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Misha Brukman	75f258e	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	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>
				46	<li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
				47	Functions</a></li>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	48	</ol></li>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	49	<li><a href="#micro">The Low Level Issues</a>
				50	<ol>
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	51	<li><a href="#ll_assert">Assert Liberally</a></li>
				52	<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]	53	<li><a href="#ll_virtual_anch">Provide a virtual method anchor for
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	54	classes in headers</a></li>
Chris Lattner	0af39ea	2009-06-30 06:13:23 +0000	[diff] [blame]	55	<li><a href="#ll_end">Don't evaluate end() every time through a
				56	loop</a></li>
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	57	<li><a href="#ll_iostream"><tt>#include <iostream></tt> is
				58	<em>forbidden</em></a></li>
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	59	<li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
Daniel Dunbar	e8530a3	2009-07-24 23:04:51 +0000	[diff] [blame]	60	<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]	61	</ol></li>
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	62
				63	<li><a href="#nano">Microscopic Details</a>
				64	<ol>
				65	<li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
				66	<li><a href="#micro_preincrement">Prefer Preincrement</a></li>
				67	<li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
				68	<li><a href="#micro_anonns">Anonymous Namespaces</a></li>
				69	</ol></li>
				70
				71
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	72	</ol></li>
				73	<li><a href="#seealso">See Also</a></li>
				74	</ol>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	75
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	76	<div class="doc_author">
Chris Lattner	219bd29	2009-07-22 05:43:01 +0000	[diff] [blame]	77	<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]	78	</div>
				79
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	80
				81	<!-- *********************************************************************** -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	82	<div class="doc_section">
				83	<a name="introduction">Introduction</a>
				84	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	85	<!-- *********************************************************************** -->
				86
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	87	<div class="doc_text">
				88
				89	<p>This document attempts to describe a few coding standards that are being used
				90	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]	91	absolute requirements to be followed in all instances, coding standards can be
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	92	useful.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	93
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	94	<p>This document intentionally does not prescribe fixed standards for religious
Misha Brukman	1b36d67	2003-10-06 19:26:00 +0000	[diff] [blame]	95	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]	96	the golden rule:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	97
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	98	<blockquote>
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	<p><b><a name="goldenrule">If you are adding a significant body of source to a
				101	project, feel free to use whatever style you are most comfortable with. If you
				102	are extending, enhancing, or bug fixing already implemented code, use the style
				103	that is already being used so that the source is uniform and easy to
				104	follow.</a></b></p>
				105
				106	</blockquote>
				107
				108	<p>The ultimate goal of these guidelines is the increase readability and
Misha Brukman	1b36d67	2003-10-06 19:26:00 +0000	[diff] [blame]	109	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]	110	be included, please mail them to <a
				111	href="mailto:sabre@nondot.org">Chris</a>.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	112
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	113	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	114
				115	<!-- *********************************************************************** -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	116	<div class="doc_section">
				117	<a name="mechanicalissues">Mechanical Source Issues</a>
				118	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	119	<!-- *********************************************************************** -->
				120
				121	<!-- ======================================================================= -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	122	<div class="doc_subsection">
				123	<a name="sourceformating">Source Code Formatting</a>
				124	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	125
				126	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	127	<div class="doc_subsubsection">
				128	<a name="scf_commenting">Commenting</a>
				129	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	130
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	131	<div class="doc_text">
				132
				133	<p>Comments are one critical part of readability and maintainability. Everyone
Chris Lattner	b927ca8	2009-07-12 00:10:24 +0000	[diff] [blame]	134	knows they should comment, so should you. When writing comments, write them as
				135	English prose, which means they should use proper capitalization, punctuation,
				136	etc. Although we all should probably
Misha Brukman	1b36d67	2003-10-06 19:26:00 +0000	[diff] [blame]	137	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]	138	documentation is very useful:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	139
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	140	<b>File Headers</b>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	141
Reid Spencer	669ed45	2007-07-09 08:04:31 +0000	[diff] [blame]	142	<p>Every source file should have a header on it that describes the basic
				143	purpose of the file. If a file does not have a header, it should not be
				144	checked into Subversion. Most source trees will probably have a standard
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	145	file header format. The standard format for the LLVM source tree looks like
				146	this:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	147
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	148	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	149	<pre>
Chris Lattner	c49fcd1	2003-10-13 14:58:11 +0000	[diff] [blame]	150	//===-- llvm/Instruction.h - Instruction class definition -------- C++ --===//
Misha Brukman	98117cd	2009-01-02 16:58:42 +0000	[diff] [blame]	151	//
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	152	// The LLVM Compiler Infrastructure
				153	//
Chris Lattner	ab2b10c	2007-12-29 19:56:08 +0000	[diff] [blame]	154	// This file is distributed under the University of Illinois Open Source
				155	// License. See LICENSE.TXT for details.
Misha Brukman	98117cd	2009-01-02 16:58:42 +0000	[diff] [blame]	156	//
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	157	//===----------------------------------------------------------------------===//
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	158	//
				159	// This file contains the declaration of the Instruction class, which is the
				160	// base class for all of the VM instructions.
				161	//
				162	//===----------------------------------------------------------------------===//
				163	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	164	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	165
Chris Lattner	ab2b10c	2007-12-29 19:56:08 +0000	[diff] [blame]	166	<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]	167	-*-</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]	168	is a C++ file, not a C file (Emacs assumes .h files are C files by default).
				169	Note that this tag is not necessary in .cpp files. The name of the file is also
				170	on the first line, along with a very short description of the purpose of the
				171	file. This is important when printing out code and flipping though lots of
				172	pages.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	173
Chris Lattner	ab2b10c	2007-12-29 19:56:08 +0000	[diff] [blame]	174	<p>The next section in the file is a concise note that defines the license
				175	that the file is released under. This makes it perfectly clear what terms the
				176	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]	177
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	178	<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]	179	Here it's only two lines. If an algorithm is being implemented or something
				180	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]	181	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]	182
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	183	<b>Class overviews</b>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	184
Jim Laskey	f55914a	2006-07-31 20:18:49 +0000	[diff] [blame]	185	<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]	186	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]	187	used for... if it's not obvious. If it's so completely obvious your grandma
				188	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]	189	something sane goes a long ways towards avoiding writing documentation.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	190
				191
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	192	<b>Method information</b>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	193
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	194	<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]	195	documented properly. A quick note about what it does any a description of the
				196	borderline behaviour is all that is necessary here (unless something
				197	particularly tricky or insideous is going on). The hope is that people can
				198	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]	199	the goal metric.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	200
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	201	<p>Good things to talk about here are what happens when something unexpected
				202	happens: does the method return null? Abort? Format your hard disk?</p>
				203
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	204	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	205
				206	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	207	<div class="doc_subsubsection">
				208	<a name="scf_commentformat">Comment Formatting</a>
				209	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	210
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	211	<div class="doc_text">
				212
				213	<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]	214	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]	215	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]	216
				217	<ol>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	218	<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]	219	comments.</li>
Misha Brukman	91d64af	2004-10-26 16:18:43 +0000	[diff] [blame]	220	<li>When writing a header file that may be <tt>#include</tt>d by a C source
				221	file.</li>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	222	<li>When writing a source file that is used by a tool that only accepts C
				223	style comments.</li>
				224	</ol>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	225
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	226	<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
				227	These nest properly and are better behaved in general than C style comments.</p>
				228
				229	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	230
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	231	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	232	<div class="doc_subsubsection">
Misha Brukman	91d64af	2004-10-26 16:18:43 +0000	[diff] [blame]	233	<a name="scf_includes"><tt>#include</tt> Style</a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	234	</div>
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	235
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	236	<div class="doc_text">
				237
				238	<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]	239	include guards if working on a header file), the <a
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	240	href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
				241	file should be listed. We prefer these <tt>#include</tt>s to be listed in this
				242	order:</p>
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	243
				244	<ol>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	245	<li><a href="#mmheader">Main Module header</a></li>
				246	<li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	247	<li><tt>llvm/*</tt></li>
				248	<li><tt>llvm/Analysis/*</tt></li>
				249	<li><tt>llvm/Assembly/*</tt></li>
				250	<li><tt>llvm/Bytecode/*</tt></li>
				251	<li><tt>llvm/CodeGen/*</tt></li>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	252	<li>...</li>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	253	<li><tt>Support/*</tt></li>
				254	<li><tt>Config/*</tt></li>
				255	<li>System <tt>#includes</tt></li>
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	256	</ol>
				257
Nick Lewycky	822293a	2008-11-29 20:13:25 +0000	[diff] [blame]	258	<p>... and each category should be sorted by name.</p>
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	259
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	260	<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]	261	which implement an interface defined by a .h file. This <tt>#include</tt>
				262	should always be included <b>first</b> regardless of where it lives on the file
				263	system. By including a header file first in the .cpp files that implement the
				264	interfaces, we ensure that the header does not have any hidden dependencies
				265	which are not explicitly #included in the header, but should be. It is also a
				266	form of documentation in the .cpp file to indicate where the interfaces it
				267	implements are defined.</p>
Chris Lattner	2e5cbc2	2003-08-07 21:45:47 +0000	[diff] [blame]	268
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	269	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	270
				271	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	272	<div class="doc_subsubsection">
				273	<a name="scf_codewidth">Source Code Width</a>
				274	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	275
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	276	<div class="doc_text">
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	<p>Write your code to fit within 80 columns of text. This helps those of us who
				279	like to print out code and look at your code in an xterm without resizing
				280	it.</p>
				281
Chris Lattner	5822e9e	2008-07-08 05:12:37 +0000	[diff] [blame]	282	<p>The longer answer is that there must be some limit to the width of the code
				283	in order to reasonably allow developers to have multiple files side-by-side in
				284	windows on a modest display. If you are going to pick a width limit, it is
				285	somewhat arbitrary but you might as well pick something standard. Going with
				286	90 columns (for example) instead of 80 columns wouldn't add any significant
				287	value and would be detrimental to printing out code. Also many other projects
				288	have standardized on 80 columns, so some people have already configured their
				289	editors for it (vs something else, like 90 columns).</p>
				290
				291	<p>This is one of many contentious issues in coding standards, but is not up
				292	for debate.</p>
				293
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	294	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	295
				296	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	297	<div class="doc_subsubsection">
				298	<a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
				299	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	300
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	301	<div class="doc_text">
				302
				303	<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]	304	prefered indentation levels, and different styles of indentation that they
				305	like... this is fine. What isn't is that different editors/viewers expand tabs
				306	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]	307	unreadable, and it is not worth dealing with.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	308
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	309	<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]	310	style of existing code if your are modifying and extending it. If you like four
				311	spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
				312	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]	313	makes for incredible diffs that are absolutely worthless.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	314
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	315	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	316
				317	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	318	<div class="doc_subsubsection">
				319	<a name="scf_indentation">Indent Code Consistently</a>
				320	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	321
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	322	<div class="doc_text">
				323
				324	<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]	325	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]	326	Just do it.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	327
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	328	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	329
				330
				331	<!-- ======================================================================= -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	332	<div class="doc_subsection">
				333	<a name="compilerissues">Compiler Issues</a>
				334	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	335
				336
				337	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	338	<div class="doc_subsubsection">
				339	<a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
				340	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	341
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	342	<div class="doc_text">
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	<p>If your code has compiler warnings in it, something is wrong: you aren't
				345	casting values correctly, your have "questionable" constructs in your code, or
				346	you are doing something legitimately wrong. Compiler warnings can cover up
				347	legitimate errors in output and make dealing with a translation unit
				348	difficult.</p>
				349
				350	<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]	351	desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
				352	a good thorough set of warnings, and stick to them. At least in the case of
				353	<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
				354	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]	355	I write code like this:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	356
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	357	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	358	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	359	if (V = getValue()) {
				360	...
				361	}
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	362	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	363	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	364
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	365	<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
				366	operator, and that I probably mistyped it. In most cases, I haven't, and I
				367	really don't want the spurious errors. To fix this particular problem, I
				368	rewrite the code like this:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	369
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	370	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	371	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	372	if ((V = getValue())) {
				373	...
				374	}
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	375	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	376	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	377
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	378	<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
				379	be fixed by massaging the code appropriately.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	380
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	381	<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
				382	-Winline -W -Wwrite-strings -Wno-unused</tt></p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	383
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	384	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	385
				386	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	387	<div class="doc_subsubsection">
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	388	<a name="ci_portable_code">Write Portable Code</a>
				389	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	390
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	391	<div class="doc_text">
				392
				393	<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]	394	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]	395	code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	396
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	397	<p>In practice, this means that you shouldn't assume much about the host
				398	compiler, including its support for "high tech" features like partial
Chris Lattner	4bd3d7e	2009-03-23 04:52:53 +0000	[diff] [blame]	399	specialization of templates. If these features are used, they should only be
				400	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]	401
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	402	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	403
Reid Spencer	b40df56	2004-09-23 16:03:48 +0000	[diff] [blame]	404	<!-- _______________________________________________________________________ -->
				405	<div class="doc_subsubsection">
				406	<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
				407	</div>
				408	<div class="doc_text">
Misha Brukman	edcc78a	2004-10-26 15:45:13 +0000	[diff] [blame]	409
				410	<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
				411	interchangeably. The only difference is when they are used to declare a class:
				412	<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
				413	all members public by default.</p>
				414
				415	<p>Unfortunately, not all compilers follow the rules and some will generate
				416	different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
				417	declare the symbol. This can lead to problems at link time.</p>
				418
				419	<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
				420	<b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
				421
Reid Spencer	b40df56	2004-09-23 16:03:48 +0000	[diff] [blame]	422	</div>
				423
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	424	<!-- *********************************************************************** -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	425	<div class="doc_section">
				426	<a name="styleissues">Style Issues</a>
				427	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	428	<!-- *********************************************************************** -->
				429
				430
				431	<!-- ======================================================================= -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	432	<div class="doc_subsection">
				433	<a name="macro">The High Level Issues</a>
				434	</div>
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	435	<!-- ======================================================================= -->
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	436
				437
				438	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	439	<div class="doc_subsubsection">
				440	<a name="hl_module">A Public Header File <b>is</b> a Module</a>
				441	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	442
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	443	<div class="doc_text">
				444
				445	<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]	446	encapsulation or data hiding (unless you use expensive protocol classes), but it
				447	is what we have to work with. When you write a public header file (in the LLVM
				448	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]	449	module of functionality.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	450
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	451	<p>Ideally, modules should be completely independent of each other, and their
Misha Brukman	2d10d75	2003-07-28 21:57:18 +0000	[diff] [blame]	452	header files should only include the absolute minimum number of headers
				453	possible. A module is not just a class, a function, or a namespace: <a
				454	href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
				455	of these</a> that defines an interface. This interface may be several
				456	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]	457	together.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	458
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	459	<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
				460	files. Each of these <tt>.cpp</tt> files should include the header that defines
				461	their interface first. This ensure that all of the dependences of the module
				462	header have been properly added to the module header itself, and are not
				463	implicit. System headers should be included after user headers for a
				464	translation unit.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	465
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	466	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	467
				468	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	469	<div class="doc_subsubsection">
Misha Brukman	91d64af	2004-10-26 16:18:43 +0000	[diff] [blame]	470	<a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	471	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	472
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	473	<div class="doc_text">
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	<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
				476	have to, especially in header files.</p>
				477
				478	<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]	479	to inherit from it. In these cases go ahead and <tt>#include</tt> that header
				480	file. Be aware however that there are many cases where you don't need to have
				481	the full definition of a class. If you are using a pointer or reference to a
				482	class, you don't need the header file. If you are simply returning a class
				483	instance from a prototyped function or method, you don't need it. In fact, for
				484	most cases, you simply don't need the definition of a class... and not
				485	<tt>#include</tt>'ing speeds up compilation.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	486
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	487	<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]	488	<b>must</b> include all of the header files that you are using -- you can
				489	include them either directly
Misha Brukman	2d10d75	2003-07-28 21:57:18 +0000	[diff] [blame]	490	or indirectly (through another header file). To make sure that you don't
				491	accidently forget to include a header file in your module header, make sure to
				492	include your module header <b>first</b> in the implementation file (as mentioned
				493	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]	494	about later...</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	495
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	496	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	497
				498	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	499	<div class="doc_subsubsection">
				500	<a name="hl_privateheaders">Keep "internal" Headers Private</a>
				501	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	502
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	503	<div class="doc_text">
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	<p>Many modules have a complex implementation that causes them to use more than
				506	one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
				507	internal communication interface (helper classes, extra functions, etc) in the
Misha Brukman	3b53a26	2004-12-04 00:32:12 +0000	[diff] [blame]	508	public module header file. Don't do this.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	509
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	510	<p>If you really need to do something like this, put a private header file in
				511	the same directory as the source files, and include it locally. This ensures
				512	that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	513
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	514	<p>Note however, that it's okay to put extra implementation methods a public
				515	class itself... just make them private (or protected), and all is well.</p>
				516
				517	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	518
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	519	<!-- _______________________________________________________________________ -->
				520	<div class="doc_subsubsection">
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	521	<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]	522	</div>
				523
				524	<div class="doc_text">
				525
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	526	<p>When reading code, keep in mind how much state and how many previous
				527	decisions have to be remembered by the reader to understand a block of code.
				528	Aim to reduce indentation where possible when it doesn't make it more difficult
				529	to understand the code. One great way to do this is by making use of early
				530	exits and the 'continue' keyword in long loops. As an example of using an early
				531	exit from a function, consider this "bad" code:</p>
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	532
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	533	<div class="doc_code">
				534	<pre>
				535	Value DoSomething(Instruction I) {
				536	if (!isa<TerminatorInst>(I) &&
				537	I->hasOneUse() && SomeOtherThing(I)) {
				538	... some long code ....
				539	}
				540
				541	return 0;
				542	}
				543	</pre>
				544	</div>
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	545
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	546	<p>This code has several problems if the body of the 'if' is large. When you're
				547	looking at the top of the function, it isn't immediately clear that this
				548	<em>only</em> does interesting things with non-terminator instructions, and only
				549	applies to things with the other predicates. Second, it is relatively difficult
				550	to describe (in comments) why these predicates are important because the if
				551	statement makes it difficult to lay out the comments. Third, when you're deep
				552	within the body of the code, it is indented an extra level. Finally, when
				553	reading the top of the function, it isn't clear what the result is if the
				554	predicate isn't true, you have to read to the end of the function to know that
				555	it returns null.</p>
Matthijs Kooijman	ed58a97	2008-07-30 12:14:10 +0000	[diff] [blame]	556
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	557	<p>It is much preferred to format the code like this:</p>
				558
				559	<div class="doc_code">
				560	<pre>
				561	Value DoSomething(Instruction I) {
				562	// Terminators never need 'something' done to them because, ...
				563	if (isa<TerminatorInst>(I))
				564	return 0;
				565
				566	// We conservatively avoid transforming instructions with multiple uses
				567	// because goats like cheese.
				568	if (!I->hasOneUse())
				569	return 0;
				570
				571	// This is really just here for example.
				572	if (!SomeOtherThing(I))
				573	return 0;
				574
				575	... some long code ....
				576	}
				577	</pre>
				578	</div>
				579
				580	<p>This fixes these problems. A similar problem frequently happens in for
				581	loops. A silly example is something like this:</p>
				582
				583	<div class="doc_code">
				584	<pre>
				585	for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
				586	if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
				587	Value *LHS = BO->getOperand(0);
				588	Value *RHS = BO->getOperand(1);
				589	if (LHS != RHS) {
				590	...
				591	}
				592	}
				593	}
				594	</pre>
				595	</div>
				596
				597	<p>When you have very very small loops, this sort of structure is fine, but if
				598	it exceeds more than 10-15 lines, it becomes difficult for people to read and
				599	understand at a glance.
				600	The problem with this sort of code is that it gets very nested very quickly,
				601	meaning that the reader of the code has to keep a lot of context in their brain
				602	to remember what is going immediately on in the loop, because they don't know
				603	if/when the if conditions will have elses etc. It is strongly preferred to
				604	structure the loop like this:</p>
				605
				606	<div class="doc_code">
				607	<pre>
				608	for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
				609	BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
				610	if (!BO) continue;
				611
				612	Value *LHS = BO->getOperand(0);
				613	Value *RHS = BO->getOperand(1);
				614	if (LHS == RHS) continue;
				615	}
				616	</pre>
				617	</div>
				618
				619	<p>This has all the benefits of using early exits from functions: it reduces
				620	nesting of the loop, it makes it easier to describe why the conditions are true,
				621	and it makes it obvious to the reader that there is no "else" coming up that
				622	they have to push context into their brain for. If a loop is large, this can
				623	be a big understandability win.</p>
				624
				625	</div>
				626
				627
				628	<!-- _______________________________________________________________________ -->
				629	<div class="doc_subsubsection">
				630	<a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
				631	</div>
				632
				633	<div class="doc_text">
				634
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	635	<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]	636	value. There are a number of ways that people commonly write these, but an
				637	example of this sort of thing is:</p>
				638
				639	<div class="doc_code">
				640	<pre>
				641	<b>bool FoundFoo = false;</b>
				642	for (unsigned i = 0, e = BarList.size(); i != e; ++i)
				643	if (BarList[i]->isFoo()) {
				644	<b>FoundFoo = true;</b>
				645	break;
				646	}
				647
				648	<b>if (FoundFoo) {</b>
				649	...
				650	}
				651	</pre>
				652	</div>
				653
				654	<p>This sort of code is awkward to write, and is almost always a bad sign.
				655	Instead of this sort of loop, we strongly prefer to use a predicate function
				656	(which may be <a href="#micro_anonns">static</a>) that uses
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	657	<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
				658	the code to be structured like this:
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	659	</p>
				660
				661
				662	<div class="doc_code">
				663	<pre>
				664	/// ListContainsFoo - Return true if the specified list has an element that is
				665	/// a foo.
				666	static bool ListContainsFoo(const std::vector<Bar*> &List) {
				667	for (unsigned i = 0, e = List.size(); i != e; ++i)
				668	if (List[i]->isFoo())
				669	return true;
				670	return false;
				671	}
				672	...
				673
				674	<b>if (ListContainsFoo(BarList)) {</b>
				675	...
				676	}
				677	</pre>
				678	</div>
				679
				680	<p>There are many reasons for doing this: it reduces indentation and factors out
				681	code which can often be shared by other code that checks for the same predicate.
				682	More importantly, it <em>forces you to pick a name</em> for the function, and
				683	forces you to write a comment for it. In this silly example, this doesn't add
				684	much value. However, if the condition is complex, this can make it a lot easier
				685	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]	686	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]	687	contains a foo, we can trust the function name and continue reading with better
				688	locality.</p>
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	689
				690	</div>
				691
				692
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	693	<!-- ======================================================================= -->
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	694	<div class="doc_subsection">
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	695	<a name="micro">The Low Level Issues</a>
				696	</div>
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	697	<!-- ======================================================================= -->
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	698
				699
				700	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	701	<div class="doc_subsubsection">
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	702	<a name="ll_assert">Assert Liberally</a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	703	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	704
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	705	<div class="doc_text">
				706
				707	<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]	708	preconditions and assumptions, you never know when a bug (not neccesarily even
				709	yours) might be caught early by an assertion, which reduces debugging time
				710	dramatically. The "<tt><cassert></tt>" header file is probably already
				711	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]	712	it.</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	713
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	714	<p>To further assist with debugging, make sure to put some kind of error message
				715	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]	716	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]	717	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]	718
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	719	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	720	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	721	inline Value *getOperand(unsigned i) {
				722	assert(i < Operands.size() && "getOperand() out of range!");
				723	return Operands[i];
				724	}
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	725	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	726	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	727
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	728	<p>Here are some examples:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	729
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	730	<div class="doc_code">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	731	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	732	assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	733
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	734	assert((Opcode == Shl \|\| Opcode == Shr) && "ShiftInst Opcode invalid!");
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	735
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	736	assert(idx < getNumSuccessors() && "Successor # out of range!");
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	737
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	738	assert(V1.getType() == V2.getType() && "Constant types must be identical!");
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	739
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	740	assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	741	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	742	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	743
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	744	<p>You get the idea...</p>
				745
Nick Lewycky	f4408e8	2008-05-31 23:54:55 +0000	[diff] [blame]	746	<p>Please be aware when adding assert statements that not all compilers are aware of
				747	the semantics of the assert. In some places, asserts are used to indicate a piece of
				748	code that should not be reached. These are typically of the form:</p>
				749
				750	<div class="doc_code">
				751	<pre>
Dan Gohman	f003276	2008-11-24 17:18:39 +0000	[diff] [blame]	752	assert(0 && "Some helpful error message");
Nick Lewycky	f4408e8	2008-05-31 23:54:55 +0000	[diff] [blame]	753	</pre>
				754	</div>
				755
				756	<p>When used in a function that returns a value, they should be followed with a return
				757	statement and a comment indicating that this line is never reached. This will prevent
				758	a compiler which is unable to deduce that the assert statement never returns from
				759	generating a warning.</p>
				760
				761	<div class="doc_code">
				762	<pre>
Dan Gohman	f003276	2008-11-24 17:18:39 +0000	[diff] [blame]	763	assert(0 && "Some helpful error message");
Nick Lewycky	f4408e8	2008-05-31 23:54:55 +0000	[diff] [blame]	764	// Not reached
				765	return 0;
				766	</pre>
				767	</div>
				768
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	769	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	770
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	771	<!-- _______________________________________________________________________ -->
				772	<div class="doc_subsubsection">
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	773	<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]	774	</div>
				775
				776	<div class="doc_text">
				777	<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]	778	namespace with an "<tt>std::</tt>" prefix, rather than rely on
				779	"<tt>using namespace std;</tt>".</p>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	780
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	781	<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]	782	the namespace of any source file that <tt>#include</tt>s the header. This is
				783	clearly a bad thing.</p>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	784
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	785	<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
				786	rule, but is still important. Basically, using explicit namespace prefixes
				787	makes the code <b>clearer</b>, because it is immediately obvious what facilities
				788	are being used and where they are coming from, and <b>more portable</b>, because
				789	namespace clashes cannot occur between LLVM code and other namespaces. The
				790	portability rule is important because different standard library implementations
				791	expose different symbols (potentially ones they shouldn't), and future revisions
				792	to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
				793	such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	794
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	795	<p>The exception to the general rule (i.e. it's not an exception for
				796	the <tt>std</tt> namespace) is for implementation files. For example, all of
				797	the code in the LLVM project implements code that lives in the 'llvm' namespace.
				798	As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
				799	namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
				800	general form of this rule is that any .cpp file that implements code in any
				801	namespace may use that namespace (and its parents'), but should not use any
				802	others.</p>
Chris Lattner	b0bff9e	2006-01-01 21:59:22 +0000	[diff] [blame]	803
				804	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	805
				806	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	807	<div class="doc_subsubsection">
Bill Wendling	4ad6d61	2006-12-09 01:35:43 +0000	[diff] [blame]	808	<a name="ll_virtual_anch">Provide a virtual method anchor for classes
				809	in headers</a>
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	810	</div>
				811
				812	<div class="doc_text">
				813
				814	<p>If a class is defined in a header file and has a v-table (either it has
				815	virtual methods or it derives from classes with virtual methods), it must
				816	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]	817	this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
				818	that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
				819	increasing link times.</p>
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	820
				821	</div>
				822
Chris Lattner	0af39ea	2009-06-30 06:13:23 +0000	[diff] [blame]	823	<!-- _______________________________________________________________________ -->
				824	<div class="doc_subsubsection">
				825	<a name="ll_end">Don't evaluate end() every time through a loop</a>
				826	</div>
				827
				828	<div class="doc_text">
				829
				830	<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
				831	with macros and may be coming in C++'0x) we end up writing a lot of loops that
				832	manually iterate from begin to end on a variety of containers or through other
				833	data structures. One common mistake is to write a loop in this style:</p>
				834
				835	<div class="doc_code">
				836	<pre>
				837	BasicBlock *BB = ...
				838	for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
				839	... use I ...
				840	</pre>
				841	</div>
				842
				843	<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
				844	every time through the loop. Instead of writing the loop like this, we strongly
				845	prefer loops to be written so that they evaluate it once before the loop starts.
				846	A convenient way to do this is like so:</p>
				847
				848	<div class="doc_code">
				849	<pre>
				850	BasicBlock *BB = ...
				851	for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
				852	... use I ...
				853	</pre>
				854	</div>
				855
				856	<p>The observant may quickly point out that these two loops may have different
				857	semantics: if the container (a basic block in this case) is being mutated, then
				858	"<tt>BB->end()</tt>" may change its value every time through the loop and the
				859	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]	860	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]	861	that you did it intentionally.</p>
				862
				863	<p>Why do we prefer the second form (when correct)? Writing the loop in the
				864	first form has two problems: First it may be less efficient than evaluating it
				865	at the start of the loop. In this case, the cost is probably minor: a few extra
				866	loads every time through the loop. However, if the base expression is more
				867	complex, then the cost can rise quickly. I've seen loops where the end
				868	expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
Chris Lattner	a44f87f	2009-06-30 06:27:54 +0000	[diff] [blame]	869	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]	870	eliminate the issue entirely and don't even have to think about it.</p>
				871
Chris Lattner	09bc65e	2009-06-30 06:20:03 +0000	[diff] [blame]	872	<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]	873	hints to the reader that the loop is mutating the container (a fact that a
				874	comment would handily confirm!). If you write the loop in the second form, it
				875	is immediately obvious without even looking at the body of the loop that the
				876	container isn't being modified, which makes it easier to read the code and
				877	understand what it does.</p>
				878
				879	<p>While the second form of the loop is a few extra keystrokes, we do strongly
				880	prefer it.</p>
				881
				882	</div>
				883
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	884	<!-- _______________________________________________________________________ -->
				885	<div class="doc_subsubsection">
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	886	<a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	887	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	888
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	889	<div class="doc_text">
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	890
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	891	<p>The use of <tt>#include <iostream></tt> in library files is
				892	hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
				893	support clients using LLVM libraries as part of larger systems. In particular,
				894	we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
				895	the static c'tors are run whenever an application start up that uses the dynamic
				896	library. There are two problems with this:</p>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	897
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	898	<ol>
				899	<li>The time to run the static c'tors impacts startup time of
				900	applications—a critical time for GUI apps.</li>
				901	<li>The static c'tors cause the app to pull many extra pages of memory off the
				902	disk: both the code for the static c'tors in each <tt>.o</tt> file and the
				903	small amount of data that gets touched. In addition, touched/dirty pages
				904	put more pressure on the VM system on low-memory machines.</li>
				905	</ol>
				906
				907	<p>Note that using the other stream headers (<tt><sstream></tt> for
				908	example) is allowed normally, it is just <tt><iostream></tt> that is
				909	causing problems.</p>
				910
Daniel Dunbar	e8530a3	2009-07-24 23:04:51 +0000	[diff] [blame]	911	<p>In addition, new code should always
				912	use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> or
				913	the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	914
				915	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	916
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	917
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	918	<!-- _______________________________________________________________________ -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	919	<div class="doc_subsubsection">
Chris Lattner	6883a88	2006-07-27 04:24:14 +0000	[diff] [blame]	920	<a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	921	</div>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	922
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	923	<div class="doc_text">
				924
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	925	<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
				926	to the output stream specified. In addition to doing this, however, it also
				927	flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	928
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	929	<div class="doc_code">
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	930	<pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	931	std::cout << std::endl;
				932	std::cout << '\n' << std::flush;
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	933	</pre>
Misha Brukman	a8c760a	2004-07-28 22:31:54 +0000	[diff] [blame]	934	</div>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	935
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	936	<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]	937	it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	938
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	939	</div>
Chris Lattner	13c4659	2002-01-20 19:01:26 +0000	[diff] [blame]	940
Bill Wendling	7f564c0	2006-12-09 01:20:34 +0000	[diff] [blame]	941
Daniel Dunbar	e8530a3	2009-07-24 23:04:51 +0000	[diff] [blame]	942	<!-- _______________________________________________________________________ -->
				943	<div class="doc_subsubsection">
				944	<a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
				945	</div>
				946
				947	<div class="doc_text">
				948
				949	<p>LLVM includes a lightweight, simple, and efficient stream implementation
				950	in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
				951	of <tt>std::iostream</tt>. All new code should use <tt>raw_ostream</tt> instead
				952	of <tt>iostream</tt>.</p>
				953
				954	<p>Unlike <tt>std::iostream</tt>, <tt>raw_ostream</tt> is not a template and can
				955	be forward declared as <tt>class raw_ostream</tt>. Public headers should
				956	generally not include the <tt>raw_ostream</tt> header, but use forward
				957	declarations and constant references to <tt>raw_ostream</tt> instances.</p>
				958
				959	</div>
				960
				961
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	962	<!-- ======================================================================= -->
				963	<div class="doc_subsection">
				964	<a name="nano">Microscopic Details</a>
				965	</div>
				966	<!-- ======================================================================= -->
				967
				968	<p>This section describes preferred low-level formatting guidelines along with
				969	reasoning on why we prefer them.</p>
				970
				971	<!-- _______________________________________________________________________ -->
				972	<div class="doc_subsubsection">
				973	<a name="micro_spaceparen">Spaces Before Parentheses</a>
				974	</div>
				975
				976	<div class="doc_text">
				977
				978	<p>We prefer to put a space before a parentheses only in control flow
				979	statements, but not in normal function call expressions and function-like
				980	macros. For example, this is good:</p>
				981
				982	<div class="doc_code">
				983	<pre>
				984	<b>if (</b>x) ...
				985	<b>for (</b>i = 0; i != 100; ++i) ...
				986	<b>while (</b>llvm_rocks) ...
				987
				988	<b>somefunc(</b>42);
				989	<b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me");
				990
				991	a = <b>foo(</b>42, 92) + <b>bar(</b>x);
				992	</pre>
				993	</div>
				994
				995	<p>... and this is bad:</p>
				996
				997	<div class="doc_code">
				998	<pre>
				999	<b>if(</b>x) ...
				1000	<b>for(</b>i = 0; i != 100; ++i) ...
				1001	<b>while(</b>llvm_rocks) ...
				1002
				1003	<b>somefunc (</b>42);
				1004	<b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me");
				1005
				1006	a = <b>foo (</b>42, 92) + <b>bar (</b>x);
				1007	</pre>
				1008	</div>
				1009
				1010	<p>The reason for doing this is not completely arbitrary. This style makes
				1011	control flow operators stand out more, and makes expressions flow better. The
				1012	function call operator binds very tightly as a postfix operator. Putting
				1013	a space after a function name (as in the last example) makes it appear that
				1014	the code might bind the arguments of the left-hand-side of a binary operator
				1015	with the argument list of a function and the name of the right side. More
				1016	specifically, it is easy to misread the "a" example as:</p>
				1017
				1018	<div class="doc_code">
				1019	<pre>
				1020	a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
				1021	</pre>
				1022	</div>
				1023
				1024	<p>... when skimming through the code. By avoiding a space in a function, we
				1025	avoid this misinterpretation.</p>
				1026
				1027	</div>
				1028
				1029	<!-- _______________________________________________________________________ -->
				1030	<div class="doc_subsubsection">
				1031	<a name="micro_preincrement">Prefer Preincrement</a>
				1032	</div>
				1033
				1034	<div class="doc_text">
				1035
				1036	<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
				1037	postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
				1038	preincrementation whenever possible.</p>
				1039
				1040	<p>The semantics of postincrement include making a copy of the value being
				1041	incremented, returning it, and then preincrementing the "work value". For
				1042	primitive types, this isn't a big deal... but for iterators, it can be a huge
				1043	issue (for example, some iterators contains stack and set objects in them...
				1044	copying an iterator could invoke the copy ctor's of these as well). In general,
				1045	get in the habit of always using preincrement, and you won't have a problem.</p>
				1046
				1047	</div>
				1048
				1049	<!-- _______________________________________________________________________ -->
				1050	<div class="doc_subsubsection">
				1051	<a name="micro_namespaceindent">Namespace Indentation</a>
				1052	</div>
				1053
				1054	<div class="doc_text">
				1055
				1056	<p>
				1057	In general, we strive to reduce indentation where ever possible. This is useful
				1058	because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
				1059	wrapping horribly, but also because it makes it easier to understand the code.
				1060	Namespaces are a funny thing: they are often large, and we often desire to put
				1061	lots of stuff into them (so they can be large). Other times they are tiny,
				1062	because they just hold an enum or something similar. In order to balance this,
				1063	we use different approaches for small versus large namespaces.
				1064	</p>
				1065
				1066	<p>
				1067	If a namespace definition is small and <em>easily</em> fits on a screen (say,
				1068	less than 35 lines of code), then you should indent its body. Here's an
				1069	example:
				1070	</p>
				1071
				1072	<div class="doc_code">
				1073	<pre>
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	1074	namespace llvm {
				1075	namespace X86 {
				1076	/// RelocationType - An enum for the x86 relocation codes. Note that
				1077	/// the terminology here doesn't follow x86 convention - word means
				1078	/// 32-bit and dword means 64-bit.
				1079	enum RelocationType {
				1080	/// reloc_pcrel_word - PC relative relocation, add the relocated value to
				1081	/// the value already in memory, after we adjust it for where the PC is.
				1082	reloc_pcrel_word = 0,
				1083
				1084	/// reloc_picrel_word - PIC base relative relocation, add the relocated
				1085	/// value to the value already in memory, after we adjust it for where the
				1086	/// PIC base is.
				1087	reloc_picrel_word = 1,
				1088
				1089	/// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
				1090	/// add the relocated value to the value already in memory.
				1091	reloc_absolute_word = 2,
				1092	reloc_absolute_dword = 3
				1093	};
				1094	}
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	1095	}
				1096	</pre>
				1097	</div>
				1098
				1099	<p>Since the body is small, indenting adds value because it makes it very clear
				1100	where the namespace starts and ends, and it is easy to take the whole thing in
				1101	in one "gulp" when reading the code. If the blob of code in the namespace is
				1102	larger (as it typically is in a header in the llvm or clang namespaces), do not
				1103	indent the code, and add a comment indicating what namespace is being closed.
				1104	For example:</p>
				1105
				1106	<div class="doc_code">
				1107	<pre>
				1108	namespace llvm {
				1109	namespace knowledge {
				1110
				1111	/// Grokable - This class represents things that Smith can have an intimate
				1112	/// understanding of and contains the data associated with it.
				1113	class Grokable {
				1114	...
				1115	public:
				1116	explicit Grokable() { ... }
				1117	virtual ~Grokable() = 0;
				1118
				1119	...
				1120
				1121	};
				1122
				1123	} // end namespace knowledge
				1124	} // end namespace llvm
				1125	</pre>
				1126	</div>
				1127
				1128	<p>Because the class is large, we don't expect that the reader can easily
				1129	understand the entire concept in a glance, and the end of the file (where the
				1130	namespaces end) may be a long ways away from the place they open. As such,
				1131	indenting the contents of the namespace doesn't add any value, and detracts from
				1132	the readability of the class. In these cases it is best to <em>not</em> indent
				1133	the contents of the namespace.</p>
				1134
				1135	</div>
				1136
				1137	<!-- _______________________________________________________________________ -->
				1138	<div class="doc_subsubsection">
				1139	<a name="micro_anonns">Anonymous Namespaces</a>
				1140	</div>
				1141
				1142	<div class="doc_text">
				1143
Chris Lattner	59fec6a	2009-07-22 16:54:14 +0000	[diff] [blame]	1144	<p>After talking about namespaces in general, you may be wondering about
				1145	anonymous namespaces in particular.
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	1146	Anonymous namespaces are a great language feature that tells the C++ compiler
				1147	that the contents of the namespace are only visible within the current
				1148	translation unit, allowing more aggressive optimization and eliminating the
				1149	possibility of symbol name collisions. Anonymous namespaces are to C++ as
				1150	"static" is to C functions and global variables. While "static" is available
				1151	in C++, anonymous namespaces are more general: they can make entire classes
				1152	private to a file.</p>
				1153
				1154	<p>The problem with anonymous namespaces is that they naturally want to
				1155	encourage indentation of their body, and they reduce locality of reference: if
				1156	you see a random function definition in a C++ file, it is easy to see if it is
				1157	marked static, but seeing if it is in an anonymous namespace requires scanning
				1158	a big chunk of the file.</p>
				1159
				1160	<p>Because of this, we have a simple guideline: make anonymous namespaces as
				1161	small as possible, and only use them for class declarations. For example, this
				1162	is good:</p>
				1163
				1164	<div class="doc_code">
				1165	<pre>
				1166	<b>namespace {</b>
				1167	class StringSort {
				1168	...
				1169	public:
				1170	StringSort(...)
				1171	bool operator<(const char *RHS) const;
				1172	};
				1173	<b>} // end anonymous namespace</b>
				1174
				1175	static void Helper() {
				1176	...
				1177	}
				1178
				1179	bool StringSort::operator<(const char *RHS) const {
				1180	...
				1181	}
				1182
				1183	</pre>
				1184	</div>
				1185
				1186	<p>This is bad:</p>
				1187
				1188
				1189	<div class="doc_code">
				1190	<pre>
				1191	<b>namespace {</b>
				1192	class StringSort {
				1193	...
				1194	public:
				1195	StringSort(...)
				1196	bool operator<(const char *RHS) const;
				1197	};
				1198
				1199	void Helper() {
				1200	...
				1201	}
				1202
				1203	bool StringSort::operator<(const char *RHS) const {
				1204	...
				1205	}
				1206
				1207	<b>} // end anonymous namespace</b>
				1208
				1209	</pre>
				1210	</div>
				1211
				1212
				1213	<p>This is bad specifically because if you're looking at "Helper" in the middle
				1214	of a large C++ file, that you have no immediate way to tell if it is local to
				1215	the file. When it is marked static explicitly, this is immediately obvious.
				1216	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]	1217	namespace just because it was declared there.
Chris Lattner	d283cb1	2009-07-22 05:40:54 +0000	[diff] [blame]	1218	</p>
				1219
				1220	</div>
				1221
				1222
				1223
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1224	<!-- *********************************************************************** -->
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1225	<div class="doc_section">
				1226	<a name="seealso">See Also</a>
				1227	</div>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1228	<!-- *********************************************************************** -->
				1229
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1230	<div class="doc_text">
				1231
				1232	<p>A lot of these comments and recommendations have been culled for other
				1233	sources. Two particularly important books for our work are:</p>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1234
				1235	<ol>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1236
Chris Lattner	788ee28	2007-11-09 21:49:08 +0000	[diff] [blame]	1237	<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
				1238	C++</a> by Scott Meyers. Also
Chris Lattner	999cf09	2004-05-23 21:05:07 +0000	[diff] [blame]	1239	interesting and useful are "More Effective C++" and "Effective STL" by the same
				1240	author.</li>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1241
Chris Lattner	788ee28	2007-11-09 21:49:08 +0000	[diff] [blame]	1242	<li>Large-Scale C++ Software Design by John Lakos</li>
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1243
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1244	</ol>
				1245
				1246	<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]	1247	something.</p>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1248
				1249	</div>
				1250
Chris Lattner	d80c43c	2001-07-09 03:27:08 +0000	[diff] [blame]	1251	<!-- *********************************************************************** -->
				1252
				1253	<hr>
Misha Brukman	6c16c4b	2004-05-12 18:37:22 +0000	[diff] [blame]	1254	<address>
				1255	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman	4440870	2008-12-11 17:34:48 +0000	[diff] [blame]	1256	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]	1257	<a href="http://validator.w3.org/check/referer"><img
Misha Brukman	4440870	2008-12-11 17:34:48 +0000	[diff] [blame]	1258	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]	1259
Misha Brukman	6c16c4b	2004-05-12 18:37:22 +0000	[diff] [blame]	1260	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer	05fe4b0	2006-03-14 05:39:39 +0000	[diff] [blame]	1261	<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1262	Last modified: $Date$
Misha Brukman	6c16c4b	2004-05-12 18:37:22 +0000	[diff] [blame]	1263	</address>
Misha Brukman	75f258e	2003-10-24 17:57:33 +0000	[diff] [blame]	1264
				1265	</body>
				1266	</html>