Fix leak for multi-output primitives which are never detached (#1059)

* fix multi output leak

* ignore arrays that will be detached

* add some comments

* stray print

mlx/array.cpp

@@ -1,5 +1,4 @@
 // Copyright © 2023-2024 Apple Inc.
-
 #include <functional>
 
 #include "mlx/array.h"
@@ -167,6 +166,39 @@ void array::move_shared_buffer(array other) {
   move_shared_buffer(other, other.strides(), other.flags(), other.data_size());
 }
 
+array::~array() {
+  if (array_desc_ == nullptr) {
+    return;
+  }
+
+  // Ignore arrays that will be detached
+  if (status() != array::Status::unscheduled) {
+    return;
+  }
+  // Break circular reference for non-detached arrays with siblings
+  if (auto n = siblings().size(); n > 0) {
+    bool do_detach = true;
+    // If all siblings have siblings.size() references except
+    // the one we are currently destroying (which has siblings.size() + 1)
+    // then there are no more external references
+    do_detach &= (array_desc_.use_count() == (n + 1));
+    for (auto& s : siblings()) {
+      do_detach &= (s.array_desc_.use_count() == n);
+      if (!do_detach) {
+        break;
+      }
+    }
+    if (do_detach) {
+      for (auto& s : siblings()) {
+        for (auto& ss : s.siblings()) {
+          ss.array_desc_ = nullptr;
+        }
+        s.array_desc_->siblings.clear();
+      }
+    }
+  }
+}
+
 void array::ArrayDesc::init() {
   strides.resize(shape.size());
   size = 1;

mlx/array.h

@@ -261,22 +261,16 @@ class array {
     return array_desc_->siblings;
   };
 
+  /** The array's siblings. */
+  std::vector<array>& siblings() {
+    return array_desc_->siblings;
+  };
+
   void set_siblings(std::vector<array> siblings, uint16_t position) {
     array_desc_->siblings = std::move(siblings);
     array_desc_->position = position;
   }
 
-  /** The i-th output of the array's primitive. */
-  const array& output(int i) const {
-    if (i == array_desc_->position) {
-      return *this;
-    } else if (i < array_desc_->position) {
-      return siblings()[i];
-    } else {
-      return siblings()[i + 1];
-    }
-  };
-
   /** The outputs of the array's primitive (i.e. this array and
    * its siblings) in the order the primitive expects. */
   std::vector<array> outputs() const {
@@ -386,6 +380,8 @@ class array {
     array_desc_ = other.array_desc_;
   }
 
+  ~array();
+
  private:
   // Initialize the arrays data
   template <typename It>

mlx/backend/common/make_compiled_preamble.sh

@@ -11,7 +11,7 @@ GCC=$2
 SRCDIR=$3
 CLANG=$4
 
-if [ $CLANG = "TRUE" ]; then
+if [ "$CLANG" = "TRUE" ]; then
   read -r -d '' INCLUDES <<- EOM
   #include <cmath>
   #include <complex>

mlx/transforms.cpp

@@ -246,7 +246,7 @@ std::pair<std::vector<array>, std::vector<array>> vjp(
       return;
     }
     a.set_tracer(false);
-    for (auto s : a.siblings()) {
+    for (auto& s : a.siblings()) {
       s.set_tracer(false);
       cache.insert(s.id());
     }
@@ -403,7 +403,7 @@ std::pair<std::vector<array>, std::vector<array>> jvp(
       return;
     }
     a.set_tracer(false);
-    for (auto s : a.siblings()) {
+    for (auto& s : a.siblings()) {
       s.set_tracer(false);
       cache.insert(s.id());
     }

python/tests/test_array.py

@@ -1694,6 +1694,22 @@ class TestArray(mlx_tests.MLXTestCase):
         b = pickle.loads(pickle.dumps(a))
         self.assertTrue(mx.array_equal(mx.array(a), mx.array(b)))
 
+    @unittest.skipIf(not mx.metal.is_available(), "Metal is not available")
+    def test_multi_output_leak(self):
+        def fun():
+            a = mx.zeros((2**20))
+            mx.eval(a)
+            b, c = mx.divmod(a, a)
+            del b, c
+
+        fun()
+        mx.synchronize()
+        peak_1 = mx.metal.get_peak_memory()
+        fun()
+        mx.synchronize()
+        peak_2 = mx.metal.get_peak_memory()
+        self.assertEqual(peak_1, peak_2)
+
 
 if __name__ == "__main__":
     unittest.main()