[llvm] [LLVM][Docs] Add language reference for `llvm.readsteadycounter` (PR #132040)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 updated https://github.com/llvm/llvm-project/pull/132040

>From f11b307dcf8afc6873869e9c5600e1c65cf6b9dc Mon Sep 17 00:00:00 2001
From: Joseph Huber <huberjn at outlook.com>
Date: Wed, 19 Mar 2025 09:41:12 -0500
Subject: [PATCH] [LLVM][Docs] Add language reference for
 `llvm.readsteadycounter`

Summary:
This was not added in the original patch, add LLVM documentation.
---
 llvm/docs/LangRef.rst | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index bda80d76dfaa5..abbbb44c08479 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -14562,6 +14562,33 @@ is lowered to a constant 0.
 Note that runtime support may be conditional on the privilege-level code is
 running at and the host platform.
 
+'``llvm.readsteadycounter``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare i64 @llvm.readsteadycounter()
+
+Overview:
+"""""""""
+
+The '``llvm.readsteadycounter``' intrinsic provides access to the fixed
+frequency clock on targets that support it. Unlike '``llvm.readcyclecounter``',
+this clock is expected to tick at a constant rate, making it suitable for
+measuring elapsed time. The actual frequency of the clock is implementation
+defined.
+
+Semantics:
+""""""""""
+
+When directly supported, reading the steady counter should not modify any
+memory. Implementations are allowed to either return an application
+specific value or a system wide value. On backends without support, this
+is lowered to a constant 0.
+
 '``llvm.clear_cache``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^