"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
+ <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link rel="stylesheet" href="llvm.css" type="text/css">
<title>LLVM Coding Standards</title>
</head>
<body>
-<div class="doc_title">
+<h1>
LLVM Coding Standards
-</div>
+</h1>
<ol>
<li><a href="#introduction">Introduction</a></li>
<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>
+ <li><a href="#ci_rtti_exceptions">Do not use RTTI or Exceptions</a></li>
+ <li><a href="#ci_class_struct">Use of <tt>class</tt>/<tt>struct</tt> Keywords</a></li>
</ol></li>
</ol></li>
<li><a href="#styleissues">Style Issues</a>
<ol>
- <li><a href="#macro">The High Level Issues</a>
+ <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_dontinclude"><tt>#include</tt> 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
+ <li><a href="#hl_earlyexit">Use Early Exits and <tt>continue</tt> 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_else_after_return">Don't use <tt>else</tt> after a
+ <tt>return</tt></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>
+ <li><a href="#micro">The Low-Level Issues</a>
<ol>
+ <li><a href="#ll_naming">Name Types, Functions, Variables, and Enumerators Properly</a></li>
<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_ns_std">Do not use '<tt>using namespace std</tt>'</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
+ <li><a href="#ll_end">Don't evaluate <tt>end()</tt> every time through a
loop</a></li>
<li><a href="#ll_iostream"><tt>#include <iostream></tt> is
<em>forbidden</em></a></li>
+ <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></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>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="introduction">Introduction</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<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
</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="mechanicalissues">Mechanical Source Issues</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
+<div>
+
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="sourceformating">Source Code Formatting</a>
-</div>
+</h3>
+
+<div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="scf_commenting">Commenting</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>Comments are one critical part of readability and maintainability. Everyone
knows they should comment, so should you. When writing comments, write them as
comment our code more than we do, there are a few very critical places that
documentation is very useful:</p>
-<b>File Headers</b>
+<h5>File Headers</h5>
+
+<div>
<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
<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
+is a C++ file, not a C file (Emacs assumes <tt>.h</tt> files are C files by default).
+Note that this tag is not necessary in <tt>.cpp</tt> 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>
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>
+</div>
+
+<h5>Class overviews</h5>
<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
something sane goes a long ways towards avoiding writing documentation.</p>
-<b>Method information</b>
+<h5>Method information</h5>
+
+<div>
<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
+documented properly. A quick note about what it does and 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
+particularly tricky or insidious 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>
</div>
+</div>
+
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="scf_commentformat">Comment Formatting</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<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
+ <li>When writing 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>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="scf_includes"><tt>#include</tt> Style</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
include guards if working on a header file), the <a
order:</p>
<ol>
- <li><a href="#mmheader">Main Module header</a></li>
+ <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/Bitcode/*</tt></li>
<li><tt>llvm/CodeGen/*</tt></li>
<li>...</li>
<li><tt>Support/*</tt></li>
<li>System <tt>#includes</tt></li>
</ol>
-<p>... and each category should be sorted by name.</p>
+<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>
+<p><a name="mmheader">The "Main Module Header"</a> file applies to <tt>.cpp</tt> files
+which implement an interface defined by a <tt>.h</tt> 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
+system. By including a header file first in the <tt>.cpp</tt> 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
+form of documentation in the <tt>.cpp</tt> file to indicate where the interfaces it
implements are defined.</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="scf_codewidth">Source Code Width</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<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
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
+<p>This is one of many contentious issues in coding standards, but it is not up
for debate.</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<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
+preferred indentation levels, and different styles of indentation that they
+like; this is fine. What isn't fine 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
+style of existing code if you 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">
+<h4>
<a name="scf_indentation">Indent Code Consistently</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
-<p>Okay, your first year of programming you were told that indentation is
+<p>Okay, in 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>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="compilerissues">Compiler Issues</a>
-</div>
+</h3>
+<div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
-<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
+<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
+a good thorough set of warnings, and stick to it. 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
+syntax of the code slightly. For example, a warning that annoys me occurs when
I write code like this:</p>
<div class="doc_code">
</pre>
</div>
-<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
+<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>
+<p>These are the <tt>gcc</tt> warnings that I prefer to enable:</p>
+
+<div class="doc_code">
+<pre>
+-Wall -Winline -W -Wwrite-strings -Wno-unused
+</pre>
+</div>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="ci_portable_code">Write Portable Code</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<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>
+compiler, and Visual Studio tends to be the lowest common denominator.
+If advanced features are used, they should only be an implementation detail of
+a library which has a simple exposed API, and preferably be buried in
+libSystem.</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
-<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
+<h4>
+<a name="ci_rtti_exceptions">Do not use RTTI or Exceptions</a>
+</h4>
+<div>
+
+<p>In an effort to reduce code and executable size, LLVM does not use RTTI
+(e.g. <tt>dynamic_cast<></tt>) or exceptions. These two language features
+violate the general C++ principle of <i>"you only pay for what you use"</i>,
+causing executable bloat even if exceptions are never used in the code base, or
+if RTTI is never used for a class. Because of this, we turn them off globally
+in the code.</p>
+
+<p>That said, LLVM does make extensive use of a hand-rolled form of RTTI that
+use templates like <a href="ProgrammersManual.html#isa"><tt>isa<></tt>,
+<tt>cast<></tt>, and <tt>dyn_cast<></tt></a>. This form of RTTI is
+opt-in and can be added to any class. It is also substantially more efficient
+than <tt>dynamic_cast<></tt>.</p>
+
</div>
-<div class="doc_text">
+
+<!-- _______________________________________________________________________ -->
+<h4>
+<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
+</h4>
+<div>
<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:
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>
+<b>all</b> members are public and the type is a C++
+<a href="http://en.wikipedia.org/wiki/Plain_old_data_structure">POD</a> type, in
+which case <tt>struct</tt> is allowed.</p>
+
+</div>
+
+</div>
</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="styleissues">Style Issues</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
+<div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="macro">The High Level Issues</a>
-</div>
+<h3>
+ <a name="macro">The High-Level Issues</a>
+</h3>
<!-- ======================================================================= -->
+<div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="hl_module">A Public Header File <b>is</b> a Module</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<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>
+source tree, they live in the top level "<tt>include</tt>" 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>
+header files should only <tt>#include</tt> 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 by 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
+their interface first. This ensures 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">
+<h4>
<a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<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
+<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
+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>
+<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 accidentally 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>
+<h4>
+ <a name="hl_privateheaders">Keep "Internal" Headers Private</a>
+</h4>
-<div class="doc_text">
+<div>
<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>
+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>
+<p>Note however, that it's okay to put extra implementation methods in 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>
+<h4>
+ <a name="hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify Code</a>
+</h4>
-<div class="doc_text">
+<div>
<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>
+exits and the <tt>continue</tt> 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>
</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>This code has several problems if the body of the '<tt>if</tt>' 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 <tt>if</tt> 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, ...
+ // Terminators never need 'something' done to them because ...
if (isa<TerminatorInst>(I))
return 0;
</pre>
</div>
-<p>This fixes these problems. A similar problem frequently happens in for
+<p>This fixes these problems. A similar problem frequently happens in <tt>for</tt>
loops. A silly example is something like this:</p>
<div class="doc_code">
</pre>
</div>
-<p>When you have very very small loops, this sort of structure is fine, but if
+<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>
+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 <tt>if</tt> conditions will have elses etc.
+It is strongly preferred to structure the loop like this:</p>
<div class="doc_code">
<pre>
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
+<p>This has all the benefits of using early exits for 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>
+and it makes it obvious to the reader that there is no <tt>else</tt> 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>
+<h4>
+ <a name="hl_else_after_return">Don't use <tt>else</tt> after a <tt>return</tt></a>
+</h4>
-<div class="doc_text">
+<div>
<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>
-
+please do not use '<tt>else</tt>' or '<tt>else if</tt>' after something that
+interrupts control flow — like <tt>return</tt>, <tt>break</tt>,
+<tt>continue</tt>, <tt>goto</tt>, etc. For example, this is <em>bad</em>:</p>
+
<div class="doc_code">
<pre>
case 'J': {
if (Type.isNull()) {
Error = ASTContext::GE_Missing_sigjmp_buf;
return QualType();
- } else {
+ <b>} else {
break;
- }
+ }</b>
} else {
Type = Context.getjmp_bufType();
if (Type.isNull()) {
Error = ASTContext::GE_Missing_jmp_buf;
return QualType();
- } else {
+ <b>} else {
break;
- }
+ }</b>
}
}
}
</pre>
</div>
-<p>It is better to write this something like:</p>
+<p>It is better to write it like this:</p>
<div class="doc_code">
<pre>
return QualType();
}
}
- break;
+ <b>break;</b>
</pre>
</div>
-<p>Or better yet (in this case), as:</p>
+<p>Or better yet (in this case) as:</p>
<div class="doc_code">
<pre>
ASTContext::GE_Missing_jmp_buf;
return QualType();
}
- break;
+ <b>break;</b>
</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>
+track of when reading the code.</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
-<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>
+<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>
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>
-
+the code to be structured like this:</p>
<div class="doc_code">
<pre>
</div>
+</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="micro">The Low Level Issues</a>
-</div>
+<h3>
+ <a name="micro">The Low-Level Issues</a>
+</h3>
<!-- ======================================================================= -->
+<div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="ll_assert">Assert Liberally</a>
+<h4>
+ <a name="ll_naming">
+ Name Types, Functions, Variables, and Enumerators Properly
+ </a>
+</h4>
+
+<div>
+
+<p>Poorly-chosen names can mislead the reader and cause bugs. We cannot stress
+enough how important it is to use <em>descriptive</em> names. Pick names that
+match the semantics and role of the underlying entities, within reason. Avoid
+abbreviations unless they are well known. After picking a good name, make sure
+to use consistent capitalization for the name, as inconsistency requires clients
+to either memorize the APIs or to look it up to find the exact spelling.</p>
+
+<p>In general, names should be in camel case (e.g. <tt>TextFileReader</tt>
+and <tt>isLValue()</tt>). Different kinds of declarations have different
+rules:</p>
+
+<ul>
+<li><p><b>Type names</b> (including classes, structs, enums, typedefs, etc)
+ should be nouns and start with an upper-case letter (e.g.
+ <tt>TextFileReader</tt>).</p></li>
+
+<li><p><b>Variable names</b> should be nouns (as they represent state). The
+ name should be camel case, and start with an upper case letter (e.g.
+ <tt>Leader</tt> or <tt>Boats</tt>).</p></li>
+
+<li><p><b>Function names</b> should be verb phrases (as they represent
+ actions), and command-like function should be imperative. The name should
+ be camel case, and start with a lower case letter (e.g. <tt>openFile()</tt>
+ or <tt>isFoo()</tt>).</p></li>
+
+<li><p><b>Enum declarations</b> (e.g. <tt>enum Foo {...}</tt>) are types, so
+ they should follow the naming conventions for types. A common use for enums
+ is as a discriminator for a union, or an indicator of a subclass. When an
+ enum is used for something like this, it should have a <tt>Kind</tt> suffix
+ (e.g. <tt>ValueKind</tt>).</p></li>
+
+<li><p><b>Enumerators</b> (e.g. <tt>enum { Foo, Bar }</tt>) and <b>public member
+ variables</b> should start with an upper-case letter, just like types.
+ Unless the enumerators are defined in their own small namespace or inside a
+ class, enumerators should have a prefix corresponding to the enum
+ declaration name. For example, <tt>enum ValueKind { ... };</tt> may contain
+ enumerators like <tt>VK_Argument</tt>, <tt>VK_BasicBlock</tt>, etc.
+ Enumerators that are just convenience constants are exempt from the
+ requirement for a prefix. For instance:</p>
+
+<div class="doc_code">
+<pre>
+enum {
+ MaxSize = 42,
+ Density = 12
+};
+</pre>
</div>
+</li>
-<div class="doc_text">
+</ul>
+
+<p>As an exception, classes that mimic STL classes can have member names in
+STL's style of lower-case words separated by underscores (e.g. <tt>begin()</tt>,
+<tt>push_back()</tt>, and <tt>empty()</tt>).</p>
+
+<p>Here are some examples of good and bad names:</p>
-<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
+<div class="doc_code">
+<pre>
+class VehicleMaker {
+ ...
+ Factory<Tire> F; // Bad -- abbreviation and non-descriptive.
+ Factory<Tire> Factory; // Better.
+ Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one
+ // kind of factories.
+};
+
+Vehicle MakeVehicle(VehicleType Type) {
+ VehicleMaker M; // Might be OK if having a short life-span.
+ Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information.
+ Light headlight = M.makeLight("head"); // Good -- descriptive.
+ ...
+}
+</pre>
+</div>
+
+</div>
+
+
+<!-- _______________________________________________________________________ -->
+<h4>
+ <a name="ll_assert">Assert Liberally</a>
+</h4>
+
+<div>
+
+<p>Use the "<tt>assert</tt>" macro to its fullest. Check all of your
+preconditions and assumptions, you never know when a bug (not necessarily 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
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
+in the assertion statement, which is printed if the assertion is tripped. This
+helps the poor debugger 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>
</div>
-<p>Here are some examples:</p>
+<p>Here are more examples:</p>
<div class="doc_code">
<pre>
</pre>
</div>
-<p>You get the idea...</p>
+<p>You get the idea.</p>
-<p>Please be aware when adding assert statements that not all compilers are aware of
+<p>Please be aware that, when adding assert statements, 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>
</pre>
</div>
+<p>Another issue is that values used only by assertions will produce an "unused
+value" warning when assertions are disabled. For example, this code will
+warn:</p>
+
+<div class="doc_code">
+<pre>
+unsigned Size = V.size();
+assert(Size > 42 && "Vector smaller than it should be");
+
+bool NewToSet = Myset.insert(Value);
+assert(NewToSet && "The value shouldn't be in the set yet");
+</pre>
</div>
-<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
+<p>These are two interesting different cases. In the first case, the call to
+V.size() is only useful for the assert, and we don't want it executed when
+assertions are disabled. Code like this should move the call into the assert
+itself. In the second case, the side effects of the call must happen whether
+the assert is enabled or not. In this case, the value should be cast to void to
+disable the warning. To be specific, it is preferred to write the code like
+this:</p>
+
+<div class="doc_code">
+<pre>
+assert(V.size() > 42 && "Vector smaller than it should be");
+
+bool NewToSet = Myset.insert(Value); (void)NewToSet;
+assert(NewToSet && "The value shouldn't be in the set yet");
+</pre>
</div>
-<div class="doc_text">
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<h4>
+ <a name="ll_ns_std">Do Not Use '<tt>using namespace std</tt>'</a>
+</h4>
+
+<div>
+
<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>
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
+<p>In implementation files (e.g. <tt>.cpp</tt> 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
+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
<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>
+As such, it is ok, and actually clearer, for the <tt>.cpp</tt> files to have a
+'<tt>using namespace llvm;</tt>' directive at the top, after the
+<tt>#include</tt>s. This reduces indentation in the body of the file for source
+editors that indent based on braces, and keeps the conceptual context cleaner.
+The general form of this rule is that any <tt>.cpp</tt> 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>
+<h4>
+ <a name="ll_virtual_anch">
+ Provide a Virtual Method Anchor for Classes in Headers
+ </a>
+</h4>
-<div class="doc_text">
+<div>
<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
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="ll_end">Don't evaluate end() every time through a loop</a>
-</div>
+<h4>
+ <a name="ll_end">Don't evaluate <tt>end()</tt> every time through a loop</a>
+</h4>
-<div class="doc_text">
+<div>
-<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>
+<p>Because C++ doesn't have a standard "<tt>foreach</tt>" 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>
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
+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>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
-</div>
+<h4>
+ <a name="ll_iostream"><tt>#include <iostream></tt> is Forbidden</a>
+</h4>
-<div class="doc_text">
+<div>
<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>
+the static constructors are run whenever an application starts 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 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
</ol>
<p>Note that using the other stream headers (<tt><sstream></tt> for
-example) is not problematic in this regard (just <tt><iostream></tt>).
-However, raw_ostream provides various APIs that are better performing for almost
-every use than std::ostream style APIs, so you should just use it for new
-code.</p>
-
-<p><b>New code should always
+example) is not problematic in this regard —
+just <tt><iostream></tt>. However, <tt>raw_ostream</tt> provides various
+APIs that are better performing for almost every use than <tt>std::ostream</tt>
+style APIs. <b>Therefore new code should always
use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or
the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
- <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
+<h4>
+ <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
+</h4>
+
+<div>
+
+<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_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>
+<!-- _______________________________________________________________________ -->
+<h4>
+ <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
+</h4>
+
+<div>
+
+<p>The <tt>std::endl</tt> modifier, when used with <tt>iostreams</tt> 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>
</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">
+<h3>
<a name="nano">Microscopic Details</a>
-</div>
+</h3>
<!-- ======================================================================= -->
+<div>
+
<p>This section describes preferred low-level formatting guidelines along with
reasoning on why we prefer them.</p>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="micro_spaceparen">Spaces Before Parentheses</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
-<p>We prefer to put a space before a parentheses only in control flow
+<p>We prefer to put a space before an open parenthesis 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>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");
+<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>
+a = <b>foo(</b>42, 92) + <b>bar(</b>x);
+</pre>
</div>
-<p>... and this is bad:</p>
+<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>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");
+<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);
+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>
+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);
+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>
+<p>when skimming through the code. By avoiding a space in a function, we avoid
+this misinterpretation.</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="micro_preincrement">Prefer Preincrement</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<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
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="micro_namespaceindent">Namespace Indentation</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>
-In general, we strive to reduce indentation where ever possible. This is useful
+In general, we strive to reduce indentation wherever 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
<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
+larger (as it typically is in a header in the <tt>llvm</tt> or <tt>clang</tt> namespaces), do not
indent the code, and add a comment indicating what namespace is being closed.
For example:</p>
</div>
<!-- _______________________________________________________________________ -->
-<div class="doc_subsubsection">
+<h4>
<a name="micro_anonns">Anonymous Namespaces</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<p>After talking about namespaces in general, you may be wondering about
anonymous namespaces in particular.
</div>
+</div>
+</div>
<!-- *********************************************************************** -->
-<div class="doc_section">
+<h2>
<a name="seealso">See Also</a>
-</div>
+</h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>A lot of these comments and recommendations have been culled for other
sources. Two particularly important books for our work are:</p>
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
- <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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</address>
projects / oota-llvm.git / blobdiff