  PCM is a popular backing memory for DRAM main memory in tiered memory
systems. PCM has asymmetric access energy; writes dominate reads. MLC asymmetry
can vary by an order of magnitude. Many schemes have been developed to take
advantage of the asymmetric patterns of 0s and 1s in the data to reduce write
energy. Because the memory is non-volatile, data can be recovered via physical
attack or across system reboot cycles. To protect information stored in PCM
against these attacks requires encryption. Unfortunately, most encryption
algorithms scramble 0s and 1s in the data, effectively removing any patterns
and negatively impacting schemes that leverage data bias and similarity to
reduce write energy. In this paper, we introduce Virtual Coset Coding (VCC) as
a workload-independent approach that reduces costly symbol transitions for
storing encrypted data. VCC is based on two ideas. First, using coset encoding
with random coset candidates, it is possible to effectively reduce the
frequency of costly bit/symbol transitions when writing encrypted data. Second,
a small set of random substrings can be used to achieve the same encoding
efficiency as a large number of random coset candidates, but at a much lower
encoding/decoding cost. Additionally, we demonstrate how VCC can be leveraged
for energy reduction in combination with fault-mitigation and fault-tolerance
to dramatically increase the lifetimes of endurance-limited NVMs, such as PCM.
We evaluate the design of VCC and demonstrate that it can be implemented
on-chip with only a nominal area overhead. VCC reduces dynamic energy by 22-28%
while maintaining the same performance. Using our multi-objective optimization
approach achieves at least a 36% improvement in lifetime over the
state-of-the-art and at least a 50% improvement in lifetime vs. an unencoded
memory, while maintaining its energy savings and system performance.
