Clang 12.0.0 (In-Progress) Release Notes

Written by the LLVM Team

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

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Improvements to Clang's diagnostics

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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

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• -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.

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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.

New Pragmas in Clang

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Attribute Changes in Clang

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Windows Support

C Language Changes in Clang

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C++ Language Changes in Clang

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C++1z Feature Support

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Objective-C Language Changes in Clang

OpenCL C Language Changes in Clang

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ABI Changes in Clang

OpenMP Support in Clang

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CUDA Support in Clang

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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=sapphirerapids was added.
• 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.

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.

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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.

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AST Matchers

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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;
  };
  

libclang

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Static Analyzer

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Undefined Behavior Sanitizer (UBSan)

Core Analysis Improvements

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New Issues Found

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Python Binding Changes

The following methods have been added:

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Significant Known Problems

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.