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>LLVM Coding Standards</title>
10 <div class="doc_title">
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"><tt>#include</tt> 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>
32 <li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>
35 <li><a href="#styleissues">Style Issues</a>
37 <li><a href="#macro">The High Level Issues</a>
39 <li><a href="#hl_module">A Public Header File <b>is</b> a
41 <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
42 <li><a href="#hl_privateheaders">Keep "internal" Headers
44 <li><a href="#hl_earlyexit">Use Early Exits and 'continue' to Simplify
46 <li><a href="#hl_else_after_return">Don't use "else" after a
48 <li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
51 <li><a href="#micro">The Low Level Issues</a>
53 <li><a href="#ll_assert">Assert Liberally</a></li>
54 <li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
55 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
56 classes in headers</a></li>
57 <li><a href="#ll_end">Don't evaluate end() every time through a
59 <li><a href="#ll_iostream"><tt>#include <iostream></tt> is
60 <em>forbidden</em></a></li>
61 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
62 <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li>
65 <li><a href="#nano">Microscopic Details</a>
67 <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
68 <li><a href="#micro_preincrement">Prefer Preincrement</a></li>
69 <li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
70 <li><a href="#micro_anonns">Anonymous Namespaces</a></li>
75 <li><a href="#seealso">See Also</a></li>
78 <div class="doc_author">
79 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
83 <!-- *********************************************************************** -->
84 <div class="doc_section">
85 <a name="introduction">Introduction</a>
87 <!-- *********************************************************************** -->
89 <div class="doc_text">
91 <p>This document attempts to describe a few coding standards that are being used
92 in the LLVM source tree. Although no coding standards should be regarded as
93 absolute requirements to be followed in all instances, coding standards can be
96 <p>This document intentionally does not prescribe fixed standards for religious
97 issues such as brace placement and space usage. For issues like this, follow
102 <p><b><a name="goldenrule">If you are adding a significant body of source to a
103 project, feel free to use whatever style you are most comfortable with. If you
104 are extending, enhancing, or bug fixing already implemented code, use the style
105 that is already being used so that the source is uniform and easy to
110 <p>The ultimate goal of these guidelines is the increase readability and
111 maintainability of our common source base. If you have suggestions for topics to
112 be included, please mail them to <a
113 href="mailto:sabre@nondot.org">Chris</a>.</p>
117 <!-- *********************************************************************** -->
118 <div class="doc_section">
119 <a name="mechanicalissues">Mechanical Source Issues</a>
121 <!-- *********************************************************************** -->
123 <!-- ======================================================================= -->
124 <div class="doc_subsection">
125 <a name="sourceformating">Source Code Formatting</a>
128 <!-- _______________________________________________________________________ -->
129 <div class="doc_subsubsection">
130 <a name="scf_commenting">Commenting</a>
133 <div class="doc_text">
135 <p>Comments are one critical part of readability and maintainability. Everyone
136 knows they should comment, so should you. When writing comments, write them as
137 English prose, which means they should use proper capitalization, punctuation,
138 etc. Although we all should probably
139 comment our code more than we do, there are a few very critical places that
140 documentation is very useful:</p>
144 <p>Every source file should have a header on it that describes the basic
145 purpose of the file. If a file does not have a header, it should not be
146 checked into Subversion. Most source trees will probably have a standard
147 file header format. The standard format for the LLVM source tree looks like
150 <div class="doc_code">
152 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
154 // The LLVM Compiler Infrastructure
156 // This file is distributed under the University of Illinois Open Source
157 // License. See LICENSE.TXT for details.
159 //===----------------------------------------------------------------------===//
161 // This file contains the declaration of the Instruction class, which is the
162 // base class for all of the VM instructions.
164 //===----------------------------------------------------------------------===//
168 <p>A few things to note about this particular format: The "<tt>-*- C++
169 -*-</tt>" string on the first line is there to tell Emacs that the source file
170 is a C++ file, not a C file (Emacs assumes .h files are C files by default).
171 Note that this tag is not necessary in .cpp files. The name of the file is also
172 on the first line, along with a very short description of the purpose of the
173 file. This is important when printing out code and flipping though lots of
176 <p>The next section in the file is a concise note that defines the license
177 that the file is released under. This makes it perfectly clear what terms the
178 source code can be distributed under and should not be modified in any way.</p>
180 <p>The main body of the description does not have to be very long in most cases.
181 Here it's only two lines. If an algorithm is being implemented or something
182 tricky is going on, a reference to the paper where it is published should be
183 included, as well as any notes or "gotchas" in the code to watch out for.</p>
185 <b>Class overviews</b>
187 <p>Classes are one fundamental part of a good object oriented design. As such,
188 a class definition should have a comment block that explains what the class is
189 used for... if it's not obvious. If it's so completely obvious your grandma
190 could figure it out, it's probably safe to leave it out. Naming classes
191 something sane goes a long ways towards avoiding writing documentation.</p>
194 <b>Method information</b>
196 <p>Methods defined in a class (as well as any global functions) should also be
197 documented properly. A quick note about what it does any a description of the
198 borderline behaviour is all that is necessary here (unless something
199 particularly tricky or insideous is going on). The hope is that people can
200 figure out how to use your interfaces without reading the code itself... that is
203 <p>Good things to talk about here are what happens when something unexpected
204 happens: does the method return null? Abort? Format your hard disk?</p>
208 <!-- _______________________________________________________________________ -->
209 <div class="doc_subsubsection">
210 <a name="scf_commentformat">Comment Formatting</a>
213 <div class="doc_text">
215 <p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
216 require less typing, don't have nesting problems, etc. There are a few cases
217 when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
220 <li>When writing a C code: Obviously if you are writing C code, use C style
222 <li>When writing a header file that may be <tt>#include</tt>d by a C source
224 <li>When writing a source file that is used by a tool that only accepts C
228 <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
229 These nest properly and are better behaved in general than C style comments.</p>
233 <!-- _______________________________________________________________________ -->
234 <div class="doc_subsubsection">
235 <a name="scf_includes"><tt>#include</tt> Style</a>
238 <div class="doc_text">
240 <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
241 include guards if working on a header file), the <a
242 href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
243 file should be listed. We prefer these <tt>#include</tt>s to be listed in this
247 <li><a href="#mmheader">Main Module header</a></li>
248 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
249 <li><tt>llvm/*</tt></li>
250 <li><tt>llvm/Analysis/*</tt></li>
251 <li><tt>llvm/Assembly/*</tt></li>
252 <li><tt>llvm/Bytecode/*</tt></li>
253 <li><tt>llvm/CodeGen/*</tt></li>
255 <li><tt>Support/*</tt></li>
256 <li><tt>Config/*</tt></li>
257 <li>System <tt>#includes</tt></li>
260 <p>... and each category should be sorted by name.</p>
262 <p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
263 which implement an interface defined by a .h file. This <tt>#include</tt>
264 should always be included <b>first</b> regardless of where it lives on the file
265 system. By including a header file first in the .cpp files that implement the
266 interfaces, we ensure that the header does not have any hidden dependencies
267 which are not explicitly #included in the header, but should be. It is also a
268 form of documentation in the .cpp file to indicate where the interfaces it
269 implements are defined.</p>
273 <!-- _______________________________________________________________________ -->
274 <div class="doc_subsubsection">
275 <a name="scf_codewidth">Source Code Width</a>
278 <div class="doc_text">
280 <p>Write your code to fit within 80 columns of text. This helps those of us who
281 like to print out code and look at your code in an xterm without resizing
284 <p>The longer answer is that there must be some limit to the width of the code
285 in order to reasonably allow developers to have multiple files side-by-side in
286 windows on a modest display. If you are going to pick a width limit, it is
287 somewhat arbitrary but you might as well pick something standard. Going with
288 90 columns (for example) instead of 80 columns wouldn't add any significant
289 value and would be detrimental to printing out code. Also many other projects
290 have standardized on 80 columns, so some people have already configured their
291 editors for it (vs something else, like 90 columns).</p>
293 <p>This is one of many contentious issues in coding standards, but is not up
298 <!-- _______________________________________________________________________ -->
299 <div class="doc_subsubsection">
300 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
303 <div class="doc_text">
305 <p>In all cases, prefer spaces to tabs in source files. People have different
306 prefered indentation levels, and different styles of indentation that they
307 like... this is fine. What isn't is that different editors/viewers expand tabs
308 out to different tab stops. This can cause your code to look completely
309 unreadable, and it is not worth dealing with.</p>
311 <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
312 style of existing code if your are modifying and extending it. If you like four
313 spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
314 with two spaces of indentation. Also, do not reindent a whole source file: it
315 makes for incredible diffs that are absolutely worthless.</p>
319 <!-- _______________________________________________________________________ -->
320 <div class="doc_subsubsection">
321 <a name="scf_indentation">Indent Code Consistently</a>
324 <div class="doc_text">
326 <p>Okay, your first year of programming you were told that indentation is
327 important. If you didn't believe and internalize this then, now is the time.
333 <!-- ======================================================================= -->
334 <div class="doc_subsection">
335 <a name="compilerissues">Compiler Issues</a>
339 <!-- _______________________________________________________________________ -->
340 <div class="doc_subsubsection">
341 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
344 <div class="doc_text">
346 <p>If your code has compiler warnings in it, something is wrong: you aren't
347 casting values correctly, your have "questionable" constructs in your code, or
348 you are doing something legitimately wrong. Compiler warnings can cover up
349 legitimate errors in output and make dealing with a translation unit
352 <p>It is not possible to prevent all warnings from all compilers, nor is it
353 desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
354 a good thorough set of warnings, and stick to them. At least in the case of
355 <tt>gcc</tt>, it is possible to work around any spurious errors by changing the
356 syntax of the code slightly. For example, an warning that annoys me occurs when
357 I write code like this:</p>
359 <div class="doc_code">
361 if (V = getValue()) {
367 <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
368 operator, and that I probably mistyped it. In most cases, I haven't, and I
369 really don't want the spurious errors. To fix this particular problem, I
370 rewrite the code like this:</p>
372 <div class="doc_code">
374 if ((V = getValue())) {
380 <p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
381 be fixed by massaging the code appropriately.</p>
383 <p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
384 -Winline -W -Wwrite-strings -Wno-unused</tt></p>
388 <!-- _______________________________________________________________________ -->
389 <div class="doc_subsubsection">
390 <a name="ci_portable_code">Write Portable Code</a>
393 <div class="doc_text">
395 <p>In almost all cases, it is possible and within reason to write completely
396 portable code. If there are cases where it isn't possible to write portable
397 code, isolate it behind a well defined (and well documented) interface.</p>
399 <p>In practice, this means that you shouldn't assume much about the host
400 compiler, including its support for "high tech" features like partial
401 specialization of templates. If these features are used, they should only be
402 an implementation detail of a library which has a simple exposed API.</p>
406 <!-- _______________________________________________________________________ -->
407 <div class="doc_subsubsection">
408 <a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
410 <div class="doc_text">
412 <p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
413 interchangeably. The only difference is when they are used to declare a class:
414 <tt>class</tt> makes all members private by default while <tt>struct</tt> makes
415 all members public by default.</p>
417 <p>Unfortunately, not all compilers follow the rules and some will generate
418 different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
419 declare the symbol. This can lead to problems at link time.</p>
421 <p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
422 <b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
426 <!-- *********************************************************************** -->
427 <div class="doc_section">
428 <a name="styleissues">Style Issues</a>
430 <!-- *********************************************************************** -->
433 <!-- ======================================================================= -->
434 <div class="doc_subsection">
435 <a name="macro">The High Level Issues</a>
437 <!-- ======================================================================= -->
440 <!-- _______________________________________________________________________ -->
441 <div class="doc_subsubsection">
442 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
445 <div class="doc_text">
447 <p>C++ doesn't do too well in the modularity department. There is no real
448 encapsulation or data hiding (unless you use expensive protocol classes), but it
449 is what we have to work with. When you write a public header file (in the LLVM
450 source tree, they live in the top level "include" directory), you are defining a
451 module of functionality.</p>
453 <p>Ideally, modules should be completely independent of each other, and their
454 header files should only include the absolute minimum number of headers
455 possible. A module is not just a class, a function, or a namespace: <a
456 href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
457 of these</a> that defines an interface. This interface may be several
458 functions, classes or data structures, but the important issue is how they work
461 <p>In general, a module should be implemented with one or more <tt>.cpp</tt>
462 files. Each of these <tt>.cpp</tt> files should include the header that defines
463 their interface first. This ensure that all of the dependences of the module
464 header have been properly added to the module header itself, and are not
465 implicit. System headers should be included after user headers for a
466 translation unit.</p>
470 <!-- _______________________________________________________________________ -->
471 <div class="doc_subsubsection">
472 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
475 <div class="doc_text">
477 <p><tt>#include</tt> hurts compile time performance. Don't do it unless you
478 have to, especially in header files.</p>
480 <p>But wait, sometimes you need to have the definition of a class to use it, or
481 to inherit from it. In these cases go ahead and <tt>#include</tt> that header
482 file. Be aware however that there are many cases where you don't need to have
483 the full definition of a class. If you are using a pointer or reference to a
484 class, you don't need the header file. If you are simply returning a class
485 instance from a prototyped function or method, you don't need it. In fact, for
486 most cases, you simply don't need the definition of a class... and not
487 <tt>#include</tt>'ing speeds up compilation.</p>
489 <p>It is easy to try to go too overboard on this recommendation, however. You
490 <b>must</b> include all of the header files that you are using -- you can
491 include them either directly
492 or indirectly (through another header file). To make sure that you don't
493 accidently forget to include a header file in your module header, make sure to
494 include your module header <b>first</b> in the implementation file (as mentioned
495 above). This way there won't be any hidden dependencies that you'll find out
500 <!-- _______________________________________________________________________ -->
501 <div class="doc_subsubsection">
502 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
505 <div class="doc_text">
507 <p>Many modules have a complex implementation that causes them to use more than
508 one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
509 internal communication interface (helper classes, extra functions, etc) in the
510 public module header file. Don't do this.</p>
512 <p>If you really need to do something like this, put a private header file in
513 the same directory as the source files, and include it locally. This ensures
514 that your private interface remains private and undisturbed by outsiders.</p>
516 <p>Note however, that it's okay to put extra implementation methods a public
517 class itself... just make them private (or protected), and all is well.</p>
521 <!-- _______________________________________________________________________ -->
522 <div class="doc_subsubsection">
523 <a name="hl_earlyexit">Use Early Exits and 'continue' to Simplify Code</a>
526 <div class="doc_text">
528 <p>When reading code, keep in mind how much state and how many previous
529 decisions have to be remembered by the reader to understand a block of code.
530 Aim to reduce indentation where possible when it doesn't make it more difficult
531 to understand the code. One great way to do this is by making use of early
532 exits and the 'continue' keyword in long loops. As an example of using an early
533 exit from a function, consider this "bad" code:</p>
535 <div class="doc_code">
537 Value *DoSomething(Instruction *I) {
538 if (!isa<TerminatorInst>(I) &&
539 I->hasOneUse() && SomeOtherThing(I)) {
540 ... some long code ....
548 <p>This code has several problems if the body of the 'if' is large. When you're
549 looking at the top of the function, it isn't immediately clear that this
550 <em>only</em> does interesting things with non-terminator instructions, and only
551 applies to things with the other predicates. Second, it is relatively difficult
552 to describe (in comments) why these predicates are important because the if
553 statement makes it difficult to lay out the comments. Third, when you're deep
554 within the body of the code, it is indented an extra level. Finally, when
555 reading the top of the function, it isn't clear what the result is if the
556 predicate isn't true, you have to read to the end of the function to know that
559 <p>It is much preferred to format the code like this:</p>
561 <div class="doc_code">
563 Value *DoSomething(Instruction *I) {
564 // Terminators never need 'something' done to them because, ...
565 if (isa<TerminatorInst>(I))
568 // We conservatively avoid transforming instructions with multiple uses
569 // because goats like cheese.
570 if (!I->hasOneUse())
573 // This is really just here for example.
574 if (!SomeOtherThing(I))
577 ... some long code ....
582 <p>This fixes these problems. A similar problem frequently happens in for
583 loops. A silly example is something like this:</p>
585 <div class="doc_code">
587 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
588 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
589 Value *LHS = BO->getOperand(0);
590 Value *RHS = BO->getOperand(1);
599 <p>When you have very very small loops, this sort of structure is fine, but if
600 it exceeds more than 10-15 lines, it becomes difficult for people to read and
601 understand at a glance.
602 The problem with this sort of code is that it gets very nested very quickly,
603 meaning that the reader of the code has to keep a lot of context in their brain
604 to remember what is going immediately on in the loop, because they don't know
605 if/when the if conditions will have elses etc. It is strongly preferred to
606 structure the loop like this:</p>
608 <div class="doc_code">
610 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
611 BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
614 Value *LHS = BO->getOperand(0);
615 Value *RHS = BO->getOperand(1);
616 if (LHS == RHS) continue;
621 <p>This has all the benefits of using early exits from functions: it reduces
622 nesting of the loop, it makes it easier to describe why the conditions are true,
623 and it makes it obvious to the reader that there is no "else" coming up that
624 they have to push context into their brain for. If a loop is large, this can
625 be a big understandability win.</p>
629 <!-- _______________________________________________________________________ -->
630 <div class="doc_subsubsection">
631 <a name="hl_else_after_return">Don't use "else" after a return</a>
634 <div class="doc_text">
636 <p>For similar reasons above (reduction of indentation and easier reading),
637 please do not use "else" or "else if" after something that interrupts
638 control flow like return, break, continue, goto, etc. For example, this is
641 <div class="doc_code">
645 Type = Context.getsigjmp_bufType();
647 Error = ASTContext::GE_Missing_sigjmp_buf;
653 Type = Context.getjmp_bufType();
655 Error = ASTContext::GE_Missing_jmp_buf;
666 <p>It is better to write this something like:</p>
668 <div class="doc_code">
672 Type = Context.getsigjmp_bufType();
674 Error = ASTContext::GE_Missing_sigjmp_buf;
678 Type = Context.getjmp_bufType();
680 Error = ASTContext::GE_Missing_jmp_buf;
688 <p>Or better yet (in this case), as:</p>
690 <div class="doc_code">
694 Type = Context.getsigjmp_bufType();
696 Type = Context.getjmp_bufType();
699 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
700 ASTContext::GE_Missing_jmp_buf;
707 <p>The idea is to reduce indentation and the amount of code you have to keep
708 track of when reading the code.</p>
712 <!-- _______________________________________________________________________ -->
713 <div class="doc_subsubsection">
714 <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
717 <div class="doc_text">
719 <p>It is very common to write small loops that just compute a boolean
720 value. There are a number of ways that people commonly write these, but an
721 example of this sort of thing is:</p>
723 <div class="doc_code">
725 <b>bool FoundFoo = false;</b>
726 for (unsigned i = 0, e = BarList.size(); i != e; ++i)
727 if (BarList[i]->isFoo()) {
728 <b>FoundFoo = true;</b>
732 <b>if (FoundFoo) {</b>
738 <p>This sort of code is awkward to write, and is almost always a bad sign.
739 Instead of this sort of loop, we strongly prefer to use a predicate function
740 (which may be <a href="#micro_anonns">static</a>) that uses
741 <a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
742 the code to be structured like this:
746 <div class="doc_code">
748 /// ListContainsFoo - Return true if the specified list has an element that is
750 static bool ListContainsFoo(const std::vector<Bar*> &List) {
751 for (unsigned i = 0, e = List.size(); i != e; ++i)
752 if (List[i]->isFoo())
758 <b>if (ListContainsFoo(BarList)) {</b>
764 <p>There are many reasons for doing this: it reduces indentation and factors out
765 code which can often be shared by other code that checks for the same predicate.
766 More importantly, it <em>forces you to pick a name</em> for the function, and
767 forces you to write a comment for it. In this silly example, this doesn't add
768 much value. However, if the condition is complex, this can make it a lot easier
769 for the reader to understand the code that queries for this predicate. Instead
770 of being faced with the in-line details of how we check to see if the BarList
771 contains a foo, we can trust the function name and continue reading with better
777 <!-- ======================================================================= -->
778 <div class="doc_subsection">
779 <a name="micro">The Low Level Issues</a>
781 <!-- ======================================================================= -->
784 <!-- _______________________________________________________________________ -->
785 <div class="doc_subsubsection">
786 <a name="ll_assert">Assert Liberally</a>
789 <div class="doc_text">
791 <p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
792 preconditions and assumptions, you never know when a bug (not neccesarily even
793 yours) might be caught early by an assertion, which reduces debugging time
794 dramatically. The "<tt><cassert></tt>" header file is probably already
795 included by the header files you are using, so it doesn't cost anything to use
798 <p>To further assist with debugging, make sure to put some kind of error message
799 in the assertion statement (which is printed if the assertion is tripped). This
800 helps the poor debugging make sense of why an assertion is being made and
801 enforced, and hopefully what to do about it. Here is one complete example:</p>
803 <div class="doc_code">
805 inline Value *getOperand(unsigned i) {
806 assert(i < Operands.size() && "getOperand() out of range!");
812 <p>Here are some examples:</p>
814 <div class="doc_code">
816 assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
818 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
820 assert(idx < getNumSuccessors() && "Successor # out of range!");
822 assert(V1.getType() == V2.getType() && "Constant types must be identical!");
824 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
828 <p>You get the idea...</p>
830 <p>Please be aware when adding assert statements that not all compilers are aware of
831 the semantics of the assert. In some places, asserts are used to indicate a piece of
832 code that should not be reached. These are typically of the form:</p>
834 <div class="doc_code">
836 assert(0 && "Some helpful error message");
840 <p>When used in a function that returns a value, they should be followed with a return
841 statement and a comment indicating that this line is never reached. This will prevent
842 a compiler which is unable to deduce that the assert statement never returns from
843 generating a warning.</p>
845 <div class="doc_code">
847 assert(0 && "Some helpful error message");
855 <!-- _______________________________________________________________________ -->
856 <div class="doc_subsubsection">
857 <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
860 <div class="doc_text">
861 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
862 namespace with an "<tt>std::</tt>" prefix, rather than rely on
863 "<tt>using namespace std;</tt>".</p>
865 <p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
866 the namespace of any source file that <tt>#include</tt>s the header. This is
867 clearly a bad thing.</p>
869 <p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
870 rule, but is still important. Basically, using explicit namespace prefixes
871 makes the code <b>clearer</b>, because it is immediately obvious what facilities
872 are being used and where they are coming from, and <b>more portable</b>, because
873 namespace clashes cannot occur between LLVM code and other namespaces. The
874 portability rule is important because different standard library implementations
875 expose different symbols (potentially ones they shouldn't), and future revisions
876 to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
877 such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
879 <p>The exception to the general rule (i.e. it's not an exception for
880 the <tt>std</tt> namespace) is for implementation files. For example, all of
881 the code in the LLVM project implements code that lives in the 'llvm' namespace.
882 As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
883 namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
884 general form of this rule is that any .cpp file that implements code in any
885 namespace may use that namespace (and its parents'), but should not use any
890 <!-- _______________________________________________________________________ -->
891 <div class="doc_subsubsection">
892 <a name="ll_virtual_anch">Provide a virtual method anchor for classes
896 <div class="doc_text">
898 <p>If a class is defined in a header file and has a v-table (either it has
899 virtual methods or it derives from classes with virtual methods), it must
900 always have at least one out-of-line virtual method in the class. Without
901 this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
902 that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
903 increasing link times.</p>
907 <!-- _______________________________________________________________________ -->
908 <div class="doc_subsubsection">
909 <a name="ll_end">Don't evaluate end() every time through a loop</a>
912 <div class="doc_text">
914 <p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
915 with macros and may be coming in C++'0x) we end up writing a lot of loops that
916 manually iterate from begin to end on a variety of containers or through other
917 data structures. One common mistake is to write a loop in this style:</p>
919 <div class="doc_code">
922 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
927 <p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
928 every time through the loop. Instead of writing the loop like this, we strongly
929 prefer loops to be written so that they evaluate it once before the loop starts.
930 A convenient way to do this is like so:</p>
932 <div class="doc_code">
935 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
940 <p>The observant may quickly point out that these two loops may have different
941 semantics: if the container (a basic block in this case) is being mutated, then
942 "<tt>BB->end()</tt>" may change its value every time through the loop and the
943 second loop may not in fact be correct. If you actually do depend on this
944 behavior, please write the loop in the first form and add a comment indicating
945 that you did it intentionally.</p>
947 <p>Why do we prefer the second form (when correct)? Writing the loop in the
948 first form has two problems: First it may be less efficient than evaluating it
949 at the start of the loop. In this case, the cost is probably minor: a few extra
950 loads every time through the loop. However, if the base expression is more
951 complex, then the cost can rise quickly. I've seen loops where the end
952 expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
953 lookups really aren't cheap. By writing it in the second form consistently, you
954 eliminate the issue entirely and don't even have to think about it.</p>
956 <p>The second (even bigger) issue is that writing the loop in the first form
957 hints to the reader that the loop is mutating the container (a fact that a
958 comment would handily confirm!). If you write the loop in the second form, it
959 is immediately obvious without even looking at the body of the loop that the
960 container isn't being modified, which makes it easier to read the code and
961 understand what it does.</p>
963 <p>While the second form of the loop is a few extra keystrokes, we do strongly
968 <!-- _______________________________________________________________________ -->
969 <div class="doc_subsubsection">
970 <a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
973 <div class="doc_text">
975 <p>The use of <tt>#include <iostream></tt> in library files is
976 hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
977 support clients using LLVM libraries as part of larger systems. In particular,
978 we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
979 the static c'tors are run whenever an application start up that uses the dynamic
980 library. There are two problems with this:</p>
983 <li>The time to run the static c'tors impacts startup time of
984 applications—a critical time for GUI apps.</li>
985 <li>The static c'tors cause the app to pull many extra pages of memory off the
986 disk: both the code for the static c'tors in each <tt>.o</tt> file and the
987 small amount of data that gets touched. In addition, touched/dirty pages
988 put more pressure on the VM system on low-memory machines.</li>
991 <p>Note that using the other stream headers (<tt><sstream></tt> for
992 example) is allowed normally, it is just <tt><iostream></tt> that is
993 causing problems.</p>
995 <p>In addition, new code should always
996 use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> or
997 the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
1002 <!-- _______________________________________________________________________ -->
1003 <div class="doc_subsubsection">
1004 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
1007 <div class="doc_text">
1009 <p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
1010 to the output stream specified. In addition to doing this, however, it also
1011 flushes the output stream. In other words, these are equivalent:</p>
1013 <div class="doc_code">
1015 std::cout << std::endl;
1016 std::cout << '\n' << std::flush;
1020 <p>Most of the time, you probably have no reason to flush the output stream, so
1021 it's better to use a literal <tt>'\n'</tt>.</p>
1026 <!-- _______________________________________________________________________ -->
1027 <div class="doc_subsubsection">
1028 <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
1031 <div class="doc_text">
1033 <p>LLVM includes a lightweight, simple, and efficient stream implementation
1034 in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
1035 of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> instead
1036 of <tt>ostream</tt>.</p>
1038 <p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
1039 be forward declared as <tt>class raw_ostream</tt>. Public headers should
1040 generally not include the <tt>raw_ostream</tt> header, but use forward
1041 declarations and constant references to <tt>raw_ostream</tt> instances.</p>
1046 <!-- ======================================================================= -->
1047 <div class="doc_subsection">
1048 <a name="nano">Microscopic Details</a>
1050 <!-- ======================================================================= -->
1052 <p>This section describes preferred low-level formatting guidelines along with
1053 reasoning on why we prefer them.</p>
1055 <!-- _______________________________________________________________________ -->
1056 <div class="doc_subsubsection">
1057 <a name="micro_spaceparen">Spaces Before Parentheses</a>
1060 <div class="doc_text">
1062 <p>We prefer to put a space before a parentheses only in control flow
1063 statements, but not in normal function call expressions and function-like
1064 macros. For example, this is good:</p>
1066 <div class="doc_code">
1069 <b>for (</b>i = 0; i != 100; ++i) ...
1070 <b>while (</b>llvm_rocks) ...
1072 <b>somefunc(</b>42);
1073 <b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me");
1075 a = <b>foo(</b>42, 92) + <b>bar(</b>x);
1079 <p>... and this is bad:</p>
1081 <div class="doc_code">
1084 <b>for(</b>i = 0; i != 100; ++i) ...
1085 <b>while(</b>llvm_rocks) ...
1087 <b>somefunc (</b>42);
1088 <b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me");
1090 a = <b>foo (</b>42, 92) + <b>bar (</b>x);
1094 <p>The reason for doing this is not completely arbitrary. This style makes
1095 control flow operators stand out more, and makes expressions flow better. The
1096 function call operator binds very tightly as a postfix operator. Putting
1097 a space after a function name (as in the last example) makes it appear that
1098 the code might bind the arguments of the left-hand-side of a binary operator
1099 with the argument list of a function and the name of the right side. More
1100 specifically, it is easy to misread the "a" example as:</p>
1102 <div class="doc_code">
1104 a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
1108 <p>... when skimming through the code. By avoiding a space in a function, we
1109 avoid this misinterpretation.</p>
1113 <!-- _______________________________________________________________________ -->
1114 <div class="doc_subsubsection">
1115 <a name="micro_preincrement">Prefer Preincrement</a>
1118 <div class="doc_text">
1120 <p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
1121 postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
1122 preincrementation whenever possible.</p>
1124 <p>The semantics of postincrement include making a copy of the value being
1125 incremented, returning it, and then preincrementing the "work value". For
1126 primitive types, this isn't a big deal... but for iterators, it can be a huge
1127 issue (for example, some iterators contains stack and set objects in them...
1128 copying an iterator could invoke the copy ctor's of these as well). In general,
1129 get in the habit of always using preincrement, and you won't have a problem.</p>
1133 <!-- _______________________________________________________________________ -->
1134 <div class="doc_subsubsection">
1135 <a name="micro_namespaceindent">Namespace Indentation</a>
1138 <div class="doc_text">
1141 In general, we strive to reduce indentation where ever possible. This is useful
1142 because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
1143 wrapping horribly, but also because it makes it easier to understand the code.
1144 Namespaces are a funny thing: they are often large, and we often desire to put
1145 lots of stuff into them (so they can be large). Other times they are tiny,
1146 because they just hold an enum or something similar. In order to balance this,
1147 we use different approaches for small versus large namespaces.
1151 If a namespace definition is small and <em>easily</em> fits on a screen (say,
1152 less than 35 lines of code), then you should indent its body. Here's an
1156 <div class="doc_code">
1160 /// RelocationType - An enum for the x86 relocation codes. Note that
1161 /// the terminology here doesn't follow x86 convention - word means
1162 /// 32-bit and dword means 64-bit.
1163 enum RelocationType {
1164 /// reloc_pcrel_word - PC relative relocation, add the relocated value to
1165 /// the value already in memory, after we adjust it for where the PC is.
1166 reloc_pcrel_word = 0,
1168 /// reloc_picrel_word - PIC base relative relocation, add the relocated
1169 /// value to the value already in memory, after we adjust it for where the
1171 reloc_picrel_word = 1,
1173 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
1174 /// add the relocated value to the value already in memory.
1175 reloc_absolute_word = 2,
1176 reloc_absolute_dword = 3
1183 <p>Since the body is small, indenting adds value because it makes it very clear
1184 where the namespace starts and ends, and it is easy to take the whole thing in
1185 in one "gulp" when reading the code. If the blob of code in the namespace is
1186 larger (as it typically is in a header in the llvm or clang namespaces), do not
1187 indent the code, and add a comment indicating what namespace is being closed.
1190 <div class="doc_code">
1193 namespace knowledge {
1195 /// Grokable - This class represents things that Smith can have an intimate
1196 /// understanding of and contains the data associated with it.
1200 explicit Grokable() { ... }
1201 virtual ~Grokable() = 0;
1207 } // end namespace knowledge
1208 } // end namespace llvm
1212 <p>Because the class is large, we don't expect that the reader can easily
1213 understand the entire concept in a glance, and the end of the file (where the
1214 namespaces end) may be a long ways away from the place they open. As such,
1215 indenting the contents of the namespace doesn't add any value, and detracts from
1216 the readability of the class. In these cases it is best to <em>not</em> indent
1217 the contents of the namespace.</p>
1221 <!-- _______________________________________________________________________ -->
1222 <div class="doc_subsubsection">
1223 <a name="micro_anonns">Anonymous Namespaces</a>
1226 <div class="doc_text">
1228 <p>After talking about namespaces in general, you may be wondering about
1229 anonymous namespaces in particular.
1230 Anonymous namespaces are a great language feature that tells the C++ compiler
1231 that the contents of the namespace are only visible within the current
1232 translation unit, allowing more aggressive optimization and eliminating the
1233 possibility of symbol name collisions. Anonymous namespaces are to C++ as
1234 "static" is to C functions and global variables. While "static" is available
1235 in C++, anonymous namespaces are more general: they can make entire classes
1236 private to a file.</p>
1238 <p>The problem with anonymous namespaces is that they naturally want to
1239 encourage indentation of their body, and they reduce locality of reference: if
1240 you see a random function definition in a C++ file, it is easy to see if it is
1241 marked static, but seeing if it is in an anonymous namespace requires scanning
1242 a big chunk of the file.</p>
1244 <p>Because of this, we have a simple guideline: make anonymous namespaces as
1245 small as possible, and only use them for class declarations. For example, this
1248 <div class="doc_code">
1255 bool operator<(const char *RHS) const;
1257 <b>} // end anonymous namespace</b>
1259 static void Helper() {
1263 bool StringSort::operator<(const char *RHS) const {
1273 <div class="doc_code">
1280 bool operator<(const char *RHS) const;
1287 bool StringSort::operator<(const char *RHS) const {
1291 <b>} // end anonymous namespace</b>
1297 <p>This is bad specifically because if you're looking at "Helper" in the middle
1298 of a large C++ file, that you have no immediate way to tell if it is local to
1299 the file. When it is marked static explicitly, this is immediately obvious.
1300 Also, there is no reason to enclose the definition of "operator<" in the
1301 namespace just because it was declared there.
1308 <!-- *********************************************************************** -->
1309 <div class="doc_section">
1310 <a name="seealso">See Also</a>
1312 <!-- *********************************************************************** -->
1314 <div class="doc_text">
1316 <p>A lot of these comments and recommendations have been culled for other
1317 sources. Two particularly important books for our work are:</p>
1321 <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
1322 C++</a> by Scott Meyers. Also
1323 interesting and useful are "More Effective C++" and "Effective STL" by the same
1326 <li>Large-Scale C++ Software Design by John Lakos</li>
1330 <p>If you get some free time, and you haven't read them: do so, you might learn
1335 <!-- *********************************************************************** -->
1339 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
1340 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
1341 <a href="http://validator.w3.org/check/referer"><img
1342 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
1344 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
1345 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
1346 Last modified: $Date$