Clang 21.0.0git (In-Progress) Release Notes¶

Written by the LLVM Team

Warning

These are in-progress notes for the upcoming Clang 21 release. Release notes for previous releases can be found on the Releases Page.

Introduction ¶

This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 21.0.0git. 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. For the libc++ release notes, see this page. 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.

Potentially Breaking Changes ¶

The Objective-C ARC migrator (ARCMigrate) has been removed.

C/C++ Language Potentially Breaking Changes ¶

C++ Specific Potentially Breaking Changes ¶

The type trait builtin __is_referenceable has been removed, since it has very few users and all the type traits that could benefit from it in the standard library already have their own bespoke builtins.

ABI Changes in This Version ¶

Return larger CXX records in memory instead of using AVX registers. Code compiled with older clang will be incompatible with newer version of the clang unless -fclang-abi-compat=20 is provided. (#120670)

AST Dumping Potentially Breaking Changes ¶

Added support for dumping template arguments of structural value kinds.

Clang Frontend Potentially Breaking Changes ¶

Clang Python Bindings Potentially Breaking Changes ¶

What’s New in Clang 21.0.0git?¶

C++ Language Changes ¶

C++2c Feature Support¶

C++23 Feature Support¶

C++20 Feature Support¶

C++17 Feature Support¶

Resolutions to C++ Defect Reports¶

The flag -frelaxed-template-template-args and its negation have been removed, having been deprecated since the previous two releases. The improvements to template template parameter matching implemented in the previous release, as described in P3310 and P3579, made this flag unnecessary.

C Language Changes ¶

Clang now allows an inline specifier on a typedef declaration of a function type in Microsoft compatibility mode. #124869
Clang now allows restrict qualifier for array types with pointer elements (#92847).

C2y Feature Support¶

C23 Feature Support¶

Non-comprehensive list of changes in this release ¶

New Compiler Flags ¶

New option -fprofile-continuous added to enable continuous profile syncing to file (#124353, docs). The feature has existed) for a while and this is just a user facing option.

Deprecated Compiler Flags ¶

Modified Compiler Flags ¶

Removed Compiler Flags ¶

Attribute Changes in Clang ¶

The no_sanitize attribute now accepts both gnu and clang names.
Clang now diagnoses use of declaration attributes on void parameters. (#108819)
Clang now allows __attribute__((model("small"))) and __attribute__((model("large"))) on non-TLS globals in x86-64 compilations. This forces the global to be considered small or large in regards to the x86-64 code model, regardless of the code model specified for the compilation.

Improvements to Clang’s diagnostics ¶

Improve the diagnostics for deleted default constructor errors for C++ class initializer lists that don’t explicitly list a class member and thus attempt to implicitly default construct that member.
The -Wunique-object-duplication warning has been added to warn about objects which are supposed to only exist once per program, but may get duplicated when built into a shared library.
Fixed a bug where Clang’s Analysis did not correctly model the destructor behavior of union members (#119415).
A statement attribute applied to a case label no longer suppresses ‘bypassing variable initialization’ diagnostics (#84072).

Improvements to Clang’s time-trace ¶

Improvements to Coverage Mapping ¶

Bug Fixes in This Version ¶

Clang now outputs correct values when #embed data contains bytes with negative signed char values (#102798).

Bug Fixes to Compiler Builtins¶

The behvaiour of __add_pointer and __remove_pointer for Objective-C++’s id and interfaces has been fixed.

Bug Fixes to Attribute Support¶

Fixed crash when a parameter to the clang::annotate attribute evaluates to void. See #119125

Bug Fixes to C++ Support¶

Clang is now better at keeping track of friend function template instance contexts. (#55509)
The initialization kind of elements of structured bindings direct-list-initialized from an array is corrected to direct-initialization.

Bug Fixes to AST Handling¶

Miscellaneous Bug Fixes¶

HTML tags in comments that span multiple lines are now parsed correctly by Clang’s comment parser. (#120843)

Miscellaneous Clang Crashes Fixed¶

OpenACC Specific Changes ¶

Target Specific Changes ¶

AMDGPU Support¶

NVPTX Support¶

Hexagon Support¶

The default compilation target has been changed from V60 to V68.

X86 Support¶

Disable -m[no-]avx10.1 and switch -m[no-]avx10.2 to alias of 512 bit options.
Change -mno-avx10.1-512 to alias of -mno-avx10.1-256 to disable both 256 and 512 bit instructions.

Arm and AArch64 Support¶

Android Support¶

Windows Support¶

LoongArch Support¶

RISC-V Support¶

Add support for -mtune=generic-ooo (a generic out-of-order model).

CUDA/HIP Language Changes¶

CUDA Support¶

AIX Support¶

NetBSD Support¶

WebAssembly Support¶

AVR Support¶

DWARF Support in Clang ¶

Floating Point Support in Clang ¶

Fixed Point Support in Clang ¶

AST Matchers ¶

clang-format ¶

Adds BreakBeforeTemplateCloser option.
Adds BinPackLongBracedList option to override bin packing options in long (20 item or more) braced list initializer lists.

libclang ¶

Code Completion ¶

Static Analyzer ¶

Clang currently support extending lifetime of object bound to reference members of aggregates in CFG and ExprEngine, that are created from default member initializer.

New features¶

A new flag - -static-libclosure was introduced to support statically linking the runtime for the Blocks extension on Windows. This flag currently only changes the code generation, and even then, only on Windows. This does not impact the linker behaviour like the other -static-* flags.

Crash and bug fixes¶

Improvements¶

Moved checkers¶

After lots of improvements, the checker alpha.security.ArrayBoundV2 is renamed to security.ArrayBound. As this checker is stable now, the old checker alpha.security.ArrayBound (which was searching for the same kind of bugs with an different, simpler and less accurate algorithm) is removed.

Sanitizers ¶

Python Binding Changes ¶

OpenMP Support ¶

Added support ‘no_openmp_constructs’ assumption clause.
Added support for ‘omp stripe’ directive.

Improvements¶

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 on the Discourse forums (Clang Frontend category).