[docs] Add release notes for the RISC-V backend in LLVM 14

[asb]

Summary

(Note: This patch is written against 14.x)

Just a first draft - I'd appreciate more eyes on this. Please speak up if I'm missing something you think is worth noting.

I haven't delved much into RVV changes - if anyone could propose anything else worth noting there (either as a comment here, or in a follow-up patch to the release notes) that would be really helpful.

[craig.topper]

llvm/docs/ReleaseNotes.rst

127

This should mention we only support constrained FP intrinsics for scalar types.

130

The B extension was also removed.

[asb]

Updated to address review comments.

[asb]

Ping.

[frasercrmck]

LGTM, I appreciate you taking the time to do this.

[asb]

Committed to the release branch (which Phabricator doesn't seen to pick up). https://github.com/llvm/llvm-project/commit/9d54fe21c67fdb76bfd95c329a31bda2cb91c520

llvm/docs/ReleaseNotes.rst

 
 * ...
 
 Changes to the PowerPC Target
 -----------------------------
 
 During this release ...
 
+Changes to the RISC-V Target
+----------------------------
+
+* Codegen improvements for RV64 around the selection of addw/subw/mulw/slliw
+  instructions and removal of redundant sext.w instructions (using the new
+  RISCVSExtWRemoval pass).
+* The various RISC-V vector extensions were updated to version 1.0 and are no
+  longer experimental.
+* The Zba, Zbb, Zbc, and Zbs bit-manipulation extensions were updated to
+  version 1.0 and are no longer experimental.
+* Added MC layer support for the ratified scalar cryptography extensions.
+* The Zfh and Zfhmin extensions for half-precision floating point were updated
+  to version 1.0 and are no longer experimental.
+* Added support for the ``.insn`` directive.
+* Various improvements to immediate materialisation, including when
+  bit-manipulation extensions are enabled. Additionally, the constant pool is
+  now used for large integers.
+* Added support for constrained FP intrinsics for scalar types.
+* Added support for CSRs introduced in the Sscofpmf, Smstateen, and Sstc
+  extensions.
+* The experimental 'Zbproposedc' extension was removed, as was the 'B'
+  extension (including all bit-manipulation sub-extensions). Individual 'Zb*'
+  extensions should be used instead.
+
 Changes to the X86 Target
 -------------------------
 
 During this release ...
 
 * Support for ``AVX512-FP16`` instructions has been added.
 * Removed incomplete support for Intel MPX.
   (`D111517 <https://reviews.llvm.org/D111517>`_)