Why does V4SI take four GPRs, that's the question. If you treat them as V2SI registers GCC ought to be able to synthesize the V4SI as two V2SIs. It might be as simple as "fixing" HARD_REGNO_NREGS... probably not, though. Nope, I remember somewhere in the code reading "it is not worth" synthesizing a vector mode out of smaller vector modes. Paolo