[RFC] Dynamic lock elision support

[Paul E. Murphy]

Elided locks can have mixed overall performance in practice. That is, there is some non-trivial tuning a user might have to do to see the positive benefits. Additionally, when tuning the adaptive lock constants on PPC, my experimentation seems to correlate tuned values with both the number of hardware threads per core, and the behavior of the application.

My initial thought is elision should be disabled by default, with an environment variable to toggle both support, and potentially override tuning constants.

Paul
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