The power of high-level languages lies in their abstraction over hardware and software complexity, leading to greater security, better reliability, and lower development costs. However, opaque abstractions are often show-stoppers for systems programmers, forcing them to either break the abstraction, or more often, simply give up and use a different language. This paper addresses the challenge of opening up a high-level language to allow practical low-level programming without forsaking integrity or performance. The contribution of this paper is three-fold: 1) we draw together common threads in a diverse literature, 2) we identify a framework for extending high-level languages for low-level programming, and 3) we show the power of this approach through concrete case studies. Our framework leverages just three core ideas: extending semantics via intrinsic methods, extending types via unboxing and architectural-width primitives, and controlling semantics via scoped semantic regimes. We develop these ideas through the context of a rich literature and substantial practical experience. We show that they provide the power necessary to implement substantial artifacts such as a high-performance virtual machine, while preserving the software engineering benefits of the host language. The time has come for high-level low-level programming to be taken more seriously: 1) more projects now use high-level languages for systems programming, 2) increasing architectural heterogeneity and parallelism heighten the need for abstraction, and 3) a new generation of high-level languages are under development and ripe to be influenced.

Flexible and efficient runtime design requires an understanding of the dependencies among the components internal to the runtime and those between the application and the runtime. These dependencies are frequently unclear. This problem exists in all runtime design, and is most vivid in a metacircular runtime — one that is implemented in terms of itself. Metacircularity blurs boundaries between application and runtime implementation, making it harder to understand and make guarantees about overall system behavior, affecting isolation, security, and resource management, as well as reducing opportunities for optimization. Our goal is to shed new light on VM interdependencies, helping all VM designers understand these dependencies and thereby engineer better runtimes. We explore these issues in the context of a high-performance Java-in-Java virtual machine. Our approach is to identify and instrument transition points into and within the runtime, which allows us to establish a dynamic execution context. Our contributions are: 1) implementing and measuring a system that dynamically maintains execution context with very low overhead, 2) demonstrating that such a framework can be used to improve the software engineering of an existing runtime, and 3) analyzing the behavior and runtime characteristics of our runtime across a wide range of benchmarks. Our solution provides clarity about execution state and allowable transitions, making it easier to develop, debug, and understand managed runtimes.