DPCT1113

Intel® DPC++ Compatibility Tool Developer Guide and Reference

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ID 768918

Date 11/07/2023

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DPCT1113

Message

Consider replacing sycl::nd_item::barrier(sycl::access::fence_space::local_space) with sycl::nd_item::barrier() if function <function name> is called in a multidimensional kernel.

Detailed Help

If there is no overlap global device memory access among each work-item in a work-group, the __syncthreads() API calls in kernel with 1D index space of a kernel execution that can be migrated to sycl::nd_item::barrier(sycl::access::fence_space::local_space). If you call the kernel with 2D/3D index space, the global memory access from each work-item in a work-group may overlap and may result in data dependency between work-items across the barrier. If this is the case, you may need to update the sycl::nd_item::barrier() call with sycl::access::fence_space::global_and_local.

Suggestions to Fix

For example, this original CUDA* code:

__global__ void kernel(float *mem) {
  unsigned int tid = threadIdx.x + blockDim.x * blockIdx.x;
  ...
  mem[tid] = ...;
  __syncthreads();
  ...
  ... = mem[tid];
}

void foo(float *mem) {
  kernel<<<16, 16>>>(mem); //1D index space of a SYCL kernel execution
}

results in the following migrated SYCL* code:

void kernel(float *mem, const sycl::nd_item<3> &item_ct1) {
  unsigned int tid = item_ct1.get_local_id(2) +
                     item_ct1.get_local_range(2) * item_ct1.get_group(2);
  ...
  mem[tid] = ...; // global memory access without overlap among each work-item in a work-group
  /*
  DPCT1113:0: Consider replacing
  sycl::nd_item::barrier(sycl::access::fence_space::local_space) with
  sycl::nd_item::barrier() if function "kernel" is called in a multidimensional
  kernel.
  */
  item_ct1.barrier(sycl::access::fence_space::local_space);
  ...
  ... = mem[tid]; // global memory access without overlap among each work-item in a work-group
}

void foo(float *mem) {
  dpct::get_default_queue().parallel_for(
      sycl::nd_range<3>(sycl::range<3>(1, 1, 16) * sycl::range<3>(1, 1, 16),
                        sycl::range<3>(1, 1, 16)),
      [=](sycl::nd_item<3> item_ct1) {
        kernel(mem, item_ct1);
      });
}

After migration, update the migrated SYCL kernel code to a 2D kernel:

void kernel(float *mem, const sycl::nd_item<3> &item_ct1) {
  unsigned int tidx = item_ct1.get_local_id(2) +
                      item_ct1.get_local_range(2) * item_ct1.get_group(2);
  unsigned int tidy = item_ct1.get_local_id(1) +
                      item_ct1.get_local_range(1) * item_ct1.get_group(1);

  ...
  mem[tidx] = ...; // global memory access with overlap among each work-item in a work-group
  mem[tidy] = ...; // global memory access with overlap among each work-item in a work-group
  /*
  DPCT1113:0: Consider replacing
  sycl::nd_item::barrier(sycl::access::fence_space::local_space) with
  sycl::nd_item::barrier() if function "kernel" is called in a multidimensional
  kernel.
  */
  item_ct1.barrier(sycl::access::fence_space::local_space);
  ...
  ... = mem[tidx]; // global memory access with overlap among each work-item in a work-group
  ... = mem[tidy]; // global memory access with overlap among each work-item in a work-group
}

void foo(float *mem) {
  dpct::get_default_queue().parallel_for(
      sycl::nd_range<3>(sycl::range<3>(1, 4, 4) * sycl::range<3>(1, 4, 4),
                        sycl::range<3>(1, 4, 4)), /*2D  index space of a SYCL kernel execution */

      [=](sycl::nd_item<3> item_ct1) {
        kernel(mem, item_ct1);
      });
}

which is rewritten to:

void kernel(float *mem, const sycl::nd_item<3> &item_ct1) {
  unsigned int tidx = item_ct1.get_local_id(2) +
                      item_ct1.get_local_range(2) * item_ct1.get_group(2);
  unsigned int tidy = item_ct1.get_local_id(1) +
                      item_ct1.get_local_range(1) * item_ct1.get_group(1);

  ...
  mem[tidx] = ...; // global memory access with overlap among each work-item in a work-group
  mem[tidy] = ...; // global memory access with overlap among each work-item in a work-group
  item_ct1.barrier(sycl::access::fence_space::global_and_local);
  ...
  ... = mem[tidx]; // global memory access with overlap among each work-item in a work-group
  ... = mem[tidy]; // global memory access with overlap among each work-item in a work-group
}

void foo(float *mem) {
  dpct::get_default_queue().parallel_for(
      sycl::nd_range<3>(sycl::range<3>(1, 4, 4) * sycl::range<3>(1, 4, 4),
                        sycl::range<3>(1, 4, 4)),
      [=](sycl::nd_item<3> item_ct1) {
        kernel(mem, item_ct1);
      });
}

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Intel® DPC++ Compatibility Tool Developer Guide and Reference

DPCT1113

Message

Detailed Help

Suggestions to Fix