This is the root-cause fix for the corrupted output on NVIDIA/Windows reported in discussions #1 and #8 (and what looks like the same symptom in #6, #10, and #11).
**What is happening.** Twelve reduction helpers in `gemma-4-e2b.js` already guard on `sgExact32`: hardware `subgroupAdd` on fixed 32-wide adapters, a 32-lane `subgroupShuffleXor` butterfly on ranged-width adapters. Eight sites (the QatMatMul family and its split-K siblings) emit a bare `subgroupAdd` unconditionally. On an RTX 5070 (D3D12, adapter range 32 to 128, Chrome 150) we captured both compiled kernel sets (subgroups on and off), ran all 59 matched pairs on identical inputs on the same device, and diffed: 53 pairs match exactly, 6 diverge, and all six are QatMatMul instantiations with `N_ROWS = 2`, the shapes that interleave subgroup reductions with the lane-divergent `if (tid == 0u)` stores. The divergence is deterministic and large (relative error up to 1.95), which is the gibberish users see.
Two details worth kno…
