  This paper proposes RCP, a new reversible coherence protocol that ensures
invisible speculative load execution (ISLE) with low overhead. RCP can be
combined with processor mechanisms that eliminate the effects of speculative
instructions on other instructions to achieve low overhead invisible
speculative execution (ISE). ISE provides protection that is at least as strong
as speculative privacy tracking (SPT) and stronger than speculative taint
tracking (STT). RCP is designed by systematically extending the existing
coherence protocol to incorporate speculative loads and states. The protocol is
implemented in Gem5 and verified with Murphi. The results show that RCP based
ISE incurs lower overhead than STT/SDO/SPT while providing similar/stronger
protection.
