Patches two cuBLAS GEMM backward operations identified by SP1 Phase B
smoke smoke-test-xvzgk (commit f139a63ee) as the F1 ep2 NaN sources:
1. apply_iqn_trunk_gradient (gpu_dqn_trainer.rs:6885+):
- Pre-call: clamp bw_d_h_s2 (the IQN DtoD-overwrite target that feeds
the GRN trunk encoder's cuBLAS Linear_a/Linear_b dW/dX/dB GEMMs;
symmetric with slot 27 source-side check) to ±1e6×ISV[96].
- Post-call: sanitize iqn_trunk_m (slot 26 output) — isfinite-or-zero
+ magnitude clamp; defence-in-depth.
2. launch_cublas_backward_to main backward path:
- Pre-call: clamp d_value_logits_buf + d_adv_logits_buf (slots 24/25
inputs that feed bw.backward_full's dueling/branch backward GEMMs)
to ±1e6×ISV[21].
- Post-call: sanitize bn_d_concat_buf (slot 32 output, gated on
bottleneck_dim > 0).
Both fixes use the new clamp_finite_f32_kernel utility (CUDA — replaces
NaN/Inf with 0 + clamps finite values to ±max_abs) + launch_clamp_finite_f32
(Rust wrapper). Kernel lives in dqn_utility_kernels.cu (already in
build.rs); loaded into the same module as the NaN check kernels in
compile_training_kernels — both call sites land inside captured children
(forward_child for slot 32, aux_child for slot 26) and use only stream-
bound launch_builder, so the kernel is graph-safe.
ISV-driven max_abs bound = 1e6 × isv[<slot>] with 1e3 ε floor for
uninitialized state (Invariant 1 carve-out per
feedback_isv_for_adaptive_bounds). Wide guard band — F0 inputs sit
several orders of magnitude below the threshold (F0 ISV[96] ≈ 1.0
LayerNorm RMS, F0 ISV[21] ≈ 1.0 Q-value scale; F0 |iqn_d_h_s2| ≤ ~10²,
F0 |d_value_logits| ≤ ~10³ post the existing 5.0 norm-clip at
line 16747), so guards are no-ops on F0; F1 overflows trigger clamp.
F0 paper-review pre-smoke confirmed.
Combined RELATED commit per feedback_no_partial_refactor — both kernels
share the same unsafe-pattern (cuBLAS GEMM extreme-intermediate-product
overflow) + the same fix template, so they ship together.
Phase B slots 24-35 remain as permanent diagnostic infrastructure;
will catch any future regression of this NaN class.
SP1 Phase D validation smoke pending.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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