1342650 - Consider adding a data structure that combines a Vector + HashSet/HashMap

Status: NEW

(Core :: JavaScript Engine, defect, P3)

Description

[Jan de Mooij [:jandem]]

See bug 1342345. Especially maps/sets that use MovableCellHasher have significant overhead and it would be nice to use a Vector up to length 30 or so to handle the small-number-of-entries cases that are pretty common.

We already have InlineSet/InlineMap in js/src/ds/InlineTable.h that stores the entries inline, maybe we should just change that to use a Vector instead of an array. Ideally we'd use something like mozilla::MaybeOneOf so we don't need space for both the Vector and HashMap/HashSet.

We could use this for the cycle detection classes for instance.