kernbench2/docs/adr-proposed/ADR-0065-prog-flat-ops-composite-softmax-merge.md

ADR-0065: Flat-ops CompositeCmd + first stateful recipe (softmax_merge)

Status

Proposed (verification gated on ADR-0064 Revision 2 land)

Supporting ADR for ADR-0060 (AHBM GQA Fused Attention). Implements the §5.6 / §8 item 4 carve-out exactly: opt2 of decode = (#1 existing GEMM composite for Q·Kᵀ) + (#2 a single composite carrying softmax + P·V + online-softmax merge). The "ex_composite" / "flash_pv_merge" name referenced in ADR-0060 §5.6 is retired — this ADR fixes the canonical shape using the existing tl.composite entry with two structural additions: (a) a flat ops tuple, and (b) a first stateful recipe softmax_merge that lowers to a sequence of MATH micro-ops.

Context

What ADR-0060 left to this ADR

ADR-0060 §5.6 shipped opt3 (software pipelining) as the recommended now, and explicitly carved out opt2 — fewer per-tile dispatches, K-before-V DMA priority, measurable only after ADR-0064's cost model — as the follow-up.

§8 item 4 sized the carve-out: "if revisited, split it into two composites — #1 = Q·Kᵀ (existing composite + scale), #2 = softmax + P·V + online-softmax accumulator merge". This ADR implements #2.

Why composite needs a structural change

Today's CompositeCmd has the shape (op, a, b, out_addr, ops=(head, *epi)) with implicit conventions:

• op selects the head engine (gemm | math)
• ops[0] is the head OpSpec, ops[1:] are epilogue OpSpecs that run after the head per-output-tile or per-K-tile (scope-determined)

For decode opt2 we need MATH ops to run before the head GEMM (the online-softmax m, l, O update + emit P for the GEMM input). The current shape has no slot for that.

Two failed paths considered earlier:

• Embedding softmax_merge as an epilogue of #1 (Q·Kᵀ) — wrong: it needs to read Sj (= #1's output) and writes a fresh P consumed by the next GEMM (#2's P·V); circular.
• A single mixed-engine recipe flash_pv_merge absorbing MATH + GEMM + MATH — violates the engine boundary, complicates PE_SCHEDULER.

The clean shape is: drop the head/epilogue distinction from the command format. Make ops a flat ordered tuple; let each op's scope + position determine its tile-loop placement. The "prologue" concept stays in the user-facing tl.composite(...) API as ergonomics — but the HW command never sees it.

Why a recipe (not a generic op list)

Decode opt2's #2 MATH chain is 8 steps with a fixed structure (online softmax merge). Letting the kernel author wire those 8 steps would be both verbose and a hazard surface (correctness depends on step order). A recipe — a single named op (softmax_merge) that TLContext expands to the 8 steps with addresses pre-filled — keeps the kernel concise and the structure under one source of truth. The recipe table lives in TLContext (the compiler analog); PE_SCHEDULER never reads it. See boundary in D5.

Decision

D1. CompositeCmd is a flat ordered op list

@dataclass(frozen=True)
class CompositeCmd:
    completion: CompletionHandle
    ops: tuple[OpSpec, ...]                       # ordered MATH/GEMM ops
    rw_handles: tuple[TensorHandle, ...] = ()     # cross-composite hazard
    data_op: bool = True

    @property
    def logical_bytes(self) -> int: ...           # ADR-0064 D2

The previous (op, a, b, out_addr, out_nbytes, math_op) fields are absorbed into ops. The first OpSpec in ops (if kind == "gemm") is the head GEMM; preceding OpSpecs are pre-loop ops, following OpSpecs are post-loop / per-tile epilogue ops depending on scope.

D2. OpSpec carries named operands

@dataclass(frozen=True)
class OpSpec:
    kind: str                                     # "gemm" | "rmax" | ...
    scope: Scope                                  # KERNEL | K_TILE | OUTPUT_TILE
    operands: dict[str, TensorHandle]             # named — see D5
    extra: dict[str, Any]                         # scalars, axes, m/k/n, etc.
    out: TensorHandle | None = None               # explicit write-back handle

    @property
    def logical_bytes(self) -> int: ...

Named operands let PE_SCHEDULER route inputs to engine ports (e.g., GEMM takes operands["a"], operands["b"]) without positional convention.

D3. Position + scope determines tile-loop placement

Semantics split. Position determines the phase (where in the tile-loop lifecycle the op fires); scope determines the repetition within that phase. Scope.KERNEL means "once per CompositeCmd invocation", not "once per kernel launch" — its single firing happens at the op's position in the sequence. The same KERNEL value carries different phases depending on whether the OpSpec sits before, at, or after the GEMM op.

PE_SCHEDULER scans cmd.ops for the GEMM op (≤1 per CompositeCmd, see D6). Calling its index g:

OpSpec position	scope	Phase	Placement in plan
0 .. g-1	KERNEL	pre-tile-loop	run once before tile loop
g	KERNEL	head GEMM	drives tile loop with extra["m"], extra["k"], extra["n"]
g+1 ..	K_TILE	in-accumulation	per K-tile epilogue (inside K accumulation loop)
g+1 ..	OUTPUT_TILE	per-output-tile	per (m, n) tile epilogue (after K accumulation)
g+1 ..	KERNEL	post-tile-loop	run once after tile loop

A composite with no GEMM op (e.g., MATH-only composite) treats all ops as KERNEL-scope sequential — phase collapses to "single-shot in order".

D4. PE_SCHEDULER auto-inserts DMAs from operand space

PE_SCHEDULER scans the GEMM's operands and out. For each:

• space == "hbm" → emit DMA_READ Stage (or DMA_WRITE for out) before the FETCH/GEMM Stages
• space == "tcm" → no DMA Stage, operand consumed in place

This is already partially modelled today via a_pinned/b_pinned; D4 makes the rule uniform and visible: the kernel never emits explicit DMA cmds for composite operands; PE_SCHEDULER infers them from handles. tl.ref(addr, shape) returns a handle with space="hbm"; tl.zeros / tl.full / tl.load outputs return space="tcm".

Non-GEMM ops are TCM-only (Phase 1 limit, enforced at TLContext emit). Every operand of every non-GEMM (MATH) OpSpec must have space == "tcm". If a kernel passes a handle with space == "hbm" as a MATH operand, TLContext raises a validation error at emit time. This keeps DMA auto-insertion bounded to GEMM operands and head output handles in Phase 1; a MATH-with-HBM path would require a future extension (split into explicit tl.load + MATH composite, or a new "DMA-as-prologue" recipe variant). This invariant is also restated in D6 #7.

D5. RECIPE_DESCRIPTORS — TLContext-internal, NOT in pe_commands.py

Recipes live in a new TLContext-adjacent module (src/kernbench/triton_emu/tl_recipes.py). PE_SCHEDULER does not import it. The first recipe:

@dataclass(frozen=True)
class RecipeDescriptor:
    # Operand contract: name → "R" (read-only) | "RW" (read-write)
    operands: dict[str, str]
    # Type / shape rule for the implicit primary output (auto-bound to
    # head's first matrix operand). None means recipe emits no value.
    primary_out: PrimaryOutSpec | None
    # Single-shot prologue (full reduction) vs tile-aligned execution.
    tile_alignment: Literal["single_shot", "tile_aligned"]
    # Internal scratch budget (bytes), function of operand dims.
    internal_scratch_bytes_fn: Callable[..., int]
    # Engine micro-op sequence: TLContext expands this into flat OpSpecs.
    engine_seq: tuple[EngineOp, ...]


RECIPE_DESCRIPTORS["softmax_merge"] = RecipeDescriptor(
    operands={"s": "R", "m": "RW", "l": "RW", "O": "RW"},
    primary_out=PrimaryOutSpec(
        from_shape="s", from_dtype="s", transform="identity",
    ),
    tile_alignment="single_shot",
    internal_scratch_bytes_fn=lambda G, TILE, d, bpe: bpe * (
        G + G + G + G * TILE + G  # m_loc + m_new + corr + P + l_loc
    ),
    engine_seq=(
        EngineOp("MATH", "rmax",       src="s",         dst="m_loc",  reduce_axis=-1),
        EngineOp("MATH", "max_elem",   src_a="m",       src_b="m_loc", dst="m_new"),
        EngineOp("MATH", "exp_diff",   src_a="m",       src_b="m_new", dst="corr"),
        EngineOp("MATH", "exp_diff",   src_a="s",       src_b="m_new", dst="P",     bcast_axis=0),
        EngineOp("MATH", "rsum",       src="P",         dst="l_loc",   reduce_axis=-1),
        EngineOp("MATH", "fma",        src_a="l",       src_b="corr",  src_c="l_loc", dst="l"),
        EngineOp("MATH", "mul_bcast",  src_a="O",       src_b="corr",  dst="O",     bcast_axis=1),
        EngineOp("MATH", "copy",       src="m_new",     dst="m"),
    ),
)

TLContext, on tl.composite(prologue=[{"op": "softmax_merge", ...}], op="gemm", ...):

1. Validates operand R/RW against RECIPE_DESCRIPTORS["softmax_merge"].
2. Allocates scratch (m_loc, m_new, corr, P, l_loc) via the scratch_scope helper (ADR-0063).
3. Derives the primary output P's shape from s.shape (identity).
4. Expands engine_seq into 8 flat MATH OpSpecs with all addresses / sizes filled. Each OpSpec gets scope=KERNEL.
5. Auto-binds (with conflict check, D6.6). If the head GEMM does not explicitly provide operands["a"], auto-bind operands["a"] = P_handle. If the kernel did provide a explicitly, emit a validation error — ambiguous primary-output replacement.
6. Emits CompositeCmd(ops=(8 MATH + 1 GEMM + epilogue), rw_handles= (m, l, O)).

RECIPE_DESCRIPTORS appears nowhere in the HW path. PE_SCHEDULER sees only the flat ops list.

D6. Invariants

1. GEMM count. 0 ≤ count(op.kind == "gemm" in cmd.ops) ≤ 1. A composite with 0 GEMMs is MATH-only.
2. softmax_merge has no V operand. RECIPE_DESCRIPTORS["softmax_merge" ].operands does not include any HBM-resident handle → no DMA inserted during the prologue → V DMA can only happen during the GEMM head → natural K-before-V DMA priority across #1 / #2 of decode opt2.
3. Strict FIFO across composites. PE_SCHEDULER tracks in-flight rw_handles. A new composite whose rw_handles (or any of its op inputs) intersect with an in-flight composite's rw_handles waits for all earlier composites to complete — no out-of-order reorder.
4. No backwards-incompatibility for legacy callers. The user-facing API tl.composite(op="gemm", a, b, epilogue=[...]) is preserved. TLContext internally lowers it to a flat-ops CompositeCmd.
5. PE_SCHEDULER recipe-free. PE_SCHEDULER does not import RECIPE_DESCRIPTORS, does not know that "softmax_merge" exists, and has no recipe-specific code branches.
6. No implicit operand replacement (auto-bind conflict). If a prologue recipe declares a primary_out AND the head op's auto-bind target operand (e.g., GEMM a) is also provided by the kernel explicitly, TLContext raises a validation error at emit time. The kernel must either let the recipe bind (omit the operand) or specify it explicitly (omit the prologue, or use a recipe without primary_out). This prevents the ambiguity of "which value wins".
7. MATH operands TCM-only. Restatement of D4: every operand of a non-GEMM OpSpec must have space == "tcm". Phase 1 enforced at TLContext emit.

D7. Boundary summary (compiler vs scheduler vs engine)

HOST                                       DEVICE
TLContext (compiler analog)          PE_SCHEDULER (scheduler/dispatcher)
  - validate against RECIPE_DESC.       - scan flat ops list
  - derive primary_out shape            - identify GEMM (≤1)
  - allocate scratch                    - position-based stage placement
  - expand recipe → flat OpSpecs        - auto-insert DMA stages from space
  - compute rw_handles                  - strict-FIFO RW hazard tracking
  - emit CompositeCmd                   - emit Stage list
  imports: RECIPE_DESCRIPTORS            imports: nothing recipe-related

                ENGINES (PE_MATH / PE_GEMM / PE_DMA)
                  - read Stage.params (op_kind, addresses, n_elements)
                  - existing latency models unchanged
                  imports: nothing recipe-related

Alternatives

A1. Three composites per tile (no prologue concept)

Split #2 further into softmax_merge + gemm(P·V) + add(O) as three separate CompositeCmds. Pros: no flat-ops restructuring (add reuses existing epilogue path). Cons: 3 dispatches per tile instead of 2 — diverges from ADR-0060 §8 item 4 sizing ("split into two"); loses ~32 ns × N_tiles of fixed-cost savings. Rejected.

A2. Mixed-engine recipe (flash_pv_merge)

One recipe absorbing MATH + GEMM + MATH (ADR-0060 §5.6 original phrasing). Pros: tightest dispatch (1 composite). Cons: PE_SCHEDULER must know engine-boundary recipes, violates the "engines see only Stage.op_kind" boundary, recipe table becomes ISA-like and pollutes HW interface. Rejected.

A3. Generic op-list DAG (tl.ex_composite([...]))

User-defined sequence of any ops with named slots and dataflow analysis. Pros: maximally flexible. Cons: a single user (softmax_merge) does not justify a generic DAG language; PE_SCHEDULER would need dataflow analysis; reintroduces ADR-0060 §8 item 4's "absorb the inner loop" trap. Rejected (Simplicity First).

A4. RW-aware scheduler reorder (not strict FIFO)

PE_SCHEDULER could allow a later CompositeCmd to overtake an in-flight one when their rw_handles don't intersect (e.g., next-tile #1 could overlap with this-tile #2 since #1 doesn't touch m/l/O). Pros: more overlap. Cons: more scheduler state, gains are bounded (next #1 waits for K DMA anyway). Deferred — strict FIFO ships first; RW-aware reorder may be a future follow-up ADR.

A5. Embedding softmax_merge as an epilogue of #1 (Q·Kᵀ)

Wrong: softmax_merge's output P is the input of #2's P·V GEMM, which runs after #1's epilogue. Embedding it would force #2 to read its own input from #1's epilogue scratch — circular tile-loop dependency. Rejected (incorrect).

Consequences

Positive

• ADR-0060 §5.6 / §8 item 4 carve-out implemented exactly: opt2 = 2 composites per tile, K-before-V DMA priority natural.
• Composite contract becomes uniform: flat ops list with scope-based placement. No structural distinction between "head" and "epilogue" in the HW cmd.
• DMAs auto-inferred from operand space; kernel surface simpler.
• HW interface (CompositeCmd shape) stays small even as recipes grow — recipe expansion happens host-side.
• New fused patterns (linear+gelu+norm, rmsnorm+linear, …) can be added by inserting one RECIPE_DESCRIPTORS entry; no PE_SCHEDULER changes.

Negative

• CompositeCmd's struct shape changes — a refactor for all current callers. Semantics preserved (D6.4); existing goldens unchanged once ADR-0064 Revision 2 lands.
• OpSpec.operands changes from positional tuple[Any, ...] to dict[str, TensorHandle] — touches the existing epilogue lowering.
• New TLContext code: recipe expansion + scratch-slot allocation + primary_out auto-bind. ~80–120 LOC.
• PE_SCHEDULER's _generate_plan gets a position-based scan + DMA auto-insertion + strict-FIFO RW tracker. ~60–100 LOC.
• Strict-FIFO RW hazard tracking is safe but conservative. When two composites share any rw_handle, the new one waits for all prior overlapping composites to fully complete — even when their compute could in principle overlap (e.g., next-tile #1 = Q·Kᵀ does not touch m/l/O and could overlap with this-tile's #2, but FIFO still serializes any path that touches the same handles down the line). This may under-expose composite-level overlap relative to a smarter scheduler. RW-aware reorder (A4) is the deferred improvement.

Open review items

1. Recipe shape derivation rule beyond identity. Future recipes may need non-identity transforms (e.g., transposed primary_out). The PrimaryOutSpec.transform field is forward-compat; add new transforms as needed.
2. Recipe scratch allocator integration with ADR-0063's scratch_scope. Initial wiring: TLContext allocates inside the current scratch_scope context if active, else in a kernel-scope persistent slot.
3. tl_recipes.py location. New module under triton_emu/. Keeps recipe knowledge co-located with the compiler analog (TLContext).
4. GEMM out=TensorHandle. New form alongside legacy out_ptr: int. Both accepted; handle form preferred for new code (enables strict-FIFO RW tracking by handle identity).

Test Requirements

1. CompositeCmd flat-ops refactor — meaning preservation. Every existing bench's op_log is byte-equal pre/post refactor (legacy API tl.composite(op="gemm", a, b, epilogue=[...]) lowers to the same Stage sequence).
2. softmax_merge recipe lowering. TLContext call tl.composite( prologue=[{"op": "softmax_merge", "s": Sj, "m": m, "l": l, "O": O}], op="gemm", b=V_ref, out=O, epilogue=[{"op": "add", "other": O}]) produces a CompositeCmd with exactly 10 ops in the expected order (8 MATH + 1 GEMM + 1 MATH(add)) and rw_handles == (m, l, O).
3. K-before-V DMA priority invariant. op_log of decode opt2 shows no V-related DMA during #2's MATH prologue (#2's V DMA begins only when the GEMM head starts).
4. Strict-FIFO RW serialization. Two consecutive composites sharing any rw_handle complete in dispatch order; their stages do not interleave.
5. DMA auto-insertion from space. A GEMM composite with b=tl.ref(...) (space=hbm) emits a DMA_READ Stage; with b from tl.zeros(...) (space=tcm) it does not. Output handle with space=hbm emits DMA_WRITE; space=tcm does not.
6. opt2 numeric parity with opt3. In data mode, opt2's final (m, l, O) matches opt3's within fp tolerance.
7. opt2 dispatch ratio (after ADR-0064 Rev2) — robust. opt3 vs opt2 per-tile PE_CPU dispatch cycles satisfies opt3 > 2 × opt2 (qualitative gate; calibration-independent). The default-calibration model expectation is ≈ 4.0× (FIXED=40 cycles, R=0.0625 cycles/byte) — see DDD-0065 §11 for the model-derived numbers; only the loose > 2× bound is the test gate so it survives calibration changes. Ratio is FIXED-dominated, reflecting command-count reduction as the primary signal.
8. GEMM-count invariant. A composite with two GEMM OpSpecs raises a validation error at TLContext emit.
9. Within composite size cap (ADR-0064 D7). Decode opt2's #2 composite (10 ops, ~322 logical bytes — per-op summation of operand handles) fits comfortably under the default MAX_COMPOSITE_LOGICAL_BYTES=1024 — no segmentation. The recipe lowering test asserts cmd.logical_bytes < 1024.
10. Auto-bind conflict. tl.composite(op="gemm", a=A_handle, prologue=[{"op": "softmax_merge", ...}], ...) where softmax_merge declares primary_out raises a validation error at emit time (D6.6).
11. MATH operand TCM invariant. tl.composite(op="math", a=tl.ref(addr, shape), ...) (passing an HBM-resident handle as a MATH operand) raises a validation error at emit time (D6.7).

Dependencies

• ADR-0060 §5.6, §8 item 4 — the carve-out this ADR implements.
• ADR-0064 Revision 2 — dispatch cost model; lands first, makes opt2's fewer-issues win measurable.
• ADR-0063 scratch_scope — recipe intermediates allocated inside; m, l, O allocated outside as persistent arena (per DDD-0060 §6.2).
• ADR-0046 tl_context contract — extended by prologue=[...] kwarg, out=TensorHandle, RECIPE_DESCRIPTORS lookup.
• ADR-0042 tile plan generators — extended (not redesigned) to process the flat ops list and auto-insert DMAs.
• ADR-0014 PE pipeline — boundary preserved: scheduler stays recipe-free, engines stay op_kind-opaque.

Migration

Land order:

1. ADR-0064 Revision 2 (separate PR): structural dispatch cost + logical_bytes property + topology config override + one-time goldens regeneration.
2. ADR-0065 Phase 1 (this ADR, PR a): CompositeCmd flat-ops restructure + legacy-API lowering. Refactor only; goldens unchanged.
3. ADR-0065 Phase 2 (this ADR, PR b): tl_recipes.py + softmax_merge recipe + PE_SCHEDULER position-scan + DMA auto-insertion + strict-FIFO RW tracker.
4. ADR-0065 Phase 3 (this ADR, PR c): _gqa_decode_long.py opt2 variant + numeric parity test + dispatch-ratio measurement against opt3 baseline.

Detailed implementation plan: see DDD-0065.