Hide and simplify backend registry internals

Differential Revision: D61928651

Pull Request resolved: #4947

Author: dbort

runtime/backend/interface.cpp

@@ -14,45 +14,44 @@ namespace runtime {
 
 PyTorchBackendInterface::~PyTorchBackendInterface() {}
 
-// TODO(T128866626): Remove global static variables.
-// We want to be able to run multiple Executor instances
-// and having a global registration isn't a viable solution
-// in the long term.
-BackendRegistry& getBackendRegistry();
-BackendRegistry& getBackendRegistry() {
-  static BackendRegistry backend_reg;
-  return backend_reg;
-}
+namespace {
 
-PyTorchBackendInterface* get_backend_class(const char* name) {
-  return getBackendRegistry().get_backend_class(name);
-}
+// The max number of backends that can be registered globally.
+constexpr size_t kMaxRegisteredBackends = 16;
+
+// TODO(T128866626): Remove global static variables. We want to be able to run
+// multiple Executor instances and having a global registration isn't a viable
+// solution in the long term.
+
+/// Global table of registered backends.
+Backend registered_backends[kMaxRegisteredBackends];
 
-PyTorchBackendInterface* BackendRegistry::get_backend_class(const char* name) {
-  for (size_t idx = 0; idx < registrationTableSize_; idx++) {
-    Backend backend = backend_table_[idx];
-    if (strcmp(backend.name_, name) == 0) {
-      return backend.interface_ptr_;
+/// The number of backends registered in the table.
+size_t num_registered_backends = 0;
+
+} // namespace
+
+PyTorchBackendInterface* get_backend_class(const char* name) {
+  for (size_t i = 0; i < num_registered_backends; i++) {
+    Backend backend = registered_backends[i];
+    if (strcmp(backend.name, name) == 0) {
+      return backend.backend;
     }
   }
   return nullptr;
 }
 
 Error register_backend(const Backend& backend) {
-  return getBackendRegistry().register_backend(backend);
-}
-
-Error BackendRegistry::register_backend(const Backend& backend) {
-  if (registrationTableSize_ >= kRegistrationTableMaxSize) {
+  if (num_registered_backends >= kMaxRegisteredBackends) {
     return Error::Internal;
   }
 
   // Check if the name already exists in the table
-  if (this->get_backend_class(backend.name_) != nullptr) {
+  if (get_backend_class(backend.name) != nullptr) {
     return Error::InvalidArgument;
   }
 
-  backend_table_[registrationTableSize_++] = backend;
+  registered_backends[num_registered_backends++] = backend;
   return Error::Ok;
 }
 

runtime/backend/interface.h

@@ -110,46 +110,6 @@ class PyTorchBackendInterface {
   virtual void destroy(ET_UNUSED DelegateHandle* handle) const {}
 };
 
-struct Backend {
-  const char* name_;
-  PyTorchBackendInterface* interface_ptr_;
-};
-
-// The max number of backends that can be registered in
-// an app. It's hard coded to 16 because it's not estimated
-// to have more than 16 backends in a system. Each table
-// element has two pointers, represented by Backend struct.
-// The memory overhead for this table is minimum (only a few bytes).
-constexpr size_t kRegistrationTableMaxSize = 16;
-
-class BackendRegistry {
- public:
-  BackendRegistry() : registrationTableSize_(0) {}
-
-  /**
-   * Registers the Backend object (i.e. string name and PyTorchBackendInterface
-   * pair) so that it could be called via the name during the runtime.
-   * @param[in] backend Backend object of the user-defined backend delegate.
-   * @retval Error code representing whether registration was successful.
-   */
-  ET_NODISCARD Error register_backend(const Backend& backend);
-
-  /**
-   * Returns the corresponding object pointer for a given string name.
-   * The mapping is populated using register_backend method.
-   *
-   * @param[in] name Name of the user-defined backend delegate.
-   * @retval Pointer to the appropriate object that implements
-   *         PyTorchBackendInterface. Nullptr if it can't find anything
-   *         with the given name.
-   */
-  PyTorchBackendInterface* get_backend_class(const char* name);
-
- private:
-  Backend backend_table_[kRegistrationTableMaxSize];
-  size_t registrationTableSize_;
-};
-
 /**
  * Returns the corresponding object pointer for a given string name.
  * The mapping is populated using register_backend method.
@@ -161,6 +121,16 @@ class BackendRegistry {
  */
 PyTorchBackendInterface* get_backend_class(const char* name);
 
+/**
+ * A named instance of a backend.
+ */
+struct Backend {
+  /// The name of the backend. Must match the string used in the PTE file.
+  const char* name;
+  /// The instance of the backend to use when loading and executing programs.
+  PyTorchBackendInterface* backend;
+};
+
 /**
  * Registers the Backend object (i.e. string name and PyTorchBackendInterface
  * pair) so that it could be called via the name during the runtime.
@@ -178,11 +148,9 @@ namespace executor {
 // TODO(T197294990): Remove these deprecated aliases once all users have moved
 // to the new `::executorch` namespaces.
 using ::executorch::runtime::Backend;
-using ::executorch::runtime::BackendRegistry;
 using ::executorch::runtime::CompileSpec;
 using ::executorch::runtime::DelegateHandle;
 using ::executorch::runtime::get_backend_class;
-// using ::executorch::runtime::kRegistrationTableMaxSize;
 using ::executorch::runtime::PyTorchBackendInterface;
 using ::executorch::runtime::register_backend;
 using ::executorch::runtime::SizedBuffer;