[async] Cache fusion results

taichi/program/async_engine.cpp

@@ -80,6 +80,44 @@ IRNode *IRBank::find(IRHandle ir_handle) {
   return result->second.get();
 }
 
+IRHandle IRBank::fuse(IRHandle handle_a, IRHandle handle_b, Kernel *kernel) {
+  auto &result = fuse_bank_[std::make_pair(handle_a, handle_b)];
+  if (!result.empty()) {
+    // assume the kernel is always the same when the ir handles are the same
+    return result;
+  }
+
+  TI_INFO("Begin uncached fusion");
+  // We are about to change both |task_a| and |task_b|. Clone them first.
+  auto cloned_task_a = handle_a.clone();
+  auto cloned_task_b = handle_b.clone();
+  auto task_a = cloned_task_a->as<OffloadedStmt>();
+  auto task_b = cloned_task_b->as<OffloadedStmt>();
+  // TODO: in certain cases this optimization can be wrong!
+  // Fuse task b into task_a
+  for (int j = 0; j < (int)task_b->body->size(); j++) {
+    task_a->body->insert(std::move(task_b->body->statements[j]));
+  }
+  task_b->body->statements.clear();
+
+  // replace all reference to the offloaded statement B to A
+  irpass::replace_all_usages_with(task_a, task_b, task_a);
+
+  irpass::full_simplify(task_a, /*after_lower_access=*/false, kernel);
+  // For now, re_id is necessary for the hash to be correct.
+  irpass::re_id(task_a);
+
+  auto h = get_hash(task_a);
+  result = IRHandle(task_a, h);
+  insert(std::move(cloned_task_a), h);
+
+  // TODO: since cloned_task_b->body is empty, can we remove this (i.e.,
+  //  simply delete cloned_task_b here)?
+  insert_to_trash_bin(std::move(cloned_task_b));
+
+  return result;
+}
+
 ParallelExecutor::ParallelExecutor(int num_threads)
     : num_threads(num_threads),
       status(ExecutorStatus::uninitialized),

taichi/program/async_engine.h

@@ -28,14 +28,16 @@ class IRBank {
   void insert_to_trash_bin(std::unique_ptr<IRNode> &&ir);
   IRNode *find(IRHandle ir_handle);
 
+  // Fuse handle_b into handle_a
+  IRHandle fuse(IRHandle handle_a, IRHandle handle_b, Kernel *kernel);
+
   std::unordered_map<IRHandle, TaskMeta> meta_bank_;
 
  private:
   std::unordered_map<IRNode *, uint64> hash_bank_;
   std::unordered_map<IRHandle, std::unique_ptr<IRNode>> ir_bank_;
   std::vector<std::unique_ptr<IRNode>> trash_bin;  // prevent IR from deleted
-  // TODO:
-  //  std::unordered_map<std::pair<IRHandle, IRHandle>, IRHandle> fuse_bank_;
+  std::unordered_map<std::pair<IRHandle, IRHandle>, IRHandle> fuse_bank_;
 };
 
 class ParallelExecutor {

taichi/program/async_utils.h

@@ -15,6 +15,9 @@ struct TaskMeta;
 
 class IRHandle {
  public:
+  IRHandle() : ir_(nullptr), hash_(0) {
+  }
+
   IRHandle(const IRNode *ir, uint64 hash) : ir_(ir), hash_(hash) {
   }
 
@@ -28,6 +31,10 @@ class IRHandle {
     return hash_;
   }
 
+  bool empty() const {
+    return ir_ == nullptr;
+  }
+
   // Two IRHandles are considered the same iff their hash values are the same.
   bool operator==(const IRHandle &other_ir_handle) const {
     return hash_ == other_ir_handle.hash_;
@@ -48,6 +55,15 @@ struct hash<taichi::lang::IRHandle> {
     return ir_handle.hash();
   }
 };
+
+template <>
+struct hash<std::pair<taichi::lang::IRHandle, taichi::lang::IRHandle>> {
+  std::size_t operator()(
+      const std::pair<taichi::lang::IRHandle, taichi::lang::IRHandle>
+          &ir_handles) const noexcept {
+    return ir_handles.first.hash() * 100000007UL + ir_handles.second.hash();
+  }
+};
 }  // namespace std
 
 TLANG_NAMESPACE_BEGIN

taichi/program/state_flow_graph.cpp

@@ -302,38 +302,14 @@ bool StateFlowGraph::fuse() {
     auto *node_a = nodes_[a].get();
     auto *node_b = nodes_[b].get();
     // TODO: remove debug output
-    TI_INFO("Fuse: {} <- {}", node_a->string(), node_b->string());
+    TI_TRACE("Fuse: {} <- {}", node_a->string(), node_b->string());
     auto &rec_a = node_a->rec;
     auto &rec_b = node_b->rec;
-    // We are about to change both |task_a| and |task_b|. Clone them first.
-    auto cloned_task_a = rec_a.ir_handle.clone();
-    auto cloned_task_b = rec_b.ir_handle.clone();
-    auto task_a = cloned_task_a->as<OffloadedStmt>();
-    auto task_b = cloned_task_b->as<OffloadedStmt>();
-    // TODO: in certain cases this optimization can be wrong!
-    // Fuse task b into task_a
-    for (int j = 0; j < (int)task_b->body->size(); j++) {
-      task_a->body->insert(std::move(task_b->body->statements[j]));
-    }
-    task_b->body->statements.clear();
-
-    // replace all reference to the offloaded statement B to A
-    irpass::replace_all_usages_with(task_a, task_b, task_a);
-
-    auto kernel = rec_a.kernel;
-    irpass::full_simplify(task_a, /*after_lower_access=*/false, kernel);
-    // For now, re_id is necessary for the hash to be correct.
-    irpass::re_id(task_a);
-
-    auto h = ir_bank_->get_hash(task_a);
-    rec_a.ir_handle = IRHandle(task_a, h);
-    ir_bank_->insert(std::move(cloned_task_a), h);
-    rec_b.ir_handle = IRHandle(nullptr, 0);
-    indices_to_delete.insert(b);
+    rec_a.ir_handle =
+        ir_bank_->fuse(rec_a.ir_handle, rec_b.ir_handle, rec_a.kernel);
+    rec_b.ir_handle = IRHandle();
 
-    // TODO: since cloned_task_b->body is empty, can we remove this (i.e.,
-    //  simply delete cloned_task_b here)?
-    ir_bank_->insert_to_trash_bin(std::move(cloned_task_b));
+    indices_to_delete.insert(b);
 
     const bool already_had_a_to_b_edge = has_path[a][b];
     if (already_had_a_to_b_edge) {