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|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<link rel="stylesheet" href="llvm.css" type="text/css">
<title>LLVM Coding Standards</title>
</head>
<body>
<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 <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>
</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 <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>
<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 and a description of the
borderline behaviour is all that is necessary here (unless something
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>
<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/Bitcode/*</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 <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 <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 <tt>.cpp</tt> 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
preferred 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, 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 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, 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>
</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 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">
<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
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>
<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<TerminatorInst>(I) &&
I->hasOneUse() && 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<TerminatorInst>(I))
return 0;
// We conservatively avoid transforming instructions with multiple uses
// because goats like cheese.
if (!I->hasOneUse())
return 0;
// This is really just here for example.
if (!SomeOtherThing(I))
return 0;
... some long code ....
}
</pre>
</div>
<p>This fixes these problems. A similar problem frequently happens in <tt>for</tt>
loops. A silly example is something like this:</p>
<div class="doc_code">
<pre>
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
Value *LHS = BO->getOperand(0);
Value *RHS = BO->getOperand(1);
if (LHS != RHS) {
...
}
}
}
</pre>
</div>
<p>When you have very very small loops, this sort of structure is fine, but if
it exceeds more than 10-15 lines, it becomes difficult for people to read and
understand at a glance.
The problem with this sort of code is that it gets very nested very quickly,
meaning that the reader of the code has to keep a lot of context in their brain
to remember what is going immediately on in the loop, because they don't know
if/when the if conditions will have elses etc. It is strongly preferred to
structure the loop like this:</p>
<div class="doc_code">
<pre>
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
if (!BO) continue;
Value *LHS = BO->getOperand(0);
Value *RHS = BO->getOperand(1);
if (LHS == RHS) continue;
...
}
</pre>
</div>
<p>This has all the benefits of using early exits from functions: it reduces
nesting of the loop, it makes it easier to describe why the conditions are true,
and it makes it obvious to the reader that there is no "else" coming up that
they have to push context into their brain for. If a loop is large, this can
be a big understandability win.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="hl_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]->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<Bar*> &List) {
for (unsigned i = 0, e = List.size(); i != e; ++i)
if (List[i]->isFoo())
return true;
return false;
}
...
<b>if (ListContainsFoo(BarList)) {</b>
...
}
</pre>
</div>
<p>There are many reasons for doing this: it reduces indentation and factors out
code which can often be shared by other code that checks for the same predicate.
More importantly, it <em>forces you to pick a name</em> for the function, and
forces you to write a comment for it. In this silly example, this doesn't add
much value. However, if the condition is complex, this can make it a lot easier
for the reader to understand the code that queries for this predicate. Instead
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>" 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 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>
inline Value *getOperand(unsigned i) {
assert(i < Operands.size() && "getOperand() out of range!");
return Operands[i];
}
</pre>
</div>
<p>Here are some examples:</p>
<div class="doc_code">
<pre>
assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
assert(idx < getNumSuccessors() && "Successor # out of range!");
assert(V1.getType() == V2.getType() && "Constant types must be identical!");
assert(isa<PHINode>(Succ->front()) && "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 && "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 && "Some helpful error message");
// Not reached
return 0;
</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>
<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>
<!-- _______________________________________________________________________ -->
<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. <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
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 <tt>.cpp</tt> files to have a
'<tt>using namespace llvm</tt>' directive at their 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>
<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 <iostream></tt> is forbidden</a>
</div>
<div class="doc_text">
<p>The use of <tt>#include <iostream></tt> in library files is
hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
support clients using LLVM libraries as part of larger systems. In particular,
we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
the static c'tors are run whenever an application start up that uses the dynamic
library. There are two problems with this:</p>
<ol>
<li>The time to run the static c'tors impacts startup time of
applications—a critical time for GUI apps.</li>
<li>The static c'tors cause the app to pull many extra pages of memory off the
disk: both the code for the static c'tors in each <tt>.o</tt> file and the
small amount of data that gets touched. In addition, touched/dirty pages
put more pressure on the VM system on low-memory machines.</li>
</ol>
<p>Note that using the other stream headers (<tt><sstream></tt> for
example) is 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>
<!-- _______________________________________________________________________ -->
<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_subsubsection">
<a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
</div>
<div class="doc_text">
<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
to the output stream specified. In addition to doing this, however, it also
flushes the output stream. In other words, these are equivalent:</p>
<div class="doc_code">
<pre>
std::cout << std::endl;
std::cout << '\n' << std::flush;
</pre>
</div>
<p>Most of the time, you probably have no reason to flush the output stream, so
it's better to use a literal <tt>'\n'</tt>.</p>
</div>
<!-- ======================================================================= -->
<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 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>somefunc(</b>42);
<b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me");
a = <b>foo(</b>42, 92) + <b>bar(</b>x);
</pre>
</div>
<p>... and this is bad:</p>
<div class="doc_code">
<pre>
<b>if(</b>x) ...
<b>for(</b>i = 0; i != 100; ++i) ...
<b>while(</b>llvm_rocks) ...
<b>somefunc (</b>42);
<b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me");
a = <b>foo (</b>42, 92) + <b>bar (</b>x);
</pre>
</div>
<p>The reason for doing this is not completely arbitrary. This style makes
control flow operators stand out more, and makes expressions flow better. The
function call operator binds very tightly as a postfix operator. Putting
a space after a function name (as in the last example) makes it appear that
the code might bind the arguments of the left-hand-side of a binary operator
with the argument list of a function and the name of the right side. More
specifically, it is easy to misread the "a" example as:</p>
<div class="doc_code">
<pre>
a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
</pre>
</div>
<p>... when skimming through the code. By avoiding a space in a function, we
avoid this misinterpretation.</p>
</div>
<!-- _______________________________________________________________________ -->
<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 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
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 <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 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<(const char *RHS) const;
};
<b>} // end anonymous namespace</b>
static void Helper() {
...
}
bool StringSort::operator<(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<(const char *RHS) const;
};
void Helper() {
...
}
bool StringSort::operator<(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<" 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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