src/third_party/llvm-project/clang/docs/ReleaseNotes.rst - cobalt - Git at Google

 =========================
 Clang 7.0.0 Release Notes
 =========================

 .. contents::
    :local:
    :depth: 2

 Written by the `LLVM Team <https://llvm.org/>`_

 Introduction
 ============

 This document contains the release notes for the Clang C/C++/Objective-C
 frontend, part of the LLVM Compiler Infrastructure, release 7.0.0. 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 <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM
 releases may be downloaded from the `LLVM releases web
 site <https://llvm.org/releases/>`_.

 For more information about Clang or LLVM, including information about the
 latest release, please see the `Clang Web Site <https://clang.llvm.org>`_ or the
 `LLVM Web Site <https://llvm.org>`_.

 What's New in Clang 7.0.0?
 ==========================

 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
 ------------------

 - A new Implicit Conversion Sanitizer (``-fsanitize=implicit-conversion``) group
   was added. Please refer to the :ref:`release-notes-ubsan` section of the
   release notes for the details.

 - Preliminary/experimental support for DWARF v5 debugging information. If you
   compile with ``-gdwarf-5 -O0`` you should get fully conforming DWARF v5
   information, including the new .debug_names accelerator table. Type units
   and split DWARF are known not to conform, and higher optimization levels
   will likely get a mix of v4 and v5 formats.

 Improvements to Clang's diagnostics
 -----------------------------------

 - ``-Wc++98-compat-extra-semi`` is a new flag, which was previously inseparable
   from ``-Wc++98-compat-pedantic``. The latter still controls the new flag.

 - ``-Wextra-semi`` now also controls ``-Wc++98-compat-extra-semi``.
   Please do note that if you pass ``-Wno-c++98-compat-pedantic``, it implies
   ``-Wno-c++98-compat-extra-semi``, so if you want that diagnostic, you need
   to explicitly re-enable it (e.g. by appending ``-Wextra-semi``).

 - ``-Wself-assign`` and ``-Wself-assign-field`` were extended to diagnose
   self-assignment operations using overloaded operators (i.e. classes).
   If you are doing such an assignment intentionally, e.g. in a unit test for
   a data structure, the first warning can be disabled by passing
   ``-Wno-self-assign-overloaded``, also the warning can be suppressed by adding
   ``*&`` to the right-hand side or casting it to the appropriate reference type.

 Non-comprehensive list of changes in this release
 -------------------------------------------------

 - Clang binary and libraries have been renamed from 7.0 to 7.
   For example, the ``clang`` binary will be called ``clang-7``
   instead of ``clang-7.0``.

 - The optimization flag to merge constants (``-fmerge-all-constants``) is no
   longer applied by default.

 - Clang implements a collection of recent fixes to the C++ standard's definition
   of "standard-layout". In particular, a class is only considered to be
   standard-layout if all base classes and the first data member (or bit-field)
   can be laid out at offset zero.

 - Clang's handling of the GCC ``packed`` class attribute in C++ has been fixed
   to apply only to non-static data members and not to base classes. This fixes
   an ABI difference between Clang and GCC, but creates an ABI difference between
   Clang 7 and earlier versions. The old behavior can be restored by setting
   ``-fclang-abi-compat`` to ``6`` or lower.

 - Clang implements the proposed resolution of LWG issue 2358, along with the
   `corresponding change to the Itanium C++ ABI
   <https://github.com/itanium-cxx-abi/cxx-abi/pull/51>`_, which make classes
   containing only unnamed non-zero-length bit-fields be considered non-empty.
   This is an ABI break compared to prior Clang releases, but makes Clang
   generate code that is ABI-compatible with other compilers. The old
   behavior can be restored by setting ``-fclang-abi-compat`` to ``6`` or
   lower.

 - An existing tool named ``diagtool`` has been added to the release. As the
   name suggests, it helps with dealing with diagnostics in ``clang``, such as
   finding out the warning hierarchy, and which of them are enabled by default
   or for a particular compiler invocation.

 - By default, Clang emits an address-significance table into
   every ELF object file when using the integrated assembler.
   Address-significance tables allow linkers to implement `safe ICF
   <https://research.google.com/pubs/archive/36912.pdf>`_ without the false
   positives that can result from other implementation techniques such as
   relocation scanning. The ``-faddrsig`` and ``-fno-addrsig`` flags can be
   used to control whether to emit the address-significance table.

 - The integrated assembler is enabled by default on OpenBSD / FreeBSD
   for MIPS 64-bit targets.

 - On MIPS FreeBSD, default CPUs have been changed to ``mips2``
   for 32-bit targets and ``mips3`` for 64-bit targets.


 New Compiler Flags
 ------------------

 - ``-fstrict-float-cast-overflow`` and ``-fno-strict-float-cast-overflow``.

   When converting a floating-point value to int and the value is not
   representable in the destination integer type,
   the code has undefined behavior according to the language standard. By
   default, Clang will not guarantee any particular result in that case. With the
   'no-strict' option, Clang attempts to match the overflowing behavior of the
   target's native float-to-int conversion instructions.

 - ``-fforce-emit-vtables`` and ``-fno-force-emit-vtables``.

   In order to improve devirtualization, forces emission of vtables even in
   modules where it isn't necessary. It causes more inline virtual functions
   to be emitted.

 - Added the ``-mcrc`` and ``-mno-crc`` flags to enable/disable using
   of MIPS Cyclic Redundancy Check instructions.

 - Added the ``-mvirt`` and ``-mno-virt`` flags to enable/disable using
   of MIPS Virtualization instructions.

 - Added the ``-mginv`` and ``-mno-ginv`` flags to enable/disable using
   of MIPS Global INValidate instructions.


 Modified Compiler Flags
 -----------------------

 - Before Clang 7, we prepended the `#` character to the ``--autocomplete``
   argument to enable cc1 flags. For example, when the ``-cc1`` or ``-Xclang`` flag
   is in the :program:`clang` invocation, the shell executed
   ``clang --autocomplete=#-<flag to be completed>``. Clang 7 now requires the
   whole invocation including all flags to be passed to the ``--autocomplete`` like
   this: ``clang --autocomplete=-cc1,-xc++,-fsyn``.


 Attribute Changes in Clang
 --------------------------

 - Clang now supports function multiversioning with attribute 'target' on ELF
   based x86/x86-64 environments by using indirect functions. This implementation
   has a few minor limitations over the GCC implementation for the sake of AST
   sanity, however it is otherwise compatible with existing code using this
   feature for GCC. Consult the `documentation for the target attribute
   <AttributeReference.html#target-gnu-target>`_ for more information.

 Windows Support
 ---------------

 - clang-cl's support for precompiled headers has been much improved:

    - When using a pch file, clang-cl now no longer redundantly emits inline
      methods that are already stored in the obj that was built together with
      the pch file (matching cl.exe).  This speeds up builds using pch files
      by around 30%.

    - The ``/Ycfoo.h`` and ``/Yufoo.h`` flags can now be used without ``/FIfoo.h`` when
      foo.h is instead included by an explicit ``#include`` directive. This means
      Visual Studio's default stdafx.h setup now uses precompiled headers with
      clang-cl.

 - The alternative entry point names
   (``wmain``/``WinMain``/``wWinMain``/``DllMain``) now are properly mangled
   as plain C names in C++ contexts when targeting MinGW, without having to
   explicit specify ``extern "C"``. (This was already the case for MSVC
   targets.)


 Objective-C Language Changes in Clang
 -------------------------------------

 Clang now supports the GNUstep Objective-C ABI v2 on ELF platforms.  This is
 enabled with the ``-fobjc-runtime=gnustep-2.0`` flag.  The new ABI is incompatible
 with the older GNUstep ABIs, which were incremental changes on the old GCC ABI.
 The new ABI provides richer reflection metadata and allows the linker to remove
 duplicate selector and protocol definitions, giving smaller binaries.  Windows
 support for the new ABI is underway, but was not completed in time for the LLVM
 7.0.0 release.

 OpenCL C/C++ Language Changes in Clang
 --------------------------------------

 Miscellaneous changes in OpenCL C:

 - Added ``cles_khr_int64`` extension.

 - Added bug fixes and simplifications to Clang blocks in OpenCL mode.

 - Added compiler flag ``-cl-uniform-work-group-size`` to allow extra compile time optimisation.

 - Propagate ``denorms-are-zero`` attribute to IR if ``-cl-denorms-are-zero`` is passed to the compiler.

 - Separated ``read_only`` and ``write_only`` pipe IR types.

 - Fixed address space for the ``__func__`` predefined macro.

 - Improved diagnostics of kernel argument types.


 Started OpenCL C++ support:

 - Added ``-std/-cl-std=c++``.

 - Added support for keywords.

 OpenMP Support in Clang
 ----------------------------------

 - Clang gained basic support for OpenMP 4.5 offloading for NVPTX target.

   To compile your program for NVPTX target use the following options:
   ``-fopenmp -fopenmp-targets=nvptx64-nvidia-cuda`` for 64 bit platforms or
   ``-fopenmp -fopenmp-targets=nvptx-nvidia-cuda`` for 32 bit platform.

 - Passing options to the OpenMP device offloading toolchain can be done using
   the ``-Xopenmp-target=<triple> -opt=val`` flag. In this way the ``-opt=val``
   option will be forwarded to the respective OpenMP device offloading toolchain
   described by the triple. For example passing the compute capability to
   the OpenMP NVPTX offloading toolchain can be done as follows:
   ``-Xopenmp-target=nvptx64-nvidia-cuda -march=sm_60``. For the case when only one
   target offload toolchain is specified under the ``-fopenmp-targets=<triples>``
   option, then the triple can be skipped: ``-Xopenmp-target -march=sm_60``.

 - Other bugfixes.

 CUDA Support in Clang
 ---------------------

 - Clang will now try to locate the CUDA installation next to :program:`ptxas`
   in the `PATH` environment variable. This behavior can be turned off by passing
   the new flag ``--cuda-path-ignore-env``.

 - Clang now supports generating object files with relocatable device code. This
   feature needs to be enabled with ``-fcuda-rdc`` and may result in performance
   penalties compared to whole program compilation. Please note that NVIDIA's
   :program:`nvcc` must be used for linking.

 Internal API Changes
 --------------------

 These are major API changes that have happened since the 6.0.0 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.

 - The methods ``getLocStart``, ``getStartLoc`` and ``getLocEnd`` in the AST
   classes are deprecated.  New APIs ``getBeginLoc`` and ``getEndLoc`` should
   be used instead.  While the old methods remain in this release, they will
   not be present in the next release of Clang.

 clang-format
 ------------

 - Clang-format will now support detecting and formatting code snippets in raw
   string literals.  This is configured through the ``RawStringFormats`` style
   option.

 Static Analyzer
 ---------------

 - The new `MmapWriteExec` checker had been introduced to detect attempts to map pages both writable and executable.

 .. _release-notes-ubsan:

 Undefined Behavior Sanitizer (UBSan)
 ------------------------------------

 * A new Implicit Conversion Sanitizer (``-fsanitize=implicit-conversion``) group
   was added.

   Currently, only one type of issues is caught - implicit integer truncation
   (``-fsanitize=implicit-integer-truncation``), also known as integer demotion.
   While there is a ``-Wconversion`` diagnostic group that catches this kind of
   issues, it is both noisy, and does not catch **all** the cases.

   .. code-block:: c++

       unsigned char store = 0;

       bool consume(unsigned int val);

       void test(unsigned long val) {
         if (consume(val)) // the value may have been silently truncated.
           store = store + 768; // before addition, 'store' was promoted to int.
         (void)consume((unsigned int)val); // OK, the truncation is explicit.
       }

   Just like other ``-fsanitize=integer`` checks, these issues are **not**
   undefined behaviour. But they are not *always* intentional, and are somewhat
   hard to track down. This group is **not** enabled by ``-fsanitize=undefined``,
   but the ``-fsanitize=implicit-integer-truncation`` check
   is enabled by ``-fsanitize=integer``.


 libc++ Changes
 ==============
 Users that wish to link together translation units built with different
 versions of libc++'s headers into the same final linked image should define the
 `_LIBCPP_HIDE_FROM_ABI_PER_TU` macro to `1` when building those translation
 units. In a future release, not defining `_LIBCPP_HIDE_FROM_ABI_PER_TU` to `1`
 and linking translation units built with different versions of libc++'s headers
 together may lead to ODR violations and ABI issues.


 Additional Information
 ======================

 A wide variety of additional information is available on the `Clang web
 page <https://clang.llvm.org/>`_. 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 <https://lists.llvm.org/mailman/listinfo/cfe-dev>`_.
	=========================
	Clang 7.0.0 Release Notes
	=========================

	.. contents::
	:local:
	:depth: 2

	Written by the `LLVM Team <https://llvm.org/>`_

	Introduction
	============

	This document contains the release notes for the Clang C/C++/Objective-C
	frontend, part of the LLVM Compiler Infrastructure, release 7.0.0. 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 <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM
	releases may be downloaded from the `LLVM releases web
	site <https://llvm.org/releases/>`_.

	For more information about Clang or LLVM, including information about the
	latest release, please see the `Clang Web Site <https://clang.llvm.org>`_ or the
	`LLVM Web Site <https://llvm.org>`_.

	What's New in Clang 7.0.0?
	==========================

	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
	------------------

	- A new Implicit Conversion Sanitizer (``-fsanitize=implicit-conversion``) group
	was added. Please refer to the :ref:`release-notes-ubsan` section of the
	release notes for the details.

	- Preliminary/experimental support for DWARF v5 debugging information. If you
	compile with ``-gdwarf-5 -O0`` you should get fully conforming DWARF v5
	information, including the new .debug_names accelerator table. Type units
	and split DWARF are known not to conform, and higher optimization levels
	will likely get a mix of v4 and v5 formats.

	Improvements to Clang's diagnostics
	-----------------------------------

	- ``-Wc++98-compat-extra-semi`` is a new flag, which was previously inseparable
	from ``-Wc++98-compat-pedantic``. The latter still controls the new flag.

	- ``-Wextra-semi`` now also controls ``-Wc++98-compat-extra-semi``.
	Please do note that if you pass ``-Wno-c++98-compat-pedantic``, it implies
	``-Wno-c++98-compat-extra-semi``, so if you want that diagnostic, you need
	to explicitly re-enable it (e.g. by appending ``-Wextra-semi``).

	- ``-Wself-assign`` and ``-Wself-assign-field`` were extended to diagnose
	self-assignment operations using overloaded operators (i.e. classes).
	If you are doing such an assignment intentionally, e.g. in a unit test for
	a data structure, the first warning can be disabled by passing
	``-Wno-self-assign-overloaded``, also the warning can be suppressed by adding
	``*&`` to the right-hand side or casting it to the appropriate reference type.

	Non-comprehensive list of changes in this release
	-------------------------------------------------

	- Clang binary and libraries have been renamed from 7.0 to 7.
	For example, the ``clang`` binary will be called ``clang-7``
	instead of ``clang-7.0``.

	- The optimization flag to merge constants (``-fmerge-all-constants``) is no
	longer applied by default.

	- Clang implements a collection of recent fixes to the C++ standard's definition
	of "standard-layout". In particular, a class is only considered to be
	standard-layout if all base classes and the first data member (or bit-field)
	can be laid out at offset zero.

	- Clang's handling of the GCC ``packed`` class attribute in C++ has been fixed
	to apply only to non-static data members and not to base classes. This fixes
	an ABI difference between Clang and GCC, but creates an ABI difference between
	Clang 7 and earlier versions. The old behavior can be restored by setting
	``-fclang-abi-compat`` to ``6`` or lower.

	- Clang implements the proposed resolution of LWG issue 2358, along with the
	`corresponding change to the Itanium C++ ABI
	<https://github.com/itanium-cxx-abi/cxx-abi/pull/51>`_, which make classes
	containing only unnamed non-zero-length bit-fields be considered non-empty.
	This is an ABI break compared to prior Clang releases, but makes Clang
	generate code that is ABI-compatible with other compilers. The old
	behavior can be restored by setting ``-fclang-abi-compat`` to ``6`` or
	lower.

	- An existing tool named ``diagtool`` has been added to the release. As the
	name suggests, it helps with dealing with diagnostics in ``clang``, such as
	finding out the warning hierarchy, and which of them are enabled by default
	or for a particular compiler invocation.

	- By default, Clang emits an address-significance table into
	every ELF object file when using the integrated assembler.
	Address-significance tables allow linkers to implement `safe ICF
	<https://research.google.com/pubs/archive/36912.pdf>`_ without the false
	positives that can result from other implementation techniques such as
	relocation scanning. The ``-faddrsig`` and ``-fno-addrsig`` flags can be
	used to control whether to emit the address-significance table.

	- The integrated assembler is enabled by default on OpenBSD / FreeBSD
	for MIPS 64-bit targets.

	- On MIPS FreeBSD, default CPUs have been changed to ``mips2``
	for 32-bit targets and ``mips3`` for 64-bit targets.


	New Compiler Flags
	------------------

	- ``-fstrict-float-cast-overflow`` and ``-fno-strict-float-cast-overflow``.

	When converting a floating-point value to int and the value is not
	representable in the destination integer type,
	the code has undefined behavior according to the language standard. By
	default, Clang will not guarantee any particular result in that case. With the
	'no-strict' option, Clang attempts to match the overflowing behavior of the
	target's native float-to-int conversion instructions.

	- ``-fforce-emit-vtables`` and ``-fno-force-emit-vtables``.

	In order to improve devirtualization, forces emission of vtables even in
	modules where it isn't necessary. It causes more inline virtual functions
	to be emitted.

	- Added the ``-mcrc`` and ``-mno-crc`` flags to enable/disable using
	of MIPS Cyclic Redundancy Check instructions.

	- Added the ``-mvirt`` and ``-mno-virt`` flags to enable/disable using
	of MIPS Virtualization instructions.

	- Added the ``-mginv`` and ``-mno-ginv`` flags to enable/disable using
	of MIPS Global INValidate instructions.


	Modified Compiler Flags
	-----------------------

	- Before Clang 7, we prepended the `#` character to the ``--autocomplete``
	argument to enable cc1 flags. For example, when the ``-cc1`` or ``-Xclang`` flag
	is in the :program:`clang` invocation, the shell executed
	``clang --autocomplete=#-<flag to be completed>``. Clang 7 now requires the
	whole invocation including all flags to be passed to the ``--autocomplete`` like
	this: ``clang --autocomplete=-cc1,-xc++,-fsyn``.


	Attribute Changes in Clang
	--------------------------

	- Clang now supports function multiversioning with attribute 'target' on ELF
	based x86/x86-64 environments by using indirect functions. This implementation
	has a few minor limitations over the GCC implementation for the sake of AST
	sanity, however it is otherwise compatible with existing code using this
	feature for GCC. Consult the `documentation for the target attribute
	<AttributeReference.html#target-gnu-target>`_ for more information.

	Windows Support
	---------------

	- clang-cl's support for precompiled headers has been much improved:

	- When using a pch file, clang-cl now no longer redundantly emits inline
	methods that are already stored in the obj that was built together with
	the pch file (matching cl.exe). This speeds up builds using pch files
	by around 30%.

	- The ``/Ycfoo.h`` and ``/Yufoo.h`` flags can now be used without ``/FIfoo.h`` when
	foo.h is instead included by an explicit ``#include`` directive. This means
	Visual Studio's default stdafx.h setup now uses precompiled headers with
	clang-cl.

	- The alternative entry point names
	(``wmain``/``WinMain``/``wWinMain``/``DllMain``) now are properly mangled
	as plain C names in C++ contexts when targeting MinGW, without having to
	explicit specify ``extern "C"``. (This was already the case for MSVC
	targets.)


	Objective-C Language Changes in Clang
	-------------------------------------

	Clang now supports the GNUstep Objective-C ABI v2 on ELF platforms. This is
	enabled with the ``-fobjc-runtime=gnustep-2.0`` flag. The new ABI is incompatible
	with the older GNUstep ABIs, which were incremental changes on the old GCC ABI.
	The new ABI provides richer reflection metadata and allows the linker to remove
	duplicate selector and protocol definitions, giving smaller binaries. Windows
	support for the new ABI is underway, but was not completed in time for the LLVM
	7.0.0 release.

	OpenCL C/C++ Language Changes in Clang
	--------------------------------------

	Miscellaneous changes in OpenCL C:

	- Added ``cles_khr_int64`` extension.

	- Added bug fixes and simplifications to Clang blocks in OpenCL mode.

	- Added compiler flag ``-cl-uniform-work-group-size`` to allow extra compile time optimisation.

	- Propagate ``denorms-are-zero`` attribute to IR if ``-cl-denorms-are-zero`` is passed to the compiler.

	- Separated ``read_only`` and ``write_only`` pipe IR types.

	- Fixed address space for the ``__func__`` predefined macro.

	- Improved diagnostics of kernel argument types.


	Started OpenCL C++ support:

	- Added ``-std/-cl-std=c++``.

	- Added support for keywords.

	OpenMP Support in Clang
	----------------------------------

	- Clang gained basic support for OpenMP 4.5 offloading for NVPTX target.

	To compile your program for NVPTX target use the following options:
	``-fopenmp -fopenmp-targets=nvptx64-nvidia-cuda`` for 64 bit platforms or
	``-fopenmp -fopenmp-targets=nvptx-nvidia-cuda`` for 32 bit platform.

	- Passing options to the OpenMP device offloading toolchain can be done using
	the ``-Xopenmp-target=<triple> -opt=val`` flag. In this way the ``-opt=val``
	option will be forwarded to the respective OpenMP device offloading toolchain
	described by the triple. For example passing the compute capability to
	the OpenMP NVPTX offloading toolchain can be done as follows:
	``-Xopenmp-target=nvptx64-nvidia-cuda -march=sm_60``. For the case when only one
	target offload toolchain is specified under the ``-fopenmp-targets=<triples>``
	option, then the triple can be skipped: ``-Xopenmp-target -march=sm_60``.

	- Other bugfixes.

	CUDA Support in Clang
	---------------------

	- Clang will now try to locate the CUDA installation next to :program:`ptxas`
	in the `PATH` environment variable. This behavior can be turned off by passing
	the new flag ``--cuda-path-ignore-env``.

	- Clang now supports generating object files with relocatable device code. This
	feature needs to be enabled with ``-fcuda-rdc`` and may result in performance
	penalties compared to whole program compilation. Please note that NVIDIA's
	:program:`nvcc` must be used for linking.

	Internal API Changes
	--------------------

	These are major API changes that have happened since the 6.0.0 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.

	- The methods ``getLocStart``, ``getStartLoc`` and ``getLocEnd`` in the AST
	classes are deprecated. New APIs ``getBeginLoc`` and ``getEndLoc`` should
	be used instead. While the old methods remain in this release, they will
	not be present in the next release of Clang.

	clang-format
	------------

	- Clang-format will now support detecting and formatting code snippets in raw
	string literals. This is configured through the ``RawStringFormats`` style
	option.

	Static Analyzer
	---------------

	- The new `MmapWriteExec` checker had been introduced to detect attempts to map pages both writable and executable.

	.. _release-notes-ubsan:

	Undefined Behavior Sanitizer (UBSan)
	------------------------------------

	* A new Implicit Conversion Sanitizer (``-fsanitize=implicit-conversion``) group
	was added.

	Currently, only one type of issues is caught - implicit integer truncation
	(``-fsanitize=implicit-integer-truncation``), also known as integer demotion.
	While there is a ``-Wconversion`` diagnostic group that catches this kind of
	issues, it is both noisy, and does not catch all the cases.

	.. code-block:: c++

	unsigned char store = 0;

	bool consume(unsigned int val);

	void test(unsigned long val) {
	if (consume(val)) // the value may have been silently truncated.
	store = store + 768; // before addition, 'store' was promoted to int.
	(void)consume((unsigned int)val); // OK, the truncation is explicit.
	}

	Just like other ``-fsanitize=integer`` checks, these issues are not
	undefined behaviour. But they are not always intentional, and are somewhat
	hard to track down. This group is not enabled by ``-fsanitize=undefined``,
	but the ``-fsanitize=implicit-integer-truncation`` check
	is enabled by ``-fsanitize=integer``.


	libc++ Changes
	==============
	Users that wish to link together translation units built with different
	versions of libc++'s headers into the same final linked image should define the
	`_LIBCPP_HIDE_FROM_ABI_PER_TU` macro to `1` when building those translation
	units. In a future release, not defining `_LIBCPP_HIDE_FROM_ABI_PER_TU` to `1`
	and linking translation units built with different versions of libc++'s headers
	together may lead to ODR violations and ABI issues.


	Additional Information
	======================

	A wide variety of additional information is available on the `Clang web
	page <https://clang.llvm.org/>`_. 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 <https://lists.llvm.org/mailman/listinfo/cfe-dev>`_.