[llvm] [RISCV][doc] Add note to RISCVUsage about supported atomics ABIs (PR #103879)

[Alex Bradbury via llvm-commits]

https://github.com/asb created https://github.com/llvm/llvm-project/pull/103879

It feels like this belongs to be documented somewhere. I've tried to avoid giving too much detailed explanation as the psABI docs are the better source for this.

>From 67c1852b9270addef08007dcc70510fe23511add Mon Sep 17 00:00:00 2001
From: Alex Bradbury <asb at igalia.com>
Date: Wed, 14 Aug 2024 13:33:16 +0100
Subject: [PATCH] [RISCV][doc] Add note to RISCVUsage about supported atomics
 ABIs

It feels like this belongs to be documented somewhere. I've tried to
avoid giving too much detailed explanation as the psABI docs are the
better source for this.
---
 llvm/docs/RISCVUsage.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/llvm/docs/RISCVUsage.rst b/llvm/docs/RISCVUsage.rst
index b3c7b0e3883d01..bf186210b4af54 100644
--- a/llvm/docs/RISCVUsage.rst
+++ b/llvm/docs/RISCVUsage.rst
@@ -280,6 +280,11 @@ Supported
 ``Za128rs``, ``Za64rs``, ``Zama16b``, ``Zic64b``, ``Ziccamoa``, ``Ziccif``, ``Zicclsm``, ``Ziccrse``, ``Shcounterenvw``, ``Shgatpa``, ``Shtvala``, ``Shvsatpa``, ``Shvstvala``, ``Shvstvecd``, ``Ssccptr``, ``Sscounterenw``, ``Ssstateen``, ``Ssstrict``, ``Sstvala``, ``Sstvecd``, ``Ssu64xl``, ``Svade``, ``Svbare``
   These extensions are defined as part of the `RISC-V Profiles specification <https://github.com/riscv/riscv-profiles/releases/tag/v1.0>`__.  They do not introduce any new features themselves, but instead describe existing hardware features.
 
+Atomics ABIs
+============
+
+At the time of writing there are three atomics mappings (ABIs) `defined for RISC-V <https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#tag_riscv_atomic_abi-14-uleb128version>__`.  As of LLVM 19, LLVM defaults to "A6S", which is compatible with both the original "A6" and the future "A7" ABI. See `the psABI atomics document <https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-atomic.adoc>`__ for more information on these mappings.
+
 Experimental Extensions
 =======================