Clang 12.0.0 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 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

  • AArch64 options -moutline-atomics, -mno-outline-atomics to enable and disable calls to helper functions implementing atomic operations. These out-of-line helpers like ‘__aarch64_cas8_relax’ will detect at runtime AArch64 Large System Extensions (LSE) availability and either use their atomic instructions, or falls back to LL/SC loop. These options do not apply if the compilation target supports LSE. Atomic instructions are used directly in that case. The option’s behaviour mirrors GCC, the helpers are implemented both in compiler-rt and libgcc.

  • New option -fbinutils-version= specifies the targeted binutils version. For example, -fbinutils-version=2.35 means compatibility with GNU as/ld before 2.35 is not needed: new features can be used and there is no need to work around old GNU as/ld bugs.

Deprecated Compiler Flags

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

  • The clang-cl /fallback flag, which made clang-cl invoke Microsoft Visual C++ on files it couldn’t compile itself, has been deprecated. It will be removed in Clang 13.

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.

  • -gsplit-dwarf no longer implies -g2.

  • -fasynchronous-unwind-tables is now the default on Linux AArch64/PowerPC. This behavior matches newer GCC. (D91760) (D92054)

  • Support has been added for the following processors (command-line identifiers in parentheses):

    • Arm Cortex-A78C (cortex-a78c).

    • Arm Cortex-R82 (cortex-r82).

    • Arm Neoverse V1 (neoverse-v1).

    • Arm Neoverse N2 (neoverse-n2).

    • Fujitsu A64FX (a64fx).

    For example, to select architecture support and tuning for Neoverse-V1 based systems, use -mcpu=neoverse-v1.

Removed Compiler Flags

The following options no longer exist.

  • clang-cl’s /Zd flag no longer exist. But -gline-tables-only still exists and does the same thing.

Modified Pragmas in Clang

  • The “#pragma clang loop vectorize_width” has been extended to support an optional ‘fixed|scalable’ argument, which can be used to indicate that the compiler should use fixed-width or scalable vectorization. Fixed-width is assumed by default.

    Scalable or vector length agnostic vectorization is an experimental feature for targets that support scalable vectors. For more information please refer to the Clang Language Extensions documentation.

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.

Windows Support

  • Implicitly add .exe suffix for MinGW targets, even when cross compiling. (This matches a change from GCC 8.)

  • Windows on Arm64: programs using the C standard library’s setjmp and longjmp functions may crash with a “Security check failure or stack buffer overrun” exception. To workaround (with reduced security), compile with /guard:cf,nolongjmp.

  • Windows on Arm64: LLVM 12 adds official binary release hosted on Windows on Arm64. The binary is built and tested by Linaro alongside AArch64 and ARM 32-bit Linux binary releases. This first WoA release includes Clang compiler, LLD Linker, and compiler-rt runtime libraries. Work on LLDB, sanitizer support, OpenMP, and other features is in progress and will be included in future Windows on Arm64 LLVM releases.

C++ Language Changes in Clang

C++1z Feature Support

OpenCL Kernel Language Changes in Clang

  • Improved online documentation: Clang Compiler User’s Manual and OpenCL Support pages.

  • Added -cl-std=CL3.0 and predefined version macro for OpenCL 3.0.

  • Added -cl-std=CL1.0 and mapped to the existing OpenCL 1.0 functionality.

  • Improved OpenCL extension handling per target.

  • Added clang extension for function pointers __cl_clang_function_pointers and variadic functions __cl_clang_variadic_functions, more details can be found in Clang Language Extensions.

  • Removed extensions without kernel language changes: cl_khr_select_fprounding_mode, cl_khr_gl_sharing, cl_khr_icd, cl_khr_gl_event, cl_khr_d3d10_sharing, cl_khr_context_abort, cl_khr_d3d11_sharing, cl_khr_dx9_media_sharing, cl_khr_image2d_from_buffer, cl_khr_initialize_memory, cl_khr_gl_depth_images, cl_khr_spir, cl_khr_egl_event, cl_khr_egl_image, cl_khr_terminate_context.

  • Improved diagnostics for unevaluated vec_step expression.

  • Allow nested pointers (e.g. pointer-to-pointer) kernel arguments beyond OpenCL 1.2.

  • Added global_device and global_host address spaces for USM allocations.

Miscellaneous improvements in C++ for OpenCL support:

  • Added diagnostics for pointers to member functions and references to functions.

  • Added support of vec_step builtin.

  • Fixed ICE on address spaces with forwarding references and templated copy constructors.

  • Removed warning for variadic macro use.

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 mapAnyOf() matcher was added. This allows convenient matching of different AST nodes which have a compatible matcher API. For example, mapAnyOf(ifStmt, forStmt).with(hasCondition(integerLiteral())) matches any IfStmt or ForStmt with a integer literal as the condition.

  • The binaryOperation() matcher allows matching expressions which appear like binary operators in the code, even if they are really CXXOperatorCallExpr for example. It is based on the mapAnyOf() matcher functionality. The matcher API for the latter node has been extended with hasLHS() etc to facilitate the abstraction.

  • Matcher API for CXXRewrittenBinaryOperator has been added. In addition to explicit matching with the cxxRewrittenBinaryOperator() matcher, the binaryOperation() matches on nodes of this type.

  • 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. This makes it consistent with the has() matcher. Uses of forEach() relying on the old behavior can now use the traverse() matcher or ignoringParenCasts().

  • Several AST Matchers have been changed to match based on the active traversal mode. For example, argumentCountIs() matches the number of arguments written in the source, ignoring default arguments represented by CXXDefaultArgExpr nodes.

  • Improvements in AST Matchers allow more matching of template declarations, independent of their template instantations.


  • 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.

  • Option IndentPragmas has been added to allow #pragma to indented with the current scope level. This is especially useful when using #pragma to mark OpenMP sections of code.

  • Option SpaceBeforeCaseColon has been added to add a space before the colon in a case or default statement.

  • Option StatementAttributeLikeMacros has been added to declare macros which are not parsed as a type in front of a statement. See the documentation for an example.

  • Options AlignConsecutiveAssignments, AlignConsecutiveBitFields, AlignConsecutiveDeclarations and AlignConsecutiveMacros have been modified to allow alignment across empty lines and/or comments.

Static Analyzer

  • Improve the analyzer’s understanding of several POSIX functions.

  • Greatly improved the analyzer’s constraint solver by better understanding when constraints are imposed on multiple symbolic values that are known to be equal or known to be non-equal. It will now also efficiently reject impossible if-branches between known comparison expressions. (Incorrectly stated as a 11.0.0 feature in the previous release notes)

  • New checker: webkit.UncountedLambdaCapturesChecker is a WebKit coding convention checker that flags raw pointers to reference-counted objects captured by lambdas and suggests using intrusive reference-counting smart pointers instead.

  • New checker: alpha.webkit.UncountedLocalVarsChecker is a WebKit coding convention checker that intends to make sure that any uncounted local variable is backed by a ref-counted object with lifetime that is strictly larger than the scope of the uncounted local variable.

  • fuchia.HandleChecker now recognizes handles in structs; All the handles referenced by the structure (direct value or ptr) would be treated as containing the release/use/acquire annotations directly.

  • Fuchsia checkers can detect the release of an unowned handle.

  • Numerous fixes and improvements to bug report generation.

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.