======================
LLVM 3.6 Release Notes
======================

.. contents::
    :local:


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

This document contains the release notes for the LLVM Compiler Infrastructure,
release 3.6.  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.cs.uiuc.edu/mailman/listinfo/llvmdev>`_ is a good place to send
them.


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

.. NOTE
   For small 1-3 sentence descriptions, just add an entry at the end of
   this list. If your description won't fit comfortably in one bullet
   point (e.g. maybe you would like to give an example of the
   functionality, or simply have a lot to talk about), see the `NOTE` below
   for adding a new subsection.

* Support for AuroraUX has been removed.

* Added support for a `native object file-based bitcode wrapper format
  <BitCodeFormat.html#native-object-file>`_.

* Added support for MSVC's ``__vectorcall`` calling convention as
  ``x86_vectorcallcc``.

.. NOTE
   If you would like to document a larger change, then you can add a
   subsection about it right here. You can copy the following boilerplate
   and un-indent it (the indentation causes it to be inside this comment).

   Special New Feature
   -------------------

   Makes programs 10x faster by doing Special New Thing.

Prefix data rework
------------------

The semantics of the ``prefix`` attribute have been changed. Users
that want the previous ``prefix`` semantics should instead use
``prologue``.  To motivate this change, let's examine the primary
usecases that these attributes aim to serve,

  1. Code sanitization metadata (e.g. Clang's undefined behavior
     sanitizer)

  2. Function hot-patching: Enable the user to insert ``nop`` operations
     at the beginning of the function which can later be safely replaced
     with a call to some instrumentation facility.

  3. Language runtime metadata: Allow a compiler to insert data for
     use by the runtime during execution. GHC is one example of a
     compiler that needs this functionality for its
     tables-next-to-code functionality.

Previously ``prefix`` served cases (1) and (2) quite well by allowing the user
to introduce arbitrary data at the entrypoint but before the function
body. Case (3), however, was poorly handled by this approach as it
required that prefix data was valid executable code.

In this release the concept of prefix data has been redefined to be
data which occurs immediately before the function entrypoint (i.e. the
symbol address). Since prefix data now occurs before the function
entrypoint, there is no need for the data to be valid code.

The previous notion of prefix data now goes under the name "prologue
data" to emphasize its duality with the function epilogue.

The intention here is to handle cases (1) and (2) with prologue data and
case (3) with prefix data. See the language reference for further details
on the semantics of these attributes.

This refactoring arose out of discussions_ with Reid Kleckner in
response to a proposal to introduce the notion of symbol offsets to
enable handling of case (3).

.. _discussions: http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-May/073235.html


Metadata is not a Value
-----------------------

Metadata nodes (``!{...}``) and strings (``!"..."``) are no longer values.
They have no use-lists, no type, cannot RAUW, and cannot be function-local.

Bridges between Value and Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

LLVM intrinsics can reference metadata using the ``metadata`` type, and
metadata nodes can reference constant values.

Function-local metadata is limited to direct arguments to LLVM intrinsics.

Metadata is typeless
^^^^^^^^^^^^^^^^^^^^

The following old IR:

.. code-block:: llvm

    @g = global i32 0

    define void @foo(i32 %v) {
    entry:
      call void @llvm.md(metadata !{i32 %v})
      call void @llvm.md(metadata !{i32* @global})
      call void @llvm.md(metadata !0)
      call void @llvm.md(metadata !{metadata !"string"})
      call void @llvm.md(metadata !{metadata !{metadata !1, metadata !"string"}})
      ret void, !bar !1, !baz !2
    }

    declare void @llvm.md(metadata)

    !0 = metadata !{metadata !1, metadata !2, metadata !3, metadata !"some string"}
    !1 = metadata !{metadata !2, null, metadata !"other", i32* @global, i32 7}
    !2 = metadata !{}

should now be written as:

.. code-block:: llvm

    @g = global i32 0

    define void @foo(i32 %v) {
    entry:
      call void @llvm.md(metadata i32 %v) ; The only legal place for function-local
                                          ; metadata.
      call void @llvm.md(metadata i32* @global)
      call void @llvm.md(metadata !0)
      call void @llvm.md(metadata !{!"string"})
      call void @llvm.md(metadata !{!{!1, !"string"}})
      ret void, !bar !1, !baz !2
    }

    declare void @llvm.md(metadata)

    !0 = !{!1, !2, !3, !"some string"}
    !1 = !{!2, null, !"other", i32* @global, i32 7}
    !2 = !{}

Distinct metadata nodes
^^^^^^^^^^^^^^^^^^^^^^^

Metadata nodes can opt-out of uniquing, using the keyword ``distinct``.
Distinct nodes are still owned by the context, but are stored in a side table,
and not uniqued.

In LLVM 3.5, metadata nodes would drop uniquing if an operand changed to
``null`` during optimizations.  This is no longer true.  However, if an operand
change causes a uniquing collision, they become ``distinct``.  Unlike LLVM 3.5,
where serializing to assembly or bitcode would re-unique the nodes, they now
remain ``distinct``.

The following IR:

.. code-block:: llvm

    !named = !{!0, !1, !2, !3, !4, !5, !6, !7, !8}

    !0 = !{}
    !1 = !{}
    !2 = distinct !{}
    !3 = distinct !{}
    !4 = !{!0}
    !5 = distinct !{!0}
    !6 = !{!4, !{}, !5}
    !7 = !{!{!0}, !0, !5}
    !8 = distinct !{!{!0}, !0, !5}

is equivalent to the following:

.. code-block:: llvm

    !named = !{!0, !0, !1, !2, !3, !4, !5, !5, !6}

    !0 = !{}
    !1 = distinct !{}
    !2 = distinct !{}
    !3 = !{!0}
    !4 = distinct !{!0}
    !5 = !{!3, !0, !4}
    !6 = distinct !{!3, !0, !4}

Constructing cyclic graphs
^^^^^^^^^^^^^^^^^^^^^^^^^^

During graph construction, if a metadata node transitively references a forward
declaration, the node itself is considered "unresolved" until the forward
declaration resolves.  An unresolved node can RAUW itself to support uniquing.
Nodes automatically resolve once all their operands have resolved.

However, cyclic graphs prevent the nodes from resolving.  An API client that
constructs a cyclic graph must call ``resolveCycles()`` to resolve nodes in the
cycle.

To save self-references from that burden, self-referencing nodes are implicitly
``distinct``.  So the following IR:

.. code-block:: llvm

    !named = !{!0, !1, !2, !3, !4}

    !0 = !{!0}
    !1 = !{!1}
    !2 = !{!2, !1}
    !3 = !{!2, !1}
    !4 = !{!2, !1}

is equivalent to:

.. code-block:: llvm

    !named = !{!0, !1, !2, !3, !3}

    !0 = distinct !{!0}
    !1 = distinct !{!1}
    !2 = distinct !{!2, !1}
    !3 = !{!2, !1}

MDLocation (aka DebugLoc aka DILocation)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

There's a new first-class metadata construct called ``MDLocation`` (to be
followed in subsequent releases by others).  It's used for the locations
referenced by ``!dbg`` metadata attachments.

For example, if an old ``!dbg`` attachment looked like this:

.. code-block:: llvm

    define i32 @foo(i32 %a, i32 %b) {
    entry:
      %add = add i32 %a, %b, !dbg !0
      ret %add, !dbg !1
    }

    !0 = metadata !{i32 10, i32 3, metadata !2, metadata !1)
    !1 = metadata !{i32 20, i32 7, metadata !3)
    !2 = metadata !{...}
    !3 = metadata !{...}

the new attachment looks like this:

.. code-block:: llvm

    define i32 @foo(i32 %a, i32 %b) {
    entry:
      %add = add i32 %a, %b, !dbg !0
      ret %add, !dbg !1
    }

    !0 = !MDLocation(line: 10, column: 3, scope: !2, inlinedAt: !1)
    !1 = !MDLocation(line: 20, column: 7, scope: !3)
    !2 = !{...}
    !3 = !{...}

The fields are named, can be reordered, and have sane defaults if left out
(although ``scope:`` is required).


Alias syntax change
-----------------------

The syntax for aliases is now closer to what is used for global variables

.. code-block:: llvm

    @a = weak global ...
    @b = weak alias ...

The order of the ``alias`` keyword and the linkage was swapped before.

The old JIT has been removed
----------------------------

All users should transition to MCJIT.


object::Binary doesn't own the file buffer
-------------------------------------------

It is now just a wrapper, which simplifies using object::Binary with other
users of the underlying file.


IR in object files is now supported
-----------------------------------

Regular object files can contain IR in a section named ``.llvmbc``.


The gold plugin has been rewritten
----------------------------------

It is now implemented directly on top of lib/Linker instead of ``lib/LTO``.
The API of ``lib/LTO`` is sufficiently different from gold's view of the
linking process that some cases could not be conveniently implemented.

The new implementation is also lazier and has a ``save-temps`` option.


Change in the representation of lazy loaded funcs
-------------------------------------------------

Lazy loaded functions are now represented in a way that ``isDeclaration``
returns the correct answer even before reading the body.


The opt option -std-compile-opts was removed
--------------------------------------------

It was effectively an alias of -O3.


Python 2.7 is now required
--------------------------

This was done to simplify compatibility with python 3.


The leak detector has been removed
----------------------------------

In practice, tools like asan and valgrind were finding way more bugs than
the old leak detector, so it was removed.


New comdat syntax
-----------------

The syntax of comdats was changed to

.. code-block:: llvm

    $c = comdat any
    @g = global i32 0, comdat($c)
    @c = global i32 0, comdat

The version without the parentheses is a syntactic sugar for a comdat with
the same name as the global.


Added support for Win64 unwind information
------------------------------------------

LLVM now obeys the `Win64 prologue and epilogue conventions
<https://msdn.microsoft.com/en-us/library/tawsa7cb.aspx>`_ documented by
Microsoft. Unwind information is also emitted into the .xdata section.

As a result of the ABI-required prologue changes, it is now no longer possible
to unwind the stack using a standard frame pointer walk on Win64. Instead,
users should call ``CaptureStackBackTrace``, or implement equivalent
functionality by consulting the unwind tables present in the binary.


Diagnostic infrastructure used by lib/Linker and lib/Bitcode
------------------------------------------------------------

These libraries now use the diagnostic handler to print errors and warnings.
This provides better error messages and simpler error handling.


The PreserveSource linker mode was removed
------------------------------------------

It was fairly broken and was removed.

The mode is currently still available in the C API for source
compatibility, but it doesn't have any effect.


Garbage Collection
------------------
A new experimental mechanism for describing a garbage collection safepoint was
added to LLVM.  The new mechanism was not complete at the point this release
was branched so it is recommended that anyone interested in using this
mechanism track the ongoing development work on tip of tree.  The hope is that
these intrinsics will be ready for general use by 3.7.  Documentation can be
found `here <http://llvm.org/docs/Statepoints.html>`_.

The existing gc.root implementation is still supported and as fully featured
as it ever was.  However, two features from GCStrategy will likely be removed
in the 3.7 release (performCustomLowering and findCustomSafePoints).  If you
have a use case for either, please mention it on llvm-dev so that it can be
considered for future development.

We are expecting to migrate away from gc.root in the 3.8 time frame,
but both mechanisms will be supported in 3.7.


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

During this release the MIPS target has reached a few major milestones. The
compiler has gained support for MIPS-II and MIPS-III; become ABI-compatible
with GCC for big and little endian O32, N32, and N64; and is now able to
compile the Linux kernel for 32-bit targets. Additionally, LLD now supports
microMIPS for the O32 ABI on little endian targets, and code generation for
microMIPS is almost completely passing the test-suite.


ABI
^^^

A large number of bugs have been fixed for big-endian MIPS targets using the
N32 and N64 ABI's as well as a small number of bugs affecting other ABI's.
Please note that some of these bugs will still affect LLVM-IR generated by
LLVM 3.5 since correct code generation depends on appropriate usage of the
``inreg``, ``signext``, and ``zeroext`` attributes on all function arguments
and returns.

There are far too many corrections to provide a complete list but here are a
few notable ones:

* Big-endian N32 and N64 now interlinks successfully with GCC compiled code.
  Previously this didn't work for the majority of cases.

* The registers used to return a structure containing a single 128-bit floating
  point member on the N32/N64 ABI's have been changed from those specified by
  the ABI documentation to match those used by GCC. The documentation specifies
  that ``$f0`` and ``$f2`` should be used but GCC has used ``$f0`` and ``$f1``
  for many years.

* Returning a zero-byte struct no longer causes arguments to be read from the
  wrong registers when using the O32 ABI.

* The exception personality has been changed for 64-bit MIPS targets to
  eliminate warnings about relocations in a read-only section.

* Incorrect usage of odd-numbered single-precision floating point registers
  has been fixed when the fastcc calling convention is used with 64-bit FPU's
  and -mno-odd-spreg.


LLVMLinux
^^^^^^^^^

It is now possible to compile the Linux kernel. This currently requires a small
number of kernel patches. See the `LLVMLinux project
<http://llvm.linuxfoundation.org/index.php/Main_Page>`_ for details.

* Added -mabicalls and -mno-abicalls. The implementation may not be complete
  but works sufficiently well for the Linux kernel.

* Fixed multiple compatibility issues between LLVM's inline assembly support
  and GCC's.

* Added support for a number of directives used by Linux to the Integrated
  Assembler.


Miscellaneous
^^^^^^^^^^^^^

* Attempting to disassemble l[wd]c[23], s[wd]c[23], cache, and pref no longer
  triggers an assertion.

* Added -muclibc and -mglibc to support toolchains that provide both uClibC and
  GLibC.

* __SIZEOF_INT128__ is no longer defined for 64-bit targets since 128-bit
  integers do not work at this time for this target.

* Using $t4-$t7 with the N32 and N64 ABI is deprecated when ``-fintegrated-as``
  is in use and will be removed in LLVM 3.7. These names have never been
  supported by the GNU Assembler for these ABI's.


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

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

* LLVM now generates the Vector-Scalar eXtension (VSX) instructions from
  version 2.06 of the Power ISA, for both big- and little-endian targets.

* LLVM now has a POWER8 instruction scheduling description.

* AddressSanitizer (ASan) support is now fully functional.

* Performance of simple atomic accesses has been greatly improved.

* Atomic fences now use light-weight syncs where possible, again providing
  significant performance benefit.

* The PowerPC target now supports PIC levels (-fPIC vs. -fpic).

* PPC32 SVR4 now supports small-model PIC.

* Experimental support for the stackmap/patchpoint intrinsics has been added.

* There have been many smaller bug fixes and performance improvements.


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

* The bindings now require OCaml >=4.00.0, ocamlfind,
  ctypes >=0.3.0 <0.4 and OUnit 2 if tests are enabled.

* The bindings can now be built using cmake as well as autoconf.

* LLVM 3.5 has, unfortunately, shipped a broken Llvm_executionengine
  implementation. In LLVM 3.6, the bindings now fully support MCJIT,
  however the interface is reworked from scratch using ctypes
  and is not backwards compatible.

* Llvm_linker.Mode was removed following the changes in LLVM.
  This breaks the interface of Llvm_linker.

* All combinations of ocamlc/ocamlc -custom/ocamlopt and shared/static
  builds of LLVM are now supported.

* Absolute paths are not embedded into the OCaml libraries anymore.
  Either OCaml >=4.02.2 must be used, which includes an rpath-like $ORIGIN
  mechanism, or META file must be updated for out-of-tree installations;
  see r221139.

* As usual, many more functions have been exposed to OCaml.


Go bindings
-----------

* A set of Go bindings based on `gollvm <https://github.com/go-llvm/llvm>`_
  was introduced in this release.


External Open Source Projects Using LLVM 3.6
============================================

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 3.6.


Portable Computing Language (pocl)
----------------------------------

In addition to producing an easily portable open source OpenCL
implementation, another major goal of `pocl <http://portablecl.org/>`_
is improving performance portability of OpenCL programs with
compiler optimizations, reducing the need for target-dependent manual
optimizations. An important part of pocl is a set of LLVM passes used to
statically parallelize multiple work-items with the kernel compiler, even in
the presence of work-group barriers. This enables static parallelization of
the fine-grained static concurrency in the work groups in multiple ways. 


TTA-based Co-design Environment (TCE)
-------------------------------------

`TCE <http://tce.cs.tut.fi/>`_ is a toolset for designing customized
exposed datapath processors based on the Transport triggered 
architecture (TTA). 

The toolset provides a complete co-design flow from C/C++
programs down to synthesizable VHDL/Verilog and parallel program binaries.
Processor customization points include the register files, function units,
supported operations, and the interconnection network.

TCE uses Clang and LLVM for C/C++/OpenCL C language support, target independent 
optimizations and also for parts of code generation. It generates
new LLVM-based code generators "on the fly" for the designed processors and
loads them in to the compiler backend as runtime libraries to avoid
per-target recompilation of larger parts of the compiler chain. 


Likely
------

`Likely <http://www.liblikely.org>`_ is an embeddable just-in-time Lisp for
image recognition and heterogeneous computing. Algorithms are just-in-time
compiled using LLVM's MCJIT infrastructure to execute on single or
multi-threaded CPUs and potentially OpenCL SPIR or CUDA enabled GPUs.
Likely seeks to explore new optimizations for statistical learning 
algorithms by moving them from an offline model generation step to the 
compile-time evaluation of a function (the learning algorithm) with constant
arguments (the training data).


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, FreeBSD and Windows and also Linux on
PowerPC (32/64 bit). Ports to other architectures like ARM, AArch64 and MIPS64
are underway.


LLVMSharp & ClangSharp
----------------------

`LLVMSharp <http://www.llvmsharp.org>`_ and
`ClangSharp <http://www.clangsharp.org>`_ are type-safe C# bindings for
Microsoft.NET and Mono that Platform Invoke into the native libraries.
ClangSharp is self-hosted and is used to generated LLVMSharp using the
LLVM-C API.

`LLVMSharp Kaleidoscope Tutorials <http://www.llvmsharp.org/Kaleidoscope/>`_
are instructive examples of writing a compiler in C#, with certain improvements
like using the visitor pattern to generate LLVM IR.

`ClangSharp PInvoke Generator <http://www.clangsharp.org/PInvoke/>`_ is the
self-hosting mechanism for LLVM/ClangSharp and is demonstrative of using
LibClang to generate Platform Invoke (PInvoke) signatures for C APIs.


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>`_.