8342692: C2: MemorySegment API slow with short running loops

[rwestrel]

To optimize a long counted loop and long range checks in a long or int counted loop, the loop is turned into a loop nest. When the loop has few iterations, the overhead of having an outer loop whose backedge is never taken, has a measurable cost. Furthermore, creating the loop nest usually causes one iteration of the loop to be peeled so predicates can be set up. If the loop is short running, then it's an extra iteration that's run with range checks (compared to an int counted loop with int range checks).

This change doesn't create a loop nest when:

1- it can be determined statically at loop nest creation time that the loop runs for a short enough number of iterations

2- profiling reports that the loop runs for no more than ShortLoopIter iterations (1000 by default).

For 2-, a guard is added which is implemented as yet another predicate.

While this change is in principle simple, I ran into a few implementation issues:

• while c2 has a way to compute the number of iterations of an int counted loop, it doesn't have that for long counted loop. The existing logic for int counted loops promotes values to long to avoid overflows. I reworked it so it now works for both long and int counted loops.

• I added a new deoptimization reason (Reason_short_running_loop) for the new predicate. Given the number of iterations is narrowed down by the predicate, the limit of the loop after transformation is a cast node that's control dependent on the short running loop predicate. Because once the counted loop is transformed, it is likely that range check predicates will be inserted and they will depend on the limit, the short running loop predicate has to be the one that's further away from the loop entry. Now it is also possible that the limit before transformation depends on a predicate (TestShortRunningLongCountedLoopPredicatesClone is an example), we can have: new predicates inserted after the transformation that depend on the casted limit that itself depend on old predicates added before the transformation. To solve this cicular dependency, parse and assert predicates are cloned between the old predicates and the loop head. The cloned short running loop parse predicate is the one that's used to insert the short running loop predicate.

• In the case of a long counted loop, the loop is transformed into a regular loop with a new limit and transformed range checks that's later turned into an in counted loop. The int counted loop doesn't need loop limit checks because of the way it's constructed. There's an assert that catches that we don't attempt to add one. I ran into test failures where, by the time the int counted loop is created, the fact that the number of iterations of the loop is small enough to not need a loop limit check gets lost. I added a cast to make sure the narrowed limit's type is not lost (I had to do something similar for loop nests). But then, I ran into the same issue again because the cast was pushed through a sub or add and the narrowed type was lost. I propose that pushing casts through sub/add be only done after loop opts are over (same as what's done for range check CastII).

On Maurizio's benchmark that's mentioned in the bug, this gives a ~30% performance increase. /cc hotspot-compiler

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Issue

• JDK-8342692: C2: MemorySegment API slow with short running loops (Enhancement - P4)

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