The Cost of Speculation: Revisiting Overheads in the V8 JavaScript Engine

Abstract

JavaScript, while already widely used in web applications, is also gaining popularity beyond the web, such as on the server as part of Node.js or on the desktop with the Electron framework (e.g., in the Slack application). However, executing code written in this weakly and dynamically typed language efficiently, requires sophisticated Just-in-time (JIT) compilation. In this paper, we revisit the execution overheads of JavaScript. We perform a detailed analysis of the JetStream2 benchmark suite on Google's modern V8 JavaScript engine. We identify micro-architectural bottlenecks and runtime overheads that result from the speculations made by the JIT compiler when it generates machine code from JavaScript. We find that checks that verify assumptions made by the compiler have an average execution overhead of 8 %, 2-4x of what an earlier study reported on an older version of V8. For the check for Small Integers (SMI), we observe that the conditional branches are not the underlying cause but rather the computation of the condition itself. This indicates that these checks provide an attractive avenue for a HW/SW codesign solution. We present an extension for the ARMv8 instruction set that optimizes SMI load instructions and checks. We can improve execution time on SMI-heavy computations by up to 10 % on a prototype implementation of the new instructions in the gem5 simulator.