Optimize elementwise_add_grad op, test=develop (#32051)

paddle/fluid/operators/elementwise/elementwise_add_op.cu

@@ -112,6 +112,22 @@ elementwise_add_grad(const framework::ExecutionContext& ctx,
                      const framework::Tensor* out,
                      const framework::Tensor* dout, framework::Tensor* dx,
                      framework::Tensor* dy) {
+  auto* dx_data = dx->mutable_data<T>(ctx.GetPlace());
+  auto* dy_data = dy->mutable_data<T>(ctx.GetPlace());
+  auto* dout_data = dout->data<T>();
+  if (dx_data == dout_data && dy_data != dout_data) {
+    VLOG(4) << "Special case when dx_data is the same as dout_data, "
+               "only need copy dout to dy";
+    framework::TensorCopy(
+        *dout, ctx.GetPlace(),
+        ctx.template device_context<platform::DeviceContext>(), dy);
+  } else if (dx_data != dout_data && dy_data == dout_data) {
+    VLOG(4) << "Special case when dy_data is the same as dout_data, "
+               "only need copy dout to dx";
+    framework::TensorCopy(
+        *dout, ctx.GetPlace(),
+        ctx.template device_context<platform::DeviceContext>(), dx);
+  } else if (dx_data != dout_data && dy_data != dout_data) {
     auto size = x->numel();
     int vec_size = max(static_cast<int>(sizeof(float4) / sizeof(T)), 1);
     dim3 block_size = dim3(PADDLE_CUDA_THREAD_SIZE, 1);
@@ -124,6 +140,11 @@ elementwise_add_grad(const framework::ExecutionContext& ctx,
              ctx.template device_context<plat::CUDADeviceContext>().stream()>>>(
         dout->data<T>(), size, vec_size, dx->mutable_data<T>(ctx.GetPlace()),
         dy->mutable_data<T>(ctx.GetPlace()));
+  } else {
+    VLOG(4) << "Special case when dy_data is the same as dout_data, "
+               "and dx_data is the same as dout_data, do not need "
+               "any operator";
+  }
 }
 
 }  // namespace operators