Pass shape and inputs by value in array's constructor (#853)

Since the shape and inputs are always saved as copy in ArrayDesc, we can
unify array's constructors to just take the arguments by value.

There are 2 cases:
1. When shape is a lvalue, it will be copied into array's constructor and
   then moved into ArrayDesc's member. So only 1 copy happens.
2. When shape is a rvalue, it will be moved into array's constructor and
   then moved into ArrayDesc's member. So no copy happens.

So having 1 constructor that takes by value is equivalent to having 2
constructors that const reference and rvalue separately.

mlx/array.cpp

@@ -36,22 +36,11 @@ array::array(const std::complex<float>& val, Dtype dtype /* = complex64 */)
   init(&cval);
 }
 
-array::array(
-    const std::vector<int>& shape,
-    Dtype dtype,
-    std::shared_ptr<Primitive> primitive,
-    const std::vector<array>& inputs)
-    : array_desc_(std::make_shared<ArrayDesc>(
-          shape,
-          dtype,
-          std::move(primitive),
-          inputs)) {}
-
 array::array(
     std::vector<int> shape,
     Dtype dtype,
     std::shared_ptr<Primitive> primitive,
-    std::vector<array>&& inputs)
+    std::vector<array> inputs)
     : array_desc_(std::make_shared<ArrayDesc>(
           std::move(shape),
           dtype,
@@ -92,10 +81,10 @@ array::array(std::initializer_list<int> data, Dtype dtype)
 /* Build an array from a shared buffer */
 array::array(
     allocator::Buffer data,
-    const std::vector<int>& shape,
+    std::vector<int> shape,
     Dtype dtype,
     deleter_t deleter)
-    : array_desc_(std::make_shared<ArrayDesc>(shape, dtype)) {
+    : array_desc_(std::make_shared<ArrayDesc>(std::move(shape), dtype)) {
   set_data(data, deleter);
 }
 
@@ -181,31 +170,16 @@ void array::move_shared_buffer(array other) {
   move_shared_buffer(other, other.strides(), other.flags(), other.data_size());
 }
 
-array::ArrayDesc::ArrayDesc(const std::vector<int>& shape, Dtype dtype)
+array::ArrayDesc::ArrayDesc(std::vector<int> shape, Dtype dtype)
-    : shape(shape), dtype(dtype) {
+    : shape(std::move(shape)), dtype(dtype) {
-  std::tie(size, strides) = cum_prod(shape);
-}
-
-array::ArrayDesc::ArrayDesc(
-    const std::vector<int>& shape,
-    Dtype dtype,
-    std::shared_ptr<Primitive> primitive,
-    const std::vector<array>& inputs)
-    : shape(shape),
-      dtype(dtype),
-      primitive(std::move(primitive)),
-      inputs(inputs) {
   std::tie(size, strides) = cum_prod(this->shape);
-  for (auto& in : this->inputs) {
-    is_tracer |= in.is_tracer();
-  }
 }
 
 array::ArrayDesc::ArrayDesc(
-    std::vector<int>&& shape,
+    std::vector<int> shape,
     Dtype dtype,
     std::shared_ptr<Primitive> primitive,
-    std::vector<array>&& inputs)
+    std::vector<array> inputs)
     : shape(std::move(shape)),
       dtype(dtype),
       primitive(std::move(primitive)),

mlx/array.h

@@ -32,7 +32,7 @@ class array {
   template <typename It>
   array(
       It data,
-      const std::vector<int>& shape,
+      std::vector<int> shape,
       Dtype dtype =
           TypeToDtype<typename std::iterator_traits<It>::value_type>());
 
@@ -48,13 +48,13 @@ class array {
   template <typename T>
   array(
       std::initializer_list<T> data,
-      const std::vector<int>& shape,
+      std::vector<int> shape,
       Dtype dtype = TypeToDtype<T>());
 
   /* Build an array from a buffer */
   array(
       allocator::Buffer data,
-      const std::vector<int>& shape,
+      std::vector<int> shape,
       Dtype dtype,
       deleter_t deleter = allocator::free);
 
@@ -173,17 +173,11 @@ class array {
    * API may change.
    */
 
-  array(
-      const std::vector<int>& shape,
-      Dtype dtype,
-      std::shared_ptr<Primitive> primitive,
-      const std::vector<array>& inputs);
-
   array(
       std::vector<int> shape,
       Dtype dtype,
       std::shared_ptr<Primitive> primitive,
-      std::vector<array>&& inputs);
+      std::vector<array> inputs);
 
   static std::vector<array> make_arrays(
       const std::vector<std::vector<int>>& shapes,
@@ -391,19 +385,13 @@ class array {
     // The arrays position in the output list
     uint32_t position{0};
 
-    explicit ArrayDesc(const std::vector<int>& shape, Dtype dtype);
+    explicit ArrayDesc(std::vector<int> shape, Dtype dtype);
 
     explicit ArrayDesc(
-        const std::vector<int>& shape,
+        std::vector<int> shape,
         Dtype dtype,
         std::shared_ptr<Primitive> primitive,
-        const std::vector<array>& inputs);
+        std::vector<array> inputs);
-
-    explicit ArrayDesc(
-        std::vector<int>&& shape,
-        Dtype dtype,
-        std::shared_ptr<Primitive> primitive,
-        std::vector<array>&& inputs);
   };
 
   // The ArrayDesc contains the details of the materialized array including the
@@ -422,9 +410,9 @@ array::array(T val, Dtype dtype /* = TypeToDtype<T>() */)
 template <typename It>
 array::array(
   It data,
-  const std::vector<int>& shape,
+  std::vector<int> shape,
   Dtype dtype /* = TypeToDtype<typename std::iterator_traits<It>::value_type>() */) :
-    array_desc_(std::make_shared<ArrayDesc>(shape, dtype)) {
+    array_desc_(std::make_shared<ArrayDesc>(std::move(shape), dtype)) {
   init(data);
 }
 
@@ -441,9 +429,9 @@ array::array(
 template <typename T>
 array::array(
     std::initializer_list<T> data,
-    const std::vector<int>& shape,
+    std::vector<int> shape,
     Dtype dtype /* = TypeToDtype<T>() */)
-    : array_desc_(std::make_shared<ArrayDesc>(shape, dtype)) {
+    : array_desc_(std::make_shared<ArrayDesc>(std::move(shape), dtype)) {
   if (data.size() != size()) {
     throw std::invalid_argument(
         "Data size and provided shape mismatch in array construction.");