We currently size the storage inside JSAutoEnterCompartment based on a platform-specific guess. This broke ted's iOS port, and it's ugly. We may be able to rely on the fact (?) that 32-bit systems with alignof(uint64) == 4 have 13-word AutoCompartments, while 32-bit systems with alignof(uint64) == 8 have 16-word AutoCompartments.
Comment on attachment 565995 [details] [diff] [review] Patch v1 I think this works.
Just for interest in case this dings other arches, on PPC this turned out to be 15 bytes, for some reason.