Clang 3.9 documentation

Clang 3.9 Release Notes

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Clang 3.9 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 3.9. 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 3.9?

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

  • Clang will no longer pass --build-id by default to the linker. In modern linkers that is a relatively expensive option. It can be passed explicitly with -Wl,--build-id. To have clang always pass it, build clang with -DENABLE_LINKER_BUILD_ID.
  • On Itanium ABI targets, attribute abi_tag is now supported for compatibility with GCC. Clang’s implementation of abi_tag is mostly compatible with GCC ABI version 10.

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 3.8 release include:

  • -Wcomma is a new warning to show most uses of the builtin comma operator.
  • -Wfloat-conversion has two new sub-warnings to give finer grain control for floating point to integer conversion warnings.
    • -Wfloat-overflow-conversion detects when a constant floating point value is converted to an integer type and will overflow the target type.
    • -Wfloat-zero-conversion detects when a non-zero floating point value is converted to a zero integer value.

Attribute Changes in Clang

  • The nodebug attribute may now be applied to static, global, and local variables (but not parameters or non-static data members). This will suppress all debugging information for the variable (and its type, if there are no other uses of the type).

Windows Support

TLS is enabled for Cygwin and defaults to -femulated-tls.

Proper support, including correct mangling and overloading, added for MS-specific “__unaligned” type qualifier.

clang-cl now has limited support for the precompiled header flags /Yc, /Yu, and /Fp. If the precompiled header is passed on the compile command with /FI, then the precompiled header flags are honored. But if the precompiled header is included by an #include <stdafx.h> in each source file instead of by a /FIstdafx.h flag, these flag continue to be ignored.

clang-cl has a new flag, /imsvc <dir>, for adding a directory to the system include search path (where warnings are disabled by default) without having to set %INCLUDE%.

C Language Changes in Clang

The -faltivec and -maltivec flags no longer silently include altivec.h on Power platforms.

RenderScript support has been added to the frontend and enabled by the ‘-x renderscript’ option or the ‘.rs’ file extension.

C++ Language Changes in Clang

  • Clang now enforces the rule that a using-declaration cannot name an enumerator of a scoped enumeration.

    namespace Foo { enum class E { e }; }
    namespace Bar {
      using Foo::E::e; // error
      constexpr auto e = Foo::E::e; // ok
  • Clang now enforces the rule that an enumerator of an unscoped enumeration declared at class scope can only be named by a using-declaration in a derived class.

    class Foo { enum E { e }; }
    using Foo::e; // error
    static constexpr auto e = Foo::e; // ok

C++1z Feature Support

Clang’s experimental support for the upcoming C++1z standard can be enabled with -std=c++1z. Changes to C++1z features since Clang 3.8:

  • The [[fallthrough]], [[nodiscard]], and [[maybe_unused]] attributes are supported in C++11 onwards, and are largely synonymous with Clang’s existing attributes [[clang::fallthrough]], [[gnu::warn_unused_result]], and [[gnu::unused]]. Use -Wimplicit-fallthrough to warn on unannotated fallthrough within switch statements.

  • In C++1z mode, aggregate initialization can be performed for classes with base classes:

    struct A { int n; };
    struct B : A { int x, y; };
    B b = { 1, 2, 3 }; // b.n == 1, b.x == 2, b.y == 3
  • The range in a range-based for statement can have different types for its begin and end iterators. This is permitted as an extension in C++11 onwards.

  • Lambda-expressions can explicitly capture *this (to capture the surrounding object by copy). This is permitted as an extension in C++11 onwards.

  • Objects of enumeration type can be direct-list-initialized from a value of the underlying type. E{n} is equivalent to E(n), except that it implies a check for a narrowing conversion.

  • Unary fold-expressions over an empty pack are now rejected for all operators other than &&, ||, and ,.

OpenCL C Language Changes in Clang

Clang now has support for all OpenCL 2.0 features. In particular, the following features have been completed since the previous release:

  • Pipe builtin functions (s6.13.16.2-4).
  • Dynamic parallelism support via the enqueue_kernel Clang builtin function, as well as the kernel query functions from s6.13.17.6.
  • Address space conversion functions to_{global/local/private}.
  • nosvm attribute support.
  • Improved diagnostic and generation of Clang Blocks used in OpenCL kernel code.
  • opencl_unroll_hint pragma.

Several miscellaneous improvements have been made:

  • Supported extensions are now part of the target representation to give correct diagnostics for unsupported target features during compilation. For example, when compiling for a target that does not support the double precision floating point extension, Clang will give an error when encountering the cl_khr_fp64 pragma. Several missing extensions were added covering up to and including OpenCL 2.0.

  • Clang now comes with the OpenCL standard headers declaring builtin types and functions up to and including OpenCL 2.0 in lib/Headers/opencl-c.h. By default, Clang will not include this header. It can be included either using the regular -I<path to header location> directive or (if the default one from installation is to be used) using the -finclude-default-header frontend flag.


    echo "bool is_wg_uniform(int i){return get_enqueued_local_size(i)==get_local_size(i);}" >
    clang -cc1 -finclude-default-header -cl-std=CL2.0

    All builtin function declarations from OpenCL 2.0 will be automatically visible in

  • Image types have been improved with better diagnostics for access qualifiers. Images with one access qualifier type cannot be used in declarations for another type. Also qualifiers are now propagated from the frontend down to libraries and backends.

  • Diagnostic improvements for OpenCL types, address spaces and vectors.

  • Half type literal support has been added. For example, 1.0h represents a floating point literal in half precision, i.e., the value 0xH3C00.

  • The Clang driver now accepts OpenCL compiler options -cl-* (following the OpenCL Spec v1.1-1.2 s5.8). For example, the -cl-std=CL1.2 option from the spec enables compilation for OpenCL 1.2, or -cl-mad-enable will enable fusing multiply-and-add operations.

  • Clang now uses function metadata instead of module metadata to propagate information related to OpenCL kernels e.g. kernel argument information.

OpenMP Support in Clang

Added support for all non-offloading features from OpenMP 4.5, including using data members in private clauses of non-static member functions. Additionally, data members can be used as loop control variables in loop-based directives.

Currently Clang supports OpenMP 3.1 and all non-offloading features of OpenMP 4.0/4.5. Offloading features are under development. Clang defines macro _OPENMP and sets it to OpenMP 3.1 (in accordance with OpenMP standard) by default. User may change this value using -fopenmp-version=[31|40|45] option.

The codegen for OpenMP constructs was significantly improved to produce much more stable and faster code.

AST Matchers

  • has and hasAnyArgument: Matchers no longer ignore parentheses and implicit casts on the argument before applying the inner matcher. The fix was done to allow for greater control by the user. In all existing checkers that use this matcher all instances of code hasAnyArgument(<inner matcher>) or has(<inner matcher>) must be changed to hasAnyArgument(ignoringParenImpCasts(<inner matcher>)) or has(ignoringParenImpCasts(<inner matcher>)).

Static Analyzer

The analyzer now checks for incorrect usage of MPI APIs in C and C++. This check can be enabled by passing the following command to scan-build: -enable-checker optin.mpi.MPI-Checker.

The analyzer now checks for improper instance cleanup up in Objective-C -dealloc methods under manual retain/release.

On Windows, checks for memory leaks, double frees, and use-after-free problems are now enabled by default.

The analyzer now includes scan-build-py, an experimental reimplementation of scan-build in Python that also creates compilation databases.

The scan-build tool now supports a --force-analyze-debug-code flag that forces projects to analyze in debug mode. This flag leaves in assertions and so typically results in fewer false positives.

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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