[LLVM][Docs] Add language reference for llvm.readsteadycounter

[jhuber6]

Summary:
This was not added in the original patch, add LLVM documentation.

[llvmbot]

@llvm/pr-subscribers-llvm-ir

Author: Joseph Huber (jhuber6)

Changes

Summary:
This was not added in the original patch, add LLVM documentation.

---

Full diff: https://github.com/llvm/llvm-project/pull/132040.diff

1 Files Affected:

• (modified) llvm/docs/LangRef.rst (+27)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index bda80d76dfaa5..f57e9ccffc72e 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^