The Phase 1.1 sp20_stats_compute kernel (de922c6a4) assumed the SP14-C aux
head emits 3-class logits {short, hold, long} with baseline 1/3. This was
an error in the SP19+20 spec — production aux head emits K=2 logits
{down, up} per gpu_aux_heads.rs:61 (AUX_NEXT_BAR_K = 2, established by
SP13 B1.1a). Wiring K=2 production aux into the K=3 kernel = OOB reads +
corrupt stats.
Retargets the kernel + launcher + tests to K=2 atomically per
feedback_no_partial_refactor:
- Kernel: SP20_K_CLASSES 3→2; SP20_UNIFORM_K3 0.333…→SP20_UNIFORM_K2 0.5f;
Pass A reads 2 logits (was 3); aux_conf range [0, 1/2] (was [0, 2/3]).
- Launcher: AUX_K_CLASSES 3→2; renamed unit test
aux_k_classes_is_three → aux_k_classes_matches_production_aux_head.
- Tests: rewrote CPU oracle for K=2 input shape and 0.5 baseline; updated
expected p50/std ranges; redesigned heterogeneous-distribution test to
use ramped (not lockstep) clusters — K=2's saturated softmax in the hot
half collapses every row to bin 255, triggering the
pearl_sp4_histogram_warp_tile_undercount trap; ramped clusters distribute
bin indices across each warp's 32 lanes so the histogram path matches
the CPU oracle within bin_width tolerance.
Phase 1.2 (sp20_emas_compute) and Phase 1.3 (sp20_controllers_compute)
consume the scalar [p50, std] outputs and do NOT carry the K dimension.
Both regression suites verified passing unmodified:
- sp20_emas_compute_test: 4 GPU + 1 unit, all pass.
- sp20_controllers_compute_test: 7 GPU, all pass.
Verification (RTX 3050 Ti, sm_86):
- sp20_stats_compute_test: 4 GPU oracle + 4 launcher unit, all pass.
- cargo check -p ml --features cuda: clean (pre-existing warnings only).
Spec + plan amended at top with "AMENDED 2026-05-09" notes; audit doc
Phase 1.1 entry has a "K=2 fixup" subsection documenting the change.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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