Clang 9.0.0 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 9.0.0. 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.

What’s New in Clang 9.0.0?

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

Non-comprehensive list of changes in this release

  • The __VERSION__ macro has been updated. Previously this macro contained the string ‘4.2.1 Compatible’ to achieve compatibility with GCC 4.2.1, but that should no longer be necessary. However, to retrieve Clang’s version, please favor the one of the macro defined in clang namespaced version macros.

New Compiler Flags

  • -ftime-trace and ftime-trace-granularity=N Emits flame chart style compilation time report in chrome://tracing and compatible format. A trace .json file is written next to the compiled object file, containing hierarchical time information about frontend activities (file parsing, template instantiation) and backend activities (modules and functions being optimized, optimization passes).

Modified Compiler Flags

  • clang -dumpversion now returns the version of Clang itself.

Windows Support

  • clang-cl now treats non-existent files as possible typos for flags, clang-cl /diagnostic:caret /c for example now produces clang: error: no such file or directory: '/diagnostic:caret'; did you mean '/diagnostics:caret'?
  • clang now parses the __declspec(allocator) specifier and generates debug information, so that memory usage can be tracked in Visual Studio.
  • The -print-search-dirs option now separates elements with semicolons, as is the norm for path lists on Windows
  • Improved handling of dllexport in conjunction with explicit template instantiations for MinGW, to allow building a shared libc++ for MinGW without --export-all-symbols to override the dllexport attributes

C Language Changes in Clang

  • The __FILE_NAME__ macro has been added as a Clang specific extension supported in all C-family languages. This macro is similar to __FILE__ except it will always provide the last path component when possible.
  • Initial support for asm goto statements (a GNU C extension) has been added for control flow from inline assembly to labels. The main consumers of this construct are the Linux kernel (CONFIG_JUMP_LABEL=y) and glib. There are still a few unsupported corner cases in Clang’s integrated assembler and IfConverter. Please file bugs for any issues you run into.

C++ Language Changes in Clang

  • Support for the address space attribute in various C++ features was improved, refer to C++ for OpenCL for more details. The following features deviated from OpenCL:
    1. Address spaces as method qualifiers are not accepted yet;
    2. There is no address space deduction.

Objective-C Language Changes in Clang

  • Fixed encoding of ObjC pointer types that are pointers to typedefs.

    typedef NSArray<NSObject *> MyArray;
    // clang used to encode this as "^{NSArray=#}" instead of "@".
    const char *s0 = @encode(MyArray *);

OpenCL Kernel Language Changes in Clang

OpenCL C

  • Enabled use of variadic macro as a Clang extension.
  • Added initial support for implicitly including OpenCL builtin fuctions using efficient trie lookup generated by TableGen. A corresponding frontend-only flag -fdeclare-opencl-builtins has been added to enable trie during parsing.
  • Refactored header file to be used for common parts between regular header added using -finclude-default-header and trie based declarations added using -fdeclare-opencl-builtins.
  • Improved string formatting diagnostics in printf for vector types.
  • Simplified the internal representation of blocks, including their generation in IR. Furthermore, indirect calls to block function has been changed to direct function calls.
  • Added diagnostics for conversions of nested pointers with different address spaces.
  • Added cl_arm_integer_dot_product extension.
  • Fixed global samplers in OpenCL v2.0.
  • Improved math builtin functions with parameters of type long long for x86.

C++ for OpenCL

Experimental support for C++17 features in OpenCL has been added and backwards compatibility with OpenCL C v2.0 was enabled. The documentation has been added for supported language features into Clang Language Extensions and Clang Compiler User’s Manual.

Implemented features are:

  • Address space behavior is improved in majority of C++ features:
    • Templates parameters and arguments;
    • Reference types;
    • Type deduction;
    • Objects and member functions, including special member functions;
    • Builtin operators;
    • Method qualifiers now include address space;
    • Address space deduction has been extended for C++ use cases;
    • Improved overload ranking rules;
    • All standard cast operators now prevent converting address spaces (except for conversions allowed implicitly). They can still be cast using C-style cast.
  • Vector types as in OpenCL C, including compound vector initialization.
  • OpenCL-specific types: images, samplers, events, pipes, are accepted. Note that blocks are not supported yet.
  • OpenCL standard header in Clang can be compiled in C++ mode.
  • Global constructors can be invoked from the host side using a specific, compiler-generated kernel.
  • Overloads with generic address space are added to all atomic builtin functions, including the ones prior to OpenCL v2.0.

OpenMP Support in Clang

  • Added emission of the debug information for NVPTX target devices.

CUDA Support in Clang

  • Added emission of the debug information for the device code.

Internal API Changes

These are major API changes that have happened since the 8.0.0 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.

Build System Changes

These are major changes to the build system that have happened since the 8.0.0 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 on Linux/Unix systems, 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.


  • Add language support for clang-formatting C# files.
  • Add Microsoft coding style to encapsulate default C# formatting style.
  • Added new option PPDIS_BeforeHash (in configuration: BeforeHash) to IndentPPDirectives which indents preprocessor directives before the hash.
  • Added new option AlignConsecutiveMacros to align the C/C++ preprocessor macros of consecutive lines.


  • When CINDEXTEST_INCLUDE_ATTRIBUTED_TYPES is not provided when making a CXType, the equivalent type of the AttributedType is returned instead of the modified type if the user does not want attribute sugar. The equivalent type represents the minimally-desugared type which the AttributedType is canonically equivalent to.

Static Analyzer

  • Fixed a bug where an incorrect checker name would be displayed for a bug report.
  • New checker: security.insecureAPI.DeprecatedOrUnsafeBufferHandling to detect uses of unsafe/deprecated buffer handling functions for C code using the C11 standard or newer.
  • New checker: osx.MIGChecker to find violations of the Mach Interface Generator calling convention
  • New checker: optin.osx.OSObjectCStyleCast to find C-style casts of of XNU libkern OSObjects
  • New package: apiModeling.llvm contains modeling checkers to improve the accuracy of reports on LLVM’s own codebase.
  • The Static Analyzer received developer documentation.
  • The UninitializedObject checker is now considered as stable. (moved from the alpha.cplusplus to the optin.cplusplus package)
  • New frontend flags: The list of available checkers are now split into 3 different frontend flags:
    • -analyzer-checker-help: The list of user-facing, stable checkers.
    • -analyzer-checker-help-alpha: The list of in-development checkers not yet advised to be turned on.
    • -analyzer-checker-help-developer: Checkers never meant to be enabled/disabled by hand + development checkers.
  • New frontend flags: While they have always been around, for the first time, checker and package options are listable:
    • -analyzer-checker-option-help: The list of user-facing, stable checker and package options.
    • -analyzer-checker-option-help-alpha: The list of in-development checker options not yet advised to be used.
    • -analyzer-checker-option-help-developer: Options never meant to be enabled/disabled by hand + development options.
  • New frontend flag: -analyzer-werror to turn analyzer warnings into errors.
  • Numerous fixes to increase the stability of the experimental cross translation unit analysis (CTU).
  • CTU now handles virtual functions as well.

Linux Kernel

With support for asm goto, the mainline Linux kernel for x86_64 is now buildable (and bootable) with Clang 9. Other architectures that don’t require CONFIG_JUMP_LABEL=y such as arm, aarch64, ppc32, ppc64le, (and possibly mips) have been supported with older releases of Clang (Clang 4 was first used with aarch64).

The Android and ChromeOS Linux distributions have moved to building their Linux kernels with Clang, and Google is currently testing Clang built kernels for their production Linux kernels.

Further, LLD, llvm-objcopy, llvm-ar, llvm-nm, llvm-objdump can all be used to build a working Linux kernel.

More information about building Linux kernels with Clang can be found:

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.