[esp8266] Keep switch lookup tables out of RAM (#20398)

esphome/components/esp8266/__init__.py

@@ -444,6 +444,8 @@ async def to_code(config: ConfigType) -> None:
     cg.add_build_flag("-DUSE_ARDUINO")
     cg.add_build_flag("-DUSE_ESP8266_FRAMEWORK_ARDUINO")
     cg.add_build_flag("-Wno-nonnull-compare")
+    # .rodata is linked into RAM on ESP8266; keep switch lookup tables (CSWTCH) out of it
+    cg.add_build_flag("-fno-tree-switch-conversion")
     if use_platformio:
         cg.add_platformio_option("framework", "arduino")
         cg.add_platformio_option("platform", conf[CONF_PLATFORM_VERSION])

tests/component_tests/esp8266/config/minimal.yaml

@@ -0,0 +1,5 @@
+esphome:
+  name: test
+
+esp8266:
+  board: d1_mini

tests/component_tests/esp8266/test_build_flags.py

@@ -0,0 +1,17 @@
+"""Tests for ESP8266 build flags."""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+from pathlib import Path
+
+from esphome.core import CORE
+
+
+def test_esp8266_keeps_switches_out_of_ram(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    generate_main(component_config_path("minimal.yaml"))
+
+    assert "-fno-tree-switch-conversion" in CORE.build_flags