The increase in computational capability of low-power Arm architectures has seen them diversify from their more traditional domain of portable battery powered devices into data center servers, personal computers, and even Supercomputers. Thus, managed languages (Java, Javascript, etc.) that require a managed runtime environment (MRE) need to be ported to the Arm architecture, requiring an understanding of different design tradeoffs.
This article studies how the lack of strong hardware support for Self Modifying Code (SMC) in low-power architectures (e.g., absence of cache coherence between instruction cache and data caches), affects Just-In-Time (JIT) compilation and runtime behavior in MREs. Specifically, we focus on the implementation and treatment of call-sites, that must maintain code consistency in the face of concurrent execution and modification to redirect control (patching) by the MRE. The lack of coherence, is compounded with the maximum distance (reach of) a call-site can jump to as the reach is more constrained (smaller distance) in Arm when compared with Intel/AMD. We present four different robust implementations for call-sites and discuss their advantages and disadvantages in the absence of strong hardware support for SMC. Finally, we evaluate each approach using a microbenchmark, further evaluating the best three techniques using three JVM benchmark suites and the open source MaxineVM showcasing performance differences up to 12%. Based on these observations, we propose extending code-cache partitioning strategies for JIT compiled code to encourage more efficient local branching for architectures with limited direct branch ranges.