# ADR-0063: `tl.scratch_scope` — per-tile scratch recycling for long context

## Status

Accepted

> Supporting ADR for **ADR-0060** (AHBM GQA Fused Attention). Long-context
> attention sweeps many K/V tiles; each tile's intermediates
> (`scores`, `P`, `exp`, partial `O`, …) currently allocate fresh
> scratch that is never freed within a kernel invocation. The 1 MiB
> per-PE scratch budget is exhausted long before a realistic context
> length.

## Context

### The bump allocator

`TLContext` allocates every math/compute output handle from a per-PE
scratch pool with a **linear bump cursor**
(`src/kernbench/triton_emu/tl_context.py`; `_scratch_alloc`):

```python
def _scratch_alloc(self, nbytes):
    aligned = (nbytes + 15) & ~15
    addr = self._scratch_base + self._scratch_cursor
    self._scratch_cursor += aligned
    if self._scratch_cursor > self._scratch_size:      # default 1 << 20 = 1 MiB
        raise RuntimeError("TLContext scratch overflow: ...")
    return addr
```

The docstring states the cursor **"resets on every kernel invocation"** —
i.e. only at kernel entry, never *within* the kernel body. Every
`tl.dot`, `tl.softmax`, `tl.exp`, `tl.sum`, `a - b`, `a * b`, … grabs a
fresh slice and nothing is reclaimed until the kernel returns.

### Why this bites the GQA kernel

The current milestone bench keeps `S_q = S_kv_per_rank = 16` **explicitly
because** of this limit
(`tests/attention/test_milestone_gqa_llama70b.py:123-148`):

> "S_q_prefill and S_kv_per_rank are deliberately small (16 each) so the
> simulator's 1 MB per-PE TCM kernel scratch is not exhausted by the
> bump-allocated handle outputs of softmax/exp/dot/sum chains over
> n_ranks ring steps."

A FlashAttention sweep over `n_tiles` tiles allocates O(`n_tiles` ×
per-tile-intermediates). For any realistic context this overflows. The
*math* of flash attention needs only **O(1)** live scratch — the running
`(m, l, O)` plus the current tile's working set — because each tile's
temporaries are dead once that tile is folded into the running state. The
allocator just doesn't know they're dead.

## Decision

Add a **scratch scope** that lets a kernel mark a region of allocations
as reclaimable, so per-tile temporaries are recycled while live running
state (and in-flight prefetch buffers) are preserved.

### D1. `tl` surface — context manager

```python
with tl.scratch_scope():
    s   = tl.dot(q, k_t)        # all handles allocated inside the `with`
    p   = tl.softmax(s)         # share a region that is rewound on exit
    o_j = tl.dot(p, v)
    # ... fold o_j into running (m,l,O) which live OUTSIDE the scope ...
# on __exit__: cursor rewinds to its value at __enter__
```

Semantics:
- `__enter__` records the current `_scratch_cursor` as a save-point.
- `__exit__` restores the cursor to the save-point, freeing everything
  allocated inside.
- Handles allocated **outside** the scope (running `m,l,O`, prefetch
  buffers held by `LoadFuture` from ADR-0062) keep their addresses —
  they were allocated before the save-point or in an enclosing scope.

### D2. Safety contract

A handle allocated inside a scope **must not** be read after the scope
exits — its bytes may be overwritten by the next scope's allocations.
The flash loop respects this naturally: the only values that survive a
tile iteration are the running accumulators, which are allocated outside
the per-tile scope and updated by ops *inside* it writing to outside
addresses (the merge writes new running state — see D3).

### D3. Interaction with running accumulators

The online-softmax merge reads the old running `(m, l, O)` and the
current tile's `(m_j, l_j, O_j)`, producing new running values. To keep
the new running values outside the recycled region, the merge writes them
to **stable scratch** allocated once before the loop (a small fixed
"running-state" arena, distinct from the per-tile scope). Concretely the
kernel keeps two arenas:

- **persistent arena** (allocated once): `m, l, O` (and their
  double-buffer if needed for the merge).
- **scoped arena** (rewound each tile): `scores, P, exp, O_j, scale_*`.

This mirrors how real flash-attention SRAM budgeting works: a small
persistent accumulator region + a recycled tile working set.

#### D3.1 The persistent-arena write mechanism: `tl.copy_to(dst, src)`

The merge ops (`tl.maximum`, `tl.exp`, binary `*` / `+`) all call
`_make_compute_out(...)` which allocates from the bump cursor (D1).
Inside a `scratch_scope`, their result handles therefore live **inside**
the scope and vanish on `__exit__`. To realise D3's two-arena split, the
kernel needs a way to **write a scoped result's bytes back to a
persistent address**.

The primitive that closes this gap:

```python
def copy_to(self, dst: TensorHandle, src: TensorHandle) -> None:
    """Copy ``src``'s bytes into ``dst``'s address (both TCM).

    Shapes and dtypes must match. ``dst`` is typically a handle
    allocated outside any active ``scratch_scope`` (the persistent
    arena); ``src`` is a scoped handle whose bytes must outlive
    scope ``__exit__``.
    """
```

Symmetric to `tl.store` (the HBM-side byte copy), kept TCM-only here so
the running-state writeback doesn't pollute op_log with spurious DMA.

**Mechanics:**
- New `CopyCmd(src, dst, nbytes, data_op=True)` command.
- op_log: `op_kind="math"`, `op_name="copy"` — runs on the vector engine.
- Latency: `pe_math._compute_ns(prod(shape))` — models on-chip register
  writeback, not HBM transfer.
- Emit-time validation: `dst.shape == src.shape`, `dst.dtype == src.dtype`,
  `dst.space == "tcm"`, `src.space == "tcm"`. Authoring errors surface in
  Phase 1, not deep in Phase 2 data execution.

**Call-site pattern:**

```python
m, l, O = init_running(...)                  # persistent (outside scope)
for j in range(n_tiles):
    with tl.scratch_scope():
        ...                                  # per-tile work (recycled)
        m_new = tl.maximum(m, mj)            # scoped scratch
        l_new = l * scale_old + l_step * scale_step
        O_new = O * scale_old + O_step * scale_step

        tl.copy_to(m, m_new)                 # ← persist new running state
        tl.copy_to(l, l_new)
        tl.copy_to(O, O_new)
    # exit: scoped m_new/l_new/O_new gone; their bytes live in persistent m/l/O
```

The copy happens **before** `__exit__`, so the read of `src` (scoped) is
valid; after exit only `dst` (persistent) is read, satisfying D2's
no-read-after-exit safety contract.

**Why a dedicated primitive rather than `dst=` kwargs on every math op**
(considered, rejected): adding `dst=` to `tl.maximum`, `tl.exp`,
`_binary_math`, `_unary_math`, `_reduction` is ~25 LOC across 5
op-families and breaks the uniform "call returns a fresh handle"
pattern. `tl.copy_to` is one primitive, one command, one executor
handler — minimal surface area for the same effect.

### D4. Nesting

Scopes nest (stack of save-points). Inner scope exit rewinds to the inner
save-point; outer exit rewinds further. This supports an outer
"per-query-block" scope around an inner "per-KV-tile" scope for prefill.

## Alternatives

### A1. Round-trip temporaries through HBM

Store intermediates to HBM and reload to "free" TCM scratch. Rejected:
turns a TCM-resident streaming kernel into an HBM-bandwidth-bound one —
the opposite of the goal, and it pollutes op_log with spurious DMA.

### A2. Tiled `tl.composite` (scheduler-managed scratch)

`tl.composite` recycles per-tile scratch inside PE_SCHEDULER
automatically. As in ADR-0062 A1, this is attractive but blocked on a
flash-capable composite kind (two GEMMs + carried `(m,l,O)`), a much
larger change. `scratch_scope` gives the same memory behaviour for the
greenlet kernel with a tiny, general primitive. The two can coexist.

### A3. Grow the scratch budget

Bump `pe_tcm.kernel_scratch_mb`. Rejected as a fix: it only pushes the
context-length ceiling out linearly while still leaking O(`n_tiles`)
scratch, and it misrepresents the hardware (real TCM is small; the point
of flash attention is O(1) working set). Useful only as a coarse knob,
not a substitute for recycling.

## Consequences

### Positive
- Removes the artificial `S = 16` validation-scale ceiling; enables
  realistic context lengths in both timing and data modes.
- Faithfully models flash attention's O(1) working-set property.
- Small, general primitive (any tiled kernel benefits).

### Negative
- A use-after-scope bug silently reads stale bytes. Mitigated by the D2
  contract, by keeping the scope discipline inside a shared attention
  helper, and (optionally) by a debug build that poisons rewound regions.
- Must coordinate with ADR-0062: prefetch buffers are live across tile
  iterations, so they belong to the persistent arena, not the scoped one.

## Test Requirements

1. **Recycling**: a loop of `N` tiles inside `tl.scratch_scope()` keeps
   peak `_scratch_cursor` bounded by one tile's footprint, independent of
   `N` (today it grows linearly and overflows).
2. **Correctness**: a flash sweep with scopes produces the same `O` as
   the same sweep without scopes at a small `N` that fits without
   recycling (Phase 2).
3. **Long context**: a sweep at an `N` that would overflow 1 MiB without
   scopes completes (the exact failure the `S=16` cap avoids today).
4. **Persistent-vs-scoped isolation**: running `(m,l,O)` allocated
   outside the scope retains correct values across `__exit__`.
5. **Nesting**: nested scopes rewind to the correct save-points.
