LLVM Coding Standards

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 a code is trying to do and why, not "how" it does it at a micro level. Here are a few critical things to document:

File Headers

//===-- 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.
//
//===----------------------------------------------------------------------===//

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

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 C code: Obviously if you are writing C code, use C style comments.
2. When writing a header file that may be #included 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.

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:

1. Main Module Header
2. Local/Private Headers
3. llvm/*
4. llvm/Analysis/*
5. llvm/Assembly/*
6. llvm/Bitcode/*
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 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 #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.

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.

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.

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.

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 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:

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.

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.

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<>. This form of RTTI is opt-in and can be added to any class. It is also substantially more efficient than dynamic_cast<>.

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 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, shaders in movies, 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:

1. The time to run the static constructors impacts startup time of applications — a critical time for GUI apps, among others.
2. 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 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.

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 type, in which case struct is allowed.

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.

#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.

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 in a public class itself. Just make them private (or protected) and all is well.

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:

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:

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:

  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 elses etc. It is strongly preferred to structure the loop like this:

  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.

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:

  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:

  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:

  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.

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:

  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:

/// 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.

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:

  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:

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.
  ...
}

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:

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

Here are more 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 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:

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;

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:

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:

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");

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 the top, after the #includes. 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.

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.

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:

  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:

  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.

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. Therefore new code should always use raw_ostream for writing, or the llvm::MemoryBuffer API for reading files.

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.

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'.

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:

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:

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:

a = foo ((42, 92) + bar) (x);

when skimming through the code. By avoiding a space in a function, we avoid this misinterpretation.

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.

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:

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:

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.

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:

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:

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.