diff options
| author | mike-m <mikem.llvm@gmail.com> | 2010-05-06 23:45:43 +0000 |
|---|---|---|
| committer | mike-m <mikem.llvm@gmail.com> | 2010-05-06 23:45:43 +0000 |
| commit | 68cb31901c590cabceee6e6356d62c84142114cb (patch) | |
| tree | 6444bddc975b662fbe47d63cd98a7b776a407c1a /docs/CodingStandards.html | |
| parent | c26ae5ab7e2d65b67c97524e66f50ce86445dec7 (diff) | |
| download | external_llvm-68cb31901c590cabceee6e6356d62c84142114cb.zip external_llvm-68cb31901c590cabceee6e6356d62c84142114cb.tar.gz external_llvm-68cb31901c590cabceee6e6356d62c84142114cb.tar.bz2 | |
Overhauled llvm/clang docs builds. Closes PR6613.
NOTE: 2nd part changeset for cfe trunk to follow.
*** PRE-PATCH ISSUES ADDRESSED
- clang api docs fail build from objdir
- clang/llvm api docs collide in install PREFIX/
- clang/llvm main docs collide in install
- clang/llvm main docs have full of hard coded destination
assumptions and make use of absolute root in static html files;
namely CommandGuide tools hard codes a website destination
for cross references and some html cross references assume
website root paths
*** IMPROVEMENTS
- bumped Doxygen from 1.4.x -> 1.6.3
- splits llvm/clang docs into 'main' and 'api' (doxygen) build trees
- provide consistent, reliable doc builds for both main+api docs
- support buid vs. install vs. website intentions
- support objdir builds
- document targets with 'make help'
- correct clean and uninstall operations
- use recursive dir delete only where absolutely necessary
- added call function fn.RMRF which safeguards against botched 'rm -rf';
if any target (or any variable is evaluated) which attempts
to remove any dirs which match a hard-coded 'safelist', a verbose
error will be printed and make will error-stop.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103213 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs/CodingStandards.html')
| -rw-r--r-- | docs/CodingStandards.html | 1353 |
1 files changed, 0 insertions, 1353 deletions
diff --git a/docs/CodingStandards.html b/docs/CodingStandards.html deleted file mode 100644 index 7815e19..0000000 --- a/docs/CodingStandards.html +++ /dev/null @@ -1,1353 +0,0 @@ -<!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_avoidendl">Avoid <tt>std::endl</tt></a></li> - <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li> - </ol></li> - - <li><a href="#nano">Microscopic Details</a> - <ol> - <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li> - <li><a href="#micro_preincrement">Prefer Preincrement</a></li> - <li><a href="#micro_namespaceindent">Namespace Indentation</a></li> - <li><a href="#micro_anonns">Anonymous Namespaces</a></li> - </ol></li> - - - </ol></li> - <li><a href="#seealso">See Also</a></li> -</ol> - -<div class="doc_author"> - <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p> -</div> - - -<!-- *********************************************************************** --> -<div class="doc_section"> - <a name="introduction">Introduction</a> -</div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p>This document attempts to describe a few coding standards that are being used -in the LLVM source tree. Although no coding standards should be regarded as -absolute requirements to be followed in all instances, coding standards can be -useful.</p> - -<p>This document intentionally does not prescribe fixed standards for religious -issues such as brace placement and space usage. For issues like this, follow -the golden rule:</p> - -<blockquote> - -<p><b><a name="goldenrule">If you are adding a significant body of source to a -project, feel free to use whatever style you are most comfortable with. If you -are extending, enhancing, or bug fixing already implemented code, use the style -that is already being used so that the source is uniform and easy to -follow.</a></b></p> - -</blockquote> - -<p>The ultimate goal of these guidelines is the increase readability and -maintainability of our common source base. If you have suggestions for topics to -be included, please mail them to <a -href="mailto:sabre@nondot.org">Chris</a>.</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"> - <a name="mechanicalissues">Mechanical Source Issues</a> -</div> -<!-- *********************************************************************** --> - -<!-- ======================================================================= --> -<div class="doc_subsection"> - <a name="sourceformating">Source Code Formatting</a> -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="scf_commenting">Commenting</a> -</div> - -<div class="doc_text"> - -<p>Comments are one critical part of readability and maintainability. Everyone -knows they should comment, so should you. When writing comments, write them as -English prose, which means they should use proper capitalization, punctuation, -etc. Although we all should probably -comment our code more than we do, there are a few very critical places that -documentation is very useful:</p> - -<b>File Headers</b> - -<p>Every source file should have a header on it that describes the basic -purpose of the file. If a file does not have a header, it should not be -checked into Subversion. Most source trees will probably have a standard -file header format. The standard format for the LLVM source tree looks like -this:</p> - -<div class="doc_code"> -<pre> -//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===// -// -// The LLVM Compiler Infrastructure -// -// This file is distributed under the University of Illinois Open Source -// License. See LICENSE.TXT for details. -// -//===----------------------------------------------------------------------===// -// -// This file contains the declaration of the Instruction class, which is the -// base class for all of the VM instructions. -// -//===----------------------------------------------------------------------===// -</pre> -</div> - -<p>A few things to note about this particular format: The "<tt>-*- C++ --*-</tt>" string on the first line is there to tell Emacs that the source file -is a C++ file, not a C file (Emacs assumes .h files are C files by default). -Note that this tag is not necessary in .cpp files. The name of the file is also -on the first line, along with a very short description of the purpose of the -file. This is important when printing out code and flipping though lots of -pages.</p> - -<p>The next section in the file is a concise note that defines the license -that the file is released under. This makes it perfectly clear what terms the -source code can be distributed under and should not be modified in any way.</p> - -<p>The main body of the description does not have to be very long in most cases. -Here it's only two lines. If an algorithm is being implemented or something -tricky is going on, a reference to the paper where it is published should be -included, as well as any notes or "gotchas" in the code to watch out for.</p> - -<b>Class overviews</b> - -<p>Classes are one fundamental part of a good object oriented design. As such, -a class definition should have a comment block that explains what the class is -used for... if it's not obvious. If it's so completely obvious your grandma -could figure it out, it's probably safe to leave it out. Naming classes -something sane goes a long ways towards avoiding writing documentation.</p> - - -<b>Method information</b> - -<p>Methods defined in a class (as well as any global functions) should also be -documented properly. A quick note about what it does 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/Bytecode/*</tt></li> - <li><tt>llvm/CodeGen/*</tt></li> - <li>...</li> - <li><tt>Support/*</tt></li> - <li><tt>Config/*</tt></li> - <li>System <tt>#includes</tt></li> -</ol> - -<p>... and each category should be sorted by name.</p> - -<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file -which implement an interface defined by a .h file. This <tt>#include</tt> -should always be included <b>first</b> regardless of where it lives on the file -system. By including a header file first in the .cpp files that implement the -interfaces, we ensure that the header does not have any hidden dependencies -which are not explicitly #included in the header, but should be. It is also a -form of documentation in the .cpp file to indicate where the interfaces it -implements are defined.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="scf_codewidth">Source Code Width</a> -</div> - -<div class="doc_text"> - -<p>Write your code to fit within 80 columns of text. This helps those of us who -like to print out code and look at your code in an xterm without resizing -it.</p> - -<p>The longer answer is that there must be some limit to the width of the code -in order to reasonably allow developers to have multiple files side-by-side in -windows on a modest display. If you are going to pick a width limit, it is -somewhat arbitrary but you might as well pick something standard. Going with -90 columns (for example) instead of 80 columns wouldn't add any significant -value and would be detrimental to printing out code. Also many other projects -have standardized on 80 columns, so some people have already configured their -editors for it (vs something else, like 90 columns).</p> - -<p>This is one of many contentious issues in coding standards, but is not up -for debate.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="scf_spacestabs">Use Spaces Instead of Tabs</a> -</div> - -<div class="doc_text"> - -<p>In all cases, prefer spaces to tabs in source files. People have different -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, your first year of programming you were told that indentation is -important. If you didn't believe and internalize this then, now is the time. -Just do it.</p> - -</div> - - -<!-- ======================================================================= --> -<div class="doc_subsection"> - <a name="compilerissues">Compiler Issues</a> -</div> - - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a> -</div> - -<div class="doc_text"> - -<p>If your code has compiler warnings in it, something is wrong: you aren't -casting values correctly, your have "questionable" constructs in your code, or -you are doing something legitimately wrong. Compiler warnings can cover up -legitimate errors in output and make dealing with a translation unit -difficult.</p> - -<p>It is not possible to prevent all warnings from all compilers, nor is it -desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides -a good thorough set of warnings, and stick to them. At least in the case of -<tt>gcc</tt>, it is possible to work around any spurious errors by changing the -syntax of the code slightly. For example, an warning that annoys me occurs when -I write code like this:</p> - -<div class="doc_code"> -<pre> -if (V = getValue()) { - ... -} -</pre> -</div> - -<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt> -operator, and that I probably mistyped it. In most cases, I haven't, and I -really don't want the spurious errors. To fix this particular problem, I -rewrite the code like this:</p> - -<div class="doc_code"> -<pre> -if ((V = getValue())) { - ... -} -</pre> -</div> - -<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can -be fixed by massaging the code appropriately.</p> - -<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall --Winline -W -Wwrite-strings -Wno-unused</tt></p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="ci_portable_code">Write Portable Code</a> -</div> - -<div class="doc_text"> - -<p>In almost all cases, it is possible and within reason to write completely -portable code. If there are cases where it isn't possible to write portable -code, isolate it behind a well defined (and well documented) interface.</p> - -<p>In practice, this means that you shouldn't assume much about the host -compiler, including its support for "high tech" features like partial -specialization of templates. If these features are used, they should only be -an implementation detail of a library which has a simple exposed API.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> -<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a> -</div> -<div class="doc_text"> - -<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost -interchangeably. The only difference is when they are used to declare a class: -<tt>class</tt> makes all members private by default while <tt>struct</tt> makes -all members public by default.</p> - -<p>Unfortunately, not all compilers follow the rules and some will generate -different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to -declare the symbol. This can lead to problems at link time.</p> - -<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless -<b>all</b> members are public and the type is a C++ "POD" type, in which case -<tt>struct</tt> is allowed.</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"> - <a name="styleissues">Style Issues</a> -</div> -<!-- *********************************************************************** --> - - -<!-- ======================================================================= --> -<div class="doc_subsection"> - <a name="macro">The High Level Issues</a> -</div> -<!-- ======================================================================= --> - - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="hl_module">A Public Header File <b>is</b> a Module</a> -</div> - -<div class="doc_text"> - -<p>C++ doesn't do too well in the modularity department. There is no real -encapsulation or data hiding (unless you use expensive protocol classes), but it -is what we have to work with. When you write a public header file (in the LLVM -source tree, they live in the top level "include" directory), you are defining a -module of functionality.</p> - -<p>Ideally, modules should be completely independent of each other, and their -header files should only include the absolute minimum number of headers -possible. A module is not just a class, a function, or a namespace: <a -href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection -of these</a> that defines an interface. This interface may be several -functions, classes or data structures, but the important issue is how they work -together.</p> - -<p>In general, a module should be implemented with one or more <tt>.cpp</tt> -files. Each of these <tt>.cpp</tt> files should include the header that defines -their interface first. This ensure that all of the dependences of the module -header have been properly added to the module header itself, and are not -implicit. System headers should be included after user headers for a -translation unit.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a> -</div> - -<div class="doc_text"> - -<p><tt>#include</tt> hurts compile time performance. Don't do it unless you -have to, especially in header files.</p> - -<p>But wait, sometimes you need to have the definition of a class to use it, or -to inherit from it. In these cases go ahead and <tt>#include</tt> that header -file. Be aware however that there are many cases where you don't need to have -the full definition of a class. If you are using a pointer or reference to a -class, you don't need the header file. If you are simply returning a class -instance from a prototyped function or method, you don't need it. In fact, for -most cases, you simply don't need the definition of a class... and not -<tt>#include</tt>'ing speeds up compilation.</p> - -<p>It is easy to try to go too overboard on this recommendation, however. You -<b>must</b> include all of the header files that you are using -- you can -include them either directly -or indirectly (through another header file). To make sure that you don't -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 for -loops. A silly example is something like this:</p> - -<div class="doc_code"> -<pre> - for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { - if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) { - Value *LHS = BO->getOperand(0); - Value *RHS = BO->getOperand(1); - if (LHS != RHS) { - ... - } - } - } -</pre> -</div> - -<p>When you have very very small loops, this sort of structure is fine, but if -it exceeds more than 10-15 lines, it becomes difficult for people to read and -understand at a glance. -The problem with this sort of code is that it gets very nested very quickly, -meaning that the reader of the code has to keep a lot of context in their brain -to remember what is going immediately on in the loop, because they don't know -if/when the if conditions will have elses etc. It is strongly preferred to -structure the loop like this:</p> - -<div class="doc_code"> -<pre> - for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { - BinaryOperator *BO = dyn_cast<BinaryOperator>(II); - if (!BO) continue; - - Value *LHS = BO->getOperand(0); - Value *RHS = BO->getOperand(1); - if (LHS == RHS) continue; - } -</pre> -</div> - -<p>This has all the benefits of using early exits from functions: it reduces -nesting of the loop, it makes it easier to describe why the conditions are true, -and it makes it obvious to the reader that there is no "else" coming up that -they have to push context into their brain for. If a loop is large, this can -be a big understandability win.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="hl_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>" function 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 -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> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a> -</div> - -<div class="doc_text"> -<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard -namespace with an "<tt>std::</tt>" prefix, rather than rely on -"<tt>using namespace std;</tt>".</p> - -<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes -the namespace of any source file that <tt>#include</tt>s the header. This is -clearly a bad thing.</p> - -<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic -rule, but is still important. Basically, using explicit namespace prefixes -makes the code <b>clearer</b>, because it is immediately obvious what facilities -are being used and where they are coming from, and <b>more portable</b>, because -namespace clashes cannot occur between LLVM code and other namespaces. The -portability rule is important because different standard library implementations -expose different symbols (potentially ones they shouldn't), and future revisions -to the C++ standard will add more symbols to the <tt>std</tt> namespace. As -such, we never use '<tt>using namespace std;</tt>' in LLVM.</p> - -<p>The exception to the general rule (i.e. it's not an exception for -the <tt>std</tt> namespace) is for implementation files. For example, all of -the code in the LLVM project implements code that lives in the 'llvm' namespace. -As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using -namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The -general form of this rule is that any .cpp file that implements code in any -namespace may use that namespace (and its parents'), but should not use any -others.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="ll_virtual_anch">Provide a virtual method anchor for classes - in headers</a> -</div> - -<div class="doc_text"> - -<p>If a class is defined in a header file and has a v-table (either it has -virtual methods or it derives from classes with virtual methods), it must -always have at least one out-of-line virtual method in the class. Without -this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file -that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and -increasing link times.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="ll_end">Don't evaluate end() every time through a loop</a> -</div> - -<div class="doc_text"> - -<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated -with macros and may be coming in C++'0x) we end up writing a lot of loops that -manually iterate from begin to end on a variety of containers or through other -data structures. One common mistake is to write a loop in this style:</p> - -<div class="doc_code"> -<pre> - BasicBlock *BB = ... - for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I) - ... use I ... -</pre> -</div> - -<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>" -every time through the loop. Instead of writing the loop like this, we strongly -prefer loops to be written so that they evaluate it once before the loop starts. -A convenient way to do this is like so:</p> - -<div class="doc_code"> -<pre> - BasicBlock *BB = ... - for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I) - ... use I ... -</pre> -</div> - -<p>The observant may quickly point out that these two loops may have different -semantics: if the container (a basic block in this case) is being mutated, then -"<tt>BB->end()</tt>" may change its value every time through the loop and the -second loop may not in fact be correct. If you actually do depend on this -behavior, please write the loop in the first form and add a comment indicating -that you did it intentionally.</p> - -<p>Why do we prefer the second form (when correct)? Writing the loop in the -first form has two problems: First it may be less efficient than evaluating it -at the start of the loop. In this case, the cost is probably minor: a few extra -loads every time through the loop. However, if the base expression is more -complex, then the cost can rise quickly. I've seen loops where the end -expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map -lookups really aren't cheap. By writing it in the second form consistently, you -eliminate the issue entirely and don't even have to think about it.</p> - -<p>The second (even bigger) issue is that writing the loop in the first form -hints to the reader that the loop is mutating the container (a fact that a -comment would handily confirm!). If you write the loop in the second form, it -is immediately obvious without even looking at the body of the loop that the -container isn't being modified, which makes it easier to read the code and -understand what it does.</p> - -<p>While the second form of the loop is a few extra keystrokes, we do strongly -prefer it.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="ll_iostream"><tt>#include <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, 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 -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_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_subsubsection"> - <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a> -</div> - -<div class="doc_text"> - -<p>LLVM includes a lightweight, simple, and efficient stream implementation -in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features -of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> instead -of <tt>ostream</tt>.</p> - -<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can -be forward declared as <tt>class raw_ostream</tt>. Public headers should -generally not include the <tt>raw_ostream</tt> header, but use forward -declarations and constant references to <tt>raw_ostream</tt> instances.</p> - -</div> - - -<!-- ======================================================================= --> -<div class="doc_subsection"> - <a name="nano">Microscopic Details</a> -</div> -<!-- ======================================================================= --> - -<p>This section describes preferred low-level formatting guidelines along with -reasoning on why we prefer them.</p> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="micro_spaceparen">Spaces Before Parentheses</a> -</div> - -<div class="doc_text"> - -<p>We prefer to put a space before a parentheses only in control flow -statements, but not in normal function call expressions and function-like -macros. For example, this is good:</p> - -<div class="doc_code"> -<pre> - <b>if (</b>x) ... - <b>for (</b>i = 0; i != 100; ++i) ... - <b>while (</b>llvm_rocks) ... - - <b>somefunc(</b>42); - <b><a href="#ll_assert">assert</a>(</b>3 != 4 && "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 where ever possible. This is useful -because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without -wrapping horribly, but also because it makes it easier to understand the code. -Namespaces are a funny thing: they are often large, and we often desire to put -lots of stuff into them (so they can be large). Other times they are tiny, -because they just hold an enum or something similar. In order to balance this, -we use different approaches for small versus large namespaces. -</p> - -<p> -If a namespace definition is small and <em>easily</em> fits on a screen (say, -less than 35 lines of code), then you should indent its body. Here's an -example: -</p> - -<div class="doc_code"> -<pre> -namespace llvm { - namespace X86 { - /// RelocationType - An enum for the x86 relocation codes. Note that - /// the terminology here doesn't follow x86 convention - word means - /// 32-bit and dword means 64-bit. - enum RelocationType { - /// reloc_pcrel_word - PC relative relocation, add the relocated value to - /// the value already in memory, after we adjust it for where the PC is. - reloc_pcrel_word = 0, - - /// reloc_picrel_word - PIC base relative relocation, add the relocated - /// value to the value already in memory, after we adjust it for where the - /// PIC base is. - reloc_picrel_word = 1, - - /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just - /// add the relocated value to the value already in memory. - reloc_absolute_word = 2, - reloc_absolute_dword = 3 - }; - } -} -</pre> -</div> - -<p>Since the body is small, indenting adds value because it makes it very clear -where the namespace starts and ends, and it is easy to take the whole thing in -in one "gulp" when reading the code. If the blob of code in the namespace is -larger (as it typically is in a header in the llvm or clang namespaces), do not -indent the code, and add a comment indicating what namespace is being closed. -For example:</p> - -<div class="doc_code"> -<pre> -namespace llvm { -namespace knowledge { - -/// Grokable - This class represents things that Smith can have an intimate -/// understanding of and contains the data associated with it. -class Grokable { -... -public: - explicit Grokable() { ... } - virtual ~Grokable() = 0; - - ... - -}; - -} // end namespace knowledge -} // end namespace llvm -</pre> -</div> - -<p>Because the class is large, we don't expect that the reader can easily -understand the entire concept in a glance, and the end of the file (where the -namespaces end) may be a long ways away from the place they open. As such, -indenting the contents of the namespace doesn't add any value, and detracts from -the readability of the class. In these cases it is best to <em>not</em> indent -the contents of the namespace.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<div class="doc_subsubsection"> - <a name="micro_anonns">Anonymous Namespaces</a> -</div> - -<div class="doc_text"> - -<p>After talking about namespaces in general, you may be wondering about -anonymous namespaces in particular. -Anonymous namespaces are a great language feature that tells the C++ compiler -that the contents of the namespace are only visible within the current -translation unit, allowing more aggressive optimization and eliminating the -possibility of symbol name collisions. Anonymous namespaces are to C++ as -"static" is to C functions and global variables. While "static" is available -in C++, anonymous namespaces are more general: they can make entire classes -private to a file.</p> - -<p>The problem with anonymous namespaces is that they naturally want to -encourage indentation of their body, and they reduce locality of reference: if -you see a random function definition in a C++ file, it is easy to see if it is -marked static, but seeing if it is in an anonymous namespace requires scanning -a big chunk of the file.</p> - -<p>Because of this, we have a simple guideline: make anonymous namespaces as -small as possible, and only use them for class declarations. For example, this -is good:</p> - -<div class="doc_code"> -<pre> -<b>namespace {</b> - class StringSort { - ... - public: - StringSort(...) - bool operator<(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> - -<!-- *********************************************************************** --> - -<hr> -<address> - <a href="http://jigsaw.w3.org/css-validator/check/referer"><img - src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> - <a href="http://validator.w3.org/check/referer"><img - 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> - Last modified: $Date$ -</address> - -</body> -</html> |