Hi Bernd,
I had a look around. There's code testing HARD_REGNO_NREGS in ree.c:combine_set_extension. It's inside #if 0, and labelled "temporarily disabled". See if enabling that helps you? (Jeff, that #if 0 was added by you).
I suspect that the code was disabled because it prevented too many useful optimization opportunities.
The code there would solve this problem, but the approach is is overly cautious, since it disables the optimization for all extensions that increase the number of hard registers used. Some of these will be viable candidates, provided that the extra hard registers are no used. (This is certainly true for the RL78, where the (patched) optimization does improve code, even though the widening does use extra registers).
Cheers Nick
