Optimize API connection memory with tagged pointers (#9203)

esphome/components/api/api_connection.h

@@ -483,55 +483,57 @@ class APIConnection : public APIServerConnection {
   // Function pointer type for message encoding
   using MessageCreatorPtr = uint16_t (*)(EntityBase *, APIConnection *, uint32_t remaining_size, bool is_single);
 
-  // Optimized MessageCreator class using union dispatch
+  // Optimized MessageCreator class using tagged pointer
   class MessageCreator {
+    // Ensure pointer alignment allows LSB tagging
+    static_assert(alignof(std::string *) > 1, "String pointer alignment must be > 1 for LSB tagging");
+
    public:
-    // Constructor for function pointer (message_type = 0)
-    MessageCreator(MessageCreatorPtr ptr) : message_type_(0) { data_.ptr = ptr; }
+    // Constructor for function pointer
+    MessageCreator(MessageCreatorPtr ptr) {
+      // Function pointers are always aligned, so LSB is 0
+      data_.ptr = ptr;
+    }
 
     // Constructor for string state capture
-    MessageCreator(const std::string &value, uint16_t msg_type) : message_type_(msg_type) {
-      data_.string_ptr = new std::string(value);
+    explicit MessageCreator(const std::string &str_value) {
+      // Allocate string and tag the pointer
+      auto *str = new std::string(str_value);
+      // Set LSB to 1 to indicate string pointer
+      data_.tagged = reinterpret_cast<uintptr_t>(str) | 1;
     }
 
     // Destructor
     ~MessageCreator() {
-      // Clean up string data for string-based message types
-      if (uses_string_data_()) {
-        delete data_.string_ptr;
+      if (has_tagged_string_ptr_()) {
+        delete get_string_ptr_();
       }
     }
 
     // Copy constructor
-    MessageCreator(const MessageCreator &other) : message_type_(other.message_type_) {
-      if (message_type_ == 0) {
-        data_.ptr = other.data_.ptr;
-      } else if (uses_string_data_()) {
-        data_.string_ptr = new std::string(*other.data_.string_ptr);
+    MessageCreator(const MessageCreator &other) {
+      if (other.has_tagged_string_ptr_()) {
+        auto *str = new std::string(*other.get_string_ptr_());
+        data_.tagged = reinterpret_cast<uintptr_t>(str) | 1;
       } else {
-        data_ = other.data_;  // For POD types
+        data_ = other.data_;
       }
     }
 
     // Move constructor
-    MessageCreator(MessageCreator &&other) noexcept : data_(other.data_), message_type_(other.message_type_) {
-      other.message_type_ = 0;  // Reset other to function pointer type
-      other.data_.ptr = nullptr;
-    }
+    MessageCreator(MessageCreator &&other) noexcept : data_(other.data_) { other.data_.ptr = nullptr; }
 
     // Assignment operators (needed for batch deduplication)
     MessageCreator &operator=(const MessageCreator &other) {
       if (this != &other) {
         // Clean up current string data if needed
-        if (uses_string_data_()) {
-          delete data_.string_ptr;
+        if (has_tagged_string_ptr_()) {
+          delete get_string_ptr_();
         }
         // Copy new data
-        message_type_ = other.message_type_;
-        if (other.message_type_ == 0) {
-          data_.ptr = other.data_.ptr;
-        } else if (other.uses_string_data_()) {
-          data_.string_ptr = new std::string(*other.data_.string_ptr);
+        if (other.has_tagged_string_ptr_()) {
+          auto *str = new std::string(*other.get_string_ptr_());
+          data_.tagged = reinterpret_cast<uintptr_t>(str) | 1;
         } else {
           data_ = other.data_;
         }
@@ -542,30 +544,35 @@ class APIConnection : public APIServerConnection {
     MessageCreator &operator=(MessageCreator &&other) noexcept {
       if (this != &other) {
         // Clean up current string data if needed
-        if (uses_string_data_()) {
-          delete data_.string_ptr;
+        if (has_tagged_string_ptr_()) {
+          delete get_string_ptr_();
         }
         // Move data
-        message_type_ = other.message_type_;
         data_ = other.data_;
         // Reset other to safe state
-        other.message_type_ = 0;
         other.data_.ptr = nullptr;
       }
       return *this;
     }
 
-    // Call operator
-    uint16_t operator()(EntityBase *entity, APIConnection *conn, uint32_t remaining_size, bool is_single) const;
+    // Call operator - now accepts message_type as parameter
+    uint16_t operator()(EntityBase *entity, APIConnection *conn, uint32_t remaining_size, bool is_single,
+                        uint16_t message_type) const;
 
    private:
-    // Helper to check if this message type uses heap-allocated strings
-    bool uses_string_data_() const { return message_type_ == EventResponse::MESSAGE_TYPE; }
-    union CreatorData {
-      MessageCreatorPtr ptr;    // 8 bytes
-      std::string *string_ptr;  // 8 bytes
-    } data_;                    // 8 bytes
-    uint16_t message_type_;     // 2 bytes (0 = function ptr, >0 = state capture)
+    // Check if this contains a string pointer
+    bool has_tagged_string_ptr_() const { return (data_.tagged & 1) != 0; }
+
+    // Get the actual string pointer (clears the tag bit)
+    std::string *get_string_ptr_() const {
+      // NOLINTNEXTLINE(performance-no-int-to-ptr)
+      return reinterpret_cast<std::string *>(data_.tagged & ~uintptr_t(1));
+    }
+
+    union {
+      MessageCreatorPtr ptr;
+      uintptr_t tagged;
+    } data_;  // 4 bytes on 32-bit
   };
 
   // Generic batching mechanism for both state updates and entity info