Failure with Link Time Optimization enabled

[unlimitedbacon]

I am trying to use Link Time Optimization (via GCC flag -flto) to reduce binary size. My sketch is too large to fit on the H7's flash otherwise. However, if I enable this then mounting a USB drive always fails.

I am using this library via Arduino_UnifiedStorage > Arduino_POSIXStorage. I don't know enough about how linking works to say whether the problem is really in this library, or one of those. However I was able to trace the issue as far as this section in Arduino_POSIXStorage.cpp.

https://github.com/arduino-libraries/Arduino_POSIXStorage/blob/2b1e7f641bbf565ebcf194cd7d24ecb812e59b63/src/Arduino_POSIXStorage.cpp#L475C1-L486C4

  if (false == (usbHostDevice->connected()))
  {
    if (false == (usbHostDevice->connect()))
    {
      // Only delete if the object was created by this function
      if (false == hotplugKeep)
      {
        deleteDevice(DEV_USB, &usb);
      }
      return ENOTBLK;
    }
  }

usbHostDevice->connect() is returning false.

To try to get more info I set #define DEBUG 4 in dgb.h. Interestingly, with LTO enabled there is no debug output at all. But with LTO off I can see all the debug output over serial and the drive works correctly. This makes me think that USBHostMSD is getting optimized out completely, but I'm not sure.