[BugFix] fix VLLM_USE_STANDALONE_COMPILE=0 (#38015)

Signed-off-by: Richard Zou <zou3519@gmail.com>

tests/compile/test_aot_compile.py

@@ -441,6 +441,37 @@ def test_partition_wrapper_applied_on_aot_load(
         )
 
 
+@create_new_process_for_each_test("spawn")
+def test_standalone_compile_correctness():
+    """Outputs must match regardless of VLLM_USE_STANDALONE_COMPILE."""
+    import json
+
+    from ..utils import compare_two_settings
+
+    compilation_config = json.dumps(
+        {
+            "mode": CompilationMode.VLLM_COMPILE,
+        }
+    )
+
+    common_args = [
+        "--dtype",
+        "float16",
+        "--max-model-len",
+        "256",
+        "--compilation_config",
+        compilation_config,
+    ]
+
+    compare_two_settings(
+        "facebook/opt-125m",
+        common_args,
+        common_args,
+        env1={"VLLM_USE_STANDALONE_COMPILE": "1"},
+        env2={"VLLM_USE_STANDALONE_COMPILE": "0"},
+    )
+
+
 @pytest.mark.skipif(not is_torch_equal_or_newer("2.10.0"), reason="requires torch 2.10")
 @create_new_process_for_each_test("spawn")
 def test_gpt2_cache_hit(monkeypatch: pytest.MonkeyPatch):

vllm/compilation/compiler_interface.py

@@ -632,6 +632,23 @@ class InductorAdaptor(CompilerInterface):
             )
             stack.enter_context(_patch_constrain_to_fx_strides())
 
+            # Clear the tracing context before calling compile_fx.
+            # vLLM calls compile_fx from within a PiecewiseCompileInterpreter
+            # that runs under Dynamo's tracing context. The tracing context
+            # has a FakeTensorMode from Dynamo, but the example inputs for
+            # this subgraph have fake tensors from a different FakeTensorMode.
+            # compile_fx's _compile_fx_main calls detect_fake_mode() which
+            # asserts all FakeTensorModes match, causing a crash.
+            # Clearing the tracing context lets compile_fx create its own.
+            saved_tracing_context = torch._guards.TracingContext.try_get()
+            if saved_tracing_context is not None:
+                torch._guards._TLS.tracing_context = None
+
+                def _restore_tracing_context():
+                    torch._guards._TLS.tracing_context = saved_tracing_context
+
+                stack.callback(_restore_tracing_context)
+
             compiled_graph = compile_fx(
                 graph,
                 example_inputs,