kernbench2/docs/adr-proposed/DDD-0065-flat-ops-composite-softmax-merge-detailed-design.md

Detailed Design Document — Flat-ops CompositeCmd + softmax_merge recipe (ADR-0065)

Status: Draft (companion to ADR-0065, Proposed) Scope: implementation-ready plan for the CompositeCmd flat-ops restructure, the softmax_merge recipe, and the decode opt2 variant in _gqa_decode_long.py. Audience: reviewer / implementer resuming after the ADR-0065 design round. Read ADR-0065 first (boundary D5 + D7 set the layering), and ADR-0064 Revision 2 (dispatch cost model that opt2's win depends on).

---

1. Goal and success criteria

Goal: make decode opt2 runnable, numerically equivalent to opt3, and measurably cheaper in PE_CPU dispatch under the ADR-0064 Rev2 cost model. Existing benches keep their (post-ADR-0064 Rev2) goldens unchanged.

Success criteria:

1. Refactor safety. CompositeCmd flat-ops restructure preserves the op_log byte-for-byte for every existing bench (tests/<existing> all green, no goldens diff).
2. Recipe correctness. opt2 final (m, l, O) matches opt3 within fp tolerance in data mode (enable_data=True).
3. Boundary preservation. PE_SCHEDULER imports nothing recipe-related; PE_MATH/PE_GEMM/PE_DMA see only Stage.params["op_kind"] (no new engine code).
4. Dispatch ratio. Per-tile PE_CPU dispatch cycles ratio(opt3 / opt2) ≈ 2.4× at default ADR-0064 calibration.
5. K-before-V invariant. op_log of decode opt2 shows zero V-related DMA during #2's MATH prologue.

Non-goals: prefill (causal if in kernel control flow, separate work); RW-aware scheduler reorder (strict FIFO ships); user-defined recipes; new engine micro-ops in PE_MATH (it stays op_kind-opaque).

---

2. Where the design sits in the current code

runtime_api (host)
  RuntimeContext.zeros/empty/launch          context.py
        │  KernelLaunchMsg
        ▼
sim_engine
  GraphEngine(enable_data=…)                 engine.py
  MemoryStore                                memory_store.py
        │
        ▼
components / PE pipeline (per PE)
  PE_CPU → PE_SCHEDULER → PE_{DMA,GEMM,MATH,IPCQ,TCM}
  greenlet kernel ↔ SimPy                    kernel_runner.py, pe_cpu.py
  CompositeCmd → tile plan → engines         pe_scheduler.py (ADR-0014 D6)
        │
        ▼
tl programming model                         tl_context.py
  composite / dot / load / store / scratch_scope
  ── ADR-0065 NEW ──────────────────────────
  tl_recipes.py                              RECIPE_DESCRIPTORS, EngineOp,
                                             RecipeDescriptor, PrimaryOutSpec

The new module tl_recipes.py sits beside tl_context.py. PE_SCHEDULER (pe_scheduler.py) and tiling.py extend to consume the flat ops list without importing tl_recipes.

---

3. File plan

3.1 Modified production files

File	Change	ADR ref
src/kernbench/common/pe_commands.py	CompositeCmd flat-ops dataclass; OpSpec.operands: dict; new Scope value semantics (KERNEL position-based); logical_bytes property on every PeCommand.	0065 D1/D2, 0064 D2
src/kernbench/triton_emu/tl_context.py	composite(prologue=[...], out=TensorHandle, …); recipe lowering pass (RECIPE_DESCRIPTORS lookup, scratch alloc, OpSpec expansion, primary_out auto-bind); legacy API preserved (op="gemm", a, b, epilogue=[...] still works, internally lowers to flat ops).	0065 D1/D5
src/kernbench/triton_emu/tl_recipes.py (NEW)	RecipeDescriptor, PrimaryOutSpec, EngineOp dataclasses; RECIPE_DESCRIPTORS["softmax_merge"] entry; expansion helper.	0065 D5
src/kernbench/components/builtin/pe_scheduler.py	_generate_plan rewritten for flat ops: scan for GEMM (≤1), position-based KERNEL placement, OUTPUT_TILE/K_TILE epilogue, strict-FIFO RW tracker keyed by handle id.	0065 D3/D6
src/kernbench/components/builtin/tiling.py	generate_plan_from_ops(ops, rw_handles, pe_prefix) replaces generate_gemm_plan / generate_math_plan (or wraps them). DMA auto-insertion from handle.space. Epilogue add stage emits other_handle in params (GAP 9 fix).	0065 D3/D4
src/kernbench/components/builtin/pe_cpu.py	dispatch cost from cmd.logical_bytes via ADR-0064 Rev2 formula; topology config override.	0064 D1/D4
topology yaml (e.g., topologies/llama70b_4sip.yaml)	optional pe_cost_model: block under PE node attrs.	0064 D4
src/kernbench/benches/_gqa_decode_long.py	opt2 variant — tl.composite(prologue=[{"op":"softmax_merge", …}], op="gemm", b=V_ref, out=O, epilogue=[{"op":"add", "other":O}]).	0065 §Migration P3

3.2 New tests

File	Covers
tests/test_composite_flat_ops_refactor.py	every existing bench's op_log byte-equal pre/post refactor (parametrized over bench list)
tests/test_tl_recipe_softmax_merge.py	recipe lowering — given inputs, asserts CompositeCmd has 10 ops in expected order + rw_handles == (m, l, O)
tests/test_pe_scheduler_strict_fifo.py	two consecutive composites sharing rw_handles complete in dispatch order; no stage interleave
tests/test_pe_scheduler_auto_dma.py	space=hbm operand → DMA_READ stage; space=tcm → none; out.space=hbm → DMA_WRITE
tests/test_pe_cost_model.py	ADR-0064 Rev2 — typical 1-GEMM composite at 45 ns; topology override path; PeCpuOverheadCmd bypass
tests/attention/test_gqa_decode_opt2.py	opt2 numeric parity with opt3; K-before-V DMA invariant; per-tile dispatch ratio measurement

---

4. Data model — CompositeCmd flat-ops

4.1 New shape

@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:
        return (
            4                                     # framing
            + 1 + sum(op.logical_bytes for op in self.ops)
            + 1 + 8 * len(self.rw_handles)
        )

The previous op, a, b, out_addr, out_nbytes, math_op fields are removed. All info lives in ops[i].

4.2 OpSpec extended

@dataclass(frozen=True)
class OpSpec:
    kind: str                                     # "gemm" | "rmax" | "exp_diff" | ...
    scope: Scope                                  # KERNEL | K_TILE | OUTPUT_TILE
    operands: dict[str, TensorHandle]
    extra: dict[str, Any] = field(default_factory=dict)
    out: TensorHandle | None = None

    @property
    def logical_bytes(self) -> int:
        return (
            1 + 1                                        # opcode + scope enum
            + 1 + 8 * len(self.operands)                 # named handles
            + (8 if self.out is not None else 0)
            + 1 + 4 * len(self.extra)                    # scalars
        )

Backward compat: existing operands: tuple[Any, ...] use sites (epilogue bias, scale, add, etc.) migrate to operands={"other": h} / operands={"src": h} per a small lookup in pe_commands.EPILOGUE_OPS.

4.3 Scope semantics (position-based for KERNEL)

Scope keeps its three values (KERNEL, K_TILE, OUTPUT_TILE). What changes: KERNEL no longer means "once anywhere"; it now means "execute at this op's position in the sequence — once". Combined with position relative to the GEMM op:

• KERNEL ops before GEMM → pre-tile-loop (single shot)
• KERNEL ops after GEMM → post-tile-loop (single shot)
• K_TILE / OUTPUT_TILE ops after GEMM → inside tile loop as before

---

5. Recipe: softmax_merge

5.1 tl_recipes.py definitions

@dataclass(frozen=True)
class PrimaryOutSpec:
    from_shape: str          # operand name to copy shape from
    from_dtype: str          # operand name to copy dtype from
    transform: str           # "identity" | "trans" (forward-compat)


@dataclass(frozen=True)
class EngineOp:
    engine: Literal["MATH", "GEMM", "DMA"]
    op_kind: str             # PE_MATH op_kind label (opaque to engine)
    # Slot names: any of "src", "src_a", "src_b", "src_c", "dst" — these
    # are RecipeDescriptor.operands names or internal scratch names.
    src: str | None = None
    src_a: str | None = None
    src_b: str | None = None
    src_c: str | None = None
    dst: str | None = None
    reduce_axis: int | None = None
    bcast_axis: int | None = None


@dataclass(frozen=True)
class RecipeDescriptor:
    operands: dict[str, str]                            # name → "R" | "RW"
    primary_out: PrimaryOutSpec | None                  # implicit output spec
    tile_alignment: Literal["single_shot", "tile_aligned"]
    internal_scratch_bytes_fn: Callable[..., int]
    engine_seq: tuple[EngineOp, ...]


RECIPE_DESCRIPTORS: dict[str, RecipeDescriptor] = { ... }

5.2 Engine sequence (8 MATH ops)

#	engine	op_kind	reads (slots)	writes (slots)	extras
1	MATH	rmax	s	m_loc	reduce_axis=-1
2	MATH	max_elem	m, m_loc	m_new	—
3	MATH	exp_diff	m, m_new	corr	—
4	MATH	exp_diff	s, m_new	P (primary_out)	bcast_axis=0
5	MATH	rsum	P	l_loc	reduce_axis=-1
6	MATH	fma	l, corr, l_loc	l	—
7	MATH	mul_bcast	O, corr	O	bcast_axis=1
8	MATH	copy	m_new	m	—

Intermediates m_loc, m_new, corr, P, l_loc are scratch slots allocated by TLContext before lowering. P is the recipe's primary output; auto- bound to the head GEMM's operands["a"].

5.3 PE_MATH op_kind additions

PE_MATH is op_kind-opaque (pe_math.py:90-99). Latency = ceil(num_elements / vector_width) / clock. Adding new op_kind labels (rmax, max_elem, exp_diff, rsum, fma, mul_bcast, copy) requires no PE_MATH code changes — they pass through as string labels in Stage.params["op_kind"]. Latency stays consistent for the same num_elements.

(Reductions like rmax / rsum have log-depth final stages in real HW not modelled by ceil(num_elements / vector_width); this is an existing approximation, not introduced by ADR-0065.)

---

6. TLContext lowering pipeline

Input (user-facing API, preserved):

tl.composite(
    op="gemm", b=tl.ref(v_tile(j), (TILE, d)), out=O,
    prologue=[{"op": "softmax_merge", "s": Sj, "m": m, "l": l, "O": O}],
    epilogue=[{"op": "add", "other": O}],
)

Lowering steps in TLContext.composite():

1. Validate prologue items. For each, lookup RECIPE_DESCRIPTORS[item["op"]]. Check all operands present, type tags (R / RW) match handle states (e.g., RW must not be tl.ref).
2. Allocate internal scratch. Call recipe.internal_scratch_bytes_fn(G, TILE, d, bpe); allocate via scratch_scope helper (ADR-0063). Get slot addresses for m_loc, m_new, corr, P, l_loc.
3. Derive primary_out handle. From recipe.primary_out: P handle = TensorHandle(shape=Sj.shape, dtype=Sj.dtype, addr=<P slot>, space="tcm").
4. Expand engine_seq → flat OpSpecs. For each EngineOp:
   • Resolve slot names (operands or scratch) to TensorHandles.
   • Build OpSpec(kind=op_kind, scope=Scope.KERNEL, operands={...}, extra={...}, out=...).
   • out is the handle for dst slot; operands is {src*: handle}.
5. Auto-bind head GEMM. GEMM OpSpec built from op="gemm", operands={"a": P_handle, "b": V_handle}, out=<output handle>, extra={"m": G, "k": TILE, "n": d}.
6. Build epilogue OpSpecs. Each epilogue=[{...}] entry maps to OpSpec(kind=..., scope=Scope.OUTPUT_TILE, operands={...}, ...). The add epilogue specifically gets operands={"other": O_handle} so the Stage later carries other_handle (GAP 9 fix).
7. Compute rw_handles. Collect all handles tagged RW in prologue recipes + the head GEMM's out (if accumulating) + any RW epilogue targets. For softmax_merge: rw_handles = (m, l, O).
8. Emit CompositeCmd. ops = (8 MATH + 1 GEMM + 1 MATH(add)), rw_handles = (m, l, O), completion = new.

Legacy lowering. Pre-existing tl.composite(op="gemm", a=A, b=B, epilogue=[{"op":"bias","bias":h}]) goes through the same emit path: no prologue, head OpSpec from op="gemm", epilogue OpSpecs built from epilogue=[...]. Output CompositeCmd has flat ops list; semantics preserved.

---

7. PE_SCHEDULER tile plan generation (flat-ops)

def _generate_plan(cmd: CompositeCmd) -> PipelinePlan:
    pp = self._pe_prefix
    bpe = 2

    # 1. Identify GEMM op (≤1).
    gemm_idx = next(
        (i for i, o in enumerate(cmd.ops) if o.kind == "gemm"),
        None,
    )

    # 2. MATH-only composite (no GEMM): single tile of all ops in order.
    if gemm_idx is None:
        return _math_only_plan(cmd.ops, pp)

    gemm = cmd.ops[gemm_idx]
    M, K, N = gemm.extra["m"], gemm.extra["k"], gemm.extra["n"]

    # 3. Partition ops by position.
    pre_loop_ops  = cmd.ops[:gemm_idx]              # KERNEL scope
    after_ops     = cmd.ops[gemm_idx+1:]            # mixed scopes

    # 4. Pre-loop MATH Stages (single-shot, before tile loop).
    pre_stages = [_math_stage_from_op(op, pp) for op in pre_loop_ops]

    # 5. Tile loop (existing generate_gemm_plan pattern, but:
    #    - DMA_READ inserted only when operand.space == "hbm"
    #    - K_TILE epilogue from after_ops with scope == K_TILE
    #    - OUTPUT_TILE epilogue from after_ops with scope == OUTPUT_TILE
    #    - DMA_WRITE inserted only when out.space == "hbm")
    tile_plans = _generate_tile_loop(
        gemm, M, K, N, after_ops, pp, bpe,
    )

    # 6. Post-loop KERNEL Stages (after tile loop completes).
    post_loop_ops = [op for op in after_ops if op.scope == Scope.KERNEL]
    post_stages   = [_math_stage_from_op(op, pp) for op in post_loop_ops]

    return PipelinePlan(
        prologue_stages=tuple(pre_stages),
        tiles=tile_plans,
        epilogue_stages=tuple(post_stages),
        m_tiles=..., k_tiles=..., n_tiles=...,
    )

PipelinePlan gets two new fields:

@dataclass(frozen=True)
class PipelinePlan:
    tiles: list[TilePlan]
    m_tiles: int
    k_tiles: int
    n_tiles: int
    prologue_stages: tuple[Stage, ...] = ()
    epilogue_stages: tuple[Stage, ...] = ()

Feeder change (_feed_loop): yield prologue_stages Stages first, then tile loop as today, then epilogue_stages. For the single-Stage case (no prologue/epilogue), behaviour identical to today.

---

8. Strict-FIFO RW hazard tracker

class _RwHazardTracker:
    def __init__(self):
        self._inflight: list[tuple[CompletionHandle, set[str]]] = []
        # (completion, set of handle ids written or RW)

    def admit(self, env, cmd: CompositeCmd) -> Generator:
        """Block until no in-flight composite has overlapping RW."""
        rw_ids = {h.id for h in cmd.rw_handles}
        # Strict FIFO: wait for ALL prior composites whose rw_set
        # intersects, in order.
        while True:
            blocking = [c for c, s in self._inflight if s & rw_ids]
            if not blocking:
                break
            # Wait on the *first* blocking composite's completion.
            yield blocking[0].done_event
        self._inflight.append((cmd.completion, rw_ids))

    def retire(self, completion: CompletionHandle) -> None:
        self._inflight = [
            (c, s) for c, s in self._inflight if c != completion
        ]

PE_SCHEDULER invokes admit() before queueing a CompositeCmd's tile plan and retire() upon done_event.succeed(). Strict FIFO: even when overlap is partial, the new composite waits for all prior overlapping composites to fully drain — simple, easy to reason about, easy to test. (RW-aware reorder deferred per ADR-0065 A4.)

---

9. Phased implementation plan (test-first per CLAUDE.md)

Each phase is an independent Phase-1 → Phase-2 cycle per CLAUDE.md Part 1.

Phase	Deliverable	Gate
P0 (= ADR-0064 Rev2)	logical_bytes property + dispatch formula + topology override; one-time goldens regeneration	typical composite ≈ 45 ns; dispatch ratio test infrastructure in place
P1	CompositeCmd flat-ops dataclass refactor + OpSpec.operands: dict + legacy tl.composite lowering preserved	every existing bench's op_log byte-equal pre/post
P2	tl_recipes.py + RECIPE_DESCRIPTORS["softmax_merge"] + TLContext lowering for prologue=[...]	recipe lowering test (10 ops, rw_handles, addresses) passes
P3	PE_SCHEDULER _generate_plan flat-ops + PipelinePlan.prologue_stages + DMA auto-insertion + _feed_loop extension	all existing bench Stage sequences unchanged; new MATH chain visible in op_log for opt2 mock
P4	Strict-FIFO RW hazard tracker + _RwHazardTracker integration	two-composite RW conflict test serializes correctly
P5	_gqa_decode_long.py opt2 variant; data-mode numeric parity check; K-before-V invariant check	opt2 matches opt3 within fp tolerance; no V DMA during prologue
P6	Dispatch-ratio measurement: opt3 vs opt2 per-tile PE_CPU cycles	ratio ≈ 2.4× ± 10% at default calibration

P0 must land first (separate ADR). P1 is pure refactor (safest). P2–P4 build the new path without touching opt3. P5 enables opt2. P6 closes the measurement loop.

---

10. Verification plan (concrete)

Mirrors ADR-0065 §Test Requirements; grounded in SPEC R2/R5, ADR-0023/ 0025, ADR-0046, ADR-0054.

Refactor safety (P1):

• Parametrize over tests/<existing bench>.py baseline op_logs.
• Pre/post refactor diff must be empty for every bench.
• Failure → revert P1 atomically; do not land partial.

Recipe lowering (P2):

• Given Sj=(8,64), m=(8,), l=(8,), O=(8,128), lower tl.composite(prologue=[{"op":"softmax_merge", "s":Sj,...}], op="gemm", b=V_ref, out=O, epilogue=[{"op":"add", "other":O}]).
• Assert len(cmd.ops) == 10, cmd.ops[0..7].kind == [rmax, max_elem, exp_diff, exp_diff, rsum, fma, mul_bcast, copy], cmd.ops[8].kind == "gemm", cmd.ops[9].kind == "add".
• Assert cmd.rw_handles == (m, l, O).
• Assert all OpSpec addresses are concrete (no None).

Scheduler plan (P3):

• For the above CompositeCmd, assert PipelinePlan has 8 prologue_stages (MATH), tile loop with FETCH/GEMM/MATH(add), no epilogue_stages.
• Assert no DMA_READ for prologue MATH stages (operands TCM-resident).
• Assert one DMA_READ for V (b.space == "hbm").
• Assert no DMA_WRITE (out.space == "tcm").

RW hazard (P4):

• Submit composite A (rw_handles=(O,)) then composite B (operands reading O). Assert B's Stages start only after A's done_event.

Numeric parity (P5):

• In data mode, run opt2 and opt3 for (G=8, T_q=1, S_kv=64, C=1, P=1).
• Assert O_opt2 ≈ O_opt3 within fp16 tolerance.

K-before-V invariant (P5):

• op_log inspection: for each tile's #2 CompositeCmd, no memory/dma_read events between #2 start and the GEMM compute begin that are not the V tile DMA itself.

Dispatch ratio (P6):

• For S_kv=64, n_tiles=16, measure total PE_CPU dispatch cycles for opt3 vs opt2 paths.
• Assert ratio in [2.0, 2.8] (centre 2.4×).

Determinism (all phases):

• Identical inputs → identical op_log + latency (SPEC §0.1).

---

11. Performance model (expected)

Per-tile PE_CPU dispatch cycles, ADR-0064 Rev2 default calibration:

opt3: 10 cmds × 32 + total_bytes(≈232) × 0.25 = 320 + 58  ≈ 378 ns
opt2:  2 cmds × 32 + total_bytes(≈380) × 0.25 =  64 + 95  ≈ 159 ns
ratio = 378 / 159 ≈ 2.4×

Per-tile engine timing dominates total latency; dispatch is one component. The ratio above is the dispatch-only relative cost. Total latency improvement depends on whether dispatch was on the critical path — measured per workload in P6.

---

12. Open items (status)

Most ADR-0065 §Open review items map to specific implementation choices already documented in §§5–8. Live remaining items:

Item	Resolution venue
Non-identity PrimaryOutSpec.transform (e.g., "trans")	Future recipe addition; current code uses string dispatch in step 3 of §6
_gqa_decode_long.py opt2 — TILE size sweep	Open (existing §B-3, §9 of ADR-0060)
Larger composite descriptors (10+ OpSpecs) and logical_bytes growth	Monitored in P6; if R term dominates, calibration revisit
scratch_scope budget vs softmax_merge intermediates (G·TILE for P)	ADR-0063 sizing check at S_kv = 256K

---

13. Risks

Risk	Likelihood	Mitigation
CompositeCmd refactor (P1) changes op_log for some bench	med	Parametrized regression over all benches; revert atomically
OpSpec.operands migration (positional → dict) breaks epilogue site	med	Per-EPILOGUE_OPS migration table; existing epilogue tests cover
_RwHazardTracker false-positive serializes too aggressively	low	Strict FIFO is the safe direction; RW-aware reorder is a future ADR
ADR-0064 Rev2 and ADR-0065 P1 land in wrong order → stale goldens	med	PR ordering enforced; CI gate requires both to be green
softmax_merge scratch budget exceeds scratch_scope at long context	med	ADR-0063 sizing check; if needed, tile_alignment promotion (future)
PE_MATH op_kind label collision with new strings	low	Labels are opaque strings — no enum conflict; just add to allowed set if validated

---

14. Glossary & references

• flat ops list — CompositeCmd.ops: tuple[OpSpec, ...] with no head/epilogue field; placement determined by op position + scope.
• prologue (user API) — tl.composite(prologue=[...]) kwarg for recipe ops that lower into pre-GEMM KERNEL-scope OpSpecs. HW command does not carry this concept.
• recipe — a named macro op (e.g., softmax_merge) that TLContext expands to multiple flat OpSpecs at emit time. Registry = RECIPE_DESCRIPTORS.
• primary_out — recipe's implicit output (e.g., P for softmax_merge). Auto-bound to head GEMM's first matrix operand.
• handle-identity strict FIFO — cross-composite serialization rule: a composite waits for all prior composites whose rw_handles intersect with its own to complete.
• logical_bytes — HW-logical byte size of a PE command, derived from a structural rule (ADR-0064 D2), not from Python's sys.getsizeof.

Source anchors: tl_context.py (tl API), pe_scheduler.py:104-143 (composite tile plan), pe_dma.py:45,89 (read channel), pe_math.py (op_kind opaque latency), pe_cpu.py (dispatch hook), pe_commands.py (CompositeCmd, OpSpec, EPILOGUE_OPS).

ADRs: 0065 (this design), 0064 Revision 2 (dispatch cost), 0060 (GQA fused attention, §5.6 / §8 item 4 carve-out), 0063 (scratch_scope), 0046 (tl_context contract), 0042 (tile plan generators), 0014 (PE pipeline execution model).