Add priority-queue eval

mlx/transforms.cpp

@@ -18,6 +18,17 @@
 
 namespace mlx::core {
 
+/* This class is only meant to be used in eval
+ * for synchronizing with the main thread. */
+class Synchronizer : public Primitive {
+ public:
+  explicit Synchronizer(Stream stream) : Primitive(stream){};
+
+  void eval_cpu(const std::vector<array>&, std::vector<array>&) override{};
+  void eval_gpu(const std::vector<array>&, std::vector<array>&) override{};
+  void print(std::ostream&) override {}
+};
+
 // Initialize the static tracing counter from transforms_impl.h .
 //
 // This is used to implement the in_tracing() function the returns true if we
@@ -188,31 +199,44 @@ void simplify(const std::vector<array>& outputs) {
 }
 
 void eval(const std::vector<array>& outputs) {
-  std::function<void(const array&)> recurse;
-  std::queue<array> tape;
-  std::unordered_set<std::uintptr_t> cache;
+  std::function<int(const array&)> recurse;
+  std::unordered_map<std::uintptr_t, int> cache;
   std::unordered_map<std::uintptr_t, std::shared_future<void>> deps;
+  struct ArrayWithPriority {
+    int depth;
+    int order;
+    array x;
+  };
+  auto cmp = [](const ArrayWithPriority& a, const ArrayWithPriority& b) {
+    return (a.depth > b.depth) || (a.depth == b.depth && a.order > b.order);
+  };
+  std::priority_queue<
+      ArrayWithPriority,
+      std::vector<ArrayWithPriority>,
+      decltype(cmp)>
+      tape(cmp);
+  int order;
 
-  std::set<std::uintptr_t> output_primitives;
-  for (auto& arr : outputs) {
-    if (!arr.is_evaled()) {
-      output_primitives.insert(arr.primitive_id());
+  // Make an effort to choose a good output stream
+  Stream stream = default_stream(default_device());
+  for (auto& o : outputs) {
+    if (!o.is_evaled() && o.has_primitive()) {
+      stream = o.primitive().stream();
+      break;
     }
   }
 
+  auto synchronizer =
+      array({}, bool_, std::make_unique<Synchronizer>(stream), outputs);
+
   recurse = [&](const array& a) {
     auto id = a.id();
-    if (cache.find(id) != cache.end()) {
-      return;
+    if (auto it = cache.find(id); it != cache.end()) {
+      return it->second;
     }
+    int input_depth = 0;
     for (auto in : a.inputs()) {
-      // Pop fake outputs from the output set so we know who to synchronize
-      // with at the end
-      if (auto it = output_primitives.find(in.primitive_id());
-          it != output_primitives.end()) {
-        output_primitives.erase(it);
-      }
-      recurse(in);
+      input_depth = std::max(input_depth, recurse(in));
       // If one of the inputs is being computed on a different
       // stream, we need to manage the dependency.
       if (!in.is_evaled()) {
@@ -221,37 +245,28 @@ void eval(const std::vector<array>& outputs) {
         }
       }
     }
-    cache.insert(id);
+    cache.insert({id, input_depth + 1});
     for (auto& s : a.siblings()) {
-      cache.insert(s.id());
+      cache.insert({s.id(), input_depth + 1});
     }
     if (!a.is_evaled() || (!a.is_tracer() && a.has_primitive())) {
       if (!a.has_primitive()) {
         throw std::invalid_argument(
             "[eval] Attempting to eval an array without a primitive.");
       }
-      tape.push(a);
+      tape.push({input_depth + 1, order++, a});
     }
+    return input_depth + 1;
   };
 
-  // We have to store the output primitive ids because the arrays are
-  // detached during eval and we need to use them for synchronization
-  // at the end of this function
-  std::vector<std::uintptr_t> output_primitive_ids;
-  for (auto& arr : outputs) {
-    if (!arr.is_evaled() || (!arr.is_tracer() && arr.has_primitive())) {
-      recurse(arr);
-    }
-  }
-
-  // Insert output dependencies
-  for (auto pid : output_primitives) {
-    deps.insert({pid, std::shared_future<void>{}});
-  }
+  recurse(synchronizer);
+  uintptr_t synch_id = synchronizer.primitive_id();
+  deps.insert({synch_id, std::shared_future<void>{}});
 
   std::vector<std::shared_ptr<std::promise<void>>> ps;
   while (!tape.empty()) {
-    auto arr = std::move(tape.front());
+    auto val = std::move(tape.top());
+    auto arr = std::move(val.x);
     tape.pop();
     if (arr.is_evaled()) {
       if (!arr.is_tracer() && arr.has_primitive()) {
@@ -305,9 +320,8 @@ void eval(const std::vector<array>& outputs) {
       scheduler::enqueue(stream, std::move(task));
     }
   }
-  for (auto id : output_primitives) {
-    deps[id].wait();
-  }
+
+  deps[synch_id].wait();
 }
 
 std::pair<std::vector<array>, std::vector<array>> vjp(