sim: only preempt testbenches on explicit wait.

Before this commit, testbenches (generators added with `add_testbench`)
were not only preemptible after any `yield`, but were *guaranteed* to
be preempted by another testbench after *every* yield. This is evil:
if you have any race condition between testbenches, which is common,
this scheduling strategy will maximize the resulting nondeterminism by
interleaving your testbench with every other one as much as possible.
This behavior is an outcome of the way `add_testbench` is implemented,
which is by yielding `Settle()` after every command.

One can observe that:
- `yield value_like` should never preempt;
- `yield assignable.eq()` in `add_process()` should not preempt, since
  it only sets a `next` signal state, or appends to `write_queue` of
  a memory state, and never wakes up processes;
- `yield assignable.eq()` in `add_testbench()` should only preempt if
  changing `assignable` wakes up an RTL process. (It could potentially
  also preempt if that wakes up another testbench, but this has no
  benefit and requires `sim.set()` from RFC 36 to be awaitable, which
  is not desirable.)

After this commit, `PySimEngine._step()` is implemented with two nested
loops instead of one. The outer loop iterates through every testbench
and runs it until an explicit wait point (`Settle()`, `Delay()`, or
`Tick()`), terminating when no testbenches are runnable. The inner loop
is the usual eval/commit loop, running whenever a testbench changes
design state.

`PySimEngine._processes` is a `set`, which doesn't have a deterministic
iteration order. This does not matter for processes, where determinism
is guaranteed by the eval/commit loop, but causes racy testbenches to
pass or fail nondeterministically (in practice depending on the memory
layout of the Python process). While it is best to not have races in
the testbenches, this commit makes `PySimEngine._testbenches` a `list`,
making the outcome of a race deterministic, and enabling a hacky work-
around to make them work: reordering calls to `add_testbench()`.

A potential future improvement is a simulation mode that, instead,
randomizes the scheduling of testbenches, exposing race conditions
early.

amaranth/sim/_base.py

@@ -65,10 +65,13 @@ class BaseSimulation:
 
 
 class BaseEngine:
+    def add_clock_process(self, clock, *, phase, period):
+        raise NotImplementedError # :nocov:
+
     def add_coroutine_process(self, process, *, default_cmd):
         raise NotImplementedError # :nocov:
 
-    def add_clock_process(self, clock, *, phase, period):
+    def add_testbench_process(self, process):
         raise NotImplementedError # :nocov:
 
     def reset(self):

amaranth/sim/_pycoro.py

@@ -1,7 +1,7 @@
 import inspect
 
 from ..hdl import *
-from ..hdl._ast import Statement, SignalSet, ValueCastable
+from ..hdl._ast import Statement, Assign, SignalSet, ValueCastable
 from ..hdl._mem import MemorySimRead, MemorySimWrite
 from .core import Tick, Settle, Delay, Passive, Active
 from ._base import BaseProcess, BaseMemoryState
@@ -12,11 +12,12 @@ __all__ = ["PyCoroProcess"]
 
 
 class PyCoroProcess(BaseProcess):
-    def __init__(self, state, domains, constructor, *, default_cmd=None):
+    def __init__(self, state, domains, constructor, *, default_cmd=None, testbench=False):
         self.state = state
         self.domains = domains
         self.constructor = constructor
         self.default_cmd = default_cmd
+        self.testbench = testbench
 
         self.reset()
 
@@ -70,7 +71,7 @@ class PyCoroProcess(BaseProcess):
             except StopIteration:
                 self.passive = True
                 self.coroutine = None
-                return
+                return False # no assignment
 
             try:
                 if command is None:
@@ -88,6 +89,8 @@ class PyCoroProcess(BaseProcess):
                 elif isinstance(command, Statement):
                     exec(_StatementCompiler.compile(self.state, command),
                         self.exec_locals)
+                    if isinstance(command, Assign) and self.testbench:
+                        return True # assignment; run a delta cycle
 
                 elif type(command) is Tick:
                     domain = command.domain
@@ -102,17 +105,20 @@ class PyCoroProcess(BaseProcess):
                     self.add_trigger(domain.clk, trigger=1 if domain.clk_edge == "pos" else 0)
                     if domain.rst is not None and domain.async_reset:
                         self.add_trigger(domain.rst, trigger=1)
-                    return
+                    return False # no assignments
+
+                elif self.testbench and (command is None or isinstance(command, Settle)):
+                    raise TypeError(f"Command {command!r} is not allowed in testbenches")
 
                 elif type(command) is Settle:
                     self.state.wait_interval(self, None)
-                    return
+                    return False # no assignments
 
                 elif type(command) is Delay:
                     # Internal timeline is in 1ps integeral units, intervals are public API and in floating point
                     interval = int(command.interval * 1e12) if command.interval is not None else None
                     self.state.wait_interval(self, interval)
-                    return
+                    return False # no assignments
 
                 elif type(command) is Passive:
                     self.passive = True
@@ -140,6 +146,8 @@ class PyCoroProcess(BaseProcess):
                     state = self.state.slots[index]
                     assert isinstance(state, BaseMemoryState)
                     state.write(addr, data)
+                    if self.testbench:
+                        return True # assignment; run a delta cycle
 
                 elif command is None: # only possible if self.default_cmd is None
                     raise TypeError("Received default command from process {!r} that was added "

amaranth/sim/core.py

@@ -115,33 +115,7 @@ class Simulator:
         self._engine.add_coroutine_process(wrapper, default_cmd=Tick(domain))
 
     def add_testbench(self, process):
-        process = self._check_process(process)
-        def wrapper():
-            generator = process()
-            # Only start a bench process after power-on reset finishes. Use object.__new__ to
-            # avoid deprecation warning.
-            yield object.__new__(Settle)
-            result = None
-            exception = None
-            while True:
-                try:
-                    if exception is None:
-                        command = generator.send(result)
-                    else:
-                        command = generator.throw(exception)
-                except StopIteration:
-                    break
-                if command is None or isinstance(command, Settle):
-                    exception = TypeError(f"Command {command!r} is not allowed in testbenches")
-                else:
-                    try:
-                        result = yield command
-                        exception = None
-                        yield object.__new__(Settle)
-                    except Exception as e:
-                        result = None
-                        exception = e
-        self._engine.add_coroutine_process(wrapper, default_cmd=None)
+        self._engine.add_testbench_process(self._check_process(process))
 
     def add_clock(self, period, *, phase=None, domain="sync", if_exists=False):
         """Add a clock process.

amaranth/sim/pysim.py

@@ -409,24 +409,27 @@ class PySimEngine(BaseEngine):
 
         self._design = design
         self._processes = _FragmentCompiler(self._state)(self._design.fragment)
+        self._testbenches = []
         self._vcd_writers = []
 
+    def add_clock_process(self, clock, *, phase, period):
+        self._processes.add(PyClockProcess(self._state, clock,
+                                           phase=phase, period=period))
+
     def add_coroutine_process(self, process, *, default_cmd):
         self._processes.add(PyCoroProcess(self._state, self._design.fragment.domains, process,
                                           default_cmd=default_cmd))
 
-    def add_clock_process(self, clock, *, phase, period):
-        self._processes.add(PyClockProcess(self._state, clock,
-                                           phase=phase, period=period))
+    def add_testbench_process(self, process):
+        self._testbenches.append(PyCoroProcess(self._state, self._design.fragment.domains, process,
+                                               testbench=True))
 
     def reset(self):
         self._state.reset()
         for process in self._processes:
             process.reset()
 
-    def _step(self):
-        changed = set() if self._vcd_writers else None
-
+    def _step_rtl(self, changed):
         # Performs the two phases of a delta cycle in a loop:
         converged = False
         while not converged:
@@ -439,6 +442,25 @@ class PySimEngine(BaseEngine):
             # 2. commit: apply every queued signal change, waking up any waiting processes
             converged = self._state.commit(changed)
 
+    def _step_tb(self):
+        changed = set() if self._vcd_writers else None
+
+        # Run processes waiting for an interval to expire (mainly `add_clock_process()``)
+        self._step_rtl(changed)
+
+        # Run testbenches waiting for an interval to expire, or for a signal to change state
+        converged = False
+        while not converged:
+            converged = True
+            # Schedule testbenches in a deterministic, predictable order by iterating a list
+            for testbench in self._testbenches:
+                if testbench.runnable:
+                    testbench.runnable = False
+                    while testbench.run():
+                        # Testbench has changed simulation state; run processes triggered by that
+                        converged = False
+                        self._step_rtl(changed)
+
         for vcd_writer in self._vcd_writers:
             for change in changed:
                 if isinstance(change, _PySignalState):
@@ -452,9 +474,9 @@ class PySimEngine(BaseEngine):
                     assert False # :nocov:
 
     def advance(self):
-        self._step()
+        self._step_tb()
         self._timeline.advance()
-        return any(not process.passive for process in self._processes)
+        return any(not process.passive for process in (*self._processes, *self._testbenches))
 
     @property
     def now(self):

tests/test_sim.py

@@ -1174,6 +1174,22 @@ class SimulatorIntegrationTestCase(FHDLTestCase):
             Coverage hit at .*test_sim\.py:\d+: Counter: 009
         """).lstrip())
 
+    def test_testbench_preemption(self):
+        sig = Signal(8)
+        def testbench_1():
+            yield sig[0:4].eq(0b1010)
+            yield sig[4:8].eq(0b0101)
+        def testbench_2():
+            yield Passive()
+            while True:
+                val = yield sig
+                assert val in (0, 0b01011010), f"{val=:#010b}"
+                yield Delay(0)
+        sim = Simulator(Module())
+        sim.add_testbench(testbench_1)
+        sim.add_testbench(testbench_2)
+        sim.run()
+
 
 class SimulatorRegressionTestCase(FHDLTestCase):
     def test_bug_325(self):