Clang 3.4 documentation

Clang 3.4 Release Notes

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Clang 3.4 Release Notes


This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 3.4. Here we describe the status of Clang in some detail, including major improvements from the previous release and new feature work. For the general LLVM release notes, see the LLVM documentation. All LLVM releases may be downloaded from the LLVM releases web site.

For more information about Clang or LLVM, including information about the latest release, please check out the main Clang Web Site or the LLVM Web Site.

Note that if you are reading this file from a Subversion checkout or the main Clang web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

What’s New in Clang 3.4?

Some of the major new features and improvements to Clang are listed here. Generic improvements to Clang as a whole or to its underlying infrastructure are described first, followed by language-specific sections with improvements to Clang’s support for those languages.

Last release which will build as C++98

This is expected to be the last release of Clang which compiles using a C++98 toolchain. We expect to start using some C++11 features in Clang starting after this release. That said, we are committed to supporting a reasonable set of modern C++ toolchains as the host compiler on all of the platforms. This will at least include Visual Studio 2012 on Windows, and Clang 3.1 or GCC 4.7.x on Mac and Linux. The final set of compilers (and the C++11 features they support) is not set in stone, but we wanted users of Clang to have a heads up that the next release will involve a substantial change in the host toolchain requirements.

Note that this change is part of a change for the entire LLVM project, not just Clang.

Major New Features

Improvements to Clang’s diagnostics

Clang’s diagnostics are constantly being improved to catch more issues, explain them more clearly, and provide more accurate source information about them. The improvements since the 3.3 release include:

  • -Wheader-guard warns on mismatches between the #ifndef and #define lines in a header guard.

    #ifndef multiple
    #define multi

    returns warning: ‘multiple’ is used as a header guard here, followed by #define of a different macro [-Wheader-guard]

  • -Wlogical-not-parentheses warns when a logical not (‘!’) only applies to the left-hand side of a comparison. This warning is part of -Wparentheses.

    int i1 = 0, i2 = 1;
    bool ret;
    ret = !i1 == i2;

    returns warning: logical not is only applied to the left hand side of this comparison [-Wlogical-not-parentheses]

  • Boolean increment, a deprecated feature, has own warning flag -Wdeprecated-increment-bool, and is still part of -Wdeprecated.

  • Clang errors on builtin enum increments and decrements in C++.

    enum A { A1, A2 };
    void test() {
        A a;

    returns error: cannot increment expression of enum type ‘A’

  • -Wloop-analysis now warns on for-loops which have the same increment or decrement in the loop header as the last statement in the loop.

    void foo(char *a, char *b, unsigned c) {
          for (unsigned i = 0; i < c; ++i) {
                a[i] = b[i];

    returns warning: variable ‘i’ is incremented both in the loop header and in the loop body [-Wloop-analysis]

  • -Wuninitialized now performs checking across field initializers to detect when one field in used uninitialized in another field initialization.

    class A {
      int x;
      int y;
      A() : x(y) {}

    returns warning: field ‘y’ is uninitialized when used here [-Wuninitialized]

  • Clang can detect initializer list use inside a macro and suggest parentheses if possible to fix.

  • Many improvements to Clang’s typo correction facilities, such as:

    • Adding global namespace qualifiers so that corrections can refer to shadowed or otherwise ambiguous or unreachable namespaces.
    • Including accessible class members in the set of typo correction candidates, so that corrections requiring a class name in the name specifier are now possible.
    • Allowing typo corrections that involve removing a name specifier.
    • In some situations, correcting function names when a function was given the wrong number of arguments, including situations where the original function name was correct but was shadowed by a lexically closer function with the same name yet took a different number of arguments.
    • Offering typo suggestions for ‘using’ declarations.
    • Providing better diagnostics and fixit suggestions in more situations when a ‘->’ was used instead of ‘.’ or vice versa.
    • Providing more relevant suggestions for typos followed by ‘.’ or ‘=’.
    • Various performance improvements when searching for typo correction candidates.
  • LeakSanitizer is an experimental memory leak detector which can be combined with AddressSanitizer.

New Compiler Flags

  • Clang no longer special cases -O4 to enable lto. Explicitly pass -flto to enable it.
  • Clang no longer fails on >= -O5. These flags are mapped to -O3 instead.
  • Command line “clang -O3 -flto a.c -c” and “clang -emit-llvm a.c -c” are no longer equivalent.
  • Clang now errors on unknown -m flags (-munknown-to-clang), unknown -f flags (-funknown-to-clang) and unknown options (-what-is-this).

C Language Changes in Clang

  • Added new checked arithmetic builtins for security critical applications.

C++ Language Changes in Clang

  • Fixed an ABI regression, introduced in Clang 3.2, which affected member offsets for classes inheriting from certain classes with tail padding. See PR16537.
  • Clang 3.4 supports the 2013-08-28 draft of the ISO WG21 SG10 feature test macro recommendations. These aim to provide a portable method to determine whether a compiler supports a language feature, much like Clang’s __has_feature macro.

C++1y Feature Support

Clang 3.4 supports all the features in the current working draft of the upcoming C++ standard, provisionally named C++1y. Support for the following major new features has been added since Clang 3.3:

  • Generic lambdas and initialized lambda captures.
  • Deduced function return types (auto f() { return 0; }).
  • Generalized constexpr support (variable mutation and loops).
  • Variable templates and static data member templates.
  • Use of ' as a digit separator in numeric literals.
  • Support for sized ::operator delete functions.

In addition, [[deprecated]] is now accepted as a synonym for Clang’s existing deprecated attribute.

Use -std=c++1y to enable C++1y mode.

OpenCL C Language Changes in Clang

  • OpenCL C “long” now always has a size of 64 bit, and all OpenCL C types are aligned as specified in the OpenCL C standard. Also, “char” is now always signed.

Internal API Changes

These are major API changes that have happened since the 3.3 release of Clang. If upgrading an external codebase that uses Clang as a library, this section should help get you past the largest hurdles of upgrading.

Wide Character Types

The ASTContext class now keeps track of two different types for wide character types: WCharTy and WideCharTy. WCharTy represents the built-in wchar_t type available in C++. WideCharTy is the type used for wide character literals; in C++ it is the same as WCharTy, but in C99, where wchar_t is a typedef, it is an integer type.

Static Analyzer

The static analyzer has been greatly improved. This impacts the overall analyzer quality and reduces a number of false positives. In particular, this release provides enhanced C++ support, reasoning about initializer lists, zeroing constructors, noreturn destructors and modeling of destructor calls on calls to delete.

Clang Format

Clang now includes a new tool clang-format which can be used to automatically format C, C++ and Objective-C source code. clang-format automatically chooses linebreaks and indentation and can be easily integrated into editors, IDEs and version control systems. It supports several pre-defined styles as well as precise style control using a multitude of formatting options. clang-format itself is just a thin wrapper around a library which can also be used directly from code refactoring and code translation tools. More information can be found on Clang Format’s site.

Windows Support

  • clang-cl provides a new driver mode that is designed for compatibility with Visual Studio’s compiler, cl.exe. This driver mode makes Clang accept the same kind of command-line options as cl.exe. The installer will attempt to expose clang-cl in any Visual Studio installations on the system as a Platform Toolset, e.g. “LLVM-vs2012”. clang-cl targets the Microsoft ABI by default. Please note that this driver mode and compatibility with the MS ABI is highly experimental.

Python Binding Changes

The following methods have been added:

Additional Information

A wide variety of additional information is available on the Clang web page. The web page contains versions of the API documentation which are up-to-date with the Subversion revision of the source code. You can access versions of these documents specific to this release by going into the “clang/docs/” directory in the Clang tree.

If you have any questions or comments about Clang, please feel free to contact us via the mailing list.

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