In or out? Putting write barriers in their place

In many garbage collected systems, the mutator performs a write barrier for every pointer update. Using generational garbage collectors, we study in depth three code placement options for remembered-set write barriers: inlined, out-of-line, and partially inlined (fast path inlined, slow path out-of-line). The fast path determines if the collector needs to remember the pointer update. The slow path records the pointer in a list when necessary. Efficient implementations minimize the instructions on the fast path, and record few pointers (from 0.16 to 3% of pointer stores in our benchmarks). We find the mutator performs best with a partially inlined barrier, by a modest 1.5% on average over full inlining. We also study the compilation cost of write-barrier code placement. We find that partial inlining reduces the compilation cost by 20 to 25% compared to full inlining. In the context of just-in-time compilation, the application is exposed to compiler activity. Regardless of the level of compiler activity, partial inlining consistently gives a total running time performance advantage over full inlining on the SPEC JVM98 benchmarks. When the compiler optimizes all application methods on demand and compiler load is highest, partial inlining improves total performance on average by 10.2%, and up to 18.5%.