Two SP1-Phase-C cuBLAS backward sanitisers at gpu_dqn_trainer.rs:7615
(bw_d_h_s2 input) and :20679 (d_value_logits + d_adv_logits) used
hardcoded `1e6 × signal.max(1.0)` formulas — outside SP4 mechanisms
1/2/5/6/9/10 but flagged by feedback_isv_for_adaptive_bounds review as
the last remaining hardcoded multipliers in the cuBLAS-backward
sanitiser consumer path after Layer A/B closed out the rest of SP4.
Migrated to the proper SP4 pattern:
- 2 new SP4 ISV slots [171, 172) extending [131..171) → [131..173):
BW_D_H_S2_BOUND_INDEX=171 (single-buffer p99) and
Q_DIR_GRAD_BOUND_INDEX=172 (multi-sub-buffer p99 over
d_value_logits ∪ d_adv_logits, n_sub=2).
- 2 new producer kernels:
bw_d_h_s2_p99_kernel.cu (mirrors h_s2_p99 single-buffer pattern)
q_dir_grad_p99_kernel.cu (mirrors param_group_oracle multi-sub
convention via shared sp4_histogram_p99_multi<256> template;
reuses oracle_subbuf_table_buf + oracle_subbuf_counts_buf).
- Pearls A+D wired via existing apply_pearls_ad_kernel chained on
same stream (GPU-only, graph-capture-compatible).
- Consumers read ISV[slot].max(EPS_CLAMP_FLOOR) directly.
Buffer growth (single source of truth in gpu_dqn_trainer.rs):
SP4_PRODUCER_COUNT 69 → 71, SP4_WIENER_TOTAL_FLOATS 207 → 213.
ISV_TOTAL_DIM 171 → 173. LAYOUT_FINGERPRINT_SEED extended (existing
checkpoints will fail-fast at load — re-train required).
StateResetRegistry: 2 new FoldReset entries (sp4_bw_d_h_s2_bound,
sp4_q_dir_grad_bound) + 2 matching reset_named_state dispatch arms;
both halves of Pearl A's sentinel contract reset together per
feedback_no_partial_refactor.
Unit tests: 2 new GPU-gated tests in sp4_producer_unit_tests.rs
exercising (single-buffer Pearl-A→Pearl-D convergence, multi-sub-
buffer p99 vs analytical |N(0,1)|). All 16 GPU tests pass on local
RTX 3050 Ti (rel-err 0.00000 / 0.00526).
Behaviour change: bound is now Pearls-smoothed p99 of the actual
gradient distribution, tighter than the pre-existing 1e6 ceiling but
appropriate-scale for observed values. Smoke validation must verify
F0/F1/F2 still converge.
Refs: task #260, baseline commit 1c150d190 (Layer A + B + fix-up +
L40S smoke pass).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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