Move output tensor resize to before XNNPACK subgraph execution (#17661)

Summary:
ET's XNNPACK delegate currently resizes output tensors after running the
XNNPACK subgraph. This is normally fine, as the buffer is large enough.
However, if there is a logic bug or corrupt model file, the current code
doesn't catch if XNNPACK's output is larger than the planned buffer
until after execution, where it may crash.

To fix this, I've moved the resize up. This fails gracefully in the
above case.

Differential Revision: D94148443

Author: GregoryComer

backends/xnnpack/runtime/XNNExecutor.cpp

@@ -127,7 +127,7 @@ ET_NODISCARD Error XNNExecutor::prepare_args(Span<EValue*> args) {
           xnn_status_to_string(status));
     }
   }
-  // // Propagate Input Shape and Memory Plan for increased allocation
+  // Propagate Input Shape and Memory Plan for increased allocation
   status = xnn_reshape_runtime(runtime_.get());
 
   ET_CHECK_OR_RETURN_ERROR(
@@ -136,6 +136,12 @@ ET_NODISCARD Error XNNExecutor::prepare_args(Span<EValue*> args) {
       "Internal Error: Propagating input shapes failed with code: %s",
       xnn_status_to_string(status));
 
+  // Resize output tensors.
+  Error err = resize_outputs(args);
+  if (err != Error::Ok) {
+    return err;
+  }
+
   return Error::Ok;
 }
 
@@ -188,14 +194,7 @@ ET_NODISCARD Error XNNExecutor::forward(BackendExecutionContext& context) {
 }
 
 /**
- * Prepares the outputs for ExecuTorch
- *
- * Resizes the output tensors based on the output shapes returned by
- * the xnnpack runtime.
- *
- * Note: For arg_max pooling, we recast the output index tensor. Since
- * XNNPACK gives the index tensor to us as int32, we need to convert it
- * back to int64 for ExecuTorch.
+ * Resizes output tensors to match XNNPACK's computed shapes.
  */
 ET_NODISCARD Error XNNExecutor::resize_outputs(Span<EValue*> args) const {
   size_t output_idx_start = input_ids_.size();
@@ -239,6 +238,22 @@ ET_NODISCARD Error XNNExecutor::resize_outputs(Span<EValue*> args) const {
       ET_LOG(Error, "Failed to resize output tensor for XNNExecutor");
       return err;
     }
+  }
+
+  return Error::Ok;
+}
+
+/**
+ * Converts output data types after XNNPACK execution.
+ *
+ * For arg_max pooling, XNNPACK outputs int32 index tensors that need
+ * to be converted to int64 for ExecuTorch.
+ */
+ET_NODISCARD Error XNNExecutor::convert_outputs(Span<EValue*> args) const {
+  size_t output_idx_start = input_ids_.size();
+  for (size_t i = output_idx_start; i < externals_.size(); ++i) {
+    uint32_t ext_id = externals_[i].id;
+    Tensor* out_tensor = &args[ext_id]->toTensor();
 
     // Output datatype is int64. However, XNNPACK doesn't support
     // int64. This means that the data was put into this tensor

backends/xnnpack/runtime/XNNExecutor.h

@@ -88,13 +88,21 @@ class XNNExecutor {
       executorch::ET_RUNTIME_NAMESPACE::BackendExecutionContext& context);
 
   /**
-   * Prepares the outputs to be returned by the delegate
+   * Resizes output tensors to match XNNPACK's computed shapes.
    *
-   * Performs any post processing of outputs like tensor resizing
    */
   ET_NODISCARD executorch::runtime::Error resize_outputs(
       executorch::runtime::Span<executorch::runtime::EValue*> args) const;
 
+  /**
+   * Converts output data types after XNNPACK execution.
+   *
+   * For arg_max pooling, XNNPACK outputs int32 index tensors that need
+   * to be converted to int64 for ExecuTorch.
+   */
+  ET_NODISCARD executorch::runtime::Error convert_outputs(
+      executorch::runtime::Span<executorch::runtime::EValue*> args) const;
+
   friend class XNNCompiler;
 };
 

backends/xnnpack/runtime/XNNPACKBackend.cpp

@@ -152,8 +152,8 @@ class XnnpackBackend final
       return err;
     }
 
-    // Resize outputs and recast pointers if necessary
-    err = executor->resize_outputs(args);
+    // Convert output data types if necessary (e.g., int32 -> int64 for Long)
+    err = executor->convert_outputs(args);
 
     return err;
   }

backends/xnnpack/test/runtime/test_xnnexecutor.cpp

@@ -174,7 +174,7 @@ TEST(XNNExecutorTest, ResizeOutputsWithLongTensorConvertsInt32ToInt64) {
   ASSERT_EQ(executor.prepare_args(span), Error::Ok);
   executorch::ET_RUNTIME_NAMESPACE::BackendExecutionContext context;
   ASSERT_EQ(executor.forward(context), Error::Ok);
-  ASSERT_EQ(executor.resize_outputs(span), Error::Ok);
+  ASSERT_EQ(executor.convert_outputs(span), Error::Ok);
 
   Tensor& result = args[2]->toTensor();
   ASSERT_EQ(result.scalar_type(), executorch::aten::ScalarType::Long);