LLVM 3.8 Release Notes

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.

For more information about LLVM, including information about the latest release, please check out the main LLVM web site. If you have questions or comments, the LLVM Developer’s Mailing List 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

  • 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 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 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. 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 ASE 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 ASE.
  • 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 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 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, in particular in the documentation 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.