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[docs][NFC] Add usage instructions for FatLTO (#75853)

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We included these in the commit message when we added
`-ffat-lto-objects`, but they should be in the documentation as well.
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Paul Kirth 2023-12-19 10:44:18 -08:00 committed by GitHub
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llvm/docs/FatLTO.rst
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@ -76,3 +76,40 @@ Supported File Formats
The current implementation only supports ELF files. At time of writing, it is
unclear if it will be useful to support other object file formats like ``COFF``
or ``Mach-O``.
Usage
=====
Clang users can specify ``-ffat-lto-objects`` with ``-flto`` or ``-flto=thin``.
Without the ``-flto`` option, ``-ffat-lto-objects`` has no effect.
Compile an object file using FatLTO:
.. code-block:: console
$ clang -flto -ffat-lto-objects example.c -c -o example.o
Link using the object code from the fat object without LTO. This turns
``-ffat-lto-objects`` into a no-op, when ``-fno-lto`` is specified:
.. code-block:: console
$ clang -fno-lto -ffat-lto-objects -fuse-ld=lld example.o
Alternatively, you can omit any references to LTO with fat objects and retain standard linker behavior:
.. code-block:: console
$ clang -fuse-ld=lld example.o
Link using the LLVM bitcode from the fat object with Full LTO:
.. code-block:: console
$ clang -flto -ffat-lto-objects -fuse-ld=lld example.o # clang will pass --lto=full --fat-lto-objects to ld.lld
Link using the LLVM bitcode from the fat object with Thin LTO:
.. code-block:: console
$ clang -flto=thin -ffat-lto-objects -fuse-ld=lld example.o # clang will pass --lto=thin --fat-lto-objects to ld.lld