What Actually Serializes GPU LZ77 Decode: Three Decoders, Three Mechanisms, and an Encode-Time Lever That Removes the Last One

Abstract

The sequential part of GPU LZ77 decode is not where the field assumes it is. Across three decoder architectures on an H100 we measure that parse, not copy, holds 64-72% of device-resident decode time; that bounding back-reference chain depth - provable, and costing 0.006% in ratio - moves latency by at most 2.8% and, for the file's own latency spike, provably by nothing at all, since a byte-level comparison of all 15,499 blocks shows the cap alters none of the 181 blocks involved; that self-overlapping matches are periodic fills rather than dependency chains, which makes them fully parallel and speeds the match layer by 2.75-8.42x bit-perfect; and that the last genuinely sequential element, a four-entry distance history, can be removed by the encoder for 0.540% of ratio, growing the dependency-free parse run from 4 commands to 706. We also report the floor the format runs into: with a median match of 7 bytes against a 128-byte cache line, bus efficiency is 4.4% and a coalesced write of the same data is 39x faster. A separate section records ten hypotheses these measurements refuted, including one methodological error of our own. Every reproducible claim carries a machine-checkable record: a fresh clone of the tagged release passes 17 of 17 checks reachable without a GPU, none failing.