The packed score-shift (v_pk_fma_f32) + packed alu1 o_acc rescale (v_pk_mul_f32)
and the fmha_alu0 rowmax/shift split, all added while bottleneck-hunting after the
1882 milestone, are a net REGRESSION on the canonical fp8 prefill shape, not a win.
Same-session, same-harness, GPU2, 3-run median (b1 sq=sk=75600 hq=hk=5 d128 fp8
non-causal):
* scalar softmax (this commit): ~1877 TF/s
* packed shift + packed alu1: ~1825 TF/s (-3%)
The softmax score-shift/rescale are hidden under the 8-wave ping-pong overlap, so
collapsing their FMAs/muls does not shorten the critical path -- it only perturbs
the scheduler and loses. The earlier "+8%" that motivated the packs was a confounded
GPU0 measurement (GPU0 sustains a lower boost clock than GPU2 on this shared box).
Reverts the fmha_alu0 rowmax/shift split entirely and flips UA_FA4_PACKED_SHIFT /
UA_FA4_PACKED_ALU1_RESCALE to default OFF (code kept behind the macros, now
documented as measured regressions, so they are not re-attempted). The production
config is unchanged: kv128 + cooperative K/V load + single-sp + wide-MMA +
conditional-rescale, scalar softmax -- i.e. the ~1882 baseline.
Co-authored-by: Cursor <cursoragent@cursor.com>
a49a273a7f