From ca59ab8f370157af31b5c44a9aaef16eb3ec139c Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Tue, 27 Jan 2026 07:47:34 -1000
Subject: [PATCH] [esp32] Eliminate dead exception class code via linker wraps
 (#13564)

---
 esphome/components/esp32/__init__.py     | 13 ++++++
 esphome/components/esp32/throw_stubs.cpp | 57 ++++++++++++++++++++++++
 2 files changed, 70 insertions(+)
 create mode 100644 esphome/components/esp32/throw_stubs.cpp

diff --git a/esphome/components/esp32/__init__.py b/esphome/components/esp32/__init__.py
index da49fdc070..b7faccaed6 100644
--- a/esphome/components/esp32/__init__.py
+++ b/esphome/components/esp32/__init__.py
@@ -1048,6 +1048,19 @@ async def to_code(config):
         cg.add_build_flag("-DUSE_ESP32_FRAMEWORK_ESP_IDF")
         if use_platformio:
             cg.add_platformio_option("framework", "espidf")
+
+        # Wrap std::__throw_* functions to abort immediately, eliminating ~3KB of
+        # exception class overhead. See throw_stubs.cpp for implementation.
+        # ESP-IDF already compiles with -fno-exceptions, so this code was dead anyway.
+        for mangled in [
+            "_ZSt20__throw_length_errorPKc",
+            "_ZSt19__throw_logic_errorPKc",
+            "_ZSt20__throw_out_of_rangePKc",
+            "_ZSt24__throw_out_of_range_fmtPKcz",
+            "_ZSt17__throw_bad_allocv",
+            "_ZSt25__throw_bad_function_callv",
+        ]:
+            cg.add_build_flag(f"-Wl,--wrap={mangled}")
     else:
         cg.add_build_flag("-DUSE_ARDUINO")
         cg.add_build_flag("-DUSE_ESP32_FRAMEWORK_ARDUINO")
diff --git a/esphome/components/esp32/throw_stubs.cpp b/esphome/components/esp32/throw_stubs.cpp
new file mode 100644
index 0000000000..e82e5645de
--- /dev/null
+++ b/esphome/components/esp32/throw_stubs.cpp
@@ -0,0 +1,57 @@
+/*
+ * Linker wrap stubs for std::__throw_* functions.
+ *
+ * ESP-IDF compiles with -fno-exceptions, so C++ exceptions always abort.
+ * However, ESP-IDF only wraps low-level functions (__cxa_throw, etc.),
+ * not the std::__throw_* functions that construct exception objects first.
+ * This pulls in ~3KB of dead exception class code that can never run.
+ *
+ * ESP8266 Arduino already solved this: their toolchain rebuilds libstdc++
+ * with throw functions that just call abort(). We achieve the same result
+ * using linker --wrap without requiring toolchain changes.
+ *
+ * These stubs abort immediately with a descriptive message, allowing
+ * the linker to dead-code eliminate the exception class infrastructure.
+ *
+ * Wrapped functions and their callers:
+ * - std::__throw_length_error: std::string::reserve, std::vector::reserve
+ * - std::__throw_logic_error: std::promise, std::packaged_task
+ * - std::__throw_out_of_range: std::string::at, std::vector::at
+ * - std::__throw_out_of_range_fmt: std::bitset::to_ulong
+ * - std::__throw_bad_alloc: operator new
+ * - std::__throw_bad_function_call: std::function::operator()
+ */
+
+#ifdef USE_ESP_IDF
+#include "esp_system.h"
+
+namespace esphome::esp32 {}
+
+// Linker wraps for std::__throw_* - must be extern "C" at global scope.
+// Names must be __wrap_ + mangled name for the linker's --wrap option.
+
+// NOLINTBEGIN(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+extern "C" {
+
+// std::__throw_length_error(char const*) - called when container size exceeds max_size()
+void __wrap__ZSt20__throw_length_errorPKc(const char *) { esp_system_abort("std::length_error"); }
+
+// std::__throw_logic_error(char const*) - called for logic errors (e.g., promise already satisfied)
+void __wrap__ZSt19__throw_logic_errorPKc(const char *) { esp_system_abort("std::logic_error"); }
+
+// std::__throw_out_of_range(char const*) - called by at() when index is out of bounds
+void __wrap__ZSt20__throw_out_of_rangePKc(const char *) { esp_system_abort("std::out_of_range"); }
+
+// std::__throw_out_of_range_fmt(char const*, ...) - called by bitset::to_ulong when value doesn't fit
+void __wrap__ZSt24__throw_out_of_range_fmtPKcz(const char *, ...) { esp_system_abort("std::out_of_range"); }
+
+// std::__throw_bad_alloc() - called when operator new fails
+void __wrap__ZSt17__throw_bad_allocv() { esp_system_abort("std::bad_alloc"); }
+
+// std::__throw_bad_function_call() - called when invoking empty std::function
+void __wrap__ZSt25__throw_bad_function_callv() { esp_system_abort("std::bad_function_call"); }
+
+}  // extern "C"
+// NOLINTEND(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+
+#endif  // USE_ESP_IDF