First implementation slice of the per-horizon attention pool design
(docs/superpowers/specs/2026-05-18-per-horizon-attention-pool-design.md).
Lands the kernel + Rust binding + numgrad verification; downstream
wiring into PerceptionTrainer's captured graph + CheckpointV2 bump +
A/B sweep are follow-up commits gated on this proving correctness.
Kernel (cuda/per_horizon_attention_pool.cu):
per_horizon_attention_pool_fwd
Q_h[N_HORIZONS, HIDDEN_DIM] × LNb[B, K, HIDDEN_DIM]
→ context_h[B, N_HORIZONS, HIDDEN_DIM]
attn_h_weights[B, N_HORIZONS, K]
Per-block math identical to the single-Q variant, looped over
N_HORIZONS sequentially within each batch's block. Grid stays
(B, 1, 1) so backward grad_ln_out writes are race-free
(per feedback_no_atomicadd.md — no cross-block contention).
Per-batch shared mem ~k_seq + BLOCK + HIDDEN_DIM floats.
per_horizon_attention_pool_bwd
Same chain-rule pattern as attention_pool_bwd but with the horizon
loop inside the block: each (b, h) slice updates grad_ln_out in
place (sequential horizon accumulation), grad_Q_h is written as
per-block scratch [B, N_HORIZONS, HIDDEN_DIM] for host reduce.
Single-writer discipline preserved.
Rust binding (src/per_horizon_attention_pool.rs):
PerHorizonAttentionPool::{new, forward, backward}. Self-contained;
doesn't yet touch PerceptionTrainer or CfcTrunk. Loads the cubin
via the standard env!("OUT_DIR") path. Dynamic shared-mem byte
count computed per launch from k_seq.
Numgrad parity test (tests/per_horizon_attention_pool_numgrad.rs):
- B=2, K=8, HIDDEN_DIM=128, N_HORIZONS=5 fixture.
- Loss = Σ context_h (so d_context = 1 everywhere — clean analytical).
- Backward kernel produces analytical grads; central-difference of
forward kernel at ±eps=1e-2 across 8 random Q_h indices + 8 random
LNb indices verifies analytical matches CD within 5e-2 rel-tol or
5e-3 abs-floor.
- Passes on RTX 3050.
build.rs picks up the new .cu file automatically via the existing
KERNELS list; cubin compiles cleanly at sm_86 + sm_89.
Same scope discipline as Phase 2D.2 (VSN numgrad) — kernel correctness
first, integration second. The follow-up commit set per the spec §3
appendix:
C22: extend multi_horizon_heads.cu signature to accept per-horizon
context input + bump head_w shape to [N_HORIZONS, 2*HIDDEN_DIM]
C23: wire PerHorizonAttentionPool into PerceptionTrainer + CfcTrunk
captured graph behind AttentionPoolVariant config flag
C24: CheckpointV1 → V2 bump with discriminant + optional q_h field
C25: smoke training (one epoch, no NaN, loss decreases)
C26: 30-epoch × 3-fold A/B sweep (#204) — decision gate per spec §0
Closes the kernel-correctness portion of #203.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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