[llvm][docs] Convert LLVM release notes to Markdown (#109107)

* Markdown is the most common format on GitHub and most contributors are
more familiar with it than RST.
* This leads to mistakes in the RST syntax and/or folks just using
Markdown syntax and assuming it works.
* The release notes have a high number of edits and a high number of
views, we should optimise for making the common path easy. That is,
adding a bullet point and a link.
* Though GitHub can render RST and Markdown, its support for Markdown is
more complete (and neither handle the Sphinx directives well).
* We already have some Markdown docs in the llvm docs.

To keep the original formatting we do need some Sphinx directives still,
and those are provided by MyST which is already enabled.

https://myst-parser.readthedocs.io/en/latest/

I did have to enable an extension so we can substitute in the release
version.


https://myst-parser.readthedocs.io/en/latest/syntax/optional.html#substitutions-with-jinja2

Needing to use MyST means there is some special knowledge needed if you
want to do advanced things, but at least the basics remain Markdown.
Even in RST form, you still had to look up Sphinx syntax.

I also make use of a nested directive

https://myst-parser.readthedocs.io/en/latest/syntax/roles-and-directives.html#nesting-directives
to implement the prerelease warning.

The note about sections referred to another note that got removed in
4c72deb. I presume accidentally, so I
have restored that.

I also removed the "Update on required toolchains to build LLVM" header
because the section is now empty.

The other difference is that the table of contents now has a heading
"Contents". This is the default and I could not find a way to remove
that name. Otherwise it's the same table as you'd get from the RST
document.

Author: DavidSpickett

llvm/docs/ReleaseNotes.md

@@ -0,0 +1,281 @@
+<!-- This document is written in Markdown and uses extra directives provided by
+MyST (https://myst-parser.readthedocs.io/en/latest/). -->
+
+LLVM {{env.config.release}} Release Notes
+=========================================
+
+```{contents}
+```
+
+````{only} PreRelease
+```{warning} These are in-progress notes for the upcoming LLVM {{env.config.release}}
+             release. Release notes for previous releases can be found on
+             [the Download Page](https://releases.llvm.org/download.html).
+```
+````
+
+Introduction
+============
+
+This document contains the release notes for the LLVM Compiler Infrastructure,
+release {{env.config.release}}.  Here we describe the status of LLVM, including
+major improvements from the previous release, improvements in various subprojects
+of LLVM, and some of the current users of the code.  All LLVM releases may be
+downloaded from the [LLVM releases web site](https://llvm.org/releases/).
+
+For more information about LLVM, including information about the latest
+release, please check out the [main LLVM web site](https://llvm.org/).  If you
+have questions or comments, the [Discourse forums](https://discourse.llvm.org)
+is a good place to ask them.
+
+Note that if you are reading this file from a Git checkout or the main
+LLVM web page, this document applies to the *next* release, not the current
+one.  To see the release notes for a specific release, please see the
+[releases page](https://llvm.org/releases/).
+
+Non-comprehensive list of changes in this release
+=================================================
+
+<!-- For small 1-3 sentence descriptions, just add an entry at the end of
+this list. If your description won't fit comfortably in one bullet
+point (e.g. maybe you would like to give an example of the
+functionality, or simply have a lot to talk about), see the comment below
+for adding a new subsection. -->
+
+* ...
+
+<!-- If you would like to document a larger change, then you can add a
+subsection about it right here. You can copy the following boilerplate:
+
+Special New Feature
+-------------------
+
+Makes programs 10x faster by doing Special New Thing.
+-->
+
+Changes to the LLVM IR
+----------------------
+
+* The `x86_mmx` IR type has been removed. It will be translated to
+  the standard vector type `<1 x i64>` in bitcode upgrade.
+* Renamed `llvm.experimental.stepvector` intrinsic to `llvm.stepvector`.
+
+* Added `usub_cond` and `usub_sat` operations to `atomicrmw`.
+
+* Remove the following intrinsics which can be replaced with a `bitcast`:
+
+  * `llvm.nvvm.bitcast.f2i`
+  * `llvm.nvvm.bitcast.i2f`
+  * `llvm.nvvm.bitcast.d2ll`
+  * `llvm.nvvm.bitcast.ll2d`
+
+* Remove the following intrinsics which can be replaced with a funnel-shift:
+
+  * `llvm.nvvm.rotate.b32`
+  * `llvm.nvvm.rotate.right.b64`
+  * `llvm.nvvm.rotate.b64`
+
+* Remove the following intrinsics which can be replaced with an
+  `addrspacecast`:
+
+  * `llvm.nvvm.ptr.gen.to.global`
+  * `llvm.nvvm.ptr.gen.to.shared`
+  * `llvm.nvvm.ptr.gen.to.constant`
+  * `llvm.nvvm.ptr.gen.to.local`
+  * `llvm.nvvm.ptr.global.to.gen`
+  * `llvm.nvvm.ptr.shared.to.gen`
+  * `llvm.nvvm.ptr.constant.to.gen`
+  * `llvm.nvvm.ptr.local.to.gen`
+
+Changes to LLVM infrastructure
+------------------------------
+
+Changes to building LLVM
+------------------------
+
+Changes to TableGen
+-------------------
+
+Changes to Interprocedural Optimizations
+----------------------------------------
+
+Changes to the AArch64 Backend
+------------------------------
+
+* `.balign N, 0`, `.p2align N, 0`, `.align N, 0` in code sections will now fill
+  the required alignment space with a sequence of `0x0` bytes (the requested
+  fill value) rather than NOPs.
+
+Changes to the AMDGPU Backend
+-----------------------------
+
+* Removed `llvm.amdgcn.flat.atomic.fadd` and
+  `llvm.amdgcn.global.atomic.fadd` intrinsics. Users should use the
+  {ref}`atomicrmw <i_atomicrmw>` instruction with `fadd` and
+  addrspace(0) or addrspace(1) instead.
+
+Changes to the ARM Backend
+--------------------------
+
+* `.balign N, 0`, `.p2align N, 0`, `.align N, 0` in code sections will now fill
+  the required alignment space with a sequence of `0x0` bytes (the requested
+  fill value) rather than NOPs.
+
+Changes to the AVR Backend
+--------------------------
+
+Changes to the DirectX Backend
+------------------------------
+
+Changes to the Hexagon Backend
+------------------------------
+
+Changes to the LoongArch Backend
+--------------------------------
+
+Changes to the MIPS Backend
+---------------------------
+
+Changes to the PowerPC Backend
+------------------------------
+
+Changes to the RISC-V Backend
+-----------------------------
+
+* `.balign N, 0`, `.p2align N, 0`, `.align N, 0` in code sections will now fill
+  the required alignment space with a sequence of `0x0` bytes (the requested
+  fill value) rather than NOPs.
+* Added Syntacore SCR4 and SCR5 CPUs: `-mcpu=syntacore-scr4/5-rv32/64`
+* `-mcpu=sifive-p470` was added.
+* Added Hazard3 CPU as taped out for RP2350: `-mcpu=rp2350-hazard3` (32-bit
+  only).
+* Fixed length vector support using RVV instructions now requires VLEN>=64. This
+  means Zve32x and Zve32f will also require Zvl64b. The prior support was
+  largely untested.
+* The `Zvbc32e` and `Zvkgs` extensions are now supported experimentally.
+* Added `Smctr` and `Ssctr` extensions.
+* `-mcpu=syntacore-scr7` was added.
+* The `Zacas` extension is no longer marked as experimental.
+
+Changes to the WebAssembly Backend
+----------------------------------
+
+Changes to the Windows Target
+-----------------------------
+
+Changes to the X86 Backend
+--------------------------
+
+* `.balign N, 0x90`, `.p2align N, 0x90`, and `.align N, 0x90` in code sections
+  now fill the required alignment space with repeating `0x90` bytes, rather than
+  using optimised NOP filling. Optimised NOP filling fills the space with NOP
+  instructions of various widths, not just those that use the `0x90` byte
+  encoding. To use optimised NOP filling in a code section, leave off the
+  "fillval" argument, i.e. `.balign N`, `.p2align N` or `.align N` respectively.
+
+* Due to the removal of the `x86_mmx` IR type, functions with
+  `x86_mmx` arguments or return values will use a different,
+  incompatible, calling convention ABI. Such functions are not
+  generally seen in the wild (Clang never generates them!), so this is
+  not expected to result in real-world compatibility problems.
+
+* Support ISA of `AVX10.2-256` and `AVX10.2-512`.
+
+Changes to the OCaml bindings
+-----------------------------
+
+Changes to the Python bindings
+------------------------------
+
+Changes to the C API
+--------------------
+
+* The following symbols are deleted due to the removal of the `x86_mmx` IR type:
+
+  * `LLVMX86_MMXTypeKind`
+  * `LLVMX86MMXTypeInContext`
+  * `LLVMX86MMXType`
+
+ * The following functions are added to further support non-null-terminated strings:
+
+  * `LLVMGetNamedFunctionWithLength`
+  * `LLVMGetNamedGlobalWithLength`
+
+* The following functions are added to access the `LLVMContextRef` associated
+   with `LLVMValueRef` and `LLVMBuilderRef` objects:
+
+  * `LLVMGetValueContext`
+  * `LLVMGetBuilderContext`
+
+* The new pass manager can now be invoked with a custom alias analysis pipeline, using
+  the `LLVMPassBuilderOptionsSetAAPipeline` function.
+
+* It is now also possible to run the new pass manager on a single function, by calling
+  `LLVMRunPassesOnFunction` instead of `LLVMRunPasses`.
+
+* Support for creating instructions with custom synchronization scopes has been added:
+
+  * `LLVMGetSyncScopeID` to map a synchronization scope name to an ID.
+  * `LLVMBuildFenceSyncScope`, `LLVMBuildAtomicRMWSyncScope` and
+    `LLVMBuildAtomicCmpXchgSyncScope` versions of the existing builder functions
+    with an additional synchronization scope ID parameter.
+  * `LLVMGetAtomicSyncScopeID` and `LLVMSetAtomicSyncScopeID` to get and set the
+    synchronization scope of any atomic instruction.
+  * `LLVMIsAtomic` to check if an instruction is atomic, for use with the above functions.
+    Because of backwards compatibility, `LLVMIsAtomicSingleThread` and
+    `LLVMSetAtomicSingleThread` continue to work with any instruction type.
+
+* The `LLVMSetPersonalityFn` and `LLVMSetInitializer` APIs now support clearing the
+  personality function and initializer respectively by passing a null pointer.
+
+* The following functions are added to allow iterating over debug records attached to
+  instructions:
+
+  * `LLVMGetFirstDbgRecord`
+  * `LLVMGetLastDbgRecord`
+  * `LLVMGetNextDbgRecord`
+  * `LLVMGetPreviousDbgRecord`
+
+* Added `LLVMAtomicRMWBinOpUSubCond` and `LLVMAtomicRMWBinOpUSubSat` to `LLVMAtomicRMWBinOp` enum for AtomicRMW instructions.
+
+Changes to the CodeGen infrastructure
+-------------------------------------
+
+Changes to the Metadata Info
+---------------------------------
+
+Changes to the Debug Info
+---------------------------------
+
+Changes to the LLVM tools
+---------------------------------
+
+Changes to LLDB
+---------------------------------
+
+Changes to BOLT
+---------------------------------
+
+Changes to Sanitizers
+---------------------
+
+Other Changes
+-------------
+
+External Open Source Projects Using LLVM {{env.config.release}}
+===============================================================
+
+* A project...
+
+Additional Information
+======================
+
+A wide variety of additional information is available on the
+[LLVM web page](https://llvm.org/), in particular in the
+[documentation](https://llvm.org/docs/) section.  The web page also contains
+versions of the API documentation which is up-to-date with the Git version of
+the source code.  You can access versions of these documents specific to this
+release by going into the `llvm/docs/` directory in the LLVM tree.
+
+If you have any questions or comments about LLVM, please feel free to contact
+us via the [Discourse forums](https://discourse.llvm.org).