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diff --git a/docs/CodingStandards.rst b/docs/CodingStandards.rst new file mode 100644 index 00000000000..4d16e2a9bd6 --- /dev/null +++ b/docs/CodingStandards.rst @@ -0,0 +1,1175 @@ +.. _coding_standards: + +===================== +LLVM Coding Standards +===================== + +.. contents:: + :local: + +Introduction +============ + +This document attempts to describe a few coding standards that are being used in +the LLVM source tree. Although no coding standards should be regarded as +absolute requirements to be followed in all instances, coding standards are +particularly important for large-scale code bases that follow a library-based +design (like LLVM). + +This document intentionally does not prescribe fixed standards for religious +issues such as brace placement and space usage. For issues like this, follow +the golden rule: + +.. _Golden Rule: + + **If you are extending, enhancing, or bug fixing already implemented code, + use the style that is already being used so that the source is uniform and + easy to follow.** + +Note that some code bases (e.g. ``libc++``) have really good reasons to deviate +from the coding standards. In the case of ``libc++``, this is because the +naming and other conventions are dictated by the C++ standard. If you think +there is a specific good reason to deviate from the standards here, please bring +it up on the LLVMdev mailing list. + +There are some conventions that are not uniformly followed in the code base +(e.g. the naming convention). This is because they are relatively new, and a +lot of code was written before they were put in place. Our long term goal is +for the entire codebase to follow the convention, but we explicitly *do not* +want patches that do large-scale reformating of existing code. On the other +hand, it is reasonable to rename the methods of a class if you're about to +change it in some other way. Just do the reformating as a separate commit from +the functionality change. + +The ultimate goal of these guidelines is the increase readability and +maintainability of our common source base. If you have suggestions for topics to +be included, please mail them to `Chris <mailto:sabre@nondot.org>`_. + +Mechanical Source Issues +======================== + +Source Code Formatting +---------------------- + +Commenting +^^^^^^^^^^ + +Comments are one critical part of readability and maintainability. Everyone +knows they should comment their code, and so should you. When writing comments, +write them as English prose, which means they should use proper capitalization, +punctuation, etc. Aim to describe what the code is trying to do and why, not +*how* it does it at a micro level. Here are a few critical things to document: + +.. _header file comment: + +File Headers +"""""""""""" + +Every source file should have a header on it that describes the basic purpose of +the file. If a file does not have a header, it should not be checked into the +tree. The standard header looks like this: + +.. code-block:: c++ + + //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===// + // + // The LLVM Compiler Infrastructure + // + // This file is distributed under the University of Illinois Open Source + // License. See LICENSE.TXT for details. + // + //===----------------------------------------------------------------------===// + // + // This file contains the declaration of the Instruction class, which is the + // base class for all of the VM instructions. + // + //===----------------------------------------------------------------------===// + +A few things to note about this particular format: The "``-*- C++ -*-``" string +on the first line is there to tell Emacs that the source file is a C++ file, not +a C file (Emacs assumes ``.h`` files are C files by default). + +.. note:: + + This tag is not necessary in ``.cpp`` files. The name of the file is also + on the first line, along with a very short description of the purpose of the + file. This is important when printing out code and flipping though lots of + pages. + +The next section in the file is a concise note that defines the license that the +file is released under. This makes it perfectly clear what terms the source +code can be distributed under and should not be modified in any way. + +The main body of the description does not have to be very long in most cases. +Here it's only two lines. If an algorithm is being implemented or something +tricky is going on, a reference to the paper where it is published should be +included, as well as any notes or *gotchas* in the code to watch out for. + +Class overviews +""""""""""""""" + +Classes are one fundamental part of a good object oriented design. As such, a +class definition should have a comment block that explains what the class is +used for and how it works. Every non-trivial class is expected to have a +``doxygen`` comment block. + +Method information +"""""""""""""""""" + +Methods defined in a class (as well as any global functions) should also be +documented properly. A quick note about what it does and a description of the +borderline behaviour is all that is necessary here (unless something +particularly tricky or insidious is going on). The hope is that people can +figure out how to use your interfaces without reading the code itself. + +Good things to talk about here are what happens when something unexpected +happens: does the method return null? Abort? Format your hard disk? + +Comment Formatting +^^^^^^^^^^^^^^^^^^ + +In general, prefer C++ style (``//``) comments. They take less space, require +less typing, don't have nesting problems, etc. There are a few cases when it is +useful to use C style (``/* */``) comments however: + +#. When writing C code: Obviously if you are writing C code, use C style + comments. + +#. When writing a header file that may be ``#include``\d by a C source file. + +#. When writing a source file that is used by a tool that only accepts C style + comments. + +To comment out a large block of code, use ``#if 0`` and ``#endif``. These nest +properly and are better behaved in general than C style comments. + +``#include`` Style +^^^^^^^^^^^^^^^^^^ + +Immediately after the `header file comment`_ (and include guards if working on a +header file), the `minimal list of #includes`_ required by the file should be +listed. We prefer these ``#include``\s to be listed in this order: + +.. _Main Module Header: +.. _Local/Private Headers: + +#. Main Module Header +#. Local/Private Headers +#. ``llvm/*`` +#. ``llvm/Analysis/*`` +#. ``llvm/Assembly/*`` +#. ``llvm/Bitcode/*`` +#. ``llvm/CodeGen/*`` +#. ... +#. ``llvm/Support/*`` +#. ``llvm/Config/*`` +#. System ``#include``\s + +and each category should be sorted by name. + +The `Main Module Header`_ file applies to ``.cpp`` files which implement an +interface defined by a ``.h`` file. This ``#include`` should always be included +**first** regardless of where it lives on the file system. By including a +header file first in the ``.cpp`` files that implement the interfaces, we ensure +that the header does not have any hidden dependencies which are not explicitly +``#include``\d in the header, but should be. It is also a form of documentation +in the ``.cpp`` file to indicate where the interfaces it implements are defined. + +.. _fit into 80 columns: + +Source Code Width +^^^^^^^^^^^^^^^^^ + +Write your code to fit within 80 columns of text. This helps those of us who +like to print out code and look at your code in an ``xterm`` without resizing +it. + +The longer answer is that there must be some limit to the width of the code in +order to reasonably allow developers to have multiple files side-by-side in +windows on a modest display. If you are going to pick a width limit, it is +somewhat arbitrary but you might as well pick something standard. Going with 90 +columns (for example) instead of 80 columns wouldn't add any significant value +and would be detrimental to printing out code. Also many other projects have +standardized on 80 columns, so some people have already configured their editors +for it (vs something else, like 90 columns). + +This is one of many contentious issues in coding standards, but it is not up for +debate. + +Use Spaces Instead of Tabs +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +In all cases, prefer spaces to tabs in source files. People have different +preferred indentation levels, and different styles of indentation that they +like; this is fine. What isn't fine is that different editors/viewers expand +tabs out to different tab stops. This can cause your code to look completely +unreadable, and it is not worth dealing with. + +As always, follow the `Golden Rule`_ above: follow the style of +existing code if you are modifying and extending it. If you like four spaces of +indentation, **DO NOT** do that in the middle of a chunk of code with two spaces +of indentation. Also, do not reindent a whole source file: it makes for +incredible diffs that are absolutely worthless. + +Indent Code Consistently +^^^^^^^^^^^^^^^^^^^^^^^^ + +Okay, in your first year of programming you were told that indentation is +important. If you didn't believe and internalize this then, now is the time. +Just do it. + +Compiler Issues +--------------- + +Treat Compiler Warnings Like Errors +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +If your code has compiler warnings in it, something is wrong --- you aren't +casting values correctly, you have "questionable" constructs in your code, or +you are doing something legitimately wrong. Compiler warnings can cover up +legitimate errors in output and make dealing with a translation unit difficult. + +It is not possible to prevent all warnings from all compilers, nor is it +desirable. Instead, pick a standard compiler (like ``gcc``) that provides a +good thorough set of warnings, and stick to it. At least in the case of +``gcc``, it is possible to work around any spurious errors by changing the +syntax of the code slightly. For example, a warning that annoys me occurs when +I write code like this: + +.. code-block:: c++ + + if (V = getValue()) { + ... + } + +``gcc`` will warn me that I probably want to use the ``==`` operator, and that I +probably mistyped it. In most cases, I haven't, and I really don't want the +spurious errors. To fix this particular problem, I rewrite the code like +this: + +.. code-block:: c++ + + if ((V = getValue())) { + ... + } + +which shuts ``gcc`` up. Any ``gcc`` warning that annoys you can be fixed by +massaging the code appropriately. + +Write Portable Code +^^^^^^^^^^^^^^^^^^^ + +In almost all cases, it is possible and within reason to write completely +portable code. If there are cases where it isn't possible to write portable +code, isolate it behind a well defined (and well documented) interface. + +In practice, this means that you shouldn't assume much about the host compiler +(and Visual Studio tends to be the lowest common denominator). If advanced +features are used, they should only be an implementation detail of a library +which has a simple exposed API, and preferably be buried in ``libSystem``. + +Do not use RTTI or Exceptions +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +In an effort to reduce code and executable size, LLVM does not use RTTI +(e.g. ``dynamic_cast<>;``) or exceptions. These two language features violate +the general C++ principle of *"you only pay for what you use"*, causing +executable bloat even if exceptions are never used in the code base, or if RTTI +is never used for a class. Because of this, we turn them off globally in the +code. + +That said, LLVM does make extensive use of a hand-rolled form of RTTI that use +templates like `isa<>, cast<>, and dyn_cast<> <ProgrammersManual.html#isa>`_. +This form of RTTI is opt-in and can be added to any class. It is also +substantially more efficient than ``dynamic_cast<>``. + +.. _static constructor: + +Do not use Static Constructors +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Static constructors and destructors (e.g. global variables whose types have a +constructor or destructor) should not be added to the code base, and should be +removed wherever possible. Besides `well known problems +<http://yosefk.com/c++fqa/ctors.html#fqa-10.12>`_ where the order of +initialization is undefined between globals in different source files, the +entire concept of static constructors is at odds with the common use case of +LLVM as a library linked into a larger application. + +Consider the use of LLVM as a JIT linked into another application (perhaps for +`OpenGL, custom languages <http://llvm.org/Users.html>`_, `shaders in movies +<http://llvm.org/devmtg/2010-11/Gritz-OpenShadingLang.pdf>`_, etc). Due to the +design of static constructors, they must be executed at startup time of the +entire application, regardless of whether or how LLVM is used in that larger +application. There are two problems with this: + +* The time to run the static constructors impacts startup time of applications + --- a critical time for GUI apps, among others. + +* The static constructors cause the app to pull many extra pages of memory off + the disk: both the code for the constructor in each ``.o`` file and the small + amount of data that gets touched. In addition, touched/dirty pages put more + pressure on the VM system on low-memory machines. + +We would really like for there to be zero cost for linking in an additional LLVM +target or other library into an application, but static constructors violate +this goal. + +That said, LLVM unfortunately does contain static constructors. It would be a +`great project <http://llvm.org/PR11944>`_ for someone to purge all static +constructors from LLVM, and then enable the ``-Wglobal-constructors`` warning +flag (when building with Clang) to ensure we do not regress in the future. + +Use of ``class`` and ``struct`` Keywords +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +In C++, the ``class`` and ``struct`` keywords can be used almost +interchangeably. The only difference is when they are used to declare a class: +``class`` makes all members private by default while ``struct`` makes all +members public by default. + +Unfortunately, not all compilers follow the rules and some will generate +different symbols based on whether ``class`` or ``struct`` was used to declare +the symbol. This can lead to problems at link time. + +So, the rule for LLVM is to always use the ``class`` keyword, unless **all** +members are public and the type is a C++ `POD +<http://en.wikipedia.org/wiki/Plain_old_data_structure>`_ type, in which case +``struct`` is allowed. + +Style Issues +============ + +The High-Level Issues +--------------------- + +A Public Header File **is** a Module +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +C++ doesn't do too well in the modularity department. There is no real +encapsulation or data hiding (unless you use expensive protocol classes), but it +is what we have to work with. When you write a public header file (in the LLVM +source tree, they live in the top level "``include``" directory), you are +defining a module of functionality. + +Ideally, modules should be completely independent of each other, and their +header files should only ``#include`` the absolute minimum number of headers +possible. A module is not just a class, a function, or a namespace: it's a +collection of these that defines an interface. This interface may be several +functions, classes, or data structures, but the important issue is how they work +together. + +In general, a module should be implemented by one or more ``.cpp`` files. Each +of these ``.cpp`` files should include the header that defines their interface +first. This ensures that all of the dependences of the module header have been +properly added to the module header itself, and are not implicit. System +headers should be included after user headers for a translation unit. + +.. _minimal list of #includes: + +``#include`` as Little as Possible +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +``#include`` hurts compile time performance. Don't do it unless you have to, +especially in header files. + +But wait! Sometimes you need to have the definition of a class to use it, or to +inherit from it. In these cases go ahead and ``#include`` that header file. Be +aware however that there are many cases where you don't need to have the full +definition of a class. If you are using a pointer or reference to a class, you +don't need the header file. If you are simply returning a class instance from a +prototyped function or method, you don't need it. In fact, for most cases, you +simply don't need the definition of a class. And not ``#include``\ing speeds up +compilation. + +It is easy to try to go too overboard on this recommendation, however. You +**must** include all of the header files that you are using --- you can include +them either directly or indirectly through another header file. To make sure +that you don't accidentally forget to include a header file in your module +header, make sure to include your module header **first** in the implementation +file (as mentioned above). This way there won't be any hidden dependencies that +you'll find out about later. + +Keep "Internal" Headers Private +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Many modules have a complex implementation that causes them to use more than one +implementation (``.cpp``) file. It is often tempting to put the internal +communication interface (helper classes, extra functions, etc) in the public +module header file. Don't do this! + +If you really need to do something like this, put a private header file in the +same directory as the source files, and include it locally. This ensures that +your private interface remains private and undisturbed by outsiders. + +.. note:: + + It's okay to put extra implementation methods in a public class itself. Just + make them private (or protected) and all is well. + +.. _early exits: + +Use Early Exits and ``continue`` to Simplify Code +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +When reading code, keep in mind how much state and how many previous decisions +have to be remembered by the reader to understand a block of code. Aim to +reduce indentation where possible when it doesn't make it more difficult to +understand the code. One great way to do this is by making use of early exits +and the ``continue`` keyword in long loops. As an example of using an early +exit from a function, consider this "bad" code: + +.. code-block:: c++ + + Value *DoSomething(Instruction *I) { + if (!isa<TerminatorInst>(I) && + I->hasOneUse() && SomeOtherThing(I)) { + ... some long code .... + } + + return 0; + } + +This code has several problems if the body of the ``'if'`` is large. When +you're looking at the top of the function, it isn't immediately clear that this +*only* does interesting things with non-terminator instructions, and only +applies to things with the other predicates. Second, it is relatively difficult +to describe (in comments) why these predicates are important because the ``if`` +statement makes it difficult to lay out the comments. Third, when you're deep +within the body of the code, it is indented an extra level. Finally, when +reading the top of the function, it isn't clear what the result is if the +predicate isn't true; you have to read to the end of the function to know that +it returns null. + +It is much preferred to format the code like this: + +.. code-block:: c++ + + Value *DoSomething(Instruction *I) { + // Terminators never need 'something' done to them because ... + if (isa<TerminatorInst>(I)) + return 0; + + // We conservatively avoid transforming instructions with multiple uses + // because goats like cheese. + if (!I->hasOneUse()) + return 0; + + // This is really just here for example. + if (!SomeOtherThing(I)) + return 0; + + ... some long code .... + } + +This fixes these problems. A similar problem frequently happens in ``for`` +loops. A silly example is something like this: + +.. code-block:: c++ + + for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { + if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) { + Value *LHS = BO->getOperand(0); + Value *RHS = BO->getOperand(1); + if (LHS != RHS) { + ... + } + } + } + +When you have very, very small loops, this sort of structure is fine. But if it +exceeds more than 10-15 lines, it becomes difficult for people to read and +understand at a glance. The problem with this sort of code is that it gets very +nested very quickly. Meaning that the reader of the code has to keep a lot of +context in their brain to remember what is going immediately on in the loop, +because they don't know if/when the ``if`` conditions will have ``else``\s etc. +It is strongly preferred to structure the loop like this: + +.. code-block:: c++ + + for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { + BinaryOperator *BO = dyn_cast<BinaryOperator>(II); + if (!BO) continue; + + Value *LHS = BO->getOperand(0); + Value *RHS = BO->getOperand(1); + if (LHS == RHS) continue; + + ... + } + +This has all the benefits of using early exits for functions: it reduces nesting +of the loop, it makes it easier to describe why the conditions are true, and it +makes it obvious to the reader that there is no ``else`` coming up that they +have to push context into their brain for. If a loop is large, this can be a +big understandability win. + +Don't use ``else`` after a ``return`` +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +For similar reasons above (reduction of indentation and easier reading), please +do not use ``'else'`` or ``'else if'`` after something that interrupts control +flow --- like ``return``, ``break``, ``continue``, ``goto``, etc. For +example, this is *bad*: + +.. code-block:: c++ + + case 'J': { + if (Signed) { + Type = Context.getsigjmp_bufType(); + if (Type.isNull()) { + Error = ASTContext::GE_Missing_sigjmp_buf; + return QualType(); + } else { + break; + } + } else { + Type = Context.getjmp_bufType(); + if (Type.isNull()) { + Error = ASTContext::GE_Missing_jmp_buf; + return QualType(); + } else { + break; + } + } + } + +It is better to write it like this: + +.. code-block:: c++ + + case 'J': + if (Signed) { + Type = Context.getsigjmp_bufType(); + if (Type.isNull()) { + Error = ASTContext::GE_Missing_sigjmp_buf; + return QualType(); + } + } else { + Type = Context.getjmp_bufType(); + if (Type.isNull()) { + Error = ASTContext::GE_Missing_jmp_buf; + return QualType(); + } + } + break; + +Or better yet (in this case) as: + +.. code-block:: c++ + + case 'J': + if (Signed) + Type = Context.getsigjmp_bufType(); + else + Type = Context.getjmp_bufType(); + + if (Type.isNull()) { + Error = Signed ? ASTContext::GE_Missing_sigjmp_buf : + ASTContext::GE_Missing_jmp_buf; + return QualType(); + } + break; + +The idea is to reduce indentation and the amount of code you have to keep track +of when reading the code. + +Turn Predicate Loops into Predicate Functions +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +It is very common to write small loops that just compute a boolean value. There +are a number of ways that people commonly write these, but an example of this +sort of thing is: + +.. code-block:: c++ + + bool FoundFoo = false; + for (unsigned i = 0, e = BarList.size(); i != e; ++i) + if (BarList[i]->isFoo()) { + FoundFoo = true; + break; + } + + if (FoundFoo) { + ... + } + +This sort of code is awkward to write, and is almost always a bad sign. Instead +of this sort of loop, we strongly prefer to use a predicate function (which may +be `static`_) that uses `early exits`_ to compute the predicate. We prefer the +code to be structured like this: + +.. code-block:: c++ + + /// ListContainsFoo - Return true if the specified list has an element that is + /// a foo. + static bool ListContainsFoo(const std::vector<Bar*> &List) { + for (unsigned i = 0, e = List.size(); i != e; ++i) + if (List[i]->isFoo()) + return true; + return false; + } + ... + + if (ListContainsFoo(BarList)) { + ... + } + +There are many reasons for doing this: it reduces indentation and factors out +code which can often be shared by other code that checks for the same predicate. +More importantly, it *forces you to pick a name* for the function, and forces +you to write a comment for it. In this silly example, this doesn't add much +value. However, if the condition is complex, this can make it a lot easier for +the reader to understand the code that queries for this predicate. Instead of +being faced with the in-line details of how we check to see if the BarList +contains a foo, we can trust the function name and continue reading with better +locality. + +The Low-Level Issues +-------------------- + +Name Types, Functions, Variables, and Enumerators Properly +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Poorly-chosen names can mislead the reader and cause bugs. We cannot stress +enough how important it is to use *descriptive* names. Pick names that match +the semantics and role of the underlying entities, within reason. Avoid +abbreviations unless they are well known. After picking a good name, make sure +to use consistent capitalization for the name, as inconsistency requires clients +to either memorize the APIs or to look it up to find the exact spelling. + +In general, names should be in camel case (e.g. ``TextFileReader`` and +``isLValue()``). Different kinds of declarations have different rules: + +* **Type names** (including classes, structs, enums, typedefs, etc) should be + nouns and start with an upper-case letter (e.g. ``TextFileReader``). + +* **Variable names** should be nouns (as they represent state). The name should + be camel case, and start with an upper case letter (e.g. ``Leader`` or + ``Boats``). + +* **Function names** should be verb phrases (as they represent actions), and + command-like function should be imperative. The name should be camel case, + and start with a lower case letter (e.g. ``openFile()`` or ``isFoo()``). + +* **Enum declarations** (e.g. ``enum Foo {...}``) are types, so they should + follow the naming conventions for types. A common use for enums is as a + discriminator for a union, or an indicator of a subclass. When an enum is + used for something like this, it should have a ``Kind`` suffix + (e.g. ``ValueKind``). + +* **Enumerators** (e.g. ``enum { Foo, Bar }``) and **public member variables** + should start with an upper-case letter, just like types. Unless the + enumerators are defined in their own small namespace or inside a class, + enumerators should have a prefix corresponding to the enum declaration name. + For example, ``enum ValueKind { ... };`` may contain enumerators like + ``VK_Argument``, ``VK_BasicBlock``, etc. Enumerators that are just + convenience constants are exempt from the requirement for a prefix. For + instance: + + .. code-block:: c++ + + enum { + MaxSize = 42, + Density = 12 + }; + +As an exception, classes that mimic STL classes can have member names in STL's +style of lower-case words separated by underscores (e.g. ``begin()``, +``push_back()``, and ``empty()``). + +Here are some examples of good and bad names: + +.. code-block:: c++ + + class VehicleMaker { + ... + Factory<Tire> F; // Bad -- abbreviation and non-descriptive. + Factory<Tire> Factory; // Better. + Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one + // kind of factories. + }; + + Vehicle MakeVehicle(VehicleType Type) { + VehicleMaker M; // Might be OK if having a short life-span. + Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information. + Light headlight = M.makeLight("head"); // Good -- descriptive. + ... + } + +Assert Liberally +^^^^^^^^^^^^^^^^ + +Use the "``assert``" macro to its fullest. Check all of your preconditions and +assumptions, you never know when a bug (not necessarily even yours) might be +caught early by an assertion, which reduces debugging time dramatically. The +"``<cassert>``" header file is probably already included by the header files you +are using, so it doesn't cost anything to use it. + +To further assist with debugging, make sure to put some kind of error message in +the assertion statement, which is printed if the assertion is tripped. This +helps the poor debugger make sense of why an assertion is being made and +enforced, and hopefully what to do about it. Here is one complete example: + +.. code-block:: c++ + + inline Value *getOperand(unsigned i) { + assert(i < Operands.size() && "getOperand() out of range!"); + return Operands[i]; + } + +Here are more examples: + +.. code-block:: c++ + + assert(Ty->isPointerType() && "Can't allocate a non pointer type!"); + + assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); + + assert(idx < getNumSuccessors() && "Successor # out of range!"); + + assert(V1.getType() == V2.getType() && "Constant types must be identical!"); + + assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!"); + +You get the idea. + +Please be aware that, when adding assert statements, not all compilers are aware +of the semantics of the assert. In some places, asserts are used to indicate a +piece of code that should not be reached. These are typically of the form: + +.. code-block:: c++ + + assert(0 && "Some helpful error message"); + +When used in a function that returns a value, they should be followed with a +return statement and a comment indicating that this line is never reached. This +will prevent a compiler which is unable to deduce that the assert statement +never returns from generating a warning. + +.. code-block:: c++ + + assert(0 && "Some helpful error message"); + return 0; + +Another issue is that values used only by assertions will produce an "unused +value" warning when assertions are disabled. For example, this code will warn: + +.. code-block:: c++ + + unsigned Size = V.size(); + assert(Size > 42 && "Vector smaller than it should be"); + + bool NewToSet = Myset.insert(Value); + assert(NewToSet && "The value shouldn't be in the set yet"); + +These are two interesting different cases. In the first case, the call to +``V.size()`` is only useful for the assert, and we don't want it executed when +assertions are disabled. Code like this should move the call into the assert +itself. In the second case, the side effects of the call must happen whether +the assert is enabled or not. In this case, the value should be cast to void to +disable the warning. To be specific, it is preferred to write the code like +this: + +.. code-block:: c++ + + assert(V.size() > 42 && "Vector smaller than it should be"); + + bool NewToSet = Myset.insert(Value); (void)NewToSet; + assert(NewToSet && "The value shouldn't be in the set yet"); + +Do Not Use ``using namespace std`` +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +In LLVM, we prefer to explicitly prefix all identifiers from the standard +namespace with an "``std::``" prefix, rather than rely on "``using namespace +std;``". + +In header files, adding a ``'using namespace XXX'`` directive pollutes the +namespace of any source file that ``#include``\s the header. This is clearly a +bad thing. + +In implementation files (e.g. ``.cpp`` files), the rule is more of a stylistic +rule, but is still important. Basically, using explicit namespace prefixes +makes the code **clearer**, because it is immediately obvious what facilities +are being used and where they are coming from. And **more portable**, because +namespace clashes cannot occur between LLVM code and other namespaces. The +portability rule is important because different standard library implementations +expose different symbols (potentially ones they shouldn't), and future revisions +to the C++ standard will add more symbols to the ``std`` namespace. As such, we +never use ``'using namespace std;'`` in LLVM. + +The exception to the general rule (i.e. it's not an exception for the ``std`` +namespace) is for implementation files. For example, all of the code in the +LLVM project implements code that lives in the 'llvm' namespace. As such, it is +ok, and actually clearer, for the ``.cpp`` files to have a ``'using namespace +llvm;'`` directive at the top, after the ``#include``\s. This reduces +indentation in the body of the file for source editors that indent based on +braces, and keeps the conceptual context cleaner. The general form of this rule +is that any ``.cpp`` file that implements code in any namespace may use that +namespace (and its parents'), but should not use any others. + +Provide a Virtual Method Anchor for Classes in Headers +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +If a class is defined in a header file and has a vtable (either it has virtual +methods or it derives from classes with virtual methods), it must always have at +least one out-of-line virtual method in the class. Without this, the compiler +will copy the vtable and RTTI into every ``.o`` file that ``#include``\s the +header, bloating ``.o`` file sizes and increasing link times. + +Use ``LLVM_DELETED_FUNCTION`` to mark uncallable methods +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Prior to C++11, a common pattern to make a class uncopyable was to declare an +unimplemented copy constructor and copy assignment operator and make them +private. This would give a compiler error for accessing a private method or a +linker error because it wasn't implemented. + +With C++11, we can mark methods that won't be implemented with ``= delete``. +This will trigger a much better error message and tell the compiler that the +method will never be implemented. This enables other checks like +``-Wunused-private-field`` to run correctly on classes that contain these +methods. + +To maintain compatibility with C++03, ``LLVM_DELETED_FUNCTION`` should be used +which will expand to ``= delete`` if the compiler supports it. These methods +should still be declared private. Example of the uncopyable pattern: + +.. code-block:: c++ + + class DontCopy { + private: + DontCopy(const DontCopy&) LLVM_DELETED_FUNCTION; + DontCopy &operator =(const DontCopy&) LLVM_DELETED_FUNCTION; + public: + ... + }; + +Don't evaluate ``end()`` every time through a loop +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Because C++ doesn't have a standard "``foreach``" loop (though it can be +emulated with macros and may be coming in C++'0x) we end up writing a lot of +loops that manually iterate from begin to end on a variety of containers or +through other data structures. One common mistake is to write a loop in this +style: + +.. code-block:: c++ + + BasicBlock *BB = ... + for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) + ... use I ... + +The problem with this construct is that it evaluates "``BB->end()``" every time +through the loop. Instead of writing the loop like this, we strongly prefer +loops to be written so that they evaluate it once before the loop starts. A +convenient way to do this is like so: + +.. code-block:: c++ + + BasicBlock *BB = ... + for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) + ... use I ... + +The observant may quickly point out that these two loops may have different +semantics: if the container (a basic block in this case) is being mutated, then +"``BB->end()``" may change its value every time through the loop and the second +loop may not in fact be correct. If you actually do depend on this behavior, +please write the loop in the first form and add a comment indicating that you +did it intentionally. + +Why do we prefer the second form (when correct)? Writing the loop in the first +form has two problems. First it may be less efficient than evaluating it at the +start of the loop. In this case, the cost is probably minor --- a few extra +loads every time through the loop. However, if the base expression is more +complex, then the cost can rise quickly. I've seen loops where the end +expression was actually something like: "``SomeMap[x]->end()``" and map lookups +really aren't cheap. By writing it in the second form consistently, you +eliminate the issue entirely and don't even have to think about it. + +The second (even bigger) issue is that writing the loop in the first form hints +to the reader that the loop is mutating the container (a fact that a comment +would handily confirm!). If you write the loop in the second form, it is +immediately obvious without even looking at the body of the loop that the +container isn't being modified, which makes it easier to read the code and +understand what it does. + +While the second form of the loop is a few extra keystrokes, we do strongly +prefer it. + +``#include <iostream>`` is Forbidden +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The use of ``#include <iostream>`` in library files is hereby **forbidden**, +because many common implementations transparently inject a `static constructor`_ +into every translation unit that includes it. + +Note that using the other stream headers (``<sstream>`` for example) is not +problematic in this regard --- just ``<iostream>``. However, ``raw_ostream`` +provides various APIs that are better performing for almost every use than +``std::ostream`` style APIs. + +.. note:: + + New code should always use `raw_ostream`_ for writing, or the + ``llvm::MemoryBuffer`` API for reading files. + +.. _raw_ostream: + +Use ``raw_ostream`` +^^^^^^^^^^^^^^^^^^^ + +LLVM includes a lightweight, simple, and efficient stream implementation in +``llvm/Support/raw_ostream.h``, which provides all of the common features of +``std::ostream``. All new code should use ``raw_ostream`` instead of +``ostream``. + +Unlike ``std::ostream``, ``raw_ostream`` is not a template and can be forward +declared as ``class raw_ostream``. Public headers should generally not include +the ``raw_ostream`` header, but use forward declarations and constant references +to ``raw_ostream`` instances. + +Avoid ``std::endl`` +^^^^^^^^^^^^^^^^^^^ + +The ``std::endl`` modifier, when used with ``iostreams`` outputs a newline to +the output stream specified. In addition to doing this, however, it also +flushes the output stream. In other words, these are equivalent: + +.. code-block:: c++ + + std::cout << std::endl; + std::cout << '\n' << std::flush; + +Most of the time, you probably have no reason to flush the output stream, so +it's better to use a literal ``'\n'``. + +Microscopic Details +------------------- + +This section describes preferred low-level formatting guidelines along with +reasoning on why we prefer them. + +Spaces Before Parentheses +^^^^^^^^^^^^^^^^^^^^^^^^^ + +We prefer to put a space before an open parenthesis only in control flow +statements, but not in normal function call expressions and function-like +macros. For example, this is good: + +.. code-block:: c++ + + if (x) ... + for (i = 0; i != 100; ++i) ... + while (llvm_rocks) ... + + somefunc(42); + assert(3 != 4 && "laws of math are failing me"); + + a = foo(42, 92) + bar(x); + +and this is bad: + +.. code-block:: c++ + + if(x) ... + for(i = 0; i != 100; ++i) ... + while(llvm_rocks) ... + + somefunc (42); + assert (3 != 4 && "laws of math are failing me"); + + a = foo (42, 92) + bar (x); + +The reason for doing this is not completely arbitrary. This style makes control +flow operators stand out more, and makes expressions flow better. The function +call operator binds very tightly as a postfix operator. Putting a space after a +function name (as in the last example) makes it appear that the code might bind +the arguments of the left-hand-side of a binary operator with the argument list +of a function and the name of the right side. More specifically, it is easy to +misread the "``a``" example as: + +.. code-block:: c++ + + a = foo ((42, 92) + bar) (x); + +when skimming through the code. By avoiding a space in a function, we avoid +this misinterpretation. + +Prefer Preincrement +^^^^^^^^^^^^^^^^^^^ + +Hard fast rule: Preincrement (``++X``) may be no slower than postincrement +(``X++``) and could very well be a lot faster than it. Use preincrementation +whenever possible. + +The semantics of postincrement include making a copy of the value being +incremented, returning it, and then preincrementing the "work value". For +primitive types, this isn't a big deal. But for iterators, it can be a huge +issue (for example, some iterators contains stack and set objects in them... +copying an iterator could invoke the copy ctor's of these as well). In general, +get in the habit of always using preincrement, and you won't have a problem. + + +Namespace Indentation +^^^^^^^^^^^^^^^^^^^^^ + +In general, we strive to reduce indentation wherever possible. This is useful +because we want code to `fit into 80 columns`_ without wrapping horribly, but +also because it makes it easier to understand the code. Namespaces are a funny +thing: they are often large, and we often desire to put lots of stuff into them +(so they can be large). Other times they are tiny, because they just hold an +enum or something similar. In order to balance this, we use different +approaches for small versus large namespaces. + +If a namespace definition is small and *easily* fits on a screen (say, less than +35 lines of code), then you should indent its body. Here's an example: + +.. code-block:: c++ + + namespace llvm { + namespace X86 { + /// RelocationType - An enum for the x86 relocation codes. Note that + /// the terminology here doesn't follow x86 convention - word means + /// 32-bit and dword means 64-bit. + enum RelocationType { + /// reloc_pcrel_word - PC relative relocation, add the relocated value to + /// the value already in memory, after we adjust it for where the PC is. + reloc_pcrel_word = 0, + + /// reloc_picrel_word - PIC base relative relocation, add the relocated + /// value to the value already in memory, after we adjust it for where the + /// PIC base is. + reloc_picrel_word = 1, + + /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just + /// add the relocated value to the value already in memory. + reloc_absolute_word = 2, + reloc_absolute_dword = 3 + }; + } + } + +Since the body is small, indenting adds value because it makes it very clear +where the namespace starts and ends, and it is easy to take the whole thing in +in one "gulp" when reading the code. If the blob of code in the namespace is +larger (as it typically is in a header in the ``llvm`` or ``clang`` namespaces), +do not indent the code, and add a comment indicating what namespace is being +closed. For example: + +.. code-block:: c++ + + namespace llvm { + namespace knowledge { + + /// Grokable - This class represents things that Smith can have an intimate + /// understanding of and contains the data associated with it. + class Grokable { + ... + public: + explicit Grokable() { ... } + virtual ~Grokable() = 0; + + ... + + }; + + } // end namespace knowledge + } // end namespace llvm + +Because the class is large, we don't expect that the reader can easily +understand the entire concept in a glance, and the end of the file (where the +namespaces end) may be a long ways away from the place they open. As such, +indenting the contents of the namespace doesn't add any value, and detracts from +the readability of the class. In these cases it is best to *not* indent the +contents of the namespace. + +.. _static: + +Anonymous Namespaces +^^^^^^^^^^^^^^^^^^^^ + +After talking about namespaces in general, you may be wondering about anonymous +namespaces in particular. Anonymous namespaces are a great language feature +that tells the C++ compiler that the contents of the namespace are only visible +within the current translation unit, allowing more aggressive optimization and +eliminating the possibility of symbol name collisions. Anonymous namespaces are +to C++ as "static" is to C functions and global variables. While "``static``" +is available in C++, anonymous namespaces are more general: they can make entire +classes private to a file. + +The problem with anonymous namespaces is that they naturally want to encourage +indentation of their body, and they reduce locality of reference: if you see a +random function definition in a C++ file, it is easy to see if it is marked +static, but seeing if it is in an anonymous namespace requires scanning a big +chunk of the file. + +Because of this, we have a simple guideline: make anonymous namespaces as small +as possible, and only use them for class declarations. For example, this is +good: + +.. code-block:: c++ + + namespace { + class StringSort { + ... + public: + StringSort(...) + bool operator<(const char *RHS) const; + }; + } // end anonymous namespace + + static void Helper() { + ... + } + + bool StringSort::operator<(const char *RHS) const { + ... + } + +This is bad: + +.. code-block:: c++ + + namespace { + class StringSort { + ... + public: + StringSort(...) + bool operator<(const char *RHS) const; + }; + + void Helper() { + ... + } + + bool StringSort::operator<(const char *RHS) const { + ... + } + + } // end anonymous namespace + +This is bad specifically because if you're looking at "``Helper``" in the middle +of a large C++ file, that you have no immediate way to tell if it is local to +the file. When it is marked static explicitly, this is immediately obvious. +Also, there is no reason to enclose the definition of "``operator<``" in the +namespace just because it was declared there. + +See Also +======== + +A lot of these comments and recommendations have been culled for other sources. +Two particularly important books for our work are: + +#. `Effective C++ + <http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876>`_ + by Scott Meyers. Also interesting and useful are "More Effective C++" and + "Effective STL" by the same author. + +#. `Large-Scale C++ Software Design + <http://www.amazon.com/Large-Scale-Software-Design-John-Lakos/dp/0201633620/ref=sr_1_1>`_ + by John Lakos + +If you get some free time, and you haven't read them: do so, you might learn +something. |