ReleaseNotes: Formatting and typo fixes.

Improvements to Clang's diagnostics

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

- -Wtautological-overlap-compare will warn on negative numbers and non-int

- ``-Wtautological-overlap-compare`` will warn on negative numbers and non-int

  types.

- -Wtautological-compare for self comparisons and

  -Wtautological-overlap-compare will now look through member and array

- ``-Wtautological-compare`` for self comparisons and

  ``-Wtautological-overlap-compare`` will now look through member and array

  access to determine if two operand expressions are the same.

- -Wtautological-bitwise-compare is a new warning group.  This group has the

- ``-Wtautological-bitwise-compare`` is a new warning group.  This group has the

  current warning which diagnoses the tautological comparison of a bitwise

  operation and a constant. The group also has the new warning which diagnoses

  when a bitwise-or with a non-negative value is converted to a bool, since

  that bool will always be true.

- -Wbitwise-conditional-parentheses will warn on operator precedence issues

- ``-Wbitwise-conditional-parentheses`` will warn on operator precedence issues

  when mixing bitwise-and (&) and bitwise-or (|) operator with the

  conditional operator (?:).

- -Wrange-loop-analysis got several improvements. It no longer warns about a

- ``-Wrange-loop-analysis`` got several improvements. It no longer warns about a

  copy being made when the result is bound to an rvalue reference. It no longer

  warns when an object of a small, trivially copyable type is copied. The

  warning now offers fixits. Excluding -Wrange-loop-bind-reference it is now

  part of -Wall. To reduce the number of false positives the diagnostic is

  warning now offers fix-its. Excluding ``-Wrange-loop-bind-reference`` it is now

  part of ``-Wall``. To reduce the number of false positives the diagnostic is

  disabled in macros and template instantiations.

- -Wmisleading-indentation has been added. This warning is similar to the GCC

- ``-Wmisleading-indentation`` has been added. This warning is similar to the GCC

  warning of the same name. It warns about statements that are indented as if

  they were part of a if/else/for/while statement but are not semantically

  part of that if/else/for/while.

- -Wbitwise-op-parentheses and -Wlogical-op-parentheses are disabled by default.

- ``-Wbitwise-op-parentheses`` and ``-Wlogical-op-parentheses`` are disabled by default.

Non-comprehensive list of changes in this release

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

  learned to sanitize pre/post increment/decrement of types with bit width

  smaller than ``int``.

* For X86 target, -march=skylake-avx512, -march=icelake-client,

  -march=icelake-server, -march=cascadelake, -march=cooperlake will default to

* For X86 target, ``-march=skylake-avx512``, ``-march=icelake-client``,

  ``-march=icelake-server``, ``-march=cascadelake``, ``-march=cooperlake`` will default to

  not using 512-bit zmm registers in vectorized code unless 512-bit intrinsics

  are used in the source code. 512-bit operations are known to cause the CPUs

  to run at a lower frequency which can impact performance. This behavior can be

  changed by passing -mprefer-vector-width=512 on the command line.

  changed by passing ``-mprefer-vector-width=512`` on the command line.

* clang now defaults to ``.init_array`` on Linux. It used to use ``.ctors`` if

  the found gcc installation is older than 4.7.0. Add ``-fno-use-init-array`` to

* Clang now defaults to ``.init_array`` on Linux. It used to use ``.ctors`` if

  the found GCC installation is older than 4.7.0. Add ``-fno-use-init-array`` to

  get the old behavior (``.ctors``).

* The behavior of the flag ``-flax-vector-conversions`` has been modified to

New Compiler Flags

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

- The -fgnuc-version= flag now controls the value of ``__GNUC__`` and related

- The ``-fgnuc-version=`` flag now controls the value of ``__GNUC__`` and related

  macros. This flag does not enable or disable any GCC extensions implemented in

  Clang. Setting the version to zero causes Clang to leave ``__GNUC__`` and

  other GNU-namespaced macros, such as ``__GXX_WEAK__``, undefined.

- vzeroupper insertion on X86 targets can now be disabled with -mno-vzeroupper.

- vzeroupper insertion on X86 targets can now be disabled with ``-mno-vzeroupper``.

  You can also force vzeroupper insertion to be used on CPUs that normally

  wouldn't with -mvzeroupper.

  wouldn't with ``-mvzeroupper``.

- The -fno-concept-satisfaction-caching can be used to disable caching for

- The ``-fno-concept-satisfaction-caching`` can be used to disable caching for

  satisfactions of Concepts. The C++2a draft standard does not currently permit

  this caching, but disabling it may incur significant compile-time costs. This

  flag is intended for experimentation purposes and may be removed at any time;

The following options are deprecated and ignored. They will be removed in

future versions of Clang.

- -mmpx used to enable the __MPX__ preprocessor define for the Intel MPX

- ``-mmpx`` used to enable the ``__MPX__`` preprocessor define for the Intel MPX

  instructions. There were no MPX intrinsics.

- -mno-mpx used to disable -mmpx and is the default behavior.

- ``-mno-mpx`` used to disable ``-mmpx`` and is the default behavior.

- -fconcepts-ts previously used to enable experimental concepts support. Use

- ``-fconcepts-ts`` previously used to enable experimental concepts support. Use

  -std=c++2a instead to enable Concepts support.

Modified Compiler Flags

ABI Changes in Clang

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

- gcc passes vectors of __int128 in memory on X86-64. Clang historically

- GCC passes vectors of __int128 in memory on X86-64. Clang historically

  broke the vectors into multiple scalars using two 64-bit values for each

  element. Clang now matches the gcc behavior on Linux and NetBSD. You can

  switch back to old API behavior with flag: -fclang-abi-compat=9.0.

  element. Clang now matches the GCC behavior on Linux and NetBSD. You can

  switch back to old API behavior with flag: ``-fclang-abi-compat=9.0``.

- RISC-V now chooses a default ``-march=`` and ``-mabi=`` to match (in almost

  all cases) the GCC defaults. On baremetal targets, where neither ``-march=``

  nor ``-mabi=`` are specified, Clang now differs from GCC by defaulting to

  ``-march=rv32imac -mabi=ilp32`` or ``-march=rv64imac -mabi=lp64`` depending on

  the architecture in the target triple. These do not always match the defaults

  in Clang 9. We strongly suggest that you explicitly pass `-march=` and

  `-mabi=` when compiling for RISC-V, due to how extensible this architecture

  is.

  ``-march=rv32imac`` ``-mabi=ilp32`` or ``-march=rv64imac`` ``-mabi=lp64``

  depending on the architecture in the target triple. These do not always match

  the defaults in Clang 9. We strongly suggest that you explicitly pass

  ``-march=`` and ``-mabi=`` when compiling for RISC-V, due to how extensible

  this architecture is.

- RISC-V now uses `target-abi` module metadata to encode the chosen psABI. This

  ensures that the correct lowering will be done by LLVM when LTO is enabled.

- Clang-format has a new option called ``--dry-run`` or ``-n`` to emit a

  warning for clang-format violations. This can be used together

  with --ferror-limit=N to limit the number of warnings per file and --Werror

  with ``--ferror-limit=N`` to limit the number of warnings per file and ``--Werror``

  to make warnings into errors.

- Option *IncludeIsMainSourceRegex* has been added to allow for additional

* Added support for Cortex-A65, Cortex-A65AE, Neoverse E1 and Neoverse N1 cores.

* With a few more bugs fixed in the LLVM 10 release, clang-cl can now target windows-on-Arm well, demonstrated by building complex pieces of software such as Chromium and the Electron framework.

* With a few more bugs fixed in the LLVM 10 release, clang-cl can now target

  Windows-on-ARM well, demonstrated by building complex pieces of software such

  as Chromium and the Electron framework.

* Support for -fpatchable-function-entry was added.

* Support for ``-fpatchable-function-entry`` was added.

Changes to the ARM Backend

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

* Optimized Armv8.1-M code generation, including generating Low Overhead Loops.

* Optimized ARMv8.1-M code generation, including generating Low Overhead Loops.

* Added auto-vectorization for the Armv8.1-M MVE vector extension.

* Added auto-vectorization for the ARMv8.1-M MVE vector extension.

* Support was added for inline asm constraints s,j,x,N,O.

* Enabled vectorization of math routines on PowerPC using MASSV (Mathematical Acceleration SubSystem) library

copiler-rt:

compiler-rt:

* Added/improved conversion functions from IBM long double to 128-bit integers

* v32i8 and v64i8 vectors with AVX512F enabled, but AVX512BW disabled will now

  be passed in ZMM registers for calls and returns. Previously they were passed

  in two YMM registers. Old behavior can be enabled by passing

  -x86-enable-old-knl-abi

  ``-x86-enable-old-knl-abi``.

* -mprefer-vector-width=256 is now the default behavior skylake-avx512 and later

  Intel CPUs. This tries to limit the use of 512-bit registers which can cause a

  decrease in CPU frequency on these CPUs. This can be re-enabled by passing

  -mprefer-vector-width=512 to clang or passing -mattr=-prefer-256-bit to llc.

* ``-mprefer-vector-width=256`` is now the default behavior skylake-avx512 and

  later Intel CPUs. This tries to limit the use of 512-bit registers which can

  cause a decrease in CPU frequency on these CPUs. This can be re-enabled by

  passing ``-mprefer-vector-width=512`` to clang or passing

  ``-mattr=-prefer-256-bit`` to llc.

* Deprecated the mpx feature flag for the Intel MPX instructions. There were no

  intrinsics for this feature. This change only this effects the results

Changes to the WebAssembly Target

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

* __attribute__((used)) no longer implies that a symbol is exported, for

* ``__attribute__((used))`` no longer implies that a symbol is exported, for

  consistency with other targets.

* Multivalue function signatures are now supported in WebAssembly object files

* There have been some small changes to the code generation for atomic

  operations.

* RISC-V no longer emits incorrect CFI directives in function prologs and

  epilogs.

* RISC-V no longer emits incorrect CFI directives in function prologues and

  epilogues.

* RV64 no longer clears the upper bits when returning complex types from

  libcalls using the LP64 psABI.

* RISC-V (both 64-bit and 32-bit) is now supported by compiler-rt, allowing

  crtbegin and crtend to be built.

* The Sanitizers now support 64-bit RISC-V on linux.

* The Sanitizers now support 64-bit RISC-V on Linux.