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Use WeakHashSet instead of HashSet for hash-consing types #12935

Merged
merged 2 commits into from
Jul 6, 2021
Merged

Use WeakHashSet instead of HashSet for hash-consing types #12935

merged 2 commits into from
Jul 6, 2021

Commits on Jun 25, 2021

  1. Make util.WeakHashSet a subclass of util.MutableSet 

    Thus making it a drop-in replacement for util.HashSet which will be
useful in the next commit where we'll use it in Uniques.

Also add `@constructorOnly` annotations for clarity.

Also remove all existing references to WeakHashSet in the code base
since it turns out they were all dead code.
    @smarter
    smarter committed Jun 25, 2021
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  2. Use WeakHashSet instead of HashSet for hash-consing types 

    This mimics what Scala 2 has been doing for a long time now and serves
the same purpose: it considerably reduces peak memory usage when
compiling some projects, for example previously compiling the Scalatest
tests required a heap of at least 11 GB, but now it fits in about 4 GB.

This required changing the implementation of WeakHashSet to have
overridable `hash` and `isEqual` methods just like HashSet, it also
required making various private methods protected since NamedTypeUniques
and AppliedUniques contain an inlined implementation of `put`.

This commit also changes the default load factor of a WeakHashSet from
0.75 to 0.5 to match the load factor we use for HashSets, though note that
Scala 2 has always been using 0.75.

For a history of the usage of WeakHashSet in Scala 2 see:
- scala/scala#247
- scala/scala#2605
- scala/scala#2901
    @smarter
    smarter committed Jun 25, 2021
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