LLVM Coding Standards

1. Introduction
2. Mechanical Source Issues
   1. Source Code Formatting
      1. Commenting
      2. Comment Formatting
      3. `#include` Style
      4. Source Code Width
      5. Use Spaces Instead of Tabs
      6. Indent Code Consistently
   2. Compiler Issues
      1. Treat Compiler Warnings Like Errors
      2. Write Portable Code
      3. Use of class/struct Keywords
3. Style Issues
   1. The High Level Issues
      1. A Public Header File is a Module
      2. #include as Little as Possible
      3. Keep "internal" Headers Private
      4. `#include <iostream>` is forbidden
   2. The Low Level Issues
      1. Assert Liberally
      2. Do not use 'using namespace std'
      3. Provide a virtual method anchor for classes in headers
      4. Prefer Preincrement
      5. Avoid `std::endl`
4. See Also

Written by Chris Lattner and Bill Wendling

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 can be useful.

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:

> If you are adding a significant body of source to a project, feel free to use whatever style you are most comfortable with. 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.

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.

Mechanical Source Issues
Source Code Formatting
Commenting

Comments are one critical part of readability and maintainability. Everyone knows they should comment, so should you. Although we all should probably comment our code more than we do, there are a few very critical places that documentation is very useful:

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 Subversion. Most source trees will probably have a standard file header format. The standard format for the LLVM source tree looks like this:

    //===-- 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 that 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... if it's not obvious. If it's so completely obvious your grandma could figure it out, it's probably safe to leave it out. Naming classes something sane goes a long ways towards avoiding writing documentation.

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 any a description of the borderline behaviour is all that is necessary here (unless something particularly tricky or insideous is going on). The hope is that people can figure out how to use your interfaces without reading the code itself... that is the goal metric.

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:

1. When writing a C code: Obviously if you are writing C code, use C style comments.
2. When writing a header file that may be `#include`d by a C source file.
3. 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 `#include`s required by the file should be listed. We prefer these `#include`s to be listed in this order:

1. Main Module header
2. Local/Private Headers
3. `llvm/*`
4. `llvm/Analysis/*`
5. `llvm/Assembly/*`
6. `llvm/Bytecode/*`
7. `llvm/CodeGen/*`
8. ...
9. `Support/*`
10. `Config/*`
11. System `#includes`

... and each category should be sorted by name.

The "Main Module Header" file applies to .cpp file 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 #included 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.

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 is not up for debate.

Use Spaces Instead of Tabs

In all cases, prefer spaces to tabs in source files. People have different prefered indentation levels, and different styles of indentation that they like... this is fine. What isn't 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 your 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, 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, your 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 them. 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, an warning that annoys me occurs when I write code like this:

    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:

    if ((V = getValue())) {
      ...
    }

...which shuts `gcc` up. Any `gcc` warning that annoys you can be fixed by massaging the code appropriately.

These are the `gcc` warnings that I prefer to enable: `-Wall -Winline -W -Wwrite-strings -Wno-unused`

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, including its support for "high tech" features like partial specialization of templates. If these features are used, they should only be an implementation detail of a library which has a simple exposed API.

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, 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 with one or more `.cpp` files. Each of these `.cpp` files should include the header that defines their interface first. This ensure 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.

`#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 accidently 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 however, that it's okay to put extra implementation methods a public class itself... just make them private (or protected), and all is well.

`#include <iostream>` is forbidden

The use of `#include <iostream>` in library files is hereby forbidden. The primary reason for doing this is to support clients using LLVM libraries as part of larger systems. In particular, we statically link LLVM into some dynamic libraries. Even if LLVM isn't used, the static c'tors are run whenever an application start up that uses the dynamic library. There are two problems with this:

1. The time to run the static c'tors impacts startup time of applications—a critical time for GUI apps.
2. The static c'tors cause the app to pull many extra pages of memory off the disk: both the code for the static c'tors 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.

Note that using the other stream headers (`<sstream>` for example) is allowed normally, it is just `<iostream>` that is causing problems.

The preferred replacement for stream functionality is the `llvm::raw_ostream` class (for writing to output streams of various sorts) and the `llvm::MemoryBuffer` API (for reading in files).

The Low Level Issues
Assert Liberally

Use the "`assert`" function to its fullest. Check all of your preconditions and assumptions, you never know when a bug (not neccesarily 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 debugging make sense of why an assertion is being made and enforced, and hopefully what to do about it. Here is one complete example:

    inline Value *getOperand(unsigned i) {
      assert(i < Operands.size() && "getOperand() out of range!");
      return Operands[i];
    }

Here are some examples:

    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 when adding assert statements that 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:

    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.

    assert(0 && "Some helpful error message");
    // Not reached
    return 0;

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 their top, after the `#include`s. 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 v-table (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.

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.

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:

    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'`.

See Also

A lot of these comments and recommendations have been culled for other sources. Two particularly important books for our work are:

1. Effective C++ by Scott Meyers. Also interesting and useful are "More Effective C++" and "Effective STL" by the same author.
2. Large-Scale C++ Software Design by John Lakos

If you get some free time, and you haven't read them: do so, you might learn something.