CONFLICTS_LOG.md

# Bundle conflicts log — flash-attn-4-sm120-sncbl

Conflicts and genuine bugs encountered while stacking PRs
#2348/2349/2389/2439 on current Dao-AILab/flash-attention main. Each
entry: location, what the two sides did, how I resolved it, and
whether an individual PR needs a backport fix at merge time.

Bundle validated on SM121a (DGX Spark GB10):
- TMA forward (dense, bf16, causal):     max_diff=0.0078
- Paged KV (varlen, bf16, causal, ps=16): max_diff=0.0039
- Dropout (bf16, causal, p=0.1, seed=42): finite, magnitude ratio 1.11
- Plain forward (bf16/fp16, causal={0,1}): max_diff 0.0005-0.008

---

## [SEMANTIC] interface.py SM120 dispatch — #2348 × #2349 × #2389 × #2439

See entry below; addressed during stacking.

## [SEMANTIC] flash_fwd.py FlashAttentionForwardSm80.call() — #2348 × #2389

See entry below; addressed during stacking.

## [MINOR] flash_fwd.py launch()/kernel() args — #2389 × #2439

See entry below; addressed during stacking.

---

## [BUG found in #2349] FlashAttentionForwardSm120Tma.__call__ stream position

**Location**: `flash_attn/cute/flash_fwd_sm120_tma.py` `__call__` signature.

**Bug**: `stream` is declared at position 7 (right after softmax_scale)
instead of at the end. The base `FlashAttentionForwardSm80.__call__`
and `FlashAttentionForwardSm100.__call__` both put `stream` last, with
a comment: *"Always keep stream as the last parameter (EnvStream:
obtained implicitly via TVM FFI)"*. `cute.compile` binds args
positionally, so the compile_args list (which ends with current_stream)
would pass `cu_seqlens_q_tensor` where the TMA kernel expects stream.

**When #2349 lands alone**: Manifests as a runtime error once the
compiled TMA kernel is invoked through `_flash_attn_fwd`:
`DSLRuntimeError: expects argument #20 (dropout_seed_hi) to be one of
(Int32, NoneType), but got _FakeStream`. The TMA PR branch's own tests
probably route the stream differently or never hit this code path.

**Fix in bundle**: Moved `stream` to the end of TMA's __call__
signature to match the base class. Also added `dropout_seed_lo` /
`dropout_seed_hi` parameters that accept and `assert` they're None
(interface dispatch already gates TMA on `dropout_p == 0`).

**PR backport recommendation**: apply the same stream-last fix to
#2349 directly. Independent of any other PR — the TMA kernel is
broken as-written.

---

## [BUG found during merge] DSL SSA collision on shared locals `row_scale` / `sO`

**Location**: `flash_attn/cute/flash_fwd.py` inside
`FlashAttentionForwardSm80.call()`. #2389 adds a block-sparse branch
that defines `row_scale` and `sO`. #2348/base dense path also defines
`row_scale` and `sO` inside a runtime `if n_block_max > n_block_min:`
gate. When both branches coexist, the DSL's SSA analysis sees the same
variable name assigned in both a compile-time branch (block-sparse)
and a dynamic-if branch (dense), then rejects the dynamic assignment:
`"row_scale is None prior to this if, and update to _Tensor inside of
this if is not supported."`

**Fix in bundle**: renamed block-sparse locals to `bs_row_scale` /
`bs_sO`. They're local-use-only, so the rename has no downstream
effect.

**Also**: replaced my initial combined gate
`if const_expr(blocksparse_tensors is None) and n_block_max > n_block_min:`
with nested `if const_expr(blocksparse_tensors is None): / if n_block_max > n_block_min:`,
so the DSL evaluates the const_expr purely at compile time.

**PR backport recommendation**: this only surfaces when #2348 and #2389
are both applied. Whichever PR merges second needs to rename its
row_scale/sO to avoid collision and nest the conditions. Either PR
can absorb the fix; easier to do in #2389 since it already introduces
the block-sparse branch.

---

## [SEMANTIC] interface.py SM120 dispatch

**Three-way conflict** (same location, four PRs touch it):
- **#2348** sets `num_stages=2` unconditionally, gates on no block-sparse.
- **#2349** adds `FlashAttentionForwardSm120Tma` dispatch.
- **#2389** enables block-sparse on SM120 — conflicts with #2348's assert.
- **#2439** passes `p_dropout=dropout_p` to SM120 constructor; TMA
  kernel doesn't implement dropout.

**Bundle resolution**:
```python
is_varlen = cu_seqlens_q is not None or cu_seqlens_k is not None
use_tma_sm120 = (
    page_table is None
    and not is_varlen
    and not use_block_sparsity
    and dropout_p == 0.0
)
if use_tma_sm120 and FlashAttentionForwardSm120Tma.can_implement(...):
    fa_fwd = FlashAttentionForwardSm120Tma(...)
else:
    num_stages_sm120 = 2 if page_table is not None else 1
    fa_fwd = FlashAttentionForwardSm120(
        ..., num_stages=num_stages_sm120, ..., p_dropout=dropout_p,
    )
```

**PR backport at merge**: whichever PR lands LAST among the four
needs to incorporate the combined dispatch. ~5-10 lines each. Flag
in PR descriptions now.

---

## [SEMANTIC] flash_fwd.py call() — #2348 × #2389

**Conflict**: #2348 wraps prologue/mainloop in
`if n_block_max > n_block_min:` (split-KV empty-range guard) + adds
`split_idx` + uses `n_block - n_block_min` bound. #2389 adds a
**separate** block-sparse mainloop path that completes to its own
epilogue, then re-gates dense on `if const_expr(blocksparse_tensors is None)`.

**Bundle resolution**:
```python
if const_expr(blocksparse_tensors is not None):
    # block-sparse complete flow (bs_row_scale, bs_sO — see BUG note above)
    ...
if const_expr(blocksparse_tensors is None):
    if n_block_max > n_block_min:
        # dense flow (split-KV-aware, split_idx in epilogue)
        ...
```

**PR backport**: same guidance as dispatch conflict. Scope ~30-40
lines of code movement. Biggest rebase cost; flag in whichever PR is
waiting.

---

## [MINOR] flash_fwd.py launch()/kernel() args — #2389 × #2439

**Conflict**: both add kwargs to the same launch call + kernel
signature + interface.py call_args list. **Bundle resolution**: keep
both (blocksparse_tensors, then dropout_nheads/lo/hi). Trivial to
backport.

---

Bundle branch: `bundle/sncbl-sm120`. Final validated commits:
1. `657e8b5` Apply PR #2348 (+#2336)
2. `fa631d1` Apply PR #2349 + merge with #2348 dispatch (amended: TMA stream-last fix)
3. `d921b6a` Apply PR #2389 + merge with #2348/#2349
4. `a865455` Apply PR #2439 + merge with #2389/dispatch (amended: bs_ rename + nested gate)

All syntax-clean and validated on SM121a end-to-end through three
distinct kernel paths (TMA, paged KV, dropout).

---

## [BUG found in bundle, fixed via #2484] GQA / MQA crd2idx error — flash_fwd_sm120{,_tma}.py

**Symptom (pre-fix)**: any call to `flash_attn_func` or
`flash_attn_varlen_func` on SM120 with `qhead_per_kvhead > 1` (i.e.,
real GQA / MQA workloads like Qwen3, LLaMA3) failed at compile time
with:

```
loc("tPrPtr[i] = utils.elem_pointer(tensor, ((h_idx, m_idx),)).toint()"
    ("flash_attn/cute/pack_gqa.py":139:20)):
error: unable to compute crd2idx with
       '!cute.layout<"(?):(?{i64 div=8})">' and
       '!cute.coord<"((?,?))">'
```

**Cause**: `Sm80.__call__`'s epilogue (which `Sm120` inherits and
`Sm120Tma` calls via `self.epilogue`) takes the `pack_gqa.store_O`
branch when `self.pack_gqa` is True (default for `qhead_per_kvhead > 1`
in interface.py). `pack_gqa.store_O` calls `compute_ptr` (pack_gqa.py:139)
which expects a packed `((qhead_per_kvhead, seqlen_q), headdim)` layout.
Sm90 and Sm100 apply `pack_gqa_layout` before handing tensors to
PackGQA, but Sm80 does not, so the layout reaching `compute_ptr` is
un-packed and crd2idx against the hierarchical coord fails. Even adding
the `pack_gqa_layout` calls is not sufficient because Sm80's mainloop
tile sizing assumes `tile_m` divides the seqlen dimension cleanly,
which fails when `qhead_per_kvhead` does not divide `tile_m`.

**Fix**: override `self.pack_gqa = False` in both
`FlashAttentionForwardSm120.__init__` and
`FlashAttentionForwardSm120Tma.__init__`, after `super().__init__()`.
This routes GQA / MQA through the non-packed epilogue branch which is
functionally correct on every shape tested. Tracked upstream as
[Dao-AILab/flash-attention#2484](https://github.com/Dao-AILab/flash-attention/pull/2484).

**Validation post-fix**: 64 / 64 configurations pass on SM121a (MHA
+ GQA Qwen3 + GQA LLaMA3 + MQA, dense + varlen, bf16 + fp16, causal +
non-causal, batched). Max diff ≤ 0.0156 against PyTorch f32 reference.