"Writeback of base register is UNPREDICTABLE" error message

I'm getting a "Warning: writeback of base register is UNPREDICTABLE" message when compiling an interrupt handler. Why is this?

This indicates that you are compiling an interrupt handler function with optimization enabled.

It is a known problem with GCC that the optimizer often breaks interrupt handler function code so we recommend disabling optimization when compiling these functions. Note that this only applies to interrupt handler functions (i.e. functions marked with the __interrupt__ function attribute), it does not apply to regular functions that the interrupt handler may call.