| A Few Coding Standards | -
+
| -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.
- -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.+
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:
-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.+
+ + ++ +
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 - |
+
+
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++ -*--=// +//===-- 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 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.+
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.
-+
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.
-+
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?-
Good things to talk about here are what happens when something unexpected +happens: does the method return null? Abort? Format your hard disk?
++
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:
+
+
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.
+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 #includes to be listed in this +order:
- -+
... and each catagory should be sorted by name.
-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 make for incredible diffs that are absolutely worthless.+
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.
++
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.
+| -Compiler Issues - |
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.
++
+
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.
-
- if (V = getValue()) {
- ..
- }
-+
+ +
+ +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())) {
- ..
- }
-+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:
-These are the gcc warnings that I prefer to enable: -Wall -Winline -W -Wwrite-strings -Wno-unused+
+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
-Compilers are finally catching up to the C++ standard. Most compilers implement most features, so you can use just about any features that you would like. In the LLVM source tree, I have chosen to not use these features:+
+ +
-Just like most of the rules in this document, this isn't a hard and fast requirement. Exceptions are used in the Parser, because it simplifies error reporting significantly, and the LLVM parser is not at all in the critical path.+
+
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.
-Other features, such as templates (without partial specialization) can be used freely. The general goal is to have clear, consise, performant code... if a technique assists with that then use it.+
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. In fact, Visual C++ 6 could be an important target +for our work in the future, and we don't want to have to rewrite all of our code +to support it.
++
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.
-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. In fact, Visual C++ 6 could be an important target for our work in the future, and we don't want to have to rewrite all of our code to support it.+
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 - |
- -Modules should be completely independent of each other, except for their dependence. 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 issues is how they work together.
- -One example of this is the llvm/include/llvm/CFG.h file. It defines a collection of global functions, template classes, and member functions that are syntactically unrelated to each other. Semantically, however, they all provide useful functionality for operating on a CFG, and so they are bound 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.
+
+ +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 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...
+ ++
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.
-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.+
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.
-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, 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...+
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.
++
+
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:
-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.+
| Old Way | +New Way | +
|---|---|
#include <iostream> |
+ #include "llvm/Support/Streams.h" |
+
DEBUG(std::cerr << ...); +DEBUG(dump(std::cerr)); |
+ DOUT << ...; +DEBUG(dump(DOUT)); |
+
std::cerr << "Hello world\n"; |
+ llvm::cerr << "Hello world\n"; |
+
std::cout << "Hello world\n"; |
+ llvm::cout << "Hello world\n"; |
+
std::cin >> Var; |
+ llvm::cin >> Var; |
+
std::ostream |
+ llvm::OStream |
+
std::istream |
+ llvm::IStream |
+
std::stringstream |
+ llvm::StringStream |
+
void print(std::ostream &Out); +// ... +print(std::cerr); |
+ void print(llvm::OStream Out);1 +// ... +print(llvm::cerr);+ + |
1llvm::OStream is a light-weight class so it should never +be passed by reference. This is important because in some configurations, +DOUT is an rvalue.
+| -The Low Level Issues - |
+
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:+
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];
- }
+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(Ty->isPointerType() && "Can't allocate a non pointer type!"); - assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); +assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); - assert(idx < getNumSuccessors() && "Successor # out of range!"); +assert(idx < getNumSuccessors() && "Successor # out of range!"); - assert(V1.getType() == V2.getType() && "Constant types must be identical!"); +assert(V1.getType() == V2.getType() && "Constant types must be identical!"); - assert(Succ->front()->isPHINode() && "Only works on PHId BBs!"); -
+assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!"); + +
+
You get the idea...
+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 includes 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 #includes. 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.
+ ++ +
+ ++
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 +#includes the header, bloating .o file sizes and increasing link times. +
+ ++
+
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.
+| -Writing Iterators - |
+
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:
+