Fix lowering bug in effectful batched cond and add tests

jax/_src/lax/control_flow/loops.py

@@ -1439,7 +1439,11 @@ def _while_lowering(ctx, *args, cond_jaxpr, body_jaxpr, cond_nconsts,
                           core.ordered_effects]
   if cond_ordered_effects:
     def cond(args):
-      return core.eval_jaxpr(cond_jaxpr.jaxpr, cond_jaxpr.consts, *args)[0]
+      # Pred can be batched
+      pred = core.eval_jaxpr(cond_jaxpr.jaxpr, cond_jaxpr.consts, *args)[0]
+      if batched:
+        pred = lax._reduce_or(pred, tuple(range(len(pred_aval.shape))))
+      return pred
     def body(args):
       return tuple(core.eval_jaxpr(body_jaxpr.jaxpr, body_jaxpr.consts, *args))
     def new_cond(pred_args):

tests/debugging_primitives_test.py

@@ -575,6 +575,66 @@ def _body(x):
       x: 10
       """))
 
+  @parameterized.named_parameters(jtu.cases_from_list(
+    dict(testcase_name="_ordered" if ordered else "", ordered=ordered)
+         for ordered in [False, True]))
+  @jtu.skip_on_devices(*disabled_backends)
+  def test_can_print_in_batched_while_cond(self, ordered):
+    def f(x):
+      def _cond(x):
+        debug_print("x: {x}", x=x, ordered=ordered)
+        return x < 5
+      def _body(x):
+        return x + 1
+      return lax.while_loop(_cond, _body, x)
+    with jtu.capture_stdout() as output:
+      jax.vmap(f)(jnp.arange(2))
+      jax.effects_barrier()
+    if ordered:
+      expected = _format_multiline("""
+      x: 0
+      x: 1
+      x: 1
+      x: 2
+      x: 2
+      x: 3
+      x: 3
+      x: 4
+      x: 4
+      x: 5
+      x: 5
+      x: 6
+      """)
+      self.assertEqual(output(), expected)
+    else:
+      # When the print is unordered, the `cond` is called an additional time
+      # after the `_body` runs, so we get more prints.
+      expected = _format_multiline("""
+      x: 0
+      x: 1
+      x: 0
+      x: 1
+      x: 1
+      x: 2
+      x: 1
+      x: 2
+      x: 2
+      x: 3
+      x: 2
+      x: 3
+      x: 3
+      x: 4
+      x: 3
+      x: 4
+      x: 4
+      x: 5
+      x: 4
+      x: 5
+      x: 5
+      x: 5
+      """)
+      self._assertLinesEqual(output(), expected)
+
   @parameterized.named_parameters(jtu.cases_from_list(
     dict(testcase_name="_ordered" if ordered else "", ordered=ordered)
          for ordered in [False, True]))