Six rounds of A/B cost 13-44 minutes per operator on S5000 (rand, 15 shapes:
13.0 min; sort, 25 shapes: 43.4 min). Screening one costs about 0.8 minutes —
21 operators in 15.0 minutes, which knocked 13 of them out of the expensive
queue and cut it from ~7 hours to ~2.7. That ratio is the whole argument for
having a separate first step.
Screening was being done by ad-hoc scripts living in the results tree, and
those got two things wrong that this codebase already knows better than:
Both sides ran `record`. Each swept autotune independently, so the two sides
compiled different configs and produced different IR for reasons that have
nothing to do with the switch — every operator would screen as "changed".
screen.sh pins configs exactly as round 1 of ab_repeat does: A records, B
replays A.
The criterion was the convert_layout count. Equal counts do not mean equal
generated code, and what the A/B measures is whether the generated code
changed. The ttgir fingerprint decides; the cvt counts are still recorded,
because they explain what kind of change it was.
Four verdicts, and three of them are not "the switch does nothing":
IR_CHANGED (measure it), NO_IR_DIFF (skip it), NO_KERNEL (the operator never
reached Triton on this backend), NO_ROWS (it did not run — usually the dtype).
The last two are runnability problems being mistaken for switch results, which
is how `fft` got written off: the screen forced DTYPES=float16 on every
operator and fft only accepts complex64. OPS entries now carry an optional
per-operator dtype and default to the operator's own set.
ir_fingerprint moves to _ir_fingerprint.sh so screening and the A/B cannot
drift apart on the criterion.
