Hi,
I recently moved Spidermonkey's js::Hash{Map,Set} classes from
js/public/HashTable.h into mfbt/HashTable.h and renamed them as
mozilla::Hash{Map,Set}. They can now be used throughout Gecko.
(js/public/HashTable.h still exists in order to provide renamings of
mozilla::Hash{Map,Set} as js::Hash{Map,Set}.)
Why might you use mozilla::Hash{Map,Set} instead of PLDHashTable (or
nsTHashtable and other descendants)?
- Better API: these types provide proper HashMap and HashSet instances, and
(in my opinion) are easier to use.
- Speed: the microbenchmark in xpcom/rust/gtest/bench-collections/Bench.cpp
shows that mozilla::HashSet is 2x or more faster than PLDHashTable. E.g.
compare "MozHash" against "PLDHash" in this graph:
https://treeherder.mozilla.org/perf.html#/graphs?timerange=604800&series=mozilla-central,1730159,1,6&series=mozilla-central,1730162,1,6&series=mozilla-central,1730164,1,6&series=mozilla-central,1732092,1,6&series=mozilla-central,1730163,1,6&series=mozilla-central,1730160,1,6
Bug 1477627 converted a hot hash table from PLDHashTable to
mozilla::HashSet and appears to have sped up cycle collection in some cases
by 7%. If you know of another PLDHashTable that is hot, it might be worth
converting it to mozilla::HashTable.
Both mozilla::Hash{Map,Set} and PLDHashTable use the same double-hashing
algorithm; the speed difference is due to mozilla::HashSet's extensive use
of templating and inlining. The downside of this is that mozilla::HashSet
will increase binary size more than PLDHashTable.
There are overview comments at the top of mfbt/HashTable.h, and the classes
themselves have more detailed comments about every method.
Nick
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