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

[Artem Belevich via cfe-commits]

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@@ -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.
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Artem-B wrote:

Should we mention that we do not guarantee any particular frequency, just that it's stable and it's up to the user to figure out the actual frequency, if they need to.

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