SpiderMonkey is on a diet
I mentioned that many Shapes are in property trees. These are N-ary trees, but most Shapes in them have zero or one child; only a small fraction have more than that, but the maximum N can be hundreds or even thousands. So there’s a long-standing space optimization where each shape contains (via a union) a single Shape pointer which is used if it has zero or one child. But if the number of children increases to 2 or more, this is changed into a pointer to a hash table, which contains pointers to the N children. Until recently, if a Shape had a child deleted and that reduced the number of children from 2 to 1, it wouldn’t be converted from the hash form back to the single-pointer. I changed this last Friday. I also reduced the minimum size of these hash tables from 16 to 4, which saves a lot of space because most of them only have 2 or 3 entries. These two changes together reduced the size of the “shapes-extra/tree-shape-kids” entry in about:memory by roughly 30–50%.
more on blog.mozilla.com
What Mozilla is doing to reduce Firefox's memory footprint.
