Enable Link Time Optimisation

ref: https://gcc.gnu.org/wiki/LinkTimeOptimization

Link Time Optimisation defers final compilation until the link
stage, which allows more aggressive inlining and optimisations
to occur, as more information is known at link time vs when
each object is compiled.

For the esp32dev platform, I have the following size reductions:
  Without LTO:
    1313024 .pio/build/esp32dev/firmware.bin

  With LTO:
    1214256 .pio/build/esp32dev/firmware.bin

Signed-off-by: Alastair D'Silva <alastair@d-silva.org>

Author: deece

pio-scripts/lto.py

@@ -0,0 +1,27 @@
+Import('env')
+
+env.Replace(AR=env['AR'].replace('elf-ar', 'elf-gcc-ar'))
+env.Replace(RANLIB=env['RANLIB'].replace('elf-ranlib', 'elf-gcc-ranlib'))
+
+# Something later clobbers AR & RANLIB, so until https://github.com/platformio/platform-espressif32/pull/1329
+# is available, wrap the replace function to protect them
+
+# Save a reference to the original env.Replace()
+original_replace = env.Replace
+
+def create_replace_wrapper(env):
+    def replace_wrapper(**kw):
+        if 'AR' in kw:
+            kw.pop("AR")
+        if 'RANLIB' in kw:
+            kw.pop("RANLIB")
+
+        original_replace(**kw)
+    
+    return replace_wrapper
+
+# Replace the env.Replace with the wrapper
+env.Replace = create_replace_wrapper(env)
+
+print(f"Updated AR: {env['AR']}")
+print(f"Updated RANLIB: {env['RANLIB']}")

platformio.ini

@@ -87,6 +87,7 @@ debug_flags = -D DEBUG=1 -D WLED_DEBUG
 #  This reduces the OTA size with ~45KB, so it's especially useful on low memory boards (512k/1m).
 # ------------------------------------------------------------------------------
 build_flags =
+  -flto
   -Wno-attributes
   -DMQTT_MAX_PACKET_SIZE=1024
   -DSECURE_CLIENT=SECURE_CLIENT_BEARSSL
@@ -103,6 +104,8 @@ build_flags =
   -D DECODE_LG=true
   -DWLED_USE_MY_CONFIG
 
+
+
 build_unflags =
 
 build_flags_esp8266 = ${common.build_flags}  ${esp8266.build_flags}
@@ -121,6 +124,7 @@ extra_scripts =
   post:pio-scripts/strip-floats.py
   pre:pio-scripts/user_config_copy.py
   pre:pio-scripts/build_ui.py
+  pre:pio-scripts/lto.py
 
 # ------------------------------------------------------------------------------
 # COMMON SETTINGS:

wled00/wled00.ino

@@ -12,10 +12,12 @@
  */
 #include "wled.h"
 
+void setup() __attribute__((used));
 void setup() {
   WLED::instance().setup();
 }
 
+void loop() __attribute__((used));
 void loop() {
   WLED::instance().loop();
 }