1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
5 <link rel="stylesheet" href="llvm.css" type="text/css">
6 <title>A Few Coding Standards</title>
10 <div class="doc_title">
11 A Few Coding Standards
15 <li><a href="#introduction">Introduction</a></li>
16 <li><a href="#mechanicalissues">Mechanical Source Issues</a>
18 <li><a href="#sourceformating">Source Code Formatting</a>
20 <li><a href="#scf_commenting">Commenting</a></li>
21 <li><a href="#scf_commentformat">Comment Formatting</a></li>
22 <li><a href="#scf_includes">#include Style</a></li>
23 <li><a href="#scf_codewidth">Source Code Width</a></li>
24 <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>
25 <li><a href="#scf_indentation">Indent Code Consistently</a></li>
27 <li><a href="#compilerissues">Compiler Issues</a>
29 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like
31 <li><a href="#ci_portable_code">Write Portable Code</a></li>
34 <li><a href="#styleissues">Style Issues</a>
36 <li><a href="#macro">The High Level Issues</a>
38 <li><a href="#hl_module">A Public Header File <b>is</b> a
40 <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
41 <li><a href="#hl_privateheaders">Keep "internal" Headers
44 <li><a href="#micro">The Low Level Issues</a>
46 <li><a href="#hl_assert">Assert Liberally</a></li>
47 <li><a href="#hl_preincrement">Prefer Preincrement</a></li>
48 <li><a href="#hl_avoidendl">Avoid std::endl</a></li>
49 <li><a href="#hl_exploitcpp">Exploit C++ to its Fullest</a></li>
52 <li><a href="#seealso">See Also</a></li>
55 <div class="doc_author">
56 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
60 <!-- *********************************************************************** -->
61 <div class="doc_section">
62 <a name="introduction">Introduction</a>
64 <!-- *********************************************************************** -->
66 <div class="doc_text">
68 <p>This document attempts to describe a few coding standards that are being used
69 in the LLVM source tree. Although no coding standards should be regarded as
70 absolute requirements to be followed in all instances, coding standards can be
73 <p>This document intentionally does not prescribe fixed standards for religious
74 issues such as brace placement and space usage. For issues like this, follow
79 <p><b><a name="goldenrule">If you are adding a significant body of source to a
80 project, feel free to use whatever style you are most comfortable with. If you
81 are extending, enhancing, or bug fixing already implemented code, use the style
82 that is already being used so that the source is uniform and easy to
87 <p>The ultimate goal of these guidelines is the increase readability and
88 maintainability of our common source base. If you have suggestions for topics to
89 be included, please mail them to <a
90 href="mailto:sabre@nondot.org">Chris</a>.</p>
94 <!-- *********************************************************************** -->
95 <div class="doc_section">
96 <a name="mechanicalissues">Mechanical Source Issues</a>
98 <!-- *********************************************************************** -->
100 <!-- ======================================================================= -->
101 <div class="doc_subsection">
102 <a name="sourceformating">Source Code Formatting</a>
105 <!-- _______________________________________________________________________ -->
106 <div class="doc_subsubsection">
107 <a name="scf_commenting">Commenting</a>
110 <div class="doc_text">
112 <p>Comments are one critical part of readability and maintainability. Everyone
113 knows they should comment, so should you. :) Although we all should probably
114 comment our code more than we do, there are a few very critical places that
115 documentation is very useful:</p>
119 <p>Every source file should have a header on it that
120 describes the basic purpose of the file. If a file does not have a header, it
121 should not be checked into CVS. Most source trees will probably have a standard
122 file header format. The standard format for the LLVM source tree looks like
126 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
128 // The LLVM Compiler Infrastructure
130 // This file was developed by the LLVM research group and is distributed under
131 // the University of Illinois Open Source License. See LICENSE.TXT for details.
133 //===----------------------------------------------------------------------===//
135 // This file contains the declaration of the Instruction class, which is the
136 // base class for all of the VM instructions.
138 //===----------------------------------------------------------------------===//
142 <p>A few things to note about this particular format: The "<tt>-*- C++
143 -*-</tt>" string on the first line is there to tell Emacs that the source file
144 is a C++ file, not a C file (Emacs assumes .h files are C files by default [Note
145 that tag this is not necessary in .cpp files]). The name of the file is also on
146 the first line, along with a very short description of the purpose of the file.
147 This is important when printing out code and flipping though lots of pages.</p>
149 <p>The next section in the file is a concise note that defines the license that
150 the file is released under. This makes it perfectly clear what terms the source
151 code can be distributed under.</p>
153 <p>The main body of the description does not have to be very long in most cases.
154 Here it's only two lines. If an algorithm is being implemented or something
155 tricky is going on, a reference to the paper where it is published should be
156 included, as well as any notes or "gotchas" in the code to watch out for.</p>
158 <b>Class overviews</b>
160 <p>Classes are one fundemental part of a good object oriented design. As such,
161 a class definition should have a comment block that explains what the class is
162 used for... if it's not obvious. If it's so completely obvious your grandma
163 could figure it out, it's probably safe to leave it out. Naming classes
164 something sane goes a long ways towards avoiding writing documentation. :)</p>
167 <b>Method information</b>
169 <p>Methods defined in a class (as well as any global functions) should also be
170 documented properly. A quick note about what it does any a description of the
171 borderline behaviour is all that is necessary here (unless something
172 particularly tricky or insideous is going on). The hope is that people can
173 figure out how to use your interfaces without reading the code itself... that is
176 <p>Good things to talk about here are what happens when something unexpected
177 happens: does the method return null? Abort? Format your hard disk?</p>
181 <!-- _______________________________________________________________________ -->
182 <div class="doc_subsubsection">
183 <a name="scf_commentformat">Comment Formatting</a>
186 <div class="doc_text">
188 <p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
189 require less typing, don't have nesting problems, etc. There are a few cases
190 when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
193 <li>When writing a C code: Obviously if you are writing C code, use C style
195 <li>When writing a header file that may be #included by a C source file.</li>
196 <li>When writing a source file that is used by a tool that only accepts C
200 <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
201 These nest properly and are better behaved in general than C style comments.</p>
205 <!-- _______________________________________________________________________ -->
206 <div class="doc_subsubsection">
207 <a name="scf_includes">#include Style</a>
210 <div class="doc_text">
212 <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
213 include guards if working on a header file), the <a
214 href="#hl_dontinclude">minimal</a> list of #includes required by the file should
215 be listed. We prefer these #includes to be listed in this order:</p>
218 <li><a href="#mmheader">Main Module header</a></li>
219 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
221 <li>llvm/Analysis/*</li>
222 <li>llvm/Assembly/*</li>
223 <li>llvm/Bytecode/*</li>
224 <li>llvm/CodeGen/*</li>
228 <li>System #includes</li>
231 <p>... and each catagory should be sorted by name.</p>
233 <p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
234 which implement an interface defined by a .h file. This #include should always
235 be included <b>first</b> regardless of where it lives on the file system. By
236 including a header file first in the .cpp files that implement the interfaces,
237 we ensure that the header does not have any hidden dependencies which are not
238 explicitly #included in the header, but should be. It is also a form of
239 documentation in the .cpp file to indicate where the interfaces it implements
244 <!-- _______________________________________________________________________ -->
245 <div class="doc_subsubsection">
246 <a name="scf_codewidth">Source Code Width</a>
249 <div class="doc_text">
251 <p>Write your code to fit within 80 columns of text. This helps those of us who
252 like to print out code and look at your code in an xterm without resizing
257 <!-- _______________________________________________________________________ -->
258 <div class="doc_subsubsection">
259 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
262 <div class="doc_text">
264 <p>In all cases, prefer spaces to tabs in source files. People have different
265 prefered indentation levels, and different styles of indentation that they
266 like... this is fine. What isn't is that different editors/viewers expand tabs
267 out to different tab stops. This can cause your code to look completely
268 unreadable, and it is not worth dealing with.</p>
270 <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
271 style of existing code if your are modifying and extending it. If you like four
272 spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
273 with two spaces of indentation. Also, do not reindent a whole source file: it
274 makes for incredible diffs that are absolutely worthless.</p>
278 <!-- _______________________________________________________________________ -->
279 <div class="doc_subsubsection">
280 <a name="scf_indentation">Indent Code Consistently</a>
283 <div class="doc_text">
285 <p>Okay, your first year of programming you were told that indentation is
286 important. If you didn't believe and internalize this then, now is the time.
292 <!-- ======================================================================= -->
293 <div class="doc_subsection">
294 <a name="compilerissues">Compiler Issues</a>
298 <!-- _______________________________________________________________________ -->
299 <div class="doc_subsubsection">
300 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
303 <div class="doc_text">
305 <p>If your code has compiler warnings in it, something is wrong: you aren't
306 casting values correctly, your have "questionable" constructs in your code, or
307 you are doing something legitimately wrong. Compiler warnings can cover up
308 legitimate errors in output and make dealing with a translation unit
311 <p>It is not possible to prevent all warnings from all compilers, nor is it
312 desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
313 a good thorough set of warnings, and stick to them. At least in the case of
314 <tt>gcc</tt>, it is possible to work around any spurious errors by changing the
315 syntax of the code slightly. For example, an warning that annoys me occurs when
316 I write code like this:</p>
319 if (V = getValue()) {
324 <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
325 operator, and that I probably mistyped it. In most cases, I haven't, and I
326 really don't want the spurious errors. To fix this particular problem, I
327 rewrite the code like this:</p>
330 if ((V = getValue())) {
335 <p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
336 be fixed by massaging the code appropriately.</p>
338 <p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
339 -Winline -W -Wwrite-strings -Wno-unused</tt></p>
343 <!-- _______________________________________________________________________ -->
344 <div class="doc_subsubsection">
345 <a name="ci_portable_code">Write Portable Code</a>
348 <div class="doc_text">
350 <p>In almost all cases, it is possible and within reason to write completely
351 portable code. If there are cases where it isn't possible to write portable
352 code, isolate it behind a well defined (and well documented) interface.</p>
354 <p>In practice, this means that you shouldn't assume much about the host
355 compiler, including its support for "high tech" features like partial
356 specialization of templates. In fact, Visual C++ 6 could be an important target
357 for our work in the future, and we don't want to have to rewrite all of our code
362 <!-- *********************************************************************** -->
363 <div class="doc_section">
364 <a name="styleissues">Style Issues</a>
366 <!-- *********************************************************************** -->
369 <!-- ======================================================================= -->
370 <div class="doc_subsection">
371 <a name="macro">The High Level Issues</a>
375 <!-- _______________________________________________________________________ -->
376 <div class="doc_subsubsection">
377 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
380 <div class="doc_text">
382 <p>C++ doesn't do too well in the modularity department. There is no real
383 encapsulation or data hiding (unless you use expensive protocol classes), but it
384 is what we have to work with. When you write a public header file (in the LLVM
385 source tree, they live in the top level "include" directory), you are defining a
386 module of functionality.</p>
388 <p>Ideally, modules should be completely independent of each other, and their
389 header files should only include the absolute minimum number of headers
390 possible. A module is not just a class, a function, or a namespace: <a
391 href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
392 of these</a> that defines an interface. This interface may be several
393 functions, classes or data structures, but the important issue is how they work
396 <p>In general, a module should be implemented with one or more <tt>.cpp</tt>
397 files. Each of these <tt>.cpp</tt> files should include the header that defines
398 their interface first. This ensure that all of the dependences of the module
399 header have been properly added to the module header itself, and are not
400 implicit. System headers should be included after user headers for a
401 translation unit.</p>
405 <!-- _______________________________________________________________________ -->
406 <div class="doc_subsubsection">
407 <a name="hl_dontinclude">#include as Little as Possible</a>
410 <div class="doc_text">
412 <p><tt>#include</tt> hurts compile time performance. Don't do it unless you
413 have to, especially in header files.</p>
415 <p>But wait, sometimes you need to have the definition of a class to use it, or
416 to inherit from it. In these cases go ahead and #include that header file. Be
417 aware however that there are many cases where you don't need to have the full
418 definition of a class. If you are using a pointer or reference to a class, you
419 don't need the header file. If you are simply returning a class instance from a
420 prototyped function or method, you don't need it. In fact, for most cases, you
421 simply don't need the definition of a class... and not <tt>#include</tt>'ing
422 speeds up compilation.</p>
424 <p>It is easy to try to go too overboard on this recommendation, however. You
425 <b>must</b> include all of the header files that you are using, either directly
426 or indirectly (through another header file). To make sure that you don't
427 accidently forget to include a header file in your module header, make sure to
428 include your module header <b>first</b> in the implementation file (as mentioned
429 above). This way there won't be any hidden dependencies that you'll find out
434 <!-- _______________________________________________________________________ -->
435 <div class="doc_subsubsection">
436 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
439 <div class="doc_text">
441 <p>Many modules have a complex implementation that causes them to use more than
442 one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
443 internal communication interface (helper classes, extra functions, etc) in the
444 public module header file. Don't do this. :)</p>
446 <p>If you really need to do something like this, put a private header file in
447 the same directory as the source files, and include it locally. This ensures
448 that your private interface remains private and undisturbed by outsiders.</p>
450 <p>Note however, that it's okay to put extra implementation methods a public
451 class itself... just make them private (or protected), and all is well.</p>
455 <!-- ======================================================================= -->
456 <div class="doc_subsection">
457 <a name="micro">The Low Level Issues</a>
461 <!-- _______________________________________________________________________ -->
462 <div class="doc_subsubsection">
463 <a name="hl_assert">Assert Liberally</a>
466 <div class="doc_text">
468 <p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
469 preconditions and assumptions, you never know when a bug (not neccesarily even
470 yours) might be caught early by an assertion, which reduces debugging time
471 dramatically. The "<tt><cassert></tt>" header file is probably already
472 included by the header files you are using, so it doesn't cost anything to use
475 <p>To further assist with debugging, make sure to put some kind of error message
476 in the assertion statement (which is printed if the assertion is tripped). This
477 helps the poor debugging make sense of why an assertion is being made and
478 enforced, and hopefully what to do about it. Here is one complete example:</p>
481 inline Value *getOperand(unsigned i) {
482 assert(i < Operands.size() && "getOperand() out of range!");
487 <p>Here are some examples:</p>
490 assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
492 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
494 assert(idx < getNumSuccessors() && "Successor # out of range!");
496 assert(V1.getType() == V2.getType() && "Constant types must be identical!");
498 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
501 <p>You get the idea...</p>
506 <!-- _______________________________________________________________________ -->
507 <div class="doc_subsubsection">
508 <a name="hl_preincrement">Prefer Preincrement</a>
511 <div class="doc_text">
513 <p>Hard fast rule: Preincrement (++X) may be no slower than postincrement (X++)
514 and could very well be a lot faster than it. Use preincrementation whenever
517 <p>The semantics of postincrement include making a copy of the value being
518 incremented, returning it, and then preincrementing the "work value". For
519 primitive types, this isn't a big deal... but for iterators, it can be a huge
520 issue (for example, some iterators contains stack and set objects in them...
521 copying an iterator could invoke the copy ctor's of these as well). In general,
522 get in the habit of always using preincrement, and you won't have a problem.</p>
527 <!-- _______________________________________________________________________ -->
528 <div class="doc_subsubsection">
529 <a name="hl_avoidendl">Avoid std::endl</a>
532 <div class="doc_text">
534 <p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
535 to the output stream specified. In addition to doing this, however, it also
536 flushes the output stream. In other words, these are equivalent:</p>
539 std::cout << std::endl;
540 std::cout << "\n" << std::flush;
543 <p>Most of the time, you probably have no reason to flush the output stream, so
544 it's better to use a literal <tt>"\n"</tt>.</p>
548 <!-- _______________________________________________________________________ -->
549 <div class="doc_subsubsection">
550 <a name="hl_exploitcpp">Exploit C++ to its Fullest</a>
553 <div class="doc_text">
555 <p>C++ is a powerful language. With a firm grasp on its capabilities, you can make
556 write effective, consise, readable and maintainable code all at the same time.
557 By staying consistent, you reduce the amount of special cases that need to be
558 remembered. Reducing the total number of lines of code you write is a good way
559 to avoid documentation, and avoid giving bugs a place to hide.</p>
561 <p>For these reasons, come to know and love the contents of your local
562 <algorithm> header file. Know about <functional> and what it can do
563 for you. C++ is just a tool that wants you to master it. :)</p>
567 <!-- *********************************************************************** -->
568 <div class="doc_section">
569 <a name="seealso">See Also</a>
571 <!-- *********************************************************************** -->
573 <div class="doc_text">
575 <p>A lot of these comments and recommendations have been culled for other
576 sources. Two particularly important books for our work are:</p>
580 <li><a href="http://www.aw-bc.com/catalog/academic/product/0,1144,0201310155,00.html">Effective
581 C++</a> by Scott Meyers. There is an online version of the book (only some
583 href="http://www.awlonline.com/cseng/meyerscddemo/">available as well</a>. Also
584 interesting and useful are "More Effective C++" and "Effective STL" by the same
587 <li><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++
588 Software Design</a> by John Lakos</li>
592 <p>If you get some free time, and you haven't read them: do so, you might learn
597 <!-- *********************************************************************** -->
601 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
602 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
603 <a href="http://validator.w3.org/check/referer"><img
604 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
606 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
607 <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
608 Last modified: $Date$