How to define "implicit extend"

[devik]

Hello,

I'd like to ask another (maybe stupid) question.
CPU I'm defining can opearate on each 64bit register
in all modes from QI to DI. All operations in
non-DI modes clears all upper bits.
So for:
int a; short b; a=b+b;

I can do:
add.16 b,b,a

and it does zero_extend to SImode by default. Is there
way to tell GCC that all registers can be zero_extended
just by pretending they already are ?

thanks, devik