compute-sanitizer caught 16 OOB errors in magma_sgemmEx_kernel
<f,f,f,1,0,6,4,6,3,4>+0xe90 surfacing as mamba2_scan_projected_bwd
LAUNCH_FAILED. Pre-existing latent, exposed when q_var_buf_trainer
allocator reshuffle (fa92cb8db) moved subsequent buffer pointers.
Root cause: DuelingWeightSet::from_flat_buffer and
BranchingWeightSet::from_flat_buffer in gpu_weights.rs used hard-coded
layout indices [0..11] / [12..23] from before the GRN trunk expansion
(Plan 4 Task 2c.3a). The expansion inserted 9 GRN tensors at indices
1..12, shifting every value/branch tensor by +9. The 12-index sequential
walk in from_flat_buffer silently slid every dueling-weight pointer
forward into adjacent tensors:
- online_dueling.w_v1 → w_residual_h_s1 (sized SH1*s1_input_dim, not
VALUE_H*SH2)
- online_dueling.b_v1 → gamma_h_s1 (sized SH1=64 floats=256 bytes,
not VALUE_H=128 floats=512 bytes)
- online_dueling.w_v2 → beta_h_s1; b_v2 → w_a_h_s2; w_a1 → b_a_h_s2; …
The pathological alias surfaced via the ensemble multi-head clone path
(forward_value_head_for_ensemble): the cuBLAS RELU_BIAS epilogue tried
to read bias[0..128] from b_v1's 64-float buffer, producing exactly the
observed 16 thread (0..7, 2..3) reads at 1..61 bytes past a 256-byte
allocation.
Production GEMMs never tripped this because they read from
params_buf via f32_weight_ptrs_from_base which has always used the
correct 163-tensor GRN layout. flatten/unflatten paths used
matched-stale self-copies (no-op when online_d.w_s1 == params_buf_ptr).
Only the ensemble clone (`clone_dueling_weights`) made independent
copies of the misaliased pointers and then handed them to the GEMM as
if they were value-head tensors.
Per feedback_no_partial_refactor: every consumer of the weight-set/
flat-buffer contract migrated in lockstep:
- New DUELING_FLAT_INDICES = [0,1,2,3, 13,14,15,16, 17,18,19,20]
and BRANCHING_FLAT_INDICES = [21..32] in gpu_weights.rs encode the
authoritative mapping from DWS/BWS slots to GRN-expanded layout
indices.
- DuelingWeightSet::from_flat_buffer + BranchingWeightSet::from_flat_buffer
rewritten to use these mappings with a full prefix-sum byte-offsets
table (matches f32_weight_ptrs_from_base byte layout).
- flatten_online_weights, unflatten_online_weights,
flatten_target_weights, unflatten_target_weights (gpu_dqn_trainer.rs)
rewritten to keyed [(ptr, layout_idx); 24] pairs and write at
byte_offsets[layout_idx] instead of sequential prefix-sum over
sizes[0..23]. The no-op zero-copy check (online_d.w_s1 == src_base)
is preserved because DUELING_FLAT_INDICES[0] == 0.
Sanitizer (RTX 3050 Ti, magnitude_distribution smoke):
magma_sgemmEx_kernel OOB count: 16 → 0
Non-sanitizer smoke completes all 20 epochs without LAUNCH_FAILED
(was crashing on epoch 1 prior to fix); MAG_DIST/EVAL_DIST results
reflect real model behavior (Q=0.349, H=0.298, F=0.353 train-mode).
The unrelated F_Full eval-cap assertion is the ongoing Kelly cap
issue (project_magnitude_eval_collapse_kelly_capped.md).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
4714c02fcf