bolt/deps/llvm-18.1.8/lld/docs/ReleaseNotes.rst

===========================
lld |release| Release Notes
===========================

.. contents::
    :local:

.. only:: PreRelease

  .. warning::
     These are in-progress notes for the upcoming LLVM |release| release.
     Release notes for previous releases can be found on
     `the Download Page <https://releases.llvm.org/download.html>`_.

Introduction
============

This document contains the release notes for the lld linker, release |release|.
Here we describe the status of lld, including major improvements
from the previous release. All lld releases may be downloaded
from the `LLVM releases web site <https://llvm.org/releases/>`_.

Non-comprehensive list of changes in this release
=================================================

ELF Improvements
----------------

* ``--fat-lto-objects`` option is added to support LLVM FatLTO.
  Without ``--fat-lto-objects``, LLD will link LLVM FatLTO objects using the
  relocatable object file. (`D146778 <https://reviews.llvm.org/D146778>`_)
* ``-Bsymbolic-non-weak`` is added to directly bind non-weak definitions.
  (`D158322 <https://reviews.llvm.org/D158322>`_)
* ``--lto-validate-all-vtables-have-type-infos``, which complements
  ``--lto-whole-program-visibility``, is added to disable unsafe whole-program
  devirtualization. ``--lto-known-safe-vtables=<glob>`` can be used
  to mark known-safe vtable symbols.
  (`D155659 <https://reviews.llvm.org/D155659>`_)
* ``--save-temps --lto-emit-asm`` now derives ELF/asm file names from bitcode file names.
  ``ld.lld --save-temps a.o d/b.o -o out`` will create ELF relocatable files
  ``out.lto.a.o``/``d/out.lto.b.o`` instead of ``out1.lto.o``/``out2.lto.o``.
  (`#78835 <https://github.com/llvm/llvm-project/pull/78835>`_)
* ``--no-allow-shlib-undefined`` now reports errors for DSO referencing
  non-exported definitions.
  (`#70769 <https://github.com/llvm/llvm-project/pull/70769>`_)
* common-page-size can now be larger than the system page-size.
  (`#57618 <https://github.com/llvm/llvm-project/issues/57618>`_)
* When call graph profile information is available due to instrumentation or
  sample PGO, input sections are now sorted using the new ``cdsort`` algorithm,
  better than the previous ``hfsort`` algorithm.
  (`D152840 <https://reviews.llvm.org/D152840>`_)
* Symbol assignments like ``a = DEFINED(a) ? a : 0;`` are now handled.
  (`#65866 <https://github.com/llvm/llvm-project/pull/65866>`_)
* ``OVERLAY`` now supports optional start address and LMA
  (`#77272 <https://github.com/llvm/llvm-project/pull/77272>`_)
* Relocations referencing a symbol defined in ``/DISCARD/`` section now lead to
  an error.
  (`#69295 <https://github.com/llvm/llvm-project/pull/69295>`_)
* For AArch64 MTE, global variable descriptors have been implemented.
  (`D152921 <https://reviews.llvm.org/D152921>`_)
* ``R_AARCH64_GOTPCREL32`` is now supported.
  (`#72584 <https://github.com/llvm/llvm-project/pull/72584>`_)
* ``R_LARCH_PCREL20_S2``/``R_LARCH_ADD6``/``R_LARCH_CALL36`` and extreme code
  model relocations are now supported.
* ``--emit-relocs`` is now supported for RISC-V linker relaxation.
  (`D159082 <https://reviews.llvm.org/D159082>`_)
* Call relaxation respects RVC when mixing +c and -c relocatable files.
  (`#73977 <https://github.com/llvm/llvm-project/pull/73977>`_)
* ``R_RISCV_GOT32_PCREL`` is now supported.
  (`#72587 <https://github.com/llvm/llvm-project/pull/72587>`_)
* ``R_RISCV_SET_ULEB128``/``R_RISCV_SUB_ULEB128`` relocations are now supported.
  (`#72610 <https://github.com/llvm/llvm-project/pull/72610>`_)
  (`#77261 <https://github.com/llvm/llvm-project/pull/77261>`_)
* RISC-V TLSDESC is now supported.
  (`#79239 <https://github.com/llvm/llvm-project/pull/79239>`_)

Breaking changes
----------------

COFF Improvements
-----------------

* Added support for ``--time-trace`` and associated ``--time-trace-granularity``.
  This generates a .json profile trace of the linker execution.
  (`#68236 <https://github.com/llvm/llvm-project/pull/68236>`_)

* The ``-dependentloadflag`` option was implemented.
  (`#71537 <https://github.com/llvm/llvm-project/pull/71537>`_)

* LLD now prefers library paths specified with ``-libpath:`` over the implicitly
  detected toolchain paths.
  (`#78039 <https://github.com/llvm/llvm-project/pull/78039>`_)

* Added new options ``-lldemit:llvm`` and ``-lldemit:asm`` for getting
  the output of LTO compilation as LLVM bitcode or assembly.
  (`#66964 <https://github.com/llvm/llvm-project/pull/66964>`_)
  (`#67079 <https://github.com/llvm/llvm-project/pull/67079>`_)

* Added a new option ``-build-id`` for generating a ``.buildid`` section
  when not generating a PDB. A new symbol ``__buildid`` is generated by
  the linker, allowing code to reference the build ID of the binary.
  (`#71433 <https://github.com/llvm/llvm-project/pull/71433>`_)
  (`#74652 <https://github.com/llvm/llvm-project/pull/74652>`_)

* A new, LLD specific option, ``-lld-allow-duplicate-weak``, was added
  for allowing duplicate weak symbols.
  (`#68077 <https://github.com/llvm/llvm-project/pull/68077>`_)

* More correctly handle LTO of files that define ``__imp_`` prefixed dllimport
  redirections.
  (`#70777 <https://github.com/llvm/llvm-project/pull/70777>`_)
  (`#71376 <https://github.com/llvm/llvm-project/pull/71376>`_)
  (`#72989 <https://github.com/llvm/llvm-project/pull/72989>`_)

* Linking undefined references to weak symbols with LTO now works.
  (`#70430 <https://github.com/llvm/llvm-project/pull/70430>`_)

* Use the ``SOURCE_DATE_EPOCH`` environment variable for the PE header and
  debug directory timestamps, if neither the ``/Brepro`` nor ``/timestamp:``
  options have been specified. This makes the linker output reproducible by
  setting this environment variable.
  (`#81326 <https://github.com/llvm/llvm-project/pull/81326>`_)

* Lots of incremental work towards supporting linking ARM64EC binaries.

MinGW Improvements
------------------

* Added support for many LTO and ThinLTO options (most LTO options supported
  by the ELF driver, that are implemented by the COFF backend as well,
  should be supported now).
  (`D158412 <https://reviews.llvm.org/D158412>`_)
  (`D158887 <https://reviews.llvm.org/D158887>`_)
  (`#77387 <https://github.com/llvm/llvm-project/pull/77387>`_)
  (`#81475 <https://github.com/llvm/llvm-project/pull/81475>`_)

* LLD no longer tries to autodetect and use library paths from MSVC/WinSDK
  installations when run in MinGW mode; that mode of operation shouldn't
  ever be needed in MinGW mode, and could be a source of unexpected
  behaviours.
  (`D144084 <https://reviews.llvm.org/D144084>`_)

* The ``--icf=safe`` option now works as expected; it was previously a no-op.
  (`#70037 <https://github.com/llvm/llvm-project/pull/70037>`_)

* The strip flags ``-S`` and ``-s`` now can be used to strip out DWARF debug
  info and symbol tables while emitting a PDB debug info file.
  (`#75181 <https://github.com/llvm/llvm-project/pull/75181>`_)

* The option ``--dll`` is handled as an alias for the ``--shared`` option.
  (`#68575 <https://github.com/llvm/llvm-project/pull/68575>`_)

* The option ``--sort-common`` is ignored now.
  (`#66336 <https://github.com/llvm/llvm-project/pull/66336>`_)

MachO Improvements
------------------

WebAssembly Improvements
------------------------

* Indexes are no longer required on archive files.  Instead symbol information
  is read from object files within the archive.  This matches the behaviour of
  the ELF linker.

SystemZ
-------

* Add target support for SystemZ (s390x).

Fixes
#####