llvm/docs/ReleaseNotes.rst

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LLVM 3.8 Release Notes
======================

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

This document contains the release notes for the LLVM Compiler Infrastructure,
release 3.8.  Here we describe the status of LLVM, including major improvements
from the previous release, improvements in various subprojects of LLVM, and
some of the current users of the code.  All LLVM releases may be downloaded
from the `LLVM releases web site <http://llvm.org/releases/>`_.

For more information about LLVM, including information about the latest
release, please check out the `main LLVM web site <http://llvm.org/>`_.  If you
have questions or comments, the `LLVM Developer's Mailing List
<http://lists.llvm.org/mailman/listinfo/llvm-dev>`_ is a good place to send
them.

Non-comprehensive list of changes in this release
=================================================
* With this release, the minimum Windows version required for running LLVM is
  Windows 7. Earlier versions, including Windows Vista and XP are no longer
  supported.

* With this release, the autoconf build system is deprecated. It will be removed
  in the 3.9 release. Please migrate to using CMake. For more information see:
  `Building LLVM with CMake <CMake.html>`_

* We have documented our C API stability guarantees for both development and
  release branches, as well as documented how to extend the C API. Please see
  the `developer documentation <DeveloperPolicy.html#c-api-changes>`_ for more
  information.

* The C API function ``LLVMLinkModules`` is deprecated. It will be removed in the
  3.9 release. Please migrate to ``LLVMLinkModules2``. Unlike the old function the
  new one

   * Doesn't take an unused parameter.
   * Destroys the source instead of only damaging it.
   * Does not record a message. Use the diagnostic handler instead.

* The C API functions ``LLVMParseBitcode``, ``LLVMParseBitcodeInContext``,
  ``LLVMGetBitcodeModuleInContext`` and ``LLVMGetBitcodeModule`` have been deprecated.
  They will be removed in 3.9. Please migrate to the versions with a 2 suffix.
  Unlike the old ones the new ones do not record a diagnostic message. Use
  the diagnostic handler instead.

* The deprecated C APIs ``LLVMGetBitcodeModuleProviderInContext`` and
  ``LLVMGetBitcodeModuleProvider`` have been removed.

* The deprecated C APIs ``LLVMCreateExecutionEngine``, ``LLVMCreateInterpreter``,
  ``LLVMCreateJITCompiler``, ``LLVMAddModuleProvider`` and ``LLVMRemoveModuleProvider``
  have been removed.

* With this release, the C API headers have been reorganized to improve build
  time. Type specific declarations have been moved to Type.h, and error
  handling routines have been moved to ErrorHandling.h. Both are included in
  Core.h so nothing should change for projects directly including the headers,
  but transitive dependencies may be affected.

* llvm-ar now supports thin archives.

* llvm doesn't produce ``.data.rel.ro.local`` or ``.data.rel`` sections anymore.

* Aliases to ``available_externally`` globals are now rejected by the verifier.

* The IR Linker has been split into ``IRMover`` that moves bits from one module to
  another and Linker proper that decides what to link.

* Support for dematerializing has been dropped.

* ``RegisterScheduler::setDefault`` was removed. Targets that used to call into the
  command line parser to set the ``DAGScheduler``, and that don't have enough
  control with ``setSchedulingPreference``, should look into overriding the
  ``SubTargetHook`` "``getDAGScheduler()``".

* ``ilist_iterator<T>`` no longer has implicit conversions to and from ``T*``,
  since ``ilist_iterator<T>`` may be pointing at the sentinel (which is usually
  not of type ``T`` at all).  To convert from an iterator ``I`` to a pointer,
  use ``&*I``; to convert from a pointer ``P`` to an iterator, use
  ``P->getIterator()``.  Alternatively, explicit conversions via
  ``static_cast<T>(U)`` are still available.

* ``ilist_node<T>::getNextNode()`` and ``ilist_node<T>::getPrevNode()`` now
  fail at compile time when the node cannot access its parent list.
  Previously, when the sentinel was was an ``ilist_half_node<T>``, this API
  could return the sentinel instead of ``nullptr``.  Frustrated callers should
  be updated to use ``iplist<T>::getNextNode(T*)`` instead.  Alternatively, if
  the node ``N`` is guaranteed not to be the last in the list, it is safe to
  call ``&*++N->getIterator()`` directly.

* The `Kaleidoscope tutorials <tutorial/index.html>`_ have been updated to use
  the ORC JIT APIs.

* ORC now has a basic set of C bindings.

* Optional support for linking clang and the LLVM tools with a single libLLVM
  shared library. To enable this, pass ``-DLLVM_LINK_LLVM_DYLIB=ON`` to CMake.
  See `Building LLVM with CMake`_ for more details.

* The optimization to move the prologue and epilogue of functions in colder
  code path (shrink-wrapping) is now enabled by default.

* A new target-independent gcc-compatible emulated Thread Local Storage mode
  is added.  When ``-femultated-tls`` flag is used, all accesses to TLS
  variables are converted to calls to ``__emutls_get_address`` in the runtime
  library.

* MSVC-compatible exception handling has been completely overhauled. New
  instructions have been introduced to facilitate this:
  `New exception handling instructions <ExceptionHandling.html#new-exception-handling-instructions>`_. 
  While we have done our best to test this feature thoroughly, it would
  not be completely surprising if there were a few lingering issues that
  early adopters might bump into.


Changes to the ARM Backends
---------------------------

During this release the AArch64 target has:

* Added support for more sanitizers (MSAN, TSAN) and made them compatible with
  all VMA kernel configurations (currently tested on 39 and 42 bits).
* Gained initial LLD support in the new ELF back-end
* Extended the Load/Store optimiser and cleaned up some of the bad decisions
  made earlier.
* Expanded LLDB support, including watchpoints, native building, Renderscript,
  LLDB-server, debugging 32-bit applications.
* Added support for the ``Exynos M1`` chip.

During this release the ARM target has:

* Gained massive performance improvements on embedded benchmarks due to finally
  running the stride vectorizer in full form, incrementing the performance gains
  that we already had in the previous releases with limited stride vectorization.
* Expanded LLDB support, including watchpoints, unwind tables
* Extended the Load/Store optimiser and cleaned up some of the bad decisions
  made earlier.
* Simplified code generation for global variable addresses in ELF, resulting in
  a significant (4% in Chromium) reduction in code size.
* Gained some additional code size improvements, though there's still a long road
  ahead, especially for older cores.
* Added some EABI floating point comparison functions to Compiler-RT
* Added support for Windows+GNU triple, ``+features`` in ``-mcpu``/``-march`` options.


Changes to the MIPS Target
--------------------------

During this release the MIPS target has:

* Significantly extended support for the Integrated Assembler. See below for
  more information
* Added support for the ``P5600`` processor.
* Added support for the ``interrupt`` attribute for MIPS32R2 and later. This
  attribute will generate a function which can be used as a interrupt handler
  on bare metal MIPS targets using the static relocation model.
* Added support for the ``ERETNC`` instruction found in MIPS32R5 and later.
* Added support for OpenCL. See http://portablecl.org/.

* Address spaces 1 to 255 are now reserved for software use and conversions
  between them are no-op casts.

* Removed the ``mips16`` value for the ``-mcpu`` option since it is an :abbr:`ASE
  (Application Specific Extension)` and not a processor. If you were using this,
  please specify another CPU and use ``-mips16`` to enable MIPS16.
* Removed ``copy_u.w`` from 32-bit MSA and ``copy_u.d`` from 64-bit MSA since
  they have been removed from the MSA specification due to forward compatibility
  issues.  For example, 32-bit MSA code containing ``copy_u.w`` would behave
  differently on a 64-bit processor supporting MSA. The corresponding intrinsics
  are still available and may expand to ``copy_s.[wd]`` where this is
  appropriate for forward compatibility purposes.
* Relaxed the ``-mnan`` option to allow ``-mnan=2008`` on MIPS32R2/MIPS64R2 for
  compatibility with GCC.
* Made MIPS64R6 the default CPU for 64-bit Android triples.

The MIPS target has also fixed various bugs including the following notable
fixes:

* Fixed reversed operands on ``mthi``/``mtlo`` in the DSP :abbr:`ASE
  (Application Specific Extension)`.
* The code generator no longer uses ``jal`` for calls to absolute immediate
  addresses.
* Disabled fast instruction selection on MIPS32R6 and MIPS64R6 since this is not
  yet supported.
* Corrected addend for ``R_MIPS_HI16`` and ``R_MIPS_PCHI16`` in MCJIT
* The code generator no longer crashes when handling subregisters of an 64-bit
  FPU register with undefined value.
* The code generator no longer attempts to use ``$zero`` for operands that do
  not permit ``$zero``.
* Corrected the opcode used for ``ll``/``sc`` when using MIPS32R6/MIPS64R6 and
  the Integrated Assembler.
* Added support for atomic load and atomic store.
* Corrected debug info when dynamically re-aligning the stack.

We have made a large number of improvements to the integrated assembler for
MIPS. In this release, the integrated assembler isn't quite production-ready
since there are a few known issues related to bare-metal support, checking
immediates on instructions, and the N32/N64 ABI's. However, the current support
should be sufficient for many users of the O32 ABI, particularly those targeting
MIPS32 on Linux or bare-metal MIPS32.

If you would like to try the integrated assembler, please use
``-fintegrated-as``.

Changes to the PowerPC Target
-----------------------------

There are numerous improvements to the PowerPC target in this release:

* Shrink wrapping optimization has been enabled for PowerPC Little Endian

* Direct move instructions are used when converting scalars to vectors

* Thread Sanitizer (TSAN) is now supported for PowerPC

* New MI peephole pass to clean up redundant XXPERMDI instructions  

* Add branch hints to highly biased branch instructions (code reaching
  unreachable terminators and exceptional control flow constructs)

* Promote boolean return values to integer to prevent excessive usage of
  condition registers

* Additional vector APIs for vector comparisons and vector merges have been
  added to altivec.h

* Many bugs have been identified and fixed


Changes to the X86 Target
-----------------------------

* TLS is enabled for Cygwin as emutls.

* Smaller code for materializing 32-bit 1 and -1 constants at ``-Os``.

* More efficient code for wide integer compares. (E.g. 64-bit compares
  on 32-bit targets.)

* Tail call support for ``thiscall``, ``stdcall``, ``vectorcall``, and
  ``fastcall`` functions.

Changes to the Hexagon Target
-----------------------------

In addition to general code size and performance improvements, Hexagon target
now has basic support for Hexagon V60 architecture and Hexagon Vector
Extensions (HVX).

Changes to the AVR Target
-------------------------

Slightly less than half of the AVR backend has been merged in at this point. It is still
missing a number large parts which cause it to be unusable, but is well on the
road to being completely merged and workable.

Changes to the OCaml bindings
-----------------------------

* The ocaml function link_modules has been replaced with link_modules' which
  uses LLVMLinkModules2.


External Open Source Projects Using LLVM 3.8
============================================

An exciting aspect of LLVM is that it is used as an enabling technology for
a lot of other language and tools projects. This section lists some of the
projects that have already been updated to work with LLVM 3.8.

LDC - the LLVM-based D compiler
-------------------------------

`D <http://dlang.org>`_ is a language with C-like syntax and static typing. It
pragmatically combines efficiency, control, and modeling power, with safety and
programmer productivity. D supports powerful concepts like Compile-Time Function
Execution (CTFE) and Template Meta-Programming, provides an innovative approach
to concurrency and offers many classical paradigms.

`LDC <http://wiki.dlang.org/LDC>`_ uses the frontend from the reference compiler
combined with LLVM as backend to produce efficient native code. LDC targets
x86/x86_64 systems like Linux, OS X and Windows and also PowerPC (32/64 bit)
and ARM. Ports to other architectures like AArch64 and MIPS64 are underway.


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

A wide variety of additional information is available on the `LLVM web page
<http://llvm.org/>`_, in particular in the `documentation
<http://llvm.org/docs/>`_ section.  The web page also contains versions of the
API documentation which is 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 ``llvm/docs/`` directory in the LLVM tree.

If you have any questions or comments about LLVM, please feel free to contact
us via the `mailing lists <http://llvm.org/docs/#maillist>`_.