The patch computes byte lookups based on the prefix. For example,
Á, É, etc all have the same prefix <<195>>, so they are lumped
together for lookup and then we just do a byte lookup later. We
tried doing the byte lookup on 64-element tuple (since the byte
is always within 0b10000000 and 0b10111111) but that's slower,
especially because we need to check the byte range for invalid
Unicode, so instead the last byte lookup is a case. Grouping the
top-level lookup makes the cost of a miss 3x cheaper albeit a
hit is 10% more expensive and reduces bytecode size.
José Valim
5a2b24d21f