tridec 0.2.0

0.2.0 — 2026-06-11

Megakernel backend (opt-in), three-platform-validated — two honest negatives
stated. A single-launch persistent megakernel runs the entire Relay-BP
decode (every BP iteration, every relay leg, in-kernel syndrome convergence +
nconv stop + lowest-weight selection) in one kernel launch per
decode_batch with per-shot early exit, replacing v0.1's host loop of
~thousands of launches. Same math — validated bit-/LER-identical to the
two-kernel path and the relay-bp Rust oracle (14/14 gates at BLOCK 128/256 on
both CUDA and ROCm, incl. fp64-vs-oracle; barriers verified honored on both —
PTX bar.sync / AMDGCN s_barrier).

Relay-BP speedups vs the v0.1 two-kernel path:

• Metal (M4 Max, triton-metal, BLOCK=32): 30.0 s → 0.46 s / 2000 shots (65×).
• NVIDIA H200 (CUDA, triton 3.0): 9–19× fp32 (fp64 to 37× at mid-batch); batch-1 62.5 → 3.44 ms.
• AMD MI300X (ROCm, torch 2.5.1+rocm6.2 / triton 3.1, gfx942): 9–32× fp32; batch-1 8.48 ms.

(Speedups are vs each platform's own v0.1 path, min–max across batch 1–16384.)

Per-arch autotuned configs (_CUDA_TUNED keyed by gcnArchName/device name);
AMD's wavefront-64 wants the opposite shape from NVIDIA warps.

Two honest negatives (detailed in the receipts + README):

1. The plain-BP megakernel loses to the two-kernel path at large batch
   (no early-exit lever) — it is a single-shot latency tool (batch-1 ~1.7× the
   two-kernel BP path); the two-kernel path stays the BP throughput default.
2. The cross-vendor latency gap widened under the megakernel: H200 leads
   MI300X 2.47× at batch-1 (vs ~9% under v0.1's two-kernel path), 1.25–1.33×
   batched. Correctness is identical; the pitch is portability + performance on
   both, not parity.

Opt-in, not yet default: the megakernel ships as
tridec.backends.megakernel.{RelayBpMegaTriton, BpMegaTriton}; from_dem
auto-dispatch is deferred to v0.2.1 (#5) — the standalone classes are gated,
but the public-API dispatch path needs its own GPU gating before the default
flips, and that discipline is not bent for the tag.

Receipts: bench/receipts/megakernel_{h200,mi300x,metal}*. Metal is BLOCK=32
pending an upstream triton-metal barrier fix (confirmed on its dev branch).