I want to use the T bit, but would like to know if the gcc compiler would ever use it.
I've never seen it, but that doesn't mean anything. I've tried to look at the source code (gcc) and search for any sign that the t bit is used, but don't see anything (not sure I'm looking at the correct code- gcc/gcc/config/avr/avr.c+avr.h?). I suspect it would be kinda odd for it to use the T bit in any way.
I want to claim it as my own little personal bit with a little set of instructions to go with it (if no one else wants to claim it). If gcc happens to use it, I better keep my paws off.