[9, 8u40] RFR (XS): 8068915: uncommon trap w/ Reason_speculate_class_check causes performance regression due to continuous deoptimizations

[Roland Westrelin]

> Roland, thanks for the feedback!
> 
>>> The fix is to (1) forbid changing uncommon trap action under the hood, and (2) consult Compile::too_many_recompiles when adding a speculative guard.
>> 
>> I’m not sure I understand what code (1) above is referring to in your webrev.
> The fix is based on 8063137 which I've sent for review earlier.
> GraphKit::uncommon_trap_exact [1] delegates to GraphKit::uncommon_trap(..., /*keep_exact_action=*/true). keep_exact_action guards the logic which rewrites the action.
> 
>> Also, why is the problem restricted to speculative traps? Wouldn’t the same checks be required for non speculative traps as well?
> Other trap types could be affected as well, but speculative traps and unstable_if are the main source of action transitions. My experiments on Octane show that all cases when action substitution happens have one of these trap reason.
> 
> With this change I wanted to address speculative traps case. I'll experiment with making unstable_if traps exact as well and get back to you with updated webrev.
> 
> I'd prefer to change the default for keep_exact_action to true (and fix only the case mentioned in GraphKit::uncommon_trap [3]), but since I consider backporting the fix into 8u40, I want to keep it very focused.

Thanks for the explanation, Vladimir. That looks good to me as it is.

Roland.