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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 .

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.

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.

, , , , , , and have been removed. The C API enum LLVMAttribute has been deleted.

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)) .

in the LLVM source were replaced with , which matches the C++17 semantics rather than gcc’s . 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.

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.

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.

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> 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.

is built on top of , and supports the same configuration options. As before (and unlike ), takes ownership of its nodes. However, it no longer supports allocating nodes, and is now equivalent to . will likely be removed in the future. 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.

) and “prev” ( ) pointer and is stored in the same allocation as . Previously, it was malloc-allocated on-demand by default, although the now-defunct 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() .

class no longer uses . 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 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

and 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

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.