UCLA

Safe languages provide programming abstractions, like type and memory safety, to improve programmer productivity. Unfortunately, in the domain of concurrent programming, mainstream safe languages instead choose to adopt complex and error-prone semantics by default in the name of performance. A memory consistency model defines the possible values that a shared-memory read may return in a multithreaded programming language. Languages today like Java and Julia have adopted a relaxed (or weak) memory model that is designed to admit most traditional compiler optimizations and obviate the need for hardware fences on most shared-memory accesses. The downside, however, is that programmers are exposed to a complex and unintuitive semantics and must carefully declare certain variables as volatile in order to enforce program orderings that are necessary for proper behavior. Not only does this performance-by-default approach conflict with the design philosophy of “safe” languages, but also surprisingly, very little is actually known about the practical performance cost of a stronger memory model.

In this thesis, I define the safe-by-default and performant-by-choice volatile-by-default semantics for Java and other JVM-based languages. volatile-by-default provides sequential consistency (SC) by default, while still allows expert programmers to avoid fence overheads on performance-critical libraries. I present VBD-HotSpot and VBDA-HotSpot, modifications of Oracle’s HotSpot JVM that enforce the volatile-by-default semantics on Intel X86 hardware and ARM hardware. I then present a novel speculative technique to optimize language-level SC. Finally, I have implemented an SC enforcing Julia compiler that guarantees SC for Julia by default. Through evaluations of the above projects, I show that it is possible to measure the practical cost of strong and simple semantics for concurrent programming, and with optimizations, it is possible to have a simpler semantics for concurrent programming at a reasonable cost for certain languages.