Clang 12.0.0 (In-Progress) Release Notes

Written by the LLVM Team

Warning

These are in-progress notes for the upcoming Clang 12 release. Release notes for previous releases can be found on the Download Page.

Introduction

This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 12.0.0. 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 Git 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 12.0.0?

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

  • The builtin intrinsics __builtin_bitreverse8, __builtin_bitreverse16, __builtin_bitreverse32 and __builtin_bitreverse64 may now be used within constant expressions.
  • The builtin intrinsics __builtin_rotateleft8, __builtin_rotateleft16, __builtin_rotateleft32 and __builtin_rotateleft64 may now be used within constant expressions.
  • The builtin intrinsics __builtin_rotateright8, __builtin_rotateright16, __builtin_rotateright32 and __builtin_rotateright64 may now be used within constant expressions.

New Compiler Flags

  • -fpch-codegen and -fpch-debuginfo generate shared code and/or debuginfo for contents of a precompiled header in a separate object file. This object file needs to be linked in, but its contents do not need to be generated for other objects using the precompiled header. This should usually save compile time. If not using clang-cl, the separate object file needs to be created explicitly from the precompiled header. Example of use:

    $ clang++ -x c++-header header.h -o header.pch -fpch-codegen -fpch-debuginfo
    $ clang++ -c header.pch -o shared.o
    $ clang++ -c source.cpp -o source.o -include-pch header.pch
    $ clang++ -o binary source.o shared.o
    
    • Using -fpch-instantiate-templates when generating the precompiled header usually increases the amount of code/debuginfo that can be shared.
    • In some cases, especially when building with optimizations enabled, using -fpch-codegen may generate so much code in the shared object that compiling it may be a net loss in build time.
    • Since headers may bring in private symbols of other libraries, it may be sometimes necessary to discard unused symbols (such as by adding -Wl,–gc-sections on ELF platforms to the linking command, and possibly adding -fdata-sections -ffunction-sections to the command generating the shared object).

Deprecated Compiler Flags

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

Modified Compiler Flags

  • On ELF, -gz now defaults to -gz=zlib with the integrated assembler. It produces SHF_COMPRESSED style compression of debug information. GNU binutils 2.26 or newer, or lld is required to link produced object files. Use -gz=zlib-gnu to get the old behavior.
  • Now that this pointers are tagged with nonnull and dereferenceable(N), -fno-delete-null-pointer-checks has gained the power to remove the nonnull attribute on this for configurations that need it to be nullable.

Attribute Changes in Clang

  • Added support for the C++20 likelihood attributes [[likely]] and [[unlikely]]. As an extension they can be used in C++11 and newer. This extension is enabled by default.

C++ Language Changes in Clang

C++1z Feature Support

X86 Support in Clang

  • The x86 intrinsics _mm_popcnt_u32, _mm_popcnt_u64, _popcnt32, _popcnt64, __popcntd and __popcntq may now be used within constant expressions.
  • The x86 intrinsics _bit_scan_forward, __bsfd and __bsfq may now be used within constant expressions.
  • The x86 intrinsics _bit_scan_reverse, __bsrd and __bsrq may now be used within constant expressions.
  • The x86 intrinsics __bswap, __bswapd, __bswap64 and __bswapq may now be used within constant expressions.
  • The x86 intrinsics _castf32_u32, _castf64_u64, _castu32_f32 and _castu64_f64 may now be used within constant expressions.
  • The x86 intrinsics __rolb, __rolw, __rold, __rolq`, ``_rotl, _rotwl and _lrotl may now be used within constant expressions.
  • The x86 intrinsics __rorb, __rorw, __rord, __rorq`, ``_rotr, _rotwr and _lrotr may now be used within constant expressions.
  • Support for -march=alderlake, -march=sapphirerapids and -march=znver3 was added.
  • Support for -march=x86-64-v[234] has been added. See Clang Compiler User’s Manual for details about these micro-architecture levels.
  • The -mtune command line option is no longer ignored for X86. This can be used to request microarchitectural optimizations independent on -march. -march=<cpu> implies -mtune=<cpu>. -mtune=generic is the default with no -march or -mtune specified.
  • Support for HRESET instructions has been added.
  • Support for UINTR instructions has been added.
  • Support for AVXVNNI instructions has been added.

Internal API Changes

These are major API changes that have happened since the 11.0.0 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 11.0.0 release of Clang. Users of the build system should adjust accordingly.

AST Matchers

  • The behavior of TK_IgnoreUnlessSpelledInSource with the traverse() matcher has been changed to no longer match on template instantiations or on implicit nodes which are not spelled in the source.
  • The TK_IgnoreImplicitCastsAndParentheses traversal kind was removed. It is recommended to use TK_IgnoreUnlessSpelledInSource instead.
  • The behavior of the forEach() matcher was changed to not internally ignore implicit and parenthesis nodes.

clang-format

  • Option BitFieldColonSpacing has been added that decides how space should be added around identifier, colon and bit-width in bitfield definitions.

    // Both (default)
    struct F {
      unsigned dscp : 6;
      unsigned ecn  : 2; // AlignConsecutiveBitFields=true
    };
    // None
    struct F {
      unsigned dscp:6;
      unsigned ecn :2;
    };
    // Before
    struct F {
      unsigned dscp :6;
      unsigned ecn  :2;
    };
    // After
    struct F {
      unsigned dscp: 6;
      unsigned ecn : 2;
    };
    
  • Experimental Support in clang-format for concepts has been improved, to aid this the follow options have been added

  • Option IndentRequires has been added to indent the requires keyword in templates.

  • Option BreakBeforeConceptDeclarations has been added to aid the formatting of concepts.

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 Git 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.