1. A computer system comprising: a processor; and a memory coupled to the processor, configured to provide the processor with a plurality of instructions including a garbage collection barrier instruction and a subsequent instruction that immediately follows the garbage collection barrier instruction; wherein the processor is configured to execute the garbage collection barrier instruction, including by: evaluating a memory reference to determine a condition associated with the garbage collection barrier instruction; and in the event that the condition is met, while maintaining the same privilege level, saving information that is based at least in part on the current value of a program counter, and setting the program counter to correspond to a target location that is other than the location of the subsequent instruction.