Update release notes for lld 7.0.

Introduction

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

This document contains the release notes for the lld linker, release 7.0.0.

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 these system

default linkers.

lld 7.0 for ELF and COFF are production-ready. lld/ELF can build the entire

FreeBSD/AMD64 and will be the default linker of the next version of the

operating system. lld/COFF is being used to build popular large programs such as

Chrome web browser. Mach-O, MinGW and WebAssembly supports are still

experimental.

Non-comprehensive list of changes in this release

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

ELF Improvements

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

* lld is now able to overcome MIPS GOT entries number limitation

  and generate multi-GOT if necessary.

* Fixed a lot of long-tail compatibility issues with GNU linkers.

* Added ``-z retpolineplt`` to emit a PLT entry that doesn't contain an indirect

  jump instruction to mitigate Spectre v2 vulnerability.

* Added experimental support for `SHT_RELR sections

  <https://groups.google.com/forum/#!topic/generic-abi/bX460iggiKg>` to create a

  compact dynamic relocation table.

* Added support for `split stacks <https://gcc.gnu.org/wiki/SplitStacks>`.

* Added support for address significance table (section with type

  SHT_LLVM_ADDRSIG) to improve Identical Code Folding (ICF). Combined with the

  ``-faddrsig`` compiler option added to Clang 7.0, lld's ``--icf=all`` can now

  safely merge functions and data to generate smaller outputs than before.

* Improved ``--gdb-index`` so that it is faster (`r336790

  <https://reviews.llvm.org/rL336790>`) and uses less memory (`r336672

  <https://reviews.llvm.org/rL336672>`).

* Reduced memory usage of ``--compress-debug-sections`` (`r338913

  <https://reviews.llvm.org/rL338913>`).

* Added linker script OVERLAY support (`r335714 <https://reviews.llvm.org/rL335714>`).

* Added ``--warn-backref`` to make it easy to identify command line option order

  that doesn't work with GNU linkers (`r329636 <https://reviews.llvm.org/rL329636>`)

* lld is now able to produce MIPS position-independent executable (PIE).

* Added ld.lld.1 man page (`r324512 <https://reviews.llvm.org/rL324512>`).

* Added support for multi-GOT.

* Added support for MIPS position-independent executable (PIE).

* Fixed MIPS TLS GOT entries for local symbols in shared libraries.

* Fixed calculation of MIPS GP relative relocations

  in case of relocatable output.

* Fixed calculation of MIPS GP relative relocations in case of relocatable

  output.

* Added support for PPCv2 ABI.

* Removed an incomplete support of PPCv1 ABI.

* Added support for Qualcomm Hexagon ISA.

* Added the following flags: ``--apply-dynamic-relocs``, ``--check-sections``,

  ``--cref``, ``--just-symbols``, ``--keep-unique``,

  ``--no-allow-multiple-definition``, ``--no-apply-dynamic-relocs``,

  ``--no-check-sections``, ``--no-gnu-unique, ``--no-pic-executable``,

  ``--no-undefined-version``, ``--no-warn-common``, ``--pack-dyn-relocs=relr``,

  ``--pop-state``, ``--print-icf-sections``, ``--push-state``,

  ``--thinlto-index-only``, ``--thinlto-object-suffix-replace``,

  ``--thinlto-prefix-replace``, ``--warn-backref``, ``-z combreloc``, ``-z

  copyreloc``, ``-z initfirst``, ``-z keep-text-section-prefix``, ``-z lazy``,

  ``-z noexecstack``, ``-z relro``, ``-z retpolineplt``, ``-z text``

COFF Improvements

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

* Added support for outputting PDB debug info for MinGW targets.

* Improved compatibility of output binaries with GNU binutils objcopy/strip.

* Sped up PDB file creation.

* Changed section layout to improve compatibility with link.exe.

* Added the following flags: ``--color-diagnostics={always,never,auto}``,

  ``--no-color-diagnostics``, ``/brepro``, ``/debug:full``, ``/debug:ghash``,

  ``/guard:cf``, ``/guard:longjmp``, ``/guard:nolongjmp``, ``/integritycheck``,

  ``/order``, ``/pdbsourcepath``, ``/timestamp``