diff --git a/jax/interpreters/xla.py b/jax/interpreters/xla.py
index 35419e85c0d3..9eb8bf2278dc 100644
--- a/jax/interpreters/xla.py
+++ b/jax/interpreters/xla.py
@@ -181,17 +181,20 @@ def stage_lazy_expr(c, lazy_expr, x):
 ArgSpec = namedtuple("ArgSpec", ["aval", "lazy_expr", "xla_shape"])
 
 def arg_spec(x):
+  # For performance reasons, the return type of this function is polymorphic:
+  # if the input has a lazy_expr attached, the return type is a raw tuple
+  # modeling an ArgSpec namedtuple; otherwise, it's an AbstractValue.
   aval = abstractify(x)
   try:
     lazy_expr = x._lazy_expr
   except AttributeError:
-    return ArgSpec(aval, None, aval_to_xla_shape(aval))
+    return aval
   else:
     if x.device_buffer is device_constant:
       xla_shape = None
     else:
       xla_shape = x.device_buffer.shape()
-    return ArgSpec(aval, x._lazy_expr, xla_shape)
+    return aval, x._lazy_expr, xla_shape
 
 
 ### handlers
@@ -282,6 +285,12 @@ def apply_primitive(prim, *args, **params):
 
 @cache()
 def xla_primitive_callable(prim, *arg_specs, **params):
+  # For performance, we allow the elements of arg_specs either to be triples
+  # modeling ArgSpec instances or abstract values. Here we canonicalize them.
+  arg_specs = [ArgSpec(x, None, aval_to_xla_shape(x))
+               if isinstance(x, core.AbstractValue) else ArgSpec(*x)
+               for x in arg_specs]
+
   if FLAGS.jax_log_compiles:
     print("Compiling {} for args {}.".format(prim.name, arg_specs))
   backend = params.get('backend', None)