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JIT off the control-flow graph, rather than instruction stream. #217
The old way the JIT worked was to keep a hashtable with the address of every instruction, so we could find all the locations of jump targets. It's super gross and there was never any rigorous error checking that the hash table was filled (i.e., that jump targets were valid).
Working directly off the CFG eliminates that whole mess. The target of a jump or switch is just a block now. No need to cache and lookup addresses.