Gitiles
Code Review Sign In
gerrit-public.fairphone.software / toolchain / llvm-project / 56c10949999790d0d055b0545420ed6bb84d5b12 / . / llvm / docs / CodingStandards.html
blob: 84503ca67958bde03e42292f05ccbbc5358f2798 [file] [log] [blame]
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
3<html>
4<head>
5 <link rel="stylesheet" href="llvm.css" type="text/css">
Misha Brukman6c2522a2008-12-10 23:07:02 +0000[diff] [blame]6 <title>LLVM Coding Standards</title>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]7</head>
8<body>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]9
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]10<div class="doc_title">
Misha Brukman6c2522a2008-12-10 23:07:02 +0000[diff] [blame]11 LLVM Coding Standards
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]12</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]13
14<ol>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]15 <li><a href="#introduction">Introduction</a></li>
Chris Lattner85014f42001-07-23 20:40:41 +0000[diff] [blame]16 <li><a href="#mechanicalissues">Mechanical Source Issues</a>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]17 <ol>
18 <li><a href="#sourceformating">Source Code Formatting</a>
Misha Brukmanf196dbb2003-10-24 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 Brukman03f87d52004-10-26 16:18:43 +0000[diff] [blame]22 <li><a href="#scf_includes"><tt>#include</tt> Style</a></li>
Misha Brukmanf196dbb2003-10-24 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 Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]27 <li><a href="#compilerissues">Compiler Issues</a>
28 <ol>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]29 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like
30 Errors</a></li>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]31 <li><a href="#ci_portable_code">Write Portable Code</a></li>
Reid Spencerbf6439f2004-09-23 16:03:48 +0000[diff] [blame]32 <li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]33 </ol></li>
34 </ol></li>
Chris Lattnerac457c42001-07-09 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 Brukmanf196dbb2003-10-24 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 Wendling2b52dc12006-12-09 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 Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]46 </ol></li>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]47 <li><a href="#micro">The Low Level Issues</a>
48 <ol>
Chris Lattner85ea83e2006-07-27 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 Wendling2b52dc12006-12-09 01:20:34 +0000[diff] [blame]51 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
Bill Wendlingb0629482006-12-09 01:35:43 +0000[diff] [blame]52 classes in headers</a></li>
Chris Lattner56c10942009-06-30 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 Lattner85ea83e2006-07-27 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 Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]57 </ol></li>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]58 </ol></li>
59 <li><a href="#seealso">See Also</a></li>
60</ol>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]61
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]62<div class="doc_author">
Bill Wendling2b52dc12006-12-09 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 Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]65</div>
66
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]67
68<!-- *********************************************************************** -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]69<div class="doc_section">
70 <a name="introduction">Introduction</a>
71</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]72<!-- *********************************************************************** -->
73
Misha Brukmanf196dbb2003-10-24 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 Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]78absolute requirements to be followed in all instances, coding standards can be
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]79useful.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]80
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]81<p>This document intentionally does not prescribe fixed standards for religious
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]82issues such as brace placement and space usage. For issues like this, follow
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]83the golden rule:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]84
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]85<blockquote>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]86
Misha Brukmanf196dbb2003-10-24 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 Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]96maintainability of our common source base. If you have suggestions for topics to
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]97be included, please mail them to <a
98href="mailto:sabre@nondot.org">Chris</a>.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]99
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]100</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]101
102<!-- *********************************************************************** -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]103<div class="doc_section">
104 <a name="mechanicalissues">Mechanical Source Issues</a>
105</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]106<!-- *********************************************************************** -->
107
108<!-- ======================================================================= -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]109<div class="doc_subsection">
110 <a name="sourceformating">Source Code Formatting</a>
111</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]112
113<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]114<div class="doc_subsubsection">
115 <a name="scf_commenting">Commenting</a>
116</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]117
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]118<div class="doc_text">
119
120<p>Comments are one critical part of readability and maintainability. Everyone
Misha Brukman6d1686c2004-12-04 00:32:12 +0000[diff] [blame]121knows they should comment, so should you. Although we all should probably
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]122comment our code more than we do, there are a few very critical places that
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]123documentation is very useful:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]124
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]125<b>File Headers</b>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]126
Reid Spencerc7f87f22007-07-09 08:04:31 +0000[diff] [blame]127<p>Every source file should have a header on it that describes the basic
128purpose of the file. If a file does not have a header, it should not be
129checked into Subversion. Most source trees will probably have a standard
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]130file header format. The standard format for the LLVM source tree looks like
131this:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]132
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]133<div class="doc_code">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]134<pre>
Chris Lattnere6f4e072003-10-13 14:58:11 +0000[diff] [blame]135//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
Misha Brukman02805a62009-01-02 16:58:42 +0000[diff] [blame]136//
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]137// The LLVM Compiler Infrastructure
138//
Chris Lattnerebf56662007-12-29 19:56:08 +0000[diff] [blame]139// This file is distributed under the University of Illinois Open Source
140// License. See LICENSE.TXT for details.
Misha Brukman02805a62009-01-02 16:58:42 +0000[diff] [blame]141//
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]142//===----------------------------------------------------------------------===//
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]143//
144// This file contains the declaration of the Instruction class, which is the
145// base class for all of the VM instructions.
146//
147//===----------------------------------------------------------------------===//
148</pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]149</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]150
Chris Lattnerebf56662007-12-29 19:56:08 +0000[diff] [blame]151<p>A few things to note about this particular format: The "<tt>-*- C++
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]152-*-</tt>" string on the first line is there to tell Emacs that the source file
Misha Brukman0d640e82004-07-28 22:37:57 +0000[diff] [blame]153is a C++ file, not a C file (Emacs assumes .h files are C files by default).
154Note that this tag is not necessary in .cpp files. The name of the file is also
155on the first line, along with a very short description of the purpose of the
156file. This is important when printing out code and flipping though lots of
157pages.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]158
Chris Lattnerebf56662007-12-29 19:56:08 +0000[diff] [blame]159<p>The next section in the file is a concise note that defines the license
160that the file is released under. This makes it perfectly clear what terms the
161source code can be distributed under and should not be modified in any way.</p>
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]162
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]163<p>The main body of the description does not have to be very long in most cases.
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]164Here it's only two lines. If an algorithm is being implemented or something
165tricky is going on, a reference to the paper where it is published should be
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]166included, as well as any notes or "gotchas" in the code to watch out for.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]167
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]168<b>Class overviews</b>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]169
Jim Laskey88b5e792006-07-31 20:18:49 +0000[diff] [blame]170<p>Classes are one fundamental part of a good object oriented design. As such,
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]171a class definition should have a comment block that explains what the class is
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]172used for... if it's not obvious. If it's so completely obvious your grandma
173could figure it out, it's probably safe to leave it out. Naming classes
Misha Brukman6d1686c2004-12-04 00:32:12 +0000[diff] [blame]174something sane goes a long ways towards avoiding writing documentation.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]175
176
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]177<b>Method information</b>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]178
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]179<p>Methods defined in a class (as well as any global functions) should also be
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]180documented properly. A quick note about what it does any a description of the
181borderline behaviour is all that is necessary here (unless something
182particularly tricky or insideous is going on). The hope is that people can
183figure out how to use your interfaces without reading the code itself... that is
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]184the goal metric.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]185
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]186<p>Good things to talk about here are what happens when something unexpected
187happens: does the method return null? Abort? Format your hard disk?</p>
188
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]189</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]190
191<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]192<div class="doc_subsubsection">
193 <a name="scf_commentformat">Comment Formatting</a>
194</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]195
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]196<div class="doc_text">
197
198<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]199require less typing, don't have nesting problems, etc. There are a few cases
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]200when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]201
202<ol>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]203 <li>When writing a C code: Obviously if you are writing C code, use C style
Misha Brukman6d1686c2004-12-04 00:32:12 +0000[diff] [blame]204 comments.</li>
Misha Brukman03f87d52004-10-26 16:18:43 +0000[diff] [blame]205 <li>When writing a header file that may be <tt>#include</tt>d by a C source
206 file.</li>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]207 <li>When writing a source file that is used by a tool that only accepts C
208 style comments.</li>
209</ol>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]210
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]211<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
212These nest properly and are better behaved in general than C style comments.</p>
213
214</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]215
Chris Lattner245b5252003-08-07 21:45:47 +0000[diff] [blame]216<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]217<div class="doc_subsubsection">
Misha Brukman03f87d52004-10-26 16:18:43 +0000[diff] [blame]218 <a name="scf_includes"><tt>#include</tt> Style</a>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]219</div>
Chris Lattner245b5252003-08-07 21:45:47 +0000[diff] [blame]220
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]221<div class="doc_text">
222
223<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
Chris Lattner245b5252003-08-07 21:45:47 +0000[diff] [blame]224include guards if working on a header file), the <a
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]225href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
226file should be listed. We prefer these <tt>#include</tt>s to be listed in this
227order:</p>
Chris Lattner245b5252003-08-07 21:45:47 +0000[diff] [blame]228
229<ol>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]230 <li><a href="#mmheader">Main Module header</a></li>
231 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]232 <li><tt>llvm/*</tt></li>
233 <li><tt>llvm/Analysis/*</tt></li>
234 <li><tt>llvm/Assembly/*</tt></li>
235 <li><tt>llvm/Bytecode/*</tt></li>
236 <li><tt>llvm/CodeGen/*</tt></li>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]237 <li>...</li>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]238 <li><tt>Support/*</tt></li>
239 <li><tt>Config/*</tt></li>
240 <li>System <tt>#includes</tt></li>
Chris Lattner245b5252003-08-07 21:45:47 +0000[diff] [blame]241</ol>
242
Nick Lewycky35847802008-11-29 20:13:25 +0000[diff] [blame]243<p>... and each category should be sorted by name.</p>
Chris Lattner245b5252003-08-07 21:45:47 +0000[diff] [blame]244
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]245<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
Misha Brukman03f87d52004-10-26 16:18:43 +0000[diff] [blame]246which implement an interface defined by a .h file. This <tt>#include</tt>
247should always be included <b>first</b> regardless of where it lives on the file
248system. By including a header file first in the .cpp files that implement the
249interfaces, we ensure that the header does not have any hidden dependencies
250which are not explicitly #included in the header, but should be. It is also a
251form of documentation in the .cpp file to indicate where the interfaces it
252implements are defined.</p>
Chris Lattner245b5252003-08-07 21:45:47 +0000[diff] [blame]253
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]254</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]255
256<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]257<div class="doc_subsubsection">
258 <a name="scf_codewidth">Source Code Width</a>
259</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]260
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]261<div class="doc_text">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]262
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]263<p>Write your code to fit within 80 columns of text. This helps those of us who
264like to print out code and look at your code in an xterm without resizing
265it.</p>
266
Chris Lattnerfc12d2e2008-07-08 05:12:37 +0000[diff] [blame]267<p>The longer answer is that there must be some limit to the width of the code
268in order to reasonably allow developers to have multiple files side-by-side in
269windows on a modest display. If you are going to pick a width limit, it is
270somewhat arbitrary but you might as well pick something standard. Going with
27190 columns (for example) instead of 80 columns wouldn't add any significant
272value and would be detrimental to printing out code. Also many other projects
273have standardized on 80 columns, so some people have already configured their
274editors for it (vs something else, like 90 columns).</p>
275
276<p>This is one of many contentious issues in coding standards, but is not up
277for debate.</p>
278
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]279</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]280
281<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]282<div class="doc_subsubsection">
283 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
284</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]285
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]286<div class="doc_text">
287
288<p>In all cases, prefer spaces to tabs in source files. People have different
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]289prefered indentation levels, and different styles of indentation that they
290like... this is fine. What isn't is that different editors/viewers expand tabs
291out to different tab stops. This can cause your code to look completely
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]292unreadable, and it is not worth dealing with.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]293
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]294<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]295style of existing code if your are modifying and extending it. If you like four
296spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
297with two spaces of indentation. Also, do not reindent a whole source file: it
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]298makes for incredible diffs that are absolutely worthless.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]299
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]300</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]301
302<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]303<div class="doc_subsubsection">
304 <a name="scf_indentation">Indent Code Consistently</a>
305</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]306
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]307<div class="doc_text">
308
309<p>Okay, your first year of programming you were told that indentation is
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]310important. If you didn't believe and internalize this then, now is the time.
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]311Just do it.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]312
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]313</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]314
315
316<!-- ======================================================================= -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]317<div class="doc_subsection">
318 <a name="compilerissues">Compiler Issues</a>
319</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]320
321
322<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]323<div class="doc_subsubsection">
324 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
325</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]326
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]327<div class="doc_text">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]328
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]329<p>If your code has compiler warnings in it, something is wrong: you aren't
330casting values correctly, your have "questionable" constructs in your code, or
331you are doing something legitimately wrong. Compiler warnings can cover up
332legitimate errors in output and make dealing with a translation unit
333difficult.</p>
334
335<p>It is not possible to prevent all warnings from all compilers, nor is it
Misha Brukman0cedb1f2003-10-06 19:26:00 +0000[diff] [blame]336desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
337a good thorough set of warnings, and stick to them. At least in the case of
338<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
339syntax of the code slightly. For example, an warning that annoys me occurs when
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]340I write code like this:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]341
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]342<div class="doc_code">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]343<pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]344if (V = getValue()) {
345 ...
346}
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]347</pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]348</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]349
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]350<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
351operator, and that I probably mistyped it. In most cases, I haven't, and I
352really don't want the spurious errors. To fix this particular problem, I
353rewrite the code like this:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]354
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]355<div class="doc_code">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]356<pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]357if ((V = getValue())) {
358 ...
359}
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]360</pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]361</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]362
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]363<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
364be fixed by massaging the code appropriately.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]365
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]366<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
367-Winline -W -Wwrite-strings -Wno-unused</tt></p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]368
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]369</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]370
371<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]372<div class="doc_subsubsection">
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]373 <a name="ci_portable_code">Write Portable Code</a>
374</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]375
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]376<div class="doc_text">
377
378<p>In almost all cases, it is possible and within reason to write completely
Misha Brukmanc3e78932003-07-28 21:57:18 +0000[diff] [blame]379portable code. If there are cases where it isn't possible to write portable
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]380code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]381
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]382<p>In practice, this means that you shouldn't assume much about the host
383compiler, including its support for "high tech" features like partial
Chris Lattner30bc9722009-03-23 04:52:53 +0000[diff] [blame]384specialization of templates. If these features are used, they should only be
385an implementation detail of a library which has a simple exposed API.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]386
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]387</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]388
Reid Spencerbf6439f2004-09-23 16:03:48 +0000[diff] [blame]389<!-- _______________________________________________________________________ -->
390<div class="doc_subsubsection">
391<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
392</div>
393<div class="doc_text">
Misha Brukmanf2499132004-10-26 15:45:13 +0000[diff] [blame]394
395<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
396interchangeably. The only difference is when they are used to declare a class:
397<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
398all members public by default.</p>
399
400<p>Unfortunately, not all compilers follow the rules and some will generate
401different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
402declare the symbol. This can lead to problems at link time.</p>
403
404<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
405<b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
406
Reid Spencerbf6439f2004-09-23 16:03:48 +0000[diff] [blame]407</div>
408
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]409<!-- *********************************************************************** -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]410<div class="doc_section">
411 <a name="styleissues">Style Issues</a>
412</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]413<!-- *********************************************************************** -->
414
415
416<!-- ======================================================================= -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]417<div class="doc_subsection">
418 <a name="macro">The High Level Issues</a>
419</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]420
421
422<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]423<div class="doc_subsubsection">
424 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
425</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]426
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]427<div class="doc_text">
428
429<p>C++ doesn't do too well in the modularity department. There is no real
Misha Brukmanc3e78932003-07-28 21:57:18 +0000[diff] [blame]430encapsulation or data hiding (unless you use expensive protocol classes), but it
431is what we have to work with. When you write a public header file (in the LLVM
432source tree, they live in the top level "include" directory), you are defining a
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]433module of functionality.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]434
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]435<p>Ideally, modules should be completely independent of each other, and their
Misha Brukmanc3e78932003-07-28 21:57:18 +0000[diff] [blame]436header files should only include the absolute minimum number of headers
437possible. A module is not just a class, a function, or a namespace: <a
438href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
439of these</a> that defines an interface. This interface may be several
440functions, classes or data structures, but the important issue is how they work
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]441together.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]442
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]443<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
444files. Each of these <tt>.cpp</tt> files should include the header that defines
445their interface first. This ensure that all of the dependences of the module
446header have been properly added to the module header itself, and are not
447implicit. System headers should be included after user headers for a
448translation unit.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]449
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]450</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]451
452<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]453<div class="doc_subsubsection">
Misha Brukman03f87d52004-10-26 16:18:43 +0000[diff] [blame]454 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]455</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]456
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]457<div class="doc_text">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]458
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]459<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
460have to, especially in header files.</p>
461
462<p>But wait, sometimes you need to have the definition of a class to use it, or
Misha Brukman03f87d52004-10-26 16:18:43 +0000[diff] [blame]463to inherit from it. In these cases go ahead and <tt>#include</tt> that header
464file. Be aware however that there are many cases where you don't need to have
465the full definition of a class. If you are using a pointer or reference to a
466class, you don't need the header file. If you are simply returning a class
467instance from a prototyped function or method, you don't need it. In fact, for
468most cases, you simply don't need the definition of a class... and not
469<tt>#include</tt>'ing speeds up compilation.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]470
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]471<p>It is easy to try to go too overboard on this recommendation, however. You
Chris Lattner756f83f2007-02-10 18:35:31 +0000[diff] [blame]472<b>must</b> include all of the header files that you are using -- you can
473include them either directly
Misha Brukmanc3e78932003-07-28 21:57:18 +0000[diff] [blame]474or indirectly (through another header file). To make sure that you don't
475accidently forget to include a header file in your module header, make sure to
476include your module header <b>first</b> in the implementation file (as mentioned
477above). This way there won't be any hidden dependencies that you'll find out
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]478about later...</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]479
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]480</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]481
482<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]483<div class="doc_subsubsection">
484 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
485</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]486
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]487<div class="doc_text">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]488
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]489<p>Many modules have a complex implementation that causes them to use more than
490one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
491internal communication interface (helper classes, extra functions, etc) in the
Misha Brukman6d1686c2004-12-04 00:32:12 +0000[diff] [blame]492public module header file. Don't do this.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]493
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]494<p>If you really need to do something like this, put a private header file in
495the same directory as the source files, and include it locally. This ensures
496that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]497
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]498<p>Note however, that it's okay to put extra implementation methods a public
499class itself... just make them private (or protected), and all is well.</p>
500
501</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]502
Bill Wendling2b52dc12006-12-09 01:20:34 +0000[diff] [blame]503<!-- _______________________________________________________________________ -->
504<div class="doc_subsubsection">
505 <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is forbidden</a>
506</div>
507
508<div class="doc_text">
509
510<p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
511hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
512support clients using LLVM libraries as part of larger systems. In particular,
513we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
514the static c'tors are run whenever an application start up that uses the dynamic
515library. There are two problems with this:</p>
516
517<ol>
518 <li>The time to run the static c'tors impacts startup time of
Bill Wendling56235a22007-11-06 09:36:34 +0000[diff] [blame]519 applications&mdash;a critical time for GUI apps.</li>
Bill Wendling2b52dc12006-12-09 01:20:34 +0000[diff] [blame]520 <li>The static c'tors cause the app to pull many extra pages of memory off the
Bill Wendling56235a22007-11-06 09:36:34 +0000[diff] [blame]521 disk: both the code for the static c'tors in each <tt>.o</tt> file and the
522 small amount of data that gets touched. In addition, touched/dirty pages
523 put more pressure on the VM system on low-memory machines.</li>
Bill Wendling2b52dc12006-12-09 01:20:34 +0000[diff] [blame]524</ol>
525
Matthijs Kooijmanca4ffaa2008-07-30 12:14:10 +0000[diff] [blame]526<p>Note that using the other stream headers (<tt>&lt;sstream&gt;</tt> for
527example) is allowed normally, it is just <tt>&lt;iostream&gt;</tt> that is
528causing problems.</p>
529
Chris Lattner30bc9722009-03-23 04:52:53 +0000[diff] [blame]530<p>The preferred replacement for stream functionality is the
Chris Lattner57be3152009-03-23 04:53:34 +0000[diff] [blame]531<tt>llvm::raw_ostream</tt> class (for writing to output streams of various
532sorts) and the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
Bill Wendling2b52dc12006-12-09 01:20:34 +0000[diff] [blame]533
534</div>
535
536
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]537<!-- ======================================================================= -->
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]538<div class="doc_subsection">
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]539 <a name="micro">The Low Level Issues</a>
540</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]541
542
543<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]544<div class="doc_subsubsection">
Chris Lattner85ea83e2006-07-27 04:24:14 +0000[diff] [blame]545 <a name="ll_assert">Assert Liberally</a>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]546</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]547
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]548<div class="doc_text">
549
550<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
Misha Brukmanc3e78932003-07-28 21:57:18 +0000[diff] [blame]551preconditions and assumptions, you never know when a bug (not neccesarily even
552yours) might be caught early by an assertion, which reduces debugging time
553dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already
554included by the header files you are using, so it doesn't cost anything to use
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]555it.</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]556
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]557<p>To further assist with debugging, make sure to put some kind of error message
558in the assertion statement (which is printed if the assertion is tripped). This
Misha Brukmanc3e78932003-07-28 21:57:18 +0000[diff] [blame]559helps the poor debugging make sense of why an assertion is being made and
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]560enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]561
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]562<div class="doc_code">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]563<pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]564inline Value *getOperand(unsigned i) {
565 assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
566 return Operands[i];
567}
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]568</pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]569</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]570
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]571<p>Here are some examples:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]572
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]573<div class="doc_code">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]574<pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]575assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]576
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]577assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]578
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]579assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]580
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]581assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]582
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]583assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]584</pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]585</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]586
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]587<p>You get the idea...</p>
588
Nick Lewyckyfb75d422008-05-31 23:54:55 +0000[diff] [blame]589<p>Please be aware when adding assert statements that not all compilers are aware of
590the semantics of the assert. In some places, asserts are used to indicate a piece of
591code that should not be reached. These are typically of the form:</p>
592
593<div class="doc_code">
594<pre>
Dan Gohman8ef44982008-11-24 17:18:39 +0000[diff] [blame]595assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyfb75d422008-05-31 23:54:55 +0000[diff] [blame]596</pre>
597</div>
598
599<p>When used in a function that returns a value, they should be followed with a return
600statement and a comment indicating that this line is never reached. This will prevent
601a compiler which is unable to deduce that the assert statement never returns from
602generating a warning.</p>
603
604<div class="doc_code">
605<pre>
Dan Gohman8ef44982008-11-24 17:18:39 +0000[diff] [blame]606assert(0 &amp;&amp; "Some helpful error message");
Nick Lewyckyfb75d422008-05-31 23:54:55 +0000[diff] [blame]607// Not reached
608return 0;
609</pre>
610</div>
611
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]612</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]613
Chris Lattner01e81e62006-01-01 21:59:22 +0000[diff] [blame]614<!-- _______________________________________________________________________ -->
615<div class="doc_subsubsection">
Bill Wendlingb0629482006-12-09 01:35:43 +0000[diff] [blame]616 <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
Chris Lattner01e81e62006-01-01 21:59:22 +0000[diff] [blame]617</div>
618
619<div class="doc_text">
620<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
Bill Wendlingb0629482006-12-09 01:35:43 +0000[diff] [blame]621namespace with an "<tt>std::</tt>" prefix, rather than rely on
622"<tt>using namespace std;</tt>".</p>
Chris Lattner01e81e62006-01-01 21:59:22 +0000[diff] [blame]623
Bill Wendlingb0629482006-12-09 01:35:43 +0000[diff] [blame]624<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
Misha Brukman586a15c2008-12-11 19:37:04 +0000[diff] [blame]625the namespace of any source file that <tt>#include</tt>s the header. This is
626clearly a bad thing.</p>
Chris Lattner01e81e62006-01-01 21:59:22 +0000[diff] [blame]627
Bill Wendlingb0629482006-12-09 01:35:43 +0000[diff] [blame]628<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
629rule, but is still important. Basically, using explicit namespace prefixes
630makes the code <b>clearer</b>, because it is immediately obvious what facilities
631are being used and where they are coming from, and <b>more portable</b>, because
632namespace clashes cannot occur between LLVM code and other namespaces. The
633portability rule is important because different standard library implementations
634expose different symbols (potentially ones they shouldn't), and future revisions
635to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
636such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
Chris Lattner01e81e62006-01-01 21:59:22 +0000[diff] [blame]637
Bill Wendlingb0629482006-12-09 01:35:43 +0000[diff] [blame]638<p>The exception to the general rule (i.e. it's not an exception for
639the <tt>std</tt> namespace) is for implementation files. For example, all of
640the code in the LLVM project implements code that lives in the 'llvm' namespace.
641As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
642namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
643general form of this rule is that any .cpp file that implements code in any
644namespace may use that namespace (and its parents'), but should not use any
645others.</p>
Chris Lattner01e81e62006-01-01 21:59:22 +0000[diff] [blame]646
647</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]648
649<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]650<div class="doc_subsubsection">
Bill Wendlingb0629482006-12-09 01:35:43 +0000[diff] [blame]651 <a name="ll_virtual_anch">Provide a virtual method anchor for classes
652 in headers</a>
Chris Lattner85ea83e2006-07-27 04:24:14 +0000[diff] [blame]653</div>
654
655<div class="doc_text">
656
657<p>If a class is defined in a header file and has a v-table (either it has
658virtual methods or it derives from classes with virtual methods), it must
659always have at least one out-of-line virtual method in the class. Without
Misha Brukman586a15c2008-12-11 19:37:04 +0000[diff] [blame]660this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
661that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
662increasing link times.</p>
Chris Lattner85ea83e2006-07-27 04:24:14 +0000[diff] [blame]663
664</div>
665
Chris Lattner56c10942009-06-30 06:13:23 +0000[diff] [blame^]666<!-- _______________________________________________________________________ -->
667<div class="doc_subsubsection">
668 <a name="ll_end">Don't evaluate end() every time through a loop</a>
669</div>
670
671<div class="doc_text">
672
673<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
674with macros and may be coming in C++'0x) we end up writing a lot of loops that
675manually iterate from begin to end on a variety of containers or through other
676data structures. One common mistake is to write a loop in this style:</p>
677
678<div class="doc_code">
679<pre>
680 BasicBlock *BB = ...
681 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
682 ... use I ...
683</pre>
684</div>
685
686<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
687every time through the loop. Instead of writing the loop like this, we strongly
688prefer loops to be written so that they evaluate it once before the loop starts.
689A convenient way to do this is like so:</p>
690
691<div class="doc_code">
692<pre>
693 BasicBlock *BB = ...
694 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
695 ... use I ...
696</pre>
697</div>
698
699<p>The observant may quickly point out that these two loops may have different
700semantics: if the container (a basic block in this case) is being mutated, then
701"<tt>BB->end()</tt>" may change its value every time through the loop and the
702second loop may not in fact be correct. If you actually do depend on this
703behavior, please write the loop in the second form and add a comment indicating
704that you did it intentionally.</p>
705
706<p>Why do we prefer the second form (when correct)? Writing the loop in the
707first form has two problems: First it may be less efficient than evaluating it
708at the start of the loop. In this case, the cost is probably minor: a few extra
709loads every time through the loop. However, if the base expression is more
710complex, then the cost can rise quickly. I've seen loops where the end
711expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
712lookups really aren't cheap. By writing it in the first form consistently, you
713eliminate the issue entirely and don't even have to think about it.</p>
714
715<p>The second (even bigger) issue is that writing the loop in the second form
716hints to the reader that the loop is mutating the container (a fact that a
717comment would handily confirm!). If you write the loop in the second form, it
718is immediately obvious without even looking at the body of the loop that the
719container isn't being modified, which makes it easier to read the code and
720understand what it does.</p>
721
722<p>While the second form of the loop is a few extra keystrokes, we do strongly
723prefer it.</p>
724
725</div>
726
Chris Lattner85ea83e2006-07-27 04:24:14 +0000[diff] [blame]727
728<!-- _______________________________________________________________________ -->
729<div class="doc_subsubsection">
730 <a name="ll_preincrement">Prefer Preincrement</a>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]731</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]732
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]733<div class="doc_text">
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]734
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]735<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
736postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
737preincrementation whenever possible.</p>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]738
739<p>The semantics of postincrement include making a copy of the value being
Misha Brukmanc3e78932003-07-28 21:57:18 +0000[diff] [blame]740incremented, returning it, and then preincrementing the "work value". For
741primitive types, this isn't a big deal... but for iterators, it can be a huge
742issue (for example, some iterators contains stack and set objects in them...
743copying an iterator could invoke the copy ctor's of these as well). In general,
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]744get in the habit of always using preincrement, and you won't have a problem.</p>
745
746</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]747
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]748<!-- _______________________________________________________________________ -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]749<div class="doc_subsubsection">
Chris Lattner85ea83e2006-07-27 04:24:14 +0000[diff] [blame]750 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]751</div>
Chris Lattner850d4f62002-01-20 19:01:26 +0000[diff] [blame]752
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]753<div class="doc_text">
754
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]755<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
756to the output stream specified. In addition to doing this, however, it also
757flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner850d4f62002-01-20 19:01:26 +0000[diff] [blame]758
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]759<div class="doc_code">
Chris Lattner850d4f62002-01-20 19:01:26 +0000[diff] [blame]760<pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]761std::cout &lt;&lt; std::endl;
762std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
Chris Lattner850d4f62002-01-20 19:01:26 +0000[diff] [blame]763</pre>
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]764</div>
Chris Lattner850d4f62002-01-20 19:01:26 +0000[diff] [blame]765
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]766<p>Most of the time, you probably have no reason to flush the output stream, so
Misha Brukmanb2246152004-07-28 22:31:54 +0000[diff] [blame]767it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner850d4f62002-01-20 19:01:26 +0000[diff] [blame]768
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]769</div>
Chris Lattner850d4f62002-01-20 19:01:26 +0000[diff] [blame]770
Bill Wendling2b52dc12006-12-09 01:20:34 +0000[diff] [blame]771
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]772<!-- *********************************************************************** -->
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]773<div class="doc_section">
774 <a name="seealso">See Also</a>
775</div>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]776<!-- *********************************************************************** -->
777
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]778<div class="doc_text">
779
780<p>A lot of these comments and recommendations have been culled for other
781sources. Two particularly important books for our work are:</p>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]782
783<ol>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]784
Chris Lattnerf9f08bf2007-11-09 21:49:08 +0000[diff] [blame]785<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
786C++</a> by Scott Meyers. Also
Chris Lattner1dab1922004-05-23 21:05:07 +0000[diff] [blame]787interesting and useful are "More Effective C++" and "Effective STL" by the same
788author.</li>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]789
Chris Lattnerf9f08bf2007-11-09 21:49:08 +0000[diff] [blame]790<li>Large-Scale C++ Software Design by John Lakos</li>
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]791
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]792</ol>
793
794<p>If you get some free time, and you haven't read them: do so, you might learn
Misha Brukman6d1686c2004-12-04 00:32:12 +0000[diff] [blame]795something.</p>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]796
797</div>
798
Chris Lattnerac457c42001-07-09 03:27:08 +0000[diff] [blame]799<!-- *********************************************************************** -->
800
801<hr>
Misha Brukmand4f290a2004-05-12 18:37:22 +0000[diff] [blame]802<address>
803 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman86242e12008-12-11 17:34:48 +0000[diff] [blame]804 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukmand4f290a2004-05-12 18:37:22 +0000[diff] [blame]805 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman86242e12008-12-11 17:34:48 +0000[diff] [blame]806 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]807
Misha Brukmand4f290a2004-05-12 18:37:22 +0000[diff] [blame]808 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencerca058542006-03-14 05:39:39 +0000[diff] [blame]809 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]810 Last modified: $Date$
Misha Brukmand4f290a2004-05-12 18:37:22 +0000[diff] [blame]811</address>
Misha Brukmanf196dbb2003-10-24 17:57:33 +0000[diff] [blame]812
813</body>
814</html>