Add API to get backends required by a method

Author: kirklandsign

extension/android/executorch_android/src/androidTest/java/org/pytorch/executorch/ModuleE2ETest.java

@@ -8,6 +8,7 @@
 
 package org.pytorch.executorch;
 
+import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.assertFalse;
@@ -89,6 +90,13 @@ public void testClassification(String filePath) throws IOException, URISyntaxExc
         assertEquals(bananaClass, argmax(scores));
     }
 
+    @Test
+    public void testXnnpackBackendRequired() {
+        Module module = Module.load(getTestFilePath(filePath));
+        String[] expectedBackends = new String[] {"xnnpack"};
+        assertArrayEquals(expectedBackends, module.getUsedBackends("forward"));
+    }
+
     @Test
     public void testMv2Fp32() throws IOException, URISyntaxException {
         testClassification("/mv2_xnnpack_fp32.pte");

extension/android/executorch_android/src/main/java/org/pytorch/executorch/Module.java

@@ -137,6 +137,16 @@ public int loadMethod(String methodName) {
     }
   }
 
+  /**
+   * Returns the names of the methods in a certain method.
+   *
+   * @param methodName method name to query
+   * @return an array of backend name
+   */
+  public String[] getUsedBackends(String methodName) {
+    return mNativePeer.getUsedBackends(methodName);
+  }
+
   /** Retrieve the in-memory log buffer, containing the most recent ExecuTorch log entries. */
   public String[] readLogBuffer() {
     return mNativePeer.readLogBuffer();

extension/android/executorch_android/src/main/java/org/pytorch/executorch/NativePeer.java

@@ -55,6 +55,11 @@ public void resetNative() {
   @DoNotStrip
   public native int loadMethod(String methodName);
 
+  /** Return the list of backends used by a method */
+  @DoNotStrip
+  public native String[] getUsedBackends(String methodName);
+
+
   /** Retrieve the in-memory log buffer, containing the most recent ExecuTorch log entries. */
   @DoNotStrip
   public native String[] readLogBuffer();

extension/android/jni/jni_layer.cpp

@@ -395,13 +395,27 @@ class ExecuTorchJni : public facebook::jni::HybridClass<ExecuTorchJni> {
 #endif
   }
 
+  facebook::jni::local_ref<facebook::jni::JArrayClass<jstring>>
+  getUsedBackends(facebook::jni::alias_ref<jstring> methodName) {
+    auto methodMeta = module_->method_meta(methodName->toStdString()).get();
+    facebook::jni::local_ref<facebook::jni::JArrayClass<jstring>> ret
+    = facebook::jni::JArrayClass<jstring>::newArray(methodMeta.num_backends());
+    for (auto i = 0; i < methodMeta.num_backends(); i++) {
+      facebook::jni::local_ref<facebook::jni::JString> backend_name =
+      facebook::jni::make_jstring(methodMeta.get_backend_name(i).get());
+      (*ret)[i] = backend_name;
+    }
+    return ret;
+  }
+
   static void registerNatives() {
     registerHybrid({
         makeNativeMethod("initHybrid", ExecuTorchJni::initHybrid),
         makeNativeMethod("forward", ExecuTorchJni::forward),
         makeNativeMethod("execute", ExecuTorchJni::execute),
         makeNativeMethod("loadMethod", ExecuTorchJni::load_method),
         makeNativeMethod("readLogBuffer", ExecuTorchJni::readLogBuffer),
+        makeNativeMethod("getUsedBackends", ExecuTorchJni::getUsedBackends),
     });
   }
 };