faster coalesce if no value provided

cuda/unique.cpp

+#include <torch/torch.h>
+
+#define CHECK_CUDA(x) AT_ASSERTM(x.type().is_cuda(), #x " must be CUDA tensor")
+
+std::tuple<at::Tensor, at::Tensor> unique_cuda(at::Tensor src);
+
+std::tuple<at::Tensor, at::Tensor> unique(at::Tensor src) {
+  CHECK_CUDA(src);
+  return unique_cuda(src);
+}
+
+PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
+  m.def("unique", &unique, "Unique (CUDA)");
+}

cuda/unique_kernel.cu

+#include <ATen/ATen.h>
+
+#define THREADS 1024
+#define BLOCKS(N) (N + THREADS - 1) / THREADS
+
+template <typename scalar_t>
+__global__ void unique_cuda_kernel(scalar_t *__restrict__ src, uint8_t *mask,
+                                   size_t numel) {
+  const size_t index = blockIdx.x * blockDim.x + threadIdx.x;
+  const size_t stride = blockDim.x * gridDim.x;
+  for (ptrdiff_t i = index; i < numel; i += stride) {
+    if (i == 0 || src[i] != src[i - 1]) {
+      mask[i] = 1;
+    }
+  }
+}
+
+std::tuple<at::Tensor, at::Tensor> unique_cuda(at::Tensor src) {
+  at::Tensor perm;
+  std::tie(src, perm) = src.sort();
+
+  auto mask = at::zeros(src.numel(), src.type().toScalarType(at::kByte));
+  AT_DISPATCH_ALL_TYPES(src.type(), "grid_cuda_kernel", [&] {
+    unique_cuda_kernel<scalar_t><<<BLOCKS(src.numel()), THREADS>>>(
+        src.data<scalar_t>(), mask.data<uint8_t>(), src.numel());
+  });
+
+  src = src.masked_select(mask);
+  perm = perm.masked_select(mask);
+
+  return std::make_tuple(src, perm);
+}

setup.py

@@ -17,7 +17,9 @@ if torch.cuda.is_available():
             'spspmm_cuda',
             ['cuda/spspmm.cpp', 'cuda/spspmm_kernel.cu'],
             extra_link_args=['-lcusparse'],
-        )
+        ),
+        CUDAExtension('unique_cuda',
+                      ['cuda/unique.cpp', 'cuda/unique_kernel.cu'])
     ]
     cmdclass['build_ext'] = BuildExtension
 

torch_sparse/coalesce.py

 import torch
 import torch_scatter
 
+from .utils.unique import unique
+
 
 def coalesce(index, value, m, n, op='add', fill_value=0):
     """Row-wise sorts :obj:`value` and removes duplicate entries. Duplicate
@@ -23,13 +25,17 @@ def coalesce(index, value, m, n, op='add', fill_value=0):
 
     row, col = index
 
-    unique, inv = torch.unique(row * n + col, sorted=True, return_inverse=True)
+    if value is None:
+        _, perm = unique(row * n + col)
+        index = torch.stack([row[perm], col[perm]], dim=0)
+        return index, value
+
+    uniq, inv = torch.unique(row * n + col, sorted=True, return_inverse=True)
 
     perm = torch.arange(inv.size(0), dtype=inv.dtype, device=inv.device)
-    perm = inv.new_empty(unique.size(0)).scatter_(0, inv, perm)
+    perm = inv.new_empty(uniq.size(0)).scatter_(0, inv, perm)
     index = torch.stack([row[perm], col[perm]], dim=0)
 
-    if value is not None:
     op = getattr(torch_scatter, 'scatter_{}'.format(op))
     value = op(value, inv, 0, None, perm.size(0), fill_value)
     if isinstance(value, tuple):

torch_sparse/utils/unique.py

+import torch
+import numpy as np
+
+if torch.cuda.is_available():
+    import unique_cuda
+
+
+def unique(src):
+    src = src.contiguous().view(-1)
+
+    if src.is_cuda:
+        out, perm = unique_cuda.unique(src)
+    else:
+        out, perm = np.unique(src.numpy(), return_index=True)
+        out, perm = torch.from_numpy(out), torch.from_numpy(perm)
+
+    return out, perm