Clang 10.0.0 (In-Progress) Release Notes

Written by the LLVM Team


These are in-progress notes for the upcoming Clang 10 release. Release notes for previous releases can be found on the Download Page.


This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 10.0.0. 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 see the 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 10.0.0?

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

Improvements to Clang’s diagnostics

Non-comprehensive list of changes in this release

  • For X86 target, -march=skylake-avx512, -march=icelake-client, -march=icelake-server, -march=cascadelake, -march=cooperlake will default to not using 512-bit zmm registers in vectorized code unless 512-bit intrinsics are used in the source code. 512-bit operations are known to cause the CPUs to run at a lower frequency which can impact performance. This behavior can be changed by passing -mprefer-vector-width=512 on the command line.

Deprecated Compiler Flags

The following options are deprecated and ignored. They will be removed in future versions of Clang.

  • -mmpx used to enable the __MPX__ preprocessor define for the Intel MPX instructions. There were no MPX intrinsics.
  • -mno-mpx used to disable -mmpx and is the default behavior.

C Language Changes in Clang

C11 Feature Support

C++ Language Changes in Clang

C++1z Feature Support

ABI Changes in Clang

  • gcc passes vectors of __int128 in memory on X86-64. Clang historically broke the vectors into multiple scalars using two 64-bit values for each element. Clang now matches the gcc behavior on Linux and NetBSD. You can switch back to old API behavior with flag: -fclang-abi-compat=9.0.

Internal API Changes

These are major API changes that have happened since the 9.0.0 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.

  • libTooling APIs that transfer ownership of FrontendAction objects now pass them by unique_ptr, making the ownership transfer obvious in the type system. FrontendActionFactory::create() now returns a unique_ptr<FrontendAction>. runToolOnCode, runToolOnCodeWithArgs, ToolInvocation::ToolInvocation() now take a unique_ptr<FrontendAction>.

Build System Changes

These are major changes to the build system that have happened since the 9.0.0 release of Clang. Users of the build system should adjust accordingly.

  • In 8.0.0 and below, the install-clang-headers target would install clang’s resource directory headers. This installation is now performed by the install-clang-resource-headers target. Users of the old install-clang-headers target should switch to the new install-clang-resource-headers target. The install-clang-headers target now installs clang’s API headers (corresponding to its libraries), which is consistent with the install-llvm-headers target.
  • In 9.0.0 and later Clang added a new target, clang-cpp, which generates a shared library comprised of all the clang component libraries and exporting the clang C++ APIs. Additionally the build system gained the new “CLANG_LINK_CLANG_DYLIB” option, which defaults Off, and when set to On, will force clang (and clang-based tools) to link the clang-cpp library instead of statically linking clang’s components. This option will reduce the size of binary distributions at the expense of compiler performance.

New Issues Found

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 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.