Apr 2, 2025

This link masking surfaced a simple but important issue with the sieve, as it gets applied naturally to the header table, as it the keys are 23 bits and the header is word aligned (32 bits). This leaves 9 non-aligned bits for the sieve. Because this results in a 7 bit screen and a 2 bit selector, the number of screens is limited to 2^2, and saturates pretty quickly. The 16 screen point filter never saturates, so I never saw the issue, and it was subtle enough to not be caught in test. When the sieve becomes "full" (last screen applied) the first bit of the sieve value must be zeroed. This corresponds to explicit mask of that sentinel bit in the mask table. If it is not masked it's possible for a full (not yet saturated) sieve to produce the empty sentinel, resulting in false negatives (which is bad). That's now resolved, and there is no impact to previous perf measurements.