LLVM 5.0.0 Release Notes

Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 5.0.0. 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

• LLVM’s WeakVH has been renamed to WeakTrackingVH and a new WeakVH has been introduced. The new WeakVH nulls itself out on deletion, but does not track values across RAUW.
• A new library named BinaryFormat has been created which holds a collection of code which previously lived in Support. This includes the file_magic structure and identify_magic functions, as well as all the structure and type definitions for DWARF, ELF, COFF, WASM, and MachO file formats.
• The tool llvm-pdbdump has been renamed llvm-pdbutil to better reflect its nature as a general purpose PDB manipulation / diagnostics tool that does more than just dumping contents.
• The BBVectorize pass has been removed. It was fully replaced and no longer used back in 2014 but we didn’t get around to removing it. Now it is gone. The SLP vectorizer is the suggested non-loop vectorization pass.
• A new tool opt-viewer.py has been added to visualize optimization remarks in HTML. The tool processes the YAML files produced by clang with the -fsave-optimization-record option.
• A new CMake macro LLVM_REVERSE_ITERATION has been added. If enabled, all supported unordered LLVM containers would be iterated in reverse order. This is useful for uncovering non-determinism caused by iteration of unordered containers. Currently, it supports reverse iteration of SmallPtrSet and DenseMap.
• A new tool llvm-dlltool has been added to create short import libraries from GNU style definition files. The tool utilizes the PE COFF SPEC Import Library Format and PE COFF Auxiliary Weak Externals Format to achieve compatibility with LLD and MSVC LINK.

Changes to the LLVM IR

• The datalayout string may now indicate an address space to use for the pointer type of alloca rather than the default of 0.
• Added speculatable attribute indicating a function which has no side-effects which could inhibit hoisting of calls.

Changes to the Arm Targets

During this release the AArch64 target has:

• A much improved Global ISel at O0.
• Support for ARMv8.1 8.2 and 8.3 instructions.
• New scheduler information for ThunderX2.
• Some SVE type changes but not much more than that.
• Made instruction fusion more aggressive, resulting in speedups for code making use of AArch64 AES instructions. AES fusion has been enabled for most Cortex-A cores and the AArch64MacroFusion pass was moved to the generic MacroFusion pass.
• Added preferred function alignments for most Cortex-A cores.
• OpenMP “offload-to-self” base support.

During this release the ARM target has:

• Improved, but still mostly broken, Global ISel.
• Scheduling models update, new schedule for Cortex-A57.
• Hardware breakpoint support in LLDB.
• New assembler error handling, with spelling corrections and multiple suggestions on how to fix problems.
• Improved mixed ARM/Thumb code generation. Some cases in which wrong relocations were emitted have been fixed.
• Added initial support for mixed ARM/Thumb link-time optimization, using the thumb-mode target feature.

Changes to the MIPS Target

• The microMIPS64R6 backend is deprecated and will be removed in the next release.
• The MIPS backend now directly supports vector types for arguments and return values (previously this required ABI specific LLVM IR).
• Added documentation for how the MIPS backend handles address lowering.
• Added a GCC compatible option -m(no-)madd4 to control the generation of four operand multiply addition/subtraction instructions.
• Added basic support for the XRay instrumentation system.
• Added support for more assembly aliases and macros.
• Added support for the micromips and nomicromips function attributes which control micromips code generation on a per function basis.
• Added the long-calls feature for non-pic environments. This feature is used where the callee is out of range of the caller using a standard call sequence. It must be enabled specifically.
• Added support for performing microMIPS code generation via function attributes.
• Added experimental support for the static relocation model for the N64 ABI.
• Added partial support for the MT ASE.
• Added basic support for code size reduction for microMIPS.
• Fixed numerous bugs including: multi-precision arithmetic support, various vectorization bugs, debug information for thread local variables, debug sections lacking the correct flags, crashing when disassembling sections whose size is not a multiple of two or four.

Changes to the PowerPC Target

• Additional support and exploitation of POWER ISA 3.0: vabsdub, vabsduh, vabsduw, modsw, moduw, modsd, modud, lxv, stxv, vextublx, vextubrx, vextuhlx, vextuhrx, vextuwlx, vextuwrx, vextsb2w, vextsb2d, vextsh2w, vextsh2d, and vextsw2d
• Implemented Optimal Code Sequences from The PowerPC Compiler Writer’s Guide.
• Enable -fomit-frame-pointer by default.
• Improved handling of bit reverse intrinsic.
• Improved handling of memcpy and memcmp functions.
• Improved handling of branches with static branch hints.
• Improved codegen for atomic load_acquire.
• Improved block placement during code layout
• Many improvements to instruction selection and code generation

Changes to the X86 Target

• Added initial AMD Ryzen (znver1) scheduler support.
• Added support for Intel Goldmont CPUs.
• Add support for avx512vpopcntdq instructions.
• Added heuristics to convert CMOV into branches when it may be profitable.
• More aggressive inlining of memcmp calls.
• Improve vXi64 shuffles on 32-bit targets.
• Improved use of PMOVMSKB for any_of/all_of comparision reductions.
• Improved Silvermont, Sandybridge, and Jaguar (btver2) schedulers.
• Improved support for AVX512 vector rotations.
• Added support for AMD Lightweight Profiling (LWP) instructions.
• Avoid using slow LEA instructions.
• Use alternative sequences for multiply by constant.
• Improved lowering of strided shuffles.
• Improved the AVX512 cost model used by the vectorizer.
• Fix scalar code performance when AVX512 is enabled by making i1’s illegal.
• Fixed many inline assembly bugs.
• Preliminary support for tracing NetBSD processes and core files with a single thread in LLDB.

Changes to the AMDGPU Target

• Initial gfx9 support

Changes to the AVR Target

This release consists mainly of bugfixes and implementations of features required for compiling basic Rust programs.

• Enable the branch relaxation pass so that we don’t crash on large stack load/stores
• Add support for lowering bit-rotations to the native ror and rol instructions
• Fix bug where function pointers were treated as pointers to RAM and not pointers to program memory
• Fix broken code generation for shift-by-variable expressions
• Support zero-sized types in argument lists; this is impossible in C, but possible in Rust

Changes to the C API

• Deprecated the LLVMAddBBVectorizePass interface since the BBVectorize pass has been removed. It is now a no-op and will be removed in the next release. Use LLVMAddSLPVectorizePass instead to get the supported SLP vectorizer.

External Open Source Projects Using LLVM 5

Zig Programming Language

Zig is an open-source programming language designed for robustness, optimality, and clarity. It integrates closely with C and is intended to eventually take the place of C. It uses LLVM to produce highly optimized native code and to cross-compile for any target out of the box. Zig is in alpha; with a beta release expected in September.

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, FreeBSD and Windows and also Linux on ARM and PowerPC (32/64 bit). 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.