RFR: 8369467: Rdtsc: Remove experimental support for non invariant tsc [v2]

[Markus Grönlund]

On Thu, 9 Oct 2025 11:41:05 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> Pushed a patch which removes the ability for `UseFastUnorderedTimeStamps` to override disabling rdtsc when it is not invariant.
> 
> However there is now a small mismatch with what the flag says it does vs what it can do. The only thing it now can do is disable rdtsc on systems with invariant tsc. It might be that we should remove this flag and introduce a new one where the behaviour of disabling rdtsc is more obvious.
> 
> But not sure what we would prefer to do here w.r.t. the advance VM option `UseFastUnorderedTimeStamps`.

The UseFastUnorderedTimeStamps is EXPERIMENTAL with the following description:

"Use platform unstable time where supported for timestamps only"

The "where supported" conditional allows us to make this change. I prefer to see it only as an opt-out flag, as rdtsc is not to be relied on directly. Also, the only user of rdtsc is, or at least has been and should be, JFR. We have talked about introducing a JFR-specific flag for this support. You can leave that concern for now; we can take on the deprecation when we decide on the JFR-specific solution.

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PR Comment: https://git.openjdk.org/jdk/pull/27713#issuecomment-3385553122