[PATCH] D133214: [docs][RISCV] Document experimental extensions

[Philip Reames via Phabricator via llvm-commits]

reames updated this revision to Diff 458771.
reames added a comment.

Fix typos


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D133214/new/

https://reviews.llvm.org/D133214

Files:
  llvm/docs/RISCVUsage.rst


Index: llvm/docs/RISCVUsage.rst
===================================================================
--- llvm/docs/RISCVUsage.rst
+++ llvm/docs/RISCVUsage.rst
@@ -89,6 +89,30 @@
   LLVM currently assumes a minimum VLEN (vector register width) of 64 bits during compilation, and as a result ``Zve32x`` and ``Zve32f`` are supported only for VLEN>=64.  Assembly support doesn't have this restriction.
 
 
+Experimental Extensions
+=======================
+
+LLVM supports (to various degrees) a number of experimental extensions.  All experimental extensions have ``experimental-`` as a prefix.  There is explicitly no compatibility promised between versions of the toolchain, and regular users are strongly advised *not* to make use of experimental extensions before they reach ratification.
+
+The primary goal of experimental support is to assist in the process of ratification by providing an existence proof of an implementation, and simplifying efforts to validate the value of a proposed extension against large code bases.  Experimental extensions are expected to either transition to ratified status, or be eventually removed.  The decision on whether to accept an experimental extension is currently done on an entirely case by case basis; if you want to propose one, attending the bi-weekly RISC-V sync-up call is strongly advised.
+
+``experimental-zbe``, ``experimental-zbf``, ``experimental-zbm``, ``experimental-zbp``, ``experimental-zbr``, ``experimental-zbt``
+  LLVM implements the `latest state of the bitmanip working branch <https://github.com/riscv/riscv-bitmanip/tree/main-history>`_, which is largely similar to the 0.93 draft specification but with some instruction naming changes.  These are individual portions of the bitmanip efforts which did *not* get ratified.  Given ratification for these sub-extensions appears stalled; they are a likely candidate for removal in the future.
+
+``experimental-zca``
+  LLVM implements the `0.70 draft specification <https://github.com/riscv/riscv-code-size-reduction/releases/tag/V0.70.1-TOOLCHAIN-DEV>`_.
+
+``experimental-zihintntl``
+  LLVM implements the `0.2 draft specification <https://github.com/riscv/riscv-isa-manual/releases/tag/draft-20220831-bf5a151>`_.
+
+``experimental-ztso``
+  LLVM implements the `v0.1 proposed specification <https://github.com/riscv/riscv-isa-manual/releases/download/draft-20220723-10eea63/riscv-spec.pdf>`_ (see Chapter 25).  Using will set appropriate ELF flags and attributes, but does not yet change code generation.
+
+``experimental-zvfh``
+  LLVM implements `this draft text <https://github.com/riscv/riscv-v-spec/pull/780>`_.
+
+To use an experimental extension from `clang`, you must add `-menable-experimental-extensions` to the command line, and specify the exact version of the experimental extension you are using.  To use an experimental extension with LLVM's internal developer tools (e.g. `llc`, `llvm-objdump`, `llvm-mc`), you must prefix the extension name with `experimental-`.  Note that you don't need to specify the version with internal tools, and shouldn't include the `experimental-` prefix with `clang`.
+
 Specification Documents
 =======================
 For ratified specifications, please refer to the `official RISC-V International


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