Canonicalize platform name for compile-only backends

jax/_src/interpreters/mlir.py

@@ -1084,7 +1084,10 @@ def lower_jaxpr_to_module(
   Handles the quirks of the argument/return value passing conventions of the
   runtime.
   """
-  platforms = tuple(map(xb.canonicalize_platform, platforms))
+  backend = (backend_or_name if isinstance(backend_or_name, xb.XlaBackend) else
+             None)
+  platforms = tuple(map(lambda p: xb.canonicalize_platform(p, backend),
+                        platforms))
 
   in_avals = (jaxpr.in_avals if arg_shardings is None else
               map(sharded_aval, jaxpr.in_avals, arg_shardings))

jax/_src/xla_bridge.py

@@ -34,6 +34,7 @@
 import traceback
 from typing import Any, Union
 import warnings
+import weakref
 
 from jax._src import config
 from jax._src import distributed
@@ -191,6 +192,8 @@ class BackendRegistration:
 _backend_factories: dict[str, BackendRegistration] = {}
 _default_backend: xla_client.Client | None = None
 _backends : dict[str, xla_client.Client] = {}
+_WeakClientToStrDict = weakref.WeakKeyDictionary[xla_client.Client, str]
+_compile_only_backend_canonical_platform = _WeakClientToStrDict()
 _backend_errors : dict[str, str] = {}
 _backend_lock = threading.Lock()
 _plugins_registered: bool = False
@@ -226,6 +229,11 @@ def register_backend_factory(name: str, factory: BackendFactory, *,
   if make_topology is not None:
     _topology_factories[name] = make_topology
 
+def register_compile_only_backend(platform: str, client: xla_client.Client):
+  # Register the compile-only backend-platform mapping as long as
+  # the platform is not an alias.
+  if _alias_to_platforms.get(platform, None) == None:
+    _compile_only_backend_canonical_platform[client] = platform
 
 def make_cpu_client(
     collectives: xla_client._xla.CpuCollectives | None = None,
@@ -813,8 +821,8 @@ def is_known_platform(platform: str) -> bool:
   # we've heard of it and it isn't, e.g., a typo.
   return platform in known_platforms()
 
-
-def canonicalize_platform(platform: str) -> str:
+def canonicalize_platform(platform: str,
+                          backend: xla_client.Client | None = None) -> str:
   """Replaces platform aliases with their concrete equivalent.
 
   In particular, replaces "gpu" with either "cuda" or "rocm", depending on which
@@ -830,6 +838,12 @@ def canonicalize_platform(platform: str) -> str:
   for p in platforms:
     if p in b.keys():
       return p
+
+  if (backend and
+      backend.platform == platform and
+      backend in _compile_only_backend_canonical_platform):
+    return _compile_only_backend_canonical_platform[backend]
+
   raise RuntimeError(f"Unknown backend: '{platform}' requested, but no "
                      f"platforms that are instances of {platform} are present. "
                      "Platforms present are: " + ",".join(b.keys()))

jax/experimental/topologies.py

@@ -45,7 +45,10 @@ def get_topology_desc(
     )
   try:
     topology = xb.make_pjrt_topology(platform, topology_name, **kwargs)
-    return TopologyDescription(topology._make_compile_only_devices())
+    devices = topology._make_compile_only_devices()
+    if platform:
+      xb.register_compile_only_backend(platform, devices[0].client)
+    return TopologyDescription(devices)
   except xla_extension.XlaRuntimeError as e:
     msg, *_ = e.args
     if msg.startswith("UNIMPLEMENTED"):

tests/BUILD

@@ -1055,6 +1055,19 @@ jax_multiplatform_test(
     ],
 )
 
+jax_multiplatform_test(
+    name = "deviceless_aot_test",
+    srcs = ["deviceless_aot_test.py"],
+    enable_backends = [ "gpu" ],
+    enable_configs = [
+        "gpu_a100",
+    ],
+    deps = [
+        "//jax:experimental",
+    ] + py_deps("numpy"),
+)
+
+
 jax_multiplatform_test(
     name = "sparsify_test",
     srcs = ["sparsify_test.py"],

tests/deviceless_aot_test.py

@@ -0,0 +1,88 @@
+# Copyright 2024 The JAX Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Tests for deviceless AOT compilation on GPU."""
+
+from absl.testing import absltest
+import jax
+from jax._src import test_util as jtu
+from jax.experimental import topologies
+from jax.experimental.serialize_executable import (
+    deserialize_and_load,
+    serialize,
+)
+import jax.numpy as jnp
+from jax.lib import xla_client as xc
+import multiprocessing
+
+class DevicelessGpuAotTest(jtu.JaxTestCase):
+
+  def compile_worker(target_config):
+    # Run without GPU.
+    jax.config.update("jax_platforms", "cpu")
+
+    topo = topologies.get_topology_desc(
+      "topo",
+      "cuda",
+      target_config=target_config,
+      topology="1x1x1")
+
+    sharding = jax.sharding.SingleDeviceSharding(topo.devices[0])
+
+    # Function to compile.
+    @jax.jit
+    def fn(x):
+      return jnp.sum(x * x)
+
+    # Provide input shape(s).
+    x_shape = jax.ShapeDtypeStruct(
+              shape=(2, 2),
+              dtype=jnp.dtype('float32'),
+              sharding=sharding)
+
+    # Lower and compile.
+    compiled = fn.lower(x_shape).compile()
+
+    # Serialize the compilation results.
+    serialized, in_tree, out_tree = serialize(compiled)
+
+    return serialized
+
+  @jtu.skip_under_pytest("Test must run in an isolated process")
+  def test_serialize_deserialize_execute(self):
+    target_config = xc.get_topology_for_devices(jax.devices()).target_config
+
+    # Call the compilation in a different process so that we
+    # can start JAX without the GPU platform there.
+    multiprocessing.set_start_method("spawn")
+    pool = multiprocessing.Pool(processes = 1)
+    [serialized] = pool.map(DevicelessGpuAotTest.compile_worker,
+                            [target_config])
+    pool.close()
+
+    # Provide the input pytree structure (0 stands for leaf).
+    _, in_tree = jax.tree_util.tree_flatten(((0,), {}))
+    # Provide the output pytree structure (here just one JAX array).
+    _, out_tree = jax.tree_util.tree_flatten(0)
+    # Deserialize the function.
+    compiled = deserialize_and_load(serialized, in_tree, out_tree)
+
+
+    # Call the deserialized function.
+    result = compiled(jnp.array([[0., 1.], [2., 3.]]))
+
+    self.assertEqual(result, 14)
+
+
+if __name__ == '__main__':
+  absltest.main(testLoader=jtu.JaxTestLoader())