Clang 3.6 documentation

Clang 3.6 Release Notes

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Clang 3.6 Release Notes

Written by the LLVM Team


This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 3.6. 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 please see the Clang Web Site or the LLVM Web Site.

What’s New in Clang 3.6?

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.

Major New Features

  • The __has_attribute built-in macro no longer queries for attributes across multiple attribute syntaxes (GNU, C++11, __declspec, etc). Instead, it only queries GNU-style attributes. With the addition of __has_cpp_attribute and __has_declspec_attribute, this allows for more precise coverage of attribute syntax querying.
  • clang-format now supports formatting Java code.

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 3.5 release include:

  • Smarter typo correction. Clang now tries a bit harder to give a usable suggestion in more cases, and can now successfully recover in more situations where the suggestion changes how an expression is parsed.

New Compiler Flags

The -fpic option now uses small pic on PowerPC.

The __EXCEPTIONS macro

__EXCEPTIONS is now defined when landing pads are emitted, not when C++ exceptions are enabled. The two can be different in Objective-C files: If C++ exceptions are disabled but Objective-C exceptions are enabled, landing pads will be emitted. Clang 3.6 is switching the behavior of __EXCEPTIONS. Clang 3.5 confusingly changed the behavior of has_feature(cxx_exceptions), which used to be set if landing pads were emitted, but is now set if C++ exceptions are enabled. So there are 3 cases:

Clang before 3.5:
__EXCEPTIONS is set if C++ exceptions are enabled, cxx_exceptions enabled if C++ or ObjC exceptions are enabled
Clang 3.5:
__EXCEPTIONS is set if C++ exceptions are enabled, cxx_exceptions enabled if C++ exceptions are enabled
Clang 3.6:
__EXCEPTIONS is set if C++ or ObjC exceptions are enabled, cxx_exceptions enabled if C++ exceptions are enabled

To reliably test if C++ exceptions are enabled, use __EXCEPTIONS && __has_feature(cxx_exceptions), else things won’t work in all versions of Clang in Objective-C++ files.

New Pragmas in Clang

Clang now supports the #pragma unroll and #pragma nounroll directives to specify loop unrolling optimization hints. Placed just prior to the desired loop, #pragma unroll directs the loop unroller to attempt to fully unroll the loop. The pragma may also be specified with a positive integer parameter indicating the desired unroll count: #pragma unroll _value_. The unroll count parameter can be optionally enclosed in parentheses. The directive #pragma nounroll indicates that the loop should not be unrolled. These unrolling hints may also be expressed using the #pragma clang loop directive. See the Clang language extensions for details.

Windows Support

  • Many, many bug fixes.
  • Clang can now self-host using the msvc environment on x86 and x64 Windows. This means that Microsoft C++ ABI is more or less feature-complete, minus exception support.
  • Added more MSVC compatibility hacks, such as allowing more lookup into dependent bases of class templates when there is a known template pattern. As a result, applications using Active Template Library (ATL) or Windows Runtime Library (WRL) headers should compile correctly.
  • Added support for the Visual C++ __super keyword.
  • Added support for MSVC’s __vectorcall calling convention, which is used in the upcoming Visual Studio 2015 STL.
  • Added basic support for DWARF debug information in COFF files.

C Language Changes in Clang

  • The default language mode for C compilations with Clang has been changed from C99 with GNU extensions to C11 with GNU extensions. C11 is largely backwards-compatible with C99, but if you want to restore the former behavior you can do so with the -std=gnu99 flag.

C11 Feature Support

  • Clang now provides an implementation of the standard C11 header <stdatomic.h>.

C++ Language Changes in Clang

  • An upcoming change to C++ <>_ changes the semantics of certain deductions of auto from a braced initializer list. Following the intent of the C++ committee, this change will be applied to our C++11 and C++14 modes as well as our experimental C++17 mode. Clang 3.6 does not yet implement this change, but to provide a transition period, it warns on constructs whose meaning will change. The fix in all cases is to add an = prior to the left brace.
  • Clang now supports putting identical constructors and destructors in the C5/D5 comdat, reducing code duplication.
  • Clang will put individual .init_array/.ctors sections in comdats, reducing code duplication and speeding up startup.

C++17 Feature Support

Clang has experimental support for some proposed C++1z (tentatively, C++17) features. This support can be enabled using the -std=c++1z flag.

New in Clang 3.6 is support for:

  • Fold expressions
  • u8 character literals
  • Nested namespace definitions: namespace A::B { ... } as a shorthand for namespace A { namespace B { ... } }
  • Attributes for namespaces and enumerators
  • Constant evaluation for all non-type template arguments

Note that these features may be changed or removed in future Clang releases without notice.

Support for for (identifier : range) as a synonym for for (auto &&identifier : range) has been removed as it is no longer currently considered for C++17.

For more details on C++ feature support, see the C++ status page.

OpenMP Language Changes in Clang

Clang 3.6 contains codegen for many individual OpenMP pragmas, but combinations are not completed yet. We plan to continue codegen code drop aiming for completion in 3.7. Please see this link for up-to-date status <>_. LLVM’s OpenMP runtime library, originally developed by Intel, has been modified to work on ARM, PowerPC, as well as X86. The Runtime Library’s compatibility with GCC 4.9 is improved - missed entry points added, barrier and fork/join code improved, one more type of barrier enabled. Support for ppc64le architecture is now available and automatically detected when using cmake system. Using makefile the new “ppc64le” arch type is available. Contributors to this work include AMD, Argonne National Lab., IBM, Intel, Texas Instruments, University of Houston and many others.

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 version 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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