SynchronizedTypes.uniques needs to be performance-tested

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We should have something better performing that a standard weak map with weak values. Uniques is a hotspot in the complier, so it ought to be a hotspot in reflection.

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Imported From: https://issues.scala-lang.org/browse/SI-6460?orig=1
Reporter: @xeno-by

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@Blaisorblade said:
The issue was born out of:
scala/scala#1439
Paul suggested that allocating an Option is a bad idea in the performance-critical SynchronizedTypes.unique, but I wonder whether that can make a difference given that you take a lock before. If that lock is uncontended and gets biased, though, my point is moot: biased locks don't even need an atomic operation to acquire them (https://blogs.oracle.com/dave/entry/biased_locking_in_hotspot).