perf(misc): use reinterpret_cast to convert valueshape

GitOrigin-RevId: ee14c7ce5e8e77a267db166a5369cd2c4e39bd58

imperative/src/include/megbrain/imperative/utils/value_shape.h

@@ -23,8 +23,7 @@ namespace mgb::imperative {
  *
  */
 struct ValueShape {
-    size_t shape[TensorShape::MAX_NDIM];
-    int ndim = 0;
+    size_t shape[TensorShape::MAX_NDIM], ndim = 0;
 
     ValueShape() = default;
     ValueShape(std::initializer_list<size_t> dims) {
@@ -70,19 +69,14 @@ struct ValueShape {
         return buffer;
     }
 
-    static ValueShape from(TensorShape tensor_shape) {
+    static const ValueShape& from(const TensorShape& tensor_shape) {
         mgb_assert(tensor_shape.ndim);
-        return Span<size_t>{tensor_shape.shape, tensor_shape.ndim};
+        return reinterpret_cast<const ValueShape&>(tensor_shape);
     }
 
-    TensorShape as_tensor_shape() const {
+    const TensorShape& as_tensor_shape() const {
         mgb_assert(ndim != 0);
-        TensorShape ret;
-        for (size_t i = 0; i < ndim; ++i) {
-            ret.shape[i] = shape[i];
-        }
-        ret.ndim = ndim;
-        return ret;
+        return reinterpret_cast<const TensorShape&>(*this);
     }
 
     bool operator==(const ValueShape& rhs) const {