Compiler optimizations for nondeferred reference

Abstract

Reference counting is a well-known technique for automatic memory management, offering unique advantages over other forms of garbage collection. However, on account of the high costs associated with the maintenance of up-to-date tallies of references from the stack, deferred variants are typically used in modern implementations. This partially sacrifices some of the benefits of non-deferred reference-counting (RC) garbage collection, like the immediate reclamation of garbage and short collector pause times.This paper presents a series of optimizations that target the stack and substantially enhance the throughput of nondeferred RC collection. A key enabler is a new static analysis and optimization called RC subsumption that significantly reduces the overhead of maintaining the stack contribution to reference counts. We report execution time improvements on a benchmark suite of ten C# programs, and show how RC subsumption, aided with other optimizations, improves the performance of nondeferred RC collection by as much as a factor of 10, making possible running times that are within 32% of that with an advanced traversal-based collector on seven programs, and 19% of that with a deferred RC collector on eight programs. This is in the context of a baseline RC implementation that is typically at least a factor of 6 slower than the tracing collector and a factor of 5 slower than the deferred RC collector.