A Few Coding Standards

Comments are one critical part of readability and maintainability. Everyone -knows they should comment, so should you. :) Although we all should probably +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 CVS. Most source trees will probably have a standard +

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 was developed by the LLVM research group and is distributed under
-// the University of Illinois Open Source License. See LICENSE.TXT for details.
-// 
+// 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 tag this 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.

+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.

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 @@ -157,11 +165,11 @@ included, as well as any notes or "gotchas" in the code to watch out for.

Class overviews -

Classes are one fundemental part of a good object oriented design. As such, +

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. :)

+something sane goes a long ways towards avoiding writing documentation.

Method information @@ -191,8 +199,9 @@ when it is useful to use C style (/* */) comments however:

When writing a C code: Obviously if you are writing C code, use C style - comments. :)
When writing a header file that may be #included by a C source file.
When writing a header file that may be #included by a C source + file.
When writing a source file that is used by a tool that only accepts C style comments.

@@ -204,40 +213,41 @@ These nest properly and are better behaved in general than C style comments.

- #include Style + #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 #includes to be listed in this order:

+href="#hl_dontinclude">minimal list of #includes required by the +file should be listed. We prefer these #includes to be listed in this +order:

Main Module header
Local/Private Headers
llvm/*
llvm/Analysis/*
llvm/Assembly/*
llvm/Bytecode/*
llvm/CodeGen/*
llvm/*
llvm/Analysis/*
llvm/Assembly/*
llvm/Bytecode/*
llvm/CodeGen/*
...
Support/*
Config/*
System #includes
Support/*
Config/*
System #includes

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

... 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.

+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.

@@ -252,6 +262,18 @@ are defined.

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.

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:

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

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.

@@ -413,16 +458,17 @@ translation unit.

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.

+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, either directly +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 @@ -441,7 +487,7 @@ 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. :)

+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 @@ -452,6 +498,40 @@ class itself... just make them private (or protected), and all is well.

The Low Level Issues @@ -460,7 +540,7 @@ class itself... just make them private (or protected), and all is well.

- Assert Liberally + Assert Liberally

@@ -477,42 +557,121 @@ 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(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
+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 #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.

+ +

+ + +

+ 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 #includes the header, bloating .o file sizes and +increasing link times.

- Prefer Preincrement + 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.

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 @@ -523,10 +682,9 @@ get in the habit of always using preincrement, and you won't have a problem.

- Avoid std::endl + Avoid std::endl

@@ -535,34 +693,18 @@ get in the habit of always using preincrement, and you won't have a problem.

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;
+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".

+it's better to use a literal '\n'.

- -

- Exploit C++ to its Fullest -

- -

C++ is a powerful language. With a firm grasp on its capabilities, you can make -write effective, consise, readable and maintainable code all at the same time. -By staying consistent, you reduce the amount of special cases that need to be -remembered. Reducing the total number of lines of code you write is a good way -to avoid documentation, and avoid giving bugs a place to hide.

- -

For these reasons, come to know and love the contents of your local -<algorithm> header file. Know about <functional> and what it can do -for you. C++ is just a tool that wants you to master it. :)

- -

@@ -577,20 +719,17 @@ sources. Two particularly important books for our work are:

Effective -C++ by Scott Meyers. There is an online version of the book (only some -chapters though) available as well. Also +
Effective +C++ by Scott Meyers. Also interesting and useful are "More Effective C++" and "Effective STL" by the same author.
Large-Scale C++ -Software Design by John Lakos
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. :)

+something.

@@ -599,12 +738,12 @@ something. :)

+ src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS">

+ src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"> Chris Lattner
- LLVM Compiler Infrastructure
+ LLVM Compiler Infrastructure
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