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<html>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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<title>LLVM Coding Standards</title>
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</head>
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<body>
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<div class="doc_title">
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LLVM Coding Standards
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</div>
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<ol>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#mechanicalissues">Mechanical Source Issues</a>
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<ol>
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<li><a href="#sourceformating">Source Code Formatting</a>
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<ol>
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<li><a href="#scf_commenting">Commenting</a></li>
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<li><a href="#scf_commentformat">Comment Formatting</a></li>
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<li><a href="#scf_includes"><tt>#include</tt> Style</a></li>
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<li><a href="#scf_codewidth">Source Code Width</a></li>
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<li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>
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<li><a href="#scf_indentation">Indent Code Consistently</a></li>
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</ol></li>
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<li><a href="#compilerissues">Compiler Issues</a>
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<ol>
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<li><a href="#ci_warningerrors">Treat Compiler Warnings Like
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Errors</a></li>
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<li><a href="#ci_portable_code">Write Portable Code</a></li>
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<li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>
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</ol></li>
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</ol></li>
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<li><a href="#styleissues">Style Issues</a>
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<ol>
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<li><a href="#macro">The High Level Issues</a>
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<ol>
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<li><a href="#hl_module">A Public Header File <b>is</b> a
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Module</a></li>
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<li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
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<li><a href="#hl_privateheaders">Keep "internal" Headers
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Private</a></li>
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<li><a href="#hl_earlyexit">Use Early Exits and 'continue' to Simplify
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Code</a></li>
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<li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
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Functions</a></li>
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</ol></li>
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<li><a href="#micro">The Low Level Issues</a>
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<ol>
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<li><a href="#ll_assert">Assert Liberally</a></li>
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<li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
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<li><a href="#ll_virtual_anch">Provide a virtual method anchor for
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classes in headers</a></li>
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<li><a href="#ll_end">Don't evaluate end() every time through a
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loop</a></li>
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<li><a href="#ll_iostream"><tt>#include <iostream></tt> is
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<em>forbidden</em></a></li>
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<li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
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<li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li>
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</ol></li>
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<li><a href="#nano">Microscopic Details</a>
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<ol>
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<li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
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<li><a href="#micro_preincrement">Prefer Preincrement</a></li>
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<li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
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<li><a href="#micro_anonns">Anonymous Namespaces</a></li>
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</ol></li>
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</ol></li>
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<li><a href="#seealso">See Also</a></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="introduction">Introduction</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This document attempts to describe a few coding standards that are being used
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in the LLVM source tree. Although no coding standards should be regarded as
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absolute requirements to be followed in all instances, coding standards can be
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useful.</p>
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<p>This document intentionally does not prescribe fixed standards for religious
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issues such as brace placement and space usage. For issues like this, follow
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the golden rule:</p>
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<blockquote>
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<p><b><a name="goldenrule">If you are adding a significant body of source to a
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project, feel free to use whatever style you are most comfortable with. If you
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are extending, enhancing, or bug fixing already implemented code, use the style
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that is already being used so that the source is uniform and easy to
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follow.</a></b></p>
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</blockquote>
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<p>The ultimate goal of these guidelines is the increase readability and
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maintainability of our common source base. If you have suggestions for topics to
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be included, please mail them to <a
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href="mailto:sabre@nondot.org">Chris</a>.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="mechanicalissues">Mechanical Source Issues</a>
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</div>
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<!-- *********************************************************************** -->
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="sourceformating">Source Code Formatting</a>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="scf_commenting">Commenting</a>
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</div>
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<div class="doc_text">
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<p>Comments are one critical part of readability and maintainability. Everyone
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knows they should comment, so should you. When writing comments, write them as
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English prose, which means they should use proper capitalization, punctuation,
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etc. Although we all should probably
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comment our code more than we do, there are a few very critical places that
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documentation is very useful:</p>
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<b>File Headers</b>
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<p>Every source file should have a header on it that describes the basic
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purpose of the file. If a file does not have a header, it should not be
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checked into Subversion. Most source trees will probably have a standard
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file header format. The standard format for the LLVM source tree looks like
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this:</p>
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2004-07-28 22:31:54 +00:00
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<div class="doc_code">
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<pre>
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//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file contains the declaration of the Instruction class, which is the
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// base class for all of the VM instructions.
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//
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//===----------------------------------------------------------------------===//
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</pre>
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</div>
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2007-12-29 19:56:08 +00:00
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<p>A few things to note about this particular format: The "<tt>-*- C++
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-*-</tt>" string on the first line is there to tell Emacs that the source file
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is a C++ file, not a C file (Emacs assumes .h files are C files by default).
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Note that this tag is not necessary in .cpp files. The name of the file is also
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on the first line, along with a very short description of the purpose of the
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file. This is important when printing out code and flipping though lots of
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pages.</p>
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<p>The next section in the file is a concise note that defines the license
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that the file is released under. This makes it perfectly clear what terms the
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source code can be distributed under and should not be modified in any way.</p>
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<p>The main body of the description does not have to be very long in most cases.
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Here it's only two lines. If an algorithm is being implemented or something
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tricky is going on, a reference to the paper where it is published should be
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included, as well as any notes or "gotchas" in the code to watch out for.</p>
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<b>Class overviews</b>
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2006-07-31 20:18:49 +00:00
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<p>Classes are one fundamental part of a good object oriented design. As such,
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a class definition should have a comment block that explains what the class is
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used for... if it's not obvious. If it's so completely obvious your grandma
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could figure it out, it's probably safe to leave it out. Naming classes
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something sane goes a long ways towards avoiding writing documentation.</p>
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<b>Method information</b>
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<p>Methods defined in a class (as well as any global functions) should also be
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documented properly. A quick note about what it does any a description of the
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borderline behaviour is all that is necessary here (unless something
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particularly tricky or insideous is going on). The hope is that people can
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figure out how to use your interfaces without reading the code itself... that is
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the goal metric.</p>
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<p>Good things to talk about here are what happens when something unexpected
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happens: does the method return null? Abort? Format your hard disk?</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="scf_commentformat">Comment Formatting</a>
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</div>
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<div class="doc_text">
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2001-07-09 03:27:08 +00:00
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2003-10-24 17:57:33 +00:00
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<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
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require less typing, don't have nesting problems, etc. There are a few cases
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when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
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<ol>
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<li>When writing a C code: Obviously if you are writing C code, use C style
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comments.</li>
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<li>When writing a header file that may be <tt>#include</tt>d by a C source
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file.</li>
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<li>When writing a source file that is used by a tool that only accepts C
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style comments.</li>
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</ol>
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<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
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These nest properly and are better behaved in general than C style comments.</p>
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</div>
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2003-08-07 21:45:47 +00:00
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="scf_includes"><tt>#include</tt> Style</a>
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</div>
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<div class="doc_text">
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2003-08-07 21:45:47 +00:00
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2003-10-24 17:57:33 +00:00
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<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
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include guards if working on a header file), the <a
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href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
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file should be listed. We prefer these <tt>#include</tt>s to be listed in this
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order:</p>
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<ol>
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<li><a href="#mmheader">Main Module header</a></li>
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|
<li><a href="#hl_privateheaders">Local/Private Headers</a></li>
|
2004-07-28 22:31:54 +00:00
|
|
|
<li><tt>llvm/*</tt></li>
|
|
|
|
|
<li><tt>llvm/Analysis/*</tt></li>
|
|
|
|
|
<li><tt>llvm/Assembly/*</tt></li>
|
|
|
|
|
<li><tt>llvm/Bytecode/*</tt></li>
|
|
|
|
|
<li><tt>llvm/CodeGen/*</tt></li>
|
2003-10-24 17:57:33 +00:00
|
|
|
<li>...</li>
|
2004-07-28 22:31:54 +00:00
|
|
|
<li><tt>Support/*</tt></li>
|
|
|
|
|
<li><tt>Config/*</tt></li>
|
|
|
|
|
<li>System <tt>#includes</tt></li>
|
2003-08-07 21:45:47 +00:00
|
|
|
</ol>
|
|
|
|
|
|
2008-11-29 20:13:25 +00:00
|
|
|
<p>... and each category should be sorted by name.</p>
|
2003-08-07 21:45:47 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
|
2004-10-26 16:18:43 +00:00
|
|
|
which implement an interface defined by a .h file. This <tt>#include</tt>
|
|
|
|
|
should always be included <b>first</b> regardless of where it lives on the file
|
|
|
|
|
system. By including a header file first in the .cpp files that implement the
|
|
|
|
|
interfaces, we ensure that the header does not have any hidden dependencies
|
|
|
|
|
which are not explicitly #included in the header, but should be. It is also a
|
|
|
|
|
form of documentation in the .cpp file to indicate where the interfaces it
|
|
|
|
|
implements are defined.</p>
|
2003-08-07 21:45:47 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="scf_codewidth">Source Code Width</a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>Write your code to fit within 80 columns of text. This helps those of us who
|
|
|
|
|
like to print out code and look at your code in an xterm without resizing
|
|
|
|
|
it.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2008-07-08 05:12:37 +00:00
|
|
|
<p>The longer answer is that there must be some limit to the width of the code
|
|
|
|
|
in order to reasonably allow developers to have multiple files side-by-side in
|
|
|
|
|
windows on a modest display. If you are going to pick a width limit, it is
|
|
|
|
|
somewhat arbitrary but you might as well pick something standard. Going with
|
|
|
|
|
90 columns (for example) instead of 80 columns wouldn't add any significant
|
|
|
|
|
value and would be detrimental to printing out code. Also many other projects
|
|
|
|
|
have standardized on 80 columns, so some people have already configured their
|
|
|
|
|
editors for it (vs something else, like 90 columns).</p>
|
|
|
|
|
|
|
|
|
|
<p>This is one of many contentious issues in coding standards, but is not up
|
|
|
|
|
for debate.</p>
|
|
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_text">
|
|
|
|
|
|
|
|
|
|
<p>In all cases, prefer spaces to tabs in source files. People have different
|
2003-10-06 19:26:00 +00:00
|
|
|
prefered indentation levels, and different styles of indentation that they
|
|
|
|
|
like... this is fine. What isn't is that different editors/viewers expand tabs
|
|
|
|
|
out to different tab stops. This can cause your code to look completely
|
2003-10-24 17:57:33 +00:00
|
|
|
unreadable, and it is not worth dealing with.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
|
2003-10-06 19:26:00 +00:00
|
|
|
style of existing code if your are modifying and extending it. If you like four
|
|
|
|
|
spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
|
|
|
|
|
with two spaces of indentation. Also, do not reindent a whole source file: it
|
2003-10-24 17:57:33 +00:00
|
|
|
makes for incredible diffs that are absolutely worthless.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="scf_indentation">Indent Code Consistently</a>
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>Okay, your first year of programming you were told that indentation is
|
|
|
|
|
important. If you didn't believe and internalize this then, now is the time.
|
|
|
|
|
Just do it.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsection">
|
|
|
|
|
<a name="compilerissues">Compiler Issues</a>
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>If your code has compiler warnings in it, something is wrong: you aren't
|
|
|
|
|
casting values correctly, your have "questionable" constructs in your code, or
|
|
|
|
|
you are doing something legitimately wrong. Compiler warnings can cover up
|
|
|
|
|
legitimate errors in output and make dealing with a translation unit
|
|
|
|
|
difficult.</p>
|
|
|
|
|
|
|
|
|
|
<p>It is not possible to prevent all warnings from all compilers, nor is it
|
2003-10-06 19:26:00 +00:00
|
|
|
desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
|
|
|
|
|
a good thorough set of warnings, and stick to them. At least in the case of
|
|
|
|
|
<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
|
|
|
|
|
syntax of the code slightly. For example, an warning that annoys me occurs when
|
2003-10-24 17:57:33 +00:00
|
|
|
I write code like this:</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
<div class="doc_code">
|
2001-07-09 03:27:08 +00:00
|
|
|
<pre>
|
2004-07-28 22:31:54 +00:00
|
|
|
if (V = getValue()) {
|
|
|
|
|
...
|
|
|
|
|
}
|
2003-10-24 17:57:33 +00:00
|
|
|
</pre>
|
2004-07-28 22:31:54 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
|
|
|
|
|
operator, and that I probably mistyped it. In most cases, I haven't, and I
|
|
|
|
|
really don't want the spurious errors. To fix this particular problem, I
|
|
|
|
|
rewrite the code like this:</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
<div class="doc_code">
|
2001-07-09 03:27:08 +00:00
|
|
|
<pre>
|
2004-07-28 22:31:54 +00:00
|
|
|
if ((V = getValue())) {
|
|
|
|
|
...
|
|
|
|
|
}
|
2003-10-24 17:57:33 +00:00
|
|
|
</pre>
|
2004-07-28 22:31:54 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
|
|
|
|
|
be fixed by massaging the code appropriately.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
|
|
|
|
|
-Winline -W -Wwrite-strings -Wno-unused</tt></p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="ci_portable_code">Write Portable Code</a>
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>In almost all cases, it is possible and within reason to write completely
|
|
|
|
|
portable code. If there are cases where it isn't possible to write portable
|
|
|
|
|
code, isolate it behind a well defined (and well documented) interface.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>In practice, this means that you shouldn't assume much about the host
|
|
|
|
|
compiler, including its support for "high tech" features like partial
|
2009-03-23 04:52:53 +00:00
|
|
|
specialization of templates. If these features are used, they should only be
|
|
|
|
|
an implementation detail of a library which has a simple exposed API.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-09-23 16:03:48 +00:00
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
|
|
|
|
|
</div>
|
|
|
|
|
<div class="doc_text">
|
2004-10-26 15:45:13 +00:00
|
|
|
|
|
|
|
|
<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
|
|
|
|
|
interchangeably. The only difference is when they are used to declare a class:
|
|
|
|
|
<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
|
|
|
|
|
all members public by default.</p>
|
|
|
|
|
|
|
|
|
|
<p>Unfortunately, not all compilers follow the rules and some will generate
|
|
|
|
|
different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
|
|
|
|
|
declare the symbol. This can lead to problems at link time.</p>
|
|
|
|
|
|
|
|
|
|
<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
|
|
|
|
|
<b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
|
|
|
|
|
|
2004-09-23 16:03:48 +00:00
|
|
|
</div>
|
|
|
|
|
|
2001-07-09 03:27:08 +00:00
|
|
|
<!-- *********************************************************************** -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="styleissues">Style Issues</a>
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsection">
|
|
|
|
|
<a name="macro">The High Level Issues</a>
|
|
|
|
|
</div>
|
2009-07-22 05:40:54 +00:00
|
|
|
<!-- ======================================================================= -->
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="hl_module">A Public Header File <b>is</b> a Module</a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>C++ doesn't do too well in the modularity department. There is no real
|
2003-07-28 21:57:18 +00:00
|
|
|
encapsulation or data hiding (unless you use expensive protocol classes), but it
|
|
|
|
|
is what we have to work with. When you write a public header file (in the LLVM
|
|
|
|
|
source tree, they live in the top level "include" directory), you are defining a
|
2003-10-24 17:57:33 +00:00
|
|
|
module of functionality.</p>
|
2003-07-28 21:57:18 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>Ideally, modules should be completely independent of each other, and their
|
2003-07-28 21:57:18 +00:00
|
|
|
header files should only include the absolute minimum number of headers
|
|
|
|
|
possible. A module is not just a class, a function, or a namespace: <a
|
|
|
|
|
href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
|
|
|
|
|
of these</a> that defines an interface. This interface may be several
|
|
|
|
|
functions, classes or data structures, but the important issue is how they work
|
2003-10-24 17:57:33 +00:00
|
|
|
together.</p>
|
2003-07-28 21:57:18 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
|
|
|
|
|
files. Each of these <tt>.cpp</tt> files should include the header that defines
|
|
|
|
|
their interface first. This ensure that all of the dependences of the module
|
|
|
|
|
header have been properly added to the module header itself, and are not
|
|
|
|
|
implicit. System headers should be included after user headers for a
|
|
|
|
|
translation unit.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
2004-10-26 16:18:43 +00:00
|
|
|
<a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
|
|
|
|
|
have to, especially in header files.</p>
|
|
|
|
|
|
|
|
|
|
<p>But wait, sometimes you need to have the definition of a class to use it, or
|
2004-10-26 16:18:43 +00:00
|
|
|
to inherit from it. In these cases go ahead and <tt>#include</tt> that header
|
|
|
|
|
file. Be aware however that there are many cases where you don't need to have
|
|
|
|
|
the full definition of a class. If you are using a pointer or reference to a
|
|
|
|
|
class, you don't need the header file. If you are simply returning a class
|
|
|
|
|
instance from a prototyped function or method, you don't need it. In fact, for
|
|
|
|
|
most cases, you simply don't need the definition of a class... and not
|
|
|
|
|
<tt>#include</tt>'ing speeds up compilation.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>It is easy to try to go too overboard on this recommendation, however. You
|
2007-02-10 18:35:31 +00:00
|
|
|
<b>must</b> include all of the header files that you are using -- you can
|
|
|
|
|
include them either directly
|
2003-07-28 21:57:18 +00:00
|
|
|
or indirectly (through another header file). To make sure that you don't
|
|
|
|
|
accidently forget to include a header file in your module header, make sure to
|
|
|
|
|
include your module header <b>first</b> in the implementation file (as mentioned
|
|
|
|
|
above). This way there won't be any hidden dependencies that you'll find out
|
2003-10-24 17:57:33 +00:00
|
|
|
about later...</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="hl_privateheaders">Keep "internal" Headers Private</a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>Many modules have a complex implementation that causes them to use more than
|
|
|
|
|
one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
|
|
|
|
|
internal communication interface (helper classes, extra functions, etc) in the
|
2004-12-04 00:32:12 +00:00
|
|
|
public module header file. Don't do this.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>If you really need to do something like this, put a private header file in
|
|
|
|
|
the same directory as the source files, and include it locally. This ensures
|
|
|
|
|
that your private interface remains private and undisturbed by outsiders.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>Note however, that it's okay to put extra implementation methods a public
|
|
|
|
|
class itself... just make them private (or protected), and all is well.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2006-12-09 01:20:34 +00:00
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
2009-07-22 05:40:54 +00:00
|
|
|
<a name="hl_earlyexit">Use Early Exits and 'continue' to Simplify Code</a>
|
2006-12-09 01:20:34 +00:00
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
|
2009-07-22 05:40:54 +00:00
|
|
|
<p>When reading code, keep in mind how much state and how many previous
|
|
|
|
|
decisions have to be remembered by the reader to understand a block of code.
|
|
|
|
|
Aim to reduce indentation where possible when it doesn't make it more difficult
|
|
|
|
|
to understand the code. One great way to do this is by making use of early
|
|
|
|
|
exits and the 'continue' keyword in long loops. As an example of using an early
|
|
|
|
|
exit from a function, consider this "bad" code:</p>
|
2006-12-09 01:20:34 +00:00
|
|
|
|
2009-07-22 05:40:54 +00:00
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
Value *DoSomething(Instruction *I) {
|
|
|
|
|
if (!isa<TerminatorInst>(I) &&
|
|
|
|
|
I->hasOneUse() && SomeOtherThing(I)) {
|
|
|
|
|
... some long code ....
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
2006-12-09 01:20:34 +00:00
|
|
|
|
2009-07-22 05:40:54 +00:00
|
|
|
<p>This code has several problems if the body of the 'if' is large. When you're
|
|
|
|
|
looking at the top of the function, it isn't immediately clear that this
|
|
|
|
|
<em>only</em> does interesting things with non-terminator instructions, and only
|
|
|
|
|
applies to things with the other predicates. Second, it is relatively difficult
|
|
|
|
|
to describe (in comments) why these predicates are important because the if
|
|
|
|
|
statement makes it difficult to lay out the comments. Third, when you're deep
|
|
|
|
|
within the body of the code, it is indented an extra level. Finally, when
|
|
|
|
|
reading the top of the function, it isn't clear what the result is if the
|
|
|
|
|
predicate isn't true, you have to read to the end of the function to know that
|
|
|
|
|
it returns null.</p>
|
2008-07-30 12:14:10 +00:00
|
|
|
|
2009-07-22 05:40:54 +00:00
|
|
|
<p>It is much preferred to format the code like this:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
Value *DoSomething(Instruction *I) {
|
|
|
|
|
// Terminators never need 'something' done to them because, ...
|
|
|
|
|
if (isa<TerminatorInst>(I))
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
// We conservatively avoid transforming instructions with multiple uses
|
|
|
|
|
// because goats like cheese.
|
|
|
|
|
if (!I->hasOneUse())
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
// This is really just here for example.
|
|
|
|
|
if (!SomeOtherThing(I))
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
... some long code ....
|
|
|
|
|
}
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>This fixes these problems. A similar problem frequently happens in for
|
|
|
|
|
loops. A silly example is something like this:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
|
|
|
|
|
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
|
|
|
|
|
Value *LHS = BO->getOperand(0);
|
|
|
|
|
Value *RHS = BO->getOperand(1);
|
|
|
|
|
if (LHS != RHS) {
|
|
|
|
|
...
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>When you have very very small loops, this sort of structure is fine, but if
|
|
|
|
|
it exceeds more than 10-15 lines, it becomes difficult for people to read and
|
|
|
|
|
understand at a glance.
|
|
|
|
|
The problem with this sort of code is that it gets very nested very quickly,
|
|
|
|
|
meaning that the reader of the code has to keep a lot of context in their brain
|
|
|
|
|
to remember what is going immediately on in the loop, because they don't know
|
|
|
|
|
if/when the if conditions will have elses etc. It is strongly preferred to
|
|
|
|
|
structure the loop like this:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
|
|
|
|
|
BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
|
|
|
|
|
if (!BO) continue;
|
|
|
|
|
|
|
|
|
|
Value *LHS = BO->getOperand(0);
|
|
|
|
|
Value *RHS = BO->getOperand(1);
|
|
|
|
|
if (LHS == RHS) continue;
|
|
|
|
|
}
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>This has all the benefits of using early exits from functions: it reduces
|
|
|
|
|
nesting of the loop, it makes it easier to describe why the conditions are true,
|
|
|
|
|
and it makes it obvious to the reader that there is no "else" coming up that
|
|
|
|
|
they have to push context into their brain for. If a loop is large, this can
|
|
|
|
|
be a big understandability win.</p>
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
|
2009-07-22 16:54:14 +00:00
|
|
|
<p>It is very common to write small loops that just compute a boolean
|
2009-07-22 05:40:54 +00:00
|
|
|
value. There are a number of ways that people commonly write these, but an
|
|
|
|
|
example of this sort of thing is:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
<b>bool FoundFoo = false;</b>
|
|
|
|
|
for (unsigned i = 0, e = BarList.size(); i != e; ++i)
|
|
|
|
|
if (BarList[i]->isFoo()) {
|
|
|
|
|
<b>FoundFoo = true;</b>
|
|
|
|
|
break;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
<b>if (FoundFoo) {</b>
|
|
|
|
|
...
|
|
|
|
|
}
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>This sort of code is awkward to write, and is almost always a bad sign.
|
|
|
|
|
Instead of this sort of loop, we strongly prefer to use a predicate function
|
|
|
|
|
(which may be <a href="#micro_anonns">static</a>) that uses
|
2009-07-22 16:54:14 +00:00
|
|
|
<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
|
|
|
|
|
the code to be structured like this:
|
2009-07-22 05:40:54 +00:00
|
|
|
</p>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
/// ListContainsFoo - Return true if the specified list has an element that is
|
|
|
|
|
/// a foo.
|
|
|
|
|
static bool ListContainsFoo(const std::vector<Bar*> &List) {
|
|
|
|
|
for (unsigned i = 0, e = List.size(); i != e; ++i)
|
|
|
|
|
if (List[i]->isFoo())
|
|
|
|
|
return true;
|
|
|
|
|
return false;
|
|
|
|
|
}
|
|
|
|
|
...
|
|
|
|
|
|
|
|
|
|
<b>if (ListContainsFoo(BarList)) {</b>
|
|
|
|
|
...
|
|
|
|
|
}
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>There are many reasons for doing this: it reduces indentation and factors out
|
|
|
|
|
code which can often be shared by other code that checks for the same predicate.
|
|
|
|
|
More importantly, it <em>forces you to pick a name</em> for the function, and
|
|
|
|
|
forces you to write a comment for it. In this silly example, this doesn't add
|
|
|
|
|
much value. However, if the condition is complex, this can make it a lot easier
|
|
|
|
|
for the reader to understand the code that queries for this predicate. Instead
|
2009-07-22 16:30:39 +00:00
|
|
|
of being faced with the in-line details of how we check to see if the BarList
|
2009-07-22 05:40:54 +00:00
|
|
|
contains a foo, we can trust the function name and continue reading with better
|
|
|
|
|
locality.</p>
|
2006-12-09 01:20:34 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
2001-07-09 03:27:08 +00:00
|
|
|
<!-- ======================================================================= -->
|
2004-05-23 21:05:07 +00:00
|
|
|
<div class="doc_subsection">
|
2003-10-24 17:57:33 +00:00
|
|
|
<a name="micro">The Low Level Issues</a>
|
|
|
|
|
</div>
|
2009-07-22 05:40:54 +00:00
|
|
|
<!-- ======================================================================= -->
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
2006-07-27 04:24:14 +00:00
|
|
|
<a name="ll_assert">Assert Liberally</a>
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
|
2003-07-28 21:57:18 +00:00
|
|
|
preconditions and assumptions, you never know when a bug (not neccesarily even
|
|
|
|
|
yours) might be caught early by an assertion, which reduces debugging time
|
|
|
|
|
dramatically. The "<tt><cassert></tt>" header file is probably already
|
|
|
|
|
included by the header files you are using, so it doesn't cost anything to use
|
2003-10-24 17:57:33 +00:00
|
|
|
it.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>To further assist with debugging, make sure to put some kind of error message
|
|
|
|
|
in the assertion statement (which is printed if the assertion is tripped). This
|
2003-07-28 21:57:18 +00:00
|
|
|
helps the poor debugging make sense of why an assertion is being made and
|
2003-10-24 17:57:33 +00:00
|
|
|
enforced, and hopefully what to do about it. Here is one complete example:</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
<div class="doc_code">
|
2001-07-09 03:27:08 +00:00
|
|
|
<pre>
|
2004-07-28 22:31:54 +00:00
|
|
|
inline Value *getOperand(unsigned i) {
|
|
|
|
|
assert(i < Operands.size() && "getOperand() out of range!");
|
|
|
|
|
return Operands[i];
|
|
|
|
|
}
|
2001-07-09 03:27:08 +00:00
|
|
|
</pre>
|
2004-07-28 22:31:54 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>Here are some examples:</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
<div class="doc_code">
|
2001-07-09 03:27:08 +00:00
|
|
|
<pre>
|
2004-07-28 22:31:54 +00:00
|
|
|
assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
assert(idx < getNumSuccessors() && "Successor # out of range!");
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
assert(V1.getType() == V2.getType() && "Constant types must be identical!");
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
|
2003-10-24 17:57:33 +00:00
|
|
|
</pre>
|
2004-07-28 22:31:54 +00:00
|
|
|
</div>
|
2003-10-24 17:57:33 +00:00
|
|
|
|
|
|
|
|
<p>You get the idea...</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2008-05-31 23:54:55 +00:00
|
|
|
<p>Please be aware when adding assert statements that not all compilers are aware of
|
|
|
|
|
the semantics of the assert. In some places, asserts are used to indicate a piece of
|
|
|
|
|
code that should not be reached. These are typically of the form:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
2008-11-24 17:18:39 +00:00
|
|
|
assert(0 && "Some helpful error message");
|
2008-05-31 23:54:55 +00:00
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>When used in a function that returns a value, they should be followed with a return
|
|
|
|
|
statement and a comment indicating that this line is never reached. This will prevent
|
|
|
|
|
a compiler which is unable to deduce that the assert statement never returns from
|
|
|
|
|
generating a warning.</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
2008-11-24 17:18:39 +00:00
|
|
|
assert(0 && "Some helpful error message");
|
2008-05-31 23:54:55 +00:00
|
|
|
// Not reached
|
|
|
|
|
return 0;
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2006-01-01 21:59:22 +00:00
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
2006-12-09 01:35:43 +00:00
|
|
|
<a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
|
2006-01-01 21:59:22 +00:00
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
|
2006-12-09 01:35:43 +00:00
|
|
|
namespace with an "<tt>std::</tt>" prefix, rather than rely on
|
|
|
|
|
"<tt>using namespace std;</tt>".</p>
|
|
|
|
|
|
|
|
|
|
<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
|
2008-12-11 19:37:04 +00:00
|
|
|
the namespace of any source file that <tt>#include</tt>s the header. This is
|
|
|
|
|
clearly a bad thing.</p>
|
2006-12-09 01:35:43 +00:00
|
|
|
|
|
|
|
|
<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
|
|
|
|
|
rule, but is still important. Basically, using explicit namespace prefixes
|
|
|
|
|
makes the code <b>clearer</b>, because it is immediately obvious what facilities
|
|
|
|
|
are being used and where they are coming from, and <b>more portable</b>, because
|
|
|
|
|
namespace clashes cannot occur between LLVM code and other namespaces. The
|
|
|
|
|
portability rule is important because different standard library implementations
|
|
|
|
|
expose different symbols (potentially ones they shouldn't), and future revisions
|
|
|
|
|
to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
|
|
|
|
|
such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
|
|
|
|
|
|
|
|
|
|
<p>The exception to the general rule (i.e. it's not an exception for
|
|
|
|
|
the <tt>std</tt> namespace) is for implementation files. For example, all of
|
|
|
|
|
the code in the LLVM project implements code that lives in the 'llvm' namespace.
|
|
|
|
|
As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
|
|
|
|
|
namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
|
|
|
|
|
general form of this rule is that any .cpp file that implements code in any
|
|
|
|
|
namespace may use that namespace (and its parents'), but should not use any
|
|
|
|
|
others.</p>
|
2006-01-01 21:59:22 +00:00
|
|
|
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
2006-12-09 01:35:43 +00:00
|
|
|
<a name="ll_virtual_anch">Provide a virtual method anchor for classes
|
|
|
|
|
in headers</a>
|
2006-07-27 04:24:14 +00:00
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
|
|
|
|
|
<p>If a class is defined in a header file and has a v-table (either it has
|
|
|
|
|
virtual methods or it derives from classes with virtual methods), it must
|
|
|
|
|
always have at least one out-of-line virtual method in the class. Without
|
2008-12-11 19:37:04 +00:00
|
|
|
this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
|
|
|
|
|
that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
|
|
|
|
|
increasing link times.</p>
|
2006-07-27 04:24:14 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
2009-06-30 06:13:23 +00:00
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="ll_end">Don't evaluate end() every time through a loop</a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
|
|
|
|
|
<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
|
|
|
|
|
with macros and may be coming in C++'0x) we end up writing a lot of loops that
|
|
|
|
|
manually iterate from begin to end on a variety of containers or through other
|
|
|
|
|
data structures. One common mistake is to write a loop in this style:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
BasicBlock *BB = ...
|
|
|
|
|
for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
|
|
|
|
|
... use I ...
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
|
|
|
|
|
every time through the loop. Instead of writing the loop like this, we strongly
|
|
|
|
|
prefer loops to be written so that they evaluate it once before the loop starts.
|
|
|
|
|
A convenient way to do this is like so:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
BasicBlock *BB = ...
|
|
|
|
|
for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
|
|
|
|
|
... use I ...
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>The observant may quickly point out that these two loops may have different
|
|
|
|
|
semantics: if the container (a basic block in this case) is being mutated, then
|
|
|
|
|
"<tt>BB->end()</tt>" may change its value every time through the loop and the
|
|
|
|
|
second loop may not in fact be correct. If you actually do depend on this
|
2009-06-30 06:20:03 +00:00
|
|
|
behavior, please write the loop in the first form and add a comment indicating
|
2009-06-30 06:13:23 +00:00
|
|
|
that you did it intentionally.</p>
|
|
|
|
|
|
|
|
|
|
<p>Why do we prefer the second form (when correct)? Writing the loop in the
|
|
|
|
|
first form has two problems: First it may be less efficient than evaluating it
|
|
|
|
|
at the start of the loop. In this case, the cost is probably minor: a few extra
|
|
|
|
|
loads every time through the loop. However, if the base expression is more
|
|
|
|
|
complex, then the cost can rise quickly. I've seen loops where the end
|
|
|
|
|
expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
|
2009-06-30 06:27:54 +00:00
|
|
|
lookups really aren't cheap. By writing it in the second form consistently, you
|
2009-06-30 06:13:23 +00:00
|
|
|
eliminate the issue entirely and don't even have to think about it.</p>
|
|
|
|
|
|
2009-06-30 06:20:03 +00:00
|
|
|
<p>The second (even bigger) issue is that writing the loop in the first form
|
2009-06-30 06:13:23 +00:00
|
|
|
hints to the reader that the loop is mutating the container (a fact that a
|
|
|
|
|
comment would handily confirm!). If you write the loop in the second form, it
|
|
|
|
|
is immediately obvious without even looking at the body of the loop that the
|
|
|
|
|
container isn't being modified, which makes it easier to read the code and
|
|
|
|
|
understand what it does.</p>
|
|
|
|
|
|
|
|
|
|
<p>While the second form of the loop is a few extra keystrokes, we do strongly
|
|
|
|
|
prefer it.</p>
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
2009-07-22 05:40:54 +00:00
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
|
|
|
|
|
<p>The use of <tt>#include <iostream></tt> in library files is
|
|
|
|
|
hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
|
|
|
|
|
support clients using LLVM libraries as part of larger systems. In particular,
|
|
|
|
|
we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
|
|
|
|
|
the static c'tors are run whenever an application start up that uses the dynamic
|
|
|
|
|
library. There are two problems with this:</p>
|
|
|
|
|
|
|
|
|
|
<ol>
|
|
|
|
|
<li>The time to run the static c'tors impacts startup time of
|
|
|
|
|
applications—a critical time for GUI apps.</li>
|
|
|
|
|
<li>The static c'tors cause the app to pull many extra pages of memory off the
|
|
|
|
|
disk: both the code for the static c'tors in each <tt>.o</tt> file and the
|
|
|
|
|
small amount of data that gets touched. In addition, touched/dirty pages
|
|
|
|
|
put more pressure on the VM system on low-memory machines.</li>
|
|
|
|
|
</ol>
|
|
|
|
|
|
|
|
|
|
<p>Note that using the other stream headers (<tt><sstream></tt> for
|
|
|
|
|
example) is allowed normally, it is just <tt><iostream></tt> that is
|
|
|
|
|
causing problems.</p>
|
|
|
|
|
|
2009-07-24 23:04:51 +00:00
|
|
|
<p>In addition, new code should always
|
|
|
|
|
use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> or
|
|
|
|
|
the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
|
2009-07-22 05:40:54 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
|
|
|
|
|
<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
|
|
|
|
|
to the output stream specified. In addition to doing this, however, it also
|
|
|
|
|
flushes the output stream. In other words, these are equivalent:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
std::cout << std::endl;
|
|
|
|
|
std::cout << '\n' << std::flush;
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>Most of the time, you probably have no reason to flush the output stream, so
|
|
|
|
|
it's better to use a literal <tt>'\n'</tt>.</p>
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
2009-07-24 23:04:51 +00:00
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
|
|
|
|
|
<p>LLVM includes a lightweight, simple, and efficient stream implementation
|
|
|
|
|
in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
|
|
|
|
|
of <tt>std::iostream</tt>. All new code should use <tt>raw_ostream</tt> instead
|
|
|
|
|
of <tt>iostream</tt>.</p>
|
|
|
|
|
|
|
|
|
|
<p>Unlike <tt>std::iostream</tt>, <tt>raw_ostream</tt> is not a template and can
|
|
|
|
|
be forward declared as <tt>class raw_ostream</tt>. Public headers should
|
|
|
|
|
generally not include the <tt>raw_ostream</tt> header, but use forward
|
|
|
|
|
declarations and constant references to <tt>raw_ostream</tt> instances.</p>
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
2009-07-22 05:40:54 +00:00
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<div class="doc_subsection">
|
|
|
|
|
<a name="nano">Microscopic Details</a>
|
|
|
|
|
</div>
|
|
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
|
|
|
|
|
<p>This section describes preferred low-level formatting guidelines along with
|
|
|
|
|
reasoning on why we prefer them.</p>
|
|
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
|
|
|
|
<a name="micro_spaceparen">Spaces Before Parentheses</a>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
|
|
|
|
|
<p>We prefer to put a space before a parentheses only in control flow
|
|
|
|
|
statements, but not in normal function call expressions and function-like
|
|
|
|
|
macros. For example, this is good:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
<b>if (</b>x) ...
|
|
|
|
|
<b>for (</b>i = 0; i != 100; ++i) ...
|
|
|
|
|
<b>while (</b>llvm_rocks) ...
|
|
|
|
|
|
|
|
|
|
<b>somefunc(</b>42);
|
|
|
|
|
<b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me");
|
|
|
|
|
|
|
|
|
|
a = <b>foo(</b>42, 92) + <b>bar(</b>x);
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>... and this is bad:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
<b>if(</b>x) ...
|
|
|
|
|
<b>for(</b>i = 0; i != 100; ++i) ...
|
|
|
|
|
<b>while(</b>llvm_rocks) ...
|
|
|
|
|
|
|
|
|
|
<b>somefunc (</b>42);
|
|
|
|
|
<b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me");
|
|
|
|
|
|
|
|
|
|
a = <b>foo (</b>42, 92) + <b>bar (</b>x);
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>The reason for doing this is not completely arbitrary. This style makes
|
|
|
|
|
control flow operators stand out more, and makes expressions flow better. The
|
|
|
|
|
function call operator binds very tightly as a postfix operator. Putting
|
|
|
|
|
a space after a function name (as in the last example) makes it appear that
|
|
|
|
|
the code might bind the arguments of the left-hand-side of a binary operator
|
|
|
|
|
with the argument list of a function and the name of the right side. More
|
|
|
|
|
specifically, it is easy to misread the "a" example as:</p>
|
|
|
|
|
|
|
|
|
|
<div class="doc_code">
|
|
|
|
|
<pre>
|
|
|
|
|
a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<p>... when skimming through the code. By avoiding a space in a function, we
|
|
|
|
|
avoid this misinterpretation.</p>
|
|
|
|
|
|
|
|
|
|
</div>
|
2006-07-27 04:24:14 +00:00
|
|
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
|
<div class="doc_subsubsection">
|
2009-07-22 05:40:54 +00:00
|
|
|
<a name="micro_preincrement">Prefer Preincrement</a>
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
|
|
|
|
|
postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
|
|
|
|
|
preincrementation whenever possible.</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<p>The semantics of postincrement include making a copy of the value being
|
2003-07-28 21:57:18 +00:00
|
|
|
incremented, returning it, and then preincrementing the "work value". For
|
|
|
|
|
primitive types, this isn't a big deal... but for iterators, it can be a huge
|
|
|
|
|
issue (for example, some iterators contains stack and set objects in them...
|
|
|
|
|
copying an iterator could invoke the copy ctor's of these as well). In general,
|
2003-10-24 17:57:33 +00:00
|
|
|
get in the habit of always using preincrement, and you won't have a problem.</p>
|
|
|
|
|
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2002-01-20 19:01:26 +00:00
|
|
|
<!-- _______________________________________________________________________ -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_subsubsection">
|
2009-07-22 05:40:54 +00:00
|
|
|
<a name="micro_namespaceindent">Namespace Indentation</a>
|
2003-10-24 17:57:33 +00:00
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
2002-01-20 19:01:26 +00:00
|
|
|
|
2009-07-22 05:40:54 +00:00
|
|
|
<p>
|
|
|
|
|
In general, we strive to reduce indentation where ever possible. This is useful
|
|
|
|
|
because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
|
|
|
|
|
wrapping horribly, but also because it makes it easier to understand the code.
|
|
|
|
|
Namespaces are a funny thing: they are often large, and we often desire to put
|
|
|
|
|
lots of stuff into them (so they can be large). Other times they are tiny,
|
|
|
|
|
because they just hold an enum or something similar. In order to balance this,
|
|
|
|
|
we use different approaches for small versus large namespaces.
|
|
|
|
|
</p>
|
|
|
|
|
|
|
|
|
|
<p>
|
|
|
|
|
If a namespace definition is small and <em>easily</em> fits on a screen (say,
|
|
|
|
|
less than 35 lines of code), then you should indent its body. Here's an
|
|
|
|
|
example:
|
|
|
|
|
</p>
|
2002-01-20 19:01:26 +00:00
|
|
|
|
2004-07-28 22:31:54 +00:00
|
|
|
<div class="doc_code">
|
2002-01-20 19:01:26 +00:00
|
|
|
<pre>
|
2009-07-22 16:54:14 +00:00
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namespace llvm {
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namespace X86 {
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/// RelocationType - An enum for the x86 relocation codes. Note that
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/// the terminology here doesn't follow x86 convention - word means
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/// 32-bit and dword means 64-bit.
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enum RelocationType {
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/// reloc_pcrel_word - PC relative relocation, add the relocated value to
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/// the value already in memory, after we adjust it for where the PC is.
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reloc_pcrel_word = 0,
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/// reloc_picrel_word - PIC base relative relocation, add the relocated
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/// value to the value already in memory, after we adjust it for where the
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/// PIC base is.
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reloc_picrel_word = 1,
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/// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
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/// add the relocated value to the value already in memory.
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reloc_absolute_word = 2,
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reloc_absolute_dword = 3
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};
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}
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2009-07-22 05:40:54 +00:00
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}
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2002-01-20 19:01:26 +00:00
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</pre>
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2004-07-28 22:31:54 +00:00
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</div>
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2002-01-20 19:01:26 +00:00
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2009-07-22 05:40:54 +00:00
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<p>Since the body is small, indenting adds value because it makes it very clear
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where the namespace starts and ends, and it is easy to take the whole thing in
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in one "gulp" when reading the code. If the blob of code in the namespace is
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larger (as it typically is in a header in the llvm or clang namespaces), do not
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indent the code, and add a comment indicating what namespace is being closed.
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For example:</p>
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<div class="doc_code">
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<pre>
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namespace llvm {
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namespace knowledge {
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/// Grokable - This class represents things that Smith can have an intimate
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/// understanding of and contains the data associated with it.
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class Grokable {
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...
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public:
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explicit Grokable() { ... }
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virtual ~Grokable() = 0;
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...
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2002-01-20 19:01:26 +00:00
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2009-07-22 05:40:54 +00:00
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};
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} // end namespace knowledge
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} // end namespace llvm
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</pre>
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</div>
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<p>Because the class is large, we don't expect that the reader can easily
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understand the entire concept in a glance, and the end of the file (where the
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namespaces end) may be a long ways away from the place they open. As such,
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indenting the contents of the namespace doesn't add any value, and detracts from
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the readability of the class. In these cases it is best to <em>not</em> indent
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the contents of the namespace.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="micro_anonns">Anonymous Namespaces</a>
|
2003-10-24 17:57:33 +00:00
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</div>
|
2002-01-20 19:01:26 +00:00
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|
2009-07-22 05:40:54 +00:00
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<div class="doc_text">
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|
2009-07-22 16:54:14 +00:00
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<p>After talking about namespaces in general, you may be wondering about
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anonymous namespaces in particular.
|
2009-07-22 05:40:54 +00:00
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Anonymous namespaces are a great language feature that tells the C++ compiler
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that the contents of the namespace are only visible within the current
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translation unit, allowing more aggressive optimization and eliminating the
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possibility of symbol name collisions. Anonymous namespaces are to C++ as
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"static" is to C functions and global variables. While "static" is available
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in C++, anonymous namespaces are more general: they can make entire classes
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private to a file.</p>
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<p>The problem with anonymous namespaces is that they naturally want to
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encourage indentation of their body, and they reduce locality of reference: if
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you see a random function definition in a C++ file, it is easy to see if it is
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marked static, but seeing if it is in an anonymous namespace requires scanning
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a big chunk of the file.</p>
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<p>Because of this, we have a simple guideline: make anonymous namespaces as
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small as possible, and only use them for class declarations. For example, this
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is good:</p>
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<div class="doc_code">
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<pre>
|
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<b>namespace {</b>
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class StringSort {
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...
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public:
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StringSort(...)
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bool operator<(const char *RHS) const;
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};
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<b>} // end anonymous namespace</b>
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static void Helper() {
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...
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}
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bool StringSort::operator<(const char *RHS) const {
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...
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}
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|
</pre>
|
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|
|
</div>
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|
<p>This is bad:</p>
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|
<div class="doc_code">
|
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|
|
<pre>
|
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|
|
<b>namespace {</b>
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|
|
class StringSort {
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|
|
...
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|
public:
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|
StringSort(...)
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|
|
bool operator<(const char *RHS) const;
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};
|
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|
|
void Helper() {
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|
|
...
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|
|
}
|
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|
|
bool StringSort::operator<(const char *RHS) const {
|
|
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|
|
...
|
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|
|
}
|
|
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|
|
<b>} // end anonymous namespace</b>
|
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|
|
|
|
|
</pre>
|
|
|
|
|
</div>
|
|
|
|
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|
|
|
|
|
|
|
|
|
|
<p>This is bad specifically because if you're looking at "Helper" in the middle
|
|
|
|
|
of a large C++ file, that you have no immediate way to tell if it is local to
|
|
|
|
|
the file. When it is marked static explicitly, this is immediately obvious.
|
|
|
|
|
Also, there is no reason to enclose the definition of "operator<" in the
|
2009-07-22 16:54:14 +00:00
|
|
|
namespace just because it was declared there.
|
2009-07-22 05:40:54 +00:00
|
|
|
</p>
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
2006-12-09 01:20:34 +00:00
|
|
|
|
2001-07-09 03:27:08 +00:00
|
|
|
<!-- *********************************************************************** -->
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="seealso">See Also</a>
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
2003-10-24 17:57:33 +00:00
|
|
|
<div class="doc_text">
|
|
|
|
|
|
|
|
|
|
<p>A lot of these comments and recommendations have been culled for other
|
|
|
|
|
sources. Two particularly important books for our work are:</p>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<ol>
|
|
|
|
|
|
2007-11-09 21:49:08 +00:00
|
|
|
<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
|
|
|
|
|
C++</a> by Scott Meyers. Also
|
2004-05-23 21:05:07 +00:00
|
|
|
interesting and useful are "More Effective C++" and "Effective STL" by the same
|
|
|
|
|
author.</li>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
2007-11-09 21:49:08 +00:00
|
|
|
<li>Large-Scale C++ Software Design by John Lakos</li>
|
2003-10-24 17:57:33 +00:00
|
|
|
|
|
|
|
|
</ol>
|
|
|
|
|
|
|
|
|
|
<p>If you get some free time, and you haven't read them: do so, you might learn
|
2004-12-04 00:32:12 +00:00
|
|
|
something.</p>
|
2003-10-24 17:57:33 +00:00
|
|
|
|
|
|
|
|
</div>
|
2001-07-09 03:27:08 +00:00
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<hr>
|
2004-05-12 18:37:22 +00:00
|
|
|
<address>
|
|
|
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
2008-12-11 17:34:48 +00:00
|
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
|
2004-05-12 18:37:22 +00:00
|
|
|
<a href="http://validator.w3.org/check/referer"><img
|
2008-12-11 17:34:48 +00:00
|
|
|
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
|
2004-05-12 18:37:22 +00:00
|
|
|
|
|
|
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
2006-03-14 05:39:39 +00:00
|
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
2003-10-24 17:57:33 +00:00
|
|
|
Last modified: $Date$
|
2004-05-12 18:37:22 +00:00
|
|
|
</address>
|
2003-10-24 17:57:33 +00:00
|
|
|
|
|
|
|
|
</body>
|
|
|
|
|
</html>
|
