More buffer donation in some cases (#1858)

* more donation

* fix

* add test

mlx/backend/metal/metal.cpp

@@ -1,6 +1,5 @@
 // Copyright © 2023-2024 Apple Inc.
 #include <memory>
-
 #include "mlx/backend/metal/device.h"
 #include "mlx/backend/metal/event.h"
 #include "mlx/backend/metal/utils.h"
@@ -54,16 +53,34 @@ std::function<void()> make_task(array arr, bool signal) {
         abort_with_exception(error);
       }
     }
-    std::vector<std::shared_ptr<array::Data>> buffers;
+
+    // Use a set to de-dup input buffers
+    std::unordered_set<std::shared_ptr<array::Data>> buffers;
     for (auto& in : arr.inputs()) {
-      buffers.push_back(in.data_shared_ptr());
+      if (in.data_shared_ptr() != nullptr) {
+        buffers.insert(in.data_shared_ptr());
+      }
     }
     for (auto& s : arr.siblings()) {
-      buffers.push_back(s.data_shared_ptr());
+      buffers.insert(s.data_shared_ptr());
     }
     if (!arr.is_tracer()) {
       arr.detach();
     }
+    // Erase any input buffers that have a use count > 1
+    // to keep them elligible for donation
+    for (auto it = buffers.begin(); it != buffers.end();) {
+      // Always hold buffers which could belong to outputs
+      // TODO: this shouldn't be necessary, but metal validation
+      // complains if buffers get released before the command buffer is
+      // finished, even if the ready signal has fired
+      if (!signal && it->use_count() > 1) {
+        it = buffers.erase(it);
+      } else {
+        ++it;
+      }
+    }
+
     for (auto& out : outputs) {
       out.set_status(array::Status::evaluated);
     }

python/tests/test_eval.py

@@ -174,6 +174,25 @@ class TestEval(mlx_tests.MLXTestCase):
         post = mx.metal.get_peak_memory()
         self.assertEqual(pre, post)
 
+    def test_donation_multiple_inputs(self):
+        def fun(its, x, y):
+            for _ in range(its):
+                a = x + y  # y should donate
+                b = x + a  # x should donate
+                x, y = a, b
+            return x, y
+
+        x = mx.zeros((128, 128))
+        y = mx.zeros((128, 128))
+        mx.metal.reset_peak_memory()
+        a, b = fun(2, x, y)
+        mx.eval(a, b)
+        mem2 = mx.metal.get_peak_memory()
+        a, b = fun(10, x, y)
+        mx.eval(a, b)
+        mem10 = mx.metal.get_peak_memory()
+        self.assertEqual(mem2, mem10)
+
 
 if __name__ == "__main__":
     unittest.main()