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ReleaseNotes: add lld/ELF notes
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Differential Revision: https://reviews.llvm.org/D86579
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@MaskRay
MaskRay committed Aug 29, 2020
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Expand Up @@ -24,22 +24,143 @@ Non-comprehensive list of changes in this release
ELF Improvements
----------------

* New ``--time-trace`` option records a time trace file that can be viewed in
* ``--lto-emit-asm`` is added to emit assembly output for debugging purposes.
(`D77231 <https://reviews.llvm.org/D77231>`_)
* ``--lto-whole-program-visibility`` is added to support LTO whole-program devirtualization.
(`D71913 <https://reviews.llvm.org/D71913>`_)
* ``--print-archive-stats=`` is added to print the number of members and the number of fetched members for each archive.
The feature is similar to GNU gold's ``--print-symbol-counts=``.
(`D78983 <https://reviews.llvm.org/D78983>`_)
* ``--shuffle-sections=`` is added to introduce randomization in the output to help reduce measurement bias and detect static initialization order fiasco.
(`D74791 <https://reviews.llvm.org/D74791>`_)
(`D74887 <https://reviews.llvm.org/D74887>`_)
* ``--time-trace`` is added. It records a time trace file that can be viewed in
chrome://tracing. The file can be specified with ``--time-trace-file``.
Trace granularity can be specified with ``--time-trace-granularity``.
(`D71060 <https://reviews.llvm.org/D71060>`_)
* For ARM architectures the default max page size was increased to 64k.
This increases compatibility with systems where a non standard page
size was configured. This also is inline with GNU ld defaults.
(`D77330 <https://reviews.llvm.org/D77330>`_)
* ...
* ``--thinlto-single-module`` is added to compile a subset of modules in ThinLTO for debugging purposes.
(`D80406 <https://reviews.llvm.org/D80406>`_)
* ``--unique`` is added to create separate output sections for orphan sections.
(`D75536 <https://reviews.llvm.org/D75536>`_)
* ``--warn-backrefs`` has been improved to emulate GNU ld's archive semantics.
If a link passes with warnings from ``--warn-backrefs``, it almost assuredly
means that the link will fail with GNU ld, or the symbol will get different
resolutions in GNU ld and LLD. ``--warn-backrefs-exclude=`` is added to
exclude known issues.
(`D77522 <https://reviews.llvm.org/D77522>`_)
(`D77630 <https://reviews.llvm.org/D77630>`_)
(`D77512 <https://reviews.llvm.org/D77512>`_)
* ``--no-relax`` is accepted but ignored. The Linux kernel's RISC-V port uses this option.
(`D81359 <https://reviews.llvm.org/D81359>`_)
* ``--rosegment`` (default) is added to complement ``--no-rosegment``.
GNU gold from 2.35 onwards support both options.
* ``--threads=N`` is added. The default uses all threads.
(`D76885 <https://reviews.llvm.org/D76885>`_)
* ``--wrap`` has better compatibility with GNU ld.
* ``-z dead-reloc-in-nonalloc=<section_glob>=<value>`` is added to resolve an absolute relocation
referencing a discarded symbol.
(`D83264 <https://reviews.llvm.org/D83264>`_)
* Changed tombstone values to (``.debug_ranges``/``.debug_loc``) 1 and (other ``.debug_*``) 0.
A tombstone value is the computed value of a relocation referencing a discarded symbol (``--gc-sections``, ICF or ``/DISCARD/``).
(`D84825 <https://reviews.llvm.org/D84825>`_)
In the future many .debug_* may switch to 0xffffffff/0xffffffffffffffff as the tombstone value.
* ``-z keep-text-section-prefix`` moves ``.text.unknown.*`` input sections to ``.text.unknown``.
* ``-z rel`` and ``-z rela`` are added to select the REL/RELA format for dynamic relocations.
The default is target specific and typically matches the form used in relocatable objects.
* ``-z start-stop-visibility={default,protected,internal,hidden}`` is added.
GNU ld/gold from 2.35 onwards support this option.
(`D55682 <https://reviews.llvm.org/D55682>`_)
* When ``-r`` or ``--emit-relocs`` is specified, the GNU ld compatible
``--discard-all`` and ``--discard-locals`` semantics are implemented.
(`D77807 <https://reviews.llvm.org/D77807>`_)
* ``--emit-relocs --strip-debug`` can now be used together.
(`D74375 <https://reviews.llvm.org/D74375>`_)
* ``--gdb-index`` supports DWARF v5.
(`D79061 <https://reviews.llvm.org/D79061>`_)
(`D85579 <https://reviews.llvm.org/D85579>`_)
* ``-r`` allows SHT_X86_64_UNWIND to be merged into SHT_PROGBITS.
This allows clang/GCC produced object files to be mixed together.
(`D85785 <https://reviews.llvm.org/D85785>`_)
* Better linker script support related to output section alignments and LMA regions.
(`D74286 <https://reviews.llvm.org/D75724>`_)
(`D74297 <https://reviews.llvm.org/D75724>`_)
(`D75724 <https://reviews.llvm.org/D75724>`_)
(`D81986 <https://reviews.llvm.org/D81986>`_)
* In a input section description, the filename can be specified in double quotes.
``archive:file`` syntax is added.
(`D72517 <https://reviews.llvm.org/D72517>`_)
(`D75100 <https://reviews.llvm.org/D75100>`_)
* Linker script specified empty ``(.init|.preinit|.fini)_array`` are allowed with RELRO.
(`D76915 <https://reviews.llvm.org/D76915>`_)
* ``INSERT AFTER`` and ``INSERT BEFORE`` work for orphan sections now.
(`D74375 <https://reviews.llvm.org/D74375>`_)
* ``INPUT_SECTION_FLAGS`` is supported in linker scripts.
(`D72745 <https://reviews.llvm.org/D72745>`_)
* ``DF_1_PIE`` is set for position-independent executables.
(`D80872 <https://reviews.llvm.org/D80872>`_)
* For a symbol assignment ``alias = aliasee;``, ``alias`` inherits the ``aliasee``'s symbol type.
(`D86263 <https://reviews.llvm.org/D86263>`_)
* ``SHT_GNU_verneed`` in shared objects are parsed, and versioned undefined symbols in shared objects are respected.
(`D80059 <https://reviews.llvm.org/D80059>`_)
* SHF_LINK_ORDER and non-SHF_LINK_ORDER sections can be mixed along as the SHF_LINK_ORDER components are contiguous.
(`D77007 <https://reviews.llvm.org/D77007>`_)
* An out-of-range relocation diagnostic mentions the referenced symbol now.
(`D73518 <https://reviews.llvm.org/D73518>`_)
* AArch64: ``R_AARCH64_PLT32`` is supported.
(`D81184 <https://reviews.llvm.org/D81184>`_)
* ARM: SBREL type relocations are supported.
(`D74375 <https://reviews.llvm.org/D74375>`_)
* ARM: ``R_ARM_ALU_PC_G0``, ``R_ARM_LDR_PC_G0``, ``R_ARM_THUMB_PC8`` and ``R_ARM_THUMB__PC12`` are supported.
(`D75349 <https://reviews.llvm.org/D75349>`_)
(`D77200 <https://reviews.llvm.org/D77200>`_)
* ARM: various improvements to .ARM.exidx: ``/DISCARD/`` support for a subset, out-of-range handling, support for non monotonic section order.
(`PR44824 <https://llvm.org/PR44824>`_)
* AVR: many relocation types are supported.
(`D78741 <https://reviews.llvm.org/D78741>`_)
* Hexagon: General Dynamic and some other relocation types are supported.
* PPC: Canonical PLT and range extension thunks with addends are supported.
(`D73399 <https://reviews.llvm.org/D73399>`_)
(`D73424 <https://reviews.llvm.org/D73424>`_)
(`D75394 <https://reviews.llvm.org/D75394>`_)
* PPC and PPC64: copy relocations.
(`D73255 <https://reviews.llvm.org/D73255>`_)
* PPC64: ``_savegpr[01]_{14..31}`` and ``_restgpr[01]_{14..31}`` can be synthesized.
(`D79977 <https://reviews.llvm.org/D79977>`_)
* PPC64: ``R_PPC64_GOT_PCREL34`` and ``R_PPC64_REL24_NOTOC`` are supported. r2 save stub is supported.
(`D81948 <https://reviews.llvm.org/D81948>`_)
(`D82950 <https://reviews.llvm.org/D82950>`_)
(`D82816 <https://reviews.llvm.org/D82816>`_)
* RISC-V: ``R_RISCV_IRELATIVE`` is supported.
(`D74022 <https://reviews.llvm.org/D74022>`_)
* RISC-V: ``R_RISCV_ALIGN`` is errored because GNU ld style linker relaxation is not supported.
(`D71820 <https://reviews.llvm.org/D71820>`_)
* SPARCv9: more relocation types are supported.
(`D77672 <https://reviews.llvm.org/D77672>`_)

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

* One-dash form of some long option (``--thinlto-*``, ``--lto-*``, ``--shuffle-sections=``)
are no longer supported.
(`D79371 <https://reviews.llvm.org/D79371>`_)
* ``--export-dynamic-symbol`` no longer implies ``-u``.
The new behavior matches GNU ld from binutils 2.35 onwards.
(`D80487 <https://reviews.llvm.org/D80487>`_)
* ARM: the default max page size was increased from 4096 to 65536.
This increases compatibility with systems where a non standard page
size was configured. This also is inline with GNU ld defaults.
(`D77330 <https://reviews.llvm.org/D77330>`_)
* ARM: for non-STT_FUNC symbols, Thumb interworking thunks are not added and BL/BLX are not substituted.
(`D73474 <https://reviews.llvm.org/D73474>`_)
(`D73542 <https://reviews.llvm.org/D73542>`_)
* AArch64: ``--force-bti`` is renamed to ``-z force-bti`. ``--pac-plt`` is renamed to ``-z pac-plt``.
This change is compatibile with GNU ld.
* A readonly ``PT_LOAD`` is created in the presence of a ``SECTIONS`` command.
The new behavior is consistent with the longstanding behavior in the absence of a SECTIONS command.
* Orphan section names like ``.rodata.foo`` and ``.text.foo`` are not grouped into ``.rodata`` and ``.text`` in the presence of a ``SECTIONS`` command.
The new behavior matches GNU ld.
(`D75225 <https://reviews.llvm.org/D75225>`_)
* ``--no-threads`` is removed. Use ``--threads=1`` instead. ``--threads`` (no-op) is removed.

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