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3 <html>
4 <head>
7 </head>
8 <body>
10 <h1>
11 LLVM Coding Standards
12 </h1>
14 <ol>
17 <ol>
19 <ol>
26 </ol></li>
28 <ol>
30 Errors</a></li>
34 </ol></li>
35 </ol></li>
37 <ol>
39 <ol>
41 Module</a></li>
44 Private</a></li>
46 Code</a></li>
50 Functions</a></li>
51 </ol></li>
53 <ol>
58 classes in headers</a></li>
60 loop</a></li>
65 </ol></li>
68 <ol>
73 </ol></li>
76 </ol></li>
78 </ol>
82 </div>
85 <!-- *********************************************************************** -->
86 <h2>
88 </h2>
89 <!-- *********************************************************************** -->
91 <div>
93 <p>This document attempts to describe a few coding standards that are being used
94 in the LLVM source tree. Although no coding standards should be regarded as
95 absolute requirements to be followed in all instances, coding standards can be
96 useful.</p>
98 <p>This document intentionally does not prescribe fixed standards for religious
99 issues such as brace placement and space usage. For issues like this, follow
100 the golden rule:</p>
102 <blockquote>
105 project, feel free to use whatever style you are most comfortable with. If you
106 are extending, enhancing, or bug fixing already implemented code, use the style
107 that is already being used so that the source is uniform and easy to
108 follow.</a></b></p>
110 </blockquote>
112 <p>The ultimate goal of these guidelines is the increase readability and
113 maintainability of our common source base. If you have suggestions for topics to
114 be included, please mail them to <a
117 </div>
119 <!-- *********************************************************************** -->
120 <h2>
122 </h2>
123 <!-- *********************************************************************** -->
125 <div>
127 <!-- ======================================================================= -->
128 <h3>
130 </h3>
132 <div>
134 <!-- _______________________________________________________________________ -->
135 <h4>
137 </h4>
139 <div>
141 <p>Comments are one critical part of readability and maintainability. Everyone
142 knows they should comment, so should you. When writing comments, write them as
143 English prose, which means they should use proper capitalization, punctuation,
144 etc. Although we all should probably
145 comment our code more than we do, there are a few very critical places that
146 documentation is very useful:</p>
150 <div>
152 <p>Every source file should have a header on it that describes the basic
153 purpose of the file. If a file does not have a header, it should not be
154 checked into Subversion. Most source trees will probably have a standard
155 file header format. The standard format for the LLVM source tree looks like
156 this:</p>
159 <pre>
160 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
161 //
162 // The LLVM Compiler Infrastructure
163 //
164 // This file is distributed under the University of Illinois Open Source
165 // License. See LICENSE.TXT for details.
166 //
167 //===----------------------------------------------------------------------===//
168 //
169 // This file contains the declaration of the Instruction class, which is the
170 // base class for all of the VM instructions.
171 //
172 //===----------------------------------------------------------------------===//
173 </pre>
174 </div>
177 -*-</tt>" string on the first line is there to tell Emacs that the source file
180 on the first line, along with a very short description of the purpose of the
181 file. This is important when printing out code and flipping though lots of
182 pages.</p>
184 <p>The next section in the file is a concise note that defines the license
185 that the file is released under. This makes it perfectly clear what terms the
186 source code can be distributed under and should not be modified in any way.</p>
188 <p>The main body of the description does not have to be very long in most cases.
189 Here it's only two lines. If an algorithm is being implemented or something
190 tricky is going on, a reference to the paper where it is published should be
193 </div>
197 <p>Classes are one fundamental part of a good object oriented design. As such,
198 a class definition should have a comment block that explains what the class is
199 used for... if it's not obvious. If it's so completely obvious your grandma
200 could figure it out, it's probably safe to leave it out. Naming classes
201 something sane goes a long ways towards avoiding writing documentation.</p>
206 <div>
208 <p>Methods defined in a class (as well as any global functions) should also be
209 documented properly. A quick note about what it does and a description of the
210 borderline behaviour is all that is necessary here (unless something
211 particularly tricky or insidious is going on). The hope is that people can
212 figure out how to use your interfaces without reading the code itself... that is
213 the goal metric.</p>
215 <p>Good things to talk about here are what happens when something unexpected
216 happens: does the method return null? Abort? Format your hard disk?</p>
218 </div>
220 </div>
222 <!-- _______________________________________________________________________ -->
223 <h4>
225 </h4>
227 <div>
230 require less typing, don't have nesting problems, etc. There are a few cases
233 <ol>
234 <li>When writing C code: Obviously if you are writing C code, use C style
235 comments.</li>
237 file.</li>
238 <li>When writing a source file that is used by a tool that only accepts C
239 style comments.</li>
240 </ol>
243 These nest properly and are better behaved in general than C style comments.</p>
245 </div>
247 <!-- _______________________________________________________________________ -->
248 <h4>
250 </h4>
252 <div>
255 include guards if working on a header file), the <a
258 order:</p>
260 <ol>
272 </ol>
280 interfaces, we ensure that the header does not have any hidden dependencies
281 which are not explicitly #included in the header, but should be. It is also a
283 implements are defined.</p>
285 </div>
287 <!-- _______________________________________________________________________ -->
288 <h4>
290 </h4>
292 <div>
295 like to print out code and look at your code in an xterm without resizing
296 it.</p>
298 <p>The longer answer is that there must be some limit to the width of the code
299 in order to reasonably allow developers to have multiple files side-by-side in
300 windows on a modest display. If you are going to pick a width limit, it is
301 somewhat arbitrary but you might as well pick something standard. Going with
303 value and would be detrimental to printing out code. Also many other projects
304 have standardized on 80 columns, so some people have already configured their
307 <p>This is one of many contentious issues in coding standards, but it is not up
308 for debate.</p>
310 </div>
312 <!-- _______________________________________________________________________ -->
313 <h4>
315 </h4>
317 <div>
319 <p>In all cases, prefer spaces to tabs in source files. People have different
320 preferred indentation levels, and different styles of indentation that they
321 like; this is fine. What isn't fine is that different editors/viewers expand
322 tabs out to different tab stops. This can cause your code to look completely
323 unreadable, and it is not worth dealing with.</p>
326 style of existing code if you are modifying and extending it. If you like four
328 with two spaces of indentation. Also, do not reindent a whole source file: it
329 makes for incredible diffs that are absolutely worthless.</p>
331 </div>
333 <!-- _______________________________________________________________________ -->
334 <h4>
336 </h4>
338 <div>
340 <p>Okay, in your first year of programming you were told that indentation is
341 important. If you didn't believe and internalize this then, now is the time.
342 Just do it.</p>
344 </div>
346 </div>
348 <!-- ======================================================================= -->
349 <h3>
351 </h3>
353 <div>
355 <!-- _______________________________________________________________________ -->
356 <h4>
358 </h4>
360 <div>
363 aren't casting values correctly, your have "questionable" constructs in your
364 code, or you are doing something legitimately wrong. Compiler warnings can
365 cover up legitimate errors in output and make dealing with a translation unit
366 difficult.</p>
368 <p>It is not possible to prevent all warnings from all compilers, nor is it
370 a good thorough set of warnings, and stick to it. At least in the case of
372 syntax of the code slightly. For example, a warning that annoys me occurs when
373 I write code like this:</p>
376 <pre>
377 if (V = getValue()) {
378 ...
379 }
380 </pre>
381 </div>
384 operator, and that I probably mistyped it. In most cases, I haven't, and I
385 really don't want the spurious errors. To fix this particular problem, I
386 rewrite the code like this:</p>
389 <pre>
390 if ((V = getValue())) {
391 ...
392 }
393 </pre>
394 </div>
397 be fixed by massaging the code appropriately.</p>
402 <pre>
403 -Wall -Winline -W -Wwrite-strings -Wno-unused
404 </pre>
405 </div>
407 </div>
409 <!-- _______________________________________________________________________ -->
410 <h4>
412 </h4>
414 <div>
416 <p>In almost all cases, it is possible and within reason to write completely
417 portable code. If there are cases where it isn't possible to write portable
418 code, isolate it behind a well defined (and well documented) interface.</p>
420 <p>In practice, this means that you shouldn't assume much about the host
421 compiler, and Visual Studio tends to be the lowest common denominator.
422 If advanced features are used, they should only be an implementation detail of
423 a library which has a simple exposed API, and preferably be buried in
424 libSystem.</p>
426 </div>
428 <!-- _______________________________________________________________________ -->
429 <h4>
431 </h4>
432 <div>
434 <p>In an effort to reduce code and executable size, LLVM does not use RTTI
437 causing executable bloat even if exceptions are never used in the code base, or
438 if RTTI is never used for a class. Because of this, we turn them off globally
439 in the code.</p>
441 <p>That said, LLVM does make extensive use of a hand-rolled form of RTTI that
444 opt-in and can be added to any class. It is also substantially more efficient
447 </div>
449 <!-- _______________________________________________________________________ -->
450 <h4>
452 </h4>
453 <div>
456 interchangeably. The only difference is when they are used to declare a class:
458 all members public by default.</p>
460 <p>Unfortunately, not all compilers follow the rules and some will generate
462 declare the symbol. This can lead to problems at link time.</p>
469 </div>
471 </div>
473 </div>
475 <!-- *********************************************************************** -->
476 <h2>
478 </h2>
479 <!-- *********************************************************************** -->
481 <div>
483 <!-- ======================================================================= -->
484 <h3>
486 </h3>
487 <!-- ======================================================================= -->
489 <div>
491 <!-- _______________________________________________________________________ -->
492 <h4>
494 </h4>
496 <div>
498 <p>C++ doesn't do too well in the modularity department. There is no real
499 encapsulation or data hiding (unless you use expensive protocol classes), but it
500 is what we have to work with. When you write a public header file (in the LLVM
501 source tree, they live in the top level "<tt>include</tt>" directory), you are
502 defining a module of functionality.</p>
504 <p>Ideally, modules should be completely independent of each other, and their
506 headers possible. A module is not just a class, a function, or a
508 a collection of these</a> that defines an interface. This interface may be
509 several functions, classes, or data structures, but the important issue is how
510 they work together.</p>
514 their interface first. This ensures that all of the dependences of the module
515 header have been properly added to the module header itself, and are not
516 implicit. System headers should be included after user headers for a
517 translation unit.</p>
519 </div>
521 <!-- _______________________________________________________________________ -->
522 <h4>
524 </h4>
526 <div>
529 have to, especially in header files.</p>
531 <p>But wait! Sometimes you need to have the definition of a class to use it, or
533 file. Be aware however that there are many cases where you don't need to have
534 the full definition of a class. If you are using a pointer or reference to a
535 class, you don't need the header file. If you are simply returning a class
536 instance from a prototyped function or method, you don't need it. In fact, for
537 most cases, you simply don't need the definition of a class. And not
540 <p>It is easy to try to go too overboard on this recommendation, however. You
542 include them either directly or indirectly (through another header file). To
543 make sure that you don't accidentally forget to include a header file in your
545 implementation file (as mentioned above). This way there won't be any hidden
546 dependencies that you'll find out about later.</p>
548 </div>
550 <!-- _______________________________________________________________________ -->
551 <h4>
553 </h4>
555 <div>
557 <p>Many modules have a complex implementation that causes them to use more than
559 internal communication interface (helper classes, extra functions, etc) in the
560 public module header file. Don't do this!</p>
562 <p>If you really need to do something like this, put a private header file in
563 the same directory as the source files, and include it locally. This ensures
564 that your private interface remains private and undisturbed by outsiders.</p>
566 <p>Note however, that it's okay to put extra implementation methods in a public
567 class itself. Just make them private (or protected) and all is well.</p>
569 </div>
571 <!-- _______________________________________________________________________ -->
572 <h4>
574 </h4>
576 <div>
578 <p>When reading code, keep in mind how much state and how many previous
579 decisions have to be remembered by the reader to understand a block of code.
580 Aim to reduce indentation where possible when it doesn't make it more difficult
581 to understand the code. One great way to do this is by making use of early
586 <pre>
587 Value *DoSomething(Instruction *I) {
590 ... some long code ....
591 }
593 return 0;
594 }
595 </pre>
596 </div>
599 When you're looking at the top of the function, it isn't immediately clear that
601 only applies to things with the other predicates. Second, it is relatively
602 difficult to describe (in comments) why these predicates are important because
604 when you're deep within the body of the code, it is indented an extra level.
605 Finally, when reading the top of the function, it isn't clear what the result is
606 if the predicate isn't true; you have to read to the end of the function to know
607 that it returns null.</p>
612 <pre>
613 Value *DoSomething(Instruction *I) {
614 // Terminators never need 'something' done to them because ...
616 return 0;
618 // We conservatively avoid transforming instructions with multiple uses
619 // because goats like cheese.
620 if (!I->hasOneUse())
621 return 0;
623 // This is really just here for example.
624 if (!SomeOtherThing(I))
625 return 0;
627 ... some long code ....
628 }
629 </pre>
630 </div>
633 loops. A silly example is something like this:</p>
636 <pre>
641 if (LHS != RHS) {
642 ...
643 }
644 }
645 }
646 </pre>
647 </div>
649 <p>When you have very, very small loops, this sort of structure is fine. But if
651 understand at a glance. The problem with this sort of code is that it gets very
652 nested very quickly. Meaning that the reader of the code has to keep a lot of
653 context in their brain to remember what is going immediately on in the loop,
655 It is strongly preferred to structure the loop like this:</p>
658 <pre>
661 if (!BO) continue;
665 if (LHS == RHS) continue;
667 ...
668 }
669 </pre>
670 </div>
672 <p>This has all the benefits of using early exits for functions: it reduces
673 nesting of the loop, it makes it easier to describe why the conditions are true,
675 that they have to push context into their brain for. If a loop is large, this
676 can be a big understandability win.</p>
678 </div>
680 <!-- _______________________________________________________________________ -->
681 <h4>
683 </h4>
685 <div>
687 <p>For similar reasons above (reduction of indentation and easier reading),
693 <pre>
694 case 'J': {
695 if (Signed) {
696 Type = Context.getsigjmp_bufType();
697 if (Type.isNull()) {
698 Error = ASTContext::GE_Missing_sigjmp_buf;
699 return QualType();
700 <b>} else {
701 break;
702 }</b>
703 } else {
704 Type = Context.getjmp_bufType();
705 if (Type.isNull()) {
706 Error = ASTContext::GE_Missing_jmp_buf;
707 return QualType();
708 <b>} else {
709 break;
710 }</b>
711 }
712 }
713 }
714 </pre>
715 </div>
720 <pre>
721 case 'J':
722 if (Signed) {
723 Type = Context.getsigjmp_bufType();
724 if (Type.isNull()) {
725 Error = ASTContext::GE_Missing_sigjmp_buf;
726 return QualType();
727 }
728 } else {
729 Type = Context.getjmp_bufType();
730 if (Type.isNull()) {
731 Error = ASTContext::GE_Missing_jmp_buf;
732 return QualType();
733 }
734 }
736 </pre>
737 </div>
742 <pre>
743 case 'J':
744 if (Signed)
745 Type = Context.getsigjmp_bufType();
746 else
747 Type = Context.getjmp_bufType();
749 if (Type.isNull()) {
750 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
751 ASTContext::GE_Missing_jmp_buf;
752 return QualType();
753 }
755 </pre>
756 </div>
758 <p>The idea is to reduce indentation and the amount of code you have to keep
759 track of when reading the code.</p>
761 </div>
763 <!-- _______________________________________________________________________ -->
764 <h4>
766 </h4>
768 <div>
770 <p>It is very common to write small loops that just compute a boolean value.
771 There are a number of ways that people commonly write these, but an example of
772 this sort of thing is:</p>
775 <pre>
777 for (unsigned i = 0, e = BarList.size(); i != e; ++i)
778 if (BarList[i]->isFoo()) {
780 break;
781 }
784 ...
785 }
786 </pre>
787 </div>
789 <p>This sort of code is awkward to write, and is almost always a bad sign.
790 Instead of this sort of loop, we strongly prefer to use a predicate function
793 the code to be structured like this:</p>
796 <pre>
797 /// ListContainsFoo - Return true if the specified list has an element that is
798 /// a foo.
800 for (unsigned i = 0, e = List.size(); i != e; ++i)
801 if (List[i]->isFoo())
802 return true;
803 return false;
804 }
805 ...
808 ...
809 }
810 </pre>
811 </div>
813 <p>There are many reasons for doing this: it reduces indentation and factors out
814 code which can often be shared by other code that checks for the same predicate.
816 forces you to write a comment for it. In this silly example, this doesn't add
817 much value. However, if the condition is complex, this can make it a lot easier
818 for the reader to understand the code that queries for this predicate. Instead
819 of being faced with the in-line details of how we check to see if the BarList
820 contains a foo, we can trust the function name and continue reading with better
821 locality.</p>
823 </div>
825 </div>
827 <!-- ======================================================================= -->
828 <h3>
830 </h3>
831 <!-- ======================================================================= -->
833 <div>
835 <!-- _______________________________________________________________________ -->
836 <h4>
838 Name Types, Functions, Variables, and Enumerators Properly
839 </a>
840 </h4>
842 <div>
844 <p>Poorly-chosen names can mislead the reader and cause bugs. We cannot stress
846 match the semantics and role of the underlying entities, within reason. Avoid
847 abbreviations unless they are well known. After picking a good name, make sure
848 to use consistent capitalization for the name, as inconsistency requires clients
849 to either memorize the APIs or to look it up to find the exact spelling.</p>
853 rules:</p>
855 <ul>
857 should be nouns and start with an upper-case letter (e.g.
861 actions), and command-like function should be imperative. The name should
866 they should follow the naming conventions for types. A common use for enums
867 is as a discriminator for a union, or an indicator of a subclass. When an
872 variables</b> should start with an upper-case letter, just like types.
873 Unless the enumerators are defined in their own small namespace or inside a
874 class, enumerators should have a prefix corresponding to the enum
877 Enumerators that are just convenience constants are exempt from the
878 requirement for a prefix. For instance:</p>
881 <pre>
882 enum {
883 MaxSize = 42,
884 Density = 12
885 };
886 </pre>
887 </div>
888 </li>
890 </ul>
892 <p>As an exception, classes that mimic STL classes can have member names in
899 <pre>
900 class VehicleMaker {
901 ...
905 // kind of factories.
906 };
908 Vehicle MakeVehicle(VehicleType Type) {
909 VehicleMaker M; // Might be OK if having a short life-span.
910 Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information.
911 Light headlight = M.makeLight("head"); // Good -- descriptive.
912 ...
913 }
914 </pre>
915 </div>
917 </div>
920 <!-- _______________________________________________________________________ -->
921 <h4>
923 </h4>
925 <div>
928 preconditions and assumptions, you never know when a bug (not necessarily even
929 yours) might be caught early by an assertion, which reduces debugging time
930 dramatically. The "<tt><cassert></tt>" header file is probably already
931 included by the header files you are using, so it doesn't cost anything to use
932 it.</p>
934 <p>To further assist with debugging, make sure to put some kind of error message
935 in the assertion statement, which is printed if the assertion is tripped. This
936 helps the poor debugger make sense of why an assertion is being made and
937 enforced, and hopefully what to do about it. Here is one complete example:</p>
940 <pre>
941 inline Value *getOperand(unsigned i) {
943 return Operands[i];
944 }
945 </pre>
946 </div>
951 <pre>
961 </pre>
962 </div>
966 <p>Please be aware that, when adding assert statements, not all compilers are aware of
967 the semantics of the assert. In some places, asserts are used to indicate a piece of
968 code that should not be reached. These are typically of the form:</p>
971 <pre>
973 </pre>
974 </div>
976 <p>When used in a function that returns a value, they should be followed with a return
977 statement and a comment indicating that this line is never reached. This will prevent
978 a compiler which is unable to deduce that the assert statement never returns from
979 generating a warning.</p>
982 <pre>
984 // Not reached
985 return 0;
986 </pre>
987 </div>
990 value" warning when assertions are disabled. For example, this code will
991 warn:</p>
994 <pre>
995 unsigned Size = V.size();
998 bool NewToSet = Myset.insert(Value);
1000 </pre>
1001 </div>
1003 <p>These are two interesting different cases. In the first case, the call to
1004 V.size() is only useful for the assert, and we don't want it executed when
1005 assertions are disabled. Code like this should move the call into the assert
1006 itself. In the second case, the side effects of the call must happen whether
1007 the assert is enabled or not. In this case, the value should be cast to void to
1008 disable the warning. To be specific, it is preferred to write the code like
1009 this:</p>
1012 <pre>
1015 bool NewToSet = Myset.insert(Value); (void)NewToSet;
1017 </pre>
1018 </div>
1021 </div>
1023 <!-- _______________________________________________________________________ -->
1024 <h4>
1026 </h4>
1028 <div>
1030 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
1031 namespace with an "<tt>std::</tt>" prefix, rather than rely on
1036 clearly a bad thing.</p>
1039 rule, but is still important. Basically, using explicit namespace prefixes
1042 namespace clashes cannot occur between LLVM code and other namespaces. The
1043 portability rule is important because different standard library implementations
1044 expose different symbols (potentially ones they shouldn't), and future revisions
1048 <p>The exception to the general rule (i.e. it's not an exception for
1050 the code in the LLVM project implements code that lives in the 'llvm' namespace.
1054 editors that indent based on braces, and keeps the conceptual context cleaner.
1056 code in any namespace may use that namespace (and its parents'), but should not
1057 use any others.</p>
1059 </div>
1061 <!-- _______________________________________________________________________ -->
1062 <h4>
1064 Provide a Virtual Method Anchor for Classes in Headers
1065 </a>
1066 </h4>
1068 <div>
1070 <p>If a class is defined in a header file and has a v-table (either it has
1071 virtual methods or it derives from classes with virtual methods), it must
1072 always have at least one out-of-line virtual method in the class. Without
1075 increasing link times.</p>
1077 </div>
1079 <!-- _______________________________________________________________________ -->
1080 <h4>
1082 </h4>
1084 <div>
1087 emulated with macros and may be coming in C++'0x) we end up writing a lot of
1088 loops that manually iterate from begin to end on a variety of containers or
1089 through other data structures. One common mistake is to write a loop in this
1090 style:</p>
1093 <pre>
1094 BasicBlock *BB = ...
1096 ... use I ...
1097 </pre>
1098 </div>
1101 every time through the loop. Instead of writing the loop like this, we strongly
1102 prefer loops to be written so that they evaluate it once before the loop starts.
1103 A convenient way to do this is like so:</p>
1106 <pre>
1107 BasicBlock *BB = ...
1109 ... use I ...
1110 </pre>
1111 </div>
1113 <p>The observant may quickly point out that these two loops may have different
1114 semantics: if the container (a basic block in this case) is being mutated, then
1115 "<tt>BB->end()</tt>" may change its value every time through the loop and the
1116 second loop may not in fact be correct. If you actually do depend on this
1117 behavior, please write the loop in the first form and add a comment indicating
1118 that you did it intentionally.</p>
1120 <p>Why do we prefer the second form (when correct)? Writing the loop in the
1121 first form has two problems. First it may be less efficient than evaluating it
1122 at the start of the loop. In this case, the cost is probably minor — a
1123 few extra loads every time through the loop. However, if the base expression is
1124 more complex, then the cost can rise quickly. I've seen loops where the end
1125 expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
1126 lookups really aren't cheap. By writing it in the second form consistently, you
1127 eliminate the issue entirely and don't even have to think about it.</p>
1129 <p>The second (even bigger) issue is that writing the loop in the first form
1130 hints to the reader that the loop is mutating the container (a fact that a
1131 comment would handily confirm!). If you write the loop in the second form, it
1132 is immediately obvious without even looking at the body of the loop that the
1133 container isn't being modified, which makes it easier to read the code and
1134 understand what it does.</p>
1136 <p>While the second form of the loop is a few extra keystrokes, we do strongly
1137 prefer it.</p>
1139 </div>
1141 <!-- _______________________________________________________________________ -->
1142 <h4>
1144 </h4>
1146 <div>
1150 support clients using LLVM libraries as part of larger systems. In particular,
1151 we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
1152 the static constructors are run whenever an application starts up that uses the
1153 dynamic library. There are two problems with this:</p>
1155 <ol>
1156 <li>The time to run the static c'tors impacts startup time of applications
1159 <li>The static c'tors cause the app to pull many extra pages of memory off the
1161 small amount of data that gets touched. In addition, touched/dirty pages
1162 put more pressure on the VM system on low-memory machines.</li>
1163 </ol>
1166 example) is not problematic in this regard —
1169 style APIs. <b>Therefore new code should always
1173 </div>
1176 <!-- _______________________________________________________________________ -->
1177 <h4>
1179 </h4>
1181 <div>
1183 <p>LLVM includes a lightweight, simple, and efficient stream implementation
1193 </div>
1196 <!-- _______________________________________________________________________ -->
1197 <h4>
1199 </h4>
1201 <div>
1204 newline to the output stream specified. In addition to doing this, however, it
1205 also flushes the output stream. In other words, these are equivalent:</p>
1208 <pre>
1209 std::cout << std::endl;
1211 </pre>
1212 </div>
1214 <p>Most of the time, you probably have no reason to flush the output stream, so
1217 </div>
1219 </div>
1221 <!-- ======================================================================= -->
1222 <h3>
1224 </h3>
1225 <!-- ======================================================================= -->
1227 <div>
1229 <p>This section describes preferred low-level formatting guidelines along with
1230 reasoning on why we prefer them.</p>
1232 <!-- _______________________________________________________________________ -->
1233 <h4>
1235 </h4>
1237 <div>
1239 <p>We prefer to put a space before an open parenthesis only in control flow
1240 statements, but not in normal function call expressions and function-like
1241 macros. For example, this is good:</p>
1244 <pre>
1253 </pre>
1254 </div>
1259 <pre>
1268 </pre>
1269 </div>
1271 <p>The reason for doing this is not completely arbitrary. This style makes
1272 control flow operators stand out more, and makes expressions flow better. The
1273 function call operator binds very tightly as a postfix operator. Putting a
1274 space after a function name (as in the last example) makes it appear that the
1275 code might bind the arguments of the left-hand-side of a binary operator with
1276 the argument list of a function and the name of the right side. More
1280 <pre>
1282 </pre>
1283 </div>
1285 <p>when skimming through the code. By avoiding a space in a function, we avoid
1286 this misinterpretation.</p>
1288 </div>
1290 <!-- _______________________________________________________________________ -->
1291 <h4>
1293 </h4>
1295 <div>
1299 preincrementation whenever possible.</p>
1301 <p>The semantics of postincrement include making a copy of the value being
1302 incremented, returning it, and then preincrementing the "work value". For
1303 primitive types, this isn't a big deal... but for iterators, it can be a huge
1304 issue (for example, some iterators contains stack and set objects in them...
1305 copying an iterator could invoke the copy ctor's of these as well). In general,
1306 get in the habit of always using preincrement, and you won't have a problem.</p>
1308 </div>
1310 <!-- _______________________________________________________________________ -->
1311 <h4>
1313 </h4>
1315 <div>
1317 <p>
1318 In general, we strive to reduce indentation wherever possible. This is useful
1320 wrapping horribly, but also because it makes it easier to understand the code.
1321 Namespaces are a funny thing: they are often large, and we often desire to put
1322 lots of stuff into them (so they can be large). Other times they are tiny,
1323 because they just hold an enum or something similar. In order to balance this,
1324 we use different approaches for small versus large namespaces.
1325 </p>
1327 <p>
1329 less than 35 lines of code), then you should indent its body. Here's an
1330 example:
1331 </p>
1334 <pre>
1335 namespace llvm {
1336 namespace X86 {
1337 /// RelocationType - An enum for the x86 relocation codes. Note that
1338 /// the terminology here doesn't follow x86 convention - word means
1340 enum RelocationType {
1341 /// reloc_pcrel_word - PC relative relocation, add the relocated value to
1342 /// the value already in memory, after we adjust it for where the PC is.
1343 reloc_pcrel_word = 0,
1345 /// reloc_picrel_word - PIC base relative relocation, add the relocated
1346 /// value to the value already in memory, after we adjust it for where the
1347 /// PIC base is.
1348 reloc_picrel_word = 1,
1350 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
1351 /// add the relocated value to the value already in memory.
1352 reloc_absolute_word = 2,
1353 reloc_absolute_dword = 3
1354 };
1355 }
1356 }
1357 </pre>
1358 </div>
1360 <p>Since the body is small, indenting adds value because it makes it very clear
1361 where the namespace starts and ends, and it is easy to take the whole thing in
1362 in one "gulp" when reading the code. If the blob of code in the namespace is
1363 larger (as it typically is in a header in the <tt>llvm</tt> or <tt>clang</tt> namespaces), do not
1364 indent the code, and add a comment indicating what namespace is being closed.
1365 For example:</p>
1368 <pre>
1369 namespace llvm {
1370 namespace knowledge {
1372 /// Grokable - This class represents things that Smith can have an intimate
1373 /// understanding of and contains the data associated with it.
1374 class Grokable {
1375 ...
1376 public:
1377 explicit Grokable() { ... }
1378 virtual ~Grokable() = 0;
1380 ...
1382 };
1384 } // end namespace knowledge
1385 } // end namespace llvm
1386 </pre>
1387 </div>
1389 <p>Because the class is large, we don't expect that the reader can easily
1390 understand the entire concept in a glance, and the end of the file (where the
1391 namespaces end) may be a long ways away from the place they open. As such,
1392 indenting the contents of the namespace doesn't add any value, and detracts from
1394 the contents of the namespace.</p>
1396 </div>
1398 <!-- _______________________________________________________________________ -->
1399 <h4>
1401 </h4>
1403 <div>
1405 <p>After talking about namespaces in general, you may be wondering about
1406 anonymous namespaces in particular.
1407 Anonymous namespaces are a great language feature that tells the C++ compiler
1408 that the contents of the namespace are only visible within the current
1409 translation unit, allowing more aggressive optimization and eliminating the
1410 possibility of symbol name collisions. Anonymous namespaces are to C++ as
1412 in C++, anonymous namespaces are more general: they can make entire classes
1413 private to a file.</p>
1415 <p>The problem with anonymous namespaces is that they naturally want to
1416 encourage indentation of their body, and they reduce locality of reference: if
1417 you see a random function definition in a C++ file, it is easy to see if it is
1418 marked static, but seeing if it is in an anonymous namespace requires scanning
1419 a big chunk of the file.</p>
1421 <p>Because of this, we have a simple guideline: make anonymous namespaces as
1422 small as possible, and only use them for class declarations. For example, this
1423 is good:</p>
1426 <pre>
1428 class StringSort {
1429 ...
1430 public:
1431 StringSort(...)
1432 bool operator<(const char *RHS) const;
1433 };
1436 static void Helper() {
1437 ...
1438 }
1440 bool StringSort::operator<(const char *RHS) const {
1441 ...
1442 }
1444 </pre>
1445 </div>
1451 <pre>
1453 class StringSort {
1454 ...
1455 public:
1456 StringSort(...)
1457 bool operator<(const char *RHS) const;
1458 };
1460 void Helper() {
1461 ...
1462 }
1464 bool StringSort::operator<(const char *RHS) const {
1465 ...
1466 }
1470 </pre>
1471 </div>
1475 of a large C++ file, that you have no immediate way to tell if it is local to
1476 the file. When it is marked static explicitly, this is immediately obvious.
1477 Also, there is no reason to enclose the definition of "operator<" in the
1478 namespace just because it was declared there.
1479 </p>
1481 </div>
1483 </div>
1485 </div>
1487 <!-- *********************************************************************** -->
1488 <h2>
1490 </h2>
1491 <!-- *********************************************************************** -->
1493 <div>
1495 <p>A lot of these comments and recommendations have been culled for other
1496 sources. Two particularly important books for our work are:</p>
1498 <ol>
1500 <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
1501 C++</a> by Scott Meyers. Also
1503 author.</li>
1507 </ol>
1509 <p>If you get some free time, and you haven't read them: do so, you might learn
1510 something.</p>
1512 </div>
1514 <!-- *********************************************************************** -->
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