Essential overheads

A key question in the CHERI research since inception has been what the essential performance overheads are — i.e., those that are innate to the CHERI protection model — as opposed to the design choices of a specific prototype or commercial implementation.

Morello has allowed us to answer this question with reasonable confidence: the widening of pointers in capability-centric code generation models is the key overhead, rather than other architectural impacts (for example) substantially harming code density or limiting instruction concurrency. Other than the register file, Morello structures (e.g., buses) were generally not widened, yet generally performed well without substantial re-tuning. One known exception to this was in branch prediction, where the lack of prediction of the bounds of PC has performance impact on some workloads, which can be addressed in a future mature implementation. As described later in this report, a Benchmark ABI allows us to utilize more mature integer-address (non-CHERI) branch prediction to explore how a better optimized implementation would enable greater performance, while retaining CHERI’s essential pointer-size overheads. We continue to explore other potential overheads arising from opportunities to better tune the Morello architecture to CHERI’s requirements, including the sizing of store queues.

Morello has also allowed us to develop new performance methodology clearly differentiating effects arising from design choices of this specific implementation from those essential costs, although limitations on software maturity (e.g., unoptimized ABI and compiler toolchain — for example, being unable to optimize certain global accesses that could occur directly via PCC rather than via the GOT) also limit accuracy on our projections. These are explored in greater detail in the remainder of this report.