@KingIcyCreamProjects caught a real artifact on the 9950X3D (#357 thread): the
nk=8192 cell reported ~75x, far above the real ~13-40x algorithm. Root cause was
two compounding bench bugs, both verified against the code:
1. ONE frozen input per cell. partial_select_desc's median-of-three pivot is
fully deterministic (no RNG), and bench_dsa_select froze the input then ran
2000 reps on that identical array -- so all 2000 reps hit the exact same
pivot sequence. A single lucky input spiked one nk row (stable across re-runs
because it's deterministic, not because it's real).
2. brng was a static global, initialized once and NEVER reset between cells.
So each (shape,nk) cell's input depended on every prior cell's brand() draws
-- reordering nks[] or adding a shape silently shifted all later inputs.
Fix:
- brng_seed() reseeds per (shape, nk, seed) so every cell is reproducible and
independent of cell ordering.
- Report the MEDIAN of N_SEEDS=11 independent inputs, each itself a median over
N_REPEAT=2000 timing reps. A lucky pivot now moves one of 11 samples, not the
reported number.
Measured on Intel Core Ultra 9 185H (this fix, median of 11 seeds):
realistic@8192 10.97x (was 13.57x single-seed on this box)
uniform@8192 9.39x (was 7.48x)
plateau@8192 16.11x (plateau ignores the RNG, unchanged as expected)
band: 6-26x across all cells, no anomalous spikes.
Correctness is unaffected (test_dsa_select, 129 cases, unchanged). This is a bench
fidelity fix only -- no engine code touched.
woolcoxm
153ee16f61