The currently planned schedule needs replanning. We have several options for the upgrade and related tasks and need to decide on the priorities. Unfortunately, the two major bugs (bug 991820, bug 984537) have not yet been fixed or not in time for v31. This means the performance improvement of the upgrade won't be as big as the best results in the first post of this thread (nearly 20% simulation performance), but if the first measurements are still valid, it should still be nearly 10%. These numbers are only valid for that specific testcase and can be different in other cases of course. I was hoping that generational GC could mitigate the impact of these bugs, but the public API is not quite ready to control generational GC yet and I haven't yet tested if it really helps. I wanted to get ready with exact stack rooting support (ticket #2415) before the upgrade because it needs a rebuild of SpiderMonkey with different flags (and new Windows binaries). It would also be good if our programmers get used to exact rooting now. Most of the changes for exact stack rooting are done, but there's still quite a bit left. Especially it needs careful testing. We have to test specifically for that because rooting issues are very hard to find by just playing the game. The same thing could work a thousand times and then suddenly fail because of garbage collection timing. There's dynamic rooting analysis which can already be used and we might also need static rooting analysis, which is probably quite a bit of work to implement (ticket #2729). I still don't know enough to tell if we really need the static analysis though. The SpiderMonkey 31 upgrade itself (without all the additional things like rooting) is a bit behind schedule. I haven't heard of any progress from Philip on the autobuilder (which we need for C++11 and SpiderMonkey) and I noticed that there's no bundled release from Mozilla yet. In addition there's still some work I have to do on the code. After all this is done, a testing period of at least one week is needed. I don't think this is all going to happen in less than two weeks (until the currently planned Alpha 17 release). I would reduce it to two options now: We aim for Alpha 17. In this case we'd probably keep exact rooting disabled but would continue with the API transition. Developers could also enable it if they want and don't mind rebuilding SpiderMonkey. This also depends a lot on Philip and whether he can/wants to complete the autobuilder work in time. The Alpha 17 release would most likely get delayed. We would have a small performance improvement. We plan the upgrade for after the Alpha 17 release. In this case we have more time for the autobuilder and exact rooting and it could be committed with exact rooting already enabled. Most of the benefits from exact rooting won't be accessible before the next SpiderMonkey version, but it would still help a bit. Some JIT compiler bugs were related to the conservative stack scanner for example. I haven't yet done performance tests with only exact rooting enabled.Here's a little visualization of the dependencies: