Morello microarchitectural limitations

The following microarchitecture limitations arose due to the constrained timeline of the Morello development process. All are believed resolvable with a full-length design and optimization cycle used with production hardware.

• PCC branch-prediction This issue prevents proper speculation around changes in PC bounds — due to limitations in changes that could be made relative to the baseline design in this research programme. As a result, additional stalls are incurred when performing inter-library pure-capability function calls or returns when the results of any updated bounds on PCC are depended on. This occurs, for example, when PC-relative accesses to globals, or function calls or returns, depend on the value of PCC. This can be worked around via the "Benchmark ABI", which avoids capability-based jumps and hence additional stalls; see below for details of this ABI.
• Data-dependent exception support during capability stores introduces a stall in address translation. This exception is motivated by MMU support for capability tracking, used for heap temporal memory safety in CHERI. The underlying Armv8.2-A architecture targeted by the initial Neoverse N1 has no concept of data-dependent exceptions on stores, and that micro-architecture does not have a structure that supports data-dependent exceptions for stores in an efficient way. Therefore for capability stores an artificial stall is introduced for address translation — which can raise an exception — until the data is available, if not already available. Arm's Memory Tagging Extensions (MTE) have a similar requirement and so more recent Arm microarchitectures are much more optimized in this regard.
• Untuned store throughput and buffer sizes due to memory buses not having been widened. Whereas store-pair instructions from 64-bit Armv8-A can execute in one cycle and occupy a single store-buffer entry, store-pair instructions for 128-bit capabilities execute in two cycles and occupy two store-buffer entries. Store throughput and the store buffer were sized based on aarch64 code measurements, and can become congested when storing capability pairs as a result of retaining that size in Morello.
• The MADD instruction, which performs a multiply-and-add, is used both for matrix operations and to calculate array and data-structure offsets. In Morello, a version operating on capabilities is omitted, leading to code-generation inefficiency when performing some types of memory accesses (seen particularly in the SPECint gobmk benchmark).

More generally, when comparing performance, consideration should be made for the differing levels of optimisation between baseline 64-bit Armv8-A instructions in the Neoverse N1, and instructions added as part of the Morello architecture. As a result, there may be further microarchitectural optimizations that do not properly apply when using CHERI-enabled code. For example, we have not yet reviewed the full set of memory prefetchers to make sure they behave well with pure-capability code. We have ongoing investigations in a number of these areas (see below for examples).