Hash Map Inlining

Abstract

Scripting languages like Javascript and PHP are widely used to implement application logic for dynamically-generated web pages. Their popularity is due in large part to their flexible syntax and dynamic type system, which enable rapid turnaround time for prototyping, releasing, and updating web site features and capabilities. The most common complex data structure in these languages is the hash map, which is used to store key-value pairs. In many cases, hash maps with a fixed set of keys are used in lieu of explicitly defined classes or structures, as would be common in compiled languages like Java or C++. Unfortunately, the runtime overhead of key lookup and value retrieval is quite high, especially relative to the direct offsets that compiled languages can use to access class members. Furthermore, key lookup and value retrieval incur high microarchitectural costs as well, since the paths they execute contain unpredictable branches and many cache accesses, leading to substantially higher numbers of branch mispredicts and cache misses per access to the hashmap. This paper quantifies these overheads, describes a compiler algorithm that discovers common use cases for hash maps and inlines them so that keys are accessed with direct offsets, and reports measured performance benefits on real hardware. A prototype implementation in the HipHop VM infrastructure shows promising performance benefits for a broad array of hash map-intensive server-side PHP applications, up to 37.6% and averaging 18.81%, improves SPECWeb throughput by 7.71% (banking) and 11.71% (e-commerce).