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Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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Misha Brukman	6c2522a	2008-12-10 23:07:02 +0000	[diff] [blame]	6	<title>LLVM Coding Standards</title>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	7	</head>
				8	<body>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	9
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	10	<div class="doc_title">
Misha Brukman	6c2522a	2008-12-10 23:07:02 +0000	[diff] [blame]	11	LLVM Coding Standards
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	12	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	13
				14	<ol>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	15	<li><a href="#introduction">Introduction</a></li>
Chris Lattner	85014f4	2001-07-23 20:40:41 +0000	[diff] [blame]	16	<li><a href="#mechanicalissues">Mechanical Source Issues</a>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	17	<ol>
				18	<li><a href="#sourceformating">Source Code Formatting</a>
Misha Brukman	f196dbb	2003-10-24 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 Brukman	03f87d5	2004-10-26 16:18:43 +0000	[diff] [blame]	22	<li><a href="#scf_includes"><tt>#include</tt> Style</a></li>
Misha Brukman	f196dbb	2003-10-24 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 Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	27	<li><a href="#compilerissues">Compiler Issues</a>
				28	<ol>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	29	<li><a href="#ci_warningerrors">Treat Compiler Warnings Like
				30	Errors</a></li>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	31	<li><a href="#ci_portable_code">Write Portable Code</a></li>
Reid Spencer	bf6439f	2004-09-23 16:03:48 +0000	[diff] [blame]	32	<li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	33	</ol></li>
				34	</ol></li>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	35	<li><a href="#styleissues">Style Issues</a>
				36	<ol>
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	37	<li><a href="#macro">The High-Level Issues</a>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	38	<ol>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	39	<li><a href="#hl_module">A Public Header File <b>is</b> a
				40	Module</a></li>
				41	<li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
				42	<li><a href="#hl_privateheaders">Keep "internal" Headers
				43	Private</a></li>
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	44	<li><a href="#hl_earlyexit">Use Early Exits and 'continue' to Simplify
				45	Code</a></li>
Chris Lattner	14abb83	2009-07-28 22:54:04 +0000	[diff] [blame]	46	<li><a href="#hl_else_after_return">Don't use "else" after a
				47	return</a></li>
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	48	<li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
				49	Functions</a></li>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	50	</ol></li>
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	51	<li><a href="#micro">The Low-Level Issues</a>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	52	<ol>
Chris Lattner	85ea83e	2006-07-27 04:24:14 +0000	[diff] [blame]	53	<li><a href="#ll_assert">Assert Liberally</a></li>
				54	<li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
Bill Wendling	2b52dc1	2006-12-09 01:20:34 +0000	[diff] [blame]	55	<li><a href="#ll_virtual_anch">Provide a virtual method anchor for
Bill Wendling	b062948	2006-12-09 01:35:43 +0000	[diff] [blame]	56	classes in headers</a></li>
Chris Lattner	56c1094	2009-06-30 06:13:23 +0000	[diff] [blame]	57	<li><a href="#ll_end">Don't evaluate end() every time through a
				58	loop</a></li>
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	59	<li><a href="#ll_iostream"><tt>#include <iostream></tt> is
				60	<em>forbidden</em></a></li>
Daniel Dunbar	fcd54f7	2009-07-24 23:04:51 +0000	[diff] [blame]	61	<li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li>
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	62	<li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	63	</ol></li>
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	64
				65	<li><a href="#nano">Microscopic Details</a>
				66	<ol>
				67	<li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
				68	<li><a href="#micro_preincrement">Prefer Preincrement</a></li>
				69	<li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
				70	<li><a href="#micro_anonns">Anonymous Namespaces</a></li>
				71	</ol></li>
				72
				73
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	74	</ol></li>
				75	<li><a href="#seealso">See Also</a></li>
				76	</ol>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	77
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	78	<div class="doc_author">
Chris Lattner	9567fff	2009-07-22 05:43:01 +0000	[diff] [blame]	79	<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	80	</div>
				81
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	82
				83	<!-- *********************************************************************** -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	84	<div class="doc_section">
				85	<a name="introduction">Introduction</a>
				86	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	87	<!-- *********************************************************************** -->
				88
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	89	<div class="doc_text">
				90
				91	<p>This document attempts to describe a few coding standards that are being used
				92	in the LLVM source tree. Although no coding standards should be regarded as
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	93	absolute requirements to be followed in all instances, coding standards can be
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	94	useful.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	95
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	96	<p>This document intentionally does not prescribe fixed standards for religious
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	97	issues such as brace placement and space usage. For issues like this, follow
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	98	the golden rule:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	99
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	100	<blockquote>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	101
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	102	<p><b><a name="goldenrule">If you are adding a significant body of source to a
				103	project, feel free to use whatever style you are most comfortable with. If you
				104	are extending, enhancing, or bug fixing already implemented code, use the style
				105	that is already being used so that the source is uniform and easy to
				106	follow.</a></b></p>
				107
				108	</blockquote>
				109
				110	<p>The ultimate goal of these guidelines is the increase readability and
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	111	maintainability of our common source base. If you have suggestions for topics to
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	112	be included, please mail them to <a
				113	href="mailto:sabre@nondot.org">Chris</a>.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	114
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	115	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	116
				117	<!-- *********************************************************************** -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	118	<div class="doc_section">
				119	<a name="mechanicalissues">Mechanical Source Issues</a>
				120	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	121	<!-- *********************************************************************** -->
				122
				123	<!-- ======================================================================= -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	124	<div class="doc_subsection">
				125	<a name="sourceformating">Source Code Formatting</a>
				126	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	127
				128	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	129	<div class="doc_subsubsection">
				130	<a name="scf_commenting">Commenting</a>
				131	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	132
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	133	<div class="doc_text">
				134
				135	<p>Comments are one critical part of readability and maintainability. Everyone
Chris Lattner	bbe228b	2009-07-12 00:10:24 +0000	[diff] [blame]	136	knows they should comment, so should you. When writing comments, write them as
				137	English prose, which means they should use proper capitalization, punctuation,
				138	etc. Although we all should probably
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	139	comment our code more than we do, there are a few very critical places that
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	140	documentation is very useful:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	141
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	142	<b>File Headers</b>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	143
Reid Spencer	c7f87f2	2007-07-09 08:04:31 +0000	[diff] [blame]	144	<p>Every source file should have a header on it that describes the basic
				145	purpose of the file. If a file does not have a header, it should not be
				146	checked into Subversion. Most source trees will probably have a standard
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	147	file header format. The standard format for the LLVM source tree looks like
				148	this:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	149
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	150	<div class="doc_code">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	151	<pre>
Chris Lattner	e6f4e07	2003-10-13 14:58:11 +0000	[diff] [blame]	152	//===-- llvm/Instruction.h - Instruction class definition -------- C++ --===//
Misha Brukman	02805a6	2009-01-02 16:58:42 +0000	[diff] [blame]	153	//
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	154	// The LLVM Compiler Infrastructure
				155	//
Chris Lattner	ebf5666	2007-12-29 19:56:08 +0000	[diff] [blame]	156	// This file is distributed under the University of Illinois Open Source
				157	// License. See LICENSE.TXT for details.
Misha Brukman	02805a6	2009-01-02 16:58:42 +0000	[diff] [blame]	158	//
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	159	//===----------------------------------------------------------------------===//
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	160	//
				161	// This file contains the declaration of the Instruction class, which is the
				162	// base class for all of the VM instructions.
				163	//
				164	//===----------------------------------------------------------------------===//
				165	</pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	166	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	167
Chris Lattner	ebf5666	2007-12-29 19:56:08 +0000	[diff] [blame]	168	<p>A few things to note about this particular format: The "<tt>-*- C++
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	169	-*-</tt>" string on the first line is there to tell Emacs that the source file
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	170	is a C++ file, not a C file (Emacs assumes <tt>.h</tt> files are C files by default).
				171	Note that this tag is not necessary in <tt>.cpp</tt> files. The name of the file is also
Misha Brukman	0d640e8	2004-07-28 22:37:57 +0000	[diff] [blame]	172	on the first line, along with a very short description of the purpose of the
				173	file. This is important when printing out code and flipping though lots of
				174	pages.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	175
Chris Lattner	ebf5666	2007-12-29 19:56:08 +0000	[diff] [blame]	176	<p>The next section in the file is a concise note that defines the license
				177	that the file is released under. This makes it perfectly clear what terms the
				178	source code can be distributed under and should not be modified in any way.</p>
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	179
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	180	<p>The main body of the description does not have to be very long in most cases.
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	181	Here it's only two lines. If an algorithm is being implemented or something
				182	tricky is going on, a reference to the paper where it is published should be
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	183	included, as well as any notes or "gotchas" in the code to watch out for.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	184
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	185	<b>Class overviews</b>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	186
Jim Laskey	88b5e79	2006-07-31 20:18:49 +0000	[diff] [blame]	187	<p>Classes are one fundamental part of a good object oriented design. As such,
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	188	a class definition should have a comment block that explains what the class is
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	189	used for... if it's not obvious. If it's so completely obvious your grandma
				190	could figure it out, it's probably safe to leave it out. Naming classes
Misha Brukman	6d1686c	2004-12-04 00:32:12 +0000	[diff] [blame]	191	something sane goes a long ways towards avoiding writing documentation.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	192
				193
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	194	<b>Method information</b>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	195
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	196	<p>Methods defined in a class (as well as any global functions) should also be
Dan Gohman	42f5691	2010-02-26 20:18:32 +0000	[diff] [blame]	197	documented properly. A quick note about what it does and a description of the
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	198	borderline behaviour is all that is necessary here (unless something
Dan Gohman	42f5691	2010-02-26 20:18:32 +0000	[diff] [blame]	199	particularly tricky or insidious is going on). The hope is that people can
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	200	figure out how to use your interfaces without reading the code itself... that is
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	201	the goal metric.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	202
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	203	<p>Good things to talk about here are what happens when something unexpected
				204	happens: does the method return null? Abort? Format your hard disk?</p>
				205
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	206	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	207
				208	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	209	<div class="doc_subsubsection">
				210	<a name="scf_commentformat">Comment Formatting</a>
				211	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	212
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	213	<div class="doc_text">
				214
				215	<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	216	require less typing, don't have nesting problems, etc. There are a few cases
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	217	when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	218
				219	<ol>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	220	<li>When writing a C code: Obviously if you are writing C code, use C style
Misha Brukman	6d1686c	2004-12-04 00:32:12 +0000	[diff] [blame]	221	comments.</li>
Misha Brukman	03f87d5	2004-10-26 16:18:43 +0000	[diff] [blame]	222	<li>When writing a header file that may be <tt>#include</tt>d by a C source
				223	file.</li>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	224	<li>When writing a source file that is used by a tool that only accepts C
				225	style comments.</li>
				226	</ol>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	227
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	228	<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
				229	These nest properly and are better behaved in general than C style comments.</p>
				230
				231	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	232
Chris Lattner	245b525	2003-08-07 21:45:47 +0000	[diff] [blame]	233	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	234	<div class="doc_subsubsection">
Misha Brukman	03f87d5	2004-10-26 16:18:43 +0000	[diff] [blame]	235	<a name="scf_includes"><tt>#include</tt> Style</a>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	236	</div>
Chris Lattner	245b525	2003-08-07 21:45:47 +0000	[diff] [blame]	237
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	238	<div class="doc_text">
				239
				240	<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
Chris Lattner	245b525	2003-08-07 21:45:47 +0000	[diff] [blame]	241	include guards if working on a header file), the <a
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	242	href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
				243	file should be listed. We prefer these <tt>#include</tt>s to be listed in this
				244	order:</p>
Chris Lattner	245b525	2003-08-07 21:45:47 +0000	[diff] [blame]	245
				246	<ol>
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	247	<li><a href="#mmheader">Main Module Header</a></li>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	248	<li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	249	<li><tt>llvm/*</tt></li>
				250	<li><tt>llvm/Analysis/*</tt></li>
				251	<li><tt>llvm/Assembly/*</tt></li>
Duncan Sands	e9da6db	2010-09-29 20:09:55 +0000	[diff] [blame]	252	<li><tt>llvm/Bitcode/*</tt></li>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	253	<li><tt>llvm/CodeGen/*</tt></li>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	254	<li>...</li>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	255	<li><tt>Support/*</tt></li>
				256	<li><tt>Config/*</tt></li>
				257	<li>System <tt>#includes</tt></li>
Chris Lattner	245b525	2003-08-07 21:45:47 +0000	[diff] [blame]	258	</ol>
				259
Nick Lewycky	3584780	2008-11-29 20:13:25 +0000	[diff] [blame]	260	<p>... and each category should be sorted by name.</p>
Chris Lattner	245b525	2003-08-07 21:45:47 +0000	[diff] [blame]	261
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	262	<p><a name="mmheader">The "Main Module Header"</a> file applies to <tt>.cpp</tt> files
				263	which implement an interface defined by a <tt>.h</tt> file. This <tt>#include</tt>
Misha Brukman	03f87d5	2004-10-26 16:18:43 +0000	[diff] [blame]	264	should always be included <b>first</b> regardless of where it lives on the file
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	265	system. By including a header file first in the <tt>.cpp</tt> files that implement the
Misha Brukman	03f87d5	2004-10-26 16:18:43 +0000	[diff] [blame]	266	interfaces, we ensure that the header does not have any hidden dependencies
				267	which are not explicitly #included in the header, but should be. It is also a
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	268	form of documentation in the <tt>.cpp</tt> file to indicate where the interfaces it
Misha Brukman	03f87d5	2004-10-26 16:18:43 +0000	[diff] [blame]	269	implements are defined.</p>
Chris Lattner	245b525	2003-08-07 21:45:47 +0000	[diff] [blame]	270
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	271	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	272
				273	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	274	<div class="doc_subsubsection">
				275	<a name="scf_codewidth">Source Code Width</a>
				276	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	277
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	278	<div class="doc_text">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	279
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	280	<p>Write your code to fit within 80 columns of text. This helps those of us who
				281	like to print out code and look at your code in an xterm without resizing
				282	it.</p>
				283
Chris Lattner	fc12d2e	2008-07-08 05:12:37 +0000	[diff] [blame]	284	<p>The longer answer is that there must be some limit to the width of the code
				285	in order to reasonably allow developers to have multiple files side-by-side in
				286	windows on a modest display. If you are going to pick a width limit, it is
				287	somewhat arbitrary but you might as well pick something standard. Going with
				288	90 columns (for example) instead of 80 columns wouldn't add any significant
				289	value and would be detrimental to printing out code. Also many other projects
				290	have standardized on 80 columns, so some people have already configured their
				291	editors for it (vs something else, like 90 columns).</p>
				292
				293	<p>This is one of many contentious issues in coding standards, but is not up
				294	for debate.</p>
				295
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	296	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	297
				298	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	299	<div class="doc_subsubsection">
				300	<a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
				301	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	302
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	303	<div class="doc_text">
				304
				305	<p>In all cases, prefer spaces to tabs in source files. People have different
Benjamin Kramer	0f42038	2009-10-12 14:46:08 +0000	[diff] [blame]	306	preferred indentation levels, and different styles of indentation that they
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	307	like... this is fine. What isn't is that different editors/viewers expand tabs
				308	out to different tab stops. This can cause your code to look completely
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	309	unreadable, and it is not worth dealing with.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	310
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	311	<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	312	style of existing code if your are modifying and extending it. If you like four
				313	spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
				314	with two spaces of indentation. Also, do not reindent a whole source file: it
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	315	makes for incredible diffs that are absolutely worthless.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	316
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	317	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	318
				319	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	320	<div class="doc_subsubsection">
				321	<a name="scf_indentation">Indent Code Consistently</a>
				322	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	323
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	324	<div class="doc_text">
				325
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	326	<p>Okay, in your first year of programming you were told that indentation is
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	327	important. If you didn't believe and internalize this then, now is the time.
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	328	Just do it.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	329
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	330	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	331
				332
				333	<!-- ======================================================================= -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	334	<div class="doc_subsection">
				335	<a name="compilerissues">Compiler Issues</a>
				336	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	337
				338
				339	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	340	<div class="doc_subsubsection">
				341	<a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
				342	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	343
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	344	<div class="doc_text">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	345
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	346	<p>If your code has compiler warnings in it, something is wrong: you aren't
				347	casting values correctly, your have "questionable" constructs in your code, or
				348	you are doing something legitimately wrong. Compiler warnings can cover up
				349	legitimate errors in output and make dealing with a translation unit
				350	difficult.</p>
				351
				352	<p>It is not possible to prevent all warnings from all compilers, nor is it
Misha Brukman	0cedb1f	2003-10-06 19:26:00 +0000	[diff] [blame]	353	desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
				354	a good thorough set of warnings, and stick to them. At least in the case of
				355	<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
				356	syntax of the code slightly. For example, an warning that annoys me occurs when
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	357	I write code like this:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	358
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	359	<div class="doc_code">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	360	<pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	361	if (V = getValue()) {
				362	...
				363	}
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	364	</pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	365	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	366
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	367	<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
				368	operator, and that I probably mistyped it. In most cases, I haven't, and I
				369	really don't want the spurious errors. To fix this particular problem, I
				370	rewrite the code like this:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	371
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	372	<div class="doc_code">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	373	<pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	374	if ((V = getValue())) {
				375	...
				376	}
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	377	</pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	378	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	379
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	380	<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
				381	be fixed by massaging the code appropriately.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	382
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	383	<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
				384	-Winline -W -Wwrite-strings -Wno-unused</tt></p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	385
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	386	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	387
				388	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	389	<div class="doc_subsubsection">
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	390	<a name="ci_portable_code">Write Portable Code</a>
				391	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	392
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	393	<div class="doc_text">
				394
				395	<p>In almost all cases, it is possible and within reason to write completely
Misha Brukman	c3e7893	2003-07-28 21:57:18 +0000	[diff] [blame]	396	portable code. If there are cases where it isn't possible to write portable
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	397	code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	398
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	399	<p>In practice, this means that you shouldn't assume much about the host
Chris Lattner	44c2241	2010-11-16 22:19:06 +0000	[diff] [blame]	400	compiler, and Visual Studio tends to be the lowest common denominator.
				401	If advanced features are used, they should only be an implementation detail of
				402	a library which has a simple exposed API, and preferably be buried in
				403	libSystem.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	404
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	405	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	406
Reid Spencer	bf6439f	2004-09-23 16:03:48 +0000	[diff] [blame]	407	<!-- _______________________________________________________________________ -->
				408	<div class="doc_subsubsection">
				409	<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
				410	</div>
				411	<div class="doc_text">
Misha Brukman	f249913	2004-10-26 15:45:13 +0000	[diff] [blame]	412
				413	<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
				414	interchangeably. The only difference is when they are used to declare a class:
				415	<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
				416	all members public by default.</p>
				417
				418	<p>Unfortunately, not all compilers follow the rules and some will generate
				419	different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
				420	declare the symbol. This can lead to problems at link time.</p>
				421
				422	<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
Chris Lattner	73e9f8c	2009-08-11 17:38:15 +0000	[diff] [blame]	423	<b>all</b> members are public and the type is a C++ "POD" type, in which case
				424	<tt>struct</tt> is allowed.</p>
Misha Brukman	f249913	2004-10-26 15:45:13 +0000	[diff] [blame]	425
Reid Spencer	bf6439f	2004-09-23 16:03:48 +0000	[diff] [blame]	426	</div>
				427
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	428	<!-- *********************************************************************** -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	429	<div class="doc_section">
				430	<a name="styleissues">Style Issues</a>
				431	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	432	<!-- *********************************************************************** -->
				433
				434
				435	<!-- ======================================================================= -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	436	<div class="doc_subsection">
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	437	<a name="macro">The High-Level Issues</a>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	438	</div>
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	439	<!-- ======================================================================= -->
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	440
				441
				442	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	443	<div class="doc_subsubsection">
				444	<a name="hl_module">A Public Header File <b>is</b> a Module</a>
				445	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	446
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	447	<div class="doc_text">
				448
				449	<p>C++ doesn't do too well in the modularity department. There is no real
Misha Brukman	c3e7893	2003-07-28 21:57:18 +0000	[diff] [blame]	450	encapsulation or data hiding (unless you use expensive protocol classes), but it
				451	is what we have to work with. When you write a public header file (in the LLVM
				452	source tree, they live in the top level "include" directory), you are defining a
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	453	module of functionality.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	454
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	455	<p>Ideally, modules should be completely independent of each other, and their
Misha Brukman	c3e7893	2003-07-28 21:57:18 +0000	[diff] [blame]	456	header files should only include the absolute minimum number of headers
				457	possible. A module is not just a class, a function, or a namespace: <a
				458	href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
				459	of these</a> that defines an interface. This interface may be several
				460	functions, classes or data structures, but the important issue is how they work
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	461	together.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	462
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	463	<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
				464	files. Each of these <tt>.cpp</tt> files should include the header that defines
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	465	their interface first. This ensures that all of the dependences of the module
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	466	header have been properly added to the module header itself, and are not
				467	implicit. System headers should be included after user headers for a
				468	translation unit.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	469
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	470	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	471
				472	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	473	<div class="doc_subsubsection">
Misha Brukman	03f87d5	2004-10-26 16:18:43 +0000	[diff] [blame]	474	<a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	475	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	476
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	477	<div class="doc_text">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	478
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	479	<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
				480	have to, especially in header files.</p>
				481
				482	<p>But wait, sometimes you need to have the definition of a class to use it, or
Misha Brukman	03f87d5	2004-10-26 16:18:43 +0000	[diff] [blame]	483	to inherit from it. In these cases go ahead and <tt>#include</tt> that header
				484	file. Be aware however that there are many cases where you don't need to have
				485	the full definition of a class. If you are using a pointer or reference to a
				486	class, you don't need the header file. If you are simply returning a class
				487	instance from a prototyped function or method, you don't need it. In fact, for
				488	most cases, you simply don't need the definition of a class... and not
				489	<tt>#include</tt>'ing speeds up compilation.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	490
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	491	<p>It is easy to try to go too overboard on this recommendation, however. You
Chris Lattner	756f83f	2007-02-10 18:35:31 +0000	[diff] [blame]	492	<b>must</b> include all of the header files that you are using -- you can
				493	include them either directly
Misha Brukman	c3e7893	2003-07-28 21:57:18 +0000	[diff] [blame]	494	or indirectly (through another header file). To make sure that you don't
Benjamin Kramer	0f42038	2009-10-12 14:46:08 +0000	[diff] [blame]	495	accidentally forget to include a header file in your module header, make sure to
Misha Brukman	c3e7893	2003-07-28 21:57:18 +0000	[diff] [blame]	496	include your module header <b>first</b> in the implementation file (as mentioned
				497	above). This way there won't be any hidden dependencies that you'll find out
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	498	about later...</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	499
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	500	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	501
				502	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	503	<div class="doc_subsubsection">
				504	<a name="hl_privateheaders">Keep "internal" Headers Private</a>
				505	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	506
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	507	<div class="doc_text">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	508
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	509	<p>Many modules have a complex implementation that causes them to use more than
				510	one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
				511	internal communication interface (helper classes, extra functions, etc) in the
Misha Brukman	6d1686c	2004-12-04 00:32:12 +0000	[diff] [blame]	512	public module header file. Don't do this.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	513
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	514	<p>If you really need to do something like this, put a private header file in
				515	the same directory as the source files, and include it locally. This ensures
				516	that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	517
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	518	<p>Note however, that it's okay to put extra implementation methods a public
				519	class itself... just make them private (or protected), and all is well.</p>
				520
				521	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	522
Bill Wendling	2b52dc1	2006-12-09 01:20:34 +0000	[diff] [blame]	523	<!-- _______________________________________________________________________ -->
				524	<div class="doc_subsubsection">
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	525	<a name="hl_earlyexit">Use Early Exits and 'continue' to Simplify Code</a>
Bill Wendling	2b52dc1	2006-12-09 01:20:34 +0000	[diff] [blame]	526	</div>
				527
				528	<div class="doc_text">
				529
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	530	<p>When reading code, keep in mind how much state and how many previous
				531	decisions have to be remembered by the reader to understand a block of code.
				532	Aim to reduce indentation where possible when it doesn't make it more difficult
				533	to understand the code. One great way to do this is by making use of early
				534	exits and the 'continue' keyword in long loops. As an example of using an early
				535	exit from a function, consider this "bad" code:</p>
Bill Wendling	2b52dc1	2006-12-09 01:20:34 +0000	[diff] [blame]	536
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	537	<div class="doc_code">
				538	<pre>
				539	Value DoSomething(Instruction I) {
				540	if (!isa<TerminatorInst>(I) &&
				541	I->hasOneUse() && SomeOtherThing(I)) {
				542	... some long code ....
				543	}
				544
				545	return 0;
				546	}
				547	</pre>
				548	</div>
Bill Wendling	2b52dc1	2006-12-09 01:20:34 +0000	[diff] [blame]	549
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	550	<p>This code has several problems if the body of the 'if' is large. When you're
				551	looking at the top of the function, it isn't immediately clear that this
				552	<em>only</em> does interesting things with non-terminator instructions, and only
				553	applies to things with the other predicates. Second, it is relatively difficult
				554	to describe (in comments) why these predicates are important because the if
				555	statement makes it difficult to lay out the comments. Third, when you're deep
				556	within the body of the code, it is indented an extra level. Finally, when
				557	reading the top of the function, it isn't clear what the result is if the
				558	predicate isn't true, you have to read to the end of the function to know that
				559	it returns null.</p>
Matthijs Kooijman	ca4ffaa	2008-07-30 12:14:10 +0000	[diff] [blame]	560
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	561	<p>It is much preferred to format the code like this:</p>
				562
				563	<div class="doc_code">
				564	<pre>
				565	Value DoSomething(Instruction I) {
				566	// Terminators never need 'something' done to them because, ...
				567	if (isa<TerminatorInst>(I))
				568	return 0;
				569
				570	// We conservatively avoid transforming instructions with multiple uses
				571	// because goats like cheese.
				572	if (!I->hasOneUse())
				573	return 0;
				574
				575	// This is really just here for example.
				576	if (!SomeOtherThing(I))
				577	return 0;
				578
				579	... some long code ....
				580	}
				581	</pre>
				582	</div>
				583
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	584	<p>This fixes these problems. A similar problem frequently happens in <tt>for</tt>
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	585	loops. A silly example is something like this:</p>
				586
				587	<div class="doc_code">
				588	<pre>
				589	for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
				590	if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
				591	Value *LHS = BO->getOperand(0);
				592	Value *RHS = BO->getOperand(1);
				593	if (LHS != RHS) {
				594	...
				595	}
				596	}
				597	}
				598	</pre>
				599	</div>
				600
				601	<p>When you have very very small loops, this sort of structure is fine, but if
				602	it exceeds more than 10-15 lines, it becomes difficult for people to read and
				603	understand at a glance.
				604	The problem with this sort of code is that it gets very nested very quickly,
				605	meaning that the reader of the code has to keep a lot of context in their brain
				606	to remember what is going immediately on in the loop, because they don't know
				607	if/when the if conditions will have elses etc. It is strongly preferred to
				608	structure the loop like this:</p>
				609
				610	<div class="doc_code">
				611	<pre>
				612	for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
				613	BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
				614	if (!BO) continue;
				615
				616	Value *LHS = BO->getOperand(0);
				617	Value *RHS = BO->getOperand(1);
				618	if (LHS == RHS) continue;
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	619	...
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	620	}
				621	</pre>
				622	</div>
				623
				624	<p>This has all the benefits of using early exits from functions: it reduces
				625	nesting of the loop, it makes it easier to describe why the conditions are true,
				626	and it makes it obvious to the reader that there is no "else" coming up that
				627	they have to push context into their brain for. If a loop is large, this can
				628	be a big understandability win.</p>
				629
				630	</div>
				631
Chris Lattner	14abb83	2009-07-28 22:54:04 +0000	[diff] [blame]	632	<!-- _______________________________________________________________________ -->
				633	<div class="doc_subsubsection">
				634	<a name="hl_else_after_return">Don't use "else" after a return</a>
				635	</div>
				636
				637	<div class="doc_text">
				638
				639	<p>For similar reasons above (reduction of indentation and easier reading),
				640	please do not use "else" or "else if" after something that interrupts
				641	control flow like return, break, continue, goto, etc. For example, this is
				642	"bad":</p>
				643
				644	<div class="doc_code">
				645	<pre>
				646	case 'J': {
				647	if (Signed) {
				648	Type = Context.getsigjmp_bufType();
				649	if (Type.isNull()) {
				650	Error = ASTContext::GE_Missing_sigjmp_buf;
				651	return QualType();
				652	} else {
				653	break;
				654	}
				655	} else {
				656	Type = Context.getjmp_bufType();
				657	if (Type.isNull()) {
				658	Error = ASTContext::GE_Missing_jmp_buf;
				659	return QualType();
				660	} else {
				661	break;
				662	}
				663	}
				664	}
				665	}
				666	</pre>
				667	</div>
				668
				669	<p>It is better to write this something like:</p>
				670
				671	<div class="doc_code">
				672	<pre>
				673	case 'J':
				674	if (Signed) {
				675	Type = Context.getsigjmp_bufType();
				676	if (Type.isNull()) {
				677	Error = ASTContext::GE_Missing_sigjmp_buf;
				678	return QualType();
				679	}
				680	} else {
				681	Type = Context.getjmp_bufType();
				682	if (Type.isNull()) {
				683	Error = ASTContext::GE_Missing_jmp_buf;
				684	return QualType();
				685	}
				686	}
				687	break;
				688	</pre>
				689	</div>
				690
				691	<p>Or better yet (in this case), as:</p>
				692
				693	<div class="doc_code">
				694	<pre>
				695	case 'J':
				696	if (Signed)
				697	Type = Context.getsigjmp_bufType();
				698	else
				699	Type = Context.getjmp_bufType();
				700
				701	if (Type.isNull()) {
				702	Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
				703	ASTContext::GE_Missing_jmp_buf;
				704	return QualType();
				705	}
				706	break;
				707	</pre>
				708	</div>
				709
				710	<p>The idea is to reduce indentation and the amount of code you have to keep
				711	track of when reading the code.</p>
				712
				713	</div>
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	714
				715	<!-- _______________________________________________________________________ -->
				716	<div class="doc_subsubsection">
				717	<a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
				718	</div>
				719
				720	<div class="doc_text">
				721
Chris Lattner	d63f1237	2009-07-22 16:54:14 +0000	[diff] [blame]	722	<p>It is very common to write small loops that just compute a boolean
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	723	value. There are a number of ways that people commonly write these, but an
				724	example of this sort of thing is:</p>
				725
				726	<div class="doc_code">
				727	<pre>
				728	<b>bool FoundFoo = false;</b>
				729	for (unsigned i = 0, e = BarList.size(); i != e; ++i)
				730	if (BarList[i]->isFoo()) {
				731	<b>FoundFoo = true;</b>
				732	break;
				733	}
				734
				735	<b>if (FoundFoo) {</b>
				736	...
				737	}
				738	</pre>
				739	</div>
				740
				741	<p>This sort of code is awkward to write, and is almost always a bad sign.
				742	Instead of this sort of loop, we strongly prefer to use a predicate function
				743	(which may be <a href="#micro_anonns">static</a>) that uses
Chris Lattner	d63f1237	2009-07-22 16:54:14 +0000	[diff] [blame]	744	<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
				745	the code to be structured like this:
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	746	</p>
				747
				748
				749	<div class="doc_code">
				750	<pre>
				751	/// ListContainsFoo - Return true if the specified list has an element that is
				752	/// a foo.
				753	static bool ListContainsFoo(const std::vector<Bar*> &List) {
				754	for (unsigned i = 0, e = List.size(); i != e; ++i)
				755	if (List[i]->isFoo())
				756	return true;
				757	return false;
				758	}
				759	...
				760
				761	<b>if (ListContainsFoo(BarList)) {</b>
				762	...
				763	}
				764	</pre>
				765	</div>
				766
				767	<p>There are many reasons for doing this: it reduces indentation and factors out
				768	code which can often be shared by other code that checks for the same predicate.
				769	More importantly, it <em>forces you to pick a name</em> for the function, and
				770	forces you to write a comment for it. In this silly example, this doesn't add
				771	much value. However, if the condition is complex, this can make it a lot easier
				772	for the reader to understand the code that queries for this predicate. Instead
Chris Lattner	611a188	2009-07-22 16:30:39 +0000	[diff] [blame]	773	of being faced with the in-line details of how we check to see if the BarList
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	774	contains a foo, we can trust the function name and continue reading with better
				775	locality.</p>
Bill Wendling	2b52dc1	2006-12-09 01:20:34 +0000	[diff] [blame]	776
				777	</div>
				778
				779
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	780	<!-- ======================================================================= -->
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	781	<div class="doc_subsection">
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	782	<a name="micro">The Low-Level Issues</a>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	783	</div>
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	784	<!-- ======================================================================= -->
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	785
				786
				787	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	788	<div class="doc_subsubsection">
Chris Lattner	85ea83e	2006-07-27 04:24:14 +0000	[diff] [blame]	789	<a name="ll_assert">Assert Liberally</a>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	790	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	791
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	792	<div class="doc_text">
				793
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	794	<p>Use the "<tt>assert</tt>" macro to its fullest. Check all of your
Benjamin Kramer	0f42038	2009-10-12 14:46:08 +0000	[diff] [blame]	795	preconditions and assumptions, you never know when a bug (not necessarily even
Misha Brukman	c3e7893	2003-07-28 21:57:18 +0000	[diff] [blame]	796	yours) might be caught early by an assertion, which reduces debugging time
				797	dramatically. The "<tt><cassert></tt>" header file is probably already
				798	included by the header files you are using, so it doesn't cost anything to use
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	799	it.</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	800
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	801	<p>To further assist with debugging, make sure to put some kind of error message
				802	in the assertion statement (which is printed if the assertion is tripped). This
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	803	helps the poor debugger make sense of why an assertion is being made and
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	804	enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	805
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	806	<div class="doc_code">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	807	<pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	808	inline Value *getOperand(unsigned i) {
				809	assert(i < Operands.size() && "getOperand() out of range!");
				810	return Operands[i];
				811	}
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	812	</pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	813	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	814
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	815	<p>Here are some examples:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	816
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	817	<div class="doc_code">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	818	<pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	819	assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	820
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	821	assert((Opcode == Shl \|\| Opcode == Shr) && "ShiftInst Opcode invalid!");
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	822
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	823	assert(idx < getNumSuccessors() && "Successor # out of range!");
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	824
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	825	assert(V1.getType() == V2.getType() && "Constant types must be identical!");
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	826
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	827	assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	828	</pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	829	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	830
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	831	<p>You get the idea...</p>
				832
Nick Lewycky	fb75d42	2008-05-31 23:54:55 +0000	[diff] [blame]	833	<p>Please be aware when adding assert statements that not all compilers are aware of
				834	the semantics of the assert. In some places, asserts are used to indicate a piece of
				835	code that should not be reached. These are typically of the form:</p>
				836
				837	<div class="doc_code">
				838	<pre>
Dan Gohman	8ef4498	2008-11-24 17:18:39 +0000	[diff] [blame]	839	assert(0 && "Some helpful error message");
Nick Lewycky	fb75d42	2008-05-31 23:54:55 +0000	[diff] [blame]	840	</pre>
				841	</div>
				842
				843	<p>When used in a function that returns a value, they should be followed with a return
				844	statement and a comment indicating that this line is never reached. This will prevent
				845	a compiler which is unable to deduce that the assert statement never returns from
				846	generating a warning.</p>
				847
				848	<div class="doc_code">
				849	<pre>
Dan Gohman	8ef4498	2008-11-24 17:18:39 +0000	[diff] [blame]	850	assert(0 && "Some helpful error message");
Nick Lewycky	fb75d42	2008-05-31 23:54:55 +0000	[diff] [blame]	851	// Not reached
				852	return 0;
				853	</pre>
				854	</div>
				855
Chris Lattner	8a7f4da	2010-11-12 00:19:41 +0000	[diff] [blame]	856	<p>Another issue is that values used only by assertions will produce an "unused
				857	value" warning when assertions are disabled. For example, this code will warn:
				858	</p>
				859
				860	<div class="doc_code">
				861	<pre>
				862	unsigned Size = V.size();
				863	assert(Size > 42 && "Vector smaller than it should be");
				864
				865	bool NewToSet = Myset.insert(Value);
				866	assert(NewToSet && "The value shouldn't be in the set yet");
				867	</pre>
				868	</div>
				869
				870	<p>These are two interesting different cases: in the first case, the call to
				871	V.size() is only useful for the assert, and we don't want it executed when
				872	assertions are disabled. Code like this should move the call into the assert
				873	itself. In the second case, the side effects of the call must happen whether
				874	the assert is enabled or not. In this case, the value should be cast to void
				875	to disable the warning. To be specific, it is preferred to write the code
				876	like this:</p>
				877
				878	<div class="doc_code">
				879	<pre>
				880	assert(V.size() > 42 && "Vector smaller than it should be");
				881
				882	bool NewToSet = Myset.insert(Value); (void)NewToSet;
				883	assert(NewToSet && "The value shouldn't be in the set yet");
				884	</pre>
				885	</div>
				886
				887
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	888	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	889
Chris Lattner	01e81e6	2006-01-01 21:59:22 +0000	[diff] [blame]	890	<!-- _______________________________________________________________________ -->
				891	<div class="doc_subsubsection">
Bill Wendling	b062948	2006-12-09 01:35:43 +0000	[diff] [blame]	892	<a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
Chris Lattner	01e81e6	2006-01-01 21:59:22 +0000	[diff] [blame]	893	</div>
				894
				895	<div class="doc_text">
				896	<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
Bill Wendling	b062948	2006-12-09 01:35:43 +0000	[diff] [blame]	897	namespace with an "<tt>std::</tt>" prefix, rather than rely on
				898	"<tt>using namespace std;</tt>".</p>
Chris Lattner	01e81e6	2006-01-01 21:59:22 +0000	[diff] [blame]	899
Bill Wendling	b062948	2006-12-09 01:35:43 +0000	[diff] [blame]	900	<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
Misha Brukman	586a15c	2008-12-11 19:37:04 +0000	[diff] [blame]	901	the namespace of any source file that <tt>#include</tt>s the header. This is
				902	clearly a bad thing.</p>
Chris Lattner	01e81e6	2006-01-01 21:59:22 +0000	[diff] [blame]	903
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	904	<p>In implementation files (e.g. <tt>.cpp</tt> files), the rule is more of a stylistic
Bill Wendling	b062948	2006-12-09 01:35:43 +0000	[diff] [blame]	905	rule, but is still important. Basically, using explicit namespace prefixes
				906	makes the code <b>clearer</b>, because it is immediately obvious what facilities
				907	are being used and where they are coming from, and <b>more portable</b>, because
				908	namespace clashes cannot occur between LLVM code and other namespaces. The
				909	portability rule is important because different standard library implementations
				910	expose different symbols (potentially ones they shouldn't), and future revisions
				911	to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
				912	such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
Chris Lattner	01e81e6	2006-01-01 21:59:22 +0000	[diff] [blame]	913
Bill Wendling	b062948	2006-12-09 01:35:43 +0000	[diff] [blame]	914	<p>The exception to the general rule (i.e. it's not an exception for
				915	the <tt>std</tt> namespace) is for implementation files. For example, all of
				916	the code in the LLVM project implements code that lives in the 'llvm' namespace.
Chris Lattner	eebbbeb	2010-11-17 19:47:20 +0000	[diff] [blame]	917	As such, it is ok, and actually clearer, for the <tt>.cpp</tt> files to have a
				918	'<tt>using namespace llvm</tt>' directive at their top, after the
				919	<tt>#include</tt>s. This reduces indentation in the body of the file for source
				920	editors that indent based on braces, and keeps the conceptual context cleaner.
				921	The general form of this rule is that any <tt>.cpp</tt> file that implements
				922	code in any namespace may use that namespace (and its parents'), but should not
				923	use any others.</p>
Chris Lattner	01e81e6	2006-01-01 21:59:22 +0000	[diff] [blame]	924
				925	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	926
				927	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	928	<div class="doc_subsubsection">
Bill Wendling	b062948	2006-12-09 01:35:43 +0000	[diff] [blame]	929	<a name="ll_virtual_anch">Provide a virtual method anchor for classes
				930	in headers</a>
Chris Lattner	85ea83e	2006-07-27 04:24:14 +0000	[diff] [blame]	931	</div>
				932
				933	<div class="doc_text">
				934
				935	<p>If a class is defined in a header file and has a v-table (either it has
				936	virtual methods or it derives from classes with virtual methods), it must
				937	always have at least one out-of-line virtual method in the class. Without
Misha Brukman	586a15c	2008-12-11 19:37:04 +0000	[diff] [blame]	938	this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
				939	that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
				940	increasing link times.</p>
Chris Lattner	85ea83e	2006-07-27 04:24:14 +0000	[diff] [blame]	941
				942	</div>
				943
Chris Lattner	56c1094	2009-06-30 06:13:23 +0000	[diff] [blame]	944	<!-- _______________________________________________________________________ -->
				945	<div class="doc_subsubsection">
				946	<a name="ll_end">Don't evaluate end() every time through a loop</a>
				947	</div>
				948
				949	<div class="doc_text">
				950
				951	<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
				952	with macros and may be coming in C++'0x) we end up writing a lot of loops that
				953	manually iterate from begin to end on a variety of containers or through other
				954	data structures. One common mistake is to write a loop in this style:</p>
				955
				956	<div class="doc_code">
				957	<pre>
				958	BasicBlock *BB = ...
				959	for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
				960	... use I ...
				961	</pre>
				962	</div>
				963
				964	<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
				965	every time through the loop. Instead of writing the loop like this, we strongly
				966	prefer loops to be written so that they evaluate it once before the loop starts.
				967	A convenient way to do this is like so:</p>
				968
				969	<div class="doc_code">
				970	<pre>
				971	BasicBlock *BB = ...
				972	for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
				973	... use I ...
				974	</pre>
				975	</div>
				976
				977	<p>The observant may quickly point out that these two loops may have different
				978	semantics: if the container (a basic block in this case) is being mutated, then
				979	"<tt>BB->end()</tt>" may change its value every time through the loop and the
				980	second loop may not in fact be correct. If you actually do depend on this
Chris Lattner	6ad089c	2009-06-30 06:20:03 +0000	[diff] [blame]	981	behavior, please write the loop in the first form and add a comment indicating
Chris Lattner	56c1094	2009-06-30 06:13:23 +0000	[diff] [blame]	982	that you did it intentionally.</p>
				983
				984	<p>Why do we prefer the second form (when correct)? Writing the loop in the
				985	first form has two problems: First it may be less efficient than evaluating it
				986	at the start of the loop. In this case, the cost is probably minor: a few extra
				987	loads every time through the loop. However, if the base expression is more
				988	complex, then the cost can rise quickly. I've seen loops where the end
				989	expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
Chris Lattner	67f2a4ac	2009-06-30 06:27:54 +0000	[diff] [blame]	990	lookups really aren't cheap. By writing it in the second form consistently, you
Chris Lattner	56c1094	2009-06-30 06:13:23 +0000	[diff] [blame]	991	eliminate the issue entirely and don't even have to think about it.</p>
				992
Chris Lattner	6ad089c	2009-06-30 06:20:03 +0000	[diff] [blame]	993	<p>The second (even bigger) issue is that writing the loop in the first form
Chris Lattner	56c1094	2009-06-30 06:13:23 +0000	[diff] [blame]	994	hints to the reader that the loop is mutating the container (a fact that a
				995	comment would handily confirm!). If you write the loop in the second form, it
				996	is immediately obvious without even looking at the body of the loop that the
				997	container isn't being modified, which makes it easier to read the code and
				998	understand what it does.</p>
				999
				1000	<p>While the second form of the loop is a few extra keystrokes, we do strongly
				1001	prefer it.</p>
				1002
				1003	</div>
				1004
Chris Lattner	85ea83e	2006-07-27 04:24:14 +0000	[diff] [blame]	1005	<!-- _______________________________________________________________________ -->
				1006	<div class="doc_subsubsection">
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1007	<a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1008	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1009
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1010	<div class="doc_text">
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1011
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1012	<p>The use of <tt>#include <iostream></tt> in library files is
				1013	hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
				1014	support clients using LLVM libraries as part of larger systems. In particular,
				1015	we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
				1016	the static c'tors are run whenever an application start up that uses the dynamic
				1017	library. There are two problems with this:</p>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1018
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1019	<ol>
				1020	<li>The time to run the static c'tors impacts startup time of
				1021	applications—a critical time for GUI apps.</li>
				1022	<li>The static c'tors cause the app to pull many extra pages of memory off the
				1023	disk: both the code for the static c'tors in each <tt>.o</tt> file and the
				1024	small amount of data that gets touched. In addition, touched/dirty pages
				1025	put more pressure on the VM system on low-memory machines.</li>
				1026	</ol>
				1027
				1028	<p>Note that using the other stream headers (<tt><sstream></tt> for
Chris Lattner	5db2ff5	2009-08-23 21:53:47 +0000	[diff] [blame]	1029	example) is not problematic in this regard (just <tt><iostream></tt>).
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	1030	However, <tt>raw_ostream</tt> provides various APIs that are better performing for almost
				1031	every use than <tt>std::ostream</tt> style APIs.
				1032	<b>Therefore new code should always
Chris Lattner	5db2ff5	2009-08-23 21:53:47 +0000	[diff] [blame]	1033	use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or
				1034	the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1035
				1036	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1037
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1038
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1039	<!-- _______________________________________________________________________ -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1040	<div class="doc_subsubsection">
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	1041	<a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
				1042	</div>
				1043
				1044	<div class="doc_text">
				1045
				1046	<p>LLVM includes a lightweight, simple, and efficient stream implementation
				1047	in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
				1048	of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> instead
				1049	of <tt>ostream</tt>.</p>
				1050
				1051	<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
				1052	be forward declared as <tt>class raw_ostream</tt>. Public headers should
				1053	generally not include the <tt>raw_ostream</tt> header, but use forward
				1054	declarations and constant references to <tt>raw_ostream</tt> instances.</p>
				1055
				1056	</div>
				1057
				1058
				1059	<!-- _______________________________________________________________________ -->
				1060	<div class="doc_subsubsection">
Chris Lattner	85ea83e	2006-07-27 04:24:14 +0000	[diff] [blame]	1061	<a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1062	</div>
Chris Lattner	850d4f6	2002-01-20 19:01:26 +0000	[diff] [blame]	1063
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1064	<div class="doc_text">
				1065
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	1066	<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
				1067	to the output stream specified. In addition to doing this, however, it also
				1068	flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner	850d4f6	2002-01-20 19:01:26 +0000	[diff] [blame]	1069
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	1070	<div class="doc_code">
Chris Lattner	850d4f6	2002-01-20 19:01:26 +0000	[diff] [blame]	1071	<pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	1072	std::cout << std::endl;
				1073	std::cout << '\n' << std::flush;
Chris Lattner	850d4f6	2002-01-20 19:01:26 +0000	[diff] [blame]	1074	</pre>
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	1075	</div>
Chris Lattner	850d4f6	2002-01-20 19:01:26 +0000	[diff] [blame]	1076
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1077	<p>Most of the time, you probably have no reason to flush the output stream, so
Misha Brukman	b224615	2004-07-28 22:31:54 +0000	[diff] [blame]	1078	it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner	850d4f6	2002-01-20 19:01:26 +0000	[diff] [blame]	1079
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1080	</div>
Chris Lattner	850d4f6	2002-01-20 19:01:26 +0000	[diff] [blame]	1081
Bill Wendling	2b52dc1	2006-12-09 01:20:34 +0000	[diff] [blame]	1082
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1083	<!-- ======================================================================= -->
				1084	<div class="doc_subsection">
				1085	<a name="nano">Microscopic Details</a>
				1086	</div>
				1087	<!-- ======================================================================= -->
				1088
				1089	<p>This section describes preferred low-level formatting guidelines along with
				1090	reasoning on why we prefer them.</p>
				1091
				1092	<!-- _______________________________________________________________________ -->
				1093	<div class="doc_subsubsection">
				1094	<a name="micro_spaceparen">Spaces Before Parentheses</a>
				1095	</div>
				1096
				1097	<div class="doc_text">
				1098
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	1099	<p>We prefer to put a space before an open parenthesis only in control flow
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1100	statements, but not in normal function call expressions and function-like
				1101	macros. For example, this is good:</p>
				1102
				1103	<div class="doc_code">
				1104	<pre>
				1105	<b>if (</b>x) ...
				1106	<b>for (</b>i = 0; i != 100; ++i) ...
				1107	<b>while (</b>llvm_rocks) ...
				1108
				1109	<b>somefunc(</b>42);
				1110	<b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me");
				1111
				1112	a = <b>foo(</b>42, 92) + <b>bar(</b>x);
				1113	</pre>
				1114	</div>
				1115
				1116	<p>... and this is bad:</p>
				1117
				1118	<div class="doc_code">
				1119	<pre>
				1120	<b>if(</b>x) ...
				1121	<b>for(</b>i = 0; i != 100; ++i) ...
				1122	<b>while(</b>llvm_rocks) ...
				1123
				1124	<b>somefunc (</b>42);
				1125	<b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me");
				1126
				1127	a = <b>foo (</b>42, 92) + <b>bar (</b>x);
				1128	</pre>
				1129	</div>
				1130
				1131	<p>The reason for doing this is not completely arbitrary. This style makes
				1132	control flow operators stand out more, and makes expressions flow better. The
				1133	function call operator binds very tightly as a postfix operator. Putting
				1134	a space after a function name (as in the last example) makes it appear that
				1135	the code might bind the arguments of the left-hand-side of a binary operator
				1136	with the argument list of a function and the name of the right side. More
				1137	specifically, it is easy to misread the "a" example as:</p>
				1138
				1139	<div class="doc_code">
				1140	<pre>
				1141	a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
				1142	</pre>
				1143	</div>
				1144
				1145	<p>... when skimming through the code. By avoiding a space in a function, we
				1146	avoid this misinterpretation.</p>
				1147
				1148	</div>
				1149
				1150	<!-- _______________________________________________________________________ -->
				1151	<div class="doc_subsubsection">
				1152	<a name="micro_preincrement">Prefer Preincrement</a>
				1153	</div>
				1154
				1155	<div class="doc_text">
				1156
				1157	<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
				1158	postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
				1159	preincrementation whenever possible.</p>
				1160
				1161	<p>The semantics of postincrement include making a copy of the value being
				1162	incremented, returning it, and then preincrementing the "work value". For
				1163	primitive types, this isn't a big deal... but for iterators, it can be a huge
				1164	issue (for example, some iterators contains stack and set objects in them...
				1165	copying an iterator could invoke the copy ctor's of these as well). In general,
				1166	get in the habit of always using preincrement, and you won't have a problem.</p>
				1167
				1168	</div>
				1169
				1170	<!-- _______________________________________________________________________ -->
				1171	<div class="doc_subsubsection">
				1172	<a name="micro_namespaceindent">Namespace Indentation</a>
				1173	</div>
				1174
				1175	<div class="doc_text">
				1176
				1177	<p>
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	1178	In general, we strive to reduce indentation wherever possible. This is useful
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1179	because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
				1180	wrapping horribly, but also because it makes it easier to understand the code.
				1181	Namespaces are a funny thing: they are often large, and we often desire to put
				1182	lots of stuff into them (so they can be large). Other times they are tiny,
				1183	because they just hold an enum or something similar. In order to balance this,
				1184	we use different approaches for small versus large namespaces.
				1185	</p>
				1186
				1187	<p>
				1188	If a namespace definition is small and <em>easily</em> fits on a screen (say,
				1189	less than 35 lines of code), then you should indent its body. Here's an
				1190	example:
				1191	</p>
				1192
				1193	<div class="doc_code">
				1194	<pre>
Chris Lattner	d63f1237	2009-07-22 16:54:14 +0000	[diff] [blame]	1195	namespace llvm {
				1196	namespace X86 {
				1197	/// RelocationType - An enum for the x86 relocation codes. Note that
				1198	/// the terminology here doesn't follow x86 convention - word means
				1199	/// 32-bit and dword means 64-bit.
				1200	enum RelocationType {
				1201	/// reloc_pcrel_word - PC relative relocation, add the relocated value to
				1202	/// the value already in memory, after we adjust it for where the PC is.
				1203	reloc_pcrel_word = 0,
				1204
				1205	/// reloc_picrel_word - PIC base relative relocation, add the relocated
				1206	/// value to the value already in memory, after we adjust it for where the
				1207	/// PIC base is.
				1208	reloc_picrel_word = 1,
				1209
				1210	/// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
				1211	/// add the relocated value to the value already in memory.
				1212	reloc_absolute_word = 2,
				1213	reloc_absolute_dword = 3
				1214	};
				1215	}
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1216	}
				1217	</pre>
				1218	</div>
				1219
				1220	<p>Since the body is small, indenting adds value because it makes it very clear
				1221	where the namespace starts and ends, and it is easy to take the whole thing in
				1222	in one "gulp" when reading the code. If the blob of code in the namespace is
Chris Lattner	8defde0	2010-11-17 17:14:55 +0000	[diff] [blame]	1223	larger (as it typically is in a header in the <tt>llvm</tt> or <tt>clang</tt> namespaces), do not
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1224	indent the code, and add a comment indicating what namespace is being closed.
				1225	For example:</p>
				1226
				1227	<div class="doc_code">
				1228	<pre>
				1229	namespace llvm {
				1230	namespace knowledge {
				1231
				1232	/// Grokable - This class represents things that Smith can have an intimate
				1233	/// understanding of and contains the data associated with it.
				1234	class Grokable {
				1235	...
				1236	public:
				1237	explicit Grokable() { ... }
				1238	virtual ~Grokable() = 0;
				1239
				1240	...
				1241
				1242	};
				1243
				1244	} // end namespace knowledge
				1245	} // end namespace llvm
				1246	</pre>
				1247	</div>
				1248
				1249	<p>Because the class is large, we don't expect that the reader can easily
				1250	understand the entire concept in a glance, and the end of the file (where the
				1251	namespaces end) may be a long ways away from the place they open. As such,
				1252	indenting the contents of the namespace doesn't add any value, and detracts from
				1253	the readability of the class. In these cases it is best to <em>not</em> indent
				1254	the contents of the namespace.</p>
				1255
				1256	</div>
				1257
				1258	<!-- _______________________________________________________________________ -->
				1259	<div class="doc_subsubsection">
				1260	<a name="micro_anonns">Anonymous Namespaces</a>
				1261	</div>
				1262
				1263	<div class="doc_text">
				1264
Chris Lattner	d63f1237	2009-07-22 16:54:14 +0000	[diff] [blame]	1265	<p>After talking about namespaces in general, you may be wondering about
				1266	anonymous namespaces in particular.
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1267	Anonymous namespaces are a great language feature that tells the C++ compiler
				1268	that the contents of the namespace are only visible within the current
				1269	translation unit, allowing more aggressive optimization and eliminating the
				1270	possibility of symbol name collisions. Anonymous namespaces are to C++ as
				1271	"static" is to C functions and global variables. While "static" is available
				1272	in C++, anonymous namespaces are more general: they can make entire classes
				1273	private to a file.</p>
				1274
				1275	<p>The problem with anonymous namespaces is that they naturally want to
				1276	encourage indentation of their body, and they reduce locality of reference: if
				1277	you see a random function definition in a C++ file, it is easy to see if it is
				1278	marked static, but seeing if it is in an anonymous namespace requires scanning
				1279	a big chunk of the file.</p>
				1280
				1281	<p>Because of this, we have a simple guideline: make anonymous namespaces as
				1282	small as possible, and only use them for class declarations. For example, this
				1283	is good:</p>
				1284
				1285	<div class="doc_code">
				1286	<pre>
				1287	<b>namespace {</b>
				1288	class StringSort {
				1289	...
				1290	public:
				1291	StringSort(...)
				1292	bool operator<(const char *RHS) const;
				1293	};
				1294	<b>} // end anonymous namespace</b>
				1295
				1296	static void Helper() {
				1297	...
				1298	}
				1299
				1300	bool StringSort::operator<(const char *RHS) const {
				1301	...
				1302	}
				1303
				1304	</pre>
				1305	</div>
				1306
				1307	<p>This is bad:</p>
				1308
				1309
				1310	<div class="doc_code">
				1311	<pre>
				1312	<b>namespace {</b>
				1313	class StringSort {
				1314	...
				1315	public:
				1316	StringSort(...)
				1317	bool operator<(const char *RHS) const;
				1318	};
				1319
				1320	void Helper() {
				1321	...
				1322	}
				1323
				1324	bool StringSort::operator<(const char *RHS) const {
				1325	...
				1326	}
				1327
				1328	<b>} // end anonymous namespace</b>
				1329
				1330	</pre>
				1331	</div>
				1332
				1333
				1334	<p>This is bad specifically because if you're looking at "Helper" in the middle
				1335	of a large C++ file, that you have no immediate way to tell if it is local to
				1336	the file. When it is marked static explicitly, this is immediately obvious.
				1337	Also, there is no reason to enclose the definition of "operator<" in the
Chris Lattner	d63f1237	2009-07-22 16:54:14 +0000	[diff] [blame]	1338	namespace just because it was declared there.
Chris Lattner	6720d7e	2009-07-22 05:40:54 +0000	[diff] [blame]	1339	</p>
				1340
				1341	</div>
				1342
				1343
				1344
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1345	<!-- *********************************************************************** -->
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1346	<div class="doc_section">
				1347	<a name="seealso">See Also</a>
				1348	</div>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1349	<!-- *********************************************************************** -->
				1350
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1351	<div class="doc_text">
				1352
				1353	<p>A lot of these comments and recommendations have been culled for other
				1354	sources. Two particularly important books for our work are:</p>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1355
				1356	<ol>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1357
Chris Lattner	f9f08bf	2007-11-09 21:49:08 +0000	[diff] [blame]	1358	<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
				1359	C++</a> by Scott Meyers. Also
Chris Lattner	1dab192	2004-05-23 21:05:07 +0000	[diff] [blame]	1360	interesting and useful are "More Effective C++" and "Effective STL" by the same
				1361	author.</li>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1362
Chris Lattner	f9f08bf	2007-11-09 21:49:08 +0000	[diff] [blame]	1363	<li>Large-Scale C++ Software Design by John Lakos</li>
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1364
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1365	</ol>
				1366
				1367	<p>If you get some free time, and you haven't read them: do so, you might learn
Misha Brukman	6d1686c	2004-12-04 00:32:12 +0000	[diff] [blame]	1368	something.</p>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1369
				1370	</div>
				1371
Chris Lattner	ac457c4	2001-07-09 03:27:08 +0000	[diff] [blame]	1372	<!-- *********************************************************************** -->
				1373
				1374	<hr>
Misha Brukman	d4f290a	2004-05-12 18:37:22 +0000	[diff] [blame]	1375	<address>
				1376	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman	86242e1	2008-12-11 17:34:48 +0000	[diff] [blame]	1377	src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman	d4f290a	2004-05-12 18:37:22 +0000	[diff] [blame]	1378	<a href="http://validator.w3.org/check/referer"><img
Misha Brukman	86242e1	2008-12-11 17:34:48 +0000	[diff] [blame]	1379	src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1380
Misha Brukman	d4f290a	2004-05-12 18:37:22 +0000	[diff] [blame]	1381	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer	ca05854	2006-03-14 05:39:39 +0000	[diff] [blame]	1382	<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1383	Last modified: $Date$
Misha Brukman	d4f290a	2004-05-12 18:37:22 +0000	[diff] [blame]	1384	</address>
Misha Brukman	f196dbb	2003-10-24 17:57:33 +0000	[diff] [blame]	1385
				1386	</body>
				1387	</html>