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=========================
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Clang 6.0.0 Release Notes
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=========================
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.. contents::
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:local:
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:depth: 2
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Written by the `LLVM Team <http://llvm.org/>`_
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Introduction
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============
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This document contains the release notes for the Clang C/C++/Objective-C
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frontend, part of the LLVM Compiler Infrastructure, release 6.0.0. Here we
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describe the status of Clang in some detail, including major
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improvements from the previous release and new feature work. For the
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general LLVM release notes, see `the LLVM
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documentation <http://llvm.org/docs/ReleaseNotes.html>`_. All LLVM
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releases may be downloaded from the `LLVM releases web
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site <http://llvm.org/releases/>`_.
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For more information about Clang or LLVM, including information about the
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latest release, please see the `Clang Web Site <http://clang.llvm.org>`_ or the
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`LLVM Web Site <http://llvm.org>`_.
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What's New in Clang 6.0.0?
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==========================
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Some of the major new features and improvements to Clang are listed
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here. Generic improvements to Clang as a whole or to its underlying
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infrastructure are described first, followed by language-specific
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sections with improvements to Clang's support for those languages.
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Non-comprehensive list of changes in this release
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-------------------------------------------------
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- Support for `retpolines <https://support.google.com/faqs/answer/7625886>`_
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was added to help mitigate "branch target injection" (variant #2) of the
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"Spectre" speculative side channels described by `Project Zero
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<https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html>`_
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and the `Spectre paper <https://spectreattack.com/spectre.pdf>`_.
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- Bitrig OS was merged back into OpenBSD, so Bitrig support has been
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removed from Clang/LLVM.
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- The default value of ``_MSC_VER`` was raised from 1800 to 1911, making it
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compatible with the Visual Studio 2015 and 2017 C++ standard library headers.
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Users should generally expect this to be regularly raised to match the most
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recently released version of the Visual C++ compiler.
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- clang now defaults to ``.init_array`` if no gcc installation can be found.
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If a gcc installation is found, it still prefers ``.ctors`` if the found
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gcc is older than 4.7.0.
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- The new builtin preprocessor macros ``__is_target_arch``,
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``__is_target_vendor``, ``__is_target_os``, and ``__is_target_environment``
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can be used to to examine the individual components of the target triple.
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Improvements to Clang's diagnostics
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-----------------------------------
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- ``-Wpragma-pack`` is a new warning that warns in the following cases:
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- When a translation unit is missing terminating ``#pragma pack (pop)``
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directives.
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- When leaving an included file that changes the current alignment value,
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i.e. when the alignment before ``#include`` is different to the alignment
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after ``#include``.
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- ``-Wpragma-pack-suspicious-include`` (disabled by default) warns on an
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``#include`` when the included file contains structures or unions affected by
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a non-default alignment that has been specified using a ``#pragma pack``
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directive prior to the ``#include``.
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- ``-Wobjc-messaging-id`` is a new, non-default warning that warns about
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message sends to unqualified ``id`` in Objective-C. This warning is useful
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for projects that would like to avoid any potential future compiler
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errors/warnings, as the system frameworks might add a method with the same
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selector which could make the message send to ``id`` ambiguous.
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- ``-Wtautological-compare`` now warns when comparing an unsigned integer and 0
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regardless of whether the constant is signed or unsigned.
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- ``-Wtautological-compare`` now warns about comparing a signed integer and 0
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when the signed integer is coerced to an unsigned type for the comparison.
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``-Wsign-compare`` was adjusted not to warn in this case.
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- ``-Wtautological-constant-compare`` is a new warning that warns on
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tautological comparisons between integer variable of the type ``T`` and the
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largest/smallest possible integer constant of that same type.
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- For C code, ``-Wsign-compare``, ``-Wsign-conversion``,
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``-Wtautological-constant-compare`` and
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``-Wtautological-constant-out-of-range-compare`` were adjusted to use the
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underlying datatype of ``enum``.
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- ``-Wnull-pointer-arithmetic`` now warns about performing pointer arithmetic
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on a null pointer. Such pointer arithmetic has an undefined behavior if the
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offset is nonzero. It also now warns about arithmetic on a null pointer
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treated as a cast from integer to pointer (GNU extension).
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- ``-Wzero-as-null-pointer-constant`` was adjusted not to warn on null pointer
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constants that originate from system macros, except ``NULL`` macro.
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- ``-Wdelete-non-virtual-dtor`` can now fire in system headers, so that
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``std::unique_ptr<>`` deleting through a non-virtual dtor is now diagnosed.
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- ``-Wunreachable-code`` can now reason about ``__try``, ``__except`` and
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``__leave``.
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New Compiler Flags
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------------------
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- Clang supports the ``-mretpoline`` flag to enable `retpolines
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<https://support.google.com/faqs/answer/7625886>`_. Code compiled with this
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flag will be hardened against variant #2 of the Spectre attack. Indirect
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branches from switches or gotos removed from the code, and indirect calls
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will be made through a "retpoline" thunk. The necessary thunks will
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automatically be inserted into the generated code. Clang also supports
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``-mretpoline-external-thunk`` which works like ``-mretpoline`` but requires
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the user to provide their own thunk definitions. The external thunk names
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start with ``__x86_indirect_thunk_`` and end in a register name. For 64-bit
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platforms, only an ``r11`` thunk is used, but for 32-bit platforms ``eax``,
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``ecx``, ``edx``, and ``edi`` thunks are used.
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- Clang now supports configuration files. These are collections of driver
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options, which can be applied by specifying the configuration file, either
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using command line option ``--config foo.cfg`` or encoding it into executable
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name ``foo-clang``. Clang behaves as if the options from this file were inserted
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before the options specified in command line. This feature is primary intended
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to facilitate cross compilation. Details can be found in
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`Clang Compiler User's Manual <UsersManual.html#configuration-files>`_.
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- The ``-fdouble-square-bracket-attributes`` and corresponding
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``-fno-double-square-bracket-attributes`` flags were added to enable or
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disable ``[[]]`` attributes in any language mode. Currently, only a limited
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number of attributes are supported outside of C++ mode. See the Clang
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`attribute documentation <AttributeReference.html>`_ for more information
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about which attributes are supported for each syntax.
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- Added the ``-std=c17``, ``-std=gnu17``, and ``-std=iso9899:2017`` language
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mode flags for compatibility with GCC. This enables support for the next
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version of the C standard, expected to be published by ISO in 2018. The only
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difference between the ``-std=c17`` and ``-std=c11`` language modes is the
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value of the ``__STDC_VERSION__`` macro, as C17 is a bug fix release.
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- Added the ``-fexperimental-isel`` and ``-fno-experimental-isel`` flags to
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enable/disable the new GlobalISel instruction selection framework. This
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feature is enabled by default for AArch64 at the ``-O0`` optimization level.
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Support for other targets or optimization levels is currently incomplete.
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- New ``-nostdlib++`` flag to disable linking the C++ standard library. Similar
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to using ``clang`` instead of ``clang++`` but doesn't disable ``-lm``.
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Attribute Changes in Clang
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--------------------------
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- Clang now supports the majority of its attributes under both the GNU-style
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spelling (``__attribute((name))``) and the double square-bracket spelling
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in the ``clang`` vendor namespace (``[[clang::name]]``). Attributes whose
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syntax is specified by some other standard (such as CUDA and OpenCL
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attributes) continue to follow their respective specification.
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- Added the ``__has_c_attribute()`` builtin preprocessor macro which allows
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users to dynamically detect whether a double square-bracket attribute is
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supported in C mode. This attribute syntax can be enabled with the
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``-fdouble-square-bracket-attributes`` flag.
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- The presence of ``__attribute__((availability(...)))`` on a declaration no
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longer implies default visibility for that declaration on macOS.
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Windows Support
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---------------
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- Clang now has initial, preliminary support for targeting Windows on
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ARM64.
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- clang-cl now exposes the ``--version`` flag.
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C++ Language Changes in Clang
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-----------------------------
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- Clang's default C++ dialect is now ``gnu++14`` instead of ``gnu++98``. This
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means Clang will by default accept code using features from C++14 and
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conforming GNU extensions. Projects incompatible with C++14 can add
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``-std=gnu++98`` to their build settings to restore the previous behaviour.
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- Added support for some features from the C++ standard after C++17
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(provisionally known as C++2a but expected to be C++20). This support can be
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enabled with the ``-std=c++2a`` flag. This enables:
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- Support for ``__VA_OPT__``, to allow variadic macros to easily provide
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different expansions when they are invoked without variadic arguments.
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- Recognition of the ``<=>`` token (the C++2a three-way comparison operator).
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- Support for default member initializers for bit-fields.
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- Lambda capture of ``*this``.
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- Pointer-to-member calls using ``const &``-qualified pointers on temporary objects.
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All of these features other than ``__VA_OPT__`` and ``<=>`` are made
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available with a warning in earlier C++ language modes.
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- A warning has been added for a ``<=`` token followed immediately by a ``>``
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character. Code containing such constructs will change meaning in C++2a due
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to the addition of the ``<=>`` operator.
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- Clang implements the "destroying operator delete" feature described in C++
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committee paper `P0722R1
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<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0722r1.html>`,
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which is targeting inclusion in C++2a but has not yet been voted into the C++
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working draft. Support for this feature is enabled by the presence of the
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standard library type ``std::destroying_delete_t``.
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OpenCL C Language Changes in Clang
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----------------------------------
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- Added subgroup builtins to enqueue kernel support.
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- Added CL2.0 atomics as Clang builtins that now accept
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an additional memory scope parameter propagated to atomic IR instructions
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(this is to align with the corresponding change in LLVM IR) (see `spec s6.13.11.4
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<https://www.khronos.org/registry/OpenCL/specs/opencl-2.0-openclc.pdf#107>`_).
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- Miscellaneous fixes in the CL header.
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- Allow per target selection of address space during CodeGen of certain OpenCL types.
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Default target implementation is provided mimicking old behavior.
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- Macro ``__IMAGE_SUPPORT__`` is now automatically added (as per `spec s6.10
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<https://www.khronos.org/registry/OpenCL/specs/opencl-2.0-openclc.pdf#55>`_).
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- Added ``cl_intel_subgroups`` and ``cl_intel_subgroups_short`` extensions.
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- All function calls are marked by `the convergent attribute
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<https://clang.llvm.org/docs/AttributeReference.html#convergent-clang-convergent>`_
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to prevent optimizations that break SPMD program semantics. This will be removed
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by LLVM passes if it can be proved that the function does not use convergent
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operations.
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- Create a kernel wrapper for enqueued blocks, which simplifies enqueue support by
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providing common functionality.
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- Added private address space explicitly in AST and refactored address space support
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with several simplifications and bug fixes (`PR33419 <https://llvm.org/pr33419>`_
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and `PR33420 <https://llvm.org/pr33420>`_).
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- OpenCL now allows functions with empty parameters to be treated as if they had a
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void parameter list (inspired from C++ support). OpenCL C spec update to follow.
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- General miscellaneous refactoring and cleanup of blocks support for OpenCL to
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remove unused parts inherited from Objective C implementation.
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- Miscellaneous improvements in vector diagnostics.
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- Added half float load and store builtins without enabling half as a legal type
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(``__builtin_store_half`` for double, ``__builtin_store_halff`` for float,
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``__builtin_load_half`` for double, ``__builtin_load_halff`` for float).
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OpenMP Support in Clang
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----------------------------------
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- Added options ``-f[no]-openmp-simd`` that support code emission only for OpenMP
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SIMD-based directives, like ``#pragma omp simd``, ``#pragma omp parallel for simd``
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etc. The code is emitted only for SIMD-based part of the combined directives
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and clauses.
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- Added support for almost all target-based directives except for
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``#pragma omp target teams distribute parallel for [simd]``. Although, please
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note that ``depend`` clauses on target-based directives are not supported yet.
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Clang supports offloading to X86_64, AArch64 and PPC64[LE] devices.
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- Added support for ``reduction``-based clauses on ``task``-based directives from
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upcoming OpenMP 5.0.
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- The LLVM OpenMP runtime ``libomp`` now supports the OpenMP Tools Interface (OMPT)
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on x86, x86_64, AArch64, and PPC64 on Linux, Windows, and macOS. If you observe
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a measurable performance impact on one of your applications without a tool
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attached, please rebuild the runtime library with ``-DLIBOMP_OMPT_SUPPORT=OFF`` and
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file a bug at `LLVM's Bugzilla <https://bugs.llvm.org/>`_ or send a message to the
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`OpenMP development list <http://lists.llvm.org/cgi-bin/mailman/listinfo/openmp-dev>`_.
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AST Matchers
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------------
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The ``hasDeclaration`` matcher now works the same for ``Type`` and ``QualType`` and only
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ever looks through one level of sugaring in a limited number of cases.
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There are two main patterns affected by this:
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- ``qualType(hasDeclaration(recordDecl(...)))``: previously, we would look through
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sugar like ``TypedefType`` to get at the underlying ``recordDecl``; now, we need
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to explicitly remove the sugaring:
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``qualType(hasUnqualifiedDesugaredType(hasDeclaration(recordDecl(...))))``
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- ``hasType(recordDecl(...))``: ``hasType`` internally uses ``hasDeclaration``; previously,
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this matcher used to match for example ``TypedefTypes`` of the ``RecordType``, but
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after the change they don't; to fix, use:
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.. code-block:: c
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hasType(hasUnqualifiedDesugaredType(
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recordType(hasDeclaration(recordDecl(...)))))
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- ``templateSpecializationType(hasDeclaration(classTemplateDecl(...)))``:
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previously, we would directly match the underlying ``ClassTemplateDecl``;
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now, we can explicitly match the ``ClassTemplateSpecializationDecl``, but that
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requires to explicitly get the ``ClassTemplateDecl``:
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.. code-block:: c
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templateSpecializationType(hasDeclaration(
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classTemplateSpecializationDecl(
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hasSpecializedTemplate(classTemplateDecl(...)))))
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clang-format
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------------
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* Option ``IndentPPDirectives`` added to indent preprocessor directives on
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conditionals.
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+----------------------+----------------------+
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| Before | After |
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+======================+======================+
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| .. code-block:: c++ | .. code-block:: c++ |
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| | |
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| #if FOO | #if FOO |
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| #if BAR | # if BAR |
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| #include <foo> | # include <foo> |
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| #endif | # endif |
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| #endif | #endif |
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+----------------------+----------------------+
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* Option ``-verbose`` added to the command line.
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Shows the list of processed files.
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* Option ``IncludeBlocks`` added to merge and regroup multiple ``#include`` blocks during sorting.
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+-------------------------+-------------------------+-------------------------+
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| Before (Preserve) | Merge | Regroup |
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+=========================+=========================+=========================+
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| .. code-block:: c++ | .. code-block:: c++ | .. code-block:: c++ |
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| | | |
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| #include "b.h" | #include "a.h" | #include "a.h" |
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| | #include "b.h" | #include "b.h" |
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| #include "a.b" | #include <lib/main.h> | |
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| #include <lib/main.h> | | #include <lib/main.h> |
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+-------------------------+-------------------------+-------------------------+
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Static Analyzer
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---------------
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- The Static Analyzer can now properly detect and diagnose unary pre-/post-
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increment/decrement on an uninitialized value.
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Undefined Behavior Sanitizer (UBSan)
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------------------------------------
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* A minimal runtime is now available. It is suitable for use in production
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environments, and has a small attack surface. It only provides very basic
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issue logging and deduplication, and does not support ``-fsanitize=vptr``
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checking.
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Additional Information
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======================
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A wide variety of additional information is available on the `Clang web
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page <http://clang.llvm.org/>`_. The web page contains versions of the
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API documentation which are up-to-date with the Subversion version of
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the source code. You can access versions of these documents specific to
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this release by going into the "``clang/docs/``" directory in the Clang
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tree.
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If you have any questions or comments about Clang, please feel free to
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contact us via the `mailing
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list <http://lists.llvm.org/mailman/listinfo/cfe-dev>`_.
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