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Misha Brukman	c1449e6	2008-12-10 23:07:02 +0000	[diff] [blame]	6	<title>LLVM Coding Standards</title>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	7	</head>
				8	<body>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	9
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	10	<div class="doc_title">
Misha Brukman	c1449e6	2008-12-10 23:07:02 +0000	[diff] [blame]	11	LLVM Coding Standards
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	12	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	13
				14	<ol>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	15	<li><a href="#introduction">Introduction</a></li>
Chris Lattner	7ae36bb	2001-07-23 20:40:41 +0000	[diff] [blame]	16	<li><a href="#mechanicalissues">Mechanical Source Issues</a>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	17	<ol>
				18	<li><a href="#sourceformating">Source Code Formatting</a>
Misha Brukman	75f258e	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	91d64af	2004-10-26 16:18:43 +0000	[diff] [blame]	22	<li><a href="#scf_includes"><tt>#include</tt> Style</a></li>
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	27	<li><a href="#compilerissues">Compiler Issues</a>
				28	<ol>
Misha Brukman	75f258e	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	31	<li><a href="#ci_portable_code">Write Portable Code</a></li>
Reid Spencer	b40df56	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	33	</ol></li>
				34	</ol></li>
Chris Lattner	d80c43c	2001-07-09 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 Brukman	75f258e	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	d283cb1	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	9eb7e0a	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	d283cb1	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	50	</ol></li>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	51	<li><a href="#micro">The Low Level Issues</a>
				52	<ol>
Chris Lattner	6883a88	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	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	55	<li><a href="#ll_virtual_anch">Provide a virtual method anchor for
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	56	classes in headers</a></li>
Chris Lattner	0af39ea	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	d283cb1	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>
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	61	<li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
Daniel Dunbar	e8530a3	2009-07-24 23:04:51 +0000	[diff] [blame]	62	<li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	63	</ol></li>
Chris Lattner	d283cb1	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	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	77
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	78	<div class="doc_author">
Chris Lattner	219bd29	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	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	80	</div>
				81
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	82
				83	<!-- *********************************************************************** -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	84	<div class="doc_section">
				85	<a name="introduction">Introduction</a>
				86	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	87	<!-- *********************************************************************** -->
				88
Misha Brukman	75f258e	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	1b36d67	2003-10-06 19:26:00 +0000	[diff] [blame]	93	absolute requirements to be followed in all instances, coding standards can be
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	94	useful.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	95
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	96	<p>This document intentionally does not prescribe fixed standards for religious
Misha Brukman	1b36d67	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	98	the golden rule:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	99
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	100	<blockquote>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	101
Misha Brukman	75f258e	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	1b36d67	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	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	114
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	115	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	116
				117	<!-- *********************************************************************** -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	121	<!-- *********************************************************************** -->
				122
				123	<!-- ======================================================================= -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	127
				128	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	132
Misha Brukman	75f258e	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	b927ca8	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	1b36d67	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	140	documentation is very useful:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	141
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	142	<b>File Headers</b>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	143
Reid Spencer	669ed45	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	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	149
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	150	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	151	<pre>
Chris Lattner	c49fcd1	2003-10-13 14:58:11 +0000	[diff] [blame]	152	//===-- llvm/Instruction.h - Instruction class definition -------- C++ --===//
Misha Brukman	98117cd	2009-01-02 16:58:42 +0000	[diff] [blame]	153	//
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	154	// The LLVM Compiler Infrastructure
				155	//
Chris Lattner	ab2b10c	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	98117cd	2009-01-02 16:58:42 +0000	[diff] [blame]	158	//
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	159	//===----------------------------------------------------------------------===//
Chris Lattner	d80c43c	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	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	166	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	167
Chris Lattner	ab2b10c	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	75f258e	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
Misha Brukman	69c5889	2004-07-28 22:37:57 +0000	[diff] [blame]	170	is a C++ file, not a C file (Emacs assumes .h files are C files by default).
				171	Note that this tag is not necessary in .cpp files. The name of the file is also
				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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	175
Chris Lattner	ab2b10c	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	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	179
Misha Brukman	75f258e	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	1b36d67	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	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	184
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	185	<b>Class overviews</b>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	186
Jim Laskey	f55914a	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	75f258e	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	1b36d67	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	3b53a26	2004-12-04 00:32:12 +0000	[diff] [blame]	191	something sane goes a long ways towards avoiding writing documentation.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	192
				193
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	194	<b>Method information</b>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	195
Misha Brukman	75f258e	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
Misha Brukman	1b36d67	2003-10-06 19:26:00 +0000	[diff] [blame]	197	documented properly. A quick note about what it does any a description of the
				198	borderline behaviour is all that is necessary here (unless something
				199	particularly tricky or insideous is going on). The hope is that people can
				200	figure out how to use your interfaces without reading the code itself... that is
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	201	the goal metric.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	202
Misha Brukman	75f258e	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	206	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	207
				208	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	212
Misha Brukman	75f258e	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	1b36d67	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	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	218
				219	<ol>
Misha Brukman	75f258e	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	3b53a26	2004-12-04 00:32:12 +0000	[diff] [blame]	221	comments.</li>
Misha Brukman	91d64af	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	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	227
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	232
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	233	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	234	<div class="doc_subsubsection">
Misha Brukman	91d64af	2004-10-26 16:18:43 +0000	[diff] [blame]	235	<a name="scf_includes"><tt>#include</tt> Style</a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	236	</div>
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	237
Misha Brukman	75f258e	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	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	241	include guards if working on a header file), the <a
Misha Brukman	a8c760a	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	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	245
				246	<ol>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	247	<li><a href="#mmheader">Main Module header</a></li>
				248	<li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukman	a8c760a	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>
				252	<li><tt>llvm/Bytecode/*</tt></li>
				253	<li><tt>llvm/CodeGen/*</tt></li>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	254	<li>...</li>
Misha Brukman	a8c760a	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	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	258	</ol>
				259
Nick Lewycky	822293a	2008-11-29 20:13:25 +0000	[diff] [blame]	260	<p>... and each category should be sorted by name.</p>
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	261
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	262	<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
Misha Brukman	91d64af	2004-10-26 16:18:43 +0000	[diff] [blame]	263	which implement an interface defined by a .h file. This <tt>#include</tt>
				264	should always be included <b>first</b> regardless of where it lives on the file
				265	system. By including a header file first in the .cpp files that implement the
				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
				268	form of documentation in the .cpp file to indicate where the interfaces it
				269	implements are defined.</p>
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	270
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	271	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	272
				273	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	277
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	278	<div class="doc_text">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	279
Misha Brukman	75f258e	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	5822e9e	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	296	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	297
				298	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	302
Misha Brukman	75f258e	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
Misha Brukman	1b36d67	2003-10-06 19:26:00 +0000	[diff] [blame]	306	prefered indentation levels, and different styles of indentation that they
				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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	309	unreadable, and it is not worth dealing with.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	310
Misha Brukman	75f258e	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	1b36d67	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	315	makes for incredible diffs that are absolutely worthless.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	316
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	317	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	318
				319	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	323
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	324	<div class="doc_text">
				325
				326	<p>Okay, your first year of programming you were told that indentation is
Misha Brukman	1b36d67	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	328	Just do it.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	329
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	330	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	331
				332
				333	<!-- ======================================================================= -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	337
				338
				339	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	343
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	344	<div class="doc_text">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	345
Misha Brukman	75f258e	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	1b36d67	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	357	I write code like this:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	358
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	359	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	360	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	361	if (V = getValue()) {
				362	...
				363	}
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	364	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	365	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	366
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	371
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	372	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	373	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	374	if ((V = getValue())) {
				375	...
				376	}
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	377	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	378	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	379
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	382
Misha Brukman	75f258e	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	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	385
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	386	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	387
				388	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	389	<div class="doc_subsubsection">
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	390	<a name="ci_portable_code">Write Portable Code</a>
				391	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	392
Misha Brukman	75f258e	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	2d10d75	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	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	397	code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	398
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	399	<p>In practice, this means that you shouldn't assume much about the host
				400	compiler, including its support for "high tech" features like partial
Chris Lattner	4bd3d7e	2009-03-23 04:52:53 +0000	[diff] [blame]	401	specialization of templates. If these features are used, they should only be
				402	an implementation detail of a library which has a simple exposed API.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	403
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	404	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	405
Reid Spencer	b40df56	2004-09-23 16:03:48 +0000	[diff] [blame]	406	<!-- _______________________________________________________________________ -->
				407	<div class="doc_subsubsection">
				408	<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
				409	</div>
				410	<div class="doc_text">
Misha Brukman	edcc78a	2004-10-26 15:45:13 +0000	[diff] [blame]	411
				412	<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
				413	interchangeably. The only difference is when they are used to declare a class:
				414	<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
				415	all members public by default.</p>
				416
				417	<p>Unfortunately, not all compilers follow the rules and some will generate
				418	different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
				419	declare the symbol. This can lead to problems at link time.</p>
				420
				421	<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
				422	<b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
				423
Reid Spencer	b40df56	2004-09-23 16:03:48 +0000	[diff] [blame]	424	</div>
				425
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	426	<!-- *********************************************************************** -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	427	<div class="doc_section">
				428	<a name="styleissues">Style Issues</a>
				429	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	430	<!-- *********************************************************************** -->
				431
				432
				433	<!-- ======================================================================= -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	434	<div class="doc_subsection">
				435	<a name="macro">The High Level Issues</a>
				436	</div>
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	437	<!-- ======================================================================= -->
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	438
				439
				440	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	441	<div class="doc_subsubsection">
				442	<a name="hl_module">A Public Header File <b>is</b> a Module</a>
				443	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	444
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	445	<div class="doc_text">
				446
				447	<p>C++ doesn't do too well in the modularity department. There is no real
Misha Brukman	2d10d75	2003-07-28 21:57:18 +0000	[diff] [blame]	448	encapsulation or data hiding (unless you use expensive protocol classes), but it
				449	is what we have to work with. When you write a public header file (in the LLVM
				450	source tree, they live in the top level "include" directory), you are defining a
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	451	module of functionality.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	452
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	453	<p>Ideally, modules should be completely independent of each other, and their
Misha Brukman	2d10d75	2003-07-28 21:57:18 +0000	[diff] [blame]	454	header files should only include the absolute minimum number of headers
				455	possible. A module is not just a class, a function, or a namespace: <a
				456	href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
				457	of these</a> that defines an interface. This interface may be several
				458	functions, classes or data structures, but the important issue is how they work
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	459	together.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	460
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	461	<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
				462	files. Each of these <tt>.cpp</tt> files should include the header that defines
				463	their interface first. This ensure that all of the dependences of the module
				464	header have been properly added to the module header itself, and are not
				465	implicit. System headers should be included after user headers for a
				466	translation unit.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	467
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	468	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	469
				470	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	471	<div class="doc_subsubsection">
Misha Brukman	91d64af	2004-10-26 16:18:43 +0000	[diff] [blame]	472	<a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	473	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	474
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	475	<div class="doc_text">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	476
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	477	<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
				478	have to, especially in header files.</p>
				479
				480	<p>But wait, sometimes you need to have the definition of a class to use it, or
Misha Brukman	91d64af	2004-10-26 16:18:43 +0000	[diff] [blame]	481	to inherit from it. In these cases go ahead and <tt>#include</tt> that header
				482	file. Be aware however that there are many cases where you don't need to have
				483	the full definition of a class. If you are using a pointer or reference to a
				484	class, you don't need the header file. If you are simply returning a class
				485	instance from a prototyped function or method, you don't need it. In fact, for
				486	most cases, you simply don't need the definition of a class... and not
				487	<tt>#include</tt>'ing speeds up compilation.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	488
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	489	<p>It is easy to try to go too overboard on this recommendation, however. You
Chris Lattner	5da59b1	2007-02-10 18:35:31 +0000	[diff] [blame]	490	<b>must</b> include all of the header files that you are using -- you can
				491	include them either directly
Misha Brukman	2d10d75	2003-07-28 21:57:18 +0000	[diff] [blame]	492	or indirectly (through another header file). To make sure that you don't
				493	accidently forget to include a header file in your module header, make sure to
				494	include your module header <b>first</b> in the implementation file (as mentioned
				495	above). This way there won't be any hidden dependencies that you'll find out
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	496	about later...</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	497
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	498	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	499
				500	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	501	<div class="doc_subsubsection">
				502	<a name="hl_privateheaders">Keep "internal" Headers Private</a>
				503	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	504
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	505	<div class="doc_text">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	506
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	507	<p>Many modules have a complex implementation that causes them to use more than
				508	one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
				509	internal communication interface (helper classes, extra functions, etc) in the
Misha Brukman	3b53a26	2004-12-04 00:32:12 +0000	[diff] [blame]	510	public module header file. Don't do this.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	511
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	512	<p>If you really need to do something like this, put a private header file in
				513	the same directory as the source files, and include it locally. This ensures
				514	that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	515
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	516	<p>Note however, that it's okay to put extra implementation methods a public
				517	class itself... just make them private (or protected), and all is well.</p>
				518
				519	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	520
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	521	<!-- _______________________________________________________________________ -->
				522	<div class="doc_subsubsection">
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	523	<a name="hl_earlyexit">Use Early Exits and 'continue' to Simplify Code</a>
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	524	</div>
				525
				526	<div class="doc_text">
				527
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	528	<p>When reading code, keep in mind how much state and how many previous
				529	decisions have to be remembered by the reader to understand a block of code.
				530	Aim to reduce indentation where possible when it doesn't make it more difficult
				531	to understand the code. One great way to do this is by making use of early
				532	exits and the 'continue' keyword in long loops. As an example of using an early
				533	exit from a function, consider this "bad" code:</p>
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	534
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	535	<div class="doc_code">
				536	<pre>
				537	Value DoSomething(Instruction I) {
				538	if (!isa<TerminatorInst>(I) &&
				539	I->hasOneUse() && SomeOtherThing(I)) {
				540	... some long code ....
				541	}
				542
				543	return 0;
				544	}
				545	</pre>
				546	</div>
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	547
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	548	<p>This code has several problems if the body of the 'if' is large. When you're
				549	looking at the top of the function, it isn't immediately clear that this
				550	<em>only</em> does interesting things with non-terminator instructions, and only
				551	applies to things with the other predicates. Second, it is relatively difficult
				552	to describe (in comments) why these predicates are important because the if
				553	statement makes it difficult to lay out the comments. Third, when you're deep
				554	within the body of the code, it is indented an extra level. Finally, when
				555	reading the top of the function, it isn't clear what the result is if the
				556	predicate isn't true, you have to read to the end of the function to know that
				557	it returns null.</p>
Matthijs Kooijman	ed58a97	2008-07-30 12:14:10 +0000	[diff] [blame]	558
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	559	<p>It is much preferred to format the code like this:</p>
				560
				561	<div class="doc_code">
				562	<pre>
				563	Value DoSomething(Instruction I) {
				564	// Terminators never need 'something' done to them because, ...
				565	if (isa<TerminatorInst>(I))
				566	return 0;
				567
				568	// We conservatively avoid transforming instructions with multiple uses
				569	// because goats like cheese.
				570	if (!I->hasOneUse())
				571	return 0;
				572
				573	// This is really just here for example.
				574	if (!SomeOtherThing(I))
				575	return 0;
				576
				577	... some long code ....
				578	}
				579	</pre>
				580	</div>
				581
				582	<p>This fixes these problems. A similar problem frequently happens in for
				583	loops. A silly example is something like this:</p>
				584
				585	<div class="doc_code">
				586	<pre>
				587	for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
				588	if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
				589	Value *LHS = BO->getOperand(0);
				590	Value *RHS = BO->getOperand(1);
				591	if (LHS != RHS) {
				592	...
				593	}
				594	}
				595	}
				596	</pre>
				597	</div>
				598
				599	<p>When you have very very small loops, this sort of structure is fine, but if
				600	it exceeds more than 10-15 lines, it becomes difficult for people to read and
				601	understand at a glance.
				602	The problem with this sort of code is that it gets very nested very quickly,
				603	meaning that the reader of the code has to keep a lot of context in their brain
				604	to remember what is going immediately on in the loop, because they don't know
				605	if/when the if conditions will have elses etc. It is strongly preferred to
				606	structure the loop like this:</p>
				607
				608	<div class="doc_code">
				609	<pre>
				610	for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
				611	BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
				612	if (!BO) continue;
				613
				614	Value *LHS = BO->getOperand(0);
				615	Value *RHS = BO->getOperand(1);
				616	if (LHS == RHS) continue;
				617	}
				618	</pre>
				619	</div>
				620
				621	<p>This has all the benefits of using early exits from functions: it reduces
				622	nesting of the loop, it makes it easier to describe why the conditions are true,
				623	and it makes it obvious to the reader that there is no "else" coming up that
				624	they have to push context into their brain for. If a loop is large, this can
				625	be a big understandability win.</p>
				626
				627	</div>
				628
Chris Lattner	9eb7e0a	2009-07-28 22:54:04 +0000	[diff] [blame]	629	<!-- _______________________________________________________________________ -->
				630	<div class="doc_subsubsection">
				631	<a name="hl_else_after_return">Don't use "else" after a return</a>
				632	</div>
				633
				634	<div class="doc_text">
				635
				636	<p>For similar reasons above (reduction of indentation and easier reading),
				637	please do not use "else" or "else if" after something that interrupts
				638	control flow like return, break, continue, goto, etc. For example, this is
				639	"bad":</p>
				640
				641	<div class="doc_code">
				642	<pre>
				643	case 'J': {
				644	if (Signed) {
				645	Type = Context.getsigjmp_bufType();
				646	if (Type.isNull()) {
				647	Error = ASTContext::GE_Missing_sigjmp_buf;
				648	return QualType();
				649	} else {
				650	break;
				651	}
				652	} else {
				653	Type = Context.getjmp_bufType();
				654	if (Type.isNull()) {
				655	Error = ASTContext::GE_Missing_jmp_buf;
				656	return QualType();
				657	} else {
				658	break;
				659	}
				660	}
				661	}
				662	}
				663	</pre>
				664	</div>
				665
				666	<p>It is better to write this something like:</p>
				667
				668	<div class="doc_code">
				669	<pre>
				670	case 'J':
				671	if (Signed) {
				672	Type = Context.getsigjmp_bufType();
				673	if (Type.isNull()) {
				674	Error = ASTContext::GE_Missing_sigjmp_buf;
				675	return QualType();
				676	}
				677	} else {
				678	Type = Context.getjmp_bufType();
				679	if (Type.isNull()) {
				680	Error = ASTContext::GE_Missing_jmp_buf;
				681	return QualType();
				682	}
				683	}
				684	break;
				685	</pre>
				686	</div>
				687
				688	<p>Or better yet (in this case), as:</p>
				689
				690	<div class="doc_code">
				691	<pre>
				692	case 'J':
				693	if (Signed)
				694	Type = Context.getsigjmp_bufType();
				695	else
				696	Type = Context.getjmp_bufType();
				697
				698	if (Type.isNull()) {
				699	Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
				700	ASTContext::GE_Missing_jmp_buf;
				701	return QualType();
				702	}
				703	break;
				704	</pre>
				705	</div>
				706
				707	<p>The idea is to reduce indentation and the amount of code you have to keep
				708	track of when reading the code.</p>
				709
				710	</div>
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	711
				712	<!-- _______________________________________________________________________ -->
				713	<div class="doc_subsubsection">
				714	<a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
				715	</div>
				716
				717	<div class="doc_text">
				718
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	719	<p>It is very common to write small loops that just compute a boolean
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	720	value. There are a number of ways that people commonly write these, but an
				721	example of this sort of thing is:</p>
				722
				723	<div class="doc_code">
				724	<pre>
				725	<b>bool FoundFoo = false;</b>
				726	for (unsigned i = 0, e = BarList.size(); i != e; ++i)
				727	if (BarList[i]->isFoo()) {
				728	<b>FoundFoo = true;</b>
				729	break;
				730	}
				731
				732	<b>if (FoundFoo) {</b>
				733	...
				734	}
				735	</pre>
				736	</div>
				737
				738	<p>This sort of code is awkward to write, and is almost always a bad sign.
				739	Instead of this sort of loop, we strongly prefer to use a predicate function
				740	(which may be <a href="#micro_anonns">static</a>) that uses
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	741	<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
				742	the code to be structured like this:
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	743	</p>
				744
				745
				746	<div class="doc_code">
				747	<pre>
				748	/// ListContainsFoo - Return true if the specified list has an element that is
				749	/// a foo.
				750	static bool ListContainsFoo(const std::vector<Bar*> &List) {
				751	for (unsigned i = 0, e = List.size(); i != e; ++i)
				752	if (List[i]->isFoo())
				753	return true;
				754	return false;
				755	}
				756	...
				757
				758	<b>if (ListContainsFoo(BarList)) {</b>
				759	...
				760	}
				761	</pre>
				762	</div>
				763
				764	<p>There are many reasons for doing this: it reduces indentation and factors out
				765	code which can often be shared by other code that checks for the same predicate.
				766	More importantly, it <em>forces you to pick a name</em> for the function, and
				767	forces you to write a comment for it. In this silly example, this doesn't add
				768	much value. However, if the condition is complex, this can make it a lot easier
				769	for the reader to understand the code that queries for this predicate. Instead
Chris Lattner	71d8f3b	2009-07-22 16:30:39 +0000	[diff] [blame]	770	of being faced with the in-line details of how we check to see if the BarList
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	771	contains a foo, we can trust the function name and continue reading with better
				772	locality.</p>
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	773
				774	</div>
				775
				776
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	777	<!-- ======================================================================= -->
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	778	<div class="doc_subsection">
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	779	<a name="micro">The Low Level Issues</a>
				780	</div>
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	781	<!-- ======================================================================= -->
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	782
				783
				784	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	785	<div class="doc_subsubsection">
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	786	<a name="ll_assert">Assert Liberally</a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	787	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	788
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	789	<div class="doc_text">
				790
				791	<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
Misha Brukman	2d10d75	2003-07-28 21:57:18 +0000	[diff] [blame]	792	preconditions and assumptions, you never know when a bug (not neccesarily even
				793	yours) might be caught early by an assertion, which reduces debugging time
				794	dramatically. The "<tt><cassert></tt>" header file is probably already
				795	included by the header files you are using, so it doesn't cost anything to use
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	796	it.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	797
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	798	<p>To further assist with debugging, make sure to put some kind of error message
				799	in the assertion statement (which is printed if the assertion is tripped). This
Misha Brukman	2d10d75	2003-07-28 21:57:18 +0000	[diff] [blame]	800	helps the poor debugging make sense of why an assertion is being made and
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	801	enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	802
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	803	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	804	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	805	inline Value *getOperand(unsigned i) {
				806	assert(i < Operands.size() && "getOperand() out of range!");
				807	return Operands[i];
				808	}
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	809	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	810	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	811
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	812	<p>Here are some examples:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	813
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	814	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	815	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	816	assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	817
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	818	assert((Opcode == Shl \|\| Opcode == Shr) && "ShiftInst Opcode invalid!");
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	819
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	820	assert(idx < getNumSuccessors() && "Successor # out of range!");
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	821
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	822	assert(V1.getType() == V2.getType() && "Constant types must be identical!");
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	823
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	824	assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	825	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	826	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	827
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	828	<p>You get the idea...</p>
				829
Nick Lewycky	f4408e8	2008-05-31 23:54:55 +0000	[diff] [blame]	830	<p>Please be aware when adding assert statements that not all compilers are aware of
				831	the semantics of the assert. In some places, asserts are used to indicate a piece of
				832	code that should not be reached. These are typically of the form:</p>
				833
				834	<div class="doc_code">
				835	<pre>
Dan Gohman	f003276	2008-11-24 17:18:39 +0000	[diff] [blame]	836	assert(0 && "Some helpful error message");
Nick Lewycky	f4408e8	2008-05-31 23:54:55 +0000	[diff] [blame]	837	</pre>
				838	</div>
				839
				840	<p>When used in a function that returns a value, they should be followed with a return
				841	statement and a comment indicating that this line is never reached. This will prevent
				842	a compiler which is unable to deduce that the assert statement never returns from
				843	generating a warning.</p>
				844
				845	<div class="doc_code">
				846	<pre>
Dan Gohman	f003276	2008-11-24 17:18:39 +0000	[diff] [blame]	847	assert(0 && "Some helpful error message");
Nick Lewycky	f4408e8	2008-05-31 23:54:55 +0000	[diff] [blame]	848	// Not reached
				849	return 0;
				850	</pre>
				851	</div>
				852
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	853	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	854
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	855	<!-- _______________________________________________________________________ -->
				856	<div class="doc_subsubsection">
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	857	<a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	858	</div>
				859
				860	<div class="doc_text">
				861	<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	862	namespace with an "<tt>std::</tt>" prefix, rather than rely on
				863	"<tt>using namespace std;</tt>".</p>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	864
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	865	<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
Misha Brukman	2d33c54	2008-12-11 19:37:04 +0000	[diff] [blame]	866	the namespace of any source file that <tt>#include</tt>s the header. This is
				867	clearly a bad thing.</p>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	868
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	869	<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
				870	rule, but is still important. Basically, using explicit namespace prefixes
				871	makes the code <b>clearer</b>, because it is immediately obvious what facilities
				872	are being used and where they are coming from, and <b>more portable</b>, because
				873	namespace clashes cannot occur between LLVM code and other namespaces. The
				874	portability rule is important because different standard library implementations
				875	expose different symbols (potentially ones they shouldn't), and future revisions
				876	to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
				877	such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	878
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	879	<p>The exception to the general rule (i.e. it's not an exception for
				880	the <tt>std</tt> namespace) is for implementation files. For example, all of
				881	the code in the LLVM project implements code that lives in the 'llvm' namespace.
				882	As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
				883	namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
				884	general form of this rule is that any .cpp file that implements code in any
				885	namespace may use that namespace (and its parents'), but should not use any
				886	others.</p>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	887
				888	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	889
				890	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	891	<div class="doc_subsubsection">
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	892	<a name="ll_virtual_anch">Provide a virtual method anchor for classes
				893	in headers</a>
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	894	</div>
				895
				896	<div class="doc_text">
				897
				898	<p>If a class is defined in a header file and has a v-table (either it has
				899	virtual methods or it derives from classes with virtual methods), it must
				900	always have at least one out-of-line virtual method in the class. Without
Misha Brukman	2d33c54	2008-12-11 19:37:04 +0000	[diff] [blame]	901	this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
				902	that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
				903	increasing link times.</p>
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	904
				905	</div>
				906
Chris Lattner	0af39ea	2009-06-30 06:13:23 +0000	[diff] [blame]	907	<!-- _______________________________________________________________________ -->
				908	<div class="doc_subsubsection">
				909	<a name="ll_end">Don't evaluate end() every time through a loop</a>
				910	</div>
				911
				912	<div class="doc_text">
				913
				914	<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
				915	with macros and may be coming in C++'0x) we end up writing a lot of loops that
				916	manually iterate from begin to end on a variety of containers or through other
				917	data structures. One common mistake is to write a loop in this style:</p>
				918
				919	<div class="doc_code">
				920	<pre>
				921	BasicBlock *BB = ...
				922	for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
				923	... use I ...
				924	</pre>
				925	</div>
				926
				927	<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
				928	every time through the loop. Instead of writing the loop like this, we strongly
				929	prefer loops to be written so that they evaluate it once before the loop starts.
				930	A convenient way to do this is like so:</p>
				931
				932	<div class="doc_code">
				933	<pre>
				934	BasicBlock *BB = ...
				935	for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
				936	... use I ...
				937	</pre>
				938	</div>
				939
				940	<p>The observant may quickly point out that these two loops may have different
				941	semantics: if the container (a basic block in this case) is being mutated, then
				942	"<tt>BB->end()</tt>" may change its value every time through the loop and the
				943	second loop may not in fact be correct. If you actually do depend on this
Chris Lattner	09bc65e	2009-06-30 06:20:03 +0000	[diff] [blame]	944	behavior, please write the loop in the first form and add a comment indicating
Chris Lattner	0af39ea	2009-06-30 06:13:23 +0000	[diff] [blame]	945	that you did it intentionally.</p>
				946
				947	<p>Why do we prefer the second form (when correct)? Writing the loop in the
				948	first form has two problems: First it may be less efficient than evaluating it
				949	at the start of the loop. In this case, the cost is probably minor: a few extra
				950	loads every time through the loop. However, if the base expression is more
				951	complex, then the cost can rise quickly. I've seen loops where the end
				952	expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
Chris Lattner	a44f87f	2009-06-30 06:27:54 +0000	[diff] [blame]	953	lookups really aren't cheap. By writing it in the second form consistently, you
Chris Lattner	0af39ea	2009-06-30 06:13:23 +0000	[diff] [blame]	954	eliminate the issue entirely and don't even have to think about it.</p>
				955
Chris Lattner	09bc65e	2009-06-30 06:20:03 +0000	[diff] [blame]	956	<p>The second (even bigger) issue is that writing the loop in the first form
Chris Lattner	0af39ea	2009-06-30 06:13:23 +0000	[diff] [blame]	957	hints to the reader that the loop is mutating the container (a fact that a
				958	comment would handily confirm!). If you write the loop in the second form, it
				959	is immediately obvious without even looking at the body of the loop that the
				960	container isn't being modified, which makes it easier to read the code and
				961	understand what it does.</p>
				962
				963	<p>While the second form of the loop is a few extra keystrokes, we do strongly
				964	prefer it.</p>
				965
				966	</div>
				967
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	968	<!-- _______________________________________________________________________ -->
				969	<div class="doc_subsubsection">
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	970	<a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	971	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	972
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	973	<div class="doc_text">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	974
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	975	<p>The use of <tt>#include <iostream></tt> in library files is
				976	hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
				977	support clients using LLVM libraries as part of larger systems. In particular,
				978	we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
				979	the static c'tors are run whenever an application start up that uses the dynamic
				980	library. There are two problems with this:</p>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	981
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	982	<ol>
				983	<li>The time to run the static c'tors impacts startup time of
				984	applications—a critical time for GUI apps.</li>
				985	<li>The static c'tors cause the app to pull many extra pages of memory off the
				986	disk: both the code for the static c'tors in each <tt>.o</tt> file and the
				987	small amount of data that gets touched. In addition, touched/dirty pages
				988	put more pressure on the VM system on low-memory machines.</li>
				989	</ol>
				990
				991	<p>Note that using the other stream headers (<tt><sstream></tt> for
				992	example) is allowed normally, it is just <tt><iostream></tt> that is
				993	causing problems.</p>
				994
Daniel Dunbar	e8530a3	2009-07-24 23:04:51 +0000	[diff] [blame]	995	<p>In addition, new code should always
				996	use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> or
				997	the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	998
				999	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1000
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	1001
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1002	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1003	<div class="doc_subsubsection">
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	1004	<a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1005	</div>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	1006
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1007	<div class="doc_text">
				1008
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	1009	<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
				1010	to the output stream specified. In addition to doing this, however, it also
				1011	flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	1012
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	1013	<div class="doc_code">
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	1014	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	1015	std::cout << std::endl;
				1016	std::cout << '\n' << std::flush;
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	1017	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	1018	</div>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	1019
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1020	<p>Most of the time, you probably have no reason to flush the output stream, so
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	1021	it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	1022
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1023	</div>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	1024
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	1025
Daniel Dunbar	e8530a3	2009-07-24 23:04:51 +0000	[diff] [blame]	1026	<!-- _______________________________________________________________________ -->
				1027	<div class="doc_subsubsection">
				1028	<a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
				1029	</div>
				1030
				1031	<div class="doc_text">
				1032
				1033	<p>LLVM includes a lightweight, simple, and efficient stream implementation
				1034	in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
Daniel Dunbar	26fe866	2009-07-24 23:54:34 +0000	[diff] [blame]	1035	of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> instead
				1036	of <tt>ostream</tt>.</p>
Daniel Dunbar	e8530a3	2009-07-24 23:04:51 +0000	[diff] [blame]	1037
Daniel Dunbar	26fe866	2009-07-24 23:54:34 +0000	[diff] [blame]	1038	<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
Daniel Dunbar	e8530a3	2009-07-24 23:04:51 +0000	[diff] [blame]	1039	be forward declared as <tt>class raw_ostream</tt>. Public headers should
				1040	generally not include the <tt>raw_ostream</tt> header, but use forward
				1041	declarations and constant references to <tt>raw_ostream</tt> instances.</p>
				1042
				1043	</div>
				1044
				1045
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	1046	<!-- ======================================================================= -->
				1047	<div class="doc_subsection">
				1048	<a name="nano">Microscopic Details</a>
				1049	</div>
				1050	<!-- ======================================================================= -->
				1051
				1052	<p>This section describes preferred low-level formatting guidelines along with
				1053	reasoning on why we prefer them.</p>
				1054
				1055	<!-- _______________________________________________________________________ -->
				1056	<div class="doc_subsubsection">
				1057	<a name="micro_spaceparen">Spaces Before Parentheses</a>
				1058	</div>
				1059
				1060	<div class="doc_text">
				1061
				1062	<p>We prefer to put a space before a parentheses only in control flow
				1063	statements, but not in normal function call expressions and function-like
				1064	macros. For example, this is good:</p>
				1065
				1066	<div class="doc_code">
				1067	<pre>
				1068	<b>if (</b>x) ...
				1069	<b>for (</b>i = 0; i != 100; ++i) ...
				1070	<b>while (</b>llvm_rocks) ...
				1071
				1072	<b>somefunc(</b>42);
				1073	<b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me");
				1074
				1075	a = <b>foo(</b>42, 92) + <b>bar(</b>x);
				1076	</pre>
				1077	</div>
				1078
				1079	<p>... and this is bad:</p>
				1080
				1081	<div class="doc_code">
				1082	<pre>
				1083	<b>if(</b>x) ...
				1084	<b>for(</b>i = 0; i != 100; ++i) ...
				1085	<b>while(</b>llvm_rocks) ...
				1086
				1087	<b>somefunc (</b>42);
				1088	<b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me");
				1089
				1090	a = <b>foo (</b>42, 92) + <b>bar (</b>x);
				1091	</pre>
				1092	</div>
				1093
				1094	<p>The reason for doing this is not completely arbitrary. This style makes
				1095	control flow operators stand out more, and makes expressions flow better. The
				1096	function call operator binds very tightly as a postfix operator. Putting
				1097	a space after a function name (as in the last example) makes it appear that
				1098	the code might bind the arguments of the left-hand-side of a binary operator
				1099	with the argument list of a function and the name of the right side. More
				1100	specifically, it is easy to misread the "a" example as:</p>
				1101
				1102	<div class="doc_code">
				1103	<pre>
				1104	a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
				1105	</pre>
				1106	</div>
				1107
				1108	<p>... when skimming through the code. By avoiding a space in a function, we
				1109	avoid this misinterpretation.</p>
				1110
				1111	</div>
				1112
				1113	<!-- _______________________________________________________________________ -->
				1114	<div class="doc_subsubsection">
				1115	<a name="micro_preincrement">Prefer Preincrement</a>
				1116	</div>
				1117
				1118	<div class="doc_text">
				1119
				1120	<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
				1121	postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
				1122	preincrementation whenever possible.</p>
				1123
				1124	<p>The semantics of postincrement include making a copy of the value being
				1125	incremented, returning it, and then preincrementing the "work value". For
				1126	primitive types, this isn't a big deal... but for iterators, it can be a huge
				1127	issue (for example, some iterators contains stack and set objects in them...
				1128	copying an iterator could invoke the copy ctor's of these as well). In general,
				1129	get in the habit of always using preincrement, and you won't have a problem.</p>
				1130
				1131	</div>
				1132
				1133	<!-- _______________________________________________________________________ -->
				1134	<div class="doc_subsubsection">
				1135	<a name="micro_namespaceindent">Namespace Indentation</a>
				1136	</div>
				1137
				1138	<div class="doc_text">
				1139
				1140	<p>
				1141	In general, we strive to reduce indentation where ever possible. This is useful
				1142	because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
				1143	wrapping horribly, but also because it makes it easier to understand the code.
				1144	Namespaces are a funny thing: they are often large, and we often desire to put
				1145	lots of stuff into them (so they can be large). Other times they are tiny,
				1146	because they just hold an enum or something similar. In order to balance this,
				1147	we use different approaches for small versus large namespaces.
				1148	</p>
				1149
				1150	<p>
				1151	If a namespace definition is small and <em>easily</em> fits on a screen (say,
				1152	less than 35 lines of code), then you should indent its body. Here's an
				1153	example:
				1154	</p>
				1155
				1156	<div class="doc_code">
				1157	<pre>
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	1158	namespace llvm {
				1159	namespace X86 {
				1160	/// RelocationType - An enum for the x86 relocation codes. Note that
				1161	/// the terminology here doesn't follow x86 convention - word means
				1162	/// 32-bit and dword means 64-bit.
				1163	enum RelocationType {
				1164	/// reloc_pcrel_word - PC relative relocation, add the relocated value to
				1165	/// the value already in memory, after we adjust it for where the PC is.
				1166	reloc_pcrel_word = 0,
				1167
				1168	/// reloc_picrel_word - PIC base relative relocation, add the relocated
				1169	/// value to the value already in memory, after we adjust it for where the
				1170	/// PIC base is.
				1171	reloc_picrel_word = 1,
				1172
				1173	/// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
				1174	/// add the relocated value to the value already in memory.
				1175	reloc_absolute_word = 2,
				1176	reloc_absolute_dword = 3
				1177	};
				1178	}
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	1179	}
				1180	</pre>
				1181	</div>
				1182
				1183	<p>Since the body is small, indenting adds value because it makes it very clear
				1184	where the namespace starts and ends, and it is easy to take the whole thing in
				1185	in one "gulp" when reading the code. If the blob of code in the namespace is
				1186	larger (as it typically is in a header in the llvm or clang namespaces), do not
				1187	indent the code, and add a comment indicating what namespace is being closed.
				1188	For example:</p>
				1189
				1190	<div class="doc_code">
				1191	<pre>
				1192	namespace llvm {
				1193	namespace knowledge {
				1194
				1195	/// Grokable - This class represents things that Smith can have an intimate
				1196	/// understanding of and contains the data associated with it.
				1197	class Grokable {
				1198	...
				1199	public:
				1200	explicit Grokable() { ... }
				1201	virtual ~Grokable() = 0;
				1202
				1203	...
				1204
				1205	};
				1206
				1207	} // end namespace knowledge
				1208	} // end namespace llvm
				1209	</pre>
				1210	</div>
				1211
				1212	<p>Because the class is large, we don't expect that the reader can easily
				1213	understand the entire concept in a glance, and the end of the file (where the
				1214	namespaces end) may be a long ways away from the place they open. As such,
				1215	indenting the contents of the namespace doesn't add any value, and detracts from
				1216	the readability of the class. In these cases it is best to <em>not</em> indent
				1217	the contents of the namespace.</p>
				1218
				1219	</div>
				1220
				1221	<!-- _______________________________________________________________________ -->
				1222	<div class="doc_subsubsection">
				1223	<a name="micro_anonns">Anonymous Namespaces</a>
				1224	</div>
				1225
				1226	<div class="doc_text">
				1227
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	1228	<p>After talking about namespaces in general, you may be wondering about
				1229	anonymous namespaces in particular.
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	1230	Anonymous namespaces are a great language feature that tells the C++ compiler
				1231	that the contents of the namespace are only visible within the current
				1232	translation unit, allowing more aggressive optimization and eliminating the
				1233	possibility of symbol name collisions. Anonymous namespaces are to C++ as
				1234	"static" is to C functions and global variables. While "static" is available
				1235	in C++, anonymous namespaces are more general: they can make entire classes
				1236	private to a file.</p>
				1237
				1238	<p>The problem with anonymous namespaces is that they naturally want to
				1239	encourage indentation of their body, and they reduce locality of reference: if
				1240	you see a random function definition in a C++ file, it is easy to see if it is
				1241	marked static, but seeing if it is in an anonymous namespace requires scanning
				1242	a big chunk of the file.</p>
				1243
				1244	<p>Because of this, we have a simple guideline: make anonymous namespaces as
				1245	small as possible, and only use them for class declarations. For example, this
				1246	is good:</p>
				1247
				1248	<div class="doc_code">
				1249	<pre>
				1250	<b>namespace {</b>
				1251	class StringSort {
				1252	...
				1253	public:
				1254	StringSort(...)
				1255	bool operator<(const char *RHS) const;
				1256	};
				1257	<b>} // end anonymous namespace</b>
				1258
				1259	static void Helper() {
				1260	...
				1261	}
				1262
				1263	bool StringSort::operator<(const char *RHS) const {
				1264	...
				1265	}
				1266
				1267	</pre>
				1268	</div>
				1269
				1270	<p>This is bad:</p>
				1271
				1272
				1273	<div class="doc_code">
				1274	<pre>
				1275	<b>namespace {</b>
				1276	class StringSort {
				1277	...
				1278	public:
				1279	StringSort(...)
				1280	bool operator<(const char *RHS) const;
				1281	};
				1282
				1283	void Helper() {
				1284	...
				1285	}
				1286
				1287	bool StringSort::operator<(const char *RHS) const {
				1288	...
				1289	}
				1290
				1291	<b>} // end anonymous namespace</b>
				1292
				1293	</pre>
				1294	</div>
				1295
				1296
				1297	<p>This is bad specifically because if you're looking at "Helper" in the middle
				1298	of a large C++ file, that you have no immediate way to tell if it is local to
				1299	the file. When it is marked static explicitly, this is immediately obvious.
				1300	Also, there is no reason to enclose the definition of "operator<" in the
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	1301	namespace just because it was declared there.
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	1302	</p>
				1303
				1304	</div>
				1305
				1306
				1307
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1308	<!-- *********************************************************************** -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1309	<div class="doc_section">
				1310	<a name="seealso">See Also</a>
				1311	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1312	<!-- *********************************************************************** -->
				1313
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1314	<div class="doc_text">
				1315
				1316	<p>A lot of these comments and recommendations have been culled for other
				1317	sources. Two particularly important books for our work are:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1318
				1319	<ol>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1320
Chris Lattner	788ee28	2007-11-09 21:49:08 +0000	[diff] [blame]	1321	<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
				1322	C++</a> by Scott Meyers. Also
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	1323	interesting and useful are "More Effective C++" and "Effective STL" by the same
				1324	author.</li>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1325
Chris Lattner	788ee28	2007-11-09 21:49:08 +0000	[diff] [blame]	1326	<li>Large-Scale C++ Software Design by John Lakos</li>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1327
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1328	</ol>
				1329
				1330	<p>If you get some free time, and you haven't read them: do so, you might learn
Misha Brukman	3b53a26	2004-12-04 00:32:12 +0000	[diff] [blame]	1331	something.</p>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1332
				1333	</div>
				1334
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1335	<!-- *********************************************************************** -->
				1336
				1337	<hr>
Misha Brukman	6c16c4b	2004-05-12 18:37:22 +0000	[diff] [blame]	1338	<address>
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Misha Brukman	4440870	2008-12-11 17:34:48 +0000	[diff] [blame]	1340	src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman	6c16c4b	2004-05-12 18:37:22 +0000	[diff] [blame]	1341	<a href="http://validator.w3.org/check/referer"><img
Misha Brukman	4440870	2008-12-11 17:34:48 +0000	[diff] [blame]	1342	src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1343
Misha Brukman	6c16c4b	2004-05-12 18:37:22 +0000	[diff] [blame]	1344	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer	05fe4b0	2006-03-14 05:39:39 +0000	[diff] [blame]	1345	<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1346	Last modified: $Date$
Misha Brukman	6c16c4b	2004-05-12 18:37:22 +0000	[diff] [blame]	1347	</address>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1348
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