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======================
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LLVM 3.9 Release Notes
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======================
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.. contents::
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:local:
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.. warning::
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These are in-progress notes for the upcoming LLVM 3.9 release. You may
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prefer the `LLVM 3.8 Release Notes <http://llvm.org/releases/3.8.0/docs
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/ReleaseNotes.html>`_.
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Introduction
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============
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This document contains the release notes for the LLVM Compiler Infrastructure,
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release 3.9. Here we describe the status of LLVM, including major improvements
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from the previous release, improvements in various subprojects of LLVM, and
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some of the current users of the code. All LLVM releases may be downloaded
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from the `LLVM releases web site <http://llvm.org/releases/>`_.
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For more information about LLVM, including information about the latest
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release, please check out the `main LLVM web site <http://llvm.org/>`_. If you
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have questions or comments, the `LLVM Developer's Mailing List
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<http://lists.llvm.org/mailman/listinfo/llvm-dev>`_ is a good place to send
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them.
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Note that if you are reading this file from a Subversion checkout or the main
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LLVM web page, this document applies to the *next* release, not the current
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one. To see the release notes for a specific release, please see the `releases
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page <http://llvm.org/releases/>`_.
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Non-comprehensive list of changes in this release
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=================================================
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* The LLVMContext gains a new runtime check (see
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LLVMContext::discardValueNames()) that can be set to discard Value names
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(other than GlobalValue). This is intended to be used in release builds by
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clients that are interested in saving CPU/memory as much as possible.
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* There is no longer a "global context" available in LLVM, except for the C API.
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* The autoconf build system has been removed in favor of CMake. LLVM 3.9
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requires CMake 3.4.3 or later to build. For information about using CMake
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please see the documentation on :doc:`CMake`. For information about the CMake
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language there is also a :doc:`CMakePrimer` document available.
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* .. note about C API functions LLVMParseBitcode,
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LLVMParseBitcodeInContext, LLVMGetBitcodeModuleInContext and
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LLVMGetBitcodeModule having been removed. LLVMGetTargetMachineData has been
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removed (use LLVMGetDataLayout instead).
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* The C API function LLVMLinkModules has been removed.
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* The C API function LLVMAddTargetData has been removed.
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* The C API function LLVMGetDataLayout is deprecated
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in favor of LLVMGetDataLayoutStr.
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* The C API enum LLVMAttribute and associated API is deprecated in favor of
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the new LLVMAttributeRef API. The deprecated functions are
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LLVMAddFunctionAttr, LLVMAddTargetDependentFunctionAttr,
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LLVMRemoveFunctionAttr, LLVMGetFunctionAttr, LLVMAddAttribute,
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LLVMRemoveAttribute, LLVMGetAttribute, LLVMAddInstrAttribute,
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LLVMRemoveInstrAttribute and LLVMSetInstrParamAlignment.
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* ``TargetFrameLowering::eliminateCallFramePseudoInstr`` now returns an
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iterator to the next instruction instead of ``void``. Targets that previously
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did ``MBB.erase(I); return;`` now probably want ``return MBB.erase(I);``.
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* ``SelectionDAGISel::Select`` now returns ``void``. Out of tree targets will
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need to be updated to replace the argument node and remove any dead nodes in
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cases where they currently return an ``SDNode *`` from this interface.
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* Raised the minimum required CMake version to 3.4.3.
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* Added the MemorySSA analysis, which hopes to replace MemoryDependenceAnalysis.
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It should provide higher-quality results than MemDep, and be algorithmically
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faster than MemDep. Currently, GVNHoist (which is off by default) makes use of
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MemorySSA.
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.. NOTE
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For small 1-3 sentence descriptions, just add an entry at the end of
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this list. If your description won't fit comfortably in one bullet
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point (e.g. maybe you would like to give an example of the
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functionality, or simply have a lot to talk about), see the `NOTE` below
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for adding a new subsection.
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* ... next change ...
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.. NOTE
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If you would like to document a larger change, then you can add a
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subsection about it right here. You can copy the following boilerplate
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and un-indent it (the indentation causes it to be inside this comment).
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Special New Feature
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-------------------
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Makes programs 10x faster by doing Special New Thing.
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GCC ABI Tag
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-----------
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Recently, many of the Linux distributions (ex. `Fedora <http://developerblog.redhat.com/2015/02/10/gcc-5-in-fedora/>`_,
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`Debian <https://wiki.debian.org/GCC5>`_, `Ubuntu <https://wiki.ubuntu.com/GCC5>`_)
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have moved on to use the new `GCC ABI <https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html>`_
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to work around `C++11 incompatibilities in libstdc++ <https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html>`_.
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This caused `incompatibility problems <https://gcc.gnu.org/ml/gcc-patches/2015-04/msg00153.html>`_
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with other compilers (ex. Clang), which needed to be fixed, but due to the
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experimental nature of GCC's own implementation, it took a long time for it to
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land in LLVM (`here <https://reviews.llvm.org/D18035>`_ and
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`here <https://reviews.llvm.org/D17567>`_), not in time for the 3.8 release.
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Those patches are now present in the 3.9.0 release and should be working on the
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majority of cases, as they have been tested thoroughly. However, some bugs were
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`filled in GCC <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71712>`_ and have not
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yet been fixed, so there may be corner cases not covered by either GCC or Clang.
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Bug fixes to those problems should be reported in Bugzilla (either LLVM or GCC),
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and patches to LLVM's trunk are very likely to be back-ported to future 3.9.x
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releases (depends on how destructive it is).
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Unfortunately, these patches won't be back-ported to 3.8.x or earlier, so we
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strongly recommend people to use 3.9.x when GCC ABI cases are at stake.
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For a more in-depth view of the issue, check our `Bugzilla entry <https://llvm.org/bugs/show_bug.cgi?id=23529>`_.
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Changes to the LLVM IR
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----------------------
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* New intrinsics ``llvm.masked.load``, ``llvm.masked.store``,
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``llvm.masked.gather`` and ``llvm.masked.scatter`` were introduced to the
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LLVM IR to allow selective memory access for vector data types.
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Changes to LLVM's IPO model
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---------------------------
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LLVM no longer does inter-procedural analysis and optimization (except
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inlining) on functions with comdat linkage. Doing IPO over such
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functions is unsound because the implementation the linker chooses at
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link-time may be differently optimized than the one what was visible
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during optimization, and may have arbitrarily different observable
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behavior. See `PR26774 <http://llvm.org/PR26774>`_ for more details.
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Support for ThinLTO
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-------------------
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LLVM now supports ThinLTO compilation, which can be invoked by compiling
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and linking with -flto=thin. The gold linker plugin, as well as linkers
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that use the new ThinLTO API in libLTO (like ld64), will transparently
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execute the ThinLTO backends in parallel threads.
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For more information on ThinLTO and the LLVM implementation, see the
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`ThinLTO blog post <http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html>`_.
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Changes to the ARM Targets
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--------------------------
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**During this release the AArch64 backend has:**
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* Gained support for Qualcomm's Kryo and Broadcom's Vulcan CPUs, including
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scheduling models.
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* Landed a scheduling model for Samsung's Exynos M1.
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* Seen a lot of work on GlobalISel.
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* Learned a few more useful combines (fadd and fmul into fmadd, adjustments to the
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stack pointer for callee-save stack memory and local stack memory etc).
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* Gained support for the Swift calling convention.
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* Switched to using SubtargetFeatures rather than testing for specific CPUs and
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to using TableGen for handling system instruction operands.
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* Like ARM, AArch64 is now using the TargetParser, so no more StringSwitches
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matching CPU, FPU or feature names will be accepted in normal code.
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* Clang can now self-host itself using LLD on AArch64.
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* Gained a big batch of tests from Halide.
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Furthermore, LLDB now supports AArch64 compact unwind tables, as used on iOS,
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tvos and watchos.
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**During this release the ARM target has:**
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* ARMv8.2-A can now be targeted directly via Clang flags.
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* Adding preliminary support for Cortex-R8.
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* LLDB can now parse EABI attributes for an ELF input.
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* Initial ARM/Thumb support was added to LLD.
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* The ExecutionEngine now supports COFF/ARM.
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* Swift calling convention was ported to ARM.
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* A large number of codegen fixes around ARMv8, DSP, correct sub-target support,
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relocations, EABI, EHABI, Windows on ARM, atomics..
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* Improved assembler support for Linux/Android/Chromium sub-projects.
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* Initial support for MUSL (libc) on ARM.
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* Support for Thumb1 targets in libunwind.
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* Gained a big batch of tests from Halide.
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Changes to the MIPS Target
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--------------------------
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**During this release the MIPS target has:**
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* Enabled the Integrated Assembler by default for all ``mips-*`` and
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``mipsel-*`` triples.
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* Significantly improved the Integrated Assembler support for the n64 ABI.
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* Added the Clang frontend ``-mcompact-branches={never,optimal,always}`` option
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that controls how LLVM generates compact branches for MIPS targets.
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* Improved performance and code size for stack pointer adjustments in functions
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with large frames.
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* Implemented many instructions from the microMIPS32R6 ISA and added CodeGen
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support for most of them.
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* Added support for the triple used by Debian Stretch for little endian
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MIPS64, ie. ``mips64el-linux-gnuabi64``.
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* Removed EABI which was neither tested nor properly supported.
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* Gained the ability to self-host on MIPS32R6.
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* Gained the ability to self-host on MIPS64R2 and MIPS64R6 when using the n64
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ABI.
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* Added support for the ``LA`` macro in PIC mode for o32.
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* Added support for safestack in compiler-rt.
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* Added support for the MIPS n64 ABI in LLD.
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* Added LLD support for TLS relocations for both o32 and n64 MIPS ABIs.
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**The MIPS target has also fixed various bugs including the following notable
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fixes:**
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* Delay slots are no longer filled multiple times when either ``-save-temps``
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or ``-via-file-asm`` are used.
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* Updated n32 and n64 to follow the standard ELF conventions for label prefixes
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(``.L``), whereas o32 still uses its own (``$``).
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* Properly sign-extend values to GPR width for instructions that expect 32-bit
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values on 64-bit ISAs.
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* Several fixes for the delay-slot filler pass, including correct
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forbidden-slot hazard handling.
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* Fixed several errors caught by the machine verifier when turned on for MIPS.
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* Fixed broken predicate for ``SELECT`` patterns in MIPS64.
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* Fixed wrong truncation of memory address for ``LL``/``SC`` seqeuences in
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MIPS64.
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* Fixed the o32, n32 and n64 handling of ``.cprestore`` directives when inside
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a ``.set noat`` region by the Integrated Assembler.
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* Fixed the ordering of ``HI``/``LO`` pairs in the relocation table.
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* Fixed the generated ELF ``EFlags`` when Octeon is the target.
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Changes to the PowerPC Target
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-----------------------------
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Moved some optimizations from O3 to O2 (D18562)
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* Enable sibling call optimization on ppc64 ELFv1/ELFv2 abi
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Changes to the X86 Target
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-------------------------
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* LLVM now supports the Intel CPU codenamed Skylake Server with AVX-512
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extensions using ``-march=skylake-avx512``. The switch enables the
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ISA extensions AVX-512{F, CD, VL, BW, DQ}.
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* LLVM now supports the Intel CPU codenamed Knights Landing with AVX-512
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extensions using ``-march=knl``. The switch enables the ISA extensions
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AVX-512{F, CD, ER, PF}.
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* LLVM will now prefer ``PUSH`` instructions rather than ``%esp``-relative
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``MOV`` instructions for function calls at all optimization levels greater
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than ``-O0``. Previously this transformation only occurred at ``-Os``.
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Changes to the AMDGPU Target
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-----------------------------
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* Added backend support for OpenGL shader image, buffer storage, atomic
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counter, and compute shader extensions (supported since Mesa 12)
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* Mesa 11.0.x is no longer supported
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Changes to the OCaml bindings
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-----------------------------
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During this release ...
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Support for attribute 'notail' has been added
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---------------------------------------------
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This marker prevents optimization passes from adding 'tail' or
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'musttail' markers to a call. It is used to prevent tail call
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optimization from being performed on the call.
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External Open Source Projects Using LLVM 3.9
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============================================
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An exciting aspect of LLVM is that it is used as an enabling technology for
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a lot of other language and tools projects. This section lists some of the
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projects that have already been updated to work with LLVM 3.9.
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* A project
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LDC - the LLVM-based D compiler
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-------------------------------
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`D <http://dlang.org>`_ is a language with C-like syntax and static typing. It
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pragmatically combines efficiency, control, and modeling power, with safety and
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programmer productivity. D supports powerful concepts like Compile-Time Function
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Execution (CTFE) and Template Meta-Programming, provides an innovative approach
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to concurrency and offers many classical paradigms.
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`LDC <http://wiki.dlang.org/LDC>`_ uses the frontend from the reference compiler
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combined with LLVM as backend to produce efficient native code. LDC targets
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x86/x86_64 systems like Linux, OS X, FreeBSD and Windows and also Linux on ARM
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and PowerPC (32/64 bit). Ports to other architectures like AArch64 and MIPS64
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are underway.
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Additional Information
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======================
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A wide variety of additional information is available on the `LLVM web page
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<http://llvm.org/>`_, in particular in the `documentation
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<http://llvm.org/docs/>`_ section. The web page also contains versions of the
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API documentation which is up-to-date with the Subversion version of the source
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code. You can access versions of these documents specific to this release by
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going into the ``llvm/docs/`` directory in the LLVM tree.
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If you have any questions or comments about LLVM, please feel free to contact
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us via the `mailing lists <http://llvm.org/docs/#maillist>`_.
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