Generalized Compare-and-Swap and Space-Efficient Universal Constructions for the Infinite-Arrival Model

Abstract

We introduce GCAS, a natural generalization of the well-known compare-and-swap (CAS) object. Intuitively, GCAS just replaces the fixed equality test of CAS with a parametrized comparator chosen from $\{<,=,>\}$. To showcase the utility of GCAS, we present two space-efficient wait-free universal constructions for systems where the number of participating processes is unknown and may be infinite (the infinite-arrival model). The first has space-complexity linear in the number of processes that have participated so far, while the second has space-complexity linear in the point contention but assumes bounded concurrency. To the best of our knowledge, these are the first wait-free universal constructions that achieve this space complexity in the infinite-arrival model. To achieve space complexity linear in the point contention, our second universal construction uses a novel memory recycling scheme that works in the infinite-arrival model with bounded concurrency. The ideas behind this recycling scheme could be of more general use.

This paper is the full version of the PODC 2026 paper with the same title, and is an extension of arXiv:2410.19102