# NVFP4 MegaMoE Debug Log

## Current State (May 15, 2026 — 23:00 UTC)

**Status:** SF remap rewritten as forward mapping with flat logical coordinates. Roundtrip verifier added. Awaiting rebuild and verification.

**Key evolution of the SF remap fix:**

1. **Original bug:** `idx2crd` reverse mapping with wrong coordinate extraction (M/K swapped, stride-0 dimension mishandled)
2. **First fix attempt:** Swap M/K groups — still cosine ≈ 0.2
3. **Second fix attempt:** Forward mapping with `layout_sf(make_coord(mn, k_sf * 16, 0))` — flat logical coordinates
4. **Critical bug found:** `if constexpr (LayoutRank == 2) { ... } else if (LayoutRank == 3) { ... }` with `int dst_idx = 0;` fallthrough — when LayoutRank didn't match 2 or 3, all threads wrote to dst[0], leaving the rest zero. This broke even uniform SF.
5. **Current approach:** Branchless `layout_sf(make_coord(mn, k_sf * 16, 0))`, explicit source strides, `filter_zeros` allocation, roundtrip verifier

### What the forward mapping does

```cpp
int mn = tid / K_sf;       // logical M/N index
int k_sf = tid % K_sf;     // compact SF group index
int k_elem = k_sf * 16;    // logical K element coordinate
int dst_idx = layout_sf(cute::make_coord(mn, k_elem, 0));
int src_idx = mn * src_stride_mn + k_sf * src_stride_ksf;
dst[dst_idx] = src[src_idx];
```

- CuTe's `layout_sf()` accepts flat logical coordinates and internally maps them to the hierarchical natural coordinate before applying strides
- `k_sf * 16` converts from compact SF group index to logical K element index (the layout expects K element coords, not SF group coords)
- `make_coord(mn, k_elem, 0)` is a 3-tuple matching the SFA/SFB layout shape `(M/N, K, L)`

### Source strides

| Tensor | Physical layout | src_stride_mn | src_stride_ksf |
|--------|----------------|---------------|-----------------|
| SFA | row-major (M, K_sf) | K_sf | 1 |
| SFB | row-major (K_sf, N) after `.T.contiguous()` | 1 | N |

### Allocation

Uses `cute::size(cute::filter_zeros(layout))` matching CUTLASS example 72a, with `cudaMemsetAsync` zero-init before remap.

### Roundtrip verifier (commit `aa209dd`)

Added `check_sf_forward_kernel` that compares `src[src_idx]` against `dst[dst_idx]` byte-by-byte. If this passes, the remap is correct. If it fails, the forward mapping is wrong. This is the key diagnostic — it separates remap correctness from other issues (A/B packing, FP8 type interpretation, stale code).

## How We Got Here

### Phase 1: Pipeline debugging
Added prints at every MoE stage. All magnitudes reasonable, no NaN. Signal present but buried. BF16 reference showed cosine ≈ 0 — the GEMM was wrong.

### Phase 2: Isolating the GEMM
Standalone tests proved: all-ones → cosine 1.0, random → cosine ≈ 0. The SF remap was the culprit.

### Phase 3: Blog post analysis
Veitner's blog (veitner.bearblog.dev) explained the CUTLASS SF layout construction in detail:
- K-major atom: `Shape<Shape<_32,_4>, Shape<SFVecSize,_4>>`, `Stride<Stride<_16,_4>, Stride<_0,_1>>`
- SFA tiled as `make_shape(M, K, L)`, SFB as `make_shape(N, K, L)` with `Step<_2,_1,_3>`
- First atom mode is M/N, second is K
- `f3` (stride-0 inner K) is within one SF group — always maps to offset 0
- `k_sf = logical_k / 16` (the /16 concept)
- Public logical access: `tensor_sfa(make_coord(m, k, 0))`

### Phase 4: Multiple failed coordinate extraction attempts
- Original: `get<0..2>` = M, `get<4..5>` = K (wrong)
- Swapped: `get<0..2>` = K, `get<3..5>` = M (still wrong — stride-0 ambiguity)
- Mike's correction: `mn = f0 + 32*f1 + 128*f2`, `k_sf = f4 + 4*f5` (correct inverse, but still ≈0.2 cosine — source stride issue)

### Phase 5: Forward mapping
Switched from reverse (`idx2crd` over dst) to forward (iterate over src, compute dst via `layout_sf()`). Multiple bugs:
- Hierarchical coordinate nesting was wrong (missing L component, wrong depth)
- Flat `make_coord(mn, k*16)` didn't auto-decompose (contrary to expectation)
- `if/else if` fallthrough wrote dst[0] for unmatched ranks, breaking uniform SF
- Source strides: `col_major_src` bool was ambiguous — replaced with explicit `src_stride_mn, src_stride_ksf`
- Allocation: `cosize` replaced with `size(filter_zeros())` to match CUTLASS examples

### Phase 6: Current approach (commit `aa209dd`)
Branchless forward mapping with flat logical coordinates, explicit strides, `filter_zeros` allocation, roundtrip verifier. This should be the correct implementation — the verifier will confirm.

## Key Commits (recent)

| Commit | Description |
|--------|-------------|
| `c384198` | Fix: SF remap uses cute::cosize() instead of cute::size() |
| `deb6b32` | Swap M/K in SF remap + add printf diagnostics |
| `a09b9b5` | Remove printf and diag function (build fix) |
| `30b6c89` | Correct k_sf = f4 + 4*f5 (Mike's patch) |
| `63e67e1` | Rewrite as forward mapping (source→dst) |
| `f6fd549` | Restore col_major_src handling |
| `7285331` | Replace col_major_src with explicit source strides |
| `6fc8fa6` | Use flat logical coordinate layout_sf(make_coord(mn, k_elem, 0)) |
| `6626b75` | Use filter_zeros for allocation + no-branch forward mapping |
| `aa209dd` | Add SF remap roundtrip verifier |

## Architecture Notes

### DeepSeek-V4 MoE Layer Forward Pass
```
residual = x
x, post, comb = hc_pre(x, hc_attn_fn, hc_attn_scale, hc_attn_base)
x = attn_norm(x)
x = attn(x)                          ← o_a_proj is BF16→NVFP4 quantized here
x = hc_post(x, residual, post, comb)

residual = x
x, post, comb = hc_pre(x, hc_ffn_fn, hc_ffn_scale, hc_ffn_base)
x = ffn_norm(x)
x = ffn(x)                           ← Our NVFP4 mega_moe kernel
x = hc_post(x, residual, post, comb)
```

### NVFP4 MoE Pipeline
```
stage_activation(hidden_states) → x_fp4, x_sf, input_global_scale
L1 GEMM: (x_fp4, x_sf) @ (l1_w, l1_sf) with alpha=igs*l1_global_sf → gate_up
SiLU(gate) * up → activated
stage_activation(activated) → l1_fp4, l1_sf, l1_igs
L2 GEMM: (l1_fp4, l1_sf) @ (l2_w, l2_sf) with alpha=l1_igs*l2_global_sf → output
scatter with routing weights → y
```

### CUTLASS SF Layout (from blog + code review)

The SM100 NVFP4 SF layout is constructed from a K-major atom:
```
SfKMajorAtom = Layout<
    Shape<Shape<_32, _4>, Shape<SFVecSize, _4>>,
    Stride<Stride<_16, _4>, Stride<_0, _1>>>
```

- First mode `(32, 4)` with stride `(16, 4)` → covers M/N dimension
- Second mode `(SFVecSize, 4)` with stride `(0, 1)` → covers K dimension
- `stride=0` means all 16 values in an SF group map to the same byte
- SFA tiled as `make_shape(M, K, L)` with `Step<_2, _1, _3>`
- SFB tiled as `make_shape(N, K, L)` with `Step<_2, _1, _3>`
- Public logical access: `tensor_sf(make_coord(m_or_n, k_element, l))`

### Checkpoint Layers (layer 0)
- **MoE experts 0-210, 212-255:** gate_proj, up_proj, down_proj — all NVFP4 (uint8 + float8 scales + float32 global scale)
- **Expert 211:** shared expert, gate_proj + up_proj only (no down_proj)
- **o_a_proj.weight:** BF16 (16384, 4096) — NOT quantized by ModelOpt
- **o_b_proj, q_a_proj, q_b_proj, kv_proj, compressor:** NVFP4
- **Gate weight, norms, sinks, position_bias:** BF16

## Next Steps

1. **Check SF-VERIFY output** — if sfa_errors=0 and sfb_errors=0, remap is correct
2. **If remap correct but cosine still low:** issue is A/B packing, FP8 type interpretation, or stale code
3. **If remap has errors:** debug the forward mapping further
4. **Run deterministic prompt** — "The capital of France is" should produce "Paris"
5. **Once working:** clean up debug prints, remove verifier
6. **Quality optimization:** investigate o_a_proj BF16→NVFP4 quantization