Fix for https://github.com/patrick-kidger/optimistix/issues/48

equinox/internal/_loop/bounded.py

@@ -32,7 +32,7 @@ def bounded_while_loop(
     - `body_fun`: As `lax.while_loop`.
     - `init_val`: As `lax.while_loop`.
     - `max_steps`: A bound on the maximum number of steps, after which the loop
-        terminates unconditionally. Can be set to `None` for arbitrarily many steps.
+        terminates unconditionally.
     - `buffers`: If passed, then every leaf of `tree_leaves(buffers(init_val))` must
         be an array; all such arrays become buffers supporting only `[]` and
         `.at[].set()`. However they will act efficiently, without spurious copies.
@@ -43,7 +43,6 @@ def bounded_while_loop(
     **Returns:**
 
     The final value; as `lax.while_loop`.
-
     """
 
     if not isinstance(max_steps, int) or max_steps < 0:

equinox/internal/_loop/common.py

@@ -429,7 +429,17 @@ def new_cond_fun(val):
             if type(max_steps) is not int:
                 raise ValueError("`max_steps` must be a Python integer")
             out = out & (step < max_steps)
-        return nonbatchable(out)
+        # Need to allow being constant across the batch. This seems to arise in some
+        # edge case when using `bounded_while_loop`, in which:
+        # - `lax.scan` saves a copy of its state (in particular `step`) for use in the
+        #    backward pass;
+        # - for some reason the `jax.checkpoint` causes such state to pick up a spurious
+        #   batch tracer.
+        # See:
+        # https://github.com/patrick-kidger/optimistix/issues/48#issuecomment-2009221739
+        return nonbatchable(
+            out, name="`equinox.internal.while_loop`", allow_constant_across_batch=True
+        )
 
     def new_body_fun(val):
         tag = object()

equinox/internal/_nontraceable.py

@@ -9,6 +9,7 @@
 import jax.interpreters.ad as ad
 import jax.interpreters.batching as batching
 import jax.interpreters.mlir as mlir
+import jax.numpy as jnp
 import jax.tree_util as jtu
 from jaxtyping import PyTree
 
@@ -122,34 +123,59 @@ def nondifferentiable_backward(
     return combine(jtu.tree_unflatten(treedef, flat), static)
 
 
-def _cannot_batch(x, b, *, msg):
+def _cannot_batch(x, b, *, msg, allow_constant_across_batch):
     (x,) = x
     (b,) = b
     if b is batching.not_mapped:
         return x, b
     else:
-        raise ValueError(msg)
+        if allow_constant_across_batch:
+            x = error_if(x, jnp.min(x) != jnp.max(x), msg)
+            return x[0], batching.not_mapped
+        else:
+            raise ValueError(msg)
 
 
 nonbatchable_p = jax.core.Primitive("nonbatchable")
-nonbatchable_p.def_impl(lambda x, *, msg: x)
-nonbatchable_p.def_abstract_eval(lambda x, *, msg: x)
+nonbatchable_p.def_impl(lambda x, *, msg, allow_constant_across_batch: x)
+nonbatchable_p.def_abstract_eval(lambda x, *, msg, allow_constant_across_batch: x)
 batching.primitive_batchers[nonbatchable_p] = _cannot_batch
 mlir.register_lowering(
-    nonbatchable_p, mlir.lower_fun(lambda x, *, msg: x, multiple_results=False)
+    nonbatchable_p,
+    mlir.lower_fun(
+        lambda x, *, msg, allow_constant_across_batch: x, multiple_results=False
+    ),
 )
 
 
 def nonbatchable(
-    x: PyTree, *, name: Optional[str] = None, msg: Optional[str] = None
+    x: PyTree,
+    *,
+    name: Optional[str] = None,
+    msg: Optional[str] = None,
+    allow_constant_across_batch: bool = False,
 ) -> PyTree:
     """Identity function. Raises a trace-time assert if it is batched."""
     dynamic, static = partition(x, is_array)
     flat, treedef = jtu.tree_flatten(dynamic)
     if msg is None:
         if name is None:
             name = "This operation"
-        msg = f"Unexpected batch tracer. {name} cannot be vmap'd."
-    bind = ft.partial(nonbatchable_p.bind, msg=msg)
+        if allow_constant_across_batch:
+            msg = (
+                f"Nonconstant batch. {name} has received a batch of values that were "
+                "expected to be constant. This is probably an internal error in the "
+                "library you are using."
+            )
+        else:
+            msg = (
+                f"Unexpected batch tracer. {name} cannot be vmap'd. This is probably "
+                "an internal error in the library you are using."
+            )
+    bind = ft.partial(
+        nonbatchable_p.bind,
+        msg=msg,
+        allow_constant_across_batch=allow_constant_across_batch,
+    )
     flat = map(bind, flat)
     return combine(jtu.tree_unflatten(treedef, flat), static)