[GraphBolt][CUDA] Refactoring SliceCSCIndptr for Neighbor Sampling. (#6799)

graphbolt/include/graphbolt/cuda_ops.h

@@ -24,6 +24,21 @@ torch::Tensor ExclusiveCumSum(torch::Tensor input);
 std::tuple<torch::Tensor, torch::Tensor> IndexSelectCSCImpl(
     torch::Tensor indptr, torch::Tensor indices, torch::Tensor nodes);
 
+/**
+ * @brief Slices the indptr tensor with nodes and returns the indegrees of the
+ * given nodes and their indptr values.
+ *
+ * @param indptr The indptr tensor.
+ * @param nodes  The nodes to read from indptr
+ *
+ * @return Tuple of tensors with values:
+ * (indptr[nodes + 1] - indptr[nodes], indptr[nodes]), the returned indegrees
+ * tensor (first one) has size nodes.size(0) + 1 so that calling ExclusiveCumSum
+ * on it gives the output indptr.
+ */
+std::tuple<torch::Tensor, torch::Tensor> SliceCSCIndptr(
+    torch::Tensor indptr, torch::Tensor nodes);
+
 std::tuple<torch::Tensor, torch::Tensor> UVAIndexSelectCSCImpl(
     torch::Tensor indptr, torch::Tensor indices, torch::Tensor nodes);
 

graphbolt/src/cuda/index_select_csc_impl.cu

@@ -102,26 +102,34 @@ struct PairSum {
 };
 
 // Returns (indptr[nodes + 1] - indptr[nodes], indptr[nodes])
-template <typename indptr_t>
-auto SliceCSCIndptr(
-    const indptr_t* const indptr, torch::Tensor nodes, cudaStream_t stream) {
+std::tuple<torch::Tensor, torch::Tensor> SliceCSCIndptr(
+    torch::Tensor indptr, torch::Tensor nodes) {
   auto allocator = cuda::GetAllocator();
-  const auto exec_policy = thrust::cuda::par_nosync(allocator).on(stream);
+  const auto exec_policy =
+      thrust::cuda::par_nosync(allocator).on(cuda::GetCurrentStream());
   const int64_t num_nodes = nodes.size(0);
   // Read indptr only once in case it is pinned and access is slow.
-  auto sliced_indptr = allocator.AllocateStorage<indptr_t>(num_nodes);
+  auto sliced_indptr =
+      torch::empty(num_nodes, nodes.options().dtype(indptr.scalar_type()));
   // compute in-degrees
-  auto in_degree = allocator.AllocateStorage<indptr_t>(num_nodes + 1);
+  auto in_degree =
+      torch::empty(num_nodes + 1, nodes.options().dtype(indptr.scalar_type()));
   thrust::counting_iterator<int64_t> iota(0);
-  AT_DISPATCH_INDEX_TYPES(nodes.scalar_type(), "IndexSelectCSCNodes", ([&] {
+  AT_DISPATCH_INTEGRAL_TYPES(
+      indptr.scalar_type(), "IndexSelectCSCIndptr", ([&] {
+        using indptr_t = scalar_t;
+        AT_DISPATCH_INDEX_TYPES(
+            nodes.scalar_type(), "IndexSelectCSCNodes", ([&] {
               using nodes_t = index_t;
               thrust::for_each(
                   exec_policy, iota, iota + num_nodes,
                   SliceFunc<indptr_t, nodes_t>{
-                                    nodes.data_ptr<nodes_t>(), indptr,
-                                    in_degree.get(), sliced_indptr.get()});
+                      nodes.data_ptr<nodes_t>(), indptr.data_ptr<indptr_t>(),
+                      in_degree.data_ptr<indptr_t>(),
+                      sliced_indptr.data_ptr<indptr_t>()});
+            }));
       }));
-  return std::make_pair(std::move(in_degree), std::move(sliced_indptr));
+  return {in_degree, sliced_indptr};
 }
 
 template <typename indptr_t, typename indices_t>
@@ -198,13 +206,14 @@ std::tuple<torch::Tensor, torch::Tensor> UVAIndexSelectCSCImpl(
   auto stream = c10::cuda::getDefaultCUDAStream();
   const int64_t num_nodes = nodes.size(0);
 
+  auto in_degree_and_sliced_indptr = SliceCSCIndptr(indptr, nodes);
   return AT_DISPATCH_INTEGRAL_TYPES(
       indptr.scalar_type(), "UVAIndexSelectCSCIndptr", ([&] {
         using indptr_t = scalar_t;
-        auto [in_degree_ptr, sliced_indptr_ptr] =
-            SliceCSCIndptr(indptr.data_ptr<indptr_t>(), nodes, stream);
-        auto in_degree = in_degree_ptr.get();
-        auto sliced_indptr = sliced_indptr_ptr.get();
+        auto in_degree =
+            std::get<0>(in_degree_and_sliced_indptr).data_ptr<indptr_t>();
+        auto sliced_indptr =
+            std::get<1>(in_degree_and_sliced_indptr).data_ptr<indptr_t>();
         return GRAPHBOLT_DISPATCH_ELEMENT_SIZES(
             indices.element_size(), "UVAIndexSelectCSCCopyIndices", ([&] {
               return UVAIndexSelectCSCCopyIndices<indptr_t, element_size_t>(
@@ -265,13 +274,14 @@ std::tuple<torch::Tensor, torch::Tensor> IndexSelectCSCImpl(
     torch::Tensor indptr, torch::Tensor indices, torch::Tensor nodes) {
   auto stream = c10::cuda::getDefaultCUDAStream();
   const int64_t num_nodes = nodes.size(0);
+  auto in_degree_and_sliced_indptr = SliceCSCIndptr(indptr, nodes);
   return AT_DISPATCH_INTEGRAL_TYPES(
       indptr.scalar_type(), "IndexSelectCSCIndptr", ([&] {
         using indptr_t = scalar_t;
-        auto [in_degree_ptr, sliced_indptr_ptr] =
-            SliceCSCIndptr(indptr.data_ptr<indptr_t>(), nodes, stream);
-        auto in_degree = in_degree_ptr.get();
-        auto sliced_indptr = sliced_indptr_ptr.get();
+        auto in_degree =
+            std::get<0>(in_degree_and_sliced_indptr).data_ptr<indptr_t>();
+        auto sliced_indptr =
+            std::get<1>(in_degree_and_sliced_indptr).data_ptr<indptr_t>();
         // Output indptr for the slice indexed by nodes.
         torch::Tensor output_indptr = torch::empty(
             num_nodes + 1, nodes.options().dtype(indptr.scalar_type()));