@skip:
In simple words, the x86 call:
  call 0x2000
placed at address 0x1000 becomes:
  call %rip + 0x1000

RIP holds the instruction pointer, which will be 0x1000 in this case
(actually, I'm ignoring the detail that when executing the call, RIP
points to the first byte of the next instruction).

If I execute the same instruction from a different location (i.e.
different RIP), things will break. So, only code for opcodes without
real calls, nor access to globals can be copied like this (inlines are OK).
With refcounting, not even POP_TOP is safe since it can call
destructors. DUP_TOP is still safe, I guess.