Functions: support cycles in lazy-function graph

Lazy-function graphs are now evaluated properly even if they contain
cycles. Note that cycles are only ok if there is no data dependency cycle.
For example, a node might output something that is fed back into itself.
As long as the output can be computed without the input that it feeds into,
everything is ok.

The code that builds the graph is responsible for making sure that there
are no actual data dependencies.

source/blender/functions/FN_lazy_function.hh

@@ -39,6 +39,7 @@
  */
 
 #include "BLI_cpp_type.hh"
+#include "BLI_function_ref.hh"
 #include "BLI_generic_pointer.hh"
 #include "BLI_linear_allocator.hh"
 #include "BLI_vector.hh"
@@ -165,7 +166,8 @@ class Params {
    * Typed utility methods that wrap the methods above.
    */
   template<typename T> T extract_input(int index);
-  template<typename T> const T &get_input(int index) const;
+  template<typename T> T &get_input(int index) const;
+  template<typename T> T *try_get_input_data_ptr(int index) const;
   template<typename T> T *try_get_input_data_ptr_or_request(int index);
   template<typename T> void set_output(int index, T &&value);
 
@@ -253,6 +255,10 @@ class LazyFunction {
   const char *debug_name_ = "<unknown>";
   Vector<Input> inputs_;
   Vector<Output> outputs_;
+  /**
+   * Allow executing the function even if previously requested values are not yet available.
+   */
+  bool allow_missing_requested_inputs_ = false;
 
  public:
   virtual ~LazyFunction() = default;
@@ -277,6 +283,13 @@ class LazyFunction {
    */
   virtual void destruct_storage(void *storage) const;
 
+  /**
+   * Calls `fn` with the input indices that the given `output_index` may depend on. By default
+   * every output depends on every input.
+   */
+  virtual void possible_output_dependencies(int output_index,
+                                            FunctionRef<void(Span<int>)> fn) const;
+
   /**
    * Inputs of the function.
    */
@@ -298,6 +311,17 @@ class LazyFunction {
    */
   bool always_used_inputs_available(const Params &params) const;
 
+  /**
+   * If true, the function can be executed even when some requested inputs are not available yet.
+   * This allows the function to make some progress and maybe to compute some outputs that are
+   * passed into this function again (lazy-function graphs may contain cycles as long as there
+   * aren't actually data dependencies).
+   */
+  bool allow_missing_requested_inputs() const
+  {
+    return allow_missing_requested_inputs_;
+  }
+
  private:
   /**
    * Needs to be implemented by subclasses. This is separate from #execute so that additional
@@ -391,11 +415,17 @@ template<typename T> inline T Params::extract_input(const int index)
   return return_value;
 }
 
-template<typename T> inline const T &Params::get_input(const int index) const
+template<typename T> inline T &Params::get_input(const int index) const
 {
-  const void *data = this->try_get_input_data_ptr(index);
+  void *data = this->try_get_input_data_ptr(index);
   BLI_assert(data != nullptr);
-  return *static_cast<const T *>(data);
+  return *static_cast<T *>(data);
+}
+
+template<typename T> inline T *Params::try_get_input_data_ptr(const int index) const
+{
+  this->assert_valid_thread();
+  return static_cast<T *>(this->try_get_input_data_ptr(index));
 }
 
 template<typename T> inline T *Params::try_get_input_data_ptr_or_request(const int index)

source/blender/functions/FN_lazy_function_execute.hh

@@ -93,6 +93,9 @@ inline void execute_lazy_function_eagerly_impl(
       fn, input_pointers, output_pointers, input_usages, output_usages, set_outputs};
   fn.execute(params, context);
   fn.destruct_storage(context.storage);
+
+  /* Make sure all outputs have been computed.  */
+  BLI_assert(!Span(set_outputs).contains(false));
 }
 
 }  // namespace detail

source/blender/functions/intern/lazy_function.cc

@@ -36,8 +36,22 @@ void LazyFunction::destruct_storage(void *storage) const
   UNUSED_VARS_NDEBUG(storage);
 }
 
+void LazyFunction::possible_output_dependencies(const int /*output_index*/,
+                                                const FunctionRef<void(Span<int>)> fn) const
+{
+  /* The output depends on all inputs by default. */
+  Vector<int, 16> indices(inputs_.size());
+  for (const int i : inputs_.index_range()) {
+    indices[i] = i;
+  }
+  fn(indices);
+}
+
 bool LazyFunction::always_used_inputs_available(const Params &params) const
 {
+  if (allow_missing_requested_inputs_) {
+    return true;
+  }
   for (const int i : inputs_.index_range()) {
     const Input &fn_input = inputs_[i];
     if (fn_input.usage == ValueUsage::Used) {

source/blender/functions/intern/lazy_function_graph_executor.cc

@@ -762,8 +762,10 @@ class Executor {
           input_state.was_ready_for_execution = true;
           continue;
         }
-        if (input_state.usage == ValueUsage::Used) {
-          return;
+        if (!fn.allow_missing_requested_inputs()) {
+          if (input_state.usage == ValueUsage::Used) {
+            return;
+          }
         }
       }
 
@@ -1031,7 +1033,10 @@ class Executor {
 
     if (input_state.usage == ValueUsage::Used) {
       node_state.missing_required_inputs -= 1;
-      if (node_state.missing_required_inputs == 0) {
+      if (node_state.missing_required_inputs == 0 ||
+          (locked_node.node.is_function() && static_cast<const FunctionNode &>(locked_node.node)
+                                                 .function()
+                                                 .allow_missing_requested_inputs())) {
         this->schedule_node(locked_node, current_task);
       }
     }
@@ -1248,6 +1253,9 @@ GraphExecutor::GraphExecutor(const Graph &graph,
       logger_(logger),
       side_effect_provider_(side_effect_provider)
 {
+  /* The graph executor can handle partial execution when there are still missing inputs. */
+  allow_missing_requested_inputs_ = true;
+
   for (const OutputSocket *socket : graph_inputs_) {
     BLI_assert(socket->node().is_dummy());
     inputs_.append({"In", socket->type(), ValueUsage::Maybe});

source/blender/functions/tests/FN_lazy_function_test.cc

@@ -112,4 +112,68 @@ TEST(lazy_function, SideEffects)
   EXPECT_EQ(dst2, 105);
 }
 
+class PartialEvaluationTestFunction : public LazyFunction {
+ public:
+  PartialEvaluationTestFunction()
+  {
+    debug_name_ = "Partial Evaluation";
+    allow_missing_requested_inputs_ = true;
+
+    inputs_.append_as("A", CPPType::get<int>(), ValueUsage::Used);
+    inputs_.append_as("B", CPPType::get<int>(), ValueUsage::Used);
+
+    outputs_.append_as("A*2", CPPType::get<int>());
+    outputs_.append_as("B*5", CPPType::get<int>());
+  }
+
+  void execute_impl(Params &params, const Context & /*context*/) const override
+  {
+    if (!params.output_was_set(0)) {
+      if (int *a = params.try_get_input_data_ptr<int>(0)) {
+        params.set_output(0, *a * 2);
+      }
+    }
+    if (!params.output_was_set(1)) {
+      if (int *b = params.try_get_input_data_ptr<int>(1)) {
+        params.set_output(1, *b * 5);
+      }
+    }
+  }
+
+  void possible_output_dependencies(const int output_index,
+                                    FunctionRef<void(Span<int>)> fn) const override
+  {
+    /* Each output only depends on the input with the same index. */
+    const int input_index = output_index;
+    fn({input_index});
+  }
+};
+
+TEST(lazy_function, GraphWithCycle)
+{
+  const PartialEvaluationTestFunction fn;
+
+  Graph graph;
+  FunctionNode &fn_node = graph.add_function(fn);
+
+  DummyNode &input_node = graph.add_dummy({}, {&CPPType::get<int>()});
+  DummyNode &output_node = graph.add_dummy({&CPPType::get<int>()}, {});
+
+  graph.add_link(input_node.output(0), fn_node.input(0));
+  /* Note: This creates a cycle in the graph. However, it should still be possible to evaluate it,
+   * because there is no actual data dependency in the cycle. */
+  graph.add_link(fn_node.output(0), fn_node.input(1));
+  graph.add_link(fn_node.output(1), output_node.input(0));
+
+  graph.update_node_indices();
+
+  GraphExecutor executor_fn{
+      graph, {&input_node.output(0)}, {&output_node.input(0)}, nullptr, nullptr};
+  int result = 0;
+  execute_lazy_function_eagerly(
+      executor_fn, nullptr, std::make_tuple(10), std::make_tuple(&result));
+
+  EXPECT_EQ(result, 10 * 2 * 5);
+}
+
 }  // namespace blender::fn::lazy_function::tests