Short-circuit dispatch

Abstract

Interpreters are widely used to implement high-level language virtual machines (VMs), especially on resource-constrained embedded platforms. Many scripting languages employ interpreter-based VMs for their advantages over native code compilers, such as portability, smaller resource footprint, and compact codes. For efficient interpretation a script (program) is first compiled into an intermediate representation, or bytecodes . The canonical interpreter then runs an infinite loop that fetches, decodes, and executes one bytecode at a time. This bytecode dispatch loop is a well-known source of inefficiency, typically featuring a large jump table with a hard-to-predict indirect jump. Most existing techniques to optimize this loop focus on reducing the misprediction rate of this indirect jump in both hardware and software. However, these techniques are much less effective on embedded processors with shallow pipelines and low IPCs. Instead, we tackle another source of inefficiency more prominent on embedded platforms--redundant computation in the dispatch loop. To this end, we propose Short-Circuit Dispatch (SCD), a low-cost architectural extension that enables fast, hardware-based bytecode dispatch with fewer instructions. The key idea of SCD is to overlay the software-created bytecode jump table on a branch target buffer (BTB). Once a bytecode is fetched, the BTB is looked up using the bytecode, instead of PC, as key. If it hits, the interpreter directly jumps to the target address retrieved from the BTB; otherwise, it goes through the original dispatch path. This effectively eliminates redundant computation in the dispatcher code for decode, bound check, and target address calculation, thus significantly reducing total instruction count. Our simulation results demonstrate that SCD achieves geomean speedups of 19.9% and 14.1% for two production-grade script interpreters for Lua and JavaScript, respectively. Moreover, our fully synthesizable RTL design based on a RISC-V embedded processor shows that SCD improves the EDP of the Lua interpreter by 24.2%, while increasing the chip area by only 0.72% at a 40nm technology node.