Fix PRNG handling in nn.jit under nn.scan.

* `nn.scan` does an abstract eval before compilation to check for constants that are then traced out. Before this change, the abstract eval increments static RNG counters, which creates a side-effect where RNG counters are not properly updated once inner functions are jitted (i.e., under `nn.jit`).
* In this fix, we cache the impact a first pass through `nn.scan` and `nn.jit` would have on rng counters and "replay" that impact on subsequent passes so that rng state remains unaffected.
* This solution doesn't affect PRNG derivations outside this isolated case and is a placeholder while a more permanent solution, which would affect PRNG derivations, is worked out.

PiperOrigin-RevId: 694463650

Author: danielsuo

flax/core/lift.py

@@ -24,6 +24,7 @@
 import warnings
 
 from flax import traceback_util
+from flax import traverse_util
 from flax.typing import (
     In,
     InOutAxis,
@@ -1499,6 +1500,81 @@ def _hashable_filter(x):
   return x
 
 
+class CountsHolder:
+
+  def __init__(self, flat_d):
+    self.flat_d = flat_d
+
+  @classmethod
+  def make(cls, d):
+    flat_d = traverse_util.flatten_dict(d)
+    flat_d = {k: v for k, v in flat_d.items()}
+    return cls(flat_d)
+
+  def sub(self, other):
+    delta_flat_d = {}
+    new_flat_d = collections.defaultdict(int, self.flat_d)
+    old_flat_d = collections.defaultdict(int, other.flat_d)
+    for k in new_flat_d:
+      delta_flat_d[k] = new_flat_d[k] - old_flat_d[k]
+    return CountsHolder(delta_flat_d)
+
+  def add(self, other):
+    delta_flat_d = {}
+    new_flat_d = collections.defaultdict(int, self.flat_d)
+    old_flat_d = collections.defaultdict(int, other.flat_d)
+    for k in new_flat_d:
+      delta_flat_d[k] = new_flat_d[k] + old_flat_d[k]
+    return CountsHolder(delta_flat_d)
+
+  def unflat(self):
+    return traverse_util.unflatten_dict(self.flat_d)
+
+
+def set_from_dict(original, updates):
+  for k in updates:
+    if k not in original:
+      original[k] = updates[k]
+    else:
+      if isinstance(updates[k], dict):
+        set_from_dict(original[k], updates[k])
+      else:
+        original[k] = updates[k]
+
+
+class _SideEffectCache(threading.local):
+
+  def __init__(self):
+    self.cache = {}
+
+
+_side_effect_cache = _SideEffectCache()
+
+
+def _restore_rng_counters(scopes, fingerprint, capture_old_counts):
+  if fingerprint not in _side_effect_cache.cache:
+    capture_new_counts = jax.tree.map(
+        lambda s: CountsHolder.make(s.rng_counters), scopes
+    )
+    capture_delta_counts = jax.tree.map(
+        lambda old, new: new.sub(old),
+        capture_old_counts,
+        capture_new_counts,
+    )
+    _side_effect_cache.cache[fingerprint] = capture_delta_counts
+  else:
+    updated_counts = jax.tree.map(
+        lambda x, y: x.add(y).unflat(),
+        _side_effect_cache.cache[fingerprint],
+        capture_old_counts,
+    )
+    jax.tree.map(
+        lambda s, u: set_from_dict(s.rng_counters, u),
+        scopes,
+        updated_counts,
+    )
+
+
 def jit(
     fn: Callable[..., Any],
     variables: CollectionFilter = True,
@@ -1599,13 +1675,18 @@ def inner(
       mutable = tuple(_hashable_filter(scope.mutable) for scope in scopes)
 
       rng_groups = jax.tree.map(
-          lambda x: x.fold() if isinstance(x, LazyRng) else x,
+          lambda x: x.clear_suffix() if isinstance(x, LazyRng) else x,
           rng_groups,
           is_leaf=lambda x: isinstance(x, LazyRng),
       )
 
       fingerprint = (mutable, module_hash_key)
-      return jitted(fingerprint, variable_groups, rng_groups, *args, **kwargs)
+      capture_old_counts = jax.tree.map(
+          lambda s: CountsHolder.make(s.rng_counters), scopes
+      )
+      res = jitted(fingerprint, variable_groups, rng_groups, *args, **kwargs)
+      _restore_rng_counters(scopes, fingerprint, capture_old_counts)
+      return res
 
   return pack(
       inner, (variables,), (variables,), (rngs,), name='jit', enable_kwargs=True
@@ -1692,3 +1773,64 @@ def inner_loop(scope, carry):
 def _unzip2(xs):
   ys = tuple(zip(*xs))
   return ys if ys else ((), ())
+
+
+def fold_rngs(
+    fn: Callable[..., Any],
+    variables: CollectionFilter = True,
+    rngs: PRNGSequenceFilter = True,
+) -> Callable[..., Any]:
+  # Close over scope_fn & repack_fn to avoid recompilation
+  # this is impure but we use the fingerprint arg to differentiate between cases
+  # where scope_fn or repack_fn actually produce non-identical results.
+  fold_rngs_context = TransformContext[tuple[Callable, Callable]]()
+
+  @functools.wraps(fn)
+  def wrapped_fold_rngs(fingerprint, variable_groups, rng_groups, *args, **kwargs):
+    scope_fn, repack_fn = fold_rngs_context.get()
+    hash_key = fingerprint[1]
+    # fingerprint is only used to differentiate the cache signature
+    # del fingerprint
+    scope = scope_fn(variable_groups, rng_groups)  # pylint: disable=not-callable
+    y = fn(scope, hash_key, *args, **kwargs)
+    return y, repack_fn(scope)  # pylint: disable=not-callable
+
+  def inner_fold_rngs(
+      scope_fn,
+      repack_fn,
+      variable_groups,
+      rng_groups,
+      module_hash_key,
+      *args,
+      **kwargs,
+  ):
+    with fold_rngs_context.push((scope_fn, repack_fn)):
+      scopes: list[Scope] = jax.tree_util.tree_leaves(
+          scope_fn(variable_groups, rng_groups)
+      )
+      mutable = tuple(_hashable_filter(scope.mutable) for scope in scopes)
+
+      rng_groups = jax.tree.map(
+          lambda x: x.clear_suffix() if isinstance(x, LazyRng) else x,
+          rng_groups,
+          is_leaf=lambda x: isinstance(x, LazyRng),
+      )
+
+      fingerprint = (mutable, module_hash_key)
+      capture_old_counts = jax.tree.map(
+          lambda s: CountsHolder.make(s.rng_counters), scopes
+      )
+      res = wrapped_fold_rngs(
+          fingerprint, variable_groups, rng_groups, *args, **kwargs
+      )
+      _restore_rng_counters(scopes, fingerprint, capture_old_counts)
+      return res
+
+  return pack(
+      inner_fold_rngs,
+      (variables,),
+      (variables,),
+      (rngs,),
+      name='fold_rngs',
+      enable_kwargs=True,
+  )

flax/core/scope.py

@@ -102,8 +102,8 @@ def create(
     else:
       return LazyRng(rng, suffix)
 
-  def fold(self):
-    key = self.as_jax_rng()
+  def clear_suffix(self):
+    key = self.rng
     return LazyRng(key, ())
 
 
@@ -583,13 +583,6 @@ def default_name(self, prefix: str) -> str:
         return name
       i += 1
 
-  def fold_rngs(self):
-    """Folds the rngs of this scope into the parent scope."""
-    self._check_valid()
-    for name, rng in self.rngs.items():
-      assert isinstance(rng, LazyRng)
-      self.rngs[name] = rng.fold()
-
   def push(
     self, name: str | None = None, prefix: str = '', reuse=False
   ) -> 'Scope':

flax/linen/__init__.py

@@ -72,8 +72,8 @@
 from .batch_apply import BatchApply as BatchApply
 from .combinators import Sequential as Sequential
 from .fp8_ops import (
-    Fp8DotGeneralOp as Fp8DotGeneralOp,
     Fp8DirectDotGeneralOp as Fp8DirectDotGeneralOp,
+    Fp8DotGeneralOp as Fp8DotGeneralOp,
     NANOOFp8DotGeneralOp as NANOOFp8DotGeneralOp,
 )
 from .initializers import (
@@ -95,8 +95,8 @@
     Module as Module,
     Variable as Variable,
     apply as apply,
-    compact as compact,
     compact_name_scope as compact_name_scope,
+    compact as compact,
     disable_named_call as disable_named_call,
     enable_named_call as enable_named_call,
     init_with_output as init_with_output,
@@ -114,19 +114,19 @@
     LayerNorm as LayerNorm,
     RMSNorm as RMSNorm,
     SpectralNorm as SpectralNorm,
-    WeightNorm as WeightNorm
+    WeightNorm as WeightNorm,
 )
 from .pooling import (avg_pool as avg_pool, max_pool as max_pool, pool as pool)
 from .recurrent import (
     Bidirectional as Bidirectional,
     ConvLSTMCell as ConvLSTMCell,
-    SimpleCell as SimpleCell,
     GRUCell as GRUCell,
-    MGUCell as MGUCell,
     LSTMCell as LSTMCell,
+    MGUCell as MGUCell,
     OptimizedLSTMCell as OptimizedLSTMCell,
     RNNCellBase as RNNCellBase,
     RNN as RNN,
+    SimpleCell as SimpleCell,
 )
 from .spmd import (
     LogicallyPartitioned as LogicallyPartitioned,
@@ -146,6 +146,8 @@
     checkpoint as checkpoint,
     cond as cond,
     custom_vjp as custom_vjp,
+    fold_rngs as fold_rngs,
+    grad as grad,
     jit as jit,
     jvp as jvp,
     map_variables as map_variables,
@@ -154,9 +156,8 @@
     remat as remat,
     scan as scan,
     switch as switch,
-    vjp as vjp,
-    grad as grad,
     value_and_grad as value_and_grad,
+    vjp as vjp,
     vmap as vmap,
     while_loop as while_loop,
 )