Clang 5 documentation

Clang 5.0.0 Release Notes

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Clang 5.0.0 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 5.0.0. 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.

What’s New in Clang 5.0.0?

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

C++ coroutines

C++ coroutines TS implementation has landed. Use -fcoroutines-ts -stdlib=libc++ to enable coroutine support. Here is an example to get you started.

Improvements to Clang’s diagnostics

  • -Wcast-qual was implemented for C++. C-style casts are now properly diagnosed.
  • -Wunused-lambda-capture warns when a variable explicitly captured by a lambda is not used in the body of the lambda.
  • -Wstrict-prototypes is a new warning that warns about non-prototype function and block declarations and types in C and Objective-C.
  • -Wunguarded-availability is a new warning that warns about uses of new APIs that were introduced in a system whose version is newer than the deployment target version. A new Objective-C expression @available has been introduced to perform system version checking at runtime. This warning is off by default to prevent unexpected warnings in existing projects. However, its less strict sibling -Wunguarded-availability-new is on by default. It warns about unguarded uses of APIs only when they were introduced in or after macOS 10.13, iOS 11, tvOS 11 or watchOS 4.
  • The -Wdocumentation warning now allows the use of \param and \returns documentation directives in the documentation comments for declarations with a function or a block pointer type.
  • The compiler no longer warns about unreachable __builtin_unreachable statements.

New Compiler Flags

  • --autocomplete was implemented to obtain a list of flags and its arguments. This is used for shell autocompletion.

Deprecated Compiler Flags

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

  • -fslp-vectorize-aggressive used to enable the BB vectorizing pass. They have been superseeded by the normal SLP vectorizer.
  • -fno-slp-vectorize-aggressive used to be the default behavior of clang.

New Pragmas in Clang

  • Clang now supports the clang attribute pragma that allows users to apply an attribute to multiple declarations.
  • pragma pack directives that are included in a precompiled header are now applied correctly to the declarations in the compilation unit that includes that precompiled header.

Attribute Changes in Clang

  • The overloadable attribute now allows at most one function with a given name to lack the overloadable attribute. This unmarked function will not have its name mangled.
  • The ms_abi attribute and the __builtin_ms_va_list types and builtins are now supported on AArch64.

C Language Changes in Clang

  • Added near complete support for implicit scalar to vector conversion, a GNU C/C++ language extension. With this extension, the following code is considered valid:
typedef unsigned v4i32 __attribute__((vector_size(16)));

v4i32 foo(v4i32 a) {
  // Here 5 is implicitly casted to an unsigned value and replicated into a
  // vector with as many elements as 'a'.
  return a + 5;

The implicit conversion of a scalar value to a vector value–in the context of a vector expression–occurs when:

  • The type of the vector is that of a __attribute__((vector_size(size))) vector, not an OpenCL __attribute__((ext_vector_type(size))) vector type.
  • The scalar value can be casted to that of the vector element’s type without the loss of precision based on the type of the scalar and the type of the vector’s elements.
  • For compile time constant values, the above rule is weakened to consider the value of the scalar constant rather than the constant’s type. However, for compatibility with GCC, floating point constants with precise integral representations are not implicitly converted to integer values.

Currently the basic integer and floating point types with the following operators are supported: +, /, -, *, %, >, <, >=, <=, ==, !=, &, |, ^ and the corresponding assignment operators where applicable.

C++ Language Changes in Clang

  • We expect this to be the last Clang release that defaults to -std=gnu++98 when using the GCC-compatible clang++ driver. From Clang 6 onwards we expect to use -std=gnu++14 or a later standard by default, to match the behavior of recent GCC releases. Users are encouraged to change their build files to explicitly specify their desired C++ standard.
  • Support for the C++17 standard has been completed. This mode can be enabled using -std=c++17 (the old flag -std=c++1z is still supported for compatibility).
  • When targeting a platform that uses the Itanium C++ ABI, Clang implements a recent change to the ABI that passes objects of class type indirectly if they have a non-trivial move constructor. Previous versions of Clang only considered the copy constructor, resulting in an ABI change in rare cases, but GCC has already implemented this change for several releases. This affects all targets other than Windows and PS4. You can opt out of this ABI change with -fclang-abi-compat=4.0.
  • As mentioned in C Language Changes in Clang, Clang’s support for implicit scalar to vector conversions also applies to C++. Additionally the following operators are also supported: && and ||.

Objective-C Language Changes in Clang

  • Clang now guarantees that a readwrite property is synthesized when an ambiguous property (i.e. a property that’s declared in multiple protocols) is synthesized. The -Wprotocol-property-synthesis-ambiguity warning that warns about incompatible property types is now promoted to an error when there’s an ambiguity between readwrite and readonly properties.
  • Clang now prohibits synthesis of ambiguous properties with incompatible explicit property attributes. The following property attributes are checked for differences: copy, retain/strong, atomic, getter and setter.

OpenCL C Language Changes in Clang

Various bug fixes and improvements:

  • Extended OpenCL-related Clang tests.
  • Improved diagnostics across several areas: scoped address space qualified variables, function pointers, atomics, type rank for overloading, block captures, reserve_id_t.
  • Several address space related fixes for constant address space function scope variables, IR generation, mangling of generic and alloca (post-fix from general Clang refactoring of address spaces).
  • Several improvements in extensions: fixed OpenCL version for cl_khr_mipmap_image, added missing cl_khr_3d_image_writes.
  • Improvements in enqueue_kernel, especially the implementation of ndrange_t and blocks.
  • OpenCL type related fixes: global samplers, the pipe_t size, internal type redefinition, and type compatibility checking in ternary and other operations.
  • The OpenCL header has been extended with missing extension guards, and direct mapping of as_type to __builtin_astype.
  • Fixed kernel_arg_type_qual and OpenCL/SPIR version in metadata.
  • Added proper use of the kernel calling convention to various targets.

The following new functionalities have been added:

  • Added documentation on OpenCL to Clang user manual.
  • Extended Clang builtins with required cl_khr_subgroups support.
  • Add intel_reqd_sub_group_size attribute support.
  • Added OpenCL types to CIndex.


  • Option BreakBeforeInheritanceComma added to break before : and , in case of multiple inheritance in a class declaration. Enabled by default in the Mozilla coding style.

    true false
    class MyClass
        : public X
        , public Y {
    class MyClass : public X, public Y {
  • Align block comment decorations.

    Before After
    /* line 1
      * line 2
    /* line 1
     * line 2
  • The Clang-Format Style Options documentation provides detailed examples for most options.

  • Namespace end comments are now added or updated automatically.

    Before After
    namespace A {
    int i;
    int j;
    namespace A {
    int i;
    int j;
    } // namespace A
  • Comment reflow support added. Overly long comment lines will now be reflown with the rest of the paragraph instead of just broken. Option ReflowComments added and enabled by default.


  • Libclang now provides code-completion results for more C++ constructs and keywords. The following keywords/identifiers are now included in the code-completion results: static_assert, alignas, constexpr, final, noexcept, override and thread_local.

  • Libclang now provides code-completion results for members from dependent classes. For example:

    template<typename T>
    void appendValue(std::vector<T> &dest, const T &value) {
        dest. // Relevant completion results are now shown after '.'

    Note that code-completion results are still not provided when the member expression includes a dependent base expression. For example:

    template<typename T>
    void appendValue(std::vector<std::vector<T>> &dest, const T &value) { // Libclang fails to provide completion results after '.'

Static Analyzer

  • The static analyzer now supports using the z3 theorem prover from Microsoft Research as an external constraint solver. This allows reasoning over more complex queries, but performance is ~15x slower than the default range-based constraint solver. To enable the z3 solver backend, clang must be built with the CLANG_ANALYZER_BUILD_Z3=ON option, and the -Xanalyzer -analyzer-constraints=z3 arguments passed at runtime.

Undefined Behavior Sanitizer (UBSan)

  • The Undefined Behavior Sanitizer has a new check for pointer overflow. This check is on by default. The flag to control this functionality is -fsanitize=pointer-overflow.

    Pointer overflow is an indicator of undefined behavior: when a pointer indexing expression wraps around the address space, or produces other unexpected results, its result may not point to a valid object.

  • UBSan has several new checks which detect violations of nullability annotations. These checks are off by default. The flag to control this group of checks is -fsanitize=nullability. The checks can be individially enabled by -fsanitize=nullability-arg (which checks calls), -fsanitize=nullability-assign (which checks assignments), and -fsanitize=nullability-return (which checks return statements).

  • UBSan can now detect invalid loads from bitfields and from ObjC BOOLs.

  • UBSan can now avoid emitting unnecessary type checks in C++ class methods and in several other cases where the result is known at compile-time. UBSan can also avoid emitting unnecessary overflow checks in arithmetic expressions with promoted integer operands.

Python Binding Changes

Python bindings now support both Python 2 and Python 3.

The following methods have been added:

  • is_scoped_enum has been added to Cursor.
  • exception_specification_kind has been added to Cursor.
  • get_address_space has been added to Type.
  • get_typedef_name has been added to Type.
  • get_exception_specification_kind has been added to Type.

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 Subversion 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.

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