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              <title>LLVM Coding Standards</title>

            
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            <div class="doc_title">

            
              LLVM Coding Standards

            
            </div>


            <ol>

            
              <li><a href="#introduction">Introduction</a></li>

            
              <li><a href="#mechanicalissues">Mechanical Source Issues</a>

            
                <ol>

            
                  <li><a href="#sourceformating">Source Code Formatting</a>

            
                    <ol>

            
                      <li><a href="#scf_commenting">Commenting</a></li>

            
                      <li><a href="#scf_commentformat">Comment Formatting</a></li>

            
                      <li><a href="#scf_includes"><tt>#include</tt> Style</a></li>

            
                      <li><a href="#scf_codewidth">Source Code Width</a></li>

            
                      <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>

            
                      <li><a href="#scf_indentation">Indent Code Consistently</a></li>

            
                    </ol></li>

            
                  <li><a href="#compilerissues">Compiler Issues</a>

            
                    <ol>

            
                      <li><a href="#ci_warningerrors">Treat Compiler Warnings Like

            
                          Errors</a></li>

            
                      <li><a href="#ci_portable_code">Write Portable Code</a></li>

            
                      <li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>

            
                    </ol></li>

            
                </ol></li>

            
              <li><a href="#styleissues">Style Issues</a>

            
                <ol>

            
                  <li><a href="#macro">The High Level Issues</a>

            
                    <ol>

            
                      <li><a href="#hl_module">A Public Header File <b>is</b> a

            
                          Module</a></li>

            
                      <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>

            
                      <li><a href="#hl_privateheaders">Keep "internal" Headers

            
                          Private</a></li>

            
                      <li><a href="#hl_earlyexit">Use Early Exits and 'continue' to Simplify

            
                          Code</a></li>

            
                      <li><a href="#hl_else_after_return">Don't use "else" after a

            
                          return</a></li>

            
                      <li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate

            
                          Functions</a></li>

            
                    </ol></li>

            
                  <li><a href="#micro">The Low Level Issues</a>

            
                    <ol>

            
                      <li><a href="#ll_assert">Assert Liberally</a></li>

            
                      <li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>

            
                      <li><a href="#ll_virtual_anch">Provide a virtual method anchor for

            
                          classes in headers</a></li>

            
                      <li><a href="#ll_end">Don't evaluate end() every time through a

            
                          loop</a></li>

            
                      <li><a href="#ll_iostream"><tt>#include &lt;iostream&gt;</tt> is

            
                          <em>forbidden</em></a></li>

            
                      <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>

            
                      <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li>

            
                    </ol></li>


                  <li><a href="#nano">Microscopic Details</a>

            
                    <ol>

            
                      <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>

            
                      <li><a href="#micro_preincrement">Prefer Preincrement</a></li>

            
                      <li><a href="#micro_namespaceindent">Namespace Indentation</a></li>

            
                      <li><a href="#micro_anonns">Anonymous Namespaces</a></li>

            
                    </ol></li>


                </ol></li>

            
              <li><a href="#seealso">See Also</a></li>

            
            </ol>


            <div class="doc_author">

            
              <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>

            
            </div>


            <!-- *********************************************************************** -->

            
            <div class="doc_section">

            
              <a name="introduction">Introduction</a>

            
            </div>

            
            <!-- *********************************************************************** -->


            <div class="doc_text">


            <p>This document attempts to describe a few coding standards that are being used

            
            in the LLVM source tree.  Although no coding standards should be regarded as

            
            absolute requirements to be followed in all instances, coding standards can be

            
            useful.</p>


            <p>This document intentionally does not prescribe fixed standards for religious

            
            issues such as brace placement and space usage.  For issues like this, follow

            
            the golden rule:</p>


            <blockquote>


            <p><b><a name="goldenrule">If you are adding a significant body of source to a

            
            project, feel free to use whatever style you are most comfortable with.  If you

            
            are extending, enhancing, or bug fixing already implemented code, use the style

            
            that is already being used so that the source is uniform and easy to

            
            follow.</a></b></p>


            </blockquote>


            <p>The ultimate goal of these guidelines is the increase readability and

            
            maintainability of our common source base. If you have suggestions for topics to

            
            be included, please mail them to <a

            
            href="mailto:sabre@nondot.org">Chris</a>.</p>


            </div>


            <!-- *********************************************************************** -->

            
            <div class="doc_section">

            
              <a name="mechanicalissues">Mechanical Source Issues</a>

            
            </div>

            
            <!-- *********************************************************************** -->


            <!-- ======================================================================= -->

            
            <div class="doc_subsection">

            
              <a name="sourceformating">Source Code Formatting</a>

            
            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_commenting">Commenting</a>

            
            </div>


            <div class="doc_text">


            <p>Comments are one critical part of readability and maintainability.  Everyone

            
            knows they should comment, so should you.  When writing comments, write them as

            
            English prose, which means they should use proper capitalization, punctuation,

            
            etc.  Although we all should probably

            
            comment our code more than we do, there are a few very critical places that

            
            documentation is very useful:</p>


            <b>File Headers</b>


            <p>Every source file should have a header on it that describes the basic

            
            purpose of the file.  If a file does not have a header, it should not be

            
            checked into Subversion.  Most source trees will probably have a standard

            
            file header format.  The standard format for the LLVM source tree looks like

            
            this:</p>


            <div class="doc_code">

            
            <pre>

            
            //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//

            
            //

            
            //                     The LLVM Compiler Infrastructure

            
            //

            
            // This file is distributed under the University of Illinois Open Source

            
            // License. See LICENSE.TXT for details.

            
            //

            
            //===----------------------------------------------------------------------===//

            
            //

            
            // This file contains the declaration of the Instruction class, which is the

            
            // base class for all of the VM instructions.

            
            //

            
            //===----------------------------------------------------------------------===//

            
            </pre>

            
            </div>


            <p>A few things to note about this particular format:  The "<tt>-*- C++

            
            -*-</tt>" string on the first line is there to tell Emacs that the source file

            
            is a C++ file, not a C file (Emacs assumes .h files are C files by default).

            
            Note that this tag is not necessary in .cpp files.  The name of the file is also

            
            on the first line, along with a very short description of the purpose of the

            
            file.  This is important when printing out code and flipping though lots of

            
            pages.</p>


            <p>The next section in the file is a concise note that defines the license

            
            that the file is released under.  This makes it perfectly clear what terms the

            
            source code can be distributed under and should not be modified in any way.</p>


            <p>The main body of the description does not have to be very long in most cases.

            
            Here it's only two lines.  If an algorithm is being implemented or something

            
            tricky is going on, a reference to the paper where it is published should be

            
            included, as well as any notes or "gotchas" in the code to watch out for.</p>


            <b>Class overviews</b>


            <p>Classes are one fundamental part of a good object oriented design.  As such,

            
            a class definition should have a comment block that explains what the class is

            
            used for... if it's not obvious.  If it's so completely obvious your grandma

            
            could figure it out, it's probably safe to leave it out.  Naming classes

            
            something sane goes a long ways towards avoiding writing documentation.</p>


            <b>Method information</b>


            <p>Methods defined in a class (as well as any global functions) should also be

            
            documented properly.  A quick note about what it does any a description of the

            
            borderline behaviour is all that is necessary here (unless something

            
            particularly tricky or insideous is going on).  The hope is that people can

            
            figure out how to use your interfaces without reading the code itself... that is

            
            the goal metric.</p>


            <p>Good things to talk about here are what happens when something unexpected

            
            happens: does the method return null?  Abort?  Format your hard disk?</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_commentformat">Comment Formatting</a>

            
            </div>


            <div class="doc_text">


            <p>In general, prefer C++ style (<tt>//</tt>) comments.  They take less space,

            
            require less typing, don't have nesting problems, etc.  There are a few cases

            
            when it is useful to use C style (<tt>/* */</tt>) comments however:</p>


            <ol>

            
              <li>When writing a C code: Obviously if you are writing C code, use C style

            
                  comments.</li>

            
              <li>When writing a header file that may be <tt>#include</tt>d by a C source

            
                  file.</li>

            
              <li>When writing a source file that is used by a tool that only accepts C

            
                  style comments.</li>

            
            </ol>


            <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.

            
            These nest properly and are better behaved in general than C style comments.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_includes"><tt>#include</tt> Style</a>

            
            </div>


            <div class="doc_text">


            <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and

            
            include guards if working on a header file), the <a

            
            href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the

            
            file should be listed.  We prefer these <tt>#include</tt>s to be listed in this

            
            order:</p>


            <ol>

            
              <li><a href="#mmheader">Main Module header</a></li>

            
              <li><a href="#hl_privateheaders">Local/Private Headers</a></li>

            
              <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>

            
              <li>...</li>

            
              <li><tt>Support/*</tt></li>

            
              <li><tt>Config/*</tt></li>

            
              <li>System <tt>#includes</tt></li>

            
            </ol>


            <p>... and each category should be sorted by name.</p>


            <p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file

            
            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>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_codewidth">Source Code Width</a>

            
            </div>


            <div class="doc_text">


            <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>


            <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>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>

            
            </div>


            <div class="doc_text">


            <p>In all cases, prefer spaces to tabs in source files.  People have different

            
            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

            
            unreadable, and it is not worth dealing with.</p>


            <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the

            
            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

            
            makes for incredible diffs that are absolutely worthless.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_indentation">Indent Code Consistently</a>

            
            </div>


            <div class="doc_text">


            <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>


            </div>


            <!-- ======================================================================= -->

            
            <div class="doc_subsection">

            
              <a name="compilerissues">Compiler Issues</a>

            
            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>

            
            </div>


            <div class="doc_text">


            <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

            
            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

            
            I write code like this:</p>


            <div class="doc_code">

            
            <pre>

            
            if (V = getValue()) {

            
              ...

            
            }

            
            </pre>

            
            </div>


            <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>


            <div class="doc_code">

            
            <pre>

            
            if ((V = getValue())) {

            
              ...

            
            }

            
            </pre>

            
            </div>


            <p>...which shuts <tt>gcc</tt> up.  Any <tt>gcc</tt> warning that annoys you can

            
            be fixed by massaging the code appropriately.</p>


            <p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall

            
            -Winline -W -Wwrite-strings -Wno-unused</tt></p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ci_portable_code">Write Portable Code</a>

            
            </div>


            <div class="doc_text">


            <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>


            <p>In practice, this means that you shouldn't assume much about the host

            
            compiler, including its support for "high tech" features like partial

            
            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>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <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">


            <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 and the type is a C++ "POD" type, in which case

            
            <tt>struct</tt> is allowed.</p>


            </div>


            <!-- *********************************************************************** -->

            
            <div class="doc_section">

            
              <a name="styleissues">Style Issues</a>

            
            </div>

            
            <!-- *********************************************************************** -->


            <!-- ======================================================================= -->

            
            <div class="doc_subsection">

            
              <a name="macro">The High Level Issues</a>

            
            </div>

            
            <!-- ======================================================================= -->


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_module">A Public Header File <b>is</b> a Module</a>

            
            </div>


            <div class="doc_text">


            <p>C++ doesn't do too well in the modularity department.  There is no real

            
            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

            
            module of functionality.</p>


            <p>Ideally, modules should be completely independent of each other, and their

            
            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

            
            together.</p>


            <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>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>

            
            </div>


            <div class="doc_text">


            <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

            
            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>


            <p>It is easy to try to go too overboard on this recommendation, however.  You

            
            <b>must</b> include all of the header files that you are using -- you can

            
            include them either directly

            
            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

            
            about later...</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_privateheaders">Keep "internal" Headers Private</a>

            
            </div>


            <div class="doc_text">


            <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

            
            public module header file.  Don't do this.</p>


            <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>


            <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>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_earlyexit">Use Early Exits and 'continue' to Simplify Code</a>

            
            </div>


            <div class="doc_text">


            <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>


            <div class="doc_code">

            
            <pre>

            
            Value *DoSomething(Instruction *I) {

            
              if (!isa&lt;TerminatorInst&gt;(I) &amp;&amp;

            
                  I-&gt;hasOneUse() &amp;&amp; SomeOtherThing(I)) {

            
                ... some long code ....

            
              }


              return 0;

            
            }

            
            </pre>

            
            </div>


            <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>


            <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&lt;TerminatorInst&gt;(I))

            
                return 0;


              // We conservatively avoid transforming instructions with multiple uses

            
              // because goats like cheese.

            
              if (!I-&gt;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-&gt;begin(), E = BB-&gt;end(); II != E; ++II) {

            
                if (BinaryOperator *BO = dyn_cast&lt;BinaryOperator&gt;(II)) {

            
                  Value *LHS = BO-&gt;getOperand(0);

            
                  Value *RHS = BO-&gt;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-&gt;begin(), E = BB-&gt;end(); II != E; ++II) {

            
                BinaryOperator *BO = dyn_cast&lt;BinaryOperator&gt;(II);

            
                if (!BO) continue;


                Value *LHS = BO-&gt;getOperand(0);

            
                Value *RHS = BO-&gt;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_else_after_return">Don't use "else" after a return</a>

            
            </div>


            <div class="doc_text">


            <p>For similar reasons above (reduction of indentation and easier reading),

            
               please do not use "else" or "else if" after something that interrupts

            
               control flow like return, break, continue, goto, etc.  For example, this is

            
               "bad":</p>


            <div class="doc_code">

            
            <pre>

            
              case 'J': {

            
                if (Signed) {

            
                  Type = Context.getsigjmp_bufType();

            
                  if (Type.isNull()) {

            
                    Error = ASTContext::GE_Missing_sigjmp_buf;

            
                    return QualType();

            
                  } else {

            
                    break;

            
                  }

            
                } else {

            
                  Type = Context.getjmp_bufType();

            
                  if (Type.isNull()) {

            
                    Error = ASTContext::GE_Missing_jmp_buf;

            
                    return QualType();

            
                  } else {

            
                    break;

            
                  }

            
                }

            
              }

            
              }

            
            </pre>

            
            </div>


            <p>It is better to write this something like:</p>


            <div class="doc_code">

            
            <pre>

            
              case 'J':

            
                if (Signed) {

            
                  Type = Context.getsigjmp_bufType();

            
                  if (Type.isNull()) {

            
                    Error = ASTContext::GE_Missing_sigjmp_buf;

            
                    return QualType();

            
                  }

            
                } else {

            
                  Type = Context.getjmp_bufType();

            
                  if (Type.isNull()) {

            
                    Error = ASTContext::GE_Missing_jmp_buf;

            
                    return QualType();

            
                  }

            
                }

            
                break;

            
            </pre>

            
            </div>


            <p>Or better yet (in this case), as:</p>


            <div class="doc_code">

            
            <pre>

            
              case 'J':

            
                if (Signed)

            
                  Type = Context.getsigjmp_bufType();

            
                else

            
                  Type = Context.getjmp_bufType();


                if (Type.isNull()) {

            
                  Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :

            
                                   ASTContext::GE_Missing_jmp_buf;

            
                  return QualType();

            
                }

            
                break;

            
            </pre>

            
            </div>


            <p>The idea is to reduce indentation and the amount of code you have to keep

            
               track of when reading the code.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>

            
            </div>


            <div class="doc_text">


            <p>It is very common to write small loops that just compute a boolean

            
               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]-&gt;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

            
            <a href="#hl_earlyexit">early exits</a> to compute the predicate.  We prefer

            
            the code to be structured like this:

            
            </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&lt;Bar*&gt; &amp;List) {

            
              for (unsigned i = 0, e = List.size(); i != e; ++i)

            
                if (List[i]-&gt;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

            
            of being faced with the in-line details of how we check to see if the BarList

            
            contains a foo, we can trust the function name and continue reading with better

            
            locality.</p>


            </div>


            <!-- ======================================================================= -->

            
            <div class="doc_subsection">

            
              <a name="micro">The Low Level Issues</a>

            
            </div>

            
            <!-- ======================================================================= -->


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ll_assert">Assert Liberally</a>

            
            </div>


            <div class="doc_text">


            <p>Use the "<tt>assert</tt>" function to its fullest.  Check all of your

            
            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>&lt;cassert&gt;</tt>" header file is probably already

            
            included by the header files you are using, so it doesn't cost anything to use

            
            it.</p>


            <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

            
            helps the poor debugging make sense of why an assertion is being made and

            
            enforced, and hopefully what to do about it.  Here is one complete example:</p>


            <div class="doc_code">

            
            <pre>

            
            inline Value *getOperand(unsigned i) {

            
              assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");

            
              return Operands[i];

            
            }

            
            </pre>

            
            </div>


            <p>Here are some examples:</p>


            <div class="doc_code">

            
            <pre>

            
            assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");


            assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");


            assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");


            assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");


            assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");

            
            </pre>

            
            </div>


            <p>You get the idea...</p>


            <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>

            
            assert(0 &amp;&amp; "Some helpful error message");

            
            </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>

            
            assert(0 &amp;&amp; "Some helpful error message");

            
            // Not reached

            
            return 0;

            
            </pre>

            
            </div>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>

            
            </div>


            <div class="doc_text">

            
            <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard

            
            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

            
            the namespace of any source file that <tt>#include</tt>s the header.  This is

            
            clearly a bad thing.</p>


            <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>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ll_virtual_anch">Provide a virtual method anchor for classes

            
              in headers</a>

            
            </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

            
            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>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <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

            
            behavior, please write the loop in the first form and add a comment indicating

            
            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

            
            lookups really aren't cheap.  By writing it in the second form consistently, you

            
            eliminate the issue entirely and don't even have to think about it.</p>


            <p>The second (even bigger) issue is that writing the loop in the first form

            
            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>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is forbidden</a>

            
            </div>


            <div class="doc_text">


            <p>The use of <tt>#include &lt;iostream&gt;</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&mdash;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>&lt;sstream&gt;</tt> for

            
            example) is allowed normally, it is just <tt>&lt;iostream&gt;</tt> that is

            
            causing problems.</p>


            <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>


            </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 &lt;&lt; std::endl;

            
            std::cout &lt;&lt; '\n' &lt;&lt; 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>


            <!-- _______________________________________________________________________ -->

            
            <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::ostream</tt>.  All new code should use <tt>raw_ostream</tt> instead

            
            of <tt>ostream</tt>.</p>


            <p>Unlike <tt>std::ostream</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>


            <!-- ======================================================================= -->

            
            <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 &amp;&amp; "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 &amp;&amp; "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>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="micro_preincrement">Prefer Preincrement</a>

            
            </div>


            <div class="doc_text">


            <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>


            <p>The semantics of postincrement include making a copy of the value being

            
            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,

            
            get in the habit of always using preincrement, and you won't have a problem.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="micro_namespaceindent">Namespace Indentation</a>

            
            </div>


            <div class="doc_text">


            <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>


            <div class="doc_code">

            
            <pre>

            
            namespace llvm {

            
              namespace X86 {

            
                /// RelocationType - An enum for the x86 relocation codes. Note that

            
                /// the terminology here doesn't follow x86 convention - word means

            
                /// 32-bit and dword means 64-bit.

            
                enum RelocationType {

            
                  /// reloc_pcrel_word - PC relative relocation, add the relocated value to

            
                  /// the value already in memory, after we adjust it for where the PC is.

            
                  reloc_pcrel_word = 0,


                  /// reloc_picrel_word - PIC base relative relocation, add the relocated

            
                  /// value to the value already in memory, after we adjust it for where the

            
                  /// PIC base is.

            
                  reloc_picrel_word = 1,


                  /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just

            
                  /// add the relocated value to the value already in memory.

            
                  reloc_absolute_word = 2,

            
                  reloc_absolute_dword = 3

            
                };

            
              }

            
            }

            
            </pre>

            
            </div>


            <p>Since the body is small, indenting adds value because it makes it very clear

            
            where the namespace starts and ends, and it is easy to take the whole thing in

            
            in one "gulp" when reading the code.  If the blob of code in the namespace is

            
            larger (as it typically is in a header in the llvm or clang namespaces), do not

            
            indent the code, and add a comment indicating what namespace is being closed.

            
            For example:</p>


            <div class="doc_code">

            
            <pre>

            
            namespace llvm {

            
            namespace knowledge {


            /// Grokable - This class represents things that Smith can have an intimate

            
            /// understanding of and contains the data associated with it.

            
            class Grokable {

            
            ...

            
            public:

            
              explicit Grokable() { ... }

            
              virtual ~Grokable() = 0;


              ...


            };


            } // end namespace knowledge

            
            } // end namespace llvm

            
            </pre>

            
            </div>


            <p>Because the class is large, we don't expect that the reader can easily

            
            understand the entire concept in a glance, and the end of the file (where the

            
            namespaces end) may be a long ways away from the place they open.  As such,

            
            indenting the contents of the namespace doesn't add any value, and detracts from

            
            the readability of the class.  In these cases it is best to <em>not</em> indent

            
            the contents of the namespace.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="micro_anonns">Anonymous Namespaces</a>

            
            </div>


            <div class="doc_text">


            <p>After talking about namespaces in general, you may be wondering about

            
            anonymous namespaces in particular.

            
            Anonymous namespaces are a great language feature that tells the C++ compiler

            
            that the contents of the namespace are only visible within the current

            
            translation unit, allowing more aggressive optimization and eliminating the

            
            possibility of symbol name collisions.  Anonymous namespaces are to C++ as

            
            "static" is to C functions and global variables.  While "static" is available

            
            in C++, anonymous namespaces are more general: they can make entire classes

            
            private to a file.</p>


            <p>The problem with anonymous namespaces is that they naturally want to

            
            encourage indentation of their body, and they reduce locality of reference: if

            
            you see a random function definition in a C++ file, it is easy to see if it is

            
            marked static, but seeing if it is in an anonymous namespace requires scanning

            
            a big chunk of the file.</p>


            <p>Because of this, we have a simple guideline: make anonymous namespaces as

            
            small as possible, and only use them for class declarations.  For example, this

            
            is good:</p>


            <div class="doc_code">

            
            <pre>

            
            <b>namespace {</b>

            
              class StringSort {

            
              ...

            
              public:

            
                StringSort(...)

            
                bool operator&lt;(const char *RHS) const;

            
              };

            
            <b>} // end anonymous namespace</b>


            static void Helper() {

            
              ...

            
            }


            bool StringSort::operator&lt;(const char *RHS) const {

            
              ...

            
            }


            </pre>

            
            </div>


            <p>This is bad:</p>


            <div class="doc_code">

            
            <pre>

            
            <b>namespace {</b>

            
            class StringSort {

            
            ...

            
            public:

            
              StringSort(...)

            
              bool operator&lt;(const char *RHS) const;

            
            };


            void Helper() {

            
              ...

            
            }


            bool StringSort::operator&lt;(const char *RHS) const {

            
              ...

            
            }


            <b>} // end anonymous namespace</b>


            </pre>

            
            </div>


            <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&lt;" in the

            
            namespace just because it was declared there.

            
            </p>


            </div>


            <!-- *********************************************************************** -->

            
            <div class="doc_section">

            
              <a name="seealso">See Also</a>

            
            </div>

            
            <!-- *********************************************************************** -->


            <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>


            <ol>


            <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective

            
            C++</a> by Scott Meyers.  Also

            
            interesting and useful are "More Effective C++" and "Effective STL" by the same

            
            author.</li>


            <li>Large-Scale C++ Software Design by John Lakos</li>


            </ol>


            <p>If you get some free time, and you haven't read them: do so, you might learn

            
            something.</p>


            </div>


            <!-- *********************************************************************** -->


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