LLVM Coding Standards



While this document may provide guidance for some mechanical formatting issues, whitespace, or other “microscopic details”, these are not fixed standards. Always follow the golden rule:



There are some conventions that are not uniformly followed in the code base (e.g. the naming convention). This is because they are relatively new, and a lot of code was written before they were put in place. Our long term goal is for the entire codebase to follow the convention, but we explicitly do not want patches that do large-scale reformatting of existing code. On the other hand, it is reasonable to rename the methods of a class if you’re about to change it in some other way. Just do the reformatting as a separate commit from the functionality change.

The ultimate goal of these guidelines is to increase the readability and maintainability of our common source base. If you have suggestions for topics to be included, please mail them to Chris.

Languages, Libraries, and Standards

Most source code in LLVM and other LLVM projects using these coding standards is C++ code. There are some places where C code is used either due to environment restrictions, historical restrictions, or due to third-party source code imported into the tree. Generally, our preference is for standards conforming, modern, and portable C++ code as the implementation language of choice.

C++ Standard Versions

LLVM, Clang, and LLD are currently written using C++11 conforming code, although we restrict ourselves to features which are available in the major toolchains supported as host compilers. The LLDB project is even more aggressive in the set of host compilers supported and thus uses still more features. Regardless of the supported features, code is expected to (when reasonable) be standard, portable, and modern C++11 code. We avoid unnecessary vendor-specific extensions, etc.

C++ Standard Library

Use the C++ standard library facilities whenever they are available for a particular task. LLVM and related projects emphasize and rely on the standard library facilities for as much as possible. Common support libraries providing functionality missing from the standard library for which there are standard interfaces or active work on adding standard interfaces will often be implemented in the LLVM namespace following the expected standard interface.

There are some exceptions such as the standard I/O streams library which are avoided. Also, there is much more detailed information on these subjects in the LLVM Programmer’s Manual.

Supported C++11 Language and Library Features

While LLVM, Clang, and LLD use C++11, not all features are available in all of the toolchains which we support. The set of features supported for use in LLVM is the intersection of those supported in the minimum requirements described in the Getting Started with the LLVM System page, section Software. The ultimate definition of this set is what build bots with those respective toolchains accept. Don’t argue with the build bots. However, we have some guidance below to help you know what to expect.

Each toolchain provides a good reference for what it accepts:

In most cases, the MSVC list will be the dominating factor. Here is a summary of the features that are expected to work. Features not on this list are unlikely to be supported by our host compilers.

  • Rvalue references: N2118
    • But not Rvalue references for *this or member qualifiers (N2439)
  • Static assert: N1720
  • auto type deduction: N1984, N1737
  • Trailing return types: N2541
  • Lambdas: N2927
    • But not lambdas with default arguments.
  • decltype: N2343
  • Nested closing right angle brackets: N1757
  • Extern templates: N1987
  • nullptr: N2431
  • Strongly-typed and forward declarable enums: N2347, N2764
  • Local and unnamed types as template arguments: N2657
  • Range-based for-loop: N2930
    • But {} are required around inner do {} while() loops. As a result, {} are required around function-like macros inside range-based for loops.
  • override and final: N2928, N3206, N3272
  • Atomic operations and the C++11 memory model: N2429
  • Variadic templates: N2242
  • Explicit conversion operators: N2437
  • Defaulted and deleted functions: N2346
  • Initializer lists: N2627
  • Delegating constructors: N1986
  • Default member initializers (non-static data member initializers): N2756
    • Feel free to use these wherever they make sense and where the = syntax is allowed. Don’t use braced initialization syntax.

The supported features in the C++11 standard libraries are less well tracked, but also much greater. Most of the standard libraries implement most of C++11’s library. The most likely lowest common denominator is Linux support. For libc++, the support is just poorly tested and undocumented but expected to be largely complete. YMMV. For libstdc++, the support is documented in detail in the libstdc++ manual. There are some very minor missing facilities that are unlikely to be common problems, and there are a few larger gaps that are worth being aware of:

  • Not all of the type traits are implemented
  • No regular expression library.
  • While most of the atomics library is well implemented, the fences are missing. Fortunately, they are rarely needed.
  • The locale support is incomplete.

Other than these areas you should assume the standard library is available and working as expected until some build bot tells you otherwise. If you’re in an uncertain area of one of the above points, but you cannot test on a Linux system, your best approach is to minimize your use of these features, and watch the Linux build bots to find out if your usage triggered a bug. For example, if you hit a type trait which doesn’t work we can then add support to LLVM’s traits header to emulate it.

Other Languages

Any code written in the Go programming language is not subject to the formatting rules below. Instead, we adopt the formatting rules enforced by the gofmt tool.

Go code should strive to be idiomatic. Two good sets of guidelines for what this means are Effective Go and Go Code Review Comments.

Mechanical Source Issues

Source Code Formatting


Comments are one critical part of readability and maintainability. Everyone knows they should comment their code, and so should you. When writing comments, write them as English prose, which means they should use proper capitalization, punctuation, etc. Aim to describe what the code is trying to do and why, not how it does it at a micro level. Here are a few critical things to document:

File Headers


// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
/// \file
/// 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).




The main body is a doxygen comment (identified by the /// comment marker instead of the usual //) describing the purpose of the file. The first sentence (or a passage beginning with \brief) is used as an abstract. Any additional information should be separated by a blank line. 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


Method information



Comment Formatting

In general, prefer C++ style comments (// for normal comments, /// for doxygen documentation 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:




  4. When documenting the significance of constants used as actual parameters in a call. This is most helpful for bool parameters, or passing 0 or nullptr. Typically you add the formal parameter name, which ought to be meaningful. For example, it’s not clear what the parameter means in this call:


    An in-line C-style comment makes the intent obvious:


Commenting out large blocks of code is discouraged, but if you really have to do this (for documentation purposes or as a suggestion for debug printing), use #if 0 and #endif. These nest properly and are better behaved in general than C style comments.

Doxygen Use in Documentation Comments

Use the \file command to turn the standard file header into a file-level comment.

Include descriptive paragraphs for all public interfaces (public classes, member and non-member functions). Don’t just restate the information that can be inferred from the API name. The first sentence (or a paragraph beginning with \brief) is used as an abstract. Try to use a single sentence as the \brief adds visual clutter. Put detailed discussion into separate paragraphs.

To refer to parameter names inside a paragraph, use the \p name command. Don’t use the \arg name command since it starts a new paragraph that contains documentation for the parameter.

Wrap non-inline code examples in \code ... \endcode.

To document a function parameter, start a new paragraph with the \param name command. If the parameter is used as an out or an in/out parameter, use the \param [out] name or \param [in,out] name command, respectively.

To describe function return value, start a new paragraph with the \returns command.

A minimal documentation comment:

/// Sets the xyzzy property to \p Baz.
void setXyzzy(bool Baz);

A documentation comment that uses all Doxygen features in a preferred way:

/// Does foo and bar.
/// Does not do foo the usual way if \p Baz is true.
/// Typical usage:
/// \code
///   fooBar(false, "quux", Res);
/// \endcode
/// \param Quux kind of foo to do.
/// \param [out] Result filled with bar sequence on foo success.
/// \returns true on success.
bool fooBar(bool Baz, StringRef Quux, std::vector<int> &Result);

Don’t duplicate the documentation comment in the header file and in the implementation file. Put the documentation comments for public APIs into the header file. Documentation comments for private APIs can go to the implementation file. In any case, implementation files can include additional comments (not necessarily in Doxygen markup) to explain implementation details as needed.

Don’t duplicate function or class name at the beginning of the comment. For humans it is obvious which function or class is being documented; automatic documentation processing tools are smart enough to bind the comment to the correct declaration.


// In Something.h:

/// Something - An abstraction for some complicated thing.
class Something {
  /// fooBar - Does foo and bar.
  void fooBar();

// In Something.cpp:

/// fooBar - Does foo and bar.
void Something::fooBar() { ... }


// In Something.h:

/// An abstraction for some complicated thing.
class Something {
  /// Does foo and bar.
  void fooBar();

// In Something.cpp:

// Builds a B-tree in order to do foo.  See paper by...
void Something::fooBar() { ... }

It is not required to use additional Doxygen features, but sometimes it might be a good idea to do so.


  • adding comments to any narrow namespace containing a collection of related functions or types;
  • using top-level groups to organize a collection of related functions at namespace scope where the grouping is smaller than the namespace;
  • using member groups and additional comments attached to member groups to organize within a class.

For example:

class Something {
  /// \name Functions that do Foo.
  /// @{
  void fooBar();
  void fooBaz();
  /// @}

#include Style


  1. Main Module Header
  2. Local/Private Headers
  3. LLVM project/subproject headers (clang/..., lldb/..., llvm/..., etc)
  4. System #includes

and each category should be sorted lexicographically by the full path.


LLVM project and subproject headers should be grouped from most specific to least specific, for the same reasons described above. For example, LLDB depends on both clang and LLVM, and clang depends on LLVM. So an LLDB source file should include lldb headers first, followed by clang headers, followed by llvm headers, to reduce the possibility (for example) of an LLDB header accidentally picking up a missing include due to the previous inclusion of that header in the main source file or some earlier header file. clang should similarly include its own headers before including llvm headers. This rule applies to all LLVM subprojects.

Source Code Width





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.


Do not commit changes that include trailing whitespace. If you find trailing whitespace in a file, do not remove it unless you’re otherwise changing that line of code. Some common editors will automatically remove trailing whitespace when saving a file which causes unrelated changes to appear in diffs and commits.

Indent Code Consistently

Okay, in your first year of programming you were told that indentation is important. If you didn’t believe and internalize this then, now is the time. Just do it. With the introduction of C++11, there are some new formatting challenges that merit some suggestions to help have consistent, maintainable, and tool-friendly formatting and indentation.

Format Lambdas Like Blocks Of Code

When formatting a multi-line lambda, format it like a block of code, that’s what it is. If there is only one multi-line lambda in a statement, and there are no expressions lexically after it in the statement, drop the indent to the standard two space indent for a block of code, as if it were an if-block opened by the preceding part of the statement:

std::sort(foo.begin(), foo.end(), [&](Foo a, Foo b) -> bool {
  if (a.blah < b.blah)
    return true;
  if (a.baz < b.baz)
    return true;
  return a.bam < b.bam;

To take best advantage of this formatting, if you are designing an API which accepts a continuation or single callable argument (be it a functor, or a std::function), it should be the last argument if at all possible.

If there are multiple multi-line lambdas in a statement, or there is anything interesting after the lambda in the statement, indent the block two spaces from the indent of the []:

           [] (PHINode *PN) {
             // process phis...
           [] (SelectInst *SI) {
             // process selects...
           [] (LoadInst *LI) {
             // process loads...
           [] (AllocaInst *AI) {
             // process allocas...
Braced Initializer Lists

With C++11, there are significantly more uses of braced lists to perform initialization. These allow you to easily construct aggregate temporaries in expressions among other niceness. They now have a natural way of ending up nested within each other and within function calls in order to build up aggregates (such as option structs) from local variables. To make matters worse, we also have many more uses of braces in an expression context that are not performing initialization.

The historically common formatting of braced initialization of aggregate variables does not mix cleanly with deep nesting, general expression contexts, function arguments, and lambdas. We suggest new code use a simple rule for formatting braced initialization lists: act as-if the braces were parentheses in a function call. The formatting rules exactly match those already well understood for formatting nested function calls. Examples:

foo({a, b, c}, {1, 2, 3});

llvm::Constant *Mask[] = {
    llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 0),
    llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 1),
    llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 2)};

This formatting scheme also makes it particularly easy to get predictable, consistent, and automatic formatting with tools like Clang Format.

Language and Compiler Issues

Treat Compiler Warnings Like Errors

If your code has compiler warnings in it, something is wrong — you aren’t casting values correctly, you have “questionable” constructs in your code, or you are doing something legitimately wrong. Compiler warnings can cover up legitimate errors in output and make dealing with a translation unit difficult.


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())) {


Write Portable Code

In almost all cases, it is possible and within reason to write completely portable code. If there are cases where it isn’t possible to write portable code, isolate it behind a well defined (and well documented) interface.

In practice, this means that you shouldn’t assume much about the host compiler (and Visual Studio tends to be the lowest common denominator). If advanced features are used, they should only be an implementation detail of a library which has a simple exposed API, and preferably be buried in libSystem.

Do not use RTTI or Exceptions

In an effort to reduce code and executable size, LLVM does not use RTTI (e.g. dynamic_cast<>;) or exceptions. These two language features violate the general C++ principle of “you only pay for what you use”, causing executable bloat even if exceptions are never used in the code base, or if RTTI is never used for a class. Because of this, we turn them off globally in the code.


Do not use Static Constructors






Use of class and struct Keywords


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 (e.g., MSVC). This can lead to problems at link time.

  • All declarations and definitions of a given class or struct must use the same keyword. For example:
class Foo;

// Breaks mangling in MSVC.
struct Foo { int Data; };
  • As a rule of thumb, struct should be kept to structures where all members are declared public.
// Foo feels like a class... this is strange.
struct Foo {
  int Data;
  Foo() : Data(0) { }
  int getData() const { return Data; }
  void setData(int D) { Data = D; }

// Bar isn't POD, but it does look like a struct.
struct Bar {
  int Data;
  Bar() : Data(0) { }

Do not use Braced Initializer Lists to Call a Constructor

In C++11 there is a “generalized initialization syntax” which allows calling constructors using braced initializer lists. Do not use these to call constructors with any interesting logic or if you care that you’re calling some particular constructor. Those should look like function calls using parentheses rather than like aggregate initialization. Similarly, if you need to explicitly name the type and call its constructor to create a temporary, don’t use a braced initializer list. Instead, use a braced initializer list (without any type for temporaries) when doing aggregate initialization or something notionally equivalent. Examples:

class Foo {
  // Construct a Foo by reading data from the disk in the whizbang format, ...
  Foo(std::string filename);

  // Construct a Foo by looking up the Nth element of some global data ...
  Foo(int N);

  // ...

// The Foo constructor call is very deliberate, no braces.
std::fill(foo.begin(), foo.end(), Foo("name"));

// The pair is just being constructed like an aggregate, use braces.
bar_map.insert({my_key, my_value});

If you use a braced initializer list when initializing a variable, use an equals before the open curly brace:

int data[] = {0, 1, 2, 3};

Use auto Type Deduction to Make Code More Readable

Some are advocating a policy of “almost always auto” in C++11, however LLVM uses a more moderate stance. Use auto if and only if it makes the code more readable or easier to maintain. Don’t “almost always” use auto, but do use auto with initializers like cast<Foo>(...) or other places where the type is already obvious from the context. Another time when auto works well for these purposes is when the type would have been abstracted away anyways, often behind a container’s typedef such as std::vector<T>::iterator.

Beware unnecessary copies with auto

The convenience of auto makes it easy to forget that its default behavior is a copy. Particularly in range-based for loops, careless copies are expensive.

As a rule of thumb, use auto & unless you need to copy the result, and use auto * when copying pointers.

// Typically there's no reason to copy.
for (const auto &Val : Container) { observe(Val); }
for (auto &Val : Container) { Val.change(); }

// Remove the reference if you really want a new copy.
for (auto Val : Container) { Val.change(); saveSomewhere(Val); }

// Copy pointers, but make it clear that they're pointers.
for (const auto *Ptr : Container) { observe(*Ptr); }
for (auto *Ptr : Container) { Ptr->change(); }

Beware of non-determinism due to ordering of pointers

In general, there is no relative ordering among pointers. As a result, when unordered containers like sets and maps are used with pointer keys the iteration order is undefined. Hence, iterating such containers may result in non-deterministic code generation. While the generated code might not necessarily be “wrong code”, this non-determinism might result in unexpected runtime crashes or simply hard to reproduce bugs on the customer side making it harder to debug and fix.

As a rule of thumb, in case an ordered result is expected, remember to sort an unordered container before iteration. Or use ordered containers like vector/MapVector/SetVector if you want to iterate pointer keys.

Beware of non-deterministic sorting order of equal elements

std::sort uses a non-stable sorting algorithm in which the order of equal elements is not guaranteed to be preserved. Thus using std::sort for a container having equal elements may result in non-determinstic behavior. To uncover such instances of non-determinism, LLVM has introduced a new llvm::sort wrapper function. For an EXPENSIVE_CHECKS build this will randomly shuffle the container before sorting. As a rule of thumb, always make sure to use llvm::sort instead of std::sort.

Style Issues

The High-Level Issues

Self-contained Headers

Header files should be self-contained (compile on their own) and end in .h. Non-header files that are meant for inclusion should end in .inc and be used sparingly.

All header files should be self-contained. Users and refactoring tools should not have to adhere to special conditions to include the header. Specifically, a header should have header guards and include all other headers it needs.

There are rare cases where a file designed to be included is not self-contained. These are typically intended to be included at unusual locations, such as the middle of another file. They might not use header guards, and might not include their prerequisites. Name such files with the .inc extension. Use sparingly, and prefer self-contained headers when possible.

In general, a header 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 header have been properly added to the header itself, and are not implicit. System headers should be included after user headers for a translation unit.

Library Layering

A directory of header files (for example include/llvm/Foo) defines a library (Foo). Dependencies between libraries are defined by the LLVMBuild.txt file in their implementation (lib/Foo). One library (both its headers and implementation) should only use things from the libraries listed in its dependencies.

Some of this constraint can be enforced by classic Unix linkers (Mac & Windows linkers, as well as lld, do not enforce this constraint). A Unix linker searches left to right through the libraries specified on its command line and never revisits a library. In this way, no circular dependencies between libraries can exist.

This doesn’t fully enforce all inter-library dependencies, and importantly doesn’t enforce header file circular dependencies created by inline functions. A good way to answer the “is this layered correctly” would be to consider whether a Unix linker would succeed at linking the program if all inline functions were defined out-of-line. (& for all valid orderings of dependencies - since linking resolution is linear, it’s possible that some implicit dependencies can sneak through: A depends on B and C, so valid orderings are “C B A” or “B C A”, in both cases the explicit dependencies come before their use. But in the first case, B could still link successfully if it implicitly depended on C, or the opposite in the second case)

#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 #includeing speeds up compilation.

It is easy to try to go too overboard on this recommendation, however. You must include all of the header files that you are using — you can include them either directly or indirectly through another header file. To make sure that you don’t accidentally forget to include a header file in your module header, make sure to include your module header first in the implementation file (as mentioned above). This way there won’t be any hidden dependencies that you’ll find out about later.

Keep “Internal” Headers Private





Use Early Exits and continue to Simplify Code

When reading code, keep in mind how much state and how many previous decisions have to be remembered by the reader to understand a block of code. Aim to reduce indentation where possible when it doesn’t make it more difficult to understand the code. One great way to do this is by making use of early exits and the continue keyword in long loops. As an example of using an early exit from a function, consider this “bad” code:

Value *doSomething(Instruction *I) {
  if (!I->isTerminator() &&
      I->hasOneUse() && doOtherThing(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) {
  if (I->isTerminator())
    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 (!doOtherThing(I))
    return 0;

  ... some long code ....


for (Instruction &I : BB) {
  if (auto *BO = dyn_cast<BinaryOperator>(&I)) {
    Value *LHS = BO->getOperand(0);
    Value *RHS = BO->getOperand(1);
    if (LHS != RHS) {


for (Instruction &I : BB) {
  auto *BO = dyn_cast<BinaryOperator>(&I);
  if (!BO) continue;

  Value *LHS = BO->getOperand(0);
  Value *RHS = BO->getOperand(1);
  if (LHS == RHS) continue;



Don’t use else after a return


case 'J': {
  if (Signed) {
    Type = Context.getsigjmp_bufType();
    if (Type.isNull()) {
      Error = ASTContext::GE_Missing_sigjmp_buf;
      return QualType();
    } else {
  } else {
    Type = Context.getjmp_bufType();
    if (Type.isNull()) {
      Error = ASTContext::GE_Missing_jmp_buf;
      return QualType();
    } else {

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();

Or better yet (in this case) as:

case 'J':
  if (Signed)
    Type = Context.getsigjmp_bufType();
    Type = Context.getjmp_bufType();

  if (Type.isNull()) {
    Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
    return QualType();


Turn Predicate Loops into Predicate Functions


bool FoundFoo = false;
for (unsigned I = 0, E = BarList.size(); I != E; ++I)
  if (BarList[I]->isFoo()) {
    FoundFoo = true;

if (FoundFoo) {


/// \returns true if the specified list has an element that is a foo.
static bool containsFoo(const std::vector<Bar*> &List) {
  for (unsigned I = 0, E = List.size(); I != E; ++I)
    if (List[I]->isFoo())
      return true;
  return false;

if (containsFoo(BarList)) {


The Low-Level Issues

Name Types, Functions, Variables, and Enumerators Properly








    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()). Classes that provide multiple iterators should add a singular prefix to begin() and end() (e.g. global_begin() and use_begin()).

Here are some examples of good and bad names:

class VehicleMaker {
  Factory<Tire> Factory;      // Better.
                              // kind of factories.

Vehicle makeVehicle(VehicleType Type) {
  Tire Tmp1 = M.makeTire();               // Bad -- 'Tmp1' provides no information.
  Light Headlight = M.makeLight("head");  // Good -- descriptive.

Assert Liberally

Use the “assert” macro to its fullest. Check all of your preconditions and assumptions, you never know when a bug (not necessarily even yours) might be caught early by an assertion, which reduces debugging time dramatically. The “<cassert>” header file is probably already included by the header files you are using, so it doesn’t cost anything to use it.


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!");





You get the idea.

In the past, asserts were used to indicate a piece of code that should not be reached. These were typically of the form:

assert(0 && "Invalid radix for integer literal");

This has a few issues, the main one being that some compilers might not understand the assertion, or warn about a missing return in builds where assertions are compiled out.

Today, we have something much better: llvm_unreachable:

llvm_unreachable("Invalid radix for integer literal");

When assertions are enabled, this will print the message if it’s ever reached and then exit the program. When assertions are disabled (i.e. in release builds), llvm_unreachable becomes a hint to compilers to skip generating code for this branch. If the compiler does not support this, it will fall back to the “abort” implementation.

Neither assertions or llvm_unreachable will abort the program on a release build. If the error condition can be triggered by user input then the recoverable error mechanism described in LLVM Programmer’s Manual should be used instead. In cases where this is not practical, report_fatal_error may be used.

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();

bool NewToSet = Myset.insert(Value);




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;”.



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.

Provide a Virtual Method Anchor for Classes in Headers


Don’t use default labels in fully covered switches over enumerations

-Wswitch warns if a switch, without a default label, over an enumeration does not cover every enumeration value. If you write a default label on a fully covered switch over an enumeration then the -Wswitch warning won’t fire when new elements are added to that enumeration. To help avoid adding these kinds of defaults, Clang has the warning -Wcovered-switch-default which is off by default but turned on when building LLVM with a version of Clang that supports the warning.

A knock-on effect of this stylistic requirement is that when building LLVM with GCC you may get warnings related to “control may reach end of non-void function” if you return from each case of a covered switch-over-enum because GCC assumes that the enum expression may take any representable value, not just those of individual enumerators. To suppress this warning, use llvm_unreachable after the switch.

Use range-based for loops wherever possible

The introduction of range-based for loops in C++11 means that explicit manipulation of iterators is rarely necessary. We use range-based for loops wherever possible for all newly added code. For example:

BasicBlock *BB = ...
for (Instruction &I : *BB)
  ... use I ...

Don’t evaluate end() every time through a loop

In cases where range-based for loops can’t be used and it is necessary to write an explicit iterator-based loop, pay close attention to whether end() is re-evaluted on each loop iteration. One common mistake is to write a loop in this style:

BasicBlock *BB = ...
for (auto 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 = ...
  ... 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.



#include <iostream> is Forbidden





Use raw_ostream



Avoid std::endl


std::cout << std::endl;

Most of the time, you probably have no reason to flush the output stream, so it’s better to use a literal '\n'.

Don’t use inline when defining a function in a class definition

A member function defined in a class definition is implicitly inline, so don’t put the inline keyword in this case.


class Foo {
  inline void bar() {
    // ...


class Foo {
  void bar() {
    // ...

Microscopic Details


Spaces Before Parentheses


if (X) ...
for (I = 0; I != 100; ++I) ...
while (LLVMRocks) ...


A = foo(42, 92) + bar(X);

and this is bad:

if(X) ...
while(LLVMRocks) ...

somefunc (42);



A = foo ((42, 92) + bar) (X);


Prefer Preincrement


The semantics of postincrement include making a copy of the value being incremented, returning it, and then preincrementing the “work value”. For primitive types, this isn’t a big deal. But for iterators, it can be a huge issue (for example, some iterators contains stack and set objects in them… copying an iterator could invoke the copy ctor’s of these as well). In general, get in the habit of always using preincrement, and you won’t have a problem.

Namespace Indentation

In general, we strive to reduce indentation wherever possible. This is useful because we want code to fit into 80 columns without wrapping horribly, but also because it makes it easier to understand the code. To facilitate this and avoid some insanely deep nesting on occasion, don’t indent namespaces. If it helps readability, feel free to add a comment indicating what namespace is being closed by a }. For example:

namespace llvm {
namespace knowledge {

/// This class represents things that Smith can have an intimate
class Grokable {
  explicit Grokable() { ... }
  virtual ~Grokable() = 0;



} // end namespace knowledge
} // end namespace llvm

Feel free to skip the closing comment when the namespace being closed is obvious for any reason. For example, the outer-most namespace in a header file is rarely a source of confusion. But namespaces both anonymous and named in source files that are being closed half way through the file probably could use clarification.

Anonymous Namespaces

After talking about namespaces in general, you may be wondering about anonymous namespaces in particular. Anonymous namespaces are a great language feature that tells the C++ compiler that the contents of the namespace are only visible within the current translation unit, allowing more aggressive optimization and eliminating the possibility of symbol name collisions. Anonymous namespaces are to C++ as “static” is to C functions and global variables. While “static” is available in C++, anonymous namespaces are more general: they can make entire classes private to a file.



namespace {
class StringSort {
  bool operator<(const char *RHS) const;
} // end anonymous namespace

static void runHelper() {


This is bad:

namespace {

class StringSort {
  bool operator<(const char *RHS) const;

void runHelper() {


} // end anonymous namespace

This is bad specifically because if you’re looking at “runHelper” in the middle of a large C++ file, that you have no immediate way to tell if it is local to the file. When it is marked static explicitly, this is immediately obvious. Also, there is no reason to enclose the definition of “operator<” in the namespace just because it was declared there.

See Also

A lot of these comments and recommendations have been culled from 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.

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