RelaseNotes: tweaks

Non-comprehensive list of changes in this release

=================================================

* The C API functions LLVMAddFunctionAttr, LLVMGetFunctionAttr,

  LLVMRemoveFunctionAttr, LLVMAddAttribute, LLVMRemoveAttribute,

  LLVMGetAttribute, LLVMAddInstrAttribute and

  LLVMRemoveInstrAttribute have been removed.

* Minimum compiler version to build has been raised to GCC 4.8 and VS 2015.

* The C API enum LLVMAttribute has been deleted.

* The C API functions ``LLVMAddFunctionAttr``, ``LLVMGetFunctionAttr``,

  ``LLVMRemoveFunctionAttr``, ``LLVMAddAttribute``, ``LLVMRemoveAttribute``,

  ``LLVMGetAttribute``, ``LLVMAddInstrAttribute`` and

  ``LLVMRemoveInstrAttribute`` have been removed.

* The definition and uses of LLVM_ATRIBUTE_UNUSED_RESULT in the LLVM source

  were replaced with LLVM_NODISCARD, which matches the C++17 [[nodiscard]]

  semantics rather than gcc's __attribute__((warn_unused_result)).

* The C API enum ``LLVMAttribute`` has been deleted.

* Minimum compiler version to build has been raised to GCC 4.8 and VS 2015.

* The definition and uses of ``LLVM_ATRIBUTE_UNUSED_RESULT`` in the LLVM source

  were replaced with ``LLVM_NODISCARD``, which matches the C++17 ``[[nodiscard]]``

  semantics rather than gcc's ``__attribute__((warn_unused_result))``.

* The Timer related APIs now expect a Name and Description. When upgrading code

  the previously used names should become descriptions and a short name in the

  style of a programming language identifier should be added.

* LLVM now handles invariant.group across different basic blocks, which makes

* LLVM now handles ``invariant.group`` across different basic blocks, which makes

  it possible to devirtualize virtual calls inside loops.

* The aggressive dead code elimination phase ("adce") now remove

* The aggressive dead code elimination phase ("adce") now removes

  branches which do not effect program behavior. Loops are retained by

  default since they may be infinite but these can also be removed

  with LLVM option -adce-remove-loops when the loop body otherwise has

  with LLVM option ``-adce-remove-loops`` when the loop body otherwise has

  no live operations.

* The GVNHoist pass is now enabled by default. The new pass based on Global

  Value Numbering detects similar computations in branch code and replaces

  multiple instances of the same computation with a unique expression.  The

  transform benefits code size and generates better schedules.  GVNHoist is

  more aggressive at -Os and -Oz, hoisting more expressions at the expense of

  execution time degradations.

  more aggressive at ``-Os`` and ``-Oz``, hoisting more expressions at the

  expense of execution time degradations.

 * The llvm-cov tool can now export coverage data as json. Its html output mode

   has also improved.

Improvements to ThinLTO (-flto=thin)

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

* Integration with profile data (PGO). When available, profile data

Integration with profile data (PGO). When available, profile data

enables more accurate function importing decisions, as well as

cross-module indirect call promotion.

* Significant build-time and binary-size improvements when compiling with

Significant build-time and binary-size improvements when compiling with

debug info (-g).

LLVM Coroutines

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

Experimental support for :doc:`Coroutines` was added, which can be enabled

with ``-enable-coroutines`` in ``opt`` command tool or using

with ``-enable-coroutines`` in ``opt`` the command tool or using the

``addCoroutinePassesToExtensionPoints`` API when building the optimization

pipeline.

Regcall and Vectorcall Calling Conventions

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

Support was added for _regcall calling convention.

Existing __vectorcall calling convention support was extended to include

Support was added for ``_regcall`` calling convention.

Existing ``__vectorcall`` calling convention support was extended to include

correct handling of HVAs.

The __vectorcall calling convention was introduced by Microsoft to

The ``__vectorcall`` calling convention was introduced by Microsoft to

enhance register usage when passing parameters.

For more information please read `__vectorcall documentation

<https://msdn.microsoft.com/en-us/library/dn375768.aspx>`_.

The __regcall calling convention was introduced by Intel to

The ``__regcall`` calling convention was introduced by Intel to

optimize parameter transfer on function call.

This calling convention ensures that as many values as possible are

passed or returned in registers.

Passes that work on the machine instruction representation can be tested with

the .mir serialization format. ``llc`` supports the ``-run-pass``,

``-stop-after``, ``-stop-before``, ``-start-after``, ``-start-before`` to to

``-stop-after``, ``-stop-before``, ``-start-after``, ``-start-before`` to

run a single pass of the code generation pipeline, or to stop or start the code

generation pipeline at a given point.

  ``&*I`` (if not ``end()``); alternatively, clients may refactor to use

  references for known-good nodes.

Changes to the LLVM IR

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

Changes to the ARM Targets

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