Weird behavior of dpc++ code after running it on FPGA device

Hello,

I am using DPC++ to accelerate knn algorithm on FPGA device. The following code is the code I wrote for the euclidean distance. The problem is that the fpga_emulation works very well with no problems while running it on fpga hardware (Intel Arria 10 OneAPI) gives -nan for all values in the resulting buffer, which means something got wrong in the parallel_for lioop. But I can't find anything wrong about it and the emulation worked.

I am using Intel Devcloud platform.

1. std::vector<double> distance_calculation_FPGA(queue& q, const std::vector<std::vector<double>>& dataset, const std::vector<double>& curr_test) {
       std::cout<<"convert 2D to 1D"<<std::endl;
       std::vector<double>linear_dataset;
       for (int i = 0; i < dataset.size(); ++i) {
           for (int j = 0; j < dataset[i].size(); ++j) {
               linear_dataset.push_back(dataset[i][j]);
           }
       }
       std::cout<<"buffering"<<std::endl;
         range<1> num_items{dataset.size()};
       std::vector<double>res;
       //std::cout << "im in" << std::endl;
   
       res.resize(dataset.size());
       buffer dataset_buf(linear_dataset);
       buffer curr_test_buf(curr_test);
       buffer res_buf(res.data(), num_items);
       
       std::cout<<"submit a job"<<std::endl;
       auto start = std::chrono::high_resolution_clock::now();
       {
       q.submit([&](handler& h) {
           accessor a(dataset_buf, h, read_only);
           accessor b(curr_test_buf, h, read_only);
   
           accessor dif(res_buf, h, write_only, no_init);
           h.parallel_for(num_items, [=](auto i) {
                   for (int j = 0; j < 5; ++j) {
                       dif[i] += (b[j] - a[i * 5 + j]) * (b[j] - a[i * 5 + j]);  
                   }
              // out << "i : " << i << " i[0]: " << i[0] << " b: " << b[0] << cl::sycl::endl;
               });
           }).wait();
       }
       auto finish = std::chrono::high_resolution_clock::now();
       std::chrono::duration<double> elapsed = finish - start;
       std::cout << "Elapsed time: " << elapsed.count() << " s\n";
       /*
           for (int i = 0; i < dataset.size(); ++i) {
               double dis = 0;
               for (int j = 0; j < dataset[i].size(); ++j) {
                   dis += (curr_test[j] - dataset[i][j]) * (curr_test[j] - dataset[i][j]);
               }
               res.push_back(dis);
           }
           */
       return res;
   }

results with fpga_emulation: ./knn.fpga_emu

results for fpga hardware: ./knn.fpga

Thank you so much!