[esp32][libretiny] Skip registering static destructors that never run (#20440)

esphome/components/esp32/__init__.py

@@ -2926,6 +2926,11 @@ async def to_code(config):
     # produce reproducible outputs and downstream tooling can reuse artifacts.
     add_idf_sdkconfig_option("CONFIG_APP_REPRODUCIBLE_BUILD", True)
 
+    # Static destructors never run, so skip registering them. See atexit_stubs.cpp.
+    # --undefined: libsrc.a is scanned before the IDF libraries that also register them.
+    cg.add_build_flag("-Wl,--wrap=__cxa_atexit")
+    cg.add_build_flag("-Wl,--undefined=__wrap___cxa_atexit")
+
     if conf[CONF_TYPE] == FRAMEWORK_ESP_IDF:
         cg.add_build_flag("-DUSE_ESP_IDF")
         cg.add_build_flag("-DUSE_ESP32_FRAMEWORK_ESP_IDF")

esphome/components/esp32/atexit_stubs.cpp

@@ -0,0 +1,20 @@
+/*
+ * Linker wrap stub for __cxa_atexit().
+ *
+ * GCC registers the destructor of every global and function local static
+ * object with __cxa_atexit(). The firmware never returns from main or calls
+ * exit(), so these destructors never run, but the first registration links
+ * newlib's exit handler tables (about 400 bytes of RAM on ESP32).
+ */
+
+#include "esphome/core/defines.h"
+
+#ifdef USE_ESP32
+
+namespace esphome::esp32 {}
+
+// NOLINTBEGIN(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+extern "C" int __wrap___cxa_atexit(void (*)(void *), void *, void *) { return 0; }
+// NOLINTEND(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+
+#endif  // USE_ESP32

esphome/components/libretiny/__init__.py

@@ -492,6 +492,9 @@ async def component_to_code(config):
         # Not enabled on RTL87xx/LN882x — costs more heap than it saves there.
         cg.add_build_flag("-DconfigSUPPORT_STATIC_ALLOCATION=1")
 
+    # Static destructors never run, so skip registering them. See atexit_stubs.cpp.
+    cg.add_build_flag("-Wl,--wrap=__cxa_atexit")
+
     # LN882x: a zero-size allocation must not trip the SDK's assert (see ln882x_zero_malloc.c).
     if config[CONF_COMPONENT_ID] == COMPONENT_LN882X:
         cg.add_build_flag("-Wl,--wrap=pvPortMalloc")

esphome/components/libretiny/atexit_stubs.cpp

@@ -0,0 +1,20 @@
+/*
+ * Linker wrap stub for __cxa_atexit().
+ *
+ * GCC registers the destructor of every global and function local static
+ * object with __cxa_atexit(). The firmware never returns from main or calls
+ * exit(), so these destructors never run, but the first registration links
+ * newlib's exit handler tables.
+ */
+
+#include "esphome/core/defines.h"
+
+#ifdef USE_LIBRETINY
+
+namespace esphome::libretiny {}
+
+// NOLINTBEGIN(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+extern "C" int __wrap___cxa_atexit(void (*)(void *), void *, void *) { return 0; }
+// NOLINTEND(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+
+#endif  // USE_LIBRETINY