fix saving for non-contiguous arrays (#389)

mlx/io/load.cpp

@@ -50,9 +50,14 @@ void save(std::shared_ptr<io::Writer> out_stream, array a, bool retain_graph) {
     throw std::invalid_argument("[save] cannot serialize an empty array");
   }
 
-  if (!a.flags().contiguous) {
+  if (!(a.flags().row_contiguous || a.flags().col_contiguous)) {
+    a = reshape(flatten(a), a.shape());
+    a.eval(retain_graph);
+  }
+  // Check once more in-case the above ops change
+  if (!(a.flags().row_contiguous || a.flags().col_contiguous)) {
     throw std::invalid_argument(
-        "[save] cannot serialize a non-contiguous array");
+        "[save] can only serialize row or col contiguous arrays");
   }
 
   ////////////////////////////////////////////////////////

mlx/io/safetensor.cpp

@@ -142,17 +142,26 @@ void save_safetensors(
   });
   size_t offset = 0;
   for (auto& [key, arr] : a) {
-    arr.eval(retain_graph_.value_or(arr.is_tracer()));
+    auto retain = retain_graph_.value_or(arr.is_tracer());
+    arr.eval(retain);
     if (arr.nbytes() == 0) {
       throw std::invalid_argument(
           "[save_safetensors] cannot serialize an empty array key: " + key);
     }
 
-    if (!arr.flags().contiguous) {
+    // Try to make it row contiguous
-      throw std::invalid_argument(
+    if (!arr.flags().row_contiguous) {
-          "[save_safetensors] cannot serialize a non-contiguous array key: " +
+      arr = reshape(flatten(arr), arr.shape());
-          key);
+      arr.eval(retain);
     }
+
+    // Has to be row-major now but, check one more time in case
+    // any of the above change in the future
+    if (!arr.flags().row_contiguous) {
+      throw std::invalid_argument(
+          "[save_safetensors] can only serialize row-major arrays");
+    }
+
     json child;
     child["dtype"] = dtype_to_safetensor_str(arr.dtype());
     child["shape"] = arr.shape();

python/tests/test_load.py

@@ -178,6 +178,29 @@ class TestLoad(mlx_tests.MLXTestCase):
                     for k, v in load_arr_mlx_npy.items():
                         self.assertTrue(np.array_equal(save_arrs_npy[k], v))
 
+    def test_non_contiguous(self):
+        if not os.path.isdir(self.test_dir):
+            os.mkdir(self.test_dir)
+
+        a = mx.broadcast_to(mx.array([1, 2]), [4, 2])
+
+        save_file = os.path.join(self.test_dir, "a.npy")
+        mx.save(save_file, a)
+        aload = mx.load(save_file)
+        self.assertTrue(mx.array_equal(a, aload))
+
+        save_file = os.path.join(self.test_dir, "a.safetensors")
+        mx.save_safetensors(save_file, {"a": a})
+        aload = mx.load(save_file)["a"]
+        self.assertTrue(mx.array_equal(a, aload))
+
+        # safetensors only works with row contiguous
+        # make sure col contiguous is handled properly
+        a = mx.arange(4).reshape(2, 2).T
+        mx.save_safetensors(save_file, {"a": a})
+        aload = mx.load(save_file)["a"]
+        self.assertTrue(mx.array_equal(a, aload))
+
 
 if __name__ == "__main__":
     unittest.main()