LLVM 16.0.0 Release Notes

Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 16.0.0. 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 Discourse forums is a good place to ask them.

Note that if you are reading this file from a Git checkout or the main LLVM web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

Non-comprehensive list of changes in this release

  • The readnone calls which are crossing suspend points in coroutines will not be merged. Since readnone calls may access thread id and thread id is not a constant in coroutines. This decision may cause unnecessary performance regressions and we plan to fix it in later versions.

  • The LoongArch target is promoted to “official” (see below for more details).

  • …

Update on required toolchains to build LLVM

LLVM is now built with C++17 by default. This means C++17 can be used in the code base.

The previous “soft” toolchain requirements have now been changed to “hard”. This means that the the following versions are now required to build LLVM and there is no way to suppress this error.

  • GCC >= 7.1

  • Clang >= 5.0

  • Apple Clang >= 10.0

  • Visual Studio 2019 >= 16.7

With LLVM 16.x we will raise the version requirement of CMake used to build LLVM. The new requirements are as follows:

  • CMake >= 3.20.0

In LLVM 16.x this requirement will be “soft”, there will only be a diagnostic.

With the release of LLVM 17.x this requirement will be hard and LLVM developers can start using CMake 3.20.0 features, making it impossible to build with older versions of CMake.

Changes to the LLVM IR

  • The readnone, readonly, writeonly, argmemonly, inaccessiblememonly and inaccessiblemem_or_argmemonly function attributes have been replaced by a single memory(...) attribute. The old attributes may be mapped to the new one as follows:

    • readnone -> memory(none)

    • readonly -> memory(read)

    • writeonly -> memory(write)

    • argmemonly -> memory(argmem: readwrite)

    • argmemonly readonly -> memory(argmem: read)

    • argmemonly writeonly -> memory(argmem: write)

    • inaccessiblememonly -> memory(inaccessiblemem: readwrite)

    • inaccessiblememonly readonly -> memory(inaccessiblemem: read)

    • inaccessiblememonly writeonly -> memory(inaccessiblemem: write)

    • inaccessiblemem_or_argmemonly -> memory(argmem: readwrite, inaccessiblemem: readwrite)

    • inaccessiblemem_or_argmemonly readonly -> memory(argmem: read, inaccessiblemem: read)

    • inaccessiblemem_or_argmemonly writeonly -> memory(argmem: write, inaccessiblemem: write)

  • The constant expression variants of the following instructions has been removed:

    • fneg

  • Target extension types have been added, which allow targets to have types that need to be preserved through the optimizer, but otherwise are not introspectable by target-independent optimizations.

  • Added uinc_wrap and udec_wrap operations to atomicrmw.

  • Renamed llvm.flt.rounds intrinsic to llvm.get.rounding.

Changes to Interprocedural Optimizations

  • Function Specialization has been integrated into IPSCCP.

  • Specialization of functions has been enabled by default at all optimization levels except Os, Oz. This has exposed a mis-compilation in SPEC/CINT2017rate/502.gcc_r when built via the LLVM Test Suite with both LTO and PGO enabled, but without the option -fno-strict-aliasing.

Changes to the AArch64 Backend

  • Added support for the Cortex-A715 CPU.

  • Added support for the Cortex-X3 CPU.

  • Added support for the Neoverse V2 CPU.

  • Added support for assembly for RME MEC (Memory Encryption Contexts).

  • Added codegen support for the Armv8.3 Complex Number extension.

  • Implemented Function Multi Versioning in accordance with Arm C Language Extensions specification. Currently in Beta state.

Changes to the ARM Backend

  • Support for targeting Armv2, Armv2A, Armv3 and Armv3M has been removed. LLVM did not, and was not ever likely to generate correct code for those architecture versions so their presence was misleading.

  • Added codegen support for the complex arithmetic instructions in MVE.

  • Added Armv4 and Armv4T compatible thunks. LLD will no longer generate BX instructions for Armv4 or BLX instructions for either Armv4 or Armv4T. Armv4T is now fully supported.

  • Added compiler-rt builtins support for Armv4T, Armv5TE and Armv6.

Changes to the Hexagon Backend

  • The Hexagon backend now support V71 and V73 ISA.

Changes to the LoongArch Backend

  • The LoongArch target is no longer “experimental”! It’s now built by default, rather than needing to be enabled with LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.

  • The backend has full codegen support for the base (both integer and floating-point) instruction set and it conforms to psABI v2. Testing has been performed with Linux, including native compilation of a large corpus of Linux applications.

  • Support GHC calling convention.

  • Initial JITLink support is added. (D141036)

Changes to the PowerPC Backend

Common PowerPC improvements

  • Supported selecting floating point and ‘sync’ family of instructions in GlobalISel PowerPC.

  • Comparison operations are now optimized by record form instructions.

  • __test_data_class built-in now accepts __float128 arguments for Power9 and newer.

  • Fixed incorrect fence insertion in atomic FP operations on PowerPC.

  • Fixed alignment of aggregate with smaller size than register in variadic function on PowerPC 64-bit.

  • CTR loops on PowerPC are now generated after instruction selection.

  • Enabled track-subreg-liveness option by default.

  • Store of link register in function prologue are now generated in location after stack pointer update instructions.

  • Code generation improvements for atomic operations, vector permutes, and constant materialization for some floating point constants.

  • Implement new register classes and a number of new instructions for mcpu=future.

  • Implement byte reverse instructions, and VSX Scalar Quad Precision compares for mcpu=pwr10.

  • Improved load-store forwarding for big-endian mode.

  • Bug fixes.

AIX improvements

  • Supported TOC-data, overflow section, R_RBR relocation in XCOFF.

  • Fixed behavior of function sections, cold attribute, and handling of TLS symbols’ name prefixes in XCOFF.

  • Fixed redundant spill and reload on AIX 64-bit when paired vector are enabled.

  • Disabled location attribution generation of TLS variables.

  • Fixed the mapping of built-in functions __builtin_frexpl, __builtin_ldexpl, and __builtin_modfl to libm routines in 64-bit long double mode.

  • Implemented libunwind function _Unwind_FindEnclosingFunction using traceback table on AIX.

  • Changed to use non-unique implementation for typeinfo comparison.

  • Codegen work done for -pg to generate correct calls to __mcount.

Changes to the RISC-V Backend

  • User Guide for RISC-V Target was introduced to document the status of support within LLVM for various RISC-V instruction set extensions.

  • Support for the unratified Zbe, Zbf, Zbm, Zbp, Zbr, and Zbt extensions have been removed.

  • i32 is now a native type in the datalayout string. This enables LoopStrengthReduce for loops with i32 induction variables, among other optimizations.

  • MC layer support was added for the experimental Zca, Zcd, Zcf, Zihintntl, Ztso, and Zawrs extensions.

  • Codegen support was added for the experimental Zca extension and for the Zfhmin extension.

  • MC layer and codegen support was added for the custom XVentanaCondOps and XTHeadVdot extensions.

  • A target feature was introduced to force-enable atomics.

  • Support was added for lowering HWASAN intrinsics.

  • The short forward branch optimisation beneficial to the SiFive Series 7 was implemented.

  • A Syntacore SCR1 CPU model was added.

  • Various codegen improvements.

Changes to the SystemZ Backend

  • The datalayout string now only depends on the target triple as expected.

  • The GNU attribute for a visible vector ABI is now emitted.

  • Align 128 bit integers to 8 bytes only, per the ABI.

Changes to the Windows Target

  • For MinGW, generate embedded -exclude-symbols: directives for symbols with hidden visibility, omitting them from automatic export of all symbols. This roughly makes hidden visibility work like it does for other object file formats.

  • When using multi-threaded LLVM tools (such as LLD) on a Windows host with a large number of processors or CPU sockets, previously the LLVM ThreadPool would span out threads to use all processors. Starting with Windows Server 2022 and Windows 11, the behavior has changed, the OS now spans out threads automatically to all processors. This also fixes an affinity mask issue. (D138747)

  • When building LLVM and related tools for Windows with Clang in MinGW mode, hidden symbol visiblity is now used to reduce the number of exports in builds with dylibs (LLVM_BUILD_LLVM_DYLIB or LLVM_LINK_LLVM_DYLIB), making such builds more manageable without running into the limit of number of exported symbols.

  • AArch64 SEH unwind info generation bugs have been fixed; there were minor cases of mismatches between the generated unwind info and actual prologues/epilogues earlier in some cases.

  • AArch64 SEH unwind info is now generated correctly for the AArch64 security features BTI (Branch Target Identification) and PAC (Pointer Authentication Code). In particular, using PAC with older versions of LLVM would generate code that would fail to unwind at runtime, if the host actually would use the pointer authentication feature.

  • Fixed stack alignment on Windows on AArch64, for stack frames with a large enough allocation that requires stack probing.

Changes to the X86 Backend

  • Add support for the RDMSRLIST and WRMSRLIST instructions.

  • Add support for the WRMSRNS instruction.

  • Support ISA of AMX-FP16 which contains tdpfp16ps instruction.

  • Support ISA of CMPCCXADD.

  • Support ISA of AVX-IFMA.

  • Support ISA of AVX-VNNI-INT8.

  • Support ISA of AVX-NE-CONVERT.

  • -mcpu=raptorlake, -mcpu=meteorlake and -mcpu=emeraldrapids are now supported.

  • -mcpu=sierraforest, -mcpu=graniterapids and -mcpu=grandridge are now supported.

Changes to the C API

  • The following functions for creating constant expressions have been removed, because the underlying constant expressions are no longer supported. Instead, an instruction should be created using the LLVMBuildXYZ APIs, which will constant fold the operands if possible and create an instruction otherwise:

    • LLVMConstFNeg

  • The following deprecated functions have been removed, because they are incompatible with opaque pointers. Use the new functions accepting a separate function/element type instead.

    • LLVMBuildLoad -> LLVMBuildLoad2

    • LLVMBuildCall -> LLVMBuildCall2

    • LLVMBuildInvoke -> LLVMBuildInvoke2

    • LLVMBuildGEP -> LLVMBuildGEP2

    • LLVMBuildInBoundsGEP -> LLVMBuildInBoundsGEP2

    • LLVMBuildStructGEP -> LLVMBuildStructGEP2

    • LLVMBuildPtrDiff -> LLVMBuildPtrDiff2

    • LLVMConstGEP -> LLVMConstGEP2

    • LLVMConstInBoundsGEP -> LLVMConstInBoundsGEP2

    • LLVMAddAlias -> LLVMAddAlias2

Changes to the Metadata Info

  • Add Module Flags Metadata stack-protector-guard-symbol which specify a symbol for addressing the stack-protector guard.

Changes to the Debug Info

Previously when emitting DWARF v4 and tuning for GDB, llc would use DWARF v2’s DW_AT_bit_offset and DW_AT_data_member_location. llc now uses DWARF v4’s DW_AT_data_bit_offset regardless of tuning.

Support for DW_AT_data_bit_offset was added in GDB 8.0. For earlier versions, you can use llc’s -dwarf-version=3 option to emit compatible DWARF.

When emitting CodeView debug information, LLVM will now emit S_CONSTANT records for variables optimized into a constant via the SROA and SCCP passes. (D138995)

DW_LANG_C11 now respects -gstrict-dwarf option.

Changes to the LLVM tools

  • llvm-readobj --elf-output-style=JSON no longer prefixes each JSON object with the file name. Previously, each object file’s output looked like "main.o":{"FileSummary":{"File":"main.o"},...} but is now {"FileSummary":{"File":"main.o"},...}. This allows each JSON object to be parsed in the same way, since each object no longer has a unique key. Tools that consume llvm-readobj’s JSON output should update their parsers accordingly.

  • llvm-readobj now supports a new option --loader-section-header to display the loader section header of XCOFF object tiles

  • llvm-readobj now supports a new option --loader-section-header to display the symbol table of the loader section of XCOFF object files

  • llvm-readobj now supports a new option --loader-section-header to display relocation entries in the loader section of XCOFF object files

  • llvm-readobj now supports a new option --exception-section to display exception section entries from XCOFF object files

  • llvm-objdump now uses --print-imm-hex by default, which brings its default behavior closer in line with objdump.

  • llvm-objcopy no longer writes corrupt addresses to empty sections if the input file had a nonzero address to an empty section.

  • llvm-nm now supports the environment variable OBJECT_MODE for the -X option on AIX OS

Changes to LLDB

  • Initial support for debugging Linux LoongArch 64-bit binaries.

  • Improvements in COFF symbol handling; previously a DLL (without any other debug info) would only use the DLL’s exported symbols, while it now also uses the full list of internal symbols, if available.

  • Avoiding duplicate DLLs in the runtime list of loaded modules on Windows.

Changes to Sanitizers

  • Many Sanitizers (asan, fuzzer, lsan, safestack, scudo, tsan, ubsan) have support for Linux LoongArch 64-bit variant. Some of them may be rudimentary.

Other Changes

  • lit no longer supports using substrings of the default target triple as feature names in UNSUPPORTED: and XFAIL: directives. These have been replaced by the target=<triple> feature, and tests can use regex matching to achieve the same effect. For example, UNSUPPORTED: arm would now be UNSUPPORTED: target=arm{{.*}} and XFAIL: windows would now be XFAIL: target={{.*}}-windows{{.*}}.

  • When cross compiling LLVM (or building with LLVM_OPTIMIZED_TABLEGEN), it is now possible to point the build to prebuilt versions of all the host tools with one CMake variable, LLVM_NATIVE_TOOL_DIR, instead of having to point out each individual tool with variables such as LLVM_TABLEGEN, CLANG_TABLEGEN, LLDB_TABLEGEN etc.

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 Git 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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