Phase 2c-1: wire chunkifies into Flits + reassembly compat layer

Wire decomposes Transactions into Flits per `_flit_bytes` but emits all
flits atomically at the same env.now — preserves single-msg timing as
infrastructure for Phase 2c-2 (per-flit timing + flit-aware routers).

Non-flit-aware components reassemble Flits in `_fan_in`; `_update_step`
sets txn.step to current component's path position so legacy
step-based routing continues working when upstream is flit-aware.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/kernbench/components/base.py

@@ -53,11 +53,51 @@ class ComponentBase(ABC):
             env.process(self._fan_in(port))
         env.process(self._worker(env))
 
+    # ADR-0033 Phase 2c: flit-aware components consume Flits directly;
+    # non-flit-aware components reassemble Flits into the parent
+    # Transaction before delivery to _inbox. Default False preserves
+    # legacy single-msg semantics during incremental rollout.
+    _FLIT_AWARE: bool = False
+
     def _fan_in(self, port: simpy.Store) -> Generator:
-        """Relay messages from one in_port into the shared inbox."""
+        """Relay messages from in_port to _inbox. For non-flit-aware
+        components (default), Flits are accumulated by parent Transaction
+        and only the reassembled Transaction is placed on _inbox once
+        ``is_last`` arrives. Step is updated to this component's path
+        position for legacy step-based routing."""
+        from kernbench.sim_engine.transaction import Flit
+
+        if self._FLIT_AWARE:
+            while True:
+                msg = yield port.get()
+                yield self._inbox.put(msg)
+            return
+
+        flit_buffers: dict[int, list[Any]] = {}
         while True:
             msg = yield port.get()
-            yield self._inbox.put(msg)
+            if isinstance(msg, Flit):
+                tid = id(msg.txn)
+                flit_buffers.setdefault(tid, []).append(msg)
+                if msg.is_last:
+                    flit_buffers.pop(tid, None)
+                    self._update_step(msg.txn)
+                    yield self._inbox.put(msg.txn)
+            else:
+                yield self._inbox.put(msg)
+
+    def _update_step(self, txn: Any) -> None:
+        """Set txn.step to this component's index in txn.path (if found).
+        Allows legacy step-based routing to work even when flit-aware
+        upstream components don't call txn.advance()."""
+        my_id = self.node.id
+        path = getattr(txn, "path", None)
+        if not path:
+            return
+        for i, n in enumerate(path):
+            if n == my_id:
+                txn.step = i
+                return
 
     def _worker(self, env: simpy.Environment) -> Generator:
         """Generic forwarding worker: spawns _forward_txn per message (pipeline)."""

src/kernbench/sim_engine/engine.py

@@ -11,7 +11,7 @@ from kernbench.components.context import ComponentContext
 from kernbench.policy.address.phyaddr import PhysAddr
 from kernbench.policy.routing.router import AddressResolver, PathRouter
 from kernbench.runtime_api.kernel import KernelLaunchMsg, MemoryReadMsg, MemoryWriteMsg, PeDmaMsg
-from kernbench.sim_engine.transaction import Transaction
+from kernbench.sim_engine.transaction import Flit, Transaction
 from kernbench.topology.types import Edge, TopologyGraph
 
 
@@ -41,6 +41,14 @@ class GraphEngine:
         for e in graph.edges:
             self._edge_map[(e.src, e.dst)] = e
         self._ns_per_mm: float = graph.spec.get("system", {}).get("ns_per_mm", 0.01)
+        # ADR-0033 Phase 2c-1: wire chunkifies into Flits (Phase 2c-2/3
+        # will graduate to per-flit timing + flit-aware components). At
+        # 2c-1 stage all flits of a Transaction are emitted atomically
+        # at the same env.now to preserve current single-msg timing —
+        # Flit transport is in place but behaviorally equivalent.
+        self._flit_bytes: int = int(
+            graph.spec.get("system", {}).get("flit_bytes", 256)
+        )
         self._results: dict[str, tuple[Completion, Trace]] = {}
         self._events: dict[str, simpy.Event] = {}
         self._counter = 0
@@ -259,18 +267,26 @@ class GraphEngine:
         available_at = 0.0
         while True:
             msg = yield out_port.get()
+            # ADR-0033 Phase 2c-1: chunkify Transactions into Flits but
+            # emit atomically (same env.now) to preserve current timing.
+            # Phase 2c-2 will graduate to per-flit timing.
+            if isinstance(msg, Transaction) and msg.nbytes > 0:
+                items = list(msg.into_flits(self._flit_bytes))
+                payload_nbytes = msg.nbytes
+            else:
+                items = [msg]
+                payload_nbytes = getattr(msg, "nbytes", 0) or 0
             # BW occupancy: wait for link to become free, then mark busy
-            if bw_gbs > 0:
-                nbytes = getattr(msg, "nbytes", 0)
-                if nbytes > 0:
-                    wait = available_at - self._env.now
-                    if wait > 0:
-                        yield self._env.timeout(wait)
-                    available_at = self._env.now + (nbytes / bw_gbs)
+            if bw_gbs > 0 and payload_nbytes > 0:
+                wait = available_at - self._env.now
+                if wait > 0:
+                    yield self._env.timeout(wait)
+                available_at = self._env.now + (payload_nbytes / bw_gbs)
             # Propagation delay
             if prop_ns > 0:
                 yield self._env.timeout(prop_ns)
-            yield in_port.put(msg)
+            for item in items:
+                yield in_port.put(item)
 
     def _process(self, key: str, request: Any, done: simpy.Event):
         if isinstance(request, PeDmaMsg):