The Structure class keeps a table of transitions to derived Structure types. Since this table commonly contains a single entry we employ an optimization where instead of holding a map, we may hold a pointer directly to a single instance of the mapped type. We use an additional bit of data to flag whether the pointer is currently pointing to a table of transitions, or a singleton transition. Previously we had commonly used a pattern of storing data in the low bits of pointers, but had moved away from this since it causes false leaks to be reported by the leaks tool. However in this case, the entries in the map are weak links - this pointer will never be responsible for keeping an object alive. As such we can use this approach provided that the bit is set when a table is not in use (otherwise the table would appear to be leaked). Additionally, the transition table currently allows to entries to exist for a given key - one specialized to a particular value, and one not specialized. This is unnecessary, wasteful, and a little inconsistent. (If you create an entry for a specialized value, then a non-specialized entry, both will exist. If you create an entry for a non-specialized value, then try to create a specialized entry, only a non-specialized form will be allowed.) This shows a small progression on v8.
Created attachment 83371 [details] The patch
Attachment 83371 [details] did not build on win: Build output: http://queues.webkit.org/results/7942469
Fixed in r79355.