[clang] [llvm] [LLVM] Add `__builtin_readsteadycounter` intrinsic and builtin for realtime clocks (PR #81331)

[Joseph Huber via cfe-commits]

================
@@ -2764,6 +2764,37 @@ Query for this feature with ``__has_builtin(__builtin_readcyclecounter)``. Note
 that even if present, its use may depend on run-time privilege or other OS
 controlled state.
 
+``__builtin_readsteadycounter``
+------------------------------
+
+``__builtin_readsteadycounter`` is used to access the fixed frequency counter
+register (or a similar steady-rate clock) on those targets that support it.
+The function is similar to ``__builtin_readcyclecounter`` above except that the
+frequency is fixed, making it suitable for measuring elapsed time.
----------------
jhuber6 wrote:

Good point, done.

https://github.com/llvm/llvm-project/pull/81331