> >That's (potentially) an awfully big table. But maybe I'm missing the point. > >If we're not trusting the bytecode to be safe, and this table is part of the > >bytecode, how do we know the table is safe? > > We vet the table at load time to make sure it's safe. That way we > avoid the cost of checking that the destination is valid for every > opcode branch and jump. Otherwise we need to check that the > destination of a branch is actually an opcode. (To avoid jumping into > data and suchlike things)
The jump target table can be generated by linear scan the bytecode. So there is no real benefit for including it in the class file, otherwise we have to verify the table itself, which is another linear algorithm (at least). The KVM of Java includes pre-verify info, which serves similar purpose. The reason behind is Java bytecode verification is (kind of) NP-complete. By using pre-verify info, the problem can be reduced to linear in most case. (No one prove it mathmatically, but you got the idea.) Hong
