A New Bytecode Format for JavaScriptCore

In the old format, instructions could be in one of two forms: unlinked, which is compact and optimized for storage and linked, which is inflated and optimized for execution, containing memory addresses of runtime objects directly in the instruction stream. The opcode was replaced by the actual pointer to the implementation of the instruction and the profiling-related metadata replaced with the memory address of the newly allocated profiling data structures. The linked bytecode had the actual pointer to the offlineasm implementation of the instruction in place of the opcode, which made executing the next instruction as simple as advancing the program counter (PC) by the size of the current instruction and making an indirect jump.

Source: webkit.org