[clang][llvm][doc] Add more information for the ABI change in FP16

[pengfei]

This is for LLVM 15.0 release per to #56854.

Forgive me my bad English. Suggestions are welcome.

[kparzysz]

Thanks!

[tstellar]

@pengfei You can commit this directly to the release/15.x branch whenever you are ready.

[pengfei]

In D131172#3699950, @tstellar wrote:

@pengfei You can commit this directly to the release/15.x branch whenever you are ready.

I see. Done. Thanks all!

[mehdi_amini]

clang/docs/ReleaseNotes.rst

633

Can you add a link to a documentation that describe the new ABI?

llvm/docs/ReleaseNotes.rst

206

(same here: users reading "switch it to the new ABI" will be confused as to what to do exactly)

[pengfei]

clang/docs/ReleaseNotes.rst

633

Here is the link for latest X86_64 psABI: https://gitlab.com/x86-psABIs/x86-64-ABI/-/wikis/uploads/01de35b2c8adc7545de52604cc45d942/x86-64-psABI-2021-05-20.pdf
I didn't put it there because the doc is esoteric for end user, and senior user should be able to find the doc theirself.
Besides, these release notes are intended for LLVM 15 release. Given we have dropped 2 versions already, I don't see much value to change it. But I'm fine if you insist it.

clang/docs/ReleaseNotes.rst

- Added the ``-m[no-]rdpru`` flag to enable/disable the RDPRU instruction
   provided by AMD Zen2 and later processors. Defined intrinsics for using
   this instruction (see rdpruintrin.h).
 - Support ``-mstack-protector-guard-symbol=[SymbolName]`` to use the given
   symbol for addressing the stack protector guard.
 - ``-mfunction-return=thunk-extern`` support was added to clang for x86. This
   will be used by Linux kernel mitigations for RETBLEED. The corresponding flag
   ``-mfunction-return=keep`` may be appended to disable the feature.
 
+The ``_Float16`` type requires SSE2 feature and above due to the instruction
+limitations. When using it on i386 targets, you need to specify ``-msse2``
+explicitly.
+
+For targets without F16C feature or above, please make sure:
+
+- Use GCC 12.0 and above if you are using libgcc.
+- If you are using compiler-rt, use the same version with the compiler.
+Early versions provided FP16 builtins in a different ABI. A workaround is to use
+a small code snippet to check the ABI if you cannot make sure of it.
+- If you are using downstream runtimes that provide FP16 conversions, update
+them with the new ABI.
+
 DWARF Support in Clang
 ----------------------
 
 - clang now adds DWARF information for inline strings in C/C++ programs,
   allowing ``line:column`` symbolization of strings. Some debugging programs may
   require updating, as this takes advantage of DWARF ``DW_TAG_variable``
   structures *without* a ``DW_AT_name`` field, which is valid DWARF, but may be
   handled incorrectly by some software (e.g. new failures with incorrect

llvm/docs/ReleaseNotes.rst

 Changes to the WebAssembly Backend
 ----------------------------------
 
 * ...
 
 Changes to the X86 Backend
 --------------------------
 
-* Support ``half`` type on SSE2 and above targets.
+* Support ``half`` type on SSE2 and above targets following X86 psABI.
 * Support ``rdpru`` instruction on Zen2 and above targets.
 
+During this release, ``half`` type has an ABI breaking change to provide the
+support for the ABI of ``_Float16`` type on SSE2 and above following X86 psABI.
+(`D107082 <https://reviews.llvm.org/D107082>`_)
+
+The change may affect the current use of ``half`` includes (but is not limited
+to):
+
+* Frontends generating ``half`` type in function passing and/or returning
+arguments.
+* Downstream runtimes providing any ``half`` conversion builtins assuming the
+old ABI.
+* Projects built with LLVM 15.0 but using early versions of compiler-rt.
+
+When you find failures with ``half`` type, check the calling conversion of the
+code and switch it to the new ABI.
+
 Changes to the OCaml bindings
 -----------------------------
 
 
 Changes to the C API
 --------------------
 
 * Add ``LLVMGetCastOpcode`` function to aid users of ``LLVMBuildCast`` in