LLVM 4.0.0 Release Notes — LLVM 4 documentation

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# LLVM 4.0.0 Release Notes

- Introduction
- New Versioning Scheme
- Non-comprehensive list of changes in this release
  - Improvements to ThinLTO (-flto=thin)
  - LLVM Coroutines
  - Regcall and Vectorcall Calling Conventions
  - Code Generation Testing
  - Intrusive list API overhaul
  - Changes to the ARM Targets
  - Changes to the AVR Target
  - Changes to the MIPS Target
  - Changes to the X86 Target
  - Changes to the OCaml bindings
- External Open Source Projects Using LLVM 4.0.0
  - LDC - the LLVM-based D compiler
  - Portable Computing Language (pocl)
  - TTA-based Co-design Environment (TCE)
- Additional Information

## Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 4.0.0. 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.

## New Versioning Scheme

Starting with this release, LLVM is using a new versioning scheme, increasing the major version number with each major release. Stable updates to this release will be versioned 4.0.x, and the next major release, six months from now, will be version 5.0.0.

## Non-comprehensive list of changes in this release

- The minimum compiler version required for building LLVM has been raised to 4.8 for GCC and 2015 for Visual Studio.
- The C API functions `LLVMAddFunctionAttr`, `LLVMGetFunctionAttr`, `LLVMRemoveFunctionAttr`, `LLVMAddAttribute`, `LLVMRemoveAttribute`, `LLVMGetAttribute`, `LLVMAddInstrAttribute` and `LLVMRemoveInstrAttribute` have been removed.
- The C API enum `LLVMAttribute` has been deleted.
- The definition and uses of `LLVM_ATRIBUTE_UNUSED_RESULT` in the LLVM source were replaced with `LLVM_NODISCARD`, which matches the C++17 `[[nodiscard]]` semantics rather than gcc’s `__attribute__((warn_unused_result))`.
- The Timer related APIs now expect a Name and Description. When upgrading code the previously used names should become descriptions and a short name in the style of a programming language identifier should be added.
- LLVM now handles `invariant.group` across different basic blocks, which makes it possible to devirtualize virtual calls inside loops.
- The aggressive dead code elimination phase (“adce”) now removes branches which do not effect program behavior. Loops are retained by default since they may be infinite but these can also be removed with LLVM option `-adce-remove-loops` when the loop body otherwise has no live operations.
- The llvm-cov tool can now export coverage data as json. Its html output mode has also improved.

### Improvements to ThinLTO (-flto=thin)

Integration with profile data (PGO). When available, profile data enables more accurate function importing decisions, as well as cross-module indirect call promotion.

Significant build-time and binary-size improvements when compiling with debug info (-g).

### LLVM Coroutines

Experimental support for Coroutines in LLVM was added, which can be enabled with `-enable-coroutines` in `opt` the command tool or using the `addCoroutinePassesToExtensionPoints` API when building the optimization pipeline.

For more information on LLVM Coroutines and the LLVM implementation, see 2016 LLVM Developers’ Meeting talk on LLVM Coroutines.

### Regcall and Vectorcall Calling Conventions

Support was added for `_regcall` calling convention. Existing `__vectorcall` calling convention support was extended to include correct handling of HVAs.

The `__vectorcall` calling convention was introduced by Microsoft to enhance register usage when passing parameters. For more information please read __vectorcall documentation.

The `__regcall` calling convention was introduced by Intel to optimize parameter transfer on function call. This calling convention ensures that as many values as possible are passed or returned in registers. For more information please read __regcall documentation.

### Code Generation Testing

Passes that work on the machine instruction representation can be tested with the .mir serialization format. `llc` supports the `-run-pass`, `-stop-after`, `-stop-before`, `-start-after`, `-start-before` to run a single pass of the code generation pipeline, or to stop or start the code generation pipeline at a given point.

Additional information can be found in the Machine IR (MIR) Format Reference Manual. The format is used by the tests ending in `.mir` in the `test/CodeGen` directory.

This feature is available since 2015. It is used more often lately and was not mentioned in the release notes yet.

### Intrusive list API overhaul

The intrusive list infrastructure was substantially rewritten over the last couple of releases, primarily to excise undefined behaviour. The biggest changes landed in this release.

- `simple_ilist<T>` is a lower-level intrusive list that never takes ownership of its nodes. New intrusive-list clients should consider using it instead of `ilist<T>`.
  - `ilist_tag<class>` allows a single data type to be inserted into two parallel intrusive lists. A type can inherit twice from `ilist_node`, first using `ilist_node<T,ilist_tag<A>>` (enabling insertion into `simple_ilist<T,ilist_tag<A>>`) and second using `ilist_node<T,ilist_tag<B>>` (enabling insertion into `simple_ilist<T,ilist_tag<B>>`), where `A` and `B` are arbitrary types.
  - `ilist_sentinel_tracking<bool>` controls whether an iterator knows whether it’s pointing at the sentinel (`end()`). By default, sentinel tracking is on when ABI-breaking checks are enabled, and off otherwise; this is used for an assertion when dereferencing `end()` (this assertion triggered often in practice, and many backend bugs were fixed). Explicitly turning on sentinel tracking also enables `iterator::isEnd()`. This is used by `MachineInstrBundleIterator` to iterate over bundles.
- `ilist<T>` is built on top of `simple_ilist<T>`, and supports the same configuration options. As before (and unlike `simple_ilist<T>`), `ilist<T>` takes ownership of its nodes. However, it no longer supports allocating nodes, and is now equivalent to `iplist<T>`. `iplist<T>` will likely be removed in the future.
  - `ilist<T>` now always uses `ilist_traits<T>`. Instead of passing a custom traits class in via a template parameter, clients that want to customize the traits should specialize `ilist_traits<T>`. Clients that want to avoid ownership can specialize `ilist_alloc_traits<T>` to inherit from `ilist_noalloc_traits<T>` (or to do something funky); clients that need callbacks can specialize `ilist_callback_traits<T>` directly.
- The underlying data structure is now a simple recursive linked list. The sentinel node contains only a “next” (`begin()`) and “prev” (`rbegin()`) pointer and is stored in the same allocation as `simple_ilist<T>`. Previously, it was malloc-allocated on-demand by default, although the now-defunct `ilist_sentinel_traits<T>` was sometimes specialized to avoid this.
- The `reverse_iterator` class no longer uses `std::reverse_iterator`. Instead, it now has a handle to the same node that it dereferences to. Reverse iterators now have the same iterator invalidation semantics as forward iterators.
  - `iterator` and `reverse_iterator` have explicit conversion constructors that match `std::reverse_iterator`‘s off-by-one semantics, so that reversing the end points of an iterator range results in the same range (albeit in reverse). I.e., `reverse_iterator(begin())` equals `rend()`.
  - `iterator::getReverse()` and `reverse_iterator::getReverse()` return an iterator that dereferences to the same node. I.e., `begin().getReverse()` equals `--rend()`.
  - `ilist_node<T>::getIterator()` and `ilist_node<T>::getReverseIterator()` return the forward and reverse iterators that dereference to the current node. I.e., `begin()->getIterator()` equals `begin()` and `rbegin()->getReverseIterator()` equals `rbegin()`.
- `iterator` now stores an `ilist_node_base*` instead of a `T*`. The implicit conversions between `ilist<T>::iterator` and `T*` have been removed. Clients may use `N->getIterator()` (if not `nullptr`) or `&*I` (if not `end()`); alternatively, clients may refactor to use references for known-good nodes.

### Changes to the ARM Targets

During this release the AArch64 target has:

- Gained support for ILP32 relocations.
- Gained support for XRay.
- Made even more progress on GlobalISel. There is still some work left before it is production-ready though.
- Refined the support for Qualcomm’s Falkor and Samsung’s Exynos CPUs.
- Learned a few new tricks for lowering multiplications by constants, folding spilled/refilled copies etc.

During this release the ARM target has:

- Gained support for ROPI (read-only position independence) and RWPI (read-write position independence), which can be used to remove the need for a dynamic linker.
- Gained support for execute-only code, which is placed in pages without read permissions.
- Gained a machine scheduler for Cortex-R52.
- Gained support for XRay.
- Gained Thumb1 implementations for several compiler-rt builtins. It also has some support for building the builtins for HF targets.
- Started using the generic bitreverse intrinsic instead of rbit.
- Gained very basic support for GlobalISel.

A lot of work has also been done in LLD for ARM, which now supports more relocations and TLS.

Note: From the next release (5.0), the “vulcan” target will be renamed to “thunderx2t99”, including command line options, assembly directives, etc. This release (4.0) will be the last one to accept “vulcan” as its name.

### Changes to the AVR Target

This marks the first release where the AVR backend has been completely merged from a fork into LLVM trunk. The backend is still marked experimental, but is generally quite usable. All downstream development has halted on GitHub, and changes now go directly into LLVM trunk.

- Instruction selector and pseudo instruction expansion pass landed
- read_register and write_register intrinsics are now supported
- Support stack stores greater than 63-bytes from the bottom of the stack
- A number of assertion errors have been fixed
- Support stores to undef locations
- Very basic support for the target has been added to clang
- Small optimizations to some 16-bit boolean expressions

Most of the work behind the scenes has been on correctness of generated assembly, and also fixing some assertions we would hit on some well-formed inputs.

### Changes to the MIPS Target

During this release the MIPS target has:

- IAS is now enabled by default for Debian mips64el.
- Added support for the two operand form for many instructions.
- Added the following macros: unaligned load/store, seq, double word load/store for O32.
- Improved the parsing of complex memory offset expressions.
- Enabled the integrated assembler by default for Debian mips64el.
- Added a generic scheduler based on the interAptiv CPU.
- Added support for thread local relocations.
- Added recip, rsqrt, evp, dvp, synci instructions in IAS.
- Optimized the generation of constants from some cases.

The following issues have been fixed:

- Thread local debug information is correctly recorded.
- MSA intrinsics are now range checked.
- Fixed an issue with MSA and the no-odd-spreg abi.
- Fixed some corner cases in handling forbidden slots for MIPSR6.
- Fixed an issue with jumps not being converted to relative branches for assembly.
- Fixed the handling of local symbols and jal instruction.
- N32/N64 no longer have their relocation tables sorted as per their ABIs.
- Fixed a crash when half-precision floating point conversion MSA intrinsics are used.
- Fixed several crashes involving FastISel.
- Corrected the corrected definitions for aui/daui/dahi/dati for MIPSR6.

### Changes to the X86 Target

During this release the X86 target has:

- Added support AMD Ryzen (znver1) CPUs.
- Gained support for using VEX encoding on AVX-512 CPUs to reduce code size when possible.
- Improved AVX-512 codegen.

### Changes to the OCaml bindings

- The attribute API was completely overhauled, following the changes to the C API.

## External Open Source Projects Using LLVM 4.0.0

### 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.

### Portable Computing Language (pocl)

In addition to producing an easily portable open source OpenCL implementation, another major goal of pocl 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 is a toolset for designing customized 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 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 TTA processors and loads them in to the compiler backend as runtime libraries to avoid per-target recompilation of larger parts of the compiler chain.

## 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.

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