Update ReleaseNotes for lld 9.0.

Introduction

============

This document contains the release notes for the lld linker, release 9.0.0.

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/>`_.

lld is a high-performance linker that supports ELF (Unix), COFF

(Windows), Mach-O (macOS), MinGW and WebAssembly. lld is

command-line-compatible with GNU linkers and Microsoft link.exe and is

significantly faster than the system default linkers.

lld 9.0 has lots of feature improvements and bug fixes.

Non-comprehensive list of changes in this release

=================================================

* ld.lld now has typo suggestions for flags:

  ``$ ld.lld --call-shared`` now prints

  ``unknown argument '--call-shared', did you mean '--call_shared'``.

  (`r361518 <https://reviews.llvm.org/rL361518>`_)

* ``--allow-shlib-undefined`` and ``--no-allow-shlib-undefined``

  options are added. ``--no-allow-shlib-undefined`` is the default for

  executables.

  (`r352826 <https://reviews.llvm.org/rL352826>`_)

* ``-nmagic`` and ``-omagic`` options are fully supported.

  (`r360593 <https://reviews.llvm.org/rL360593>`_)

* Segment layout has changed. PT_GNU_RELRO, which was previously

  placed in the middle of readable/writable PT_LOAD segments, is now

  placed at the beginning of them. This change permits lld-produced

  ELF files to be read correctly by GNU strip older than 2.31, which

  has a bug to discard a PT_GNU_RELRO in the former layout.

* ``-z common-page-size`` is supported.

  (`r360593 <https://reviews.llvm.org/rL360593>`_)

* Diagnostics messages have improved. A new flag ``--vs-diagnostics``

  alters the format of diagnostic output to enable source hyperlinks

  in Microsoft Visual Studio IDE.

* Linker script compatibility with GNU BFD linker has generally improved.

* The clang ``--dependent-library`` form of autolinking is supported.

  This feature is added to implement the Windows-style autolinking for

  Unix. On Unix, in order to use a library, you usually have to

  include a header file provided by the library and then explicitly

  link the library with the linker ``-l`` option. On Windows, header

  files usually contain pragmas that list needed libraries. Compilers

  copies that information to object files, so that linkers can

  automatically link needed libraries. ``--dependent-library`` is

  added for implementing that Windows semantics on Unix.

  (`r360984 <https://reviews.llvm.org/rL360984>`_)

* AArch64 BTI and PAC are supported.

  (`r362793 <https://reviews.llvm.org/rL362793>`_)

* lld now supports replacing ``JAL`` with ``JALX`` instructions in case

  of MIPS - microMIPS cross-mode jumps.

  of MIPS ↔ microMIPS cross-mode jumps.

  (`r354311 <https://reviews.llvm.org/rL354311>`_)

* lld now creates LA25 thunks for MIPS R6 code.

  (`r354312 <https://reviews.llvm.org/rL354312>`_)

* Put MIPS-specific .reginfo, .MIPS.options, and .MIPS.abiflags sections

  into corresponding PT_MIPS_REGINFO, PT_MIPS_OPTIONS, and PT_MIPS_ABIFLAGS

  segments.

* The quality of RISC-V and PowerPC ports have greatly improved. Many

  applications can now be linked by lld. PowerPC64 is now almost

  production ready.

* The Linux kernel for arm32_7, arm64, ppc64le and x86_64 can now be

  linked by lld.

* x86-64 TLSDESC is supported.

  (`r361911 <https://reviews.llvm.org/rL361911>`_,

  `r362078 <https://reviews.llvm.org/rL362078>`_)

* DF_STATIC_TLS flag is set for i386 and x86-64 when needed.

  (`r353293 <https://reviews.llvm.org/rL353293>`_,

  `r353378 <https://reviews.llvm.org/rL353378>`_)

* The experimental partitioning feature is added to allow a program to

  be split into multiple pieces.

  The feature allows you semi-automatically split a single program

  into multiple ELF files called "partitions". Since all partitions

  share the same memory address space and don't use PLT/GOT, split

  programs run as fast as regular programs.

  With the mechanism, you can start a program only with a "main"

  partition and load remaining partitions on-demand. For example, you

  can split a web browser into a main partition and a PDF reader

  sub-partition and load the PDF reader partition only when a user

  tries to open a PDF file.

  See https://lld.llvm.org/Partitions.html for more information.

* If "-" is given as an output filename, lld write the final result

  to the standard output. Previously, it created a file "-" in the

  current directory.

  (`r351852 <https://reviews.llvm.org/rL351852>`_)

* ``-z ifunc-noplt`` option is added to reduce IFunc function call

  overhead in a freestanding environment such as the OS kernel.

  Functions resolved by the IFunc mechanism are usually dispatched via

  PLT and thus slower than regular functions because of the cost of

  indirection. With ``-z ifunc-noplt``, you can eliminate it by doing

  text relocations at load-time. You need a special loader to utilize

  this feature. This feature is added for the FreeBSD kernel but can

  be used by any operating systems.

  (`r360685 <https://reviews.llvm.org/rL360685>`_)

* ``--undefined-glob`` option is added. The new option is an extension

  to ``--undefined`` to take a glob pattern instead of a single symbol

  name.

  (`r363396 <https://reviews.llvm.org/rL363396>`_)

COFF Improvements

-----------------

* Like the ELF driver, lld-link now has typo suggestions for flags.

  (`r361518 <https://reviews.llvm.org/rL361518>`_)

* lld-link now correctly reports duplicate symbol errors for obj files

  that were compiled with /Gy.

* lld-link now correctly reports duplicate symbol errors for object

  files that were compiled with ``/Gy``.

  (`r352590 <https://reviews.llvm.org/rL352590>`_)

* lld-link now correctly reports duplicate symbol errors when several res

  input files define resources with the same type, name, and language.

  This can be demoted to a warning using ``/force:multipleres``.

* lld-link now correctly reports duplicate symbol errors when several

  resource (.res) input files define resources with the same type,

  name and language.  This can be demoted to a warning using

  ``/force:multipleres``.

  (`r359829 <https://reviews.llvm.org/rL359829>`_)

* lld-link now rejects more than one resource obj input files, matching

  link.exe. Previously, lld-link would silently ignore all but one.

  If you hit this: Don't pass resource obj files to the linker, instead pass

  res files to the linker directly. Don't put res files in static libraries,

  pass them on the command line.

* lld-link now rejects more than one resource object input files,

  matching link.exe. Previously, lld-link would silently ignore all

  but one.  If you hit this: Don't pass resource object files to the

  linker, instead pass res files to the linker directly. Don't put

  resource files in static libraries, pass them on the command line.

  (`r359749 <https://reviews.llvm.org/rL359749>`_)

* Having more than two ``/natvis:`` now works correctly; it used to not

  work for larger binaries before.

  (`r327895 <https://reviews.llvm.org/rL327895>`_)

* Undefined symbols are now printed only in demangled form. Pass

  ``/demangle:no`` to see raw symbol names instead.

* The following flags have been added: ``/functionpadmin``, ``/swaprun:``,

  ``/threads:no``

  (`r355878 <https://reviews.llvm.org/rL355878>`_)

* Several speed and memory usage improvements.

* Range extension thunks are now created for ARM64, if needed

* Range extension thunks are now created for ARM64, if needed.

* lld-link now supports resource object files created by GNU windres and

  MS cvtres, not only llvm-cvtres

  MS cvtres, not only llvm-cvtres.

* The generated thunks for delayimports now share the majority of code

  among thunks, significantly reducing the overhead of using delayimport

  among thunks, significantly reducing the overhead of using delayimport.

  (`r365823 <https://reviews.llvm.org/rL365823>`_)

* ``IMAGE_REL_ARM{,64}_REL32`` relocations are supported.

  (`r352325 <https://reviews.llvm.org/rL352325>`_)

* Range extension thunks for AArch64 are now supported, so lld can

  create large executables for Windows/ARM64.

  (`r352929 <https://reviews.llvm.org/rL352929>`_)

* The following flags have been added:

  ``/functionpadmin`` (`r354716 <https://reviews.llvm.org/rL354716>`_),

  ``/swaprun:`` (`r359192 <https://reviews.llvm.org/rL359192>`_),

  ``/threads:no`` (`r355029 <https://reviews.llvm.org/rL355029>`_),

  ``/filealign`` (`r361634 <https://reviews.llvm.org/rL361634>`_)

WebAssembly Improvements

------------------------

* Imports from custom module names are supported.

  (`r352828 <https://reviews.llvm.org/rL352828>`_)

* Symbols that are in llvm.used are no exported by default.

  (`r353364 <https://reviews.llvm.org/rL353364>`_)

* Initial support for PIC and dynamic linking has landed.

  (`r357022 <https://reviews.llvm.org/rL357022>`_)

* wasm-ld now add ``__start_``/``__stop_`` symbols for data sections.

  (`r361236 <https://reviews.llvm.org/rL361236>`_)

* wasm-ld now doesn't report an error on archives without a symbol index.

  (`r364338 <https://reviews.llvm.org/rL364338>`_)

* The following flags have been added:

  ``--emit-relocs`` (`r361635 <https://reviews.llvm.org/rL361635>`_),

  ``--wrap`` (`r361639 <https://reviews.llvm.org/rL361639>`_),

  ``--trace`` and ``--trace-symbol``

  (`r353264 <https://reviews.llvm.org/rL353264>`_).

MinGW Improvements

------------------

  DWARF exception handling with libgcc and gcc's crtend.o.

* lld now also handles DWARF unwind info generated by GCC, when linking

  with libgcc

* Many more GNU ld options are now supported, which e.g. allows the lld

  MinGW frontend to be called by GCC

  with libgcc.

* PDB output can be requested without manually specifying the PDB file

  name, with the new option ``-pdb=`` with an empty value to the option.

  (The old existing syntax ``-pdb <filename>`` was more cumbersome to use

  with an empty parameter value.)

* ``--no-insert-timestamp`` option is added as an alias to ``/timestamp:0``.

  (`r353145 <https://reviews.llvm.org/rL353145>`_)

* Many more GNU ld options are now supported, which e.g. allows the lld

  MinGW frontend to be called by GCC.

* The following options are added: ``--exclude-all-symbols``,

  ``--appcontainer``, ``--emit-relocs``, ``--wrap``, ``--undefined``