LLVM 6.0.0 Release Notes

Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 6.0.0. 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

  • Support for retpolines was added to help mitigate “branch target injection” (variant #2) of the “Spectre” speculative side channels described by Project Zero and the Spectre paper.
  • The Redirects argument of llvm::sys::ExecuteAndWait and llvm::sys::ExecuteNoWait was changed to an ArrayRef of optional StringRef’s to make it safer and more convenient to use.
  • The backend name was added to the Target Registry to allow run-time information to be fed back into TableGen. Out-of-tree targets will need to add the name used in the def X : Target definition to the call to RegisterTarget.
  • The Debugify pass was added to opt to facilitate testing of debug info preservation. This pass attaches synthetic DILocations and DIVariables to the instructions in a Module. The CheckDebugify pass determines how much of the metadata is lost.
  • Significantly improved quality of CodeView debug info for Windows.
  • Preliminary support for Sanitizers and sibling features on X86(_64) NetBSD (ASan, UBsan, TSan, MSan, SafeStack, libFuzzer).

Changes to the LLVM IR

  • The fast-math-flags (FMF) have been updated. Previously, the ‘fast’ flag indicated that floating-point reassociation was allowed and all other flags were set too. The ‘fast’ flag still exists, but there is a new flag called ‘reassoc’ to indicate specifically that reassociation is allowed. A new bit called ‘afn’ was also added to selectively allow approximations for common mathlib functions like square-root. The new flags provide more flexibility to enable/disable specific floating-point optimizations. Making the optimizer respond appropriately to these flags is an ongoing effort.

Changes to the AArch64 Target

  • Enabled the new GlobalISel instruction selection framework by default at -O0.

Changes to the ARM Target

  • Support for enabling SjLj exception handling on platforms where it isn’t the default.

Changes to the Hexagon Target

  • The Hexagon backend now supports V65 ISA.
  • The -mhvx option now takes an optional value that specifies the ISA version of the HVX coprocessor. The available values are v60, v62 and v65. By default, the value is set to be the same as the CPU version.
  • The compiler option -mhvx-double is deprecated and will be removed in the next release of the compiler. Programmers should use the -mhvx-length option to specify the desired vector length: -mhvx-length=64b for 64-byte vectors and -mhvx-length=128b for 128-byte vectors. While the current default vector length is 64 bytes, users should always specify the length explicitly, since the default value may change in the future.
  • The target feature hvx-double is deprecated and will be removed in the next release. LLVM IR generators should use target features hvx-length64b and hvx-length128b to indicate the vector length. The length should always be specified when HVX code generation is enabled.

Changes to the MIPS Target

Fixed numerous bugs:

  • fpowi on MIPS64 giving incorrect results when used with a negative integer.
  • Usage of the asm ‘c’ constraint with the wrong datatype causing an assert/crash.
  • Fixed a conversion bug when using the DSP ASE.
  • Fixed an inconsistency where objects were not marked as using the microMIPS as when the micromips function attribute or the “.set micromips” directive was used.
  • Reordered the MIPSR6 specific hazard scheduler pass to after the delay slot filler, fixing a class of rare edge case bugs where the delay slot filler would violate ISA restrictions.
  • Fixed a crash when using a type of unknown size with gp relative addressing.
  • Corrected the j macro for microMIPS.
  • Corrected the encoding of movep for microMIPS32r6.
  • Fixed an issue with the usage of insert instructions having an invalid set of operands.
  • Fixed an issue where TLS symbols were not marked as such.
  • Enabled the usage of register scavenging with MSA, due to its shorter offsets for loads and stores.
  • Corrected the ELF headers when using the DSP ASE.

New features:

  • The long branch pass now generates some R6 specific instructions when targeting MIPSR6.
  • The delay slot filler now performs more branch conversions if delay slots cannot be filled.
  • The MIPS MT ASE is now fully supported.
  • Added support for the lapc pseudo instruction.
  • Improved the selection of multiple instructions (dext, nmadd, nmsub).
  • Further improved microMIPS codesize reduction.

Deprecation notices:

  • microMIPS64R6 support was been deprecated since 5.0, and has now been completely removed.

Changes to the SystemZ Target

During this release the SystemZ target has:

  • Added support for 128-bit atomic operations.
  • Added support for the “o” constraint for inline asm statements.

Changes to the X86 Target

During this release the X86 target has:

  • Added support for enabling SjLj exception handling on platforms where it isn’t the default.
  • Added intrinsics for Intel Extensions: VAES, GFNI, VPCLMULQDQ, AVX512VBMI2, AVX512BITALG, AVX512VNNI.
  • Added support for Intel Icelake CPU.
  • Fixed some X87 codegen bugs.
  • Added instruction scheduling information for Intel Sandy Bridge, Ivy Bridge, Haswell, Broadwell, and Skylake CPUs.
  • Improved scheduler model for AMD Jaguar CPUs.
  • Improved llvm-mc’s disassembler for some EVEX encoded instructions.
  • Add support for i8 and i16 vector signed/unsigned min/max horizontal reductions.
  • Improved codegen for memory comparisons
  • Improved codegen for i32 vector multiplies
  • Improved codegen for scalar integer absolute values
  • Improved codegen for vector integer rotations (XOP and AVX512)
  • Improved codegen of data being transferred between GPRs and K-registers.
  • Improved codegen for vector truncations.
  • Improved folding of address computations into gather/scatter instructions.
  • Gained initial support recognizing variable shuffles from vector element extracts and inserts.
  • Improved documentation for SSE/AVX intrinsics in intrin.h header files.
  • Gained support for emitting retpolines, including automatic insertion of the necessary thunks or using external thunks.

External Open Source Projects Using LLVM 6

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.

JFS - JIT Fuzzing Solver

JFS is an experimental constraint solver designed to investigate using coverage guided fuzzing as an incomplete strategy for solving boolean, BitVector, and floating-point constraints. It is built on top of LLVM, Clang, LibFuzzer, and Z3.

The solver works by generating a C++ program where the reachability of an abort() statement is equivalent to finding a satisfying assignment to the constraints. This program is then compiled by Clang with SanitizerCoverage instrumentation and then fuzzed using LibFuzzer.

Zig Programming Language

Zig is an open-source programming language designed for robustness, optimality, and clarity. It is intended to replace C. It provides high level features such as Generics, Compile Time Function Execution, and Partial Evaluation, yet exposes low level LLVM IR features such as Aliases. Zig uses Clang to provide automatic import of .h symbols - even inline functions and macros. Zig uses LLD combined with lazily building compiler-rt to provide out-of-the-box cross-compiling for all supported targets.

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.

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