Reshape improvement (#818)

mlx/backend/common/utils.h

@@ -28,4 +28,70 @@ inline size_t elem_to_loc(int elem, const array& a) {
   return elem_to_loc(elem, a.shape(), a.strides());
 }
 
+// Collapse dims that are contiguous to possibly route to a better kernel
+// e.g. for x = transpose(array({0, 1, 2, 3, 4, 5, 6, 7}, {2, 2, 2}), {2, 0, 1})
+// should return {{2, 4}, {{1, 2}}}.
+//
+// When multiple arrays are passed they should all have the same shape. The
+// collapsed axes are also the same so one shape is returned.
+inline std::tuple<std::vector<int>, std::vector<std::vector<size_t>>>
+collapse_contiguous_dims(
+    const std::vector<int>& shape,
+    const std::vector<std::vector<size_t>> strides) {
+  // Make a vector that has axes separated with -1. Collapse all axes between
+  // -1.
+  std::vector<int> to_collapse;
+  if (shape.size() > 0) {
+    to_collapse.push_back(0);
+    for (int i = 1; i < shape.size(); i++) {
+      bool contiguous = true;
+      for (const std::vector<size_t>& st : strides) {
+        if (st[i] * shape[i] != st[i - 1]) {
+          contiguous = false;
+        }
+        if (!contiguous) {
+          break;
+        }
+      }
+      if (!contiguous) {
+        to_collapse.push_back(-1);
+      }
+      to_collapse.push_back(i);
+    }
+    to_collapse.push_back(-1);
+  }
+
+  std::vector<int> out_shape;
+  std::vector<std::vector<size_t>> out_strides(strides.size());
+  for (int i = 0; i < to_collapse.size(); i++) {
+    int current_shape = shape[to_collapse[i]];
+    while (to_collapse[++i] != -1) {
+      current_shape *= shape[to_collapse[i]];
+    }
+    out_shape.push_back(current_shape);
+    for (int j = 0; j < strides.size(); j++) {
+      const std::vector<size_t>& st = strides[j];
+      out_strides[j].push_back(st[to_collapse[i - 1]]);
+    }
+  }
+
+  return std::make_tuple(out_shape, out_strides);
+}
+
+inline std::tuple<std::vector<int>, std::vector<std::vector<size_t>>>
+collapse_contiguous_dims(const std::vector<array>& xs) {
+  std::vector<std::vector<size_t>> strides;
+  for (auto& x : xs) {
+    strides.emplace_back(x.strides());
+  }
+  return collapse_contiguous_dims(xs[0].shape(), strides);
+}
+
+template <typename... Arrays>
+inline std::tuple<std::vector<int>, std::vector<std::vector<size_t>>>
+collapse_contiguous_dims(Arrays... xs) {
+  return collapse_contiguous_dims(
+      std::vector<array>{std::forward<Arrays>(xs)...});
+}
+
 } // namespace mlx::core