part of readability and maintainability. Everyone
Chris Lattnerb927ca82009-07-12 00:10:24 +0000[diff] [blame]121knows they should comment, so should you. When writing comments, write them as
122English prose, which means they should use proper capitalization, punctuation,
123etc. Although we all should probably
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]124comment our code more than we do, there are a few very critical places that
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]125documentation is very useful:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]126
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]127<b>File Headers</b>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]128
Reid Spencer669ed452007-07-09 08:04:31 +0000[diff] [blame]129<p>Every source file should have a header on it that describes the basic
130purpose of the file. If a file does not have a header, it should not be
131checked into Subversion. Most source trees will probably have a standard
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]132file header format. The standard format for the LLVM source tree looks like
133this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]134
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]135<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]136<pre>
Chris Lattnerc49fcd12003-10-13 14:58:11 +0000[diff] [blame]137//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
Misha Brukman98117cd2009-01-02 16:58:42 +0000[diff] [blame]138//
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]139// The LLVM Compiler Infrastructure
140//
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]141// This file is distributed under the University of Illinois Open Source
142// License. See LICENSE.TXT for details.
Misha Brukman98117cd2009-01-02 16:58:42 +0000[diff] [blame]143//
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]144//===----------------------------------------------------------------------===//
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]145//
146// This file contains the declaration of the Instruction class, which is the
147// base class for all of the VM instructions.
148//
149//===----------------------------------------------------------------------===//
150</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]151</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]152
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]153<p>A few things to note about this particular format: The "<tt>-*- C++
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]154-*-</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]155is a C++ file, not a C file (Emacs assumes .h files are C files by default).
156Note that this tag is not necessary in .cpp files. The name of the file is also
157on the first line, along with a very short description of the purpose of the
158file. This is important when printing out code and flipping though lots of
159pages.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]160
Chris Lattnerab2b10c2007-12-29 19:56:08 +0000[diff] [blame]161<p>The next section in the file is a concise note that defines the license
162that the file is released under. This makes it perfectly clear what terms the
163source code can be distributed under and should not be modified in any way.</p>
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]164
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]165<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]166Here it's only two lines. If an algorithm is being implemented or something
167tricky is going on, a reference to the paper where it is published should be
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]168included, 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]169
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]170<b>Class overviews</b>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]171
Jim Laskeyf55914a2006-07-31 20:18:49 +0000[diff] [blame]172<p>Classes are one fundamental part of a good object oriented design. As such,
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]173a class definition should have a comment block that explains what the class is
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]174used for... if it's not obvious. If it's so completely obvious your grandma
175could figure it out, it's probably safe to leave it out. Naming classes
Misha Brukman3b53a262004-12-04 00:32:12 +0000[diff] [blame]176something sane goes a long ways towards avoiding writing documentation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]177
178
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]179<b>Method information</b>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]180
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]181<p>Methods defined in a class (as well as any global functions) should also be
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]182documented properly. A quick note about what it does any a description of the
183borderline behaviour is all that is necessary here (unless something
184particularly tricky or insideous is going on). The hope is that people can
185figure out how to use your interfaces without reading the code itself... that is
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]186the goal metric.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]187
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]188<p>Good things to talk about here are what happens when something unexpected
189happens: does the method return null? Abort? Format your hard disk?</p>
190
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]191</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]192
193<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]194<div class="doc_subsubsection">
195 <a name="scf_commentformat">Comment Formatting</a>
196</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]197
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]198<div class="doc_text">
199
200<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]201require less typing, don't have nesting problems, etc. There are a few cases
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]202when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]203
204<ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]205 <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]206 comments.</li>
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]207 <li>When writing a header file that may be <tt>#include</tt>d by a C source
208 file.</li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]209 <li>When writing a source file that is used by a tool that only accepts C
210 style comments.</li>
211</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]212
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]213<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
214These nest properly and are better behaved in general than C style comments.</p>
215
216</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]217
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]218<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]219<div class="doc_subsubsection">
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]220 <a name="scf_includes"><tt>#include</tt> Style</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]221</div>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]222
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]223<div class="doc_text">
224
225<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]226include guards if working on a header file), the <a
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]227href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
228file should be listed. We prefer these <tt>#include</tt>s to be listed in this
229order:</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]230
231<ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]232 <li><a href="#mmheader">Main Module header</a></li>
233 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]234 <li><tt>llvm/*</tt></li>
235 <li><tt>llvm/Analysis/*</tt></li>
236 <li><tt>llvm/Assembly/*</tt></li>
237 <li><tt>llvm/Bytecode/*</tt></li>
238 <li><tt>llvm/CodeGen/*</tt></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]239 <li>...</li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]240 <li><tt>Support/*</tt></li>
241 <li><tt>Config/*</tt></li>
242 <li>System <tt>#includes</tt></li>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]243</ol>
244
Nick Lewycky822293a2008-11-29 20:13:25 +0000[diff] [blame]245<p>... and each category should be sorted by name.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]246
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]247<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]248which implement an interface defined by a .h file. This <tt>#include</tt>
249should always be included <b>first</b> regardless of where it lives on the file
250system. By including a header file first in the .cpp files that implement the
251interfaces, we ensure that the header does not have any hidden dependencies
252which are not explicitly #included in the header, but should be. It is also a
253form of documentation in the .cpp file to indicate where the interfaces it
254implements are defined.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000[diff] [blame]255
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]256</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]257
258<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]259<div class="doc_subsubsection">
260 <a name="scf_codewidth">Source Code Width</a>
261</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]262
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]263<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]264
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]265<p>Write your code to fit within 80 columns of text. This helps those of us who
266like to print out code and look at your code in an xterm without resizing
267it.</p>
268
Chris Lattner5822e9e2008-07-08 05:12:37 +0000[diff] [blame]269<p>The longer answer is that there must be some limit to the width of the code
270in order to reasonably allow developers to have multiple files side-by-side in
271windows on a modest display. If you are going to pick a width limit, it is
272somewhat arbitrary but you might as well pick something standard. Going with
27390 columns (for example) instead of 80 columns wouldn't add any significant
274value and would be detrimental to printing out code. Also many other projects
275have standardized on 80 columns, so some people have already configured their
276editors for it (vs something else, like 90 columns).</p>
277
278<p>This is one of many contentious issues in coding standards, but is not up
279for debate.</p>
280
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]281</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]282
283<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]284<div class="doc_subsubsection">
285 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
286</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]287
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]288<div class="doc_text">
289
290<p>In all cases, prefer spaces to tabs in source files. People have different
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]291prefered indentation levels, and different styles of indentation that they
292like... this is fine. What isn't is that different editors/viewers expand tabs
293out to different tab stops. This can cause your code to look completely
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]294unreadable, and it is not worth dealing with.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]295
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]296<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]297style of existing code if your are modifying and extending it. If you like four
298spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
299with two spaces of indentation. Also, do not reindent a whole source file: it
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]300makes for incredible diffs that are absolutely worthless.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]301
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]302</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]303
304<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]305<div class="doc_subsubsection">
306 <a name="scf_indentation">Indent Code Consistently</a>
307</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]308
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]309<div class="doc_text">
310
311<p>Okay, your first year of programming you were told that indentation is
Misha Brukman1b36d672003-10-06 19:26:00 +0000[diff] [blame]312important. If you didn't believe and internalize this then, now is the time.
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]313Just do it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]314
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]315</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]316
317
318<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]319<div class="doc_subsection">
320 <a name="compilerissues">Compiler Issues</a>
321</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]322
323
324<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]325<div class="doc_subsubsection">
326 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
327</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]328
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]329<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]330
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]331<p>If your code has compiler warnings in it, something is wrong: you aren't
332casting values correctly, your have "questionable" constructs in your code, or
333you are doing something legitimately wrong. Compiler warnings can cover up
334legitimate errors in output and make dealing with a translation unit
335difficult.</p>
336
337<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]338desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
339a good thorough set of warnings, and stick to them. At least in the case of
340<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
341syntax of the code slightly. For example, an warning that annoys me occurs when
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]342I write code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]343
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]344<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]345<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]346if (V = getValue()) {
347 ...
348}
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]349</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]350</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]351
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]352<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
353operator, and that I probably mistyped it. In most cases, I haven't, and I
354really don't want the spurious errors. To fix this particular problem, I
355rewrite the code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]356
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]357<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]358<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]359if ((V = getValue())) {
360 ...
361}
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]362</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]363</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]364
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]365<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
366be fixed by massaging the code appropriately.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]367
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]368<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
369-Winline -W -Wwrite-strings -Wno-unused</tt></p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]370
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]371</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]372
373<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]374<div class="doc_subsubsection">
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]375 <a name="ci_portable_code">Write Portable Code</a>
376</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]377
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]378<div class="doc_text">
379
380<p>In almost all cases, it is possible and within reason to write completely
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]381portable code. If there are cases where it isn't possible to write portable
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]382code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]383
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]384<p>In practice, this means that you shouldn't assume much about the host
385compiler, including its support for "high tech" features like partial
Chris Lattner4bd3d7e2009-03-23 04:52:53 +0000[diff] [blame]386specialization of templates. If these features are used, they should only be
387an implementation detail of a library which has a simple exposed API.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]388
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]389</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]390
Reid Spencerb40df562004-09-23 16:03:48 +0000[diff] [blame]391<!-- _______________________________________________________________________ -->
392<div class="doc_subsubsection">
393<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
394</div>
395<div class="doc_text">
Misha Brukmanedcc78a2004-10-26 15:45:13 +0000[diff] [blame]396
397<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
398interchangeably. The only difference is when they are used to declare a class:
399<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
400all members public by default.</p>
401
402<p>Unfortunately, not all compilers follow the rules and some will generate
403different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
404declare the symbol. This can lead to problems at link time.</p>
405
406<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
407<b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
408
Reid Spencerb40df562004-09-23 16:03:48 +0000[diff] [blame]409</div>
410
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]411<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]412<div class="doc_section">
413 <a name="styleissues">Style Issues</a>
414</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]415<!-- *********************************************************************** -->
416
417
418<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]419<div class="doc_subsection">
420 <a name="macro">The High Level Issues</a>
421</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]422
423
424<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]425<div class="doc_subsubsection">
426 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
427</div>
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_text">
430
431<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]432encapsulation or data hiding (unless you use expensive protocol classes), but it
433is what we have to work with. When you write a public header file (in the LLVM
434source tree, they live in the top level "include" directory), you are defining a
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]435module of functionality.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]436
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]437<p>Ideally, modules should be completely independent of each other, and their
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]438header files should only include the absolute minimum number of headers
439possible. A module is not just a class, a function, or a namespace: <a
440href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
441of these</a> that defines an interface. This interface may be several
442functions, classes or data structures, but the important issue is how they work
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]443together.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]444
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]445<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
446files. Each of these <tt>.cpp</tt> files should include the header that defines
447their interface first. This ensure that all of the dependences of the module
448header have been properly added to the module header itself, and are not
449implicit. System headers should be included after user headers for a
450translation unit.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]451
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]452</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]453
454<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]455<div class="doc_subsubsection">
Misha Brukman91d64af2004-10-26 16:18:43 +0000[diff] [blame]456 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]457</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]458
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]459<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]460
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]461<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
462have to, especially in header files.</p>
463
464<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]465to inherit from it. In these cases go ahead and <tt>#include</tt> that header
466file. Be aware however that there are many cases where you don't need to have
467the full definition of a class. If you are using a pointer or reference to a
468class, you don't need the header file. If you are simply returning a class
469instance from a prototyped function or method, you don't need it. In fact, for
470most cases, you simply don't need the definition of a class... and not
471<tt>#include</tt>'ing speeds up compilation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]472
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]473<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]474<b>must</b> include all of the header files that you are using -- you can
475include them either directly
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]476or indirectly (through another header file). To make sure that you don't
477accidently forget to include a header file in your module header, make sure to
478include your module header <b>first</b> in the implementation file (as mentioned
479above). This way there won't be any hidden dependencies that you'll find out
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]480about later...</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]481
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]482</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]483
484<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]485<div class="doc_subsubsection">
486 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
487</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]488
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]489<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]490
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]491<p>Many modules have a complex implementation that causes them to use more than
492one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
493internal communication interface (helper classes, extra functions, etc) in the
Misha Brukman3b53a262004-12-04 00:32:12 +0000[diff] [blame]494public module header file. Don't do this.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]495
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]496<p>If you really need to do something like this, put a private header file in
497the same directory as the source files, and include it locally. This ensures
498that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]499
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]500<p>Note however, that it's okay to put extra implementation methods a public
501class itself... just make them private (or protected), and all is well.</p>
502
503</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]504
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]505<!-- _______________________________________________________________________ -->
506<div class="doc_subsubsection">
507 <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is forbidden</a>
508</div>
509
510<div class="doc_text">
511
512<p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
513hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
514support clients using LLVM libraries as part of larger systems. In particular,
515we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
516the static c'tors are run whenever an application start up that uses the dynamic
517library. There are two problems with this:</p>
518
519<ol>
520 <li>The time to run the static c'tors impacts startup time of
Bill Wendling4f6766d2007-11-06 09:36:34 +0000[diff] [blame]521 applications&mdash;a critical time for GUI apps.</li>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]522 <li>The static c'tors cause the app to pull many extra pages of memory off the
Bill Wendling4f6766d2007-11-06 09:36:34 +0000[diff] [blame]523 disk: both the code for the static c'tors in each <tt>.o</tt> file and the
524 small amount of data that gets touched. In addition, touched/dirty pages
525 put more pressure on the VM system on low-memory machines.</li>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]526</ol>
527
Matthijs Kooijmaned58a972008-07-30 12:14:10 +0000[diff] [blame]528<p>Note that using the other stream headers (<tt>&lt;sstream&gt;</tt> for
529example) is allowed normally, it is just <tt>&lt;iostream&gt;</tt> that is
530causing problems.</p>
531
Chris Lattner4bd3d7e2009-03-23 04:52:53 +0000[diff] [blame]532<p>The preferred replacement for stream functionality is the
Chris Lattner640e7c62009-03-23 04:53:34 +0000[diff] [blame]533<tt>llvm::raw_ostream</tt> class (for writing to output streams of various
534sorts) and the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]535
536</div>
537
538
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]539<!-- ======================================================================= -->
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]540<div class="doc_subsection">
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]541 <a name="micro">The Low Level Issues</a>
542</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]543
544
545<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]546<div class="doc_subsubsection">
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]547 <a name="ll_assert">Assert Liberally</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]548</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]549
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]550<div class="doc_text">
551
552<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]553preconditions and assumptions, you never know when a bug (not neccesarily even
554yours) might be caught early by an assertion, which reduces debugging time
555dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already
556included 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]557it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]558
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]559<p>To further assist with debugging, make sure to put some kind of error message
560in the assertion statement (which is printed if the assertion is tripped). This
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]561helps the poor debugging make sense of why an assertion is being made and
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]562enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]563
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]564<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]565<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]566inline Value *getOperand(unsigned i) {
567 assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
568 return Operands[i];
569}
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]570</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]571</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]572
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]573<p>Here are some examples:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]574
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]575<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]576<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]577assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]578
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]579assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]580
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]581assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]582
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]583assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]584
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]585assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]586</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]587</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]588
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]589<p>You get the idea...</p>
590
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]591<p>Please be aware when adding assert statements that not all compilers are aware of
592the semantics of the assert. In some places, asserts are used to indicate a piece of
593code that should not be reached. These are typically of the form:</p>
594
595<div class="doc_code">
596<pre>
Dan Gohmanf0032762008-11-24 17:18:39 +0000[diff] [blame]597assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]598</pre>
599</div>
600
601<p>When used in a function that returns a value, they should be followed with a return
602statement and a comment indicating that this line is never reached. This will prevent
603a compiler which is unable to deduce that the assert statement never returns from
604generating a warning.</p>
605
606<div class="doc_code">
607<pre>
Dan Gohmanf0032762008-11-24 17:18:39 +0000[diff] [blame]608assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyf4408e82008-05-31 23:54:55 +0000[diff] [blame]609// Not reached
610return 0;
611</pre>
612</div>
613
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]614</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]615
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]616<!-- _______________________________________________________________________ -->
617<div class="doc_subsubsection">
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]618 <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]619</div>
620
621<div class="doc_text">
622<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]623namespace with an "<tt>std::</tt>" prefix, rather than rely on
624"<tt>using namespace std;</tt>".</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]625
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]626<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
Misha Brukman2d33c542008-12-11 19:37:04 +0000[diff] [blame]627the namespace of any source file that <tt>#include</tt>s the header. This is
628clearly a bad thing.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]629
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]630<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
631rule, but is still important. Basically, using explicit namespace prefixes
632makes the code <b>clearer</b>, because it is immediately obvious what facilities
633are being used and where they are coming from, and <b>more portable</b>, because
634namespace clashes cannot occur between LLVM code and other namespaces. The
635portability rule is important because different standard library implementations
636expose different symbols (potentially ones they shouldn't), and future revisions
637to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
638such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]639
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]640<p>The exception to the general rule (i.e. it's not an exception for
641the <tt>std</tt> namespace) is for implementation files. For example, all of
642the code in the LLVM project implements code that lives in the 'llvm' namespace.
643As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
644namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
645general form of this rule is that any .cpp file that implements code in any
646namespace may use that namespace (and its parents'), but should not use any
647others.</p>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000[diff] [blame]648
649</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]650
651<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]652<div class="doc_subsubsection">
Bill Wendling4ad6d612006-12-09 01:35:43 +0000[diff] [blame]653 <a name="ll_virtual_anch">Provide a virtual method anchor for classes
654 in headers</a>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]655</div>
656
657<div class="doc_text">
658
659<p>If a class is defined in a header file and has a v-table (either it has
660virtual methods or it derives from classes with virtual methods), it must
661always have at least one out-of-line virtual method in the class. Without
Misha Brukman2d33c542008-12-11 19:37:04 +0000[diff] [blame]662this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
663that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
664increasing link times.</p>
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]665
666</div>
667
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]668<!-- _______________________________________________________________________ -->
669<div class="doc_subsubsection">
670 <a name="ll_end">Don't evaluate end() every time through a loop</a>
671</div>
672
673<div class="doc_text">
674
675<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
676with macros and may be coming in C++'0x) we end up writing a lot of loops that
677manually iterate from begin to end on a variety of containers or through other
678data structures. One common mistake is to write a loop in this style:</p>
679
680<div class="doc_code">
681<pre>
682 BasicBlock *BB = ...
683 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
684 ... use I ...
685</pre>
686</div>
687
688<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
689every time through the loop. Instead of writing the loop like this, we strongly
690prefer loops to be written so that they evaluate it once before the loop starts.
691A convenient way to do this is like so:</p>
692
693<div class="doc_code">
694<pre>
695 BasicBlock *BB = ...
696 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
697 ... use I ...
698</pre>
699</div>
700
701<p>The observant may quickly point out that these two loops may have different
702semantics: if the container (a basic block in this case) is being mutated, then
703"<tt>BB->end()</tt>" may change its value every time through the loop and the
704second loop may not in fact be correct. If you actually do depend on this
Chris Lattner09bc65e2009-06-30 06:20:03 +0000[diff] [blame]705behavior, please write the loop in the first form and add a comment indicating
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]706that you did it intentionally.</p>
707
708<p>Why do we prefer the second form (when correct)? Writing the loop in the
709first form has two problems: First it may be less efficient than evaluating it
710at the start of the loop. In this case, the cost is probably minor: a few extra
711loads every time through the loop. However, if the base expression is more
712complex, then the cost can rise quickly. I've seen loops where the end
713expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
Chris Lattnera44f87f2009-06-30 06:27:54 +0000[diff] [blame]714lookups really aren't cheap. By writing it in the second form consistently, you
Chris Lattner0af39ea2009-06-30 06:13:23 +0000[diff] [blame]715eliminate the issue entirely and don't even have to think about it.</p>
716
Chris Lattner09bc65e2009-06-30 06:20:03 +0000[diff] [blame]717<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]718hints to the reader that the loop is mutating the container (a fact that a
719comment would handily confirm!). If you write the loop in the second form, it
720is immediately obvious without even looking at the body of the loop that the
721container isn't being modified, which makes it easier to read the code and
722understand what it does.</p>
723
724<p>While the second form of the loop is a few extra keystrokes, we do strongly
725prefer it.</p>
726
727</div>
728
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]729
730<!-- _______________________________________________________________________ -->
731<div class="doc_subsubsection">
732 <a name="ll_preincrement">Prefer Preincrement</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]733</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]734
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]735<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]736
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]737<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
738postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
739preincrementation whenever possible.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]740
741<p>The semantics of postincrement include making a copy of the value being
Misha Brukman2d10d752003-07-28 21:57:18 +0000[diff] [blame]742incremented, returning it, and then preincrementing the "work value". For
743primitive types, this isn't a big deal... but for iterators, it can be a huge
744issue (for example, some iterators contains stack and set objects in them...
745copying an iterator could invoke the copy ctor's of these as well). In general,
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]746get in the habit of always using preincrement, and you won't have a problem.</p>
747
748</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]749
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]750<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]751<div class="doc_subsubsection">
Chris Lattner6883a882006-07-27 04:24:14 +0000[diff] [blame]752 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]753</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]754
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]755<div class="doc_text">
756
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]757<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
758to the output stream specified. In addition to doing this, however, it also
759flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]760
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]761<div class="doc_code">
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]762<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]763std::cout &lt;&lt; std::endl;
764std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]765</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000[diff] [blame]766</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]767
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]768<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]769it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]770
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]771</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000[diff] [blame]772
Bill Wendling7f564c02006-12-09 01:20:34 +0000[diff] [blame]773
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]774<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]775<div class="doc_section">
776 <a name="seealso">See Also</a>
777</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]778<!-- *********************************************************************** -->
779
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]780<div class="doc_text">
781
782<p>A lot of these comments and recommendations have been culled for other
783sources. Two particularly important books for our work are:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]784
785<ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]786
Chris Lattner788ee282007-11-09 21:49:08 +0000[diff] [blame]787<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
788C++</a> by Scott Meyers. Also
Chris Lattner999cf092004-05-23 21:05:07 +0000[diff] [blame]789interesting and useful are "More Effective C++" and "Effective STL" by the same
790author.</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]791
Chris Lattner788ee282007-11-09 21:49:08 +0000[diff] [blame]792<li>Large-Scale C++ Software Design by John Lakos</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]793
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]794</ol>
795
796<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]797something.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]798
799</div>
800
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000[diff] [blame]801<!-- *********************************************************************** -->
802
803<hr>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]804<address>
805 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]806 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]807 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]808 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]809
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]810 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]811 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]812 Last modified: $Date$
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000[diff] [blame]813</address>
Misha Brukman75f258e2003-10-24 17:57:33 +0000[diff] [blame]814
815</body>
816</html>