Clang

Written by the LLVM Team

PreRelease

Warning

These are in-progress notes for the upcoming Clang 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 . 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

These changes are ones which we think may surprise users when upgrading to Clang because of the opportunity they pose for disruption to existing code bases.

C/C++ Language Potentially Breaking Changes

Indirect edges of asm goto statements under certain circumstances may now be split. In previous releases of clang, that means for the following code the two inputs may have compared equal in the inline assembly. This is no longer guaranteed (and necessary to support outputs along indirect edges, which is now supported as of this release). This change is more consistent with the behavior of GCC.
```
foo: asm goto ("# %0 %1"::"i"(&&foo)::foo);
```

C++ Specific Potentially Breaking Changes

Clang won't search for coroutine_traits in std::experimental namespace any more. Clang will only search for std::coroutine_traits for coroutines then.

ABI Changes in This Version

__is_trivial has changed for a small category of classes with constrained default constructors (#60697). FIXME: Remove this note if we've backported this change to the Clang 16 branch.

What's New in Clang ?

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.

C++ Language Changes

Improved -O0 code generation for calls to std::forward_like. Similarly to std::move, std::forward et al. it is now treated as a compiler builtin and implemented directly rather than instantiating the definition from the standard library.
Implemented CWG2518 which allows static_assert(false) to not be ill-formed when its condition is evaluated in the context of a template definition.

C++20 Feature Support

Support for out-of-line definitions of constrained templates has been improved. This partially fixes #49620.
Lambda templates with a requires clause directly after the template parameters now parse correctly if the requires clause consists of a variable with a dependent type. (#61278)
Announced C++20 Coroutines is fully supported on all targets except Windows, which still has some stability and ABI issues.
Downgraded use of a reserved identifier in a module export declaration from an error to a warning under the -Wreserved-module-identifier warning group. This warning is enabled by default. This addresses #61446 and allows easier building of precompiled modules. This diagnostic may be strengthened into an error again in the future once there is a less fragile way to mark a module as being part of the implementation rather than a user module.

C++2b Feature Support

Implemented P2036R3: Change scope of lambda trailing-return-type and P2579R0 Mitigation strategies for P2036. These proposals modify how variables captured in lambdas can appear in trailing return type expressions and how their types are deduced therein, in all C++ language versions.

Resolutions to C++ Defect Reports

C Language Changes

Support for outputs from asm goto statements along indirect edges has been added. (#53562)

C2x Feature Support

Implemented the unreachable macro in freestanding <stddef.h> for WG14 N2826
Removed the ATOMIC_VAR_INIT macro in C2x and later standards modes, which implements WG14 N2886
Implemented WG14 N2934 which introduces the bool, static_assert, alignas, alignof, and thread_local keywords in C2x.
Implemented WG14 N2900 and WG14 N3011 which allows for empty braced initialization in C.
```
struct S { int x, y } s = {}; // Initializes s.x and s.y to 0
```
As part of this change, the -Wgnu-empty-initializer warning group was removed, as this is no longer a GNU extension but a C2x extension. You can use -Wno-c2x-extensions to silence the extension warning instead.

Non-comprehensive list of changes in this release

Clang now saves the address of ABI-indirect function parameters on the stack, improving the debug information available in programs compiled without optimizations.
Clang now supports __builtin_nondeterministic_value that returns a nondeterministic value of the same type as the provided argument.
Clang now supports __builtin_FILE_NAME() which returns the same information as the __FILE_NAME__ macro (the presumed file name from the invocation point, with no path components included).
Clang now supports __builtin_assume_separate_storage that indicates that its arguments point to objects in separate storage allocations.
Clang now supports expressions in #pragma clang __debug dump.
Clang now supports declaration of multi-dimensional arrays with __declspec(property).

New Compiler Flags

Deprecated Compiler Flags

Modified Compiler Flags

Removed Compiler Flags

The deprecated flag -fmodules-ts is removed. Please use -std=c++20 or higher to use standard C++ modules instead.
The deprecated flag -fcoroutines-ts is removed. Please use -std=c++20 or higher to use standard C++ coroutines instead.
The CodeGen flag -lower-global-dtors-via-cxa-atexit which affects how global destructors are lowered for MachO is removed without replacement. The default of -lower-global-dtors-via-cxa-atexit=true is now the only supported way.

Attribute Changes in Clang

Introduced a new function attribute __attribute__((unsafe_buffer_usage)) to be worn by functions containing buffer operations that could cause out of bounds memory accesses. It emits warnings at call sites to such functions when the flag -Wunsafe-buffer-usage is enabled.
__declspec attributes can now be used together with the using keyword. Before the attributes on __declspec was ignored, while now it will be forwarded to the point where the alias is used.
Introduced a new USR (unified symbol resolution) clause inside of the existing __attribute__((external_source_symbol)) attribute. Clang's indexer uses the optional USR value when indexing Clang's AST. This value is expected to be generated by an external compiler when generating C++ bindings during the compilation of the foreign language sources (e.g. Swift).
The __has_attribute, __has_c_attribute and __has_cpp_attribute preprocessor operators now return 1 also for attributes defined by plugins.

Improvements to Clang's diagnostics

We now generate a diagnostic for signed integer overflow due to unary minus in a non-constant expression context. (#31643)
Clang now warns by default for C++20 and later about deprecated capture of this with a capture default of =. This warning can be disabled with -Wno-deprecated-this-capture.
Clang had failed to emit some -Wundefined-internal for members of a local class if that class was first introduced with a forward declaration.
Diagnostic notes and fix-its are now generated for ifunc/alias attributes which point to functions whose names are mangled.
Diagnostics relating to macros on the command line of a preprocessed assembly file are now reported as coming from the file <command line> instead of <built-in>.
Clang constexpr evaluator now provides a more concise diagnostic when calling function pointer that is known to be null.
Clang now avoids duplicate warnings on unreachable [[fallthrough]]; statements previously issued from -Wunreachable-code and -Wunreachable-code-fallthrough by prioritizing -Wunreachable-code-fallthrough.
Clang now correctly diagnoses statement attributes [[clang::always_inine]] and [[clang::noinline]] when used on a statement with dependent call expressions.
Clang now checks for completeness of the second and third arguments in the conditional operator. (#59718)

Bug Fixes in This Version

Fix crash when diagnosing incorrect usage of _Nullable involving alias templates. (#60344)
Fix confusing warning message when /clang:-x is passed in clang-cl driver mode and emit an error which suggests using /TC or /TP clang-cl options instead. (#59307)
Fix assert that fails when the expression causing the this pointer to be captured by a block is part of a constexpr if statement's branch and instantiation of the enclosing method causes the branch to be discarded.
Fix __VA_OPT__ implementation so that it treats the concatenation of a non-placemaker token and placemaker token as a non-placemaker token. (#60268)
Fix crash when taking the address of a consteval lambda call operator. (#57682)
Clang now support export declarations in the language linkage. (#60405)
Fix aggregate initialization inside lambda constexpr. (#60936)
No longer issue a false positive diagnostic about a catch handler that cannot be reached despite being reachable. This fixes #61177 in anticipation of CWG2699 <https://wg21.link/CWG2699>_ being accepted by WG21.
Fix crash when parsing fold expression containing a delayed typo correction. (#61326)
Fix crash when dealing with some member accesses outside of class or member function context. (#37792) and (#48405)
Fix crash when using [[clang::always_inline]] or [[clang::noinline]] statement attributes on a call to a template function in the body of a template function.
Fix coroutines issue where get_return_object() result was always eargerly converted to the return type. Eager initialization (allowing RVO) is now only perfomed when these types match, otherwise deferred initialization is used, enabling short-circuiting coroutines use cases. This fixes (#56532) in antecipation of CWG2563 <https://cplusplus.github.io/CWG/issues/2563.html>_.
Fix highlighting issue with _Complex and initialization list with more than 2 items. (#61518)
Fix getSourceRange on VarTemplateSpecializationDecl and VarTemplatePartialSpecializationDecl, which represents variable with the initializer, so it behaves consistently with other VarDecls and ends on the last token of initializer, instead of right angle bracket of the template argument list.
Fix false-positive diagnostic issued for consteval initializers of temporary objects. (#60286)
Correct restriction of trailing requirements clauses on a templated function. Previously we only rejected non-'templated' things, but the restrictions ALSO need to limit non-defined/non-member functions as well. Additionally, we now diagnose requires on lambdas when not allowed, which we previously missed. (#61748)
Fix confusing diagnostic for incorrect use of qualified concepts names.
Fix handling of comments in function like macros so they are ignored in -CC mode. (#60887)
Fix incorrect merging of lambdas across modules. (#60985)
Fix crash when handling nested immediate invocations in initializers of global variables. (#58207)
Fix crash when generating code coverage information for PseudoObjectExpr in Clang AST. (#45481)

Bug Fixes to Compiler Builtins

Bug Fixes to Attribute Support

Bug Fixes to C++ Support

Fix crash on invalid code when looking up a destructor in a templated class inside a namespace. (#59446)
Fix crash when evaluating consteval constructor of derived class whose base has more than one field. (#60166)
Fix an issue about decltype in the members of class templates derived from templates with related parameters. (#58674)
Fix incorrect deletion of the default constructor of unions in some cases. (#48416)
No longer issue a pre-C++2b compatibility warning in -pedantic mode regading overloaded operator[] with more than one parmeter or for static lambdas. (#61582)
Stop stripping CV qualifiers from the type of this when capturing it by value in a lambda. (#50866)
Fix ordering of function templates by constraints when they have template template parameters with different nested constraints.
Fix type equivalence comparison between auto types to take constraints into account.
Fix bug in the computation of the __has_unique_object_representations builtin for types with unnamed bitfields. (#61336)

Bug Fixes to AST Handling

Miscellaneous Bug Fixes

Miscellaneous Clang Crashes Fixed

Target Specific Changes

AMDGPU Support

Linking for AMDGPU now uses --no-undefined by default. This causes undefined symbols in the created module to be a linker error. To prevent this, pass -Wl,--undefined if compiling directly, or -Xoffload-linker --undefined if using an offloading language.
The deprecated -mcode-object-v3 and -mno-code-object-v3 command-line options have been removed.

X86 Support

Add ISA of AMX-COMPLEX which supports tcmmimfp16ps and tcmmrlfp16ps.

Arm and AArch64 Support

The hard-float ABI is now available in Armv8.1-M configurations that have integer MVE instructions (and therefore have FP registers) but no scalar or vector floating point computation. Previously, trying to select the hard-float ABI on such a target (via -mfloat-abi=hard or a triple ending in hf) would silently use the soft-float ABI instead.
Clang builtin __arithmetic_fence and the command line option -fprotect-parens are now enabled for AArch64.

Windows Support

LoongArch Support

Patchable function entry (-fpatchable-function-entry) is now supported on LoongArch.

RISC-V Support

Added -mrvv-vector-bits= option to give an upper and lower bound on vector length. Valid values are powers of 2 between 64 and 65536. A value of 32 should eventually be supported. We also accept "zvl" to use the Zvl*b extension from -march or -mcpu to the be the upper and lower bound.
Fixed incorrect ABI lowering of _Float16 in the case of structs containing _Float16 that are eligible for passing via GPR+FPR or FPR+FPR.

CUDA/HIP Language Changes

CUDA Support

AIX Support

Add an AIX-only link-time option, -mxcoff-build-id=0xHEXSTRING, to allow users to embed a hex id in their binary such that it's readable by the program itself. This option is an alternative to the --build-id=0xHEXSTRING GNU linker option which is currently not supported by the AIX linker.

WebAssembly Support

AVR Support

The definition of USHRT_MAX in the freestanding <limits.h> no longer overflows on AVR (where sizeof(int) == sizeof(unsigned short)). The type of USHRT_MAX is now unsigned int instead of int, as required by the C standard.

DWARF Support in Clang

Floating Point Support in Clang

Add __builtin_elementwise_log builtin for floating point types only.
Add __builtin_elementwise_log10 builtin for floating point types only.
Add __builtin_elementwise_log2 builtin for floating point types only.
Add __builtin_elementwise_exp builtin for floating point types only.
Add __builtin_elementwise_exp2 builtin for floating point types only.
Add __builtin_set_flt_rounds builtin for X86, x86_64, Arm and AArch64 only.

AST Matchers

Add coroutineBodyStmt matcher.
The hasBody matcher now matches coroutine body nodes in CoroutineBodyStmts.

clang-format

Add NextLineOnly style to option PackConstructorInitializers. Compared to NextLine style, NextLineOnly style will not try to put the initializers on the current line first, instead, it will try to put the initializers on the next line only.
Add additional Qualifier Ordering support for special cases such as templates, requires clauses, long qualified names.
Fix all known issues associated with LambdaBodyIndentation: OuterScope.

libclang

Introduced the new function clang_CXXMethod_isExplicit, which identifies whether a constructor or conversion function cursor was marked with the explicit identifier.
Introduced the new CXIndex constructor function clang_createIndexWithOptions, which allows storing precompiled preambles in memory or overriding the precompiled preamble storage path.
Deprecated two functions clang_CXIndex_setGlobalOptions and clang_CXIndex_setInvocationEmissionPathOption in favor of the new function clang_createIndexWithOptions in order to improve thread safety.
Added check in clang_getFieldDeclBitWidth for whether a bit-field has an evaluable bit width. Fixes undefined behavior when called on a bit-field whose width depends on a template paramter.

Static Analyzer

Fix incorrect alignment attribute on the this parameter of certain non-complete destructors when using the Microsoft ABI. (#60465)

Sanitizers

Python Binding Changes

The following methods have been added:

clang_Location_isInSystemHeader exposed via the is_in_system_header property of the Location class.

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).

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