11 This document attempts to describe a few coding standards that are being used in
12 the LLVM source tree. Although no coding standards should be regarded as
13 absolute requirements to be followed in all instances, coding standards are
14 particularly important for large-scale code bases that follow a library-based
17 While this document may provide guidance for some mechanical formatting issues,
18 whitespace, or other "microscopic details", these are not fixed standards.
19 Always follow the golden rule:
23 **If you are extending, enhancing, or bug fixing already implemented code,
24 use the style that is already being used so that the source is uniform and
27 Note that some code bases (e.g. ``libc++``) have really good reasons to deviate
28 from the coding standards. In the case of ``libc++``, this is because the
29 naming and other conventions are dictated by the C++ standard. If you think
30 there is a specific good reason to deviate from the standards here, please bring
31 it up on the LLVM-dev mailing list.
33 There are some conventions that are not uniformly followed in the code base
34 (e.g. the naming convention). This is because they are relatively new, and a
35 lot of code was written before they were put in place. Our long term goal is
36 for the entire codebase to follow the convention, but we explicitly *do not*
37 want patches that do large-scale reformatting of existing code. On the other
38 hand, it is reasonable to rename the methods of a class if you're about to
39 change it in some other way. Just do the reformatting as a separate commit
40 from the functionality change.
42 The ultimate goal of these guidelines is to increase the readability and
43 maintainability of our common source base. If you have suggestions for topics to
44 be included, please mail them to `Chris <mailto:sabre@nondot.org>`_.
46 Languages, Libraries, and Standards
47 ===================================
49 Most source code in LLVM and other LLVM projects using these coding standards
50 is C++ code. There are some places where C code is used either due to
51 environment restrictions, historical restrictions, or due to third-party source
52 code imported into the tree. Generally, our preference is for standards
53 conforming, modern, and portable C++ code as the implementation language of
59 LLVM, Clang, and LLD are currently written using C++11 conforming code,
60 although we restrict ourselves to features which are available in the major
61 toolchains supported as host compilers. The LLDB project is even more
62 aggressive in the set of host compilers supported and thus uses still more
63 features. Regardless of the supported features, code is expected to (when
64 reasonable) be standard, portable, and modern C++11 code. We avoid unnecessary
65 vendor-specific extensions, etc.
70 Use the C++ standard library facilities whenever they are available for
71 a particular task. LLVM and related projects emphasize and rely on the standard
72 library facilities for as much as possible. Common support libraries providing
73 functionality missing from the standard library for which there are standard
74 interfaces or active work on adding standard interfaces will often be
75 implemented in the LLVM namespace following the expected standard interface.
77 There are some exceptions such as the standard I/O streams library which are
78 avoided. Also, there is much more detailed information on these subjects in the
79 :doc:`ProgrammersManual`.
81 Supported C++11 Language and Library Features
82 ---------------------------------------------
84 While LLVM, Clang, and LLD use C++11, not all features are available in all of
85 the toolchains which we support. The set of features supported for use in LLVM
86 is the intersection of those supported in the minimum requirements described
87 in the :doc:`GettingStarted` page, section `Software`.
88 The ultimate definition of this set is what build bots with those respective
89 toolchains accept. Don't argue with the build bots. However, we have some
90 guidance below to help you know what to expect.
92 Each toolchain provides a good reference for what it accepts:
94 * Clang: http://clang.llvm.org/cxx_status.html
95 * GCC: http://gcc.gnu.org/projects/cxx0x.html
96 * MSVC: http://msdn.microsoft.com/en-us/library/hh567368.aspx
98 In most cases, the MSVC list will be the dominating factor. Here is a summary
99 of the features that are expected to work. Features not on this list are
100 unlikely to be supported by our host compilers.
102 * Rvalue references: N2118_
104 * But *not* Rvalue references for ``*this`` or member qualifiers (N2439_)
106 * Static assert: N1720_
107 * ``auto`` type deduction: N1984_, N1737_
108 * Trailing return types: N2541_
111 * But *not* lambdas with default arguments.
113 * ``decltype``: N2343_
114 * Nested closing right angle brackets: N1757_
115 * Extern templates: N1987_
116 * ``nullptr``: N2431_
117 * Strongly-typed and forward declarable enums: N2347_, N2764_
118 * Local and unnamed types as template arguments: N2657_
119 * Range-based for-loop: N2930_
121 * But ``{}`` are required around inner ``do {} while()`` loops. As a result,
122 ``{}`` are required around function-like macros inside range-based for
125 * ``override`` and ``final``: N2928_, N3206_, N3272_
126 * Atomic operations and the C++11 memory model: N2429_
127 * Variadic templates: N2242_
128 * Explicit conversion operators: N2437_
129 * Defaulted and deleted functions: N2346_
130 * Initializer lists: N2627_
131 * Delegating constructors: N1986_
132 * Default member initializers (non-static data member initializers): N2756_
134 * Feel free to use these wherever they make sense and where the `=`
135 syntax is allowed. Don't use braced initialization syntax.
137 .. _N2118: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2118.html
138 .. _N2439: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2439.htm
139 .. _N1720: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1720.html
140 .. _N1984: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1984.pdf
141 .. _N1737: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1737.pdf
142 .. _N2541: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2541.htm
143 .. _N2927: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2927.pdf
144 .. _N2343: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2343.pdf
145 .. _N1757: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1757.html
146 .. _N1987: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1987.htm
147 .. _N2431: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2431.pdf
148 .. _N2347: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2347.pdf
149 .. _N2764: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2764.pdf
150 .. _N2657: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm
151 .. _N2930: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2930.html
152 .. _N2928: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2928.htm
153 .. _N3206: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3206.htm
154 .. _N3272: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3272.htm
155 .. _N2429: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2429.htm
156 .. _N2242: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2242.pdf
157 .. _N2437: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2437.pdf
158 .. _N2346: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2346.htm
159 .. _N2627: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2672.htm
160 .. _N1986: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1986.pdf
161 .. _N2756: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2756.htm
163 The supported features in the C++11 standard libraries are less well tracked,
164 but also much greater. Most of the standard libraries implement most of C++11's
165 library. The most likely lowest common denominator is Linux support. For
166 libc++, the support is just poorly tested and undocumented but expected to be
167 largely complete. YMMV. For libstdc++, the support is documented in detail in
168 `the libstdc++ manual`_. There are some very minor missing facilities that are
169 unlikely to be common problems, and there are a few larger gaps that are worth
172 * Not all of the type traits are implemented
173 * No regular expression library.
174 * While most of the atomics library is well implemented, the fences are
175 missing. Fortunately, they are rarely needed.
176 * The locale support is incomplete.
178 Other than these areas you should assume the standard library is available and
179 working as expected until some build bot tells you otherwise. If you're in an
180 uncertain area of one of the above points, but you cannot test on a Linux
181 system, your best approach is to minimize your use of these features, and watch
182 the Linux build bots to find out if your usage triggered a bug. For example, if
183 you hit a type trait which doesn't work we can then add support to LLVM's
184 traits header to emulate it.
186 .. _the libstdc++ manual:
187 http://gcc.gnu.org/onlinedocs/gcc-4.8.0/libstdc++/manual/manual/status.html#status.iso.2011
192 Any code written in the Go programming language is not subject to the
193 formatting rules below. Instead, we adopt the formatting rules enforced by
196 Go code should strive to be idiomatic. Two good sets of guidelines for what
197 this means are `Effective Go`_ and `Go Code Review Comments`_.
200 https://golang.org/cmd/gofmt/
203 https://golang.org/doc/effective_go.html
205 .. _Go Code Review Comments:
206 https://github.com/golang/go/wiki/CodeReviewComments
208 Mechanical Source Issues
209 ========================
211 Source Code Formatting
212 ----------------------
217 Comments are one critical part of readability and maintainability. Everyone
218 knows they should comment their code, and so should you. When writing comments,
219 write them as English prose, which means they should use proper capitalization,
220 punctuation, etc. Aim to describe what the code is trying to do and why, not
221 *how* it does it at a micro level. Here are a few critical things to document:
223 .. _header file comment:
228 Every source file should have a header on it that describes the basic purpose of
229 the file. If a file does not have a header, it should not be checked into the
230 tree. The standard header looks like this:
234 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
236 // The LLVM Compiler Infrastructure
238 // This file is distributed under the University of Illinois Open Source
239 // License. See LICENSE.TXT for details.
241 //===----------------------------------------------------------------------===//
244 /// This file contains the declaration of the Instruction class, which is the
245 /// base class for all of the VM instructions.
247 //===----------------------------------------------------------------------===//
249 A few things to note about this particular format: The "``-*- C++ -*-``" string
250 on the first line is there to tell Emacs that the source file is a C++ file, not
251 a C file (Emacs assumes ``.h`` files are C files by default).
255 This tag is not necessary in ``.cpp`` files. The name of the file is also
256 on the first line, along with a very short description of the purpose of the
257 file. This is important when printing out code and flipping though lots of
260 The next section in the file is a concise note that defines the license that the
261 file is released under. This makes it perfectly clear what terms the source
262 code can be distributed under and should not be modified in any way.
264 The main body is a ``doxygen`` comment (identified by the ``///`` comment
265 marker instead of the usual ``//``) describing the purpose of the file. The
266 first sentence (or a passage beginning with ``\brief``) is used as an abstract.
267 Any additional information should be separated by a blank line. If an
268 algorithm is being implemented or something tricky is going on, a reference
269 to the paper where it is published should be included, as well as any notes or
270 *gotchas* in the code to watch out for.
275 Classes are one fundamental part of a good object oriented design. As such, a
276 class definition should have a comment block that explains what the class is
277 used for and how it works. Every non-trivial class is expected to have a
278 ``doxygen`` comment block.
283 Methods defined in a class (as well as any global functions) should also be
284 documented properly. A quick note about what it does and a description of the
285 borderline behaviour is all that is necessary here (unless something
286 particularly tricky or insidious is going on). The hope is that people can
287 figure out how to use your interfaces without reading the code itself.
289 Good things to talk about here are what happens when something unexpected
290 happens: does the method return null? Abort? Format your hard disk?
295 In general, prefer C++ style comments (``//`` for normal comments, ``///`` for
296 ``doxygen`` documentation comments). They take less space, require
297 less typing, don't have nesting problems, etc. There are a few cases when it is
298 useful to use C style (``/* */``) comments however:
300 #. When writing C code: Obviously if you are writing C code, use C style
303 #. When writing a header file that may be ``#include``\d by a C source file.
305 #. When writing a source file that is used by a tool that only accepts C style
308 Commenting out large blocks of code is discouraged, but if you really have to do
309 this (for documentation purposes or as a suggestion for debug printing), use
310 ``#if 0`` and ``#endif``. These nest properly and are better behaved in general
311 than C style comments.
313 Doxygen Use in Documentation Comments
314 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
316 Use the ``\file`` command to turn the standard file header into a file-level
319 Include descriptive paragraphs for all public interfaces (public classes,
320 member and non-member functions). Don't just restate the information that can
321 be inferred from the API name. The first sentence (or a paragraph beginning
322 with ``\brief``) is used as an abstract. Try to use a single sentence as the
323 ``\brief`` adds visual clutter. Put detailed discussion into separate
326 To refer to parameter names inside a paragraph, use the ``\p name`` command.
327 Don't use the ``\arg name`` command since it starts a new paragraph that
328 contains documentation for the parameter.
330 Wrap non-inline code examples in ``\code ... \endcode``.
332 To document a function parameter, start a new paragraph with the
333 ``\param name`` command. If the parameter is used as an out or an in/out
334 parameter, use the ``\param [out] name`` or ``\param [in,out] name`` command,
337 To describe function return value, start a new paragraph with the ``\returns``
340 A minimal documentation comment:
344 /// Sets the xyzzy property to \p Baz.
345 void setXyzzy(bool Baz);
347 A documentation comment that uses all Doxygen features in a preferred way:
351 /// Does foo and bar.
353 /// Does not do foo the usual way if \p Baz is true.
357 /// fooBar(false, "quux", Res);
360 /// \param Quux kind of foo to do.
361 /// \param [out] Result filled with bar sequence on foo success.
363 /// \returns true on success.
364 bool fooBar(bool Baz, StringRef Quux, std::vector<int> &Result);
366 Don't duplicate the documentation comment in the header file and in the
367 implementation file. Put the documentation comments for public APIs into the
368 header file. Documentation comments for private APIs can go to the
369 implementation file. In any case, implementation files can include additional
370 comments (not necessarily in Doxygen markup) to explain implementation details
373 Don't duplicate function or class name at the beginning of the comment.
374 For humans it is obvious which function or class is being documented;
375 automatic documentation processing tools are smart enough to bind the comment
376 to the correct declaration.
384 /// Something - An abstraction for some complicated thing.
387 /// fooBar - Does foo and bar.
393 /// fooBar - Does foo and bar.
394 void Something::fooBar() { ... }
402 /// An abstraction for some complicated thing.
405 /// Does foo and bar.
411 // Builds a B-tree in order to do foo. See paper by...
412 void Something::fooBar() { ... }
414 It is not required to use additional Doxygen features, but sometimes it might
415 be a good idea to do so.
419 * adding comments to any narrow namespace containing a collection of
420 related functions or types;
422 * using top-level groups to organize a collection of related functions at
423 namespace scope where the grouping is smaller than the namespace;
425 * using member groups and additional comments attached to member
426 groups to organize within a class.
433 /// \name Functions that do Foo.
444 Immediately after the `header file comment`_ (and include guards if working on a
445 header file), the `minimal list of #includes`_ required by the file should be
446 listed. We prefer these ``#include``\s to be listed in this order:
448 .. _Main Module Header:
449 .. _Local/Private Headers:
451 #. Main Module Header
452 #. Local/Private Headers
453 #. LLVM project/subproject headers (``clang/...``, ``lldb/...``, ``llvm/...``, etc)
454 #. System ``#include``\s
456 and each category should be sorted lexicographically by the full path.
458 The `Main Module Header`_ file applies to ``.cpp`` files which implement an
459 interface defined by a ``.h`` file. This ``#include`` should always be included
460 **first** regardless of where it lives on the file system. By including a
461 header file first in the ``.cpp`` files that implement the interfaces, we ensure
462 that the header does not have any hidden dependencies which are not explicitly
463 ``#include``\d in the header, but should be. It is also a form of documentation
464 in the ``.cpp`` file to indicate where the interfaces it implements are defined.
466 LLVM project and subproject headers should be grouped from most specific to least
467 specific, for the same reasons described above. For example, LLDB depends on
468 both clang and LLVM, and clang depends on LLVM. So an LLDB source file should
469 include ``lldb`` headers first, followed by ``clang`` headers, followed by
470 ``llvm`` headers, to reduce the possibility (for example) of an LLDB header
471 accidentally picking up a missing include due to the previous inclusion of that
472 header in the main source file or some earlier header file. clang should
473 similarly include its own headers before including llvm headers. This rule
474 applies to all LLVM subprojects.
476 .. _fit into 80 columns:
481 Write your code to fit within 80 columns of text. This helps those of us who
482 like to print out code and look at your code in an ``xterm`` without resizing
485 The longer answer is that there must be some limit to the width of the code in
486 order to reasonably allow developers to have multiple files side-by-side in
487 windows on a modest display. If you are going to pick a width limit, it is
488 somewhat arbitrary but you might as well pick something standard. Going with 90
489 columns (for example) instead of 80 columns wouldn't add any significant value
490 and would be detrimental to printing out code. Also many other projects have
491 standardized on 80 columns, so some people have already configured their editors
492 for it (vs something else, like 90 columns).
494 This is one of many contentious issues in coding standards, but it is not up for
497 Use Spaces Instead of Tabs
498 ^^^^^^^^^^^^^^^^^^^^^^^^^^
500 In all cases, prefer spaces to tabs in source files. People have different
501 preferred indentation levels, and different styles of indentation that they
502 like; this is fine. What isn't fine is that different editors/viewers expand
503 tabs out to different tab stops. This can cause your code to look completely
504 unreadable, and it is not worth dealing with.
506 As always, follow the `Golden Rule`_ above: follow the style of
507 existing code if you are modifying and extending it. If you like four spaces of
508 indentation, **DO NOT** do that in the middle of a chunk of code with two spaces
509 of indentation. Also, do not reindent a whole source file: it makes for
510 incredible diffs that are absolutely worthless.
512 Indent Code Consistently
513 ^^^^^^^^^^^^^^^^^^^^^^^^
515 Okay, in your first year of programming you were told that indentation is
516 important. If you didn't believe and internalize this then, now is the time.
517 Just do it. With the introduction of C++11, there are some new formatting
518 challenges that merit some suggestions to help have consistent, maintainable,
519 and tool-friendly formatting and indentation.
521 Format Lambdas Like Blocks Of Code
522 """"""""""""""""""""""""""""""""""
524 When formatting a multi-line lambda, format it like a block of code, that's
525 what it is. If there is only one multi-line lambda in a statement, and there
526 are no expressions lexically after it in the statement, drop the indent to the
527 standard two space indent for a block of code, as if it were an if-block opened
528 by the preceding part of the statement:
532 std::sort(foo.begin(), foo.end(), [&](Foo a, Foo b) -> bool {
537 return a.bam < b.bam;
540 To take best advantage of this formatting, if you are designing an API which
541 accepts a continuation or single callable argument (be it a functor, or
542 a ``std::function``), it should be the last argument if at all possible.
544 If there are multiple multi-line lambdas in a statement, or there is anything
545 interesting after the lambda in the statement, indent the block two spaces from
546 the indent of the ``[]``:
550 dyn_switch(V->stripPointerCasts(),
554 [] (SelectInst *SI) {
555 // process selects...
560 [] (AllocaInst *AI) {
561 // process allocas...
564 Braced Initializer Lists
565 """"""""""""""""""""""""
567 With C++11, there are significantly more uses of braced lists to perform
568 initialization. These allow you to easily construct aggregate temporaries in
569 expressions among other niceness. They now have a natural way of ending up
570 nested within each other and within function calls in order to build up
571 aggregates (such as option structs) from local variables. To make matters
572 worse, we also have many more uses of braces in an expression context that are
573 *not* performing initialization.
575 The historically common formatting of braced initialization of aggregate
576 variables does not mix cleanly with deep nesting, general expression contexts,
577 function arguments, and lambdas. We suggest new code use a simple rule for
578 formatting braced initialization lists: act as-if the braces were parentheses
579 in a function call. The formatting rules exactly match those already well
580 understood for formatting nested function calls. Examples:
584 foo({a, b, c}, {1, 2, 3});
586 llvm::Constant *Mask[] = {
587 llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 0),
588 llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 1),
589 llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 2)};
591 This formatting scheme also makes it particularly easy to get predictable,
592 consistent, and automatic formatting with tools like `Clang Format`_.
594 .. _Clang Format: http://clang.llvm.org/docs/ClangFormat.html
596 Language and Compiler Issues
597 ----------------------------
599 Treat Compiler Warnings Like Errors
600 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
602 If your code has compiler warnings in it, something is wrong --- you aren't
603 casting values correctly, you have "questionable" constructs in your code, or
604 you are doing something legitimately wrong. Compiler warnings can cover up
605 legitimate errors in output and make dealing with a translation unit difficult.
607 It is not possible to prevent all warnings from all compilers, nor is it
608 desirable. Instead, pick a standard compiler (like ``gcc``) that provides a
609 good thorough set of warnings, and stick to it. At least in the case of
610 ``gcc``, it is possible to work around any spurious errors by changing the
611 syntax of the code slightly. For example, a warning that annoys me occurs when
612 I write code like this:
616 if (V = getValue()) {
620 ``gcc`` will warn me that I probably want to use the ``==`` operator, and that I
621 probably mistyped it. In most cases, I haven't, and I really don't want the
622 spurious errors. To fix this particular problem, I rewrite the code like
627 if ((V = getValue())) {
631 which shuts ``gcc`` up. Any ``gcc`` warning that annoys you can be fixed by
632 massaging the code appropriately.
637 In almost all cases, it is possible and within reason to write completely
638 portable code. If there are cases where it isn't possible to write portable
639 code, isolate it behind a well defined (and well documented) interface.
641 In practice, this means that you shouldn't assume much about the host compiler
642 (and Visual Studio tends to be the lowest common denominator). If advanced
643 features are used, they should only be an implementation detail of a library
644 which has a simple exposed API, and preferably be buried in ``libSystem``.
646 Do not use RTTI or Exceptions
647 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
649 In an effort to reduce code and executable size, LLVM does not use RTTI
650 (e.g. ``dynamic_cast<>;``) or exceptions. These two language features violate
651 the general C++ principle of *"you only pay for what you use"*, causing
652 executable bloat even if exceptions are never used in the code base, or if RTTI
653 is never used for a class. Because of this, we turn them off globally in the
656 That said, LLVM does make extensive use of a hand-rolled form of RTTI that use
657 templates like :ref:`isa\<>, cast\<>, and dyn_cast\<> <isa>`.
658 This form of RTTI is opt-in and can be
659 :doc:`added to any class <HowToSetUpLLVMStyleRTTI>`. It is also
660 substantially more efficient than ``dynamic_cast<>``.
662 .. _static constructor:
664 Do not use Static Constructors
665 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
667 Static constructors and destructors (e.g. global variables whose types have a
668 constructor or destructor) should not be added to the code base, and should be
669 removed wherever possible. Besides `well known problems
670 <http://yosefk.com/c++fqa/ctors.html#fqa-10.12>`_ where the order of
671 initialization is undefined between globals in different source files, the
672 entire concept of static constructors is at odds with the common use case of
673 LLVM as a library linked into a larger application.
675 Consider the use of LLVM as a JIT linked into another application (perhaps for
676 `OpenGL, custom languages <http://llvm.org/Users.html>`_, `shaders in movies
677 <http://llvm.org/devmtg/2010-11/Gritz-OpenShadingLang.pdf>`_, etc). Due to the
678 design of static constructors, they must be executed at startup time of the
679 entire application, regardless of whether or how LLVM is used in that larger
680 application. There are two problems with this:
682 * The time to run the static constructors impacts startup time of applications
683 --- a critical time for GUI apps, among others.
685 * The static constructors cause the app to pull many extra pages of memory off
686 the disk: both the code for the constructor in each ``.o`` file and the small
687 amount of data that gets touched. In addition, touched/dirty pages put more
688 pressure on the VM system on low-memory machines.
690 We would really like for there to be zero cost for linking in an additional LLVM
691 target or other library into an application, but static constructors violate
694 That said, LLVM unfortunately does contain static constructors. It would be a
695 `great project <http://llvm.org/PR11944>`_ for someone to purge all static
696 constructors from LLVM, and then enable the ``-Wglobal-constructors`` warning
697 flag (when building with Clang) to ensure we do not regress in the future.
699 Use of ``class`` and ``struct`` Keywords
700 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
702 In C++, the ``class`` and ``struct`` keywords can be used almost
703 interchangeably. The only difference is when they are used to declare a class:
704 ``class`` makes all members private by default while ``struct`` makes all
705 members public by default.
707 Unfortunately, not all compilers follow the rules and some will generate
708 different symbols based on whether ``class`` or ``struct`` was used to declare
709 the symbol (e.g., MSVC). This can lead to problems at link time.
711 * All declarations and definitions of a given ``class`` or ``struct`` must use
712 the same keyword. For example:
718 // Breaks mangling in MSVC.
719 struct Foo { int Data; };
721 * As a rule of thumb, ``struct`` should be kept to structures where *all*
722 members are declared public.
726 // Foo feels like a class... this is strange.
732 int getData() const { return Data; }
733 void setData(int D) { Data = D; }
736 // Bar isn't POD, but it does look like a struct.
742 Do not use Braced Initializer Lists to Call a Constructor
743 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
745 In C++11 there is a "generalized initialization syntax" which allows calling
746 constructors using braced initializer lists. Do not use these to call
747 constructors with any interesting logic or if you care that you're calling some
748 *particular* constructor. Those should look like function calls using
749 parentheses rather than like aggregate initialization. Similarly, if you need
750 to explicitly name the type and call its constructor to create a temporary,
751 don't use a braced initializer list. Instead, use a braced initializer list
752 (without any type for temporaries) when doing aggregate initialization or
753 something notionally equivalent. Examples:
759 // Construct a Foo by reading data from the disk in the whizbang format, ...
760 Foo(std::string filename);
762 // Construct a Foo by looking up the Nth element of some global data ...
768 // The Foo constructor call is very deliberate, no braces.
769 std::fill(foo.begin(), foo.end(), Foo("name"));
771 // The pair is just being constructed like an aggregate, use braces.
772 bar_map.insert({my_key, my_value});
774 If you use a braced initializer list when initializing a variable, use an equals before the open curly brace:
778 int data[] = {0, 1, 2, 3};
780 Use ``auto`` Type Deduction to Make Code More Readable
781 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
783 Some are advocating a policy of "almost always ``auto``" in C++11, however LLVM
784 uses a more moderate stance. Use ``auto`` if and only if it makes the code more
785 readable or easier to maintain. Don't "almost always" use ``auto``, but do use
786 ``auto`` with initializers like ``cast<Foo>(...)`` or other places where the
787 type is already obvious from the context. Another time when ``auto`` works well
788 for these purposes is when the type would have been abstracted away anyways,
789 often behind a container's typedef such as ``std::vector<T>::iterator``.
791 Beware unnecessary copies with ``auto``
792 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
794 The convenience of ``auto`` makes it easy to forget that its default behavior
795 is a copy. Particularly in range-based ``for`` loops, careless copies are
798 As a rule of thumb, use ``auto &`` unless you need to copy the result, and use
799 ``auto *`` when copying pointers.
803 // Typically there's no reason to copy.
804 for (const auto &Val : Container) { observe(Val); }
805 for (auto &Val : Container) { Val.change(); }
807 // Remove the reference if you really want a new copy.
808 for (auto Val : Container) { Val.change(); saveSomewhere(Val); }
810 // Copy pointers, but make it clear that they're pointers.
811 for (const auto *Ptr : Container) { observe(*Ptr); }
812 for (auto *Ptr : Container) { Ptr->change(); }
814 Beware of non-determinism due to ordering of pointers
815 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
817 In general, there is no relative ordering among pointers. As a result,
818 when unordered containers like sets and maps are used with pointer keys
819 the iteration order is undefined. Hence, iterating such containers may
820 result in non-deterministic code generation. While the generated code
821 might not necessarily be "wrong code", this non-determinism might result
822 in unexpected runtime crashes or simply hard to reproduce bugs on the
823 customer side making it harder to debug and fix.
825 As a rule of thumb, in case an ordered result is expected, remember to
826 sort an unordered container before iteration. Or use ordered containers
827 like vector/MapVector/SetVector if you want to iterate pointer keys.
832 The High-Level Issues
833 ---------------------
835 Self-contained Headers
836 ^^^^^^^^^^^^^^^^^^^^^^
838 Header files should be self-contained (compile on their own) and end in .h.
839 Non-header files that are meant for inclusion should end in .inc and be used
842 All header files should be self-contained. Users and refactoring tools should
843 not have to adhere to special conditions to include the header. Specifically, a
844 header should have header guards and include all other headers it needs.
846 There are rare cases where a file designed to be included is not
847 self-contained. These are typically intended to be included at unusual
848 locations, such as the middle of another file. They might not use header
849 guards, and might not include their prerequisites. Name such files with the
850 .inc extension. Use sparingly, and prefer self-contained headers when possible.
852 In general, a header should be implemented by one or more ``.cpp`` files. Each
853 of these ``.cpp`` files should include the header that defines their interface
854 first. This ensures that all of the dependences of the header have been
855 properly added to the header itself, and are not implicit. System headers
856 should be included after user headers for a translation unit.
861 A directory of header files (for example ``include/llvm/Foo``) defines a
862 library (``Foo``). Dependencies between libraries are defined by the
863 ``LLVMBuild.txt`` file in their implementation (``lib/Foo``). One library (both
864 its headers and implementation) should only use things from the libraries
865 listed in its dependencies.
867 Some of this constraint can be enforced by classic Unix linkers (Mac & Windows
868 linkers, as well as lld, do not enforce this constraint). A Unix linker
869 searches left to right through the libraries specified on its command line and
870 never revisits a library. In this way, no circular dependencies between
873 This doesn't fully enforce all inter-library dependencies, and importantly
874 doesn't enforce header file circular dependencies created by inline functions.
875 A good way to answer the "is this layered correctly" would be to consider
876 whether a Unix linker would succeed at linking the program if all inline
877 functions were defined out-of-line. (& for all valid orderings of dependencies
878 - since linking resolution is linear, it's possible that some implicit
879 dependencies can sneak through: A depends on B and C, so valid orderings are
880 "C B A" or "B C A", in both cases the explicit dependencies come before their
881 use. But in the first case, B could still link successfully if it implicitly
882 depended on C, or the opposite in the second case)
884 .. _minimal list of #includes:
886 ``#include`` as Little as Possible
887 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
889 ``#include`` hurts compile time performance. Don't do it unless you have to,
890 especially in header files.
892 But wait! Sometimes you need to have the definition of a class to use it, or to
893 inherit from it. In these cases go ahead and ``#include`` that header file. Be
894 aware however that there are many cases where you don't need to have the full
895 definition of a class. If you are using a pointer or reference to a class, you
896 don't need the header file. If you are simply returning a class instance from a
897 prototyped function or method, you don't need it. In fact, for most cases, you
898 simply don't need the definition of a class. And not ``#include``\ing speeds up
901 It is easy to try to go too overboard on this recommendation, however. You
902 **must** include all of the header files that you are using --- you can include
903 them either directly or indirectly through another header file. To make sure
904 that you don't accidentally forget to include a header file in your module
905 header, make sure to include your module header **first** in the implementation
906 file (as mentioned above). This way there won't be any hidden dependencies that
907 you'll find out about later.
909 Keep "Internal" Headers Private
910 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
912 Many modules have a complex implementation that causes them to use more than one
913 implementation (``.cpp``) file. It is often tempting to put the internal
914 communication interface (helper classes, extra functions, etc) in the public
915 module header file. Don't do this!
917 If you really need to do something like this, put a private header file in the
918 same directory as the source files, and include it locally. This ensures that
919 your private interface remains private and undisturbed by outsiders.
923 It's okay to put extra implementation methods in a public class itself. Just
924 make them private (or protected) and all is well.
928 Use Early Exits and ``continue`` to Simplify Code
929 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
931 When reading code, keep in mind how much state and how many previous decisions
932 have to be remembered by the reader to understand a block of code. Aim to
933 reduce indentation where possible when it doesn't make it more difficult to
934 understand the code. One great way to do this is by making use of early exits
935 and the ``continue`` keyword in long loops. As an example of using an early
936 exit from a function, consider this "bad" code:
940 Value *doSomething(Instruction *I) {
941 if (!isa<TerminatorInst>(I) &&
942 I->hasOneUse() && doOtherThing(I)) {
943 ... some long code ....
949 This code has several problems if the body of the ``'if'`` is large. When
950 you're looking at the top of the function, it isn't immediately clear that this
951 *only* does interesting things with non-terminator instructions, and only
952 applies to things with the other predicates. Second, it is relatively difficult
953 to describe (in comments) why these predicates are important because the ``if``
954 statement makes it difficult to lay out the comments. Third, when you're deep
955 within the body of the code, it is indented an extra level. Finally, when
956 reading the top of the function, it isn't clear what the result is if the
957 predicate isn't true; you have to read to the end of the function to know that
960 It is much preferred to format the code like this:
964 Value *doSomething(Instruction *I) {
965 // Terminators never need 'something' done to them because ...
966 if (isa<TerminatorInst>(I))
969 // We conservatively avoid transforming instructions with multiple uses
970 // because goats like cheese.
974 // This is really just here for example.
975 if (!doOtherThing(I))
978 ... some long code ....
981 This fixes these problems. A similar problem frequently happens in ``for``
982 loops. A silly example is something like this:
986 for (Instruction &I : BB) {
987 if (auto *BO = dyn_cast<BinaryOperator>(&I)) {
988 Value *LHS = BO->getOperand(0);
989 Value *RHS = BO->getOperand(1);
996 When you have very, very small loops, this sort of structure is fine. But if it
997 exceeds more than 10-15 lines, it becomes difficult for people to read and
998 understand at a glance. The problem with this sort of code is that it gets very
999 nested very quickly. Meaning that the reader of the code has to keep a lot of
1000 context in their brain to remember what is going immediately on in the loop,
1001 because they don't know if/when the ``if`` conditions will have ``else``\s etc.
1002 It is strongly preferred to structure the loop like this:
1006 for (Instruction &I : BB) {
1007 auto *BO = dyn_cast<BinaryOperator>(&I);
1010 Value *LHS = BO->getOperand(0);
1011 Value *RHS = BO->getOperand(1);
1012 if (LHS == RHS) continue;
1017 This has all the benefits of using early exits for functions: it reduces nesting
1018 of the loop, it makes it easier to describe why the conditions are true, and it
1019 makes it obvious to the reader that there is no ``else`` coming up that they
1020 have to push context into their brain for. If a loop is large, this can be a
1021 big understandability win.
1023 Don't use ``else`` after a ``return``
1024 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1026 For similar reasons above (reduction of indentation and easier reading), please
1027 do not use ``'else'`` or ``'else if'`` after something that interrupts control
1028 flow --- like ``return``, ``break``, ``continue``, ``goto``, etc. For
1029 example, this is *bad*:
1035 Type = Context.getsigjmp_bufType();
1036 if (Type.isNull()) {
1037 Error = ASTContext::GE_Missing_sigjmp_buf;
1043 Type = Context.getjmp_bufType();
1044 if (Type.isNull()) {
1045 Error = ASTContext::GE_Missing_jmp_buf;
1053 It is better to write it like this:
1059 Type = Context.getsigjmp_bufType();
1060 if (Type.isNull()) {
1061 Error = ASTContext::GE_Missing_sigjmp_buf;
1065 Type = Context.getjmp_bufType();
1066 if (Type.isNull()) {
1067 Error = ASTContext::GE_Missing_jmp_buf;
1073 Or better yet (in this case) as:
1079 Type = Context.getsigjmp_bufType();
1081 Type = Context.getjmp_bufType();
1083 if (Type.isNull()) {
1084 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
1085 ASTContext::GE_Missing_jmp_buf;
1090 The idea is to reduce indentation and the amount of code you have to keep track
1091 of when reading the code.
1093 Turn Predicate Loops into Predicate Functions
1094 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1096 It is very common to write small loops that just compute a boolean value. There
1097 are a number of ways that people commonly write these, but an example of this
1102 bool FoundFoo = false;
1103 for (unsigned I = 0, E = BarList.size(); I != E; ++I)
1104 if (BarList[I]->isFoo()) {
1113 This sort of code is awkward to write, and is almost always a bad sign. Instead
1114 of this sort of loop, we strongly prefer to use a predicate function (which may
1115 be `static`_) that uses `early exits`_ to compute the predicate. We prefer the
1116 code to be structured like this:
1120 /// \returns true if the specified list has an element that is a foo.
1121 static bool containsFoo(const std::vector<Bar*> &List) {
1122 for (unsigned I = 0, E = List.size(); I != E; ++I)
1123 if (List[I]->isFoo())
1129 if (containsFoo(BarList)) {
1133 There are many reasons for doing this: it reduces indentation and factors out
1134 code which can often be shared by other code that checks for the same predicate.
1135 More importantly, it *forces you to pick a name* for the function, and forces
1136 you to write a comment for it. In this silly example, this doesn't add much
1137 value. However, if the condition is complex, this can make it a lot easier for
1138 the reader to understand the code that queries for this predicate. Instead of
1139 being faced with the in-line details of how we check to see if the BarList
1140 contains a foo, we can trust the function name and continue reading with better
1143 The Low-Level Issues
1144 --------------------
1146 Name Types, Functions, Variables, and Enumerators Properly
1147 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1149 Poorly-chosen names can mislead the reader and cause bugs. We cannot stress
1150 enough how important it is to use *descriptive* names. Pick names that match
1151 the semantics and role of the underlying entities, within reason. Avoid
1152 abbreviations unless they are well known. After picking a good name, make sure
1153 to use consistent capitalization for the name, as inconsistency requires clients
1154 to either memorize the APIs or to look it up to find the exact spelling.
1156 In general, names should be in camel case (e.g. ``TextFileReader`` and
1157 ``isLValue()``). Different kinds of declarations have different rules:
1159 * **Type names** (including classes, structs, enums, typedefs, etc) should be
1160 nouns and start with an upper-case letter (e.g. ``TextFileReader``).
1162 * **Variable names** should be nouns (as they represent state). The name should
1163 be camel case, and start with an upper case letter (e.g. ``Leader`` or
1166 * **Function names** should be verb phrases (as they represent actions), and
1167 command-like function should be imperative. The name should be camel case,
1168 and start with a lower case letter (e.g. ``openFile()`` or ``isFoo()``).
1170 * **Enum declarations** (e.g. ``enum Foo {...}``) are types, so they should
1171 follow the naming conventions for types. A common use for enums is as a
1172 discriminator for a union, or an indicator of a subclass. When an enum is
1173 used for something like this, it should have a ``Kind`` suffix
1174 (e.g. ``ValueKind``).
1176 * **Enumerators** (e.g. ``enum { Foo, Bar }``) and **public member variables**
1177 should start with an upper-case letter, just like types. Unless the
1178 enumerators are defined in their own small namespace or inside a class,
1179 enumerators should have a prefix corresponding to the enum declaration name.
1180 For example, ``enum ValueKind { ... };`` may contain enumerators like
1181 ``VK_Argument``, ``VK_BasicBlock``, etc. Enumerators that are just
1182 convenience constants are exempt from the requirement for a prefix. For
1192 As an exception, classes that mimic STL classes can have member names in STL's
1193 style of lower-case words separated by underscores (e.g. ``begin()``,
1194 ``push_back()``, and ``empty()``). Classes that provide multiple
1195 iterators should add a singular prefix to ``begin()`` and ``end()``
1196 (e.g. ``global_begin()`` and ``use_begin()``).
1198 Here are some examples of good and bad names:
1202 class VehicleMaker {
1204 Factory<Tire> F; // Bad -- abbreviation and non-descriptive.
1205 Factory<Tire> Factory; // Better.
1206 Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one
1207 // kind of factories.
1210 Vehicle makeVehicle(VehicleType Type) {
1211 VehicleMaker M; // Might be OK if having a short life-span.
1212 Tire Tmp1 = M.makeTire(); // Bad -- 'Tmp1' provides no information.
1213 Light Headlight = M.makeLight("head"); // Good -- descriptive.
1220 Use the "``assert``" macro to its fullest. Check all of your preconditions and
1221 assumptions, you never know when a bug (not necessarily even yours) might be
1222 caught early by an assertion, which reduces debugging time dramatically. The
1223 "``<cassert>``" header file is probably already included by the header files you
1224 are using, so it doesn't cost anything to use it.
1226 To further assist with debugging, make sure to put some kind of error message in
1227 the assertion statement, which is printed if the assertion is tripped. This
1228 helps the poor debugger make sense of why an assertion is being made and
1229 enforced, and hopefully what to do about it. Here is one complete example:
1233 inline Value *getOperand(unsigned I) {
1234 assert(I < Operands.size() && "getOperand() out of range!");
1238 Here are more examples:
1242 assert(Ty->isPointerType() && "Can't allocate a non-pointer type!");
1244 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
1246 assert(idx < getNumSuccessors() && "Successor # out of range!");
1248 assert(V1.getType() == V2.getType() && "Constant types must be identical!");
1250 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
1254 In the past, asserts were used to indicate a piece of code that should not be
1255 reached. These were typically of the form:
1259 assert(0 && "Invalid radix for integer literal");
1261 This has a few issues, the main one being that some compilers might not
1262 understand the assertion, or warn about a missing return in builds where
1263 assertions are compiled out.
1265 Today, we have something much better: ``llvm_unreachable``:
1269 llvm_unreachable("Invalid radix for integer literal");
1271 When assertions are enabled, this will print the message if it's ever reached
1272 and then exit the program. When assertions are disabled (i.e. in release
1273 builds), ``llvm_unreachable`` becomes a hint to compilers to skip generating
1274 code for this branch. If the compiler does not support this, it will fall back
1275 to the "abort" implementation.
1277 Neither assertions or ``llvm_unreachable`` will abort the program on a release
1278 build. If the error condition can be triggered by user input then the
1279 recoverable error mechanism described in :doc:`ProgrammersManual` should be
1280 used instead. In cases where this is not practical, ``report_fatal_error`` may
1283 Another issue is that values used only by assertions will produce an "unused
1284 value" warning when assertions are disabled. For example, this code will warn:
1288 unsigned Size = V.size();
1289 assert(Size > 42 && "Vector smaller than it should be");
1291 bool NewToSet = Myset.insert(Value);
1292 assert(NewToSet && "The value shouldn't be in the set yet");
1294 These are two interesting different cases. In the first case, the call to
1295 ``V.size()`` is only useful for the assert, and we don't want it executed when
1296 assertions are disabled. Code like this should move the call into the assert
1297 itself. In the second case, the side effects of the call must happen whether
1298 the assert is enabled or not. In this case, the value should be cast to void to
1299 disable the warning. To be specific, it is preferred to write the code like
1304 assert(V.size() > 42 && "Vector smaller than it should be");
1306 bool NewToSet = Myset.insert(Value); (void)NewToSet;
1307 assert(NewToSet && "The value shouldn't be in the set yet");
1309 Do Not Use ``using namespace std``
1310 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1312 In LLVM, we prefer to explicitly prefix all identifiers from the standard
1313 namespace with an "``std::``" prefix, rather than rely on "``using namespace
1316 In header files, adding a ``'using namespace XXX'`` directive pollutes the
1317 namespace of any source file that ``#include``\s the header. This is clearly a
1320 In implementation files (e.g. ``.cpp`` files), the rule is more of a stylistic
1321 rule, but is still important. Basically, using explicit namespace prefixes
1322 makes the code **clearer**, because it is immediately obvious what facilities
1323 are being used and where they are coming from. And **more portable**, because
1324 namespace clashes cannot occur between LLVM code and other namespaces. The
1325 portability rule is important because different standard library implementations
1326 expose different symbols (potentially ones they shouldn't), and future revisions
1327 to the C++ standard will add more symbols to the ``std`` namespace. As such, we
1328 never use ``'using namespace std;'`` in LLVM.
1330 The exception to the general rule (i.e. it's not an exception for the ``std``
1331 namespace) is for implementation files. For example, all of the code in the
1332 LLVM project implements code that lives in the 'llvm' namespace. As such, it is
1333 ok, and actually clearer, for the ``.cpp`` files to have a ``'using namespace
1334 llvm;'`` directive at the top, after the ``#include``\s. This reduces
1335 indentation in the body of the file for source editors that indent based on
1336 braces, and keeps the conceptual context cleaner. The general form of this rule
1337 is that any ``.cpp`` file that implements code in any namespace may use that
1338 namespace (and its parents'), but should not use any others.
1340 Provide a Virtual Method Anchor for Classes in Headers
1341 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1343 If a class is defined in a header file and has a vtable (either it has virtual
1344 methods or it derives from classes with virtual methods), it must always have at
1345 least one out-of-line virtual method in the class. Without this, the compiler
1346 will copy the vtable and RTTI into every ``.o`` file that ``#include``\s the
1347 header, bloating ``.o`` file sizes and increasing link times.
1349 Don't use default labels in fully covered switches over enumerations
1350 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1352 ``-Wswitch`` warns if a switch, without a default label, over an enumeration
1353 does not cover every enumeration value. If you write a default label on a fully
1354 covered switch over an enumeration then the ``-Wswitch`` warning won't fire
1355 when new elements are added to that enumeration. To help avoid adding these
1356 kinds of defaults, Clang has the warning ``-Wcovered-switch-default`` which is
1357 off by default but turned on when building LLVM with a version of Clang that
1358 supports the warning.
1360 A knock-on effect of this stylistic requirement is that when building LLVM with
1361 GCC you may get warnings related to "control may reach end of non-void function"
1362 if you return from each case of a covered switch-over-enum because GCC assumes
1363 that the enum expression may take any representable value, not just those of
1364 individual enumerators. To suppress this warning, use ``llvm_unreachable`` after
1367 Use range-based ``for`` loops wherever possible
1368 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1370 The introduction of range-based ``for`` loops in C++11 means that explicit
1371 manipulation of iterators is rarely necessary. We use range-based ``for``
1372 loops wherever possible for all newly added code. For example:
1376 BasicBlock *BB = ...
1377 for (Instruction &I : *BB)
1380 Don't evaluate ``end()`` every time through a loop
1381 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1383 In cases where range-based ``for`` loops can't be used and it is necessary
1384 to write an explicit iterator-based loop, pay close attention to whether
1385 ``end()`` is re-evaluted on each loop iteration. One common mistake is to
1386 write a loop in this style:
1390 BasicBlock *BB = ...
1391 for (auto I = BB->begin(); I != BB->end(); ++I)
1394 The problem with this construct is that it evaluates "``BB->end()``" every time
1395 through the loop. Instead of writing the loop like this, we strongly prefer
1396 loops to be written so that they evaluate it once before the loop starts. A
1397 convenient way to do this is like so:
1401 BasicBlock *BB = ...
1402 for (auto I = BB->begin(), E = BB->end(); I != E; ++I)
1405 The observant may quickly point out that these two loops may have different
1406 semantics: if the container (a basic block in this case) is being mutated, then
1407 "``BB->end()``" may change its value every time through the loop and the second
1408 loop may not in fact be correct. If you actually do depend on this behavior,
1409 please write the loop in the first form and add a comment indicating that you
1410 did it intentionally.
1412 Why do we prefer the second form (when correct)? Writing the loop in the first
1413 form has two problems. First it may be less efficient than evaluating it at the
1414 start of the loop. In this case, the cost is probably minor --- a few extra
1415 loads every time through the loop. However, if the base expression is more
1416 complex, then the cost can rise quickly. I've seen loops where the end
1417 expression was actually something like: "``SomeMap[X]->end()``" and map lookups
1418 really aren't cheap. By writing it in the second form consistently, you
1419 eliminate the issue entirely and don't even have to think about it.
1421 The second (even bigger) issue is that writing the loop in the first form hints
1422 to the reader that the loop is mutating the container (a fact that a comment
1423 would handily confirm!). If you write the loop in the second form, it is
1424 immediately obvious without even looking at the body of the loop that the
1425 container isn't being modified, which makes it easier to read the code and
1426 understand what it does.
1428 While the second form of the loop is a few extra keystrokes, we do strongly
1431 ``#include <iostream>`` is Forbidden
1432 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1434 The use of ``#include <iostream>`` in library files is hereby **forbidden**,
1435 because many common implementations transparently inject a `static constructor`_
1436 into every translation unit that includes it.
1438 Note that using the other stream headers (``<sstream>`` for example) is not
1439 problematic in this regard --- just ``<iostream>``. However, ``raw_ostream``
1440 provides various APIs that are better performing for almost every use than
1441 ``std::ostream`` style APIs.
1445 New code should always use `raw_ostream`_ for writing, or the
1446 ``llvm::MemoryBuffer`` API for reading files.
1453 LLVM includes a lightweight, simple, and efficient stream implementation in
1454 ``llvm/Support/raw_ostream.h``, which provides all of the common features of
1455 ``std::ostream``. All new code should use ``raw_ostream`` instead of
1458 Unlike ``std::ostream``, ``raw_ostream`` is not a template and can be forward
1459 declared as ``class raw_ostream``. Public headers should generally not include
1460 the ``raw_ostream`` header, but use forward declarations and constant references
1461 to ``raw_ostream`` instances.
1466 The ``std::endl`` modifier, when used with ``iostreams`` outputs a newline to
1467 the output stream specified. In addition to doing this, however, it also
1468 flushes the output stream. In other words, these are equivalent:
1472 std::cout << std::endl;
1473 std::cout << '\n' << std::flush;
1475 Most of the time, you probably have no reason to flush the output stream, so
1476 it's better to use a literal ``'\n'``.
1478 Don't use ``inline`` when defining a function in a class definition
1479 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1481 A member function defined in a class definition is implicitly inline, so don't
1482 put the ``inline`` keyword in this case.
1509 This section describes preferred low-level formatting guidelines along with
1510 reasoning on why we prefer them.
1512 Spaces Before Parentheses
1513 ^^^^^^^^^^^^^^^^^^^^^^^^^
1515 We prefer to put a space before an open parenthesis only in control flow
1516 statements, but not in normal function call expressions and function-like
1517 macros. For example, this is good:
1522 for (I = 0; I != 100; ++I) ...
1523 while (LLVMRocks) ...
1526 assert(3 != 4 && "laws of math are failing me");
1528 A = foo(42, 92) + bar(X);
1535 for(I = 0; I != 100; ++I) ...
1536 while(LLVMRocks) ...
1539 assert (3 != 4 && "laws of math are failing me");
1541 A = foo (42, 92) + bar (X);
1543 The reason for doing this is not completely arbitrary. This style makes control
1544 flow operators stand out more, and makes expressions flow better. The function
1545 call operator binds very tightly as a postfix operator. Putting a space after a
1546 function name (as in the last example) makes it appear that the code might bind
1547 the arguments of the left-hand-side of a binary operator with the argument list
1548 of a function and the name of the right side. More specifically, it is easy to
1549 misread the "``A``" example as:
1553 A = foo ((42, 92) + bar) (X);
1555 when skimming through the code. By avoiding a space in a function, we avoid
1556 this misinterpretation.
1561 Hard fast rule: Preincrement (``++X``) may be no slower than postincrement
1562 (``X++``) and could very well be a lot faster than it. Use preincrementation
1565 The semantics of postincrement include making a copy of the value being
1566 incremented, returning it, and then preincrementing the "work value". For
1567 primitive types, this isn't a big deal. But for iterators, it can be a huge
1568 issue (for example, some iterators contains stack and set objects in them...
1569 copying an iterator could invoke the copy ctor's of these as well). In general,
1570 get in the habit of always using preincrement, and you won't have a problem.
1573 Namespace Indentation
1574 ^^^^^^^^^^^^^^^^^^^^^
1576 In general, we strive to reduce indentation wherever possible. This is useful
1577 because we want code to `fit into 80 columns`_ without wrapping horribly, but
1578 also because it makes it easier to understand the code. To facilitate this and
1579 avoid some insanely deep nesting on occasion, don't indent namespaces. If it
1580 helps readability, feel free to add a comment indicating what namespace is
1581 being closed by a ``}``. For example:
1586 namespace knowledge {
1588 /// This class represents things that Smith can have an intimate
1589 /// understanding of and contains the data associated with it.
1593 explicit Grokable() { ... }
1594 virtual ~Grokable() = 0;
1600 } // end namespace knowledge
1601 } // end namespace llvm
1604 Feel free to skip the closing comment when the namespace being closed is
1605 obvious for any reason. For example, the outer-most namespace in a header file
1606 is rarely a source of confusion. But namespaces both anonymous and named in
1607 source files that are being closed half way through the file probably could use
1612 Anonymous Namespaces
1613 ^^^^^^^^^^^^^^^^^^^^
1615 After talking about namespaces in general, you may be wondering about anonymous
1616 namespaces in particular. Anonymous namespaces are a great language feature
1617 that tells the C++ compiler that the contents of the namespace are only visible
1618 within the current translation unit, allowing more aggressive optimization and
1619 eliminating the possibility of symbol name collisions. Anonymous namespaces are
1620 to C++ as "static" is to C functions and global variables. While "``static``"
1621 is available in C++, anonymous namespaces are more general: they can make entire
1622 classes private to a file.
1624 The problem with anonymous namespaces is that they naturally want to encourage
1625 indentation of their body, and they reduce locality of reference: if you see a
1626 random function definition in a C++ file, it is easy to see if it is marked
1627 static, but seeing if it is in an anonymous namespace requires scanning a big
1630 Because of this, we have a simple guideline: make anonymous namespaces as small
1631 as possible, and only use them for class declarations. For example, this is
1641 bool operator<(const char *RHS) const;
1643 } // end anonymous namespace
1645 static void runHelper() {
1649 bool StringSort::operator<(const char *RHS) const {
1663 bool operator<(const char *RHS) const;
1670 bool StringSort::operator<(const char *RHS) const {
1674 } // end anonymous namespace
1676 This is bad specifically because if you're looking at "``runHelper``" in the middle
1677 of a large C++ file, that you have no immediate way to tell if it is local to
1678 the file. When it is marked static explicitly, this is immediately obvious.
1679 Also, there is no reason to enclose the definition of "``operator<``" in the
1680 namespace just because it was declared there.
1685 A lot of these comments and recommendations have been culled from other sources.
1686 Two particularly important books for our work are:
1689 <http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876>`_
1690 by Scott Meyers. Also interesting and useful are "More Effective C++" and
1691 "Effective STL" by the same author.
1693 #. `Large-Scale C++ Software Design
1694 <http://www.amazon.com/Large-Scale-Software-Design-John-Lakos/dp/0201633620/ref=sr_1_1>`_
1697 If you get some free time, and you haven't read them: do so, you might learn