Add LTO support for nrf52840 while preserving interrupt handlers

extra_scripts/nrf52_lto.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+# trunk-ignore-all(ruff/F821)
+# trunk-ignore-all(flake8/F821)
+#
+# Whole-image LTO for nrf52840, EXCEPT the interrupt handlers.
+#
+# Every interrupt/exception handler is referenced only from the assembly vector
+# table (gcc_startup_nrf52840.S), which LTO cannot see. So whole-program LTO judges
+# the handlers dead, removes them, and the weak `b .` Default_Handler stubs prevail
+# -> the first IRQ (FreeRTOS scheduler-start SVC, then GPIOTE/RTC/USBD/...) lands in
+# an infinite loop and the chip hangs. Compiling the handler-bearing objects WITHOUT
+# LTO lets ordinary linking keep the strong handlers (exactly like a non-LTO build):
+#   - framework core (/FrameworkArduino/, /cores/nRF5/): every nrfx ISR + the
+#     FreeRTOS SVC/PendSV port (vPortSVCHandler / xPortPendSVHandler).
+#   - TinyUSB's nrf port: USBD_IRQHandler -- without it USB enumerates but the data
+#     path hangs (the host's first transfer interrupt is never serviced).
+# Everything else (app src + RadioLib/Crypto/nanopb/sensor libs/...) stays LTO'd.
+import os
+
+Import("env")
+
+env.Append(LINKFLAGS=["-flto", "-flto-partition=1to1"])
+
+# Private include dir for the TinyUSB nrf-port re-compile below (tusb_option.h etc.).
+_fw = env.PioPlatform().get_package_dir("framework-arduinoadafruitnrf52") or ""
+_tinyusb_src = os.path.join(_fw, "libraries", "Adafruit_TinyUSB_Arduino", "src")
+
+FRAMEWORK = ("/FrameworkArduino/", "/cores/nRF5/")
+USB_ISR = "Adafruit_TinyUSB_nrf"  # defines USBD_IRQHandler
+
+
+def _no_lto(node):
+    try:
+        path = node.get_abspath()
+    except Exception:
+        path = str(node)
+    if USB_ISR in path:
+        return env.Object(
+            node,
+            CCFLAGS=env["CCFLAGS"] + ["-fno-lto"],
+            CPPPATH=env["CPPPATH"] + [_tinyusb_src],
+        )
+    if any(s in path for s in FRAMEWORK):
+        return env.Object(node, CCFLAGS=env["CCFLAGS"] + ["-fno-lto"])
+    return node
+
+
+env.AddBuildMiddleware(_no_lto)

variants/nrf52840/rak4631/platformio.ini

@@ -22,6 +22,10 @@ build_flags = ${nrf52840_base.build_flags}
   -DRADIOLIB_EXCLUDE_SX127X=1
   -DRADIOLIB_EXCLUDE_LR11X0=1
   -DRADIOLIB_EXCLUDE_LR2021=1
+  -fmerge-all-constants ; fold identical constants across the image (~0.7KB; same flag stm32 uses)
+  -flto                 ; whole-image LTO (~-57KB); nrf52_lto.py keeps the hardware layer out of LTO so the vector ISRs survive
+extra_scripts = ${nrf52_base.extra_scripts}
+  pre:extra_scripts/nrf52_lto.py
 build_src_filter = ${nrf52_base.build_src_filter} \
   +<../variants/nrf52840/rak4631> \
   +<mesh/eth/> \