Effect handlers and multishot continuations are powerful abstractions for managing control flow; together, they offer concise and modular ways to express and handle nondeterminism, randomness, and more. However, implementing multishot continuations in the presence of stack-allocated lexical resources—lexical effect handlers in particular—is challenging, since stack copying invalidates references to these resources. We present a novel implementation strategy for lexical effect handlers that fully supports multishot continuations. The key idea is to virtualize the stack space used by continuations. Each stack-allocated handler instance is assigned a virtual address, and all effect invocations through these virtual addresses are mediated by an address translation mechanism. A software-based memory management unit in the runtime system performs these translations efficiently, exploiting the lexical scoping discipline of effect handlers. We capture the essence of our approach via a new operational semantics for lexical effect handlers and prove it correct with respect to the standard semantics. We also implement it in a compiler and runtime system. Compared to prior languages with lexical effect handlers, our implementation increases expressivity by fully supporting multishot continuations—and, as a happy consequence, unlocks significant performance gains by enabling parallel execution of multishot continuations.

Exception handlers—and effect handlers more generally—are language mechanisms for structured nonlocal control flow. A recent trend in language-design research has introduced lexically scoped handlers, which address a modularity problem with dynamic scoping. While dynamically scoped handlers allow zero-overhead implementations when no effects are raised, existing implementations of lexically scoped handlers require programs to pay a cost just for having handlers in the lexical context. In this paper, we present a novel approach to implementing lexically scoped handlers of exceptional effects. It satisfies the zero-overhead principle—a property otherwise met by most modern compilers supporting dynamically scoped exception handlers. The key idea is a type-directed translation that emits information indicating how handlers come into the lexical context. This information guides the runtime in walking the stack to locate the right handler. Crucially, no reified lexical identifiers of handlers are needed, and mainline code is not slowed down by the presence of handlers in the program text. We formalize the essential aspects of this compilation scheme and prove it correct. We integrate our approach into the Lexa language, allowing the compilation strategy to be customized for each declared effect based on its expected invocation rate. Empirical results suggest that the new Lexa compiler reduces run-time overhead in low-effect or no-effect scenarios while preserving competitive performance for effect-heavy workloads.

Lexically scoping effect handlers is a language-design idea that equips algebraic effects with a modular semantics: it enables local-reasoning principles without giving up on the control-flow expressiveness that makes effect handlers powerful. However, we observe that existing implementations risk incurring costs akin to the run-time search for dynamically scoped handlers. This paper presents a compilation strategy for lexical effect handlers, adhering to the lexical scoping principle and targeting a language with low-level control over stack layout. Key aspects of this approach are formalized and proven correct. We embody the ideas in a language called L exa : the L exa compiler translates high-level effect handling to low-level stack switching. We evaluate the L exa compiler on a set of benchmarks; the results suggest that it generates efficient code, reducing running-time complexity from quadratic to linear in some cases.

Cache side-channel attacks leverage secret-dependent footprints in CPU cache to steal confidential information, such as encryption keys. Due to the lack of a proper abstraction for reasoning about cache side channels, existing static program analysis tools that can quantify or mitigate cache side channels are built on very different kinds of abstractions. As a consequence, it is hard to bridge advances in quantification and mitigation research. Moreover, existing abstractions lead to imprecise results. In this paper, we present a novel abstraction, called differential set, for analyzing cache side channels at compile time. A distinguishing feature of differential sets is that it allows compositional and precise reasoning about cache side channels. Moreover, it is the first abstraction that carries sufficient information for both side channel quantification and mitigation. Based on this new abstraction, we develop a static analysis tool DSA that automatically quantifies and mitigates cache side channel leakage at the same time. Experimental evaluation on a set of commonly used benchmarks shows that DSA can produce more precise leakage bound as well as mitigated code with fewer memory footprints, when compared with state-of-the-art tools that only quantify or mitigate cache side channel leakage.