[xla:ffi] Add auto-binding for FFI results

xla/ffi/api/api.h

@@ -352,6 +352,20 @@ struct ArgBinding {
   using Arg = void;
 };
 
+// XLA FFI binding for a returned result.
+//
+// Example: binding for the `MyType` result
+//
+//   template <>
+//   struct RetBinding<MyType> {
+//     using Ret = MyType;
+//   };
+//
+template <typename T>
+struct RetBinding {
+  using Ret = void;
+};
+
 // XLA FFI binding for a named attribute.
 //
 // Example: binding for the `MyType` attribute
@@ -394,6 +408,10 @@ template <typename Param>
 inline constexpr bool is_arg_binding_v =
     !std::is_void_v<typename ArgBinding<Param>::Arg>;
 
+template <typename Param>
+inline constexpr bool is_ret_binding_v =
+    !std::is_void_v<typename RetBinding<Param>::Ret>;
+
 template <typename Param>
 inline constexpr bool is_attr_binding_v =
     !std::is_void_v<typename AttrBinding<Param>::Attr>;
@@ -422,6 +440,11 @@ struct BindOne<Fn, Param, Params...> {
       return BindOne<Fn, Params...>::To(
           std::move(fn),
           std::move(binding).template Arg<typename ArgBinding<Param>::Arg>());
+    } else if constexpr (is_ret_binding_v<Param>) {
+      // Bind parameter as an FFI handler result.
+      return BindOne<Fn, Params...>::To(
+          std::move(fn),
+          std::move(binding).template Ret<typename RetBinding<Param>::Ret>());
 
     } else if constexpr (is_attr_binding_v<Param>) {
       // Bind parameter as a named FFI handler attribute.

xla/ffi/api/ffi.h

@@ -216,6 +216,20 @@ struct ArgBinding<Buffer<dtype, rank>> {
   using Arg = Buffer<dtype, rank>;
 };
 
+//===----------------------------------------------------------------------===//
+// Results binding
+//===----------------------------------------------------------------------===//
+
+template <>
+struct RetBinding<Result<BufferBase>> {
+  using Ret = BufferBase;
+};
+
+template <DataType dtype, size_t rank>
+struct RetBinding<Result<Buffer<dtype, rank>>> {
+  using Ret = Buffer<dtype, rank>;
+};
+
 //===----------------------------------------------------------------------===//
 // Arguments decoding
 //===----------------------------------------------------------------------===//

xla/ffi/api/ffi_test.cc

@@ -189,6 +189,18 @@ TEST(FfiTest, AutoBinding) {
   TF_ASSERT_OK(status);
 }
 
+TEST(FfiTest, AutoBindingResult) {
+  auto handler =
+      Ffi::BindTo(+[](Result<BufferBase> buffer) { return Error::Success(); });
+
+  CallFrameBuilder builder;
+  builder.AddBufferRet(se::DeviceMemoryBase(), PrimitiveType::F32, /*dims=*/{});
+  auto call_frame = builder.Build();
+
+  auto status = Call(*handler, call_frame);
+  TF_ASSERT_OK(status);
+}
+
 struct I32AndF32 {
   int32_t i32;
   float f32;