Mixed backend

lib/api/onnx-impl.ts

@@ -8,6 +8,7 @@ import {WebGLBackend} from '../backends/backend-webgl';
 import {Environment} from './env';
 import {envImpl} from './env-impl';
 import {Backend} from './onnx';
+import {MixedBackend} from '../backends/backend-mixed';
 
 export * from './env';
 export * from './onnx';
@@ -17,7 +18,8 @@ export * from './inference-session';
 export const backend: Backend = {
   cpu: new CpuBackend(),
   wasm: new WasmBackend(),
-  webgl: new WebGLBackend()
+  webgl: new WebGLBackend(),
+  mixed: new MixedBackend()
 };
 
 export const ENV: Environment = envImpl;

lib/backend.ts

@@ -82,7 +82,7 @@ const backendsCache: Map<string, Backend> = new Map();
  */
 export async function Backend(hint?: string|ReadonlyArray<string>): Promise<Backend> {
   if (!hint) {
-    return Backend(['webgl', 'wasm', 'cpu']);
+    return Backend(['webgl', 'mixed', 'wasm', 'cpu']);
   } else {
     const hints = typeof hint === 'string' ? [hint] : hint;
 

lib/backends/backend-mixed.ts

@@ -0,0 +1,14 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+import {SessionHandler} from '../backend';
+import {Session} from '../session';
+import {MixedSessionHandler} from './mixed-session-handler';
+import {WebGLBackend} from './backend-webgl';
+
+
+export class MixedBackend extends WebGLBackend {
+  createSessionHandler(context: Session.Context): SessionHandler {
+    return new MixedSessionHandler(this, context);
+  }
+}

lib/backends/mixed-session-handler.ts

@@ -0,0 +1,24 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+import {WebGLSessionHandler} from './webgl/session-handler';
+import {Graph} from '../graph';
+import {OpSet, resolveOperator} from '../opset';
+import {Operator} from '../operators';
+import {CPU_OP_RESOLVE_RULES} from './cpu/op-resolve-rules';
+import {Logger} from '../instrument';
+
+export class MixedSessionHandler extends WebGLSessionHandler {
+    resolve(node: Graph.Node, opsets: ReadonlyArray<OpSet>): Operator {
+        try {
+            return super.resolve(node, opsets);
+        } catch (e) {
+            Logger.warning(
+                'MixedSessionHandler',
+                `Unable to initialize operator '${node.opType}' with webgl. trying with cpu...`);
+            const op = resolveOperator(node, opsets, CPU_OP_RESOLVE_RULES);
+            op.initialize(node.attributes);
+            return op;
+        }
+    }
+}