Clang 4 documentation

Clang 4.0.0 Release Notes

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Clang 4.0.0 Release Notes

Written by the LLVM Team


This document contains the release notes for the Clang C/C++/Objective-C/OpenCL frontend, part of the LLVM Compiler Infrastructure, release 4.0.0. 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 4.0.0?

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 diagnose_if attribute has been added to clang. This attribute allows clang to emit a warning or error if a function call meets one or more user-specified conditions.

  • Enhanced devirtualization with -fstrict-vtable-pointers. Clang devirtualizes across different basic blocks, like loops:

    struct A {
        virtual void foo();
    void indirect(A &a, int n) {
        for (int i = 0 ; i < n; i++)
    void test(int n) {
        A a;
        indirect(a, n);

Improvements to ThinLTO (-flto=thin)

  • Integration with profile data (PGO). When available, profile data enables more accurate function importing decisions, as well as cross-module indirect call promotion.
  • Significant build-time and binary-size improvements when compiling with debug info (-g).

New Compiler Flags

  • The option -Og has been added to optimize the debugging experience. For now, this option is exactly the same as -O1. However, in the future, some other optimizations might be enabled or disabled.
  • The option -MJ has been added to simplify adding JSON compilation database output into existing build systems.

OpenCL C Language Changes in Clang

The following bugs in the OpenCL header have been fixed:

  • Added missing overloadable and convergent attributes.
  • Removed some erroneous extra native_* functions.

The following bugs in the generation of metadata have been fixed:

  • Corrected the SPIR version depending on the OpenCL version.
  • Source level address spaces are taken from the SPIR specification.
  • Image types now contain no access qualifier.

The following bugs in the AMD target have been fixed:

  • Corrected the bitwidth of size_t and NULL pointer value with respect to address spaces.
  • Added cl_khr_subgroups, cl_amd_media_ops and cl_amd_media_ops2 extensions.
  • Added cl-denorms-are-zero support.
  • Changed address spaces for image objects to be constant.
  • Added little-endian.

The following bugs in OpenCL 2.0 have been fixed:

  • Fixed pipe builtin function return type, added extra argument to generated IR intrinsics to propagate size and alignment information of the pipe packed type.
  • Improved pipe type to accommodate access qualifiers.
  • Added correct address space to the ObjC block generation and enqueue_kernel prototype.
  • Improved handling of integer parameters of enqueue_kernel prototype. We now allow size_t instead of int for specifying block parameter sizes.
  • Allow using NULL (aka CLK_NULL_QUEUE) with queue_t.

Improved the following diagnostics:

  • Disallow address spaces other than global for kernel pointer parameters.
  • Correct the use of half type argument and pointer assignment with dereferencing.
  • Disallow variadic arguments in functions and blocks.
  • Allow partial initializer for array and struct.

Some changes to OpenCL extensions have been made:

  • Added cl_khr_mipmap_image.

  • Added -cl-ext flag to allow overwriting supported extensions otherwise set by the target compiled for (Example: -cl-ext=-all,+cl_khr_fp16).

  • New types and functions can now be flexibly added to extensions using the following pragmas instead of modifying the Clang source code:

    #pragma OPENCL EXTENSION the_new_extension_name : begin
    // declare types and functions associated with the extension here
    #pragma OPENCL EXTENSION the_new_extension_name : end

Miscellaneous changes:

  • Fix __builtin_astype to cast between different address space objects.
  • Allow using opencl_unroll_hint with earlier OpenCL versions than 2.0.
  • Improved handling of floating point literal to default to single precision if fp64 extension is not enabled.
  • Refactor sampler_t implementation to simplify initializer representation which is now handled as a compiler builtin function with an integer value passed into it.
  • Change fake address space map to use the SPIR convention.
  • Added the OpenCL manual to Clang documentation.

Static Analyzer

With the option --show-description, scan-build’s list of defects will also show the description of the defects.

The analyzer now provides better support of code that uses gtest.

Several new checks were added:

  • The analyzer warns when virtual calls are made from constructors or destructors. This check is off by default but can be enabled by passing the following command to scan-build: -enable-checker optin.cplusplus.VirtualCall.
  • The analyzer checks for synthesized copy properties of mutable types in Objective C, such as NSMutableArray. Calling the setter for these properties will store an immutable copy of the value.
  • The analyzer checks for calls to dispatch_once() that use an Objective-C instance variable as the predicate. Using an instance variable as a predicate may result in the passed-in block being executed multiple times or not at all. These calls should be rewritten either to use a lock or to store the predicate in a global or static variable.
  • The analyzer checks for unintended comparisons of NSNumber, CFNumberRef, and other Cocoa number objects to scalar values.

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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